explanation blue bibcodes open ADS page with paths to full text
Author name code: carlsson
ADS astronomy entries on 2022-09-14
author:"Carlsson, Mats"
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Title: Quiet Sun Center to Limb Variation of the Linear Polarization
Observed by CLASP2 Across the Mg II h and k Lines
Authors: Rachmeler, L. A.; Bueno, J. Trujillo; McKenzie, D. E.;
Ishikawa, R.; Auchère, F.; Kobayashi, K.; Kano, R.; Okamoto,
T. J.; Bethge, C. W.; Song, D.; Ballester, E. Alsina; Belluzzi,
L.; Pino Alemán, T. del; Ramos, A. Asensio; Yoshida, M.; Shimizu,
T.; Winebarger, A.; Kobelski, A. R.; Vigil, G. D.; Pontieu, B. De;
Narukage, N.; Kubo, M.; Sakao, T.; Hara, H.; Suematsu, Y.; Štěpán,
J.; Carlsson, M.; Leenaarts, J.
2022ApJ...936...67R Altcode: 2022arXiv220701788R
The CLASP2 (Chromospheric LAyer Spectro-Polarimeter 2) sounding rocket
mission was launched on 2019 April 11. CLASP2 measured the four Stokes
parameters of the Mg II h and k spectral region around 2800 Å along a
200″ slit at three locations on the solar disk, achieving the first
spatially and spectrally resolved observations of the solar polarization
in this near-ultraviolet region. The focus of the work presented here
is the center-to-limb variation of the linear polarization across these
resonance lines, which is produced by the scattering of anisotropic
radiation in the solar atmosphere. The linear polarization signals of
the Mg II h and k lines are sensitive to the magnetic field from the
low to the upper chromosphere through the Hanle and magneto-optical
effects. We compare the observations to theoretical predictions
from radiative transfer calculations in unmagnetized semiempirical
models, arguing that magnetic fields and horizontal inhomogeneities
are needed to explain the observed polarization signals and spatial
variations. This comparison is an important step in both validating and
refining our understanding of the physical origin of these polarization
signatures, and also in paving the way toward future space telescopes
for probing the magnetic fields of the solar upper atmosphere via
ultraviolet spectropolarimetry.
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Title: Formation and heating of chromospheric fibrils in a
radiation-MHD simulation
Authors: Druett, M. K.; Leenaarts, J.; Carlsson, M.; Szydlarski, M.
2022A&A...665A...6D Altcode: 2021arXiv211208245D
<BR /> Aims: We examine the movements of mass elements within dense
fibrils using passive tracer particles (corks) in order to understand
the creation and destruction processes of fibrils. <BR /> Methods:
Simulated fibrils were selected at times when they were visible in a
Hα image proxy. The corks were selected within fibril Hα formation
regions. From this set, we selected a cork and constructed the field
line passing through it. Other fibrilar corks close to this field
line were also selected and pathlines were constructed, revealing the
locations of the mass elements forwards and backwards in time. Finally,
we analysed the forces acting on these mass elements. <BR /> Results:
The main process of fibrilar loading in the simulation is different
to the mass loading scenario in which waves steepen into shocks and
push material upwards along the field lines from locations near their
footpoints. The twisted, low-lying field lines were destabilised
and then they untwisted, lifting the material trapped above their
apexes via the Lorentz force. Subsequently, the majority of the mass
drained down the field lines towards one or both footpoints under the
influence of gravity. Material with large horizontal velocities could
also be elevated in rising field lines, creating somewhat parabolic
motions, but the material was not generally moving upward along a
stationary magnetic field line during loading. <BR /> Conclusions:
The processes observed in the simulation are additional scenarios that
are plausible. The criteria for observing such events are described
in this work. We note that it is desirable for our simulations to
also be able to form more densely packed fibrils from material fed
from the base of field footpoints. The experimental parameters
required to achieve this are also discussed in this paper. <P
/>Movies associated to Figs. 1, 4, 9, 14 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202142399/olm">https://www.aanda.org</A>
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Title: Coronal oscillations in the self-consistent 3D MHD simulations
of the solar atmosphere
Authors: Kohutova, Petra; Antolin, Patrick; Carlsson, Mats; Popovas,
Andrius
2022cosp...44.2494K Altcode:
Solar coronal loops are commonly subject to oscillations. Coronal
oscillations are typically studied using highly idealised models of
magnetic flux-tubes. In order to improve our understanding of coronal
oscillations, it is necessary to consider the effect of realistic
magnetic field topology and evolution. To do this, we study excitation,
evolution and damping of coronal oscillations in three-dimensional
self-consistent simulations of solar atmosphere spanning from convection
zone to solar corona using the radiation-MHD code Bifrost. We use
forward-modelled EUV emission and three-dimensional tracing of magnetic
field to analyse oscillatory behaviour of individual magnetic loops. We
show that coronal loop oscillations are abundant in such models and
the oscillation modes and characteristics match those detected in solar
observations. Finally, we discuss the dynamics and variability of the
oscillating loops and the implications for coronal seismology.
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Title: Acoustic-gravity wave propagation characteristics in 3D
radiation hydrodynamic simulations of the solar atmosphere
Authors: Fleck, Bernhard; Khomenko, Elena; Carlsson, Mats; Rempel,
Matthias; Steiner, Oskar; Riva, Fabio; Vigeesh, Gangadharan
2022cosp...44.2503F Altcode:
There has been tremendous progress in the degree of realism of
three-dimensional radiation magneto-hydrodynamic simulations of the
solar atmosphere in the past decades. Four of the most frequently
used numerical codes are Bifrost, CO5BOLD, MANCHA3D, and MURaM. Here
we test and compare the wave propagation characteristics in model
runs from these four codes by measuring the dispersion relation
of acoustic-gravity waves at various heights. We find considerable
differences between the various models.
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Title: Stirring the Base of the Solar Wind: On Heat Transfer and
Vortex Formation
Authors: Finley, Adam J.; Brun, Sacha A.; Carlsson, Mats; Szydlarski,
Mikolaj; Hansteen, Viggo; Shoda, Munehito
2022arXiv220702878F Altcode:
Current models of the solar wind must approximate (or ignore) the
small-scale dynamics within the solar atmosphere, however these are
likely important in shaping the emerging wave-turbulence spectrum and
ultimately heating/accelerating the coronal plasma. The Bifrost code
produces realistic simulations of the solar atmosphere that facilitate
the analysis of spatial and temporal scales which are currently at,
or beyond, the limit of modern solar telescopes. For this study, the
Bifrost simulation is configured to represent the solar atmosphere in
a coronal hole region, from which the fast solar wind emerges. The
simulation extends from the upper-convection zone (2.5 Mm below the
photosphere) to the low-corona (14.5 Mm above the photosphere), with
a horizontal extent of 24 Mm x 24 Mm. The twisting of the coronal
magnetic field by photospheric flows, efficiently injects energy
into the low-corona. Poynting fluxes of up to $2-4$ kWm$^{-2}$ are
commonly observed inside twisted magnetic structures with diameters
in the low-corona of 1 - 5 Mm. Torsional Alfvén waves are favourably
transmitted along these structures, and will subsequently escape into
the solar wind. However, reflections of these waves from the upper
boundary condition make it difficult to unambiguously quantify the
emerging Alfvén wave-energy flux. This study represents a first step
in quantifying the conditions at the base of the solar wind using
Bifrost simulations. It is shown that the coronal magnetic field is
readily braided and twisted by photospheric flows. Temperature and
density contrasts form between regions with active stirring motions
and those without. Stronger whirlpool-like flows in the convection,
concurrent with magnetic concentrations, launch torsional Alfvén waves
up through the magnetic funnel network, which are expected to enhance
the turbulent generation of magnetic switchbacks in the solar wind.
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Title: Abundance diagnostics in active regions with Solar
Orbiter/SPICE
Authors: Giunta, Alessandra; Peter, Hardi; Parenti, Susanna; Buchlin,
Eric; Thompson, William; Auchere, Frederic; Kucera, Therese; Carlsson,
Mats; Janvier, Miho; Fludra, Andrzej; Hassler, Donald M.; Grundy,
Timothy; Sidher, Sunil; Guest, Steve; Leeks, Sarah; Fredvik, Terje;
Young, Peter
2022cosp...44.2583G Altcode:
With the launch of Solar Orbiter in February 2020, we are now able to
fully explore the link between the solar activity on the Sun and the
inner heliosphere. Elemental abundance measurements provide a key tracer
to probe the source regions of the solar wind and to track it from the
solar surface and corona to the heliosphere. Abundances of elements
with low first ionisation potential (FIP) are enhanced in the corona
relative to high-FIP elements, with respect to the photosphere. This is
known as the FIP effect, which is measured as abundance bias (FIP bias)
of low and high FIP elements. This effect is vital for understanding the
flow of mass and energy through the solar atmosphere. The comparison
between in-situ and remote sensing composition data, coupled with
modelling, will allow us to trace back the source of heliospheric
plasma. Solar Orbiter has a unique combination of in-situ and remote
sensing instruments that will help to make such a comparison. In
particular, the SPICE (Spectral Imaging of the Coronal Environment)
EUV spectrometer records spectra in two wavelength bands, 70.4-79.0
nm and 97.3-104.9 nm. SPICE is designed to provide spectroheliograms
using a core set of emission lines arising from ions of both low-FIP
and high-FIP elements such as C, N, O, Ne, Mg, S and Fe. These lines
are formed over a wide range of temperatures from 20,000 K to over 1
million K, enabling the analysis of the different layers of the solar
atmosphere. SPICE spectroheliograms can be processed to produce FIP
bias maps, which can be compared to in-situ measurements of the solar
wind composition of the same elements. During the Solar Orbiter Cruise
Phase, SPICE observed several active regions. We will present some of
these observations and discuss the SPICE diagnostic potential to derive
relative abundances (e.g., Mg/Ne) and the FIP bias in those regions.
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Title: The SPICE spectrograph on Solar Orbiter: an introduction and
results from the first Orbits
Authors: Auchère, Frédéric; Peter, Hardi; Parenti, Susanna; Buchlin,
Eric; Thompson, William; Auchere, Frederic; Teriaca, Luca; Kucera,
Therese; Carlsson, Mats; Janvier, Miho; Fludra, Andrzej; Giunta,
Alessandra; Schuehle, Udo; Hassler, Donald M.; Grundy, Timothy;
Sidher, Sunil; Fredvik, Terje; Plowman, Joseph; Aznar Cuadrado, Regina
2022cosp...44.1338A Altcode:
The Spectral Imaging of the Coronal Environment (SPICE) instrument is
the EUV imaging spectrometer on board the Solar Orbiter mission. With
its ability to derive physical properties of the coronal plasma,
SPICE is a key component of the payload to establish the connection
between the source regions and the in-situ measurements of the solar
wind. The spacecraft was successfully launched in February 2020 and
completed its cruise phase in December 2021. During this period,
the remote sensing instruments were mostly operated during limited
periods of time for 'checkout' engineering activities and synoptic
observations. Nonetheless, several of these periods provided enough
opportunities already to obtain new insights on coronal physics. During
the march 2022 perihelion - close to 0.3 AU - SPICE will provide
its highest spatial resolution data so far. Coordinated observations
between the remote sensing and in-situ instruments will provide the
first opportunity to use the full potential of the Solar Orbiter
mission. We will review the instrument characteristics and present
initial results from the cruise phase and first close encounter.
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Title: Effects of spatial resolution on inferences of atmospheric
quantities from simulations
Authors: Moe, Thore E.; Pereira, Tiago M. D.; Carlsson, Mats
2022A&A...662A..80M Altcode: 2022arXiv220408849M
Context. Small-scale processes are thought to be important for
the dynamics of the solar atmosphere. While numerical resolution
fundamentally limits their inclusion in magnetohydronamic (MHD)
simulations, real observations at the same nominal resolution should
still contain imprints of subresolution effects. This means that
the synthetic observables from a simulation of a given resolution
might not be directly comparable to real observables at the same
resolution. It is thus of interest to investigate how inferences
based on synthetic spectra from simulations with different numerical
resolutions compare, and whether these differences persist after the
spectra have been spatially degraded to a common resolution <BR />
Aims: We aim to compare synthetic spectra obtained from realistic 3D
radiative magnetohydrodynamic (rMHD) simulations run at different
numerical resolutions from the same initial atmosphere, using very
simple methods for inferring line-of-sight velocities and magnetic
fields. Additionally we examine how the differing spatial resolution
impacts the results retrieved from the STiC inversion code. <BR />
Methods: We used the RH 1.5D code to synthesize the photospheric
Fe I 617.33 line in local thermodynamic equilibrium (LTE), and the
chromospheric Ca II 854.209 line in non-LTE from three MHD simulation
snapshots of differing spatial resolution. The simulations were produced
by the Bifrost code, using horizontal grid spacing of 6 km, 12 km, and
23 km, respectively. They were started from the exact same atmosphere,
and the snapshots were taken after the same exact elapsed time. The
spectra obtained from the high-resolution snapshots were spatially
degraded to match the lowest resolution. Simple methods, such as the
center-of-gravity approach and the weak field approximation, were then
used to estimate line-of-sight velocities and magnetic fields for the
three cases after degradation. Finally, the spectra were input into
the STiC inversion code and the retrieved line-of-sight velocities
and magnetic field strengths, as well as the temperatures, from
the inversions were compared. <BR /> Results: We find that while the
simple inferences for all three simulations reveal the same large-scale
tendencies, the higher resolutions yield more fine-grained structures
and more extreme line-of-sight velocities and magnetic fields in
concentrated spots even after spatial smearing. We also see indications
that the imprints of subresolution effects on the degraded spectra
result in systematic errors in the inversions, and that these errors
increase with the amount of subresolution effects included. Fortunately,
however, we find that successively including more subresolution yields
smaller additional effects; that is to say, there is a clear trend of
diminishing importance for progressively finer subresolution effects.
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Title: An approximate recipe of chromospheric radiative losses for
solar flares
Authors: Hong, J.; Carlsson, M.; Ding, M. D.
2022A&A...661A..77H Altcode: 2022arXiv220307630H
Context. Radiative losses in the chromosphere are very important for the
energy balance of the Sun. There have been efforts to make simple lookup
tables for chromospheric radiative losses in the quiet Sun. During
solar flares, the atmospheric conditions are quite different, and
the currently available recipe is constructed from semi-empirical
models. How these recipes work in flare conditions remains to be
evaluated. <BR /> Aims: We aim to construct an approximate recipe
of chromospheric radiative losses for solar flares. <BR /> Methods:
We tabulate the optically thin radiative loss, escape probability,
and ionization fraction using a grid of flare models from radiative
hydrodynamic simulations as our dataset. <BR /> Results: We provide
new lookup tables to calculate chromospheric radiative losses for
flares. Compared with previous recipes, our recipe provides a better
approximation of the detailed radiative losses for flares.
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Title: The Atmospheric Response to High Nonthermal Electron-beam
Fluxes in Solar Flares. II. Hydrogen-broadening Predictions for
Solar Flare Observations with the Daniel K. Inouye Solar Telescope
Authors: Kowalski, Adam F.; Allred, Joel C.; Carlsson, Mats; Kerr,
Graham S.; Tremblay, Pier-Emmanuel; Namekata, Kosuke; Kuridze, David;
Uitenbroek, Han
2022ApJ...928..190K Altcode: 2022arXiv220113349K
Redshifted components of chromospheric emission lines in the hard X-ray
impulsive phase of solar flares have recently been studied through
their 30 s evolution with the high resolution of the Interface Region
Imaging Spectrograph. Radiative-hydrodynamic flare models show that
these redshifts are generally reproduced by electron-beam-generated
chromospheric condensations. The models produce large ambient electron
densities, and the pressure broadening of the hydrogen Balmer series
should be readily detected in observations. To accurately interpret
the upcoming spectral data of flares with the DKIST, we incorporate
nonideal, nonadiabatic line-broadening profiles of hydrogen into the
RADYN code. These improvements allow time-dependent predictions for
the extreme Balmer line wing enhancements in solar flares. We study two
chromospheric condensation models, which cover a range of electron-beam
fluxes (1 - 5 × 10<SUP>11</SUP> erg s<SUP>-1</SUP> cm<SUP>-2</SUP>) and
ambient electron densities (1 - 60 × 10<SUP>13</SUP> cm<SUP>-3</SUP>)
in the flare chromosphere. Both models produce broadening and
redshift variations within 10 s of the onset of beam heating. In the
chromospheric condensations, there is enhanced spectral broadening due
to large optical depths at Hα, Hβ, and Hγ, while the much lower
optical depth of the Balmer series H12-H16 provides a translucent
window into the smaller electron densities in the beam-heated layers
below the condensation. The wavelength ranges of typical DKIST/ViSP
spectra of solar flares will be sufficient to test the predictions
of extreme hydrogen wing broadening and accurately constrain large
densities in chromospheric condensations.
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Title: Chromospheric emission from nanoflare heating in RADYN
simulations
Authors: Bakke, H.; Carlsson, M.; Rouppe van der Voort, L.; Gudiksen,
B. V.; Polito, V.; Testa, P.; De Pontieu, B.
2022A&A...659A.186B Altcode: 2022arXiv220111961B
Context. Heating signatures from small-scale magnetic reconnection
events in the solar atmosphere have proven to be difficult to
detect through observations. Numerical models that reproduce flaring
conditions are essential in understanding how nanoflares may act as a
heating mechanism of the corona. <BR /> Aims: We study the effects of
non-thermal electrons in synthetic spectra from 1D hydrodynamic RADYN
simulations of nanoflare heated loops to investigate the diagnostic
potential of chromospheric emission from small-scale events. <BR />
Methods: The Mg II h and k, Ca II H and K, Ca II 854.2 nm, and Hα and
Hβ chromospheric lines were synthesised from various RADYN models of
coronal loops subject to electron beams of nanoflare energies. The
contribution function to the line intensity was computed to better
understand how the atmospheric response to the non-thermal electrons
affects the formation of spectral lines and the detailed shape of
their spectral profiles. <BR /> Results: The spectral line signatures
arising from the electron beams highly depend on the density of the
loop and the lower cutoff energy of the electrons. Low-energy (5 keV)
electrons deposit their energy in the corona and transition region,
producing strong plasma flows that cause both redshifts and blueshifts
of the chromospheric spectra. Higher-energy (10 and 15 keV) electrons
deposit their energy in the lower transition region and chromosphere,
resulting in increased emission from local heating. Our results indicate
that effects from small-scale events can be observed with ground-based
telescopes, expanding the list of possible diagnostics for the presence
and properties of nanoflares.
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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE). I. Coronal Heating
Authors: De Pontieu, Bart; Testa, Paola; Martínez-Sykora, Juan;
Antolin, Patrick; Karampelas, Konstantinos; Hansteen, Viggo; Rempel,
Matthias; Cheung, Mark C. M.; Reale, Fabio; Danilovic, Sanja; Pagano,
Paolo; Polito, Vanessa; De Moortel, Ineke; Nóbrega-Siverio, Daniel;
Van Doorsselaere, Tom; Petralia, Antonino; Asgari-Targhi, Mahboubeh;
Boerner, Paul; Carlsson, Mats; Chintzoglou, Georgios; Daw, Adrian;
DeLuca, Edward; Golub, Leon; Matsumoto, Takuma; Ugarte-Urra, Ignacio;
McIntosh, Scott W.; the MUSE Team
2022ApJ...926...52D Altcode: 2021arXiv210615584D
The Multi-slit Solar Explorer (MUSE) is a proposed mission composed of
a multislit extreme ultraviolet (EUV) spectrograph (in three spectral
bands around 171 Å, 284 Å, and 108 Å) and an EUV context imager (in
two passbands around 195 Å and 304 Å). MUSE will provide unprecedented
spectral and imaging diagnostics of the solar corona at high spatial
(≤0.″5) and temporal resolution (down to ~0.5 s for sit-and-stare
observations), thanks to its innovative multislit design. By obtaining
spectra in four bright EUV lines (Fe IX 171 Å, Fe XV 284 Å, Fe XIX-Fe
XXI 108 Å) covering a wide range of transition regions and coronal
temperatures along 37 slits simultaneously, MUSE will, for the first
time, "freeze" (at a cadence as short as 10 s) with a spectroscopic
raster the evolution of the dynamic coronal plasma over a wide range of
scales: from the spatial scales on which energy is released (≤0.″5)
to the large-scale (~170″ × 170″) atmospheric response. We use
numerical modeling to showcase how MUSE will constrain the properties of
the solar atmosphere on spatiotemporal scales (≤0.″5, ≤20 s) and
the large field of view on which state-of-the-art models of the physical
processes that drive coronal heating, flares, and coronal mass ejections
(CMEs) make distinguishing and testable predictions. We describe the
synergy between MUSE, the single-slit, high-resolution Solar-C EUVST
spectrograph, and ground-based observatories (DKIST and others), and
the critical role MUSE plays because of the multiscale nature of the
physical processes involved. In this first paper, we focus on coronal
heating mechanisms. An accompanying paper focuses on flares and CMEs.
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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE). II. Flares and Eruptions
Authors: Cheung, Mark C. M.; Martínez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham S.; Reeves, Katharine K.; Fletcher, Lyndsay; Jin,
Meng; Nóbrega-Siverio, Daniel; Danilovic, Sanja; Antolin, Patrick;
Allred, Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward;
Longcope, Dana; Takasao, Shinsuke; DeRosa, Marc L.; Boerner, Paul;
Jaeggli, Sarah; Nitta, Nariaki V.; Daw, Adrian; Carlsson, Mats; Golub,
Leon; The
2022ApJ...926...53C Altcode: 2021arXiv210615591C
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (subarcseconds) and temporal (less than a few tens of
seconds) scales of the coronal dynamics of solar flares and eruptive
phenomena. The highest-resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by the Interface Region Imaging
Spectrograph for the low solar atmosphere, we need high-resolution
spectroscopic measurements with simultaneous imaging to understand the
dominant processes. In this paper: (1) we introduce the Multi-slit Solar
Explorer (MUSE), a spaceborne observatory to fill this observational
gap by providing high-cadence (<20 s), subarcsecond-resolution
spectroscopic rasters over an active region size of the solar transition
region and corona; (2) using advanced numerical models, we demonstrate
the unique diagnostic capabilities of MUSE for exploring solar coronal
dynamics and for constraining and discriminating models of solar flares
and eruptions; (3) we discuss the key contributions MUSE would make
in addressing the science objectives of the Next Generation Solar
Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme
Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope
(and other ground-based observatories) can operate as a distributed
implementation of the NGSPM. This is a companion paper to De Pontieu
et al., which focuses on investigating coronal heating with MUSE.
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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE): II. Flares and Eruptions
Authors: Cheung, Chun Ming Mark; Martinez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham; Reeves, Katharine; Fletcher, Lyndsay; Jin,
Meng; Nobrega, Daniel; Danilovic, Sanja; Antolin, Patrick; Allred,
Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward; Longcope,
Dana; Takasao, Shinsuke; DeRosa, Marc; Boerner, Paul; Jaeggli, Sarah;
Nitta, Nariaki; Daw, Adrian; Carlsson, Mats; Golub, Leon
2021AGUFMSH51A..08C Altcode:
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (sub-arcseconds) and temporal scales (less than a few tens
of seconds) of the coronal dynamics of solar flares and eruptive
phenomena. The highest resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by IRIS for the low solar atmosphere,
we need high-resolution spectroscopic measurements with simultaneous
imaging to understand the dominant processes. In this paper: (1)
we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne
observatory to fill this observational gap by providing high-cadence
(<20 s), sub-arcsecond resolution spectroscopic rasters over an
active region size of the solar transition region and corona; (2)
using advanced numerical models, we demonstrate the unique diagnostic
capabilities of MUSE for exploring solar coronal dynamics, and for
constraining and discriminating models of solar flares and eruptions;
(3) we discuss the key contributions MUSE would make in addressing the
science objectives of the Next Generation Solar Physics Mission (NGSPM),
and how MUSE, the high-throughput EUV Solar Telescope (EUVST) and the
Daniel K Inouye Solar Telescope (and other ground-based observatories)
can operate as a distributed implementation of the NGSPM. This is a
companion paper to De Pontieu et al. (2021, also submitted to SH-17),
which focuses on investigating coronal heating with MUSE.
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Title: First observations from the SPICE EUV spectrometer on Solar
Orbiter
Authors: Fludra, A.; Caldwell, M.; Giunta, A.; Grundy, T.; Guest,
S.; Leeks, S.; Sidher, S.; Auchère, F.; Carlsson, M.; Hassler, D.;
Peter, H.; Aznar Cuadrado, R.; Buchlin, É.; Caminade, S.; DeForest,
C.; Fredvik, T.; Haberreiter, M.; Harra, L.; Janvier, M.; Kucera, T.;
Müller, D.; Parenti, S.; Schmutz, W.; Schühle, U.; Solanki, S. K.;
Teriaca, L.; Thompson, W. T.; Tustain, S.; Williams, D.; Young, P. R.;
Chitta, L. P.
2021A&A...656A..38F Altcode: 2021arXiv211011252F
<BR /> Aims: We present first science observations taken during the
commissioning activities of the Spectral Imaging of the Coronal
Environment (SPICE) instrument on the ESA/NASA Solar Orbiter
mission. SPICE is a high-resolution imaging spectrometer operating at
extreme ultraviolet (EUV) wavelengths. In this paper we illustrate
the possible types of observations to give prospective users a
better understanding of the science capabilities of SPICE. <BR />
Methods: We have reviewed the data obtained by SPICE between April
and June 2020 and selected representative results obtained with
different slits and a range of exposure times between 5 s and 180
s. Standard instrumental corrections have been applied to the raw
data. <BR /> Results: The paper discusses the first observations
of the Sun on different targets and presents an example of the full
spectra from the quiet Sun, identifying over 40 spectral lines from
neutral hydrogen and ions of carbon, oxygen, nitrogen, neon, sulphur,
magnesium, and iron. These lines cover the temperature range between
20 000 K and 1 million K (10 MK in flares), providing slices of the
Sun's atmosphere in narrow temperature intervals. We provide a list
of count rates for the 23 brightest spectral lines. We show examples
of raster images of the quiet Sun in several strong transition region
lines, where we have found unusually bright, compact structures in the
quiet Sun network, with extreme intensities up to 25 times greater
than the average intensity across the image. The lifetimes of these
structures can exceed 2.5 hours. We identify them as a transition
region signature of coronal bright points and compare their areas and
intensity enhancements. We also show the first above-limb measurements
with SPICE above the polar limb in C III, O VI, and Ne VIII lines, and
far off limb measurements in the equatorial plane in Mg IX, Ne VIII,
and O VI lines. We discuss the potential to use abundance diagnostics
methods to study the variability of the elemental composition that can
be compared with in situ measurements to help confirm the magnetic
connection between the spacecraft location and the Sun's surface,
and locate the sources of the solar wind. <BR /> Conclusions: The
SPICE instrument successfully performs measurements of EUV spectra
and raster images that will make vital contributions to the scientific
success of the Solar Orbiter mission.
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Title: Solar oxygen abundance
Authors: Bergemann, Maria; Hoppe, Richard; Semenova, Ekaterina;
Carlsson, Mats; Yakovleva, Svetlana A.; Voronov, Yaroslav V.;
Bautista, Manuel; Nemer, Ahmad; Belyaev, Andrey K.; Leenaarts, Jorrit;
Mashonkina, Lyudmila; Reiners, Ansgar; Ellwarth, Monika
2021MNRAS.508.2236B Altcode: 2021arXiv210901143B; 2021MNRAS.tmp.1964B
Motivated by the controversy over the surface metallicity of the
Sun, we present a re-analysis of the solar photospheric oxygen (O)
abundance. New atomic models of O and Ni are used to perform non-local
thermodynamic equilibrium (NLTE) calculations with 1D hydrostatic
(MARCS) and 3D hydrodynamical (Stagger and Bifrost) models. The
Bifrost 3D MHD simulations are used to quantify the influence of
the chromosphere. We compare the 3D NLTE line profiles with new
high-resolution, R$\approx 700\, 000$, spatially resolved spectra
of the Sun obtained using the IAG FTS instrument. We find that the
O I lines at 777 nm yield the abundance of log A(O) = 8.74 ± 0.03
dex, which depends on the choice of the H-impact collisional data and
oscillator strengths. The forbidden [O I] line at 630 nm is less model
dependent, as it forms nearly in LTE and is only weakly sensitive to
convection. However, the oscillator strength for this transition is more
uncertain than for the 777 nm lines. Modelled in 3D NLTE with the Ni I
blend, the 630 nm line yields an abundance of log A(O) = 8.77 ± 0.05
dex. We compare our results with previous estimates in the literature
and draw a conclusion on the most likely value of the solar photospheric
O abundance, which we estimate at log A(O) = 8.75 ± 0.03 dex.
---------------------------------------------------------
Title: Diagnostic capabilities of spectropolarimetric observations for
understanding solar phenomena. I. Zeeman-sensitive photospheric lines
Authors: Quintero Noda, C.; Barklem, P. S.; Gafeira, R.; Ruiz Cobo,
B.; Collados, M.; Carlsson, M.; Martínez Pillet, V.; Orozco Suárez,
D.; Uitenbroek, H.; Katsukawa, Y.
2021A&A...652A.161Q Altcode: 2021arXiv210605084Q
Future ground-based telescopes will expand our capabilities for
simultaneous multi-line polarimetric observations in a wide range of
wavelengths, from the near-ultraviolet to the near-infrared. This
creates a strong demand to compare candidate spectral lines to
establish a guideline of the lines that are most appropriate for each
observation target. We focused in this first work on Zeeman-sensitive
photospheric lines in the visible and infrared. We first examined their
polarisation signals and response functions using a 1D semi-empirical
atmosphere. Then we studied the spatial distribution of the line core
intensity and linear and circular polarisation signals using a realistic
3D numerical simulation. We ran inversions of synthetic profiles, and
we compared the heights at which we obtain a high correlation between
the input and the inferred atmosphere. We also used this opportunity
to revisit the atomic information we have on these lines and computed
the broadening cross-sections due to collisions with neutral hydrogen
atoms for all the studied spectral lines. The results reveal that
four spectral lines stand out from the rest for quiet-Sun and network
conditions: Fe I 5250.2, 6302, 8468, and 15 648 Å. The first three
form higher in the atmosphere, and the last line is mainly sensitive to
the atmospheric parameters at the bottom of the photosphere. However,
as they reach different heights, we strongly recommend using at least
one of the first three candidates together with the Fe I 15 648 Å line
to optimise our capabilities for inferring the thermal and magnetic
properties of the lower atmosphere.
---------------------------------------------------------
Title: Mapping of Solar Magnetic Fields from the Photosphere to the
Top of the Chromosphere with CLASP2
Authors: McKenzie, D.; Ishikawa, R.; Trujillo Bueno, J.; Auchere, F.;
del Pino Aleman, T.; Okamoto, T.; Kano, R.; Song, D.; Yoshida, M.;
Rachmeler, L.; Kobayashi, K.; Narukage, N.; Kubo, M.; Ishikawa, S.;
Hara, H.; Suematsu, Y.; Sakao, T.; Bethge, C.; De Pontieu, B.; Vigil,
G.; Winebarger, A.; Alsina Ballester, E.; Belluzzi, L.; Stepan, J.;
Asensio Ramos, A.; Carlsson, M.; Leenaarts, J.
2021AAS...23810603M Altcode:
Coronal heating, chromospheric heating, and the heating &
acceleration of the solar wind, are well-known problems in solar
physics. Additionally, knowledge of the magnetic energy that
powers solar flares and coronal mass ejections, important drivers
of space weather, is handicapped by imperfect determination of the
magnetic field in the sun's atmosphere. Extrapolation of photospheric
magnetic measurements into the corona is fraught with difficulties and
uncertainties, partly due to the vastly different plasma beta between
the photosphere and the corona. Better results in understanding
the coronal magnetic field should be derived from measurements of
the magnetic field in the chromosphere. To that end, we are pursuing
quantitative determination of the magnetic field in the chromosphere,
where plasma beta transitions from greater than unity to less than
unity, via ultraviolet spectropolarimetry. The CLASP2 mission, flown
on a sounding rocket in April 2019, succeeded in measuring all four
Stokes polarization parameters in UV spectral lines formed by singly
ionized Magnesium and neutral Manganese. Because these ions produce
spectral lines under different conditions, CLASP2 thus was able to
quantify the magnetic field properties at multiple heights in the
chromosphere simultaneously, as shown in the recent paper by Ishikawa
et al. In this presentation we will report the findings of CLASP2,
demonstrating the variation of magnetic fields along a track on
the solar surface and as a function of height in the chromosphere;
and we will illustrate what is next for the CLASP missions and the
demonstration of UV spectropolarimetry in the solar chromosphere.
---------------------------------------------------------
Title: Chromospheric Heating by Magnetohydrodynamic Waves and
Instabilities
Authors: Srivastava, A. K.; Ballester, J. L.; Cally, P. S.; Carlsson,
M.; Goossens, M.; Jess, D. B.; Khomenko, E.; Mathioudakis, M.;
Murawski, K.; Zaqarashvili, T. V.
2021JGRA..12629097S Altcode: 2021arXiv210402010S
The importance of the chromosphere in the mass and energy transport
within the solar atmosphere is now widely recognized. This review
discusses the physics of magnetohydrodynamic waves and instabilities
in large-scale chromospheric structures as well as in magnetic flux
tubes. We highlight a number of key observational aspects that have
helped our understanding of the role of the solar chromosphere
in various dynamic processes and wave phenomena, and the heating
scenario of the solar chromosphere is also discussed. The review
focuses on the physics of waves and invokes the basics of plasma
instabilities in the context of this important layer of the solar
atmosphere. Potential implications, future trends and outstanding
questions are also delineated.
---------------------------------------------------------
Title: A New View of the Solar Interface Region from the Interface
Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, Bart; Polito, Vanessa; Hansteen, Viggo; Testa,
Paola; Reeves, Katharine K.; Antolin, Patrick; Nóbrega-Siverio,
Daniel Elias; Kowalski, Adam F.; Martinez-Sykora, Juan; Carlsson,
Mats; McIntosh, Scott W.; Liu, Wei; Daw, Adrian; Kankelborg, Charles C.
2021SoPh..296...84D Altcode: 2021arXiv210316109D
The Interface Region Imaging Spectrograph (IRIS) has been obtaining
near- and far-ultraviolet images and spectra of the solar atmosphere
since July 2013. IRIS is the highest resolution observatory to provide
seamless coverage of spectra and images from the photosphere into the
low corona. The unique combination of near- and far-ultraviolet spectra
and images at sub-arcsecond resolution and high cadence allows the
tracing of mass and energy through the critical interface between the
surface and the corona or solar wind. IRIS has enabled research into the
fundamental physical processes thought to play a role in the low solar
atmosphere such as ion-neutral interactions, magnetic reconnection, the
generation, propagation, and dissipation of waves, the acceleration of
non-thermal particles, and various small-scale instabilities. IRIS has
provided insights into a wide range of phenomena including the discovery
of non-thermal particles in coronal nano-flares, the formation and
impact of spicules and other jets, resonant absorption and dissipation
of Alfvénic waves, energy release and jet-like dynamics associated
with braiding of magnetic-field lines, the role of turbulence and the
tearing-mode instability in reconnection, the contribution of waves,
turbulence, and non-thermal particles in the energy deposition during
flares and smaller-scale events such as UV bursts, and the role of flux
ropes and various other mechanisms in triggering and driving CMEs. IRIS
observations have also been used to elucidate the physical mechanisms
driving the solar irradiance that impacts Earth's upper atmosphere,
and the connections between solar and stellar physics. Advances in
numerical modeling, inversion codes, and machine-learning techniques
have played a key role. With the advent of exciting new instrumentation
both on the ground, e.g. the Daniel K. Inouye Solar Telescope (DKIST)
and the Atacama Large Millimeter/submillimeter Array (ALMA), and
space-based, e.g. the Parker Solar Probe and the Solar Orbiter, we aim
to review new insights based on IRIS observations or related modeling,
and highlight some of the outstanding challenges.
---------------------------------------------------------
Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
2021SoPh..296...70R Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.
---------------------------------------------------------
Title: Mapping solar magnetic fields from the photosphere to the
base of the corona
Authors: Ishikawa, Ryohko; Bueno, Javier Trujillo; del Pino Alemán,
Tanausú; Okamoto, Takenori J.; McKenzie, David E.; Auchère,
Frédéric; Kano, Ryouhei; Song, Donguk; Yoshida, Masaki; Rachmeler,
Laurel A.; Kobayashi, Ken; Hara, Hirohisa; Kubo, Masahito; Narukage,
Noriyuki; Sakao, Taro; Shimizu, Toshifumi; Suematsu, Yoshinori; Bethge,
Christian; De Pontieu, Bart; Dalda, Alberto Sainz; Vigil, Genevieve D.;
Winebarger, Amy; Ballester, Ernest Alsina; Belluzzi, Luca; Štěpán,
Jiří; Ramos, Andrés Asensio; Carlsson, Mats; Leenaarts, Jorrit
2021SciA....7.8406I Altcode: 2021arXiv210301583I
Routine ultraviolet imaging of the Sun's upper atmosphere shows the
spectacular manifestation of solar activity; yet we remain blind to
its main driver, the magnetic field. Here we report unprecedented
spectropolarimetric observations of an active region plage and
its surrounding enhanced network, showing circular polarization in
ultraviolet (Mg II $h$ & $k$ and Mn I) and visible (Fe I) lines. We
infer the longitudinal magnetic field from the photosphere to the
very upper chromosphere. At the top of the plage chromosphere the
field strengths reach more than 300 gauss, strongly correlated with
the Mg II $k$ line core intensity and the electron pressure. This
unique mapping shows how the magnetic field couples the different
atmospheric layers and reveals the magnetic origin of the heating in
the plage chromosphere.
---------------------------------------------------------
Title: Acoustic-gravity wave propagation characteristics in
three-dimensional radiation hydrodynamic simulations of the solar
atmosphere
Authors: Fleck, B.; Carlsson, M.; Khomenko, E.; Rempel, M.; Steiner,
O.; Vigeesh, G.
2021RSPTA.37900170F Altcode: 2020arXiv200705847F
There has been tremendous progress in the degree of realism of
three-dimensional radiation magneto-hydrodynamic simulations of the
solar atmosphere in the past decades. Four of the most frequently
used numerical codes are Bifrost, CO5BOLD, MANCHA3D and MURaM. Here
we test and compare the wave propagation characteristics in model
runs from these four codes by measuring the dispersion relation of
acoustic-gravity waves at various heights. We find considerable
differences between the various models. The height dependence of
wave power, in particular of high-frequency waves, varies by up to
two orders of magnitude between the models, and the phase difference
spectra of several models show unexpected features, including ±180°
phase jumps. <P />This article is part of the Theo Murphy meeting issue
`High-resolution wave dynamics in the lower solar atmosphere'.
---------------------------------------------------------
Title: Characterization of shock wave signatures at millimetre
wavelengths from Bifrost simulations
Authors: Eklund, Henrik; Wedemeyer, Sven; Snow, Ben; Jess, David B.;
Jafarzadeh, Shahin; Grant, Samuel D. T.; Carlsson, Mats; Szydlarski,
Mikołaj
2021RSPTA.37900185E Altcode: 2020arXiv200805324E
Observations at millimetre wavelengths provide a valuable tool to study
the small-scale dynamics in the solar chromosphere. We evaluate the
physical conditions of the atmosphere in the presence of a propagating
shock wave and link that to the observable signatures in mm-wavelength
radiation, providing valuable insights into the underlying physics
of mm-wavelength observations. A realistic numerical simulation from
the three-dimensional radiative magnetohydrodynamic code Bifrost
is used to interpret changes in the atmosphere caused by shock wave
propagation. High-cadence (1 s) time series of brightness temperature
(T<SUB>b</SUB>) maps are calculated with the Advanced Radiative Transfer
code at the wavelengths 1.309 mm and 1.204 mm, which represents opposite
sides of spectral band 6 of the Atacama Large Millimeter/submillimeter
Array (ALMA). An example of shock wave propagation is presented. The
brightness temperatures show a strong shock wave signature with large
variation in formation height between approximately 0.7 and 1.4 Mm. The
results demonstrate that millimetre brightness temperatures efficiently
track upwardly propagating shock waves in the middle chromosphere. In
addition, we show that the gradient of the brightness temperature
between wavelengths within ALMA band 6 can potentially be used as
a diagnostics tool in understanding the small-scale dynamics at the
sampled layers. <P />This article is part of the Theo Murphy meeting
issue `High-resolution wave dynamics in the lower solar atmosphere'.
---------------------------------------------------------
Title: Relative coronal abundance diagnostics with Solar Orbiter/SPICE
Authors: Zambrana Prado, N.; Buchlin, E.; Peter, H.; Young, P. R.;
Auchere, F.; Carlsson, M.; Fludra, A.; Hassler, D.; Aznar Cuadrado,
R.; Caminade, S.; Caldwell, M.; DeForest, C.; Fredvik, T.; Harra,
L.; Janvier, M.; Kucera, T. A.; Giunta, A. S.; Grundy, T.; Müller,
D.; Parenti, S.; Schmutz, W. K.; Schühle, U.; Sidher, S.; Teriaca,
L.; Thompson, W. T.; Williams, D.
2020AGUFMSH038..09Z Altcode:
Linking solar activity on the surface and in the corona to the inner
heliosphere is one of Solar Orbiter's main goals. Its UV spectrometer
SPICE (SPectral Imaging of the Coronal Environment) will provide
relative abundance measurements which will be key in this quest
as different structures on the Sun have different abundances as a
consequence of the FIP (First Ionization Potential) effect. Solar
Orbiter's unique combination of remote sensing and in-situ instruments
coupled with observation from other missions such as Parker Solar
Probe will allow us to compare in-situ and remote sensing composition
data. With the addition of modeling, these new results will allow us
to trace back the source of heliospheric plasma. As high telemetry
will not always be available with SPICE, we have developed a method
for measuring relative abundances that is both telemetry efficient
and reliable. Unlike methods based on Differential Emission Measure
(DEM) inversion, the Linear Combination Ratio (LCR) method does not
require a large number of spectral lines. This new method is based
on linear combinations of UV spectral lines. The coefficients of
the combinations are optimized such that the ratio of two linear
combinations of radiances would yield the relative abundance of two
elements. We present some abundance diagnostics tested on different
combinations of spectral lines observable by SPICE.
---------------------------------------------------------
Title: Dynamics and thermal structure in the quiet Sun seen by SPICE
Authors: Peter, H.; Aznar Cuadrado, R.; Schühle, U.; Teriaca, L.;
Auchere, F.; Carlsson, M.; Fludra, A.; Hassler, D.; Buchlin, E.;
Caminade, S.; Caldwell, M.; DeForest, C.; Fredvik, T.; Harra, L. K.;
Janvier, M.; Kucera, T. A.; Giunta, A. S.; Grundy, T.; Müller, D.;
Parenti, S.; Schmutz, W. K.; Sidher, S.; Thompson, W. T.; Williams,
D.; Young, P. R.
2020AGUFMSH038..03P Altcode:
We will present some of the early data of the Spectral Imaging of the
Coronal Environment (SPICE) instrument on Solar Orbiter. One of the
unique features of SPICE is its capability to record a wide range of
wavelengths in the extreme UV with the possibility to record spectral
lines giving access to a continuous plasma temperature range from 10.000
K to well above 1 MK. The data taken so far were for commissioning
purposes and they can be used for a preliminary evaluation of the
science performance of the instrument. Here we will concentrate on
sample spectra covering the whole wavelength region and on the early
raster maps acquired in bright lines in the quiet Sun close to disk
center. Looking at different quiet Sun features we investigate the
thermal structure of the atmosphere and flow structures. For this
we apply fits to the spectral profiles and check the performance in
terms of Doppler shifts and line widths to retrieve the structure of
the network in terms of dynamics. While the amount of data available
so far is limited, we will have a first look on how quiet Sun plasma
responds to heating events. For this, we will compare spectral lines
forming at different temperatures recorded at strictly the same time.
---------------------------------------------------------
Title: A Journey of Exploration to the Polar Regions of a Star:
Probing the Solar Poles and the Heliosphere from High Helio-Latitude
Authors: Finsterle, W.; Harra, L.; Andretta, V.; Appourchaux, T.;
Baudin, F.; Bellot Rubio, L.; Birch, A.; Boumier, P.; Cameron, R. H.;
Carlsson, M.; Corbard, T.; Davies, J. A.; Fazakerley, A. N.; Fineschi,
S.; Gizon, L. C.; Harrison, R. A.; Hassler, D.; Leibacher, J. W.;
Liewer, P. C.; Macdonald, M.; Maksimovic, M.; Murphy, N.; Naletto, G.;
Nigro, G.; Owen, C. J.; Martinez-Pillet, V.; Rochus, P. L.; Romoli,
M.; Sekii, T.; Spadaro, D.; Veronig, A.
2020AGUFMSH0110005F Altcode:
A mission to view the solar poles from high helio-latitudes (above
60°) will build on the experience of Solar Orbiter as well as a long
heritage of successful solar missions and instrumentation (e.g. SOHO,
STEREO, Hinode, SDO), but will focus for the first time on the solar
poles, enabling scientific investigations that cannot be done by
any other mission. One of the major mysteries of the Sun is the solar
cycle. The activity cycle of the Sun drives the structure and behaviour
of the heliosphere and is, of course, the driver of space weather. In
addition, solar activity and variability provides fluctuating input
into the Earth climate models, and these same physical processes
are applicable to stellar systems hosting exoplanets. One of the
main obstructions to understanding the solar cycle, and hence all
solar activity, is our current lack of understanding of the polar
regions. We describe a mission concept that aims to address this
fundamental issue. In parallel, we recognise that viewing the Sun
from above the polar regions enables further scientific advantages,
beyond those related to the solar cycle, such as unique and powerful
studies of coronal mass ejection processes, from a global perspective,
and studies of coronal structure and activity in polar regions. Not
only will these provide important scientific advances for fundamental
stellar physics research, they will feed into our understanding of
impacts on the Earth and other planets' space environment.
---------------------------------------------------------
Title: First Results From SPICE EUV Spectrometer on Solar Orbiter
Authors: Fludra, A.; Caldwell, M.; Giunta, A. S.; Grundy, T.; Guest,
S.; Sidher, S.; Auchere, F.; Carlsson, M.; Hassler, D.; Peter, H.;
Aznar Cuadrado, R.; Buchlin, E.; Caminade, S.; DeForest, C.; Fredvik,
T.; Harra, L. K.; Janvier, M.; Kucera, T. A.; Leeks, S.; Mueller,
D.; Parenti, S.; Schmutz, W. K.; Schühle, U.; Teriaca, L.; Thompson,
W. T.; Tustain, S.; Williams, D.; Young, P. R.
2020AGUFMSH038..02F Altcode:
SPICE (Spectral Imaging of Coronal Environment) is one of the remote
sensing instruments onboard Solar Orbiter. It is an EUV imaging
spectrometer observing the Sun in two wavelength bands: 69.6-79.4 nm
and 96.6-105.1 nm. SPICE is capable of recording full spectra in these
bands with exposures as short as 1s. SPICE is the only Solar Orbiter
instrument that can measure EUV spectra from the disk and low corona
of the Sun and record all spectral lines simultaneously. SPICE uses
one of three narrow slits, 2"x11', 4”x11', 6”x11', or a wide slit
30”x14'. The primary mirror can be scanned in a direction perpendicular
to the slit, allowing raster images of up to 16' in size. <P />We
present an overview of the first SPICE data taken on several days
during the instrument commissioning carried out by the RAL Space team
between 2020 April 21 and 2020 June 14. We also include results from
SPICE observations at the first Solar Orbiter perihelion at 0.52AU,
taken between June 16-21<SUP>st</SUP>. We give examples of full spectra
from the quiet Sun near disk centre and provide a list of key spectral
lines emitted in a range of temperatures between 10,000 K and over 1
million K, from neutral hydrogen and ions of carbon, oxygen, nitrogen,
neon, sulphur and magnesium. We show examples of first raster images
in several strong lines, obtained with different slits and a range
of exposure times between 5s and 180s. We describe the temperature
coverage and density diagnostics, determination of plasma flows, and
discuss possible applications to studies of the elemental abundances
in the corona. We also show the first off-limb measurements with SPICE,
as obtained when the spacecraft pointed at the limb.
---------------------------------------------------------
Title: Calibrating optical distortions in the Solar Orbiter SPICE
spectrograph
Authors: Thompson, W. T.; Schühle, U.; Young, P. R.; Auchere, F.;
Carlsson, M.; Fludra, A.; Hassler, D.; Peter, H.; Aznar Cuadrado, R.;
Buchlin, E.; Caldwell, M.; DeForest, C.; Fredvik, T.; Harra, L. K.;
Janvier, M.; Kucera, T. A.; Giunta, A. S.; Grundy, T.; Müller, D.;
Parenti, S.; Caminade, S.; Schmutz, W. K.; Teriaca, L.; Williams,
D.; Sidher, S.
2020AGUFMSH0360029T Altcode:
The Spectral Imaging of the Coronal Environment (SPICE) instrument on
Solar Orbiter is a high-resolution imaging spectrometer operating
at extreme ultraviolet (EUV) wavelengths from 70.4-79.0 nm and
97.3-104.9 nm. A single-mirror off-axis paraboloid focuses the solar
image onto the entrance slit of the spectrometer section. A Toroidal
Variable Line Space (TVLS) grating images the entrance slit onto a
pair of MCP-intensified APS detectors. Ray-tracing analysis prior
to launch showed that the instrument was subject to a number of
small image distortions which need to be corrected in the final data
product. We compare the ray tracing results with measurements made in
flight. Co-alignment with other telescopes on Solar Orbiter will also
be examined.
---------------------------------------------------------
Title: First results from the EUI and SPICE observations of Alpha
Leo near Solar Orbiter first perihelion
Authors: Buchlin, E.; Teriaca, L.; Giunta, A. S.; Grundy, T.; Andretta,
V.; Auchere, F.; Peter, H.; Berghmans, D.; Carlsson, M.; Fludra, A.;
Harra, L.; Hassler, D.; Long, D.; Rochus, P. L.; Schühle, U.; Aznar
Cuadrado, R.; Caldwell, M.; Caminade, S.; DeForest, C.; Fredvik, T.;
Gissot, S.; Heerlein, K.; Janvier, M.; Kraaikamp, E.; Kucera, T. A.;
Müller, D.; Parenti, S.; Schmutz, W. K.; Sidher, S.; Smith, P.;
Stegen, K.; Thompson, W. T.; Verbeeck, C.; Williams, D.; Young, P. R.
2020AGUFMSH0360024B Altcode:
On June 16th 2020 Solar Orbiter made a dedicated observing campaign
where the spacecraft pointed to the solar limb to allow some of the
high resolution instruments to observe the ingress (at the east limb)
and later the egress (west limb) of the occultation of the star Alpha
Leonis by the solar disk. The star was chosen because its luminosity and
early spectral type ensure high and stable flux at wavelengths between
100 and 122 nanometers, a range observed by the High Resolution EUI
Lyman alpha telescope (HRI-LYA) and by the long wavelength channel
of the SPICE spectrograph. Star observations, when feasible, allow
to gather a great deal of information on the instrument performances,
such as the radiometric performance and the instrument optical point
spread function (PSF). <P />We report here the first results from the
above campaign for the two instruments.
---------------------------------------------------------
Title: Solar Orbiter: connecting remote sensing and in situ
measurements
Authors: Horbury, T. S.; Auchere, F.; Antonucci, E.; Berghmans, D.;
Bruno, R.; Carlsson, M.; del Toro Iniesta, J. C.; Fludra, A.; Harra,
L.; Hassler, D.; Heinzel, P.; Howard, R. A.; Krucker, S.; Livi, S. A.;
Long, D.; Louarn, P.; Maksimovic, M.; Mueller, D.; Owen, C. J.; Peter,
H.; Rochus, P. L.; Rodriguez-Pacheco, J.; Romoli, M.; Schühle, U.;
Solanki, S. K.; Teriaca, L.; Wimmer-Schweingruber, R. F.; Zouganelis,
Y.; Laker, R.
2020AGUFMSH038..10H Altcode:
A key science goal of the Solar Orbiter mission is to make connections
between phenomena on the Sun and their manifestations in interplanetary
space. To that end, the spacecraft carries a carefully tailored
payload of six remote sensing instruments and four making in situ
measurements. During June 2020, while the spacecraft was around 0.5
AU from the Sun, the remote sensing instruments operated for several
days. While this was primarily an engineering activity, the resulting
observations provided outstanding measurements and represent the ideal
first opportunity to investigate the potential for making connections
between the remote sensing and in situ payloads on Solar Orbiter. <P
/>We present a preliminary analysis of the available remote sensing and
in situ observations, showing how connections can be made, and discuss
the potential for further, more precise mapping to be performed as
the mission progresses.
---------------------------------------------------------
Title: First results from combined EUI and SPICE observations of
Lyman lines of Hydrogen and He II
Authors: Teriaca, L.; Aznar Cuadrado, R.; Giunta, A. S.; Grundy, T.;
Parenti, S.; Auchere, F.; Vial, J. C.; Fludra, A.; Berghmans, D.;
Carlsson, M.; Harra, L.; Hassler, D.; Long, D.; Peter, H.; Rochus,
P. L.; Schühle, U.; Buchlin, E.; Caldwell, M.; Caminade, S.; DeForest,
C.; Fredvik, T.; Gissot, S.; Heerlein, K.; Janvier, M.; Kraaikamp,
E.; Kucera, T. A.; Mueller, D.; Schmutz, W. K.; Sidher, S.; Smith, P.;
Stegen, K.; Thompson, W. T.; Verbeeck, C.; Williams, D.; Young, P. R.
2020AGUFMSH0360003T Altcode:
The Solar Orbiter spacecraft carries a powerful set of remote
sensing instruments that allow studying the solar atmosphere with
unprecedented diagnostic capabilities. Many such diagnostics require
the simultaneous usage of more than one instrument. One example of that
is the capability, for the first time, to obtain (near) simultaneous
spatially resolved observations of the emission from the first three
lines of the Lyman series of hydrogen and of He II Lyman alpha. In fact,
the SPectral Imaging of the Coronal Environment (SPICE) spectrometer
can observe the Lyman beta and gamma lines in its long wavelength
(SPICE-LW) channel, the High Resolution Lyman Alpha (HRI-LYA) telescope
of the Extreme Ultraviolet Imager (EUI) acquires narrow band images in
the Lyman alpha line while the Full Disk Imager (FSI) of EUI can take
images dominated by the Lyman alpha line of ionized Helium at 30.4 nm
(FSI-304). Being hydrogen and helium the main components of our star,
these very bright transitions play an important role in the energy
budget of the outer atmosphere via radiative losses and the measurement
of their profiles and radiance ratios is a fundamental constraint to
any comprehensive modelization effort of the upper solar chromosphere
and transition region. Additionally, monitoring their average ratios
can serve as a check out for the relative radiometric performance of
the two instruments throughout the mission. Although the engineering
data acquired so far are far from ideal in terms of time simultaneity
(often only within about 1 h) and line coverage (often only Lyman beta
was acquired by SPICE and not always near simultaneous images from all
three telescopes are available) the analysis we present here still
offers a great opportunity to have a first look at the potential of
this diagnostic from the two instruments. In fact, we have identified
a series of datasets obtained at disk center and at various positions
at the solar limb that allow studying the Lyman alpha to beta radiance
ratio and their relation to He II 30.4 as a function of the position
on the Sun (disk center versus limb and quiet Sun versus coronal holes).
---------------------------------------------------------
Title: Coordination within the remote sensing payload on the Solar
Orbiter mission
Authors: Auchère, F.; Andretta, V.; Antonucci, E.; Bach, N.;
Battaglia, M.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Caminade,
S.; Carlsson, M.; Carlyle, J.; Cerullo, J. J.; Chamberlin, P. C.;
Colaninno, R. C.; Davila, J. M.; De Groof, A.; Etesi, L.; Fahmy,
S.; Fineschi, S.; Fludra, A.; Gilbert, H. R.; Giunta, A.; Grundy,
T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.;
Howard, R. A.; Hurford, G.; Kleint, L.; Kolleck, M.; Krucker, S.;
Lagg, A.; Landini, F.; Long, D. M.; Lefort, J.; Lodiot, S.; Mampaey,
B.; Maloney, S.; Marliani, F.; Martinez-Pillet, V.; McMullin, D. R.;
Müller, D.; Nicolini, G.; Orozco Suarez, D.; Pacros, A.; Pancrazzi,
M.; Parenti, S.; Peter, H.; Philippon, A.; Plunkett, S.; Rich, N.;
Rochus, P.; Rouillard, A.; Romoli, M.; Sanchez, L.; Schühle, U.;
Sidher, S.; Solanki, S. K.; Spadaro, D.; St Cyr, O. C.; Straus, T.;
Tanco, I.; Teriaca, L.; Thompson, W. T.; del Toro Iniesta, J. C.;
Verbeeck, C.; Vourlidas, A.; Watson, C.; Wiegelmann, T.; Williams,
D.; Woch, J.; Zhukov, A. N.; Zouganelis, I.
2020A&A...642A...6A Altcode:
Context. To meet the scientific objectives of the mission, the Solar
Orbiter spacecraft carries a suite of in-situ (IS) and remote sensing
(RS) instruments designed for joint operations with inter-instrument
communication capabilities. Indeed, previous missions have shown that
the Sun (imaged by the RS instruments) and the heliosphere (mainly
sampled by the IS instruments) should be considered as an integrated
system rather than separate entities. Many of the advances expected
from Solar Orbiter rely on this synergistic approach between IS and
RS measurements. <BR /> Aims: Many aspects of hardware development,
integration, testing, and operations are common to two or more
RS instruments. In this paper, we describe the coordination effort
initiated from the early mission phases by the Remote Sensing Working
Group. We review the scientific goals and challenges, and give an
overview of the technical solutions devised to successfully operate
these instruments together. <BR /> Methods: A major constraint for the
RS instruments is the limited telemetry (TM) bandwidth of the Solar
Orbiter deep-space mission compared to missions in Earth orbit. Hence,
many of the strategies developed to maximise the scientific return from
these instruments revolve around the optimisation of TM usage, relying
for example on onboard autonomy for data processing, compression,
and selection for downlink. The planning process itself has been
optimised to alleviate the dynamic nature of the targets, and an
inter-instrument communication scheme has been implemented which can
be used to autonomously alter the observing modes. We also outline the
plans for in-flight cross-calibration, which will be essential to the
joint data reduction and analysis. <BR /> Results: The RS instrument
package on Solar Orbiter will carry out comprehensive measurements
from the solar interior to the inner heliosphere. Thanks to the close
coordination between the instrument teams and the European Space
Agency, several challenges specific to the RS suite were identified
and addressed in a timely manner.
---------------------------------------------------------
Title: The Solar Orbiter SPICE instrument. An extreme UV imaging
spectrometer
Authors: SPICE Consortium; Anderson, M.; Appourchaux, T.; Auchère, F.;
Aznar Cuadrado, R.; Barbay, J.; Baudin, F.; Beardsley, S.; Bocchialini,
K.; Borgo, B.; Bruzzi, D.; Buchlin, E.; Burton, G.; Büchel, V.;
Caldwell, M.; Caminade, S.; Carlsson, M.; Curdt, W.; Davenne, J.;
Davila, J.; Deforest, C. E.; Del Zanna, G.; Drummond, D.; Dubau,
J.; Dumesnil, C.; Dunn, G.; Eccleston, P.; Fludra, A.; Fredvik, T.;
Gabriel, A.; Giunta, A.; Gottwald, A.; Griffin, D.; Grundy, T.; Guest,
S.; Gyo, M.; Haberreiter, M.; Hansteen, V.; Harrison, R.; Hassler,
D. M.; Haugan, S. V. H.; Howe, C.; Janvier, M.; Klein, R.; Koller,
S.; Kucera, T. A.; Kouliche, D.; Marsch, E.; Marshall, A.; Marshall,
G.; Matthews, S. A.; McQuirk, C.; Meining, S.; Mercier, C.; Morris,
N.; Morse, T.; Munro, G.; Parenti, S.; Pastor-Santos, C.; Peter, H.;
Pfiffner, D.; Phelan, P.; Philippon, A.; Richards, A.; Rogers, K.;
Sawyer, C.; Schlatter, P.; Schmutz, W.; Schühle, U.; Shaughnessy,
B.; Sidher, S.; Solanki, S. K.; Speight, R.; Spescha, M.; Szwec, N.;
Tamiatto, C.; Teriaca, L.; Thompson, W.; Tosh, I.; Tustain, S.; Vial,
J. -C.; Walls, B.; Waltham, N.; Wimmer-Schweingruber, R.; Woodward,
S.; Young, P.; de Groof, A.; Pacros, A.; Williams, D.; Müller, D.
2020A&A...642A..14S Altcode: 2019arXiv190901183A; 2019arXiv190901183S
<BR /> Aims: The Spectral Imaging of the Coronal Environment (SPICE)
instrument is a high-resolution imaging spectrometer operating at
extreme ultraviolet wavelengths. In this paper, we present the concept,
design, and pre-launch performance of this facility instrument on the
ESA/NASA Solar Orbiter mission. <BR /> Methods: The goal of this paper
is to give prospective users a better understanding of the possible
types of observations, the data acquisition, and the sources that
contribute to the instrument's signal. <BR /> Results: The paper
discusses the science objectives, with a focus on the SPICE-specific
aspects, before presenting the instrument's design, including optical,
mechanical, thermal, and electronics aspects. This is followed by a
characterisation and calibration of the instrument's performance. The
paper concludes with descriptions of the operations concept and data
processing. <BR /> Conclusions: The performance measurements of the
various instrument parameters meet the requirements derived from the
mission's science objectives. The SPICE instrument is ready to perform
measurements that will provide vital contributions to the scientific
success of the Solar Orbiter mission.
---------------------------------------------------------
Title: The Solar Orbiter mission. Science overview
Authors: Müller, D.; St. Cyr, O. C.; Zouganelis, I.; Gilbert, H. R.;
Marsden, R.; Nieves-Chinchilla, T.; Antonucci, E.; Auchère, F.;
Berghmans, D.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic,
M.; Owen, C. J.; Rochus, P.; Rodriguez-Pacheco, J.; Romoli, M.;
Solanki, S. K.; Bruno, R.; Carlsson, M.; Fludra, A.; Harra, L.;
Hassler, D. M.; Livi, S.; Louarn, P.; Peter, H.; Schühle, U.;
Teriaca, L.; del Toro Iniesta, J. C.; Wimmer-Schweingruber, R. F.;
Marsch, E.; Velli, M.; De Groof, A.; Walsh, A.; Williams, D.
2020A&A...642A...1M Altcode: 2020arXiv200900861M
<BR /> Aims: Solar Orbiter, the first mission of ESA's Cosmic Vision
2015-2025 programme and a mission of international collaboration between
ESA and NASA, will explore the Sun and heliosphere from close up and
out of the ecliptic plane. It was launched on 10 February 2020 04:03
UTC from Cape Canaveral and aims to address key questions of solar and
heliospheric physics pertaining to how the Sun creates and controls
the Heliosphere, and why solar activity changes with time. To answer
these, the mission carries six remote-sensing instruments to observe
the Sun and the solar corona, and four in-situ instruments to measure
the solar wind, energetic particles, and electromagnetic fields. In
this paper, we describe the science objectives of the mission, and how
these will be addressed by the joint observations of the instruments
onboard. <BR /> Methods: The paper first summarises the mission-level
science objectives, followed by an overview of the spacecraft and
payload. We report the observables and performance figures of each
instrument, as well as the trajectory design. This is followed by a
summary of the science operations concept. The paper concludes with a
more detailed description of the science objectives. <BR /> Results:
Solar Orbiter will combine in-situ measurements in the heliosphere
with high-resolution remote-sensing observations of the Sun to address
fundamental questions of solar and heliospheric physics. The performance
of the Solar Orbiter payload meets the requirements derived from the
mission's science objectives. Its science return will be augmented
further by coordinated observations with other space missions and
ground-based observatories. <P />ARRAY(0x207ce98)
---------------------------------------------------------
Title: On the Formation of Lyman β and the O I 1027 and 1028 Å
Spectral Lines
Authors: Hasegawa, Takahiro; Noda, Carlos Quintero; Shimizu, Toshifumi;
Carlsson, Mats
2020ApJ...900...34H Altcode: 2020arXiv200812556H
We study the potential of Lyman β and the O I 1027 and 1028
Å spectral lines to help in understanding the properties of the
chromosphere and transition region (TR). The oxygen transitions are
located in the wing of Lyman β, which is a candidate spectral line
for the solar missions Solar Orbiter/Spectral Imaging of the Coronal
Environment and Solar-C (EUVST). We examine the general spectroscopic
properties of the three transitions in the quiet Sun by synthesizing
them assuming nonlocal thermal equilibrium and taking into account
partial redistribution effects. We estimate the heights where the
spectral lines are sensitive to the physical parameters, computing the
response functions to temperature and velocity using a 1D semiempirical
atmospheric model. We also synthesize the intensity spectrum using the
3D enhanced network simulation computed with the BIFROST code. The
results indicate that Lyman β is sensitive to the temperature from
the middle chromosphere to the TR, while it is mainly sensitive to
the line-of-sight (LOS) velocity at the lower atmospheric layers,
around 2000 km above the optical surface. The O I lines form lower
in the middle chromosphere, being sensitive to the LOS velocities at
heights lower than those covered by Lyman β. The spatial distribution
of the intensity signals computed with the BIFROST atmosphere, as
well as the inferred velocities from the line core Doppler shift,
confirms the previous results. Therefore, these results indicate that
the spectral window at 1025 Å contains several spectral lines that
complement each other to seamlessly trace the thermal structure and
gas dynamics from the middle chromosphere to the lower TR.
---------------------------------------------------------
Title: High-resolution observations of the solar photosphere,
chromosphere, and transition region. A database of coordinated IRIS
and SST observations
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson,
M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.;
Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques,
V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.;
Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.;
Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.;
Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski,
M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P.
2020A&A...641A.146R Altcode: 2020arXiv200514175R
NASA's Interface Region Imaging Spectrograph (IRIS) provides
high-resolution observations of the solar atmosphere through ultraviolet
spectroscopy and imaging. Since the launch of IRIS in June 2013, we
have conducted systematic observation campaigns in coordination with
the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides
complementary high-resolution observations of the photosphere and
chromosphere. The SST observations include spectropolarimetric imaging
in photospheric Fe I lines and spectrally resolved imaging in the
chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present
a database of co-aligned IRIS and SST datasets that is open for
analysis to the scientific community. The database covers a variety
of targets including active regions, sunspots, plages, the quiet Sun,
and coronal holes.
---------------------------------------------------------
Title: Ambipolar diffusion in the Bifrost code
Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis,
F.; Carlsson, M.
2020A&A...638A..79N Altcode: 2020arXiv200411927N
Context. Ambipolar diffusion is a physical mechanism related to the
drift between charged and neutral particles in a partially ionized
plasma that is key to many different astrophysical systems. However,
understanding its effects is challenging due to basic uncertainties
concerning relevant microphysical aspects and the strong constraints it
imposes on the numerical modeling. <BR /> Aims: Our aim is to introduce
a numerical tool that allows us to address complex problems involving
ambipolar diffusion in which, additionally, departures from ionization
equilibrium are important or high resolution is needed. The primary
application of this tool is for solar atmosphere calculations, but the
methods and results presented here may also have a potential impact
on other astrophysical systems. <BR /> Methods: We have developed a
new module for the stellar atmosphere Bifrost code that improves its
computational capabilities of the ambipolar diffusion term in the
generalized Ohm's law. This module includes, among other things,
collision terms adequate to processes in the coolest regions in
the solar chromosphere. As the main feature of the module, we have
implemented the super time stepping (STS) technique, which allows an
important acceleration of the calculations. We have also introduced
hyperdiffusion terms to guarantee the stability of the code. <BR />
Results: We show that to have an accurate value for the ambipolar
diffusion coefficient in the solar atmosphere it is necessary to
include as atomic elements in the equation of state not only hydrogen
and helium, but also the main electron donors like sodium, silicon,
and potassium. In addition, we establish a range of criteria to set
up an automatic selection of the free parameters of the STS method
that guarantees the best performance, optimizing the stability and
speed for the ambipolar diffusion calculations. We validate the STS
implementation by comparison with a self-similar analytical solution.
---------------------------------------------------------
Title: Chromospheric Bubbles in Solar Flares
Authors: Reid, A.; Zhigulin, B.; Carlsson, M.; Mathioudakis, M.
2020ApJ...894L..21R Altcode: 2020arXiv200510586R
We analyze a grid of radiative hydrodynamic simulations of solar flares
to study the energy balance and response of the atmosphere to nonthermal
electron beam heating. The appearance of chromospheric bubbles is one
of the most notable features that we find in the simulations. These
pockets of chromospheric plasma get trapped between the transition
region and the lower atmosphere as it is superheated by the particle
beam. The chromospheric bubbles are seen in the synthetic spectra,
appearing as an additional component to Balmer line profiles with high
Doppler velocities as high as 200 km s<SUP>-1</SUP>. Their signatures
are also visible in the wings of Ca II 8542 Å line profiles. These
bubbles of chromospheric plasma are driven upward by a wave front that
is induced by the shock of energy deposition, and require a specific
heating rate and atmospheric location to manifest.
---------------------------------------------------------
Title: The Sun at millimeter wavelengths. I. Introduction to ALMA
Band 3 observations
Authors: Wedemeyer, Sven; Szydlarski, Mikolaj; Jafarzadeh, Shahin;
Eklund, Henrik; Guevara Gomez, Juan Camilo; Bastian, Tim; Fleck,
Bernhard; de la Cruz Rodriguez, Jaime; Rodger, Andrew; Carlsson, Mats
2020A&A...635A..71W Altcode: 2020arXiv200102185W
Context. The Atacama Large Millimeter/submillimeter Array (ALMA) started
regular observations of the Sun in 2016, first offering receiver Band
3 at wavelengths near 3 mm (100 GHz) and Band 6 at wavelengths around
1.25 mm (239 GHz). <BR /> Aims: Here we present an initial study
of one of the first ALMA Band 3 observations of the Sun. Our aim is
to characterise the diagnostic potential of brightness temperatures
measured with ALMA on the Sun. <BR /> Methods: The observation covers
a duration of 48 min at a cadence of 2 s targeting a quiet Sun region
at disc-centre. Corresponding time series of brightness temperature
maps are constructed with the first version of the Solar ALMA Pipeline
and compared to simultaneous observations with the Solar Dynamics
Observatory (SDO). <BR /> Results: The angular resolution of the
observations is set by the synthesised beam, an elliptical Gaussian
that is approximately 1.4″ × 2.1″ in size. The ALMA maps exhibit
network patches, internetwork regions, and elongated thin features
that are connected to large-scale magnetic loops, as confirmed by a
comparison with SDO maps. The ALMA Band 3 maps correlate best with
the SDO/AIA 171 Å, 131 Å, and 304 Å channels in that they exhibit
network features and, although very weak in the ALMA maps, imprints
of large-scale loops. A group of compact magnetic loops is very
clearly visible in ALMA Band 3. The brightness temperatures in the
loop tops reach values of about 8000-9000 K and in extreme moments
up to 10 000 K. <BR /> Conclusions: ALMA Band 3 interferometric
observations from early observing cycles already reveal temperature
differences in the solar chromosphere. The weak imprint of magnetic
loops and the correlation with the 171, 131, and 304 SDO channels
suggests, however, that the radiation mapped in ALMA Band 3 might
have contributions from a wider range of atmospheric heights than
previously assumed, but the exact formation height of Band 3 needs to
be investigated in more detail. The absolute brightness temperature
scale as set by total power measurements remains less certain and
must be improved in the future. Despite these complications and the
limited angular resolution, ALMA Band 3 observations have a large
potential for quantitative studies of the small-scale structure and
dynamics of the solar chromosphere. <P />Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201937122/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Observational constraints on the origin of the elements. II. 3D
non-LTE formation of Ba II lines in the solar atmosphere
Authors: Gallagher, A. J.; Bergemann, M.; Collet, R.; Plez, B.;
Leenaarts, J.; Carlsson, M.; Yakovleva, S. A.; Belyaev, A. K.
2020A&A...634A..55G Altcode: 2019arXiv191003898G
Context. The pursuit of more realistic spectroscopic modelling
and consistent abundances has led us to begin a new series of
papers designed to improve current solar and stellar abundances of
various atomic species. To achieve this, we have begun updating the
three-dimensional (3D) non-local thermodynamic equilibrium (non-LTE)
radiative transfer code, MULTI3D, and the equivalent one-dimensional
(1D) non-LTE radiative transfer code, MULTI 2.3. <BR /> Aims: We
examine our improvements to these codes by redetermining the solar
barium abundance. Barium was chosen for this test as it is an important
diagnostic element of the s-process in the context of galactic chemical
evolution. New Ba II + H collisional data for excitation and charge
exchange reactions computed from first principles had recently become
available and were included in the model atom. The atom also includes
the effects of isotopic line shifts and hyperfine splitting. <BR />
Methods: A grid of 1D LTE barium lines were constructed with MULTI 2.3
and fit to the four Ba II lines available to us in the optical region
of the solar spectrum. Abundance corrections were then determined in
1D non-LTE, 3D LTE, and 3D non-LTE. A new 3D non-LTE solar barium
abundance was computed from these corrections. <BR /> Results: We
present for the first time the full 3D non-LTE barium abundance of
A(Ba) = 2.27 ± 0.02 ± 0.01, which was derived from four individual
fully consistent barium lines. Errors here represent the systematic
and random errors, respectively.
---------------------------------------------------------
Title: Ion-neutral Interactions and Nonequilibrium Ionization in
the Solar Chromosphere
Authors: Martínez-Sykora, Juan; Leenaarts, Jorrit; De Pontieu,
Bart; Nóbrega-Siverio, Daniel; Hansteen, Viggo H.; Carlsson, Mats;
Szydlarski, Mikolaj
2020ApJ...889...95M Altcode: 2019arXiv191206682M
The thermal structure of the chromosphere is regulated through a
complex interaction of various heating processes, radiative cooling,
and the ionization degree of the plasma. Here, we study the impact on
the thermal properties of the chromosphere when including the combined
action of nonequilibrium ionization (NEI) of hydrogen and helium and
ion-neutral interaction effects. We have performed a 2.5D radiative
magnetohydrodynamic simulation using the Bifrost code. This model
includes ion-neutral interaction effects by solving the generalized
Ohm' s law (GOL) as well as NEI for hydrogen and helium. The GOL
equation includes ambipolar diffusion and the Hall term. We compare
this simulation with another simulation that computes the ionization in
local thermodynamic equilibrium (LTE) including ion-neutral interaction
effects. Our numerical models reveal substantial thermal differences
in magneto-acoustic shocks, the wake behind the shocks, spicules,
low-lying magnetic loops, and the transition region. In particular,
we find that heating through ambipolar diffusion in shock wakes is
substantially less efficient, while in the shock fronts themselves it
is more efficient, under NEI conditions than when assuming LTE.
---------------------------------------------------------
Title: Cosmic ray interactions in the solar atmosphere
Authors: Hudson, Hugh S.; MacKinnon, Alec; Szydlarski, Mikolaj;
Carlsson, Mats
2020MNRAS.491.4852H Altcode: 2019arXiv191001186H; 2019MNRAS.tmp.3116H
High-energy particles enter the solar atmosphere from Galactic or
solar coronal sources, and produce 'albedo' emission from the quiet
Sun that is now observable across a wide range of photon energies. The
interaction of high-energy particles in a stellar atmosphere depends
essentially upon the joint variation of the magnetic field and plasma
density, which heretofore has been characterized parametrically as
P ∝ B<SUP>α</SUP> with P the gas pressure and B the magnitude of
the magnetic field. We re-examine that parametrization by using a
self-consistent 3D MHD model (Bifrost) and show that this relationship
tends to P ∝ B<SUP>3.5 ± 0.1</SUP> based on the visible portions
of the sample of open-field flux tubes in such a model, but with large
variations from point to point. This scatter corresponds to the strong
meandering of the open-field flux tubes in the lower atmosphere, which
will have a strong effect on the prediction of the emission anisotropy
(limb brightening). The simulations show that much of the open flux
in coronal holes originates in weak-field regions within the granular
pattern of the convective motions seen in the simulations.
---------------------------------------------------------
Title: Capabilities of bisector analysis of the Si I 10 827 Å line
for estimating line-of-sight velocities in the quiet Sun
Authors: González Manrique, S. J.; Quintero Noda, C.; Kuckein, C.;
Ruiz Cobo, B.; Carlsson, M.
2020A&A...634A..19G Altcode: 2020arXiv200100508G
We examine the capabilities of a fast and simple method to infer
line-of-sight (LOS) velocities from observations of the photospheric
Si I 10 827 Å line. This spectral line is routinely observed together
with the chromospheric He I 10 830 Å triplet as it helps to constrain
the atmospheric parameters. We study the accuracy of bisector analysis
and a line core fit of Si I 10 827 Å. We employ synthetic profiles
starting from the Bifrost enhanced network simulation. The profiles are
computed solving the radiative transfer equation, including non-local
thermodynamic equilibrium effects on the determination of the atomic
level populations of Si I. We found a good correlation between the
inferred velocities from bisectors taken at different line profile
intensities and the original simulation velocity at given optical
depths. This good correlation means that we can associate bisectors
taken at different line-profile percentages with atmospheric layers
that linearly increase as we scan lower spectral line intensities. We
also determined that a fit to the line-core intensity is robust and
reliable, providing information about atmospheric layers that are
above those accessible through bisectors. Therefore, by combining
both methods on the Si I 10 827 Å line, we can seamlessly trace the
quiet-Sun LOS velocity stratification from the deep photosphere to
higher layers until around logτ = -3.5 in a fast and straightforward
way. This method is ideal for generating quick-look reference images
for future missions like the Daniel K. Inoue Solar Telescope and the
European Solar Telescope, for example.
---------------------------------------------------------
Title: Nonequilibrium ionization and ambipolar diffusion in solar
magnetic flux emergence processes
Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora,
J.; Carlsson, M.; Szydlarski, M.
2020A&A...633A..66N Altcode: 2019arXiv191201015N
Context. Magnetic flux emergence from the solar interior has
been shown to be a key mechanism for unleashing a wide variety of
phenomena. However, there are still open questions concerning the
rise of the magnetized plasma through the atmosphere, mainly in
the chromosphere, where the plasma departs from local thermodynamic
equilibrium (LTE) and is partially ionized. <BR /> Aims: We aim to
investigate the impact of the nonequilibrium (NEQ) ionization and
recombination and molecule formation of hydrogen, as well as ambipolar
diffusion, on the dynamics and thermodynamics of the flux emergence
process. <BR /> Methods: Using the radiation-magnetohydrodynamic
Bifrost code, we performed 2.5D numerical experiments of magnetic flux
emergence from the convection zone up to the corona. The experiments
include the NEQ ionization and recombination of atomic hydrogen, the NEQ
formation and dissociation of H<SUB>2</SUB> molecules, and the ambipolar
diffusion term of the generalized Ohm's law. <BR /> Results: Our
experiments show that the LTE assumption substantially underestimates
the ionization fraction in most of the emerged region, leading to an
artificial increase in the ambipolar diffusion and, therefore, in the
heating and temperatures as compared to those found when taking the
NEQ effects on the hydrogen ion population into account. We see that
LTE also overestimates the number density of H<SUB>2</SUB> molecules
within the emerged region, thus mistakenly magnifying the exothermic
contribution of the H<SUB>2</SUB> molecule formation to the thermal
energy during the flux emergence process. We find that the ambipolar
diffusion does not significantly affect the amount of total unsigned
emerged magnetic flux, but it is important in the shocks that cross
the emerged region, heating the plasma on characteristic times ranging
from 0.1 to 100 s. We also briefly discuss the importance of including
elements heavier than hydrogen in the equation of state so as not to
overestimate the role of ambipolar diffusion in the atmosphere. <P
/>Movies associated to Figs. 2-5, 8, 9, and A.1 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936944/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: The SPICE (Spectral Imaging of the Coronal Environment)
Ultraviolet Imaging Spectrograph Investigation
Authors: Hassler, D.; Auchere, F.; Carlsson, M.; Fludra, A.; Giunta,
A. S.; Mueller, D.; Peter, H.; Parenti, S.; Teriaca, L.; Fredvik, T.
2019AGUFMSH24A..02H Altcode:
One of the primary objectives of the Solar Orbiter mission is to link
remote sensing observations of the solar surface structures with in-situ
observations of solar wind streams. The SPICE (Spectral Imaging of the
Coronal Environment) instrument will characterize the plasma properties
of regions near the Sun to directly compare with in-situ measurements
from both Solar Orbiter & Parker Solar Probe. Specifically, SPICE
will map outflow velocities of surface features to solar wind structures
with similar composition (FIP, M/q) measured in-situ by the SWA/HIS
instrument on Solar Orbiter. These observations will help discriminate
models of solar wind origin by matching composition signatures in
solar wind streams to surface feature composition, and discriminate
physical processes that inject material from closed structures into
solar wind streams. <P />This presentation will provide an overview of
the SPICE investigation, including science & measurement objective,
instrument design, capabilities and performance as measured during
calibration prior to delivery to the Solar Orbiter spacecraft. The
presentation will also provide a description of the operations concept
and data processing during the mission.
---------------------------------------------------------
Title: Science Requirement Document (SRD) for the European Solar
Telescope (EST) (2nd edition, December 2019)
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.;
Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.;
Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse,
K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.;
Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller,
T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno,
J.; Utz, D.; Zuccarello, F.
2019arXiv191208650S Altcode:
The European Solar Telescope (EST) is a research infrastructure
for solar physics. It is planned to be an on-axis solar telescope
with an aperture of 4 m and equipped with an innovative suite of
spectro-polarimetric and imaging post-focus instrumentation. The EST
project was initiated and is driven by EAST, the European Association
for Solar Telescopes. EAST was founded in 2006 as an association
of 14 European countries. Today, as of December 2019, EAST consists
of 26 European research institutes from 18 European countries. The
Preliminary Design Phase of EST was accomplished between 2008 and
2011. During this phase, in 2010, the first version of the EST Science
Requirement Document (SRD) was published. After EST became a project
on the ESFRI roadmap 2016, the preparatory phase started. The goal
of the preparatory phase is to accomplish a final design for the
telescope and the legal governance structure of EST. A major milestone
on this path is to revisit and update the Science Requirement Document
(SRD). The EST Science Advisory Group (SAG) has been constituted by
EAST and the Board of the PRE-EST EU project in November 2017 and has
been charged with the task of providing with a final statement on the
science requirements for EST. Based on the conceptual design, the SRD
update takes into account recent technical and scientific developments,
to ensure that EST provides significant advancement beyond the current
state-of-the-art. The present update of the EST SRD has been developed
and discussed during a series of EST SAG meetings. The SRD develops
the top-level science objectives of EST into individual science
cases. Identifying critical science requirements is one of its main
goals. Those requirements will define the capabilities of EST and the
post-focus instrument suite. The technical requirements for the final
design of EST will be derived from the SRD.
---------------------------------------------------------
Title: Cosmic Rays Across the Rainbow Bridge: Particle Interactions
in a Magnetized Plasma Atmosphere
Authors: Hudson, H. S.; MacKinnon, A.; Szydlarski, M.; Carlsson, M.
2019AGUFMSH21B..02H Altcode:
High-energy particles enter the solar atmosphere from Galactic or solar
sources, producing gamma-ray "albedo” radiations. These emissions come
from the quiet Sun and from the large-scale corona, as well as from
the vicinity of flares, and have been observed across a wide range of
photon energies (MeV to GeV). The interaction of high-energy particles
in the solar atmosphere depends essentially upon the joint variation of
the magnetic field and the gas, and this has heretofore generally been
characterized parametrically as P ~ B<SUP>alpha</SUP> (Zweibel &
Haber 1983), with P the gas pressure and B the magnitude of the magnetic
field. We have checked this relationship with a Bifrost 3D MHD model,
approximating the particle transport as a guiding-center motion along
"open” (large-scale) fieldlines. We find alpha ~ 2.2 in the strongest
(kG) fields in the simulation, but individual field structures have
widely disparate relationships. The scatter corresponds to the strong
meandering of the open-field flux tubes in the lower atmosphere and
to their incessant dynamics. We discuss this and other aspects of
the interactions of high-energy particles with the Sun (the "solar
Størmer problem”).
---------------------------------------------------------
Title: Co-spatial velocity and magnetic swirls in the simulated
solar photosphere
Authors: Liu, Jiajia; Carlsson, Mats; Nelson, Chris J.; Erdélyi,
Robert
2019A&A...632A..97L Altcode: 2019arXiv191110923L
Context. Velocity or intensity swirls have now been shown to be
widely present throughout the photosphere and chromosphere. It has
been suggested that these events could contribute to the heating
of the upper solar atmosphere, via exciting Alfvén pulses, which
could carry significant amounts of energy. However, the conjectured
necessary physical conditions for their excitation, that the magnetic
field rotates co-spatially and co-temporally with the velocity field,
has not been verified. <BR /> Aims: We aim to understand whether
photospheric velocity swirls exist co-spatially and co-temporally with
photospheric magnetic swirls, in order to demonstrate the link between
swirls and pulses. <BR /> Methods: The automated swirl detection
algorithm (ASDA) is applied to the photospheric horizontal velocity
and vertical magnetic fields obtained from a series of realistic
numerical simulations using the radiative magnetohydrodynamics (RMHD)
code Bifrost. The spatial relationship between the detected velocity
and magnetic swirls is further investigated via a well-defined
correlation index (CI) study. <BR /> Results: On average, there are
∼63 short-lived photospheric velocity swirls (with lifetimes mostly
less than 20 s, and average radius of ∼37 km and rotating speeds of
∼2.5 km s<SUP>-1</SUP>) detected in a field of view (FOV) of 6 ×
6 Mm<SUP>-2</SUP>, implying a total population of velocity swirls of
∼1.06 × 10<SUP>7</SUP> in the solar photosphere. More than 80% of the
detected velocity swirls are found to be accompanied by local magnetic
concentrations in intergranular lanes. On average, ∼71% of the
detected velocity swirls have been found to co-exist with photospheric
magnetic swirls with the same rotating direction. <BR /> Conclusions:
The co-temporal and co-spatial rotation in the photospheric velocity
and magnetic fields provide evidence that the conjectured condition for
the excitation of Alfvén pulses by photospheric swirls is fulfilled.
---------------------------------------------------------
Title: Modeling Mg II during Solar Flares. II. Nonequilibrium Effects
Authors: Kerr, Graham S.; Carlsson, Mats; Allred, Joel C.
2019ApJ...885..119K Altcode: 2019arXiv190913300K
To extract the information that the Mg II NUV spectra (observed by the
Interface Region Imaging Spectrograph) carry about the chromosphere
during solar flares, and to validate models of energy transport via
model-data comparison, forward modeling is required. The assumption
of statistical equilibrium (SE) is typically used to obtain the
atomic level populations from snapshots of flare atmospheres, due
to computational necessity. However, it is possible that relying
on SE could lead to spurious results. We compare solving the atomic
level populations via SE versus a nonequilibrium (NEQ) time-dependent
approach. This was achieved using flare simulations from RADYN alongside
the minority species version MS_RADYN from which the time-dependent
Mg II atomic level populations and radiation transfer were computed in
complete frequency redistribution. The impacts on the emergent profiles,
lightcurves, line ratios, and formation heights are discussed. In
summary we note that NEQ effects during flares are typically important
only in the initial stages and for a short period following the
cessation of the energy injection. An analysis of the timescales of
ionization equilibrium reveals that for most of the duration of the
flare, when the temperatures and densities are sufficiently enhanced,
the relaxation timescales are short (τ <SUB>relax</SUB> < 0.1 s),
so that the equilibrium solution is an adequate approximation. These
effects vary with the size of the flare, however. In weaker flares,
effects can be more pronounced. We recommend that NEQ effects be
considered when possible but that SE is sufficient at most stages of
the flare.
---------------------------------------------------------
Title: Observational constraints on the origin of the elements. I. 3D
NLTE formation of Mn lines in late-type stars
Authors: Bergemann, Maria; Gallagher, Andrew J.; Eitner, Philipp;
Bautista, Manuel; Collet, Remo; Yakovleva, Svetlana A.; Mayriedl,
Anja; Plez, Bertrand; Carlsson, Mats; Leenaarts, Jorrit; Belyaev,
Andrey K.; Hansen, Camilla
2019A&A...631A..80B Altcode: 2019arXiv190505200B
Manganese (Mn) is a key Fe-group element, commonly employed in
stellar population and nucleosynthesis studies to explore the role
of SN Ia. We have developed a new non-local thermodynamic equilibrium
(NLTE) model of Mn, including new photo-ionisation cross-sections and
new transition rates caused by collisions with H and H<SUP>-</SUP>
atoms. We applied the model in combination with one-dimensional (1D)
LTE model atmospheres and 3D hydrodynamical simulations of stellar
convection to quantify the impact of NLTE and convection on the line
formation. We show that the effects of NLTE are present in Mn I and, to
a lesser degree, in Mn II lines, and these increase with metallicity and
with the effective temperature of a model. Employing 3D NLTE radiative
transfer, we derive a new abundance of Mn in the Sun, A(Mn) = 5.52 ±
0.03 dex, consistent with the element abundance in C I meteorites. We
also applied our methods to the analysis of three metal-poor benchmark
stars. We find that 3D NLTE abundances are significantly higher than 1D
LTE. For dwarfs, the differences between 1D NLTE and 3D NLTE abundances
are typically within 0.15 dex, however, the effects are much larger in
the atmospheres of giants owing to their more vigorous convection. We
show that 3D NLTE successfully solves the ionisation and excitation
balance for the RGB star HD 122563 that cannot be achieved by 1D LTE or
1D NLTE modelling. For HD 84937 and HD 140283, the ionisation balance is
satisfied, however, the resonance Mn I triplet lines still show somewhat
lower abundances compared to the high-excitation lines. Our results for
the benchmark stars confirm that 1D LTE modelling leads to significant
systematic biases in Mn abundances across the full wavelength range
from the blue to the IR. We also produce a list of Mn lines that are
not significantly biased by 3D and can be reliably, within the 0.1 dex
uncertainty, modelled in 1D NLTE. <P />The new cross-sections and rate
coefficients are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A80">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A80</A>The
atomic model is available at <A
href="https://keeper.mpdl.mpg.de/f/1ce2a838074b49fc9424/?dl=1">https://keeper.mpdl.mpg.de/f/1ce2a838074b49fc9424/?dl=1</A>
---------------------------------------------------------
Title: A comprehensive three-dimensional radiative magnetohydrodynamic
simulation of a solar flare
Authors: Cheung, M. C. M.; Rempel, M.; Chintzoglou, G.; Chen, F.;
Testa, P.; Martínez-Sykora, J.; Sainz Dalda, A.; DeRosa, M. L.;
Malanushenko, A.; Hansteen, V.; De Pontieu, B.; Carlsson, M.; Gudiksen,
B.; McIntosh, S. W.
2019NatAs...3..160C Altcode: 2018NatAs...3..160C
Solar and stellar flares are the most intense emitters of X-rays and
extreme ultraviolet radiation in planetary systems<SUP>1,2</SUP>. On
the Sun, strong flares are usually found in newly emerging sunspot
regions<SUP>3</SUP>. The emergence of these magnetic sunspot groups
leads to the accumulation of magnetic energy in the corona. When
the magnetic field undergoes abrupt relaxation, the energy released
powers coronal mass ejections as well as heating plasma to temperatures
beyond tens of millions of kelvins. While recent work has shed light
on how magnetic energy and twist accumulate in the corona<SUP>4</SUP>
and on how three-dimensional magnetic reconnection allows for rapid
energy release<SUP>5,6</SUP>, a self-consistent model capturing how
such magnetic changes translate into observable diagnostics has remained
elusive. Here, we present a comprehensive radiative magnetohydrodynamics
simulation of a solar flare capturing the process from emergence to
eruption. The simulation has sufficient realism for the synthesis of
remote sensing measurements to compare with observations at visible,
ultraviolet and X-ray wavelengths. This unifying model allows us to
explain a number of well-known features of solar flares<SUP>7</SUP>,
including the time profile of the X-ray flux during flares, origin
and temporal evolution of chromospheric evaporation and condensation,
and sweeping of flare ribbons in the lower atmosphere. Furthermore,
the model reproduces the apparent non-thermal shape of coronal X-ray
spectra, which is the result of the superposition of multi-component
super-hot plasmas<SUP>8</SUP> up to and beyond 100 million K.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Mn lines 3D NLTE formation in
late-type stars (Bergemann+, 2019)
Authors: Bergemann, M.; Gallagher, A. G.; Eitner, P.; Bautista, M.;
Collet, R.; Yakovleva, S.; Mayriedl, A.; Plez, B.; Carlsson, M.;
Leenaarts, J.; Belyaev, A. K.; Hansen, C.
2019yCat..36310080B Altcode:
Model rate coefficients in cm<SUP>3</SUP>/s for neutralization and
de-exci processes in collisions of MnI+H and MnII+H<SUP>-</SUP>, as
well as MnII+H and MnIII+H<SUP>-</SUP> for temperatures from T=1000K to
T=10000K. <P />Total photoionization cross section for MnI levels. <P
/>(24 data files).
---------------------------------------------------------
Title: Modeling Mg II During Solar Flares. I. Partial Frequency
Redistribution, Opacity, and Coronal Irradiation
Authors: Kerr, Graham S.; Allred, Joel C.; Carlsson, Mats
2019ApJ...883...57K Altcode: 2019arXiv190805329K
The Interface Region Imaging Spectrograph has routinely observed the
flaring Mg II near-ultraviolet (NUV) spectrum, offering excellent
diagnostic potential and a window into the location of energy
deposition. A number of studies have forward-modeled both the general
properties of these lines and specific flare observations. Generally
these have forward-modeled radiation via post-processing of snapshots
from hydrodynamic flare simulations through radiation transfer
codes. There has, however, not been a study of how the physics
included in these radiation transport codes affects the solution. A
baseline setup for forward-modeling Mg II in flares is presented and
contrasted with approaches that add or remove complexity. It is shown
for Mg II that (1) partial frequency distribution (PRD) is still
required during flare simulations despite the increased densities;
(2) using full angle-dependent PRD affects the solution but takes
significantly longer to process a snapshot; (3) including Mg I in
non-LTE (NLTE) results in negligible differences to the Mg II lines
but does affect the NUV quasi-continuum; (4) only hydrogen and Mg II
need to be included in NLTE; (5) ideally the nonequilibrium hydrogen
populations, with nonthermal collisional rates, should be used rather
than the statistical equilibrium populations; (6) an atom consisting
of only the ground state, h and k upper levels, and continuum level
is insufficient to model the resonance lines; and (7) irradiation from
a hot, dense flaring transition region can affect the formation of Mg
II. We discuss modifications to the RH code allowing straightforward
inclusion of the transition region and coronal irradiation in flares.
---------------------------------------------------------
Title: New View of the Solar Chromosphere
Authors: Carlsson, Mats; De Pontieu, Bart; Hansteen, Viggo H.
2019ARA&A..57..189C Altcode:
The solar chromosphere forms a crucial, yet complex and until recently
poorly understood, interface between the solar photosphere and the
heliosphere. Advances in high-resolution instrumentation, adaptive
optics, image reconstruction techniques, and space-based observatories
allow unprecedented high-resolution views of the finely structured and
highly dynamic chromosphere. Dramatic progress in numerical computations
allows 3D radiative magnetohydrodynamic forward models to take the place
of the previous generation of 1D semiempirical atmosphere models. These
new models provide deep insight into complex nonlocal thermodynamic
equilibrium chromospheric diagnostics and enable physics-based
interpretations of observations. This combination of modeling and
observations has led to new insights into the role of shock waves,
transverse magnetic waves, magnetic reconnection and flux emergence
in the chromospheric energy balance, the formation of spicules,
the impact of ion-neutral interactions, and the connectivity between
chromosphere and transition region. During the next few years, the
advent of new instrumentation (integral-field-unit spectropolarimetry)
and observatories (ALMA, DKIST), coupled with novel inversion codes and
expansion of existing numerical models to deal with ever more complex
physical processes (including multifluid approaches), is expected to
lead to major new insights into the dominant heating processes in the
chromosphere and beyond.
---------------------------------------------------------
Title: Modelling of Flare Processes: A Comparison of the Two RHD
Codes FLARIX and RADYN
Authors: Kašparová, Jana; Carlsson, Mats; Heinzel, Petr; Varady,
Michal
2019ASPC..519..141K Altcode:
We present a comparison of two autonomous, methodologically different
radiation hydrodynamic codes, FLARIX and RADYN, and their use to model
the solar flare processes. Both codes can model the time evolution
of a 1D atmosphere heated by a specified process, e.g. by electron
beams propagating from the injection site in the corona down to the
lower atmosphere. In such a scenario time scales can be rather short
and lead to fast heating on even sub-second time scales. Our aim is
to compare the FLARIX and RADYN codes using exactly the same setup
and model conditions. Although such a comparison has never been done
successfully before for this type of codes, we will present a close
agreement between the time evolution of the modelled atmospheric
structure for a test case of electron beam heating.
---------------------------------------------------------
Title: Modeling Mg II h, k and Triplet Lines at Solar Flare Ribbons
Authors: Zhu, Yingjie; Kowalski, Adam F.; Tian, Hui; Uitenbroek, Han;
Carlsson, Mats; Allred, Joel C.
2019ApJ...879...19Z Altcode: 2019arXiv190412285Z
Observations from the Interface Region Imaging Spectrograph often
reveal significantly broadened and non-reversed profiles of the Mg II
h, k and triplet lines at flare ribbons. To understand the formation
of these optically thick Mg II lines, we perform plane-parallel
radiative hydrodynamics modeling with the RADYN code, and then
recalculate the Mg II line profiles from RADYN atmosphere snapshots
using the radiative transfer code RH. We find that the current RH
code significantly underestimates the Mg II h and k Stark widths. By
implementing semiclassical perturbation approximation results of
quadratic Stark broadening from the STARK-B database in the RH code,
the Stark broadenings are found to be one order of magnitude larger
than those calculated from the current RH code. However, the improved
Stark widths are still too small, and another factor of 30 has to be
multiplied to reproduce the significantly broadened lines and adjacent
continuum seen in observations. Nonthermal electrons, magnetic fields,
three-dimensional effects, or electron density effects may account
for this factor. Without modifying the RADYN atmosphere, we have also
reproduced non-reversed Mg II h and k profiles, which appear when the
electron beam energy flux is decreasing. These profiles are formed at
an electron density of ∼8 × 10<SUP>14</SUP> cm<SUP>-3</SUP> and a
temperature of ∼1.4 × 10<SUP>4</SUP> K, where the source function
slightly deviates from the Planck function. Our investigation also
demonstrates that at flare ribbons the triplet lines are formed in the
upper chromosphere, close to the formation heights of the h and k lines.
---------------------------------------------------------
Title: The Response of the Lyα Line in Different Flare Heating Models
Authors: Hong, Jie; Li, Ying; Ding, M. D.; Carlsson, Mats
2019ApJ...879..128H Altcode: 2019arXiv190513356H
The solar Lyα line is the strongest line in the ultraviolet waveband,
and is greatly enhanced during solar flares. Here we present
radiative hydrodynamic simulations of solar flares under different
heating models, and calculate the response of this line taking into
account nonequilibrium ionization of hydrogen and partial frequency
redistribution. We find that in nonthermal heating models, the Lyα line
can show a red or blue asymmetry corresponding to the chromospheric
evaporation or condensation, respectively. The asymmetry may change
from red to blue if the electron beam flux is large enough to produce
a significant chromospheric condensation region. In the Lyα intensity
light curve, a dip appears when the change of asymmetry occurs. In
thermal models, the Lyα line intensity peaks quickly and then falls,
and the profile has an overall red asymmetry, which is similar to the
profiles from heating by a soft electron beam. The Lyα profile shows
a single red peak at the end of thermal heating, and the whole line
is formed in a very small height range.
---------------------------------------------------------
Title: Chromospheric polarimetry through multiline observations of
the 850 nm spectral region III: Chromospheric jets driven by twisted
magnetic fields
Authors: Quintero Noda, C.; Iijima, H.; Katsukawa, Y.; Shimizu,
T.; Carlsson, M.; de la Cruz Rodríguez, J.; Ruiz Cobo, B.; Orozco
Suárez, D.; Oba, T.; Anan, T.; Kubo, M.; Kawabata, Y.; Ichimoto,
K.; Suematsu, Y.
2019MNRAS.486.4203Q Altcode: 2019MNRAS.tmp.1081N; 2019arXiv190409151Q
We investigate the diagnostic potential of the spectral lines at 850
nm for understanding the magnetism of the lower atmosphere. For that
purpose, we use a newly developed 3D simulation of a chromospheric
jet to check the sensitivity of the spectral lines to this phenomenon
as well as our ability to infer the atmospheric information through
spectropolarimetric inversions of noisy synthetic data. We start
comparing the benefits of inverting the entire spectrum at 850 nm versus
only the Ca II 8542 Å spectral line. We found a better match of the
input atmosphere for the former case, mainly at lower heights. However,
the results at higher layers were not accurate. After several tests,
we determined that we need to weight more the chromospheric lines
than the photospheric ones in the computation of the goodness of the
fit. The new inversion configuration allows us to obtain better fits and
consequently more accurate physical parameters. Therefore, to extract
the most from multiline inversions, a proper set of weights needs to
be estimated. Besides that, we conclude again that the lines at 850
nm, or a similar arrangement with Ca II 8542 Å plus Zeeman-sensitive
photospheric lines, pose the best-observing configuration for examining
the thermal and magnetic properties of the lower solar atmosphere.
---------------------------------------------------------
Title: Dissecting bombs and bursts: non-LTE inversions of
low-atmosphere reconnection in SST and IRIS observations
Authors: Vissers, G. J. M.; de la Cruz Rodríguez, J.; Libbrecht,
T.; Rouppe van der Voort, L. H. M.; Scharmer, G. B.; Carlsson, M.
2019A&A...627A.101V Altcode: 2019arXiv190502035V
Ellerman bombs and UV bursts are transient brightenings that are
ubiquitously observed in the lower atmospheres of active and emerging
flux regions. As they are believed to pinpoint sites of magnetic
reconnection in reconfiguring fields, understanding their occurrence
and detailed evolution may provide useful insight into the overall
evolution of active regions. Here we present results from inversions
of SST/CRISP and CHROMIS, as well as IRIS data of such transient
events. Combining information from the Mg II h & k, Si IV, and Ca
II 8542 Å and Ca II H & K lines, we aim to characterise their
temperature and velocity stratification, as well as their magnetic
field configuration. We find average temperature enhancements of
a few thousand kelvin, close to the classical temperature minimum
and similar to previous studies, but localised peak temperatures
of up to 10 000-15 000 K from Ca II inversions. Including Mg II
appears to generally dampen these temperature enhancements to below
8000 K, while Si IV requires temperatures in excess of 10 000 K at
low heights, but may also be reproduced with secondary temperature
enhancements of 35 000-60 000 K higher up. However, reproducing Si
IV comes at the expense of overestimating the Mg II emission. The
line-of-sight velocity maps show clear bi-directional jet signatures
for some events and strong correlation with substructure in the
intensity images in general. Absolute line-of-sight velocities range
between 5 and 20 km s<SUP>-1</SUP> on average, with slightly larger
velocities towards, rather than away from, the observer. The inverted
magnetic field parameters show an enhancement of the horizontal
field co-located with the brightenings at heights similar to that of
the temperature increase. We are thus able to largely reproduce the
observational properties of Ellerman bombs with the UV burst signature
(e.g. intensities, profile asymmetries, morphology, and bi-directional
jet signatures), with temperature stratifications peaking close
to the classical temperature minimum. Correctly modelling the Si
IV emission in agreement with all other diagnostics is however an
outstanding issue and remains paramount in explaining its apparent
coincidence with Hα emission. Fine-tuning the approach (accounting for
resolution differences, fitting localised temperature enhancements,
and/or performing spatially coupled inversions) is likely necessary
in order to obtain better agreement between all considered diagnostics.
---------------------------------------------------------
Title: Radiative MHD Simulation of a Solar Flare
Authors: Cheung, Mark; Rempel, Matthias D.; Chintzoglou, Georgios;
Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto;
DeRosa, Marc L.; Malanushenko, Anna; Hansteen, Viggo; Carlsson, Mats;
De Pontieu, Bart; Gudiksen, Boris; McIntosh, Scott W.
2019AAS...23431005C Altcode:
We present a radiative MHD simulation of a solar flare. The
computational domain captures the near-surface layers of the convection
zone and overlying atmosphere. Inspired by the observed evolution of
NOAA Active Region (AR) 12017, a parasitic bipolar region is imposed
to emerge in the vicinity of a pre-existing sunspot. The emergence of
twisted magnetic flux generates shear flows that create a pre-existing
flux rope underneath the canopy field of the sunspot. Following erosion
of the overlying bootstrapping field, the flux rope erupts. Rapid
release of magnetic energy results in multi-wavelength synthetic
observables (including X-ray spectra, narrowband EUV images, Doppler
shifts of EUV lines) that are consistent with flare observations. This
works suggests the super-position of multi-thermal, superhot (up
to 100 MK) plasma may be partially responsible for the apparent
non-thermal shape of coronal X-ray sources in flares. Implications
for remote sensing observations of other astrophysical objects is also
discussed. This work is an important stepping stone toward high-fidelity
data-driven MHD models.
---------------------------------------------------------
Title: Ellerman bombs and UV bursts: transient events in chromospheric
current sheets
Authors: Hansteen, V.; Ortiz, A.; Archontis, V.; Carlsson, M.; Pereira,
T. M. D.; Bjørgen, J. P.
2019A&A...626A..33H Altcode: 2019arXiv190411524H
Context. Ellerman bombs (EBs), observed in the photospheric wings
of the Hα line, and UV bursts, observed in the transition region Si
IV line, are both brightenings related to flux emergence regions and
specifically to magnetic flux of opposite polarity that meet in the
photosphere. These two reconnection-related phenomena, nominally formed
far apart, occasionally occur in the same location and at the same
time, thus challenging our understanding of reconnection and heating
of the lower solar atmosphere. <BR /> Aims: We consider the formation
of an active region, including long fibrils and hot and dense coronal
plasma. The emergence of a untwisted magnetic flux sheet, injected 2.5
Mm below the photosphere, is studied as it pierces the photosphere
and interacts with the preexisting ambient field. Specifically, we
aim to study whether EBs and UV bursts are generated as a result of
such flux emergence and examine their physical relationship. <BR />
Methods: The Bifrost radiative magnetohydrodynamics code was used
to model flux emerging into a model atmosphere that contained a
fairly strong ambient field, constraining the emerging field to
a limited volume wherein multiple reconnection events occur as
the field breaks through the photosphere and expands into the outer
atmosphere. Synthetic spectra of the different reconnection events were
computed using the 1.5D RH code and the fully 3D MULTI3D code. <BR
/> Results: The formation of UV bursts and EBs at intensities and
with line profiles that are highly reminiscent of observed spectra
are understood to be a result of the reconnection of emerging flux
with itself in a long-lasting current sheet that extends over several
scale heights through the chromosphere. Synthetic spectra in the Hα
and Si IV 139.376 nm lines both show characteristics that are typical
of the observations. These synthetic diagnostics suggest that there
are no compelling reasons to assume that UV bursts occur in the
photosphere. Instead, EBs and UV bursts are occasionally formed at
opposite ends of a long current sheet that resides in an extended bubble
of cool gas. <P />The movie associated to Fig. 3 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935376/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: On the Origin of the Magnetic Energy in the Quiet Solar
Chromosphere
Authors: Martínez-Sykora, Juan; Hansteen, Viggo H.; Gudiksen, Boris;
Carlsson, Mats; De Pontieu, Bart; Gošić, Milan
2019ApJ...878...40M Altcode: 2019arXiv190404464M
The presence of magnetic field is crucial in the transport of energy
through the solar atmosphere. Recent ground-based and space-borne
observations of the quiet Sun have revealed that magnetic field
accumulates at photospheric heights, via a local dynamo or from
small-scale flux emergence events. However, most of this small-scale
magnetic field may not expand into the chromosphere due to the entropy
drop with height at the photosphere. Here we present a study that uses
a high-resolution 3D radiative MHD simulation of the solar atmosphere
with non-gray and non-LTE radiative transfer and thermal conduction
along the magnetic field to reveal that (1) the net magnetic flux
from the simulated quiet photosphere is not sufficient to maintain a
chromospheric magnetic field (on average), (2) processes in the lower
chromosphere, in the region dominated by magnetoacoustic shocks,
are able to convert kinetic energy into magnetic energy, (3) the
magnetic energy in the chromosphere increases linearly in time until
the rms of the magnetic field strength saturates at roughly 4-30 G
(horizontal average) due to conversion from kinetic energy, (4) and
that the magnetic features formed in the chromosphere are localized
to this region.
---------------------------------------------------------
Title: Mg II NUV Spectra in Solar Flares: Modelling approaches and
Velocity Diagnostics
Authors: Kerr, Graham S.; Allred, Joel C.; Carlsson, Mats
2019AAS...23420413K Altcode:
The Interface Region Imaging Spectrograph (IRIS) has observed the Mg II
NUV spectrum (the h & k resonance and the subordinate triplet), in
hundreds of solar flares. These lines form throughout the chromosphere,
offering excellent diagnostic potential and a window into the location
of flare energy deposition. A number of studies have attempted to
forward model both the general properties of these lines in flares,
and specific flare observations. Generally, these investigations have
post-processed snapshots of solar atmospheres from hydrodynamic or
radiation hydrodynamic flare simulations through advanced radiation
transport codes. There has, however, not been a survey of how the
physics included in these radiation transport codes affects the
solution. We present here a detailed study showing what physics must
be included when forward modelling these lines, including the use
of partial redistribution (PRD), the specific treatment of (PRD),
the need for a sufficiently large model atom, the inclusion of Mg
I in NLTE, the inclusion of other species in NLTE, and the impact
of using non-equilibrium hydrogen populations. Futher, we contrast
Mg II computed in statistical equilibrium with a computation that
includes non-equilibrium effects. Finally, we apply commonly used
observational metrics for inferring Doppler speeds to the synthetic
spectra, demonstrating that optical depth and radiation transfer
effects can confuse the interpretation of those measurements.
---------------------------------------------------------
Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter
Authors: McKenzie, D. E.; Ishikawa, R.; Trujillo Bueno, J.; Auchére,
F.; Rachmeler, L. A.; Kubo, M.; Kobayashi, K.; Winebarger, A. R.;
Bethge, C. W.; Narukage, N.; Kano, R.; Ishikawa, S.; de Pontieu,
B.; Carlsson, M.; Yoshida, M.; Belluzzi, L.; Štěpán, J.; del Pino
Alemán, T.; Alsina Ballester, E.; Asensio Ramos, A.
2019ASPC..526..361M Altcode:
The hydrogen Lyman-α line at 121.6 nm and the Mg k line at 279.5
nm are especially relevant for deciphering the magnetic structure
of the chromosphere since their line-center signals are formed in
the chromosphere and transition region, with unique sensitivities to
magnetic fields. We propose the Chromospheric LAyer Spectro-Polarimeter
(CLASP2), to build upon the success of the first CLASP flight, which
measured the linear polarization in H I Lyman-α. The existing CLASP
instrument will be refitted to measure all four Stokes parameters in
the 280 nm range, including variations due to the anisotropic radiation
pumping, the Hanle effect, and the Zeeman effect.
---------------------------------------------------------
Title: Comparison of Scattering Polarization Signals Observed by
CLASP: Possible Indication of the Hanle Effect
Authors: Ishikawa, R.; Trujillo Bueno, J.; Uitenbroek, H.; Kubo, M.;
Tsuneta, S.; Goto, M.; Kano, R.; Narukage, N.; Bando, T.; Katsukawa,
Y.; Ishikawa, S.; Giono, G.; Suematsu, Y.; Hara, H.; Shimizu, T.;
Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.;
Auchère, F.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso
Sainz, R.; De Pomtieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.
2019ASPC..526..305I Altcode:
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP; Kano et
al. 2012; Kobayashi et al. 2012; Kubo et al. 2014) observed, for the
first time, the linear polarization produced by scattering processes
in the hydrogen Lyman-α (121.57 nm) and Si III (120.56 nm) lines of
the solar disk radiation. The complexity of the observed scattering
polarization (i.e., conspicuous spatial variations in Q/I and U/I
at spatial scales of 10″-20″ and the absence of center-to- limb
variation at the Lyman-α center; see Kano et al. 2017) motivated us
to search for possible hints of the operation of the Hanle effect by
comparing: (a) the Lyman-α line center signal, for which the critical
field strength (B<SUB>H</SUB>) for the onset of the Hanle effect is
53 G, (b) the Lyman-α wing, which is insensitive to the Hanle effect,
and (c) the Si III line, whose B<SUB>H</SUB> = 290 G. We focus on four
regions with different total unsigned photospheric magnetic fluxes
(estimated from SDO/HMI observations), and compare the corresponding
U/I spatial variations in the Lyman-α wing, Lyman-α center, and Si III
line. The U/I signal in the Lyman-α wing shows an antisymmetric spatial
distribution, which is caused by the presence of a bright structure in
all the selected regions, regardless of the total unsigned photospheric
magnetic flux. In an internetwork region, the Lyman-α center shows an
antisymmetric spatial variation across the selected bright structure,
but it does not show it in other more magnetized regions. In the Si III
line, the spatial variation of U/I deviates from the above-mentioned
antisymmetric shape as the total unsigned photospheric magnetic flux
increases. We argue that a plausible explanation of this differential
behavior is the operation of the Hanle effect. <P />This work, presented
in an oral contribution at this Workshop, has been published on The
Astrophysical Journal (Ishikawa et al. 2017).
---------------------------------------------------------
Title: The solar chromosphere at millimetre and ultraviolet
wavelengths. I. Radiation temperatures and a detailed comparison
Authors: Jafarzadeh, S.; Wedemeyer, S.; Szydlarski, M.; De Pontieu,
B.; Rezaei, R.; Carlsson, M.
2019A&A...622A.150J Altcode: 2019arXiv190105763J
Solar observations with the Atacama Large Millimeter/submillimeter
Array (ALMA) provide us with direct measurements of the brightness
temperature in the solar chromosphere. We study the temperature
distributions obtained with ALMA Band 6 (in four sub-bands at 1.21,
1.22, 1.29, and 1.3 mm) for various areas at, and in the vicinity of,
a sunspot, comprising quasi-quiet and active regions with different
amounts of underlying magnetic fields. We compare these temperatures
with those obtained at near- and far-ultraviolet (UV) wavelengths
(and with the line-core intensities of the optically-thin far-UV
spectra), co-observed with the Interface Region Imaging Spectrograph
(IRIS) explorer. These include the emission peaks and cores of the Mg
II k 279.6 nm and Mg II h 280.4 nm lines as well as the line cores
of C II 133.4 nm, O I 135.6 nm, and Si IV 139.4 nm, sampling the
mid-to-high chromosphere and the low transition region. Splitting the
ALMA sub-bands resulted in an slight increase of spatial resolution in
individual temperature maps, thus, resolving smaller-scale structures
compared to those produced with the standard averaging routines. We
find that the radiation temperatures have different, though somewhat
overlapping, distributions in different wavelengths and in the various
magnetic regions. Comparison of the ALMA temperatures with those of
the UV diagnostics should, however, be interpreted with great caution,
the former is formed under the local thermodynamic equilibrium (LTE)
conditions, the latter under non-LTE. The mean radiation temperature
of the ALMA Band 6 is similar to that extracted from the IRIS C II
line in all areas with exception of the sunspot and pores where the C
II poses higher radiation temperatures. In all magnetic regions, the
Mg II lines associate with the lowest mean radiation temperatures in
our sample. These will provide constraints for future numerical models.
---------------------------------------------------------
Title: SI IV Resonance Line Emission during Solar Flares: Non-LTE,
Nonequilibrium, Radiation Transfer Simulations
Authors: Kerr, Graham S.; Carlsson, Mats; Allred, Joel C.; Young,
Peter R.; Daw, Adrian N.
2019ApJ...871...23K Altcode: 2018arXiv181111075K
The Interface Region Imaging Spectrograph routinely observes the Si
IV resonance lines. When analyzing quiescent observations of these
lines, it has typically been assumed that they form under optically
thin conditions. This is likely valid for the quiescent Sun, but
this assumption has also been applied to the more extreme flaring
scenario. We used 36 electron-beam-driven radiation hydrodynamic
solar flare simulations, computed using the RADYN code, to probe
the validity of this assumption. Using these simulated atmospheres,
we solved the radiation transfer equations to obtain the non-LTE,
nonequilibrium populations, line profiles, and opacities for a model
silicon atom, including charge exchange processes. This was achieved
using the “minority species” version of RADYN. The inclusion of
charge exchange resulted in a substantial fraction of Si IV at cooler
temperatures than those predicted by ionization equilibrium. All
simulations with an injected energy flux F> 5× {10}<SUP>10</SUP>
erg cm<SUP>-2</SUP> s<SUP>-1</SUP> resulted in optical depth effects on
the Si IV emission, with differences in both intensity and line shape
compared to the optically thin calculation. Weaker flares (down to F ≈
5 × 10<SUP>9</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>) also resulted
in Si IV emission forming under optically thick conditions, depending on
the other beam parameters. When opacity was significant, the atmospheres
generally had column masses in excess of 5 × 10<SUP>-6</SUP> g
cm<SUP>-2</SUP> over the temperature range 40-100 kK, and the Si IV
formation temperatures were between 30 and 60 kK. We urge caution
when analyzing Si IV flare observations, or when computing synthetic
emission without performing a full radiation transfer calculation.
---------------------------------------------------------
Title: Observationally Based Models of Penumbral Microjets
Authors: Esteban Pozuelo, S.; de la Cruz Rodríguez, J.; Drews, A.;
Rouppe van der Voort, L.; Scharmer, G. B.; Carlsson, M.
2019ApJ...870...88E Altcode: 2018arXiv181107881E
We study the polarization signals and physical parameters of penumbral
microjets (PMJs) by using high spatial resolution data taken in the
Fe I 630 nm pair, Ca II 854.2 nm, and Ca II K lines with the CRISP
and CHROMIS instruments at the Swedish 1 m Solar Telescope. We
infer their physical parameters, such as physical observables in
the photosphere and chromospheric velocity diagnostics, by different
methods, including inversions of the observed Stokes profiles with
the STiC code. PMJs harbor overall brighter Ca II K line profiles
and conspicuous polarization signals in Ca II 854.2 nm, specifically
in circular polarization that often shows multiple lobes mainly due
to the shape of Stokes I. They usually overlap photospheric regions
with a sheared magnetic field configuration, suggesting that magnetic
reconnections could play an important role in the origin of PMJs. The
discrepancy between their low LOS velocities and the high apparent
speeds reported on earlier, as well as the existence of different
vertical velocity gradients in the chromosphere, indicate that PMJs
might not be entirely related to mass motions. Instead, PMJs could
be due to perturbation fronts induced by magnetic reconnections
occurring in the deep photosphere that propagate through the
chromosphere. This reconnection may be associated with current heating
that produces temperature enhancements from the temperature minimum
region. Furthermore, enhanced collisions with electrons could also
increase the coupling to the local conditions at higher layers during
the PMJ phase, giving a possible explanation for the enhanced emission
in the overall Ca II K profiles emerging from these transients.
---------------------------------------------------------
Title: Study of the polarization produced by the Zeeman effect in
the solar Mg I b lines
Authors: Quintero Noda, C.; Uitenbroek, H.; Carlsson, M.; Orozco
Suárez, D.; Katsukawa, Y.; Shimizu, T.; Ruiz Cobo, B.; Kubo, M.; Oba,
T.; Kawabata, Y.; Hasegawa, T.; Ichimoto, K.; Anan, T.; Suematsu, Y.
2018MNRAS.481.5675Q Altcode: 2018arXiv181001067Q; 2018MNRAS.tmp.2566Q
The next generation of solar observatories aim to understand the
magnetism of the solar chromosphere. Therefore, it is crucial to
understand the polarimetric signatures of chromospheric spectral
lines. For this purpose, we here examine the suitability of the three
Fraunhofer Mg I b<SUB>1</SUB>, b<SUB>2</SUB>, and b<SUB>4</SUB> lines
at 5183.6, 5172.7, and 5167.3 Å, respectively. We start by describing
a simplified atomic model of only six levels and three line transitions
for computing the atomic populations of the 3p-4s (multiplet number
2) levels involved in the Mg I b line transitions assuming non-local
thermodynamic conditions and considering only the Zeeman effect using
the field-free approximation. We test this simplified atom against
more complex ones finding that, although there are differences in the
computed profiles, they are small compared with the advantages provided
by the simple atom in terms of speed and robustness. After comparing
the three Mg I lines, we conclude that the most capable one is the
b<SUB>2</SUB> line as b<SUB>1</SUB> forms at similar heights and always
shows weaker polarization signals, while b<SUB>4</SUB> is severely
blended with photospheric lines. We also compare Mg I b<SUB>2</SUB>
with the K I D<SUB>1</SUB> and Ca II 8542 Å lines finding that the
former is sensitive to the atmospheric parameters at heights that
are in between those covered by the latter two lines. This makes Mg I
b<SUB>2</SUB> an excellent candidate for future multiline observations
that aim to seamlessly infer the thermal and magnetic properties of
different features in the lower solar atmosphere.
---------------------------------------------------------
Title: CLASP Constraints on the Magnetization and Geometrical
Complexity of the Chromosphere-Corona Transition Region
Authors: Trujillo Bueno, J.; Štěpán, J.; Belluzzi, L.; Asensio
Ramos, A.; Manso Sainz, R.; del Pino Alemán, T.; Casini, R.; Ishikawa,
R.; Kano, R.; Winebarger, A.; Auchère, F.; Narukage, N.; Kobayashi,
K.; Bando, T.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Giono, G.; Hara,
H.; Suematsu, Y.; Shimizu, T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.;
Cirtain, J.; Champey, P.; De Pontieu, B.; Carlsson, M.
2018ApJ...866L..15T Altcode: 2018arXiv180908865T
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a
suborbital rocket experiment that on 2015 September 3 measured
the linear polarization produced by scattering processes in the
hydrogen Lyα line of the solar disk radiation. The line-center
photons of this spectral line radiation mostly stem from the
chromosphere-corona transition region (TR). These unprecedented
spectropolarimetric observations revealed an interesting surprise,
namely that there is practically no center-to-limb variation (CLV) in
the Q/I line-center signals. Using an analytical model, we first show
that the geometric complexity of the corrugated surface that delineates
the TR has a crucial impact on the CLV of the Q/I and U/I line-center
signals. Second, we introduce a statistical description of the solar
atmosphere based on a 3D model derived from a state-of-the-art radiation
magnetohydrodynamic simulation. Each realization of the statistical
ensemble is a 3D model characterized by a given degree of magnetization
and corrugation of the TR, and for each such realization we solve the
full 3D radiative transfer problem taking into account the impact
of the CLASP instrument degradation on the calculated polarization
signals. Finally, we apply the statistical inference method presented
in a previous paper to show that the TR of the 3D model that produces
the best agreement with the CLASP observations has a relatively weak
magnetic field and a relatively high degree of corrugation. We emphasize
that a suitable way to validate or refute numerical models of the upper
solar chromosphere is by confronting calculations and observations
of the scattering polarization in ultraviolet lines sensitive to the
Hanle effect.
---------------------------------------------------------
Title: A Statistical Inference Method for Interpreting the CLASP
Observations
Authors: Štěpán, J.; Trujillo Bueno, J.; Belluzzi, L.; Asensio
Ramos, A.; Manso Sainz, R.; del Pino Alemán, T.; Casini, R.; Kano, R.;
Winebarger, A.; Auchère, F.; Ishikawa, R.; Narukage, N.; Kobayashi,
K.; Bando, T.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Giono, G.; Hara,
H.; Suematsu, Y.; Shimizu, T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.;
Cirtain, J.; Champey, P.; De Pontieu, B.; Carlsson, M.
2018ApJ...865...48S Altcode: 2018arXiv180802725S
On 2015 September 3, the Chromospheric Lyman-Alpha SpectroPolarimeter
(CLASP) successfully measured the linear polarization produced by
scattering processes in the hydrogen Lyα line of the solar disk
radiation, revealing conspicuous spatial variations in the Q/I and U/I
signals. Via the Hanle effect, the line-center Q/I and U/I amplitudes
encode information on the magnetic field of the chromosphere-corona
transition region, but they are also sensitive to the three-dimensional
structure of this corrugated interface region. With the help of a simple
line-formation model, here we propose a statistical inference method
for interpreting the Lyα line-center polarization observed by CLASP.
---------------------------------------------------------
Title: Current State of UV Spectro-Polarimetry and its Future
Direction
Authors: Ishikawa, Ryohko; Sakao, Taro; Katsukawa, Yukio; Hara,
Hirohisa; Ichimoto, Kiyoshi; Shimizu, Toshifumi; Kubo, Masahito;
Auchere, Frederic; De Pontieu, Bart; Winebarger, Amy; Kobayashi,
. Ken; Kano, Ryouhei; Narukage, Noriyuki; Trujillo Bueno, Javier;
Song, Dong-uk; Manso Sainz, Rafael; Asensio Ramos, Andres; Leenaarts,
Jorritt; Carlsson, Mats; Bando, Takamasa; Ishikawa, Shin-nosuke;
Tsuneta, Saku; Belluzzi, Luca; Suematsu, Yoshinori; Giono, Gabriel;
Yoshida, Masaki; Goto, Motoshi; Del Pino Aleman, Tanausu; Stepan,
Jiri; Okamoto, Joten; Tsuzuki, Toshihiro; Uraguchi, Fumihiro; Champey,
Patrick; Alsina Ballester, Ernest; Casini, Roberto; McKenzie, David;
Rachmeler, Laurel; Bethge, Christian
2018cosp...42E1564I Altcode:
To obtain quantitative information on the magnetic field in low beta
regions (i.e., upper chromosphere and above) has been increasingly
important to understand the energetic phenomena of the outer
solar atmosphere such as flare, coronal heating, and the solar wind
acceleration. In the UV range, there are abundant spectral lines that
originate in the upper chromosphere and transition region. However,
the Zeeman effect in these spectral lines does not give rise to easily
measurable polarization signals because of the weak magnetic field
strength and the larger Doppler broadening compared with the Zeeman
effect. Instead, the Hanle effect in UV lines is expected to be a
suitable diagnostic tool of the magnetic field in the upper atmospheric
layers. To investigate the validity of UV spectro-polarimetry and
the Hanle effect, the Chromospheric Lyman-Alpha Spectro-Polarimeter
(CLASP), which is a NASA sounding- rocket experiment, was launched at
White Sands in US on September 3, 2015. During its 5 minutes ballistic
flight, it successfully performed spectro-polarimetric observations
of the hydrogen Lyman-alpha line (121.57 nm) with an unprecedentedly
high polarization sensitivity of 0.1% in this wavelength range. CLASP
observed the linear polarization produced by scattering process in VUV
lines for the first time and detected the polarization signals which
indicate the operation of the Hanle effect. Following the success
of CLASP, we are confident that UV spectro-polarimetry is the way
to proceed, and we are planning the second flight of CLASP (CLASP2:
Chromospheric LAyer SpectroPolarimeter 2). For this second flight we
will carry out spectro-polarimetry in the Mg II h and k lines around
280 nm, with minimum modifications of the CLASP1 instrument. The linear
polarization in the Mg II k line is induced by scattering processes and
the Hanle effect, being sensitive to magnetic field strengths of 5 to 50
G. In addition, the circular polarizations in the Mg II h and k lines
induced by the Zeeman effect can be measurable in at least plage and
active regions. The combination of the Hanle and Zeeman effects could
help us to more reliably infer the magnetic fields of the upper solar
chromosphere. CLASP2 was selected for flight and is being developed for
launch in the spring of 2019.Based on these sounding rocket experiments
(CLASP1 and 2), we aim at establishing the strategy and refining the
instrument concept for future space missions to explore the enigmatic
atmospheric layers via UV spectro-polarimetry.
---------------------------------------------------------
Title: Impact of Type II Spicules in the Corona: Simulations and
Synthetic Observables
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; De Moortel, Ineke;
Hansteen, Viggo H.; Carlsson, Mats
2018ApJ...860..116M Altcode: 2018arXiv180506475M
The role of type II spicules in the corona has been a much debated topic
in recent years. This paper aims to shed light on the impact of type
II spicules in the corona using novel 2.5D radiative MHD simulations,
including ion-neutral interaction effects with the Bifrost code. We
find that the formation of simulated type II spicules, driven by
the release of magnetic tension, impacts the corona in various
manners. Associated with the formation of spicules, the corona
exhibits (1) magneto-acoustic shocks and flows, which supply mass
to coronal loops, and (2) transversal magnetic waves and electric
currents that propagate at Alfvén speeds. The transversal waves and
electric currents, generated by the spicule’s driver and lasting
for many minutes, are dissipated and heat the associated loop. These
complex interactions in the corona can be connected with blueshifted
secondary components in coronal spectral lines (red-blue asymmetries)
observed with Hinode/EIS and SOHO/SUMER, as well as the EUV counterpart
of type II spicules and propagating coronal disturbances observed with
the 171 Å and 193 Å SDO/AIA channels.
---------------------------------------------------------
Title: Disentangling flows in the solar transition region
Authors: Zacharias, P.; Hansteen, V. H.; Leenaarts, J.; Carlsson,
M.; Gudiksen, B. V.
2018A&A...614A.110Z Altcode: 2018arXiv180407513Z
Context. The measured average velocities in solar and stellar spectral
lines formed at transition region temperatures have been difficult
to interpret. The dominant redshifts observed in the lower transition
region naturally leads to the question of how the upper layers of the
solar (and stellar) atmosphere can be maintained. Likewise, no ready
explanation has been made for the average blueshifts often found in
upper transition region lines. However, realistic three-dimensional
radiation magnetohydrodynamics (3D rMHD) models of the solar atmosphere
are able to reproduce the observed dominant line shifts and may thus
hold the key to resolve these issues. <BR /> Aims: These new 3D rMHD
simulations aim to shed light on how mass flows between the chromosphere
and corona and on how the coronal mass is maintained. These simulations
give new insights into the coupling of various atmospheric layers
and the origin of Doppler shifts in the solar transition region and
corona. <BR /> Methods: The passive tracer particles, so-called corks,
allow the tracking of parcels of plasma over time and thus the study of
changes in plasma temperature and velocity not only locally, but also
in a co-moving frame. By following the trajectories of the corks, we
can investigate mass and energy flows and understand the composition
of the observed velocities. <BR /> Results: Our findings show that
most of the transition region mass is cooling. The preponderance of
transition region redshifts in the model can be explained by the higher
percentage of downflowing mass in the lower and middle transition
region. The average upflows in the upper transition region can be
explained by a combination of both stronger upflows than downflows
and a higher percentage of upflowing mass. The most common combination
at lower and middle transition region temperatures are corks that are
cooling and traveling downward. For these corks, a strong correlation
between the pressure gradient along the magnetic field line and the
velocity along the magnetic field line has been observed, indicating a
formation mechanism that is related to downward propagating pressure
disturbances. Corks at upper transition region temperatures are
subject to a rather slow and highly variable but continuous heating
process. <BR /> Conclusions: Corks are shown to be an essential tool
in 3D rMHD models in order to study mass and energy flows. We have
shown that most transition region plasma is cooling after having been
heated slowly to upper transition region temperatures several minutes
before. Downward propagating pressure disturbances are identified as
one of the main mechanisms responsible for the observed redshifts at
transition region temperatures. <P />The movie associated to Fig. 3
is available at <A href="https://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Spectropolarimetric Inversions of the Ca II 8542 Å Line in
an M-class Solar Flare
Authors: Kuridze, D.; Henriques, V. M. J.; Mathioudakis, M.; Rouppe
van der Voort, L.; de la Cruz Rodríguez, J.; Carlsson, M.
2018ApJ...860...10K Altcode: 2018arXiv180500487K
We study the M1.9-class solar flare SOL2015-09-27T10:40 UT using
high-resolution full Stokes imaging spectropolarimetry of the Ca II
8542 Å line obtained with the CRISP imaging spectropolarimeter at the
Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the
non-LTE code NICOLE are used to construct semiempirical models of the
flaring atmosphere to investigate the structure and evolution of the
flare temperature and magnetic field. A comparison of the temperature
stratification in flaring and nonflaring areas reveals strong heating
of the flare ribbon during the flare peak. The polarization signals
of the ribbon in the chromosphere during the flare maximum become
stronger when compared to its surroundings and to pre- and post-flare
profiles. Furthermore, a comparison of the response functions to
perturbations in the line-of-sight magnetic field and temperature in
flaring and nonflaring atmospheres shows that during the flare, the
Ca II 8542 Å line is more sensitive to the lower atmosphere where the
magnetic field is expected to be stronger. The chromospheric magnetic
field was also determined with the weak-field approximation, which
led to results similar to those obtained with the NICOLE inversions.
---------------------------------------------------------
Title: Chromospheric Heating due to Cancellation of Quiet Sun
Internetwork Fields
Authors: Gošić, M.; de la Cruz Rodríguez, J.; De Pontieu, B.; Bellot
Rubio, L. R.; Carlsson, M.; Esteban Pozuelo, S.; Ortiz, A.; Polito, V.
2018ApJ...857...48G Altcode: 2018arXiv180207392G
The heating of the solar chromosphere remains one of the most
important questions in solar physics. Our current understanding is that
small-scale internetwork (IN) magnetic fields play an important role
as a heating agent. Indeed, cancellations of IN magnetic elements in
the photosphere can produce transient brightenings in the chromosphere
and transition region. These bright structures might be the signature
of energy release and plasma heating, probably driven by the magnetic
reconnection of IN field lines. Although single events are not expected
to release large amounts of energy, their global contribution to the
chromosphere may be significant due to their ubiquitous presence
in quiet Sun regions. In this paper, we study cancellations of IN
elements and analyze their impact on the energetics and dynamics of
the quiet Sun atmosphere. We use high-resolution, multiwavelength,
coordinated observations obtained with the Interface Region Imaging
Spectrograph and the Swedish 1 m Solar Telescope (SST) to identify
cancellations of IN magnetic flux patches and follow their evolution. We
find that, on average, these events live for ∼3 minutes in the
photosphere and ∼12 minutes in the chromosphere and/or transition
region. Employing multi-line inversions of the Mg II h and k lines,
we show that cancellations produce clear signatures of heating in the
upper atmospheric layers. However, at the resolution and sensitivity
accessible to the SST, their number density still seems to be one
order of magnitude too low to explain the global chromospheric heating.
---------------------------------------------------------
Title: Chromospheric heating during flux emergence in the solar
atmosphere
Authors: Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Danilovic,
Sanja; Scharmer, Göran; Carlsson, Mats
2018A&A...612A..28L Altcode: 2017arXiv171200474L
Context. The radiative losses in the solar chromosphere vary from
4 kW m<SUP>-2</SUP> in the quiet Sun, to 20 kW m<SUP>-2</SUP> in
active regions. The mechanisms that transport non-thermal energy to
and deposit it in the chromosphere are still not understood. Aim. We
aim to investigate the atmospheric structure and heating of the solar
chromosphere in an emerging flux region. <BR /> Methods: We have used
observations taken with the CHROMIS and CRISP instruments on the
Swedish 1-m Solar Telescope in the Ca II K , Ca II 854.2 nm, Hα,
and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line
profiles and in addition perform multi-line, multi-species, non-local
thermodynamic equilibrium (non-LTE) inversions to estimate the spatial
and temporal variation of the chromospheric structure. <BR /> Results:
We investigate which spectral features of Ca II K contribute to the
frequency-integrated Ca II K brightness, which we use as a tracer
of chromospheric radiative losses. The majority of the radiative
losses are not associated with localised high-Ca II K-brightness
events, but instead with a more gentle, spatially extended, and
persistent heating. The frequency-integrated Ca II K brightness
correlates strongly with the total linear polarization in the Ca II
854.2 nm, while the Ca II K profile shapes indicate that the bulk
of the radiative losses occur in the lower chromosphere. Non-LTE
inversions indicate a transition from heating concentrated around
photospheric magnetic elements below log τ<SUB>500</SUB> = -3 to a more
space-filling and time-persistent heating above log τ<SUB>500</SUB>
= -4. The inferred gas temperature at log τ<SUB>500</SUB> = -3.8
correlates strongly with the total linear polarization in the Ca
II 854.2 nm line, suggesting that that the heating rate correlates
with the strength of the horizontal magnetic field in the low
chromosphere. <P />Movies attached to Figs. 1 and 4 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201732027/olm">https://www.aanda.org/</A>
---------------------------------------------------------
Title: Non-LTE Calculations of the Fe I 6173 Å Line in a Flaring
Atmosphere
Authors: Hong, Jie; Ding, M. D.; Li, Ying; Carlsson, Mats
2018ApJ...857L...2H Altcode: 2018arXiv180309912H
The Fe I 6173 Å line is widely used in the measurements of vector
magnetic fields by instruments including the Helioseismic and
Magnetic Imager (HMI). We perform non-local thermodynamic equilibrium
calculations of this line based on radiative hydrodynamic simulations
in a flaring atmosphere. We employ both a quiet-Sun atmosphere and
a penumbral atmosphere as the initial one in our simulations. We find
that, in the quiet-Sun atmosphere, the line center is obviously enhanced
during an intermediate flare. The enhanced emission is contributed from
both radiative backwarming in the photosphere and particle beam heating
in the lower chromosphere. A blue asymmetry of the line profile also
appears due to an upward mass motion in the lower chromosphere. If we
take a penumbral atmosphere as the initial atmosphere, the line has
a more significant response to the flare heating, showing a central
emission and an obvious asymmetry. The low spectral resolution of
HMI would indicate some loss of information, but the enhancement
and line asymmetry are still kept. By calculating polarized line
profiles, we find that the Stokes I and V profiles can be altered
as a result of flare heating. Thus the distortion of this line has
a crucial influence on the magnetic field measured from this line,
and one should be cautious in interpreting the magnetic transients
observed frequently in solar flares.
---------------------------------------------------------
Title: Investigating the Response of Loop Plasma to Nanoflare Heating
Using RADYN Simulations
Authors: Polito, V.; Testa, P.; Allred, J.; De Pontieu, B.; Carlsson,
M.; Pereira, T. M. D.; Gošić, Milan; Reale, Fabio
2018ApJ...856..178P Altcode: 2018arXiv180405970P
We present the results of 1D hydrodynamic simulations of coronal
loops that are subject to nanoflares, caused by either in situ
thermal heating or nonthermal electron (NTE) beams. The synthesized
intensity and Doppler shifts can be directly compared with Interface
Region Imaging Spectrograph (IRIS) and Atmospheric Imaging Assembly
(AIA) observations of rapid variability in the transition region (TR)
of coronal loops, associated with transient coronal heating. We find
that NTEs with high enough low-energy cutoff ({E}<SUB>{{C</SUB>}})
deposit energy in the lower TR and chromosphere, causing blueshifts
(up to ∼20 km s<SUP>-1</SUP>) in the IRIS Si IV lines, which
thermal conduction cannot reproduce. The {E}<SUB>{{C</SUB>}} threshold
value for the blueshifts depends on the total energy of the events
(≈5 keV for 10<SUP>24</SUP> erg, up to 15 keV for 10<SUP>25</SUP>
erg). The observed footpoint emission intensity and flows, combined
with the simulations, can provide constraints on both the energy of the
heating event and {E}<SUB>{{C</SUB>}}. The response of the loop plasma
to nanoflares depends crucially on the electron density: significant
Si IV intensity enhancements and flows are observed only for initially
low-density loops (<10<SUP>9</SUP> cm<SUP>-3</SUP>). This provides
a possible explanation of the relative scarcity of observations of
significant moss variability. While the TR response to single heating
episodes can be clearly observed, the predicted coronal emission (AIA
94 Å) for single strands is below current detectability and can only
be observed when several strands are heated closely in time. Finally,
we show that the analysis of the IRIS Mg II chromospheric lines can
help further constrain the properties of the heating mechanisms.
---------------------------------------------------------
Title: Three-dimensional modeling of the Ca II H and K lines in the
solar atmosphere
Authors: Bjørgen, Johan P.; Sukhorukov, Andrii V.; Leenaarts, Jorrit;
Carlsson, Mats; de la Cruz Rodríguez, Jaime; Scharmer, Göran B.;
Hansteen, Viggo H.
2018A&A...611A..62B Altcode: 2017arXiv171201045B
Context. CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar
Telescope (SST), can observe the chromosphere in the H and K lines of
Ca II at high spatial and spectral resolution. Accurate modeling as
well as an understanding of the formation of these lines are needed to
interpret the SST/CHROMIS observations. Such modeling is computationally
challenging because these lines are influenced by strong departures from
local thermodynamic equilibrium, three-dimensional radiative transfer,
and partially coherent resonance scattering of photons. Aim. We aim to
model the Ca II H and K lines in 3D model atmospheres to understand
their formation and to investigate their diagnostic potential for
probing the chromosphere. <BR /> Methods: We model the synthetic
spectrum of Ca II using the radiative transfer code Multi3D in three
different radiation-magnetohydrodynamic model atmospheres computed with
the Bifrost code. We classify synthetic intensity profiles according
to their shapes and study how their features are related to the
physical properties in the model atmospheres. We investigate whether
the synthetic data reproduce the observed spatially-averaged line
shapes, center-to-limb variation and compare this data with SST/CHROMIS
images. <BR /> Results: The spatially-averaged synthetic line profiles
show too low central emission peaks, and too small separation between
the peaks. The trends of the observed center-to-limb variation of
the profiles properties are reproduced by the models. The Ca II H and
K line profiles provide a temperature diagnostic of the temperature
minimum and the temperature at the formation height of the emission
peaks. The Doppler shift of the central depression is an excellent
probe of the velocity in the upper chromosphere.
---------------------------------------------------------
Title: Hα and Hβ emission in a C3.3 solar flare: comparison between
observations and simulations
Authors: Zuccarello, F.; Simoes, P. J. D. A.; Capparelli, V.; Fletcher,
L.; Romano, P.; Mathioudakis, M.; Cauzzi, G.; Carlsson, M.; Kuridze,
D.; Keys, P.
2017AGUFMSH41A2742Z Altcode:
This work is based on the analysis of an extremely rare set of
simultaneous observations of a C3.3 solar flare in the Hα and Hβ lines
at high spatial and temporal resolution, which were acquired at the
Dunn Solar Telescope. Images of the C3.3 flare (SOL2014-04-22T15:22)
made at various wavelengths along the Hα line profile by the
Interferometric Bidimensional Spectrometer (IBIS) and in the Hβ
with the Rapid Oscillations in the Solar Atmosphere (ROSA) broadband
imager are analyzed to obtain the intensity evolution. The analysis
shows that Hα and Hβ intensity excesses in three identified flare
footpoints are well correlated in time. In the stronger footpoints,
the typical value of the the Hα/Hβ intensity ratio observed is ∼
0.4 - 0.5, in broad agreement with values obtained from a RADYN non-LTE
simulation driven by an electron beam with parameters constrained by
observations. The weaker footpoint has a larger Hα/Hβ ratio, again
consistent with a RADYN simulation but with a smaller energy flux.
---------------------------------------------------------
Title: An IRIS Optically Thin View of the Dynamics of the Solar
Chromosphere
Authors: Carlsson, M.
2017AGUFMSH41C..04C Altcode:
We analyze the formation of the O I 1356 and Cl I 1351 lines and
show that they are formed in the mid-chromosphere and are optically
thin. Their non-thermal line-widths are thus a direct measure of the
velocity field along the line of sight. We use this insight to analyze a
large set of observations from the Interface Region Imaging Spectrograph
(IRIS) to study the dynamics of the Solar Chromosphere.
---------------------------------------------------------
Title: Constraints on active region coronal heating properties
from observations and modeling of chromospheric, transition region,
and coronal emission
Authors: Testa, P.; Polito, V.; De Pontieu, B.; Carlsson, M.; Reale,
F.; Allred, J. C.; Hansteen, V. H.
2017AGUFMSH43A2804T Altcode:
We investigate coronal heating properties in active region cores in
non-flaring conditions, using high spatial, spectral, and temporal
resolution chromospheric/transition region/coronal observations coupled
with detailed modeling. We will focus, in particular, on observations
with the Interface Region Imaging Spectrograph (IRIS), joint with
observations with Hinode (XRT and EIS) and SDO/AIA. We will discuss
how these observations and models (1D HD and 3D MHD, with the RADYN
and Bifrost codes) provide useful diagnostics of the coronal heating
processes and mechanisms of energy transport.
---------------------------------------------------------
Title: Intermittent Reconnection and Plasmoids in UV Bursts in the
Low Solar Atmosphere
Authors: Rouppe van der Voort, L.; De Pontieu, B.; Scharmer, G. B.;
de la Cruz Rodríguez, J.; Martínez-Sykora, J.; Nóbrega-Siverio,
D.; Guo, L. J.; Jafarzadeh, S.; Pereira, T. M. D.; Hansteen, V. H.;
Carlsson, M.; Vissers, G.
2017ApJ...851L...6R Altcode: 2017arXiv171104581R
Magnetic reconnection is thought to drive a wide variety of dynamic
phenomena in the solar atmosphere. Yet, the detailed physical mechanisms
driving reconnection are difficult to discern in the remote sensing
observations that are used to study the solar atmosphere. In this
Letter, we exploit the high-resolution instruments Interface Region
Imaging Spectrograph and the new CHROMIS Fabry-Pérot instrument at
the Swedish 1-m Solar Telescope (SST) to identify the intermittency
of magnetic reconnection and its association with the formation of
plasmoids in so-called UV bursts in the low solar atmosphere. The Si IV
1403 Å UV burst spectra from the transition region show evidence of
highly broadened line profiles with often non-Gaussian and triangular
shapes, in addition to signatures of bidirectional flows. Such profiles
had previously been linked, in idealized numerical simulations, to
magnetic reconnection driven by the plasmoid instability. Simultaneous
CHROMIS images in the chromospheric Ca II K 3934 Å line now provide
compelling evidence for the presence of plasmoids by revealing highly
dynamic and rapidly moving brightenings that are smaller than 0.″2 and
that evolve on timescales of the order of seconds. Our interpretation
of the observations is supported by detailed comparisons with synthetic
observables from advanced numerical simulations of magnetic reconnection
and associated plasmoids in the chromosphere. Our results highlight
how subarcsecond imaging spectroscopy sensitive to a wide range of
temperatures combined with advanced numerical simulations that are
realistic enough to compare with observations can directly reveal the
small-scale physical processes that drive the wide range of phenomena
in the solar atmosphere.
---------------------------------------------------------
Title: Chromospheric polarimetry through multiline observations of
the 850-nm spectral region - II. A magnetic flux tube scenario
Authors: Quintero Noda, C.; Kato, Y.; Katsukawa, Y.; Oba, T.; de la
Cruz Rodríguez, J.; Carlsson, M.; Shimizu, T.; Orozco Suárez, D.;
Ruiz Cobo, B.; Kubo, M.; Anan, T.; Ichimoto, K.; Suematsu, Y.
2017MNRAS.472..727Q Altcode: 2017arXiv170801333Q
In this publication, we continue the work started in Quintero Noda et
al., examining this time a numerical simulation of a magnetic flux
tube concentration. Our goal is to study if the physical phenomena
that take place in it, in particular, the magnetic pumping, leaves
a specific imprint on the examined spectral lines. We find that the
profiles from the interior of the flux tube are periodically doppler
shifted following an oscillation pattern that is also reflected in
the amplitude of the circular polarization signals. In addition, we
analyse the properties of the Stokes profiles at the edges of the flux
tube discovering the presence of linear polarization signals for the Ca
II lines, although they are weak with an amplitude around 0.5 per cent
of the continuum intensity. Finally, we compute the response functions
to perturbations in the longitudinal field, and we estimate the field
strength using the weak-field approximation. Our results indicate
that the height of formation of the spectral lines changes during the
magnetic pumping process, which makes the interpretation of the inferred
magnetic field strength and its evolution more difficult. These results
complement those from previous works, demonstrating the capabilities and
limitations of the 850-nm spectrum for chromospheric Zeeman polarimetry
in a very dynamic and complex atmosphere.
---------------------------------------------------------
Title: Hα and Hβ Emission in a C3.3 Solar Flare: Comparison between
Observations and Simulations
Authors: Capparelli, Vincenzo; Zuccarello, Francesca; Romano, Paolo;
Simões, Paulo J. A.; Fletcher, Lyndsay; Kuridze, David; Mathioudakis,
Mihalis; Keys, Peter H.; Cauzzi, Gianna; Carlsson, Mats
2017ApJ...850...36C Altcode: 2017arXiv171004067C
The hydrogen Balmer series is a basic radiative loss channel from
the flaring solar chromosphere. We report here on the analysis of an
extremely rare set of simultaneous observations of a solar flare in the
{{H}}α and {{H}}β lines, at high spatial and temporal resolutions,
that were acquired at the Dunn Solar Telescope. Images of the C3.3 flare
(SOL2014-04-22T15:22) made at various wavelengths along the {{H}}α line
profile by the Interferometric Bidimensional Spectrometer (IBIS) and in
the {{H}}β with the Rapid Oscillations in the Solar Atmosphere (ROSA)
broadband imager are analyzed to obtain the intensity evolution. The
{{H}}α and {{H}}β intensity excesses in three identified flare
footpoints are well-correlated in time. We examine the ratio of {{H}}α
to {{H}}β flare excess, which was proposed by previous authors as
a possible diagnostic of the level of electron-beam energy input. In
the stronger footpoints, the typical value of the the {{H}}α /H β
intensity ratio observed is ∼0.4-0.5, in broad agreement with values
obtained from a RADYN non-LTE simulation driven by an electron beam
with parameters constrained (as far as possible) by observation. The
weaker footpoint has a larger {{H}}α /H β ratio, again consistent
with a RADYN simulation, but with a smaller energy flux. The {{H}}α
line profiles observed have a less prominent central reversal than is
predicted by the RADYN results, but can be brought into agreement if
the {{H}}α -emitting material has a filling factor of around 0.2-0.3.
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. IX. The Formation of the
C I 135.58 NM Line in the Solar Atmosphere
Authors: Lin, Hsiao-Hsuan; Carlsson, Mats; Leenaarts, Jorrit
2017ApJ...846...40L Altcode: 2017arXiv170809426L
The C I 135.58 nm line is located in the wavelength range of NASA’s
Interface Region Imaging Spectrograph (IRIS) small explorer mission. We
study the formation and diagnostic potential of this line by means of
non local-thermodynamic-equilibrium modeling, employing both 1D and 3D
radiation-magnetohydrodynamic models. The C I/C II ionization balance is
strongly influenced by photoionization by Lyα emission. The emission
in the C I 135.58 nm line is dominated by a recombination cascade and
the line forming region is optically thick. The Doppler shift of the
line correlates strongly with the vertical velocity in its line forming
region, which is typically located at 1.5 Mm height. With IRIS, the C
I 135.58 nm line is usually observed together with the O I 135.56 nm
line, and from the Doppler shift of both lines, we obtain the velocity
difference between the line forming regions of the two lines. From
the ratio of the C I/O I line core intensity, we can determine the
distance between the C I and the O I forming layers. Combined with the
velocity difference, the velocity gradient at mid-chromospheric heights
can be derived. The C I/O I total intensity line ratio is correlated
with the inverse of the electron density in the mid-chromosphere. We
conclude that the C I 135.58 nm line is an excellent probe of the
middle chromosphere by itself, and together with the O I 135.56 nm
line the two lines provide even more information, which complements
other powerful chromospheric diagnostics of IRIS such as the Mg II h
and k lines and the C II lines around 133.5 nm.
---------------------------------------------------------
Title: Solar polarimetry through the K I lines at 770 nm
Authors: Quintero Noda, C.; Uitenbroek, H.; Katsukawa, Y.; Shimizu,
T.; Oba, T.; Carlsson, M.; Orozco Suárez, D.; Ruiz Cobo, B.; Kubo,
M.; Anan, T.; Ichimoto, K.; Suematsu, Y.
2017MNRAS.470.1453Q Altcode: 2017arXiv170510002Q
We characterize the K I D<SUB>1</SUB> & D<SUB>2</SUB> lines in
order to determine whether they could complement the 850 nm window,
containing the Ca II infrared triplet lines and several Zeeman sensitive
photospheric lines, that was studied previously. We investigate the
effect of partial redistribution on the intensity profiles, their
sensitivity to changes in different atmospheric parameters, and
the spatial distribution of Zeeman polarization signals employing a
realistic magnetohydrodynamic simulation. The results show that these
lines form in the upper photosphere at around 500 km, and that they
are sensitive to the line-of-sight velocity and magnetic field strength
at heights where neither the photospheric lines nor the Ca II infrared
lines are. However, at the same time, we found that their sensitivity
to the temperature essentially comes from the photosphere. Then, we
conclude that the K I lines provide a complement to the lines in the
850 nm window for the determination of atmospheric parameters in the
upper photosphere, especially for the line-of-sight velocity and the
magnetic field.
---------------------------------------------------------
Title: Two-dimensional Radiative Magnetohydrodynamic Simulations of
Partial Ionization in the Chromosphere. II. Dynamics and Energetics
of the Low Solar Atmosphere
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats;
Hansteen, Viggo H.; Nóbrega-Siverio, Daniel; Gudiksen, Boris V.
2017ApJ...847...36M Altcode: 2017arXiv170806781M
We investigate the effects of interactions between ions and
neutrals on the chromosphere and overlying corona using 2.5D
radiative MHD simulations with the Bifrost code. We have extended
the code capabilities implementing ion-neutral interaction effects
using the generalized Ohm’s law, I.e., we include the Hall term
and the ambipolar diffusion (Pedersen dissipation) in the induction
equation. Our models span from the upper convection zone to the corona,
with the photosphere, chromosphere, and transition region partially
ionized. Our simulations reveal that the interactions between ionized
particles and neutral particles have important consequences for the
magnetothermodynamics of these modeled layers: (1) ambipolar diffusion
increases the temperature in the chromosphere; (2) sporadically the
horizontal magnetic field in the photosphere is diffused into the
chromosphere, due to the large ambipolar diffusion; (3) ambipolar
diffusion concentrates electrical currents, leading to more violent
jets and reconnection processes, resulting in (3a) the formation of
longer and faster spicules, (3b) heating of plasma during the spicule
evolution, and (3c) decoupling of the plasma and magnetic field in
spicules. Our results indicate that ambipolar diffusion is a critical
ingredient for understanding the magnetothermodynamic properties in the
chromosphere and transition region. The numerical simulations have been
made publicly available, similar to previous Bifrost simulations. This
will allow the community to study realistic numerical simulations with
a wider range of magnetic field configurations and physics modules
than previously possible.
---------------------------------------------------------
Title: Impact of Type II Spicules into the Corona
Authors: Martinez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats;
Hansteen, Viggo H.; Pereira, Tiago M. D.
2017SPD....4810403M Altcode:
In the lower solar atmosphere, the chromosphere is permeated by jets,
in which plasma is propelled at speeds of 50-150 km/s into the Sun’s
atmosphere or corona. Although these spicules may play a role in heating
the million-degree corona and are associated with Alfvén waves that
help drive the solar wind, their generation remains mysterious. We
implemented in the radiative MHD Bifrost code the effects of partial
ionization using the generalized Ohm’s law. This code also solves
the full MHD equations with non-grey and non-LTE radiative transfer
and thermal conduction along magnetic field lines. The ion-neutral
collision frequency is computed using recent studies that improved the
estimation of the cross sections under chromospheric conditions (Vranjes
& Krstic 2013). Self-consistently driven jets (spicules type II)
in magnetohydrodynamic simulations occur ubiquitously when magnetic
tension is confined and transported upwards through interactions
between ions and neutrals, and impulsively released to drive flows,
heat plasma, generate Alfvén waves, and may play an important role in
maintaining the substructure of loop fans. This mechanism explains how
spicular plasma can be heated to millions of degrees and how Alfvén
waves are generated in the chromosphere.
---------------------------------------------------------
Title: Realistic radiative MHD simulation of a solar flare
Authors: Rempel, Matthias D.; Cheung, Mark; Chintzoglou, Georgios;
Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto;
DeRosa, Marc L.; Viktorovna Malanushenko, Anna; Hansteen, Viggo H.;
De Pontieu, Bart; Carlsson, Mats; Gudiksen, Boris; McIntosh, Scott W.
2017SPD....4840001R Altcode:
We present a recently developed version of the MURaM radiative
MHD code that includes coronal physics in terms of optically thin
radiative loss and field aligned heat conduction. The code employs
the "Boris correction" (semi-relativistic MHD with a reduced speed
of light) and a hyperbolic treatment of heat conduction, which allow
for efficient simulations of the photosphere/corona system by avoiding
the severe time-step constraints arising from Alfven wave propagation
and heat conduction. We demonstrate that this approach can be used
even in dynamic phases such as a flare. We consider a setup in which
a flare is triggered by flux emergence into a pre-existing bipolar
active region. After the coronal energy release, efficient transport
of energy along field lines leads to the formation of flare ribbons
within seconds. In the flare ribbons we find downflows for temperatures
lower than ~5 MK and upflows at higher temperatures. The resulting
soft X-ray emission shows a fast rise and slow decay, reaching a peak
corresponding to a mid C-class flare. The post reconnection energy
release in the corona leads to average particle energies reaching 50
keV (500 MK under the assumption of a thermal plasma). We show that
hard X-ray emission from the corona computed under the assumption of
thermal bremsstrahlung can produce a power-law spectrum due to the
multi-thermal nature of the plasma. The electron energy flux into the
flare ribbons (classic heat conduction with free streaming limit) is
highly inhomogeneous and reaches peak values of about 3x10<SUP>11</SUP>
erg/cm<SUP>2</SUP>/s in a small fraction of the ribbons, indicating
regions that could potentially produce hard X-ray footpoint sources. We
demonstrate that these findings are robust by comparing simulations
computed with different values of the saturation heat flux as well as
the "reduced speed of light".
---------------------------------------------------------
Title: RADYN Simulations of Non-thermal and Thermal Models of
Ellerman Bombs
Authors: Hong, Jie; Carlsson, Mats; Ding, M. D.
2017ApJ...845..144H Altcode: 2017arXiv170705514H
Ellerman bombs (EBs) are brightenings in the Hα line wings that
are believed to be caused by magnetic reconnection in the lower
atmosphere. To study the response and evolution of the chromospheric
line profiles, we perform radiative hydrodynamic simulations of EBs
using both non-thermal and thermal models. Overall, these models can
generate line profiles that are similar to observations. However, in
non-thermal models we find dimming in the Hα line wings and continuum
when the heating begins, while for the thermal models dimming occurs
only in the Hα line core, and with a longer lifetime. This difference
in line profiles can be used to determine whether an EB is dominated by
non-thermal heating or thermal heating. In our simulations, if a higher
heating rate is applied, then the Hα line will be unrealistically
strong and there are still no clear UV burst signatures.
---------------------------------------------------------
Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter
Authors: Rachmeler, Laurel; E McKenzie, David; Ishikawa, Ryohko;
Trujillo Bueno, Javier; Auchère, Frédéric; Kobayashi, Ken;
Winebarger, Amy; Bethge, Christian; Kano, Ryouhei; Kubo, Masahito;
Song, Donguk; Narukage, Noriyuki; Ishikawa, Shin-nosuke; De Pontieu,
Bart; Carlsson, Mats; Yoshida, Masaki; Belluzzi, Luca; Stepan, Jiri;
del Pino Alemná, Tanausú; Ballester, Ernest Alsina; Asensio Ramos,
Andres
2017SPD....4811010R Altcode:
We present the instrument, science case, and timeline of the CLASP2
sounding rocket mission. The successful CLASP (Chromospheric Lyman-Alpha
Spectro-Polarimeter) sounding rocket flight in 2015 resulted in
the first-ever linear polarization measurements of solar hydrogen
Lyman-alpha line, which is sensitive to the Hanle effect and can be used
to constrain the magnetic field and geometric complexity of the upper
chromosphere. Ly-alpha is one of several upper chromospheric lines that
contain magnetic information. In the spring of 2019, we will re-fly
the modified CLASP telescope to measure the full Stokes profile of Mg
II h & k near 280 nm. This set of lines is sensitive to the upper
chromospheric magnetic field via both the Hanle and the Zeeman effects.
---------------------------------------------------------
Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter
Authors: Rachmeler, Laurel A.; McKenzie, D. E.; Ishikawa, R.;
Trujillo-Bueno, J.; Auchere, F.; Kobayashi, K.; Winebarger, A.;
Bethge, C.; Kano, R.; Kubo, M.; Song, D.; Narukage, N.; Ishikawa, S.;
De Pontieu, B.; Carlsson, M.; Yoshida, M.; Belluzzi, L.; Stepan, J.;
del Pino Alemán, T.; Alsina Ballester, E.; Asensio Ramos, A.
2017shin.confE..79R Altcode:
We present the instrument, science case, and timeline of the CLASP2
sounding rocket mission. The successful CLASP (Chromospheric Lyman-Alpha
Spectro-Polarimeter) sounding rocket flight in 2015 resulted in
the first-ever linear polarization measurements of solar hydrogen
Lyman-alpha line, which is sensitive to the Hanle effect and can be used
to constrain the magnetic field and geometric complexity of the upper
chromosphere. Ly-alpha is one of several upper chromospheric lines that
contain magnetic information. In the spring of 2019, we will re-fly
the modified CLASP telescope to measure the full Stokes profile of Mg
II h & k near 280 nm. This set of lines is sensitive to the upper
chromospheric magnetic field via both the Hanle and the Zeeman effects.
---------------------------------------------------------
Title: On the generation of solar spicules and Alfvénic waves
Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.;
Rouppe van der Voort, L.; Carlsson, M.; Pereira, T. M. D.
2017Sci...356.1269M Altcode: 2017arXiv171007559M
In the lower solar atmosphere, the chromosphere is permeated by jets
known as spicules, in which plasma is propelled at speeds of 50 to
150 kilometers per second into the corona. The origin of the spicules
is poorly understood, although they are expected to play a role in
heating the million-degree corona and are associated with Alfvénic
waves that help drive the solar wind. We compare magnetohydrodynamic
simulations of spicules with observations from the Interface Region
Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules
are shown to occur when magnetic tension is amplified and transported
upward through interactions between ions and neutrals or ambipolar
diffusion. The tension is impulsively released to drive flows, heat
plasma (through ambipolar diffusion), and generate Alfvénic waves.
---------------------------------------------------------
Title: Millimeter radiation from a 3D model of the solar
atmosphere. II. Chromospheric magnetic field
Authors: Loukitcheva, M.; White, S. M.; Solanki, S. K.; Fleishman,
G. D.; Carlsson, M.
2017A&A...601A..43L Altcode: 2017arXiv170206018L
<BR /> Aims: We use state-of-the-art, three-dimensional non-local
thermodynamic equilibrium (non-LTE) radiative magnetohydrodynamic
simulations of the quiet solar atmosphere to carry out detailed tests
of chromospheric magnetic field diagnostics from free-free radiation at
millimeter and submillimeter wavelengths (mm/submm). <BR /> Methods:
The vertical component of the magnetic field was deduced from the
mm/submm brightness spectra and the degree of circular polarization
synthesized at millimeter frequencies. We used the frequency bands
observed by the Atacama Large Millimeter/Submillimeter Array (ALMA)
as a convenient reference. The magnetic field maps obtained describe
the longitudinal magnetic field at the effective formation heights of
the relevant wavelengths in the solar chromosphere. <BR /> Results:
The comparison of the deduced and model chromospheric magnetic fields
at the spatial resolution of both the model and current observations
demonstrates a good correlation, but has a tendency to underestimate
the model field. The systematic discrepancy of about 10% is probably
due to averaging of the restored field over the heights contributing
to the radiation, weighted by the strength of the contribution. On
the whole, the method of probing the longitudinal component of the
magnetic field with free-free emission at mm/submm wavelengths
is found to be applicable to measurements of the weak quiet-Sun
magnetic fields. However, successful exploitation of this technique
requires very accurate measurements of the polarization properties
(primary beam and receiver polarization response) of the antennas,
which will be the principal factor that determines the level to which
chromospheric magnetic fields can be measured. <BR /> Conclusions:
Consequently, high-resolution and high-precision observations of
circularly polarized radiation at millimeter wavelengths can be a
powerful tool for producing chromospheric longitudinal magnetograms.
---------------------------------------------------------
Title: Indication of the Hanle Effect by Comparing the Scattering
Polarization Observed by CLASP in the Lyα and Si III 120.65 nm Lines
Authors: Ishikawa, R.; Trujillo Bueno, J.; Uitenbroek, H.; Kubo, M.;
Tsuneta, S.; Goto, M.; Kano, R.; Narukage, N.; Bando, T.; Katsukawa,
Y.; Ishikawa, S.; Giono, G.; Suematsu, Y.; Hara, H.; Shimizu, T.;
Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.;
Auchère, F.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso
Sainz, R.; De Pontieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.
2017ApJ...841...31I Altcode:
The Chromospheric Lyman-Alpha Spectro-Polarimeter is a sounding
rocket experiment that has provided the first successful measurement
of the linear polarization produced by scattering processes in
the hydrogen Lyα line (121.57 nm) radiation of the solar disk. In
this paper, we report that the Si III line at 120.65 nm also shows
scattering polarization and we compare the scattering polarization
signals observed in the Lyα and Si III lines in order to search for
observational signatures of the Hanle effect. We focus on four selected
bright structures and investigate how the U/I spatial variations vary
between the Lyα wing, the Lyα core, and the Si III line as a function
of the total unsigned photospheric magnetic flux estimated from Solar
Dynamics Observatory/Helioseismic and Magnetic Imager observations. In
an internetwork region, the Lyα core shows an antisymmetric spatial
variation across the selected bright structure, but it does not show
it in other more magnetized regions. In the Si III line, the spatial
variation of U/I deviates from the above-mentioned antisymmetric
shape as the total unsigned photospheric magnetic flux increases. A
plausible explanation of this difference is the operation of the Hanle
effect. We argue that diagnostic techniques based on the scattering
polarization observed simultaneously in two spectral lines with very
different sensitivities to the Hanle effect, like Lyα and Si III,
are of great potential interest for exploring the magnetism of the
upper solar chromosphere and transition region.
---------------------------------------------------------
Title: Bombs and Flares at the Surface and Lower Atmosphere of the Sun
Authors: Hansteen, V. H.; Archontis, V.; Pereira, T. M. D.; Carlsson,
M.; Rouppe van der Voort, L.; Leenaarts, J.
2017ApJ...839...22H Altcode: 2017arXiv170402872H
A spectacular manifestation of solar activity is the appearance of
transient brightenings in the far wings of the Hα line, known as
Ellerman bombs (EBs). Recent observations obtained by the Interface
Region Imaging Spectrograph have revealed another type of plasma
“bombs” (UV bursts) with high temperatures of perhaps up to 8 ×
10<SUP>4</SUP> K within the cooler lower solar atmosphere. Realistic
numerical modeling showing such events is needed to explain
their nature. Here, we report on 3D radiative magnetohydrodynamic
simulations of magnetic flux emergence in the solar atmosphere. We
find that ubiquitous reconnection between emerging bipolar magnetic
fields can trigger EBs in the photosphere, UV bursts in the mid/low
chromosphere and small (nano-/micro-) flares (10<SUP>6</SUP> K) in
the upper chromosphere. These results provide new insights into the
emergence and build up of the coronal magnetic field and the dynamics
and heating of the solar surface and lower atmosphere.
---------------------------------------------------------
Title: Polarization Calibration of the Chromospheric Lyman-Alpha
SpectroPolarimeter for a 0.1% Polarization Sensitivity in the VUV
Range. Part II: In-Flight Calibration
Authors: Giono, G.; Ishikawa, R.; Narukage, N.; Kano, R.; Katsukawa,
Y.; Kubo, M.; Ishikawa, S.; Bando, T.; Hara, H.; Suematsu, Y.;
Winebarger, A.; Kobayashi, K.; Auchère, F.; Trujillo Bueno, J.;
Tsuneta, S.; Shimizu, T.; Sakao, T.; Cirtain, J.; Champey, P.; Asensio
Ramos, A.; Štěpán, J.; Belluzzi, L.; Manso Sainz, R.; De Pontieu,
B.; Ichimoto, K.; Carlsson, M.; Casini, R.; Goto, M.
2017SoPh..292...57G Altcode:
The Chromospheric Lyman-Alpha SpectroPolarimeter is a sounding
rocket instrument designed to measure for the first time the linear
polarization of the hydrogen Lyman-α line (121.6 nm). The instrument
was successfully launched on 3 September 2015 and observations were
conducted at the solar disc center and close to the limb during the
five-minutes flight. In this article, the disc center observations are
used to provide an in-flight calibration of the instrument spurious
polarization. The derived in-flight spurious polarization is consistent
with the spurious polarization levels determined during the pre-flight
calibration and a statistical analysis of the polarization fluctuations
from solar origin is conducted to ensure a 0.014% precision on the
spurious polarization. The combination of the pre-flight and the
in-flight polarization calibrations provides a complete picture of
the instrument response matrix, and a proper error transfer method
is used to confirm the achieved polarization accuracy. As a result,
the unprecedented 0.1% polarization accuracy of the instrument in the
vacuum ultraviolet is ensured by the polarization calibration.
---------------------------------------------------------
Title: Discovery of Scattering Polarization in the Hydrogen Lyα
Line of the Solar Disk Radiation
Authors: Kano, R.; Trujillo Bueno, J.; Winebarger, A.; Auchère, F.;
Narukage, N.; Ishikawa, R.; Kobayashi, K.; Bando, T.; Katsukawa, Y.;
Kubo, M.; Ishikawa, S.; Giono, G.; Hara, H.; Suematsu, Y.; Shimizu,
T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.; Goto, M.; Belluzzi, L.;
Štěpán, J.; Asensio Ramos, A.; Manso Sainz, R.; Champey, P.;
Cirtain, J.; De Pontieu, B.; Casini, R.; Carlsson, M.
2017ApJ...839L..10K Altcode: 2017arXiv170403228K
There is a thin transition region (TR) in the solar atmosphere where
the temperature rises from 10,000 K in the chromosphere to millions
of degrees in the corona. Little is known about the mechanisms that
dominate this enigmatic region other than the magnetic field plays a
key role. The magnetism of the TR can only be detected by polarimetric
measurements of a few ultraviolet (UV) spectral lines, the Lyα line
of neutral hydrogen at 121.6 nm (the strongest line of the solar UV
spectrum) being of particular interest given its sensitivity to the
Hanle effect (the magnetic-field-induced modification of the scattering
line polarization). We report the discovery of linear polarization
produced by scattering processes in the Lyα line, obtained with
the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket
experiment. The Stokes profiles observed by CLASP in quiet regions of
the solar disk show that the Q/I and U/I linear polarization signals are
of the order of 0.1% in the line core and up to a few percent in the
nearby wings, and that both have conspicuous spatial variations with
scales of ∼10 arcsec. These observations help constrain theoretical
models of the chromosphere-corona TR and extrapolations of the
magnetic field from photospheric magnetograms. In fact, the observed
spatial variation from disk to limb of polarization at the line core
and wings already challenge the predictions from three-dimensional
magnetohydrodynamical models of the upper solar chromosphere.
---------------------------------------------------------
Title: Hydrogen Balmer Line Broadening in Solar and Stellar Flares
Authors: Kowalski, Adam F.; Allred, Joel C.; Uitenbroek, Han; Tremblay,
Pier-Emmanuel; Brown, Stephen; Carlsson, Mats; Osten, Rachel A.;
Wisniewski, John P.; Hawley, Suzanne L.
2017ApJ...837..125K Altcode: 2017arXiv170203321K
The broadening of the hydrogen lines during flares is thought to
result from increased charge (electron, proton) density in the flare
chromosphere. However, disagreements between theory and modeling
prescriptions have precluded an accurate diagnostic of the degree
of ionization and compression resulting from flare heating in the
chromosphere. To resolve this issue, we have incorporated the unified
theory of electric pressure broadening of the hydrogen lines into
the non-LTE radiative-transfer code RH. This broadening prescription
produces a much more realistic spectrum of the quiescent, A0 star Vega
compared to the analytic approximations used as a damping parameter
in the Voigt profiles. We test recent radiative-hydrodynamic (RHD)
simulations of the atmospheric response to high nonthermal electron
beam fluxes with the new broadening prescription and find that
the Balmer lines are overbroadened at the densest times in the
simulations. Adding many simultaneously heated and cooling model
loops as a “multithread” model improves the agreement with the
observations. We revisit the three-component phenomenological flare
model of the YZ CMi Megaflare using recent and new RHD models. The
evolution of the broadening, line flux ratios, and continuum flux
ratios are well-reproduced by a multithread model with high-flux
nonthermal electron beam heating, an extended decay phase model, and a
“hot spot” atmosphere heated by an ultrarelativistic electron beam
with reasonable filling factors: ∼0.1%, 1%, and 0.1% of the visible
stellar hemisphere, respectively. The new modeling motivates future
work to understand the origin of the extended gradual phase emission.
---------------------------------------------------------
Title: Chromospheric polarimetry through multiline observations of
the 850-nm spectral region
Authors: Quintero Noda, C.; Shimizu, T.; Katsukawa, Y.; de la Cruz
Rodríguez, J.; Carlsson, M.; Anan, T.; Oba, T.; Ichimoto, K.;
Suematsu, Y.
2017MNRAS.464.4534Q Altcode: 2016arXiv161006651Q
Future solar missions and ground-based telescopes aim to understand the
magnetism of the solar chromosphere. We performed a supporting study in
Quintero Noda et al. focused on the infrared Ca II 8542 Å line and we
concluded that it is one of the best candidates because it is sensitive
to a large range of atmospheric heights, from the photosphere to the
middle chromosphere. However, we believe that it is worth trying to
improve the results produced by this line observing additional spectral
lines. In that regard, we examined the neighbourhood solar spectrum
looking for spectral lines which could increase the sensitivity to
the atmospheric parameters. Interestingly, we discovered several
photospheric lines which greatly improve the photospheric sensitivity
to the magnetic field vector. Moreover, they are located close to a
second chromospheric line which also belongs to the Ca II infrared
triplet, I.e. the Ca II 8498 Å line, and enhances the sensitivity to
the atmospheric parameters at chromospheric layers. We conclude that the
lines in the vicinity of the Ca II 8542 Å line not only increase its
sensitivity to the atmospheric parameters at all layers, but also they
constitute an excellent spectral window for chromospheric polarimetry.
---------------------------------------------------------
Title: The Atmospheric Response to High Nonthermal Electron Beam
Fluxes in Solar Flares. I. Modeling the Brightest NUV Footpoints in
the X1 Solar Flare of 2014 March 29
Authors: Kowalski, Adam F.; Allred, Joel C.; Daw, Adrian; Cauzzi,
Gianna; Carlsson, Mats
2017ApJ...836...12K Altcode: 2016arXiv160907390K
The 2014 March 29 X1 solar flare (SOL20140329T17:48) produced bright
continuum emission in the far- and near-ultraviolet (NUV) and highly
asymmetric chromospheric emission lines, providing long-sought
constraints on the heating mechanisms of the lower atmosphere in
solar flares. We analyze the continuum and emission line data from
the Interface Region Imaging Spectrograph (IRIS) of the brightest
flaring magnetic footpoints in this flare. We compare the NUV spectra
of the brightest pixels to new radiative-hydrodynamic predictions
calculated with the RADYN code using constraints on a nonthermal
electron beam inferred from the collisional thick-target modeling of
hard X-ray data from Reuven Ramaty High Energy Solar Spectroscopic
Imager. We show that the atmospheric response to a high beam flux
density satisfactorily achieves the observed continuum brightness
in the NUV. The NUV continuum emission in this flare is consistent
with hydrogen (Balmer) recombination radiation that originates from
low optical depth in a dense chromospheric condensation and from the
stationary beam-heated layers just below the condensation. A model
producing two flaring regions (a condensation and stationary layers)
in the lower atmosphere is also consistent with the asymmetric Fe II
chromospheric emission line profiles observed in the impulsive phase.
---------------------------------------------------------
Title: On the generation of solar spicules and Alfvén waves
Authors: Carlsson, Mats
2017psio.confE..46C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Formation of the helium extreme-UV resonance lines
Authors: Golding, T. P.; Leenaarts, J.; Carlsson, M.
2017A&A...597A.102G Altcode: 2016arXiv161000352G
Context. While classical models successfully reproduce intensities
of many transition region lines, they predict helium extreme-UV
(EUV) line intensities roughly an order of magnitude lower than the
observed value. <BR /> Aims: Our aim is to determine the relevant
formation mechanism(s) of the helium EUV resonance lines capable of
explaining the high intensities under quiet Sun conditions. <BR />
Methods: We synthesised and studied the emergent spectra from a 3D
radiation-magnetohydrodynamics simulation model. The effects of coronal
illumination and non-equilibrium ionisation of hydrogen and helium
are included self-consistently in the numerical simulation. <BR />
Results: Radiative transfer calculations result in helium EUV line
intensities that are an order of magnitude larger than the intensities
calculated under the classical assumptions. The enhanced intensity
of He Iλ584 is primarily caused by He II recombination cascades. The
enhanced intensity of He IIλ304 and He IIλ256 is caused primarily by
non-equilibrium helium ionisation. <BR /> Conclusions: The analysis
shows that the long standing problem of the high helium EUV line
intensities disappears when taking into account optically thick
radiative transfer and non-equilibrium ionisation effects.
---------------------------------------------------------
Title: A Chromospheric Flare Model Consisting of Two Dynamical Layers:
Critical Tests from IRIS Data of Solar Flares
Authors: Kowalski, Adam; Allred, Joel C.; Daw, Adrian N.; Cauzzi,
Gianna; Carlsson, Mats; Inglis, Andrew; O'Neill, Aaron; Mathioudakis,
Mihalis; Uitenbroek, Han
2017AAS...22933902K Altcode:
Recent 1D radiative-hydrodynamic simulations of flares have shown that
a heated, chromospheric compression layer and a stationary layer, just
below the compression, are produced in response to high flux electron
beam heating. The hot blackbody-like continuum and redshifted intensity
in singly ionized chromospheric lines in these model predictions are
generally consistent with broad wavelength coverage spectra of M dwarf
flares and with high spectral resolution observations of solar flares,
respectively. We critically test this two-component chromospheric
flare model against the Fe II profiles and NUV continuum brightness
for several X-class solar flares observed with the Interface Region
Imaging Spectrograph (IRIS). We present several new predictions for
the Daniel K. Inoue Solar Telescope (DKIST).
---------------------------------------------------------
Title: SOLARNET WP 100: Access to Science Data Centres. Space missions
Authors: Carlsson, Mats
2017psio.confE.101C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Discovery of Ubiquitous Fast-Propagating Intensity Disturbances
by the Chromospheric Lyman Alpha Spectropolarimeter (CLASP)
Authors: Kubo, M.; Katsukawa, Y.; Suematsu, Y.; Kano, R.; Bando,
T.; Narukage, N.; Ishikawa, R.; Hara, H.; Giono, G.; Tsuneta, S.;
Ishikawa, S.; Shimizu, T.; Sakao, T.; Winebarger, A.; Kobayashi, K.;
Cirtain, J.; Champey, P.; Auchère, F.; Trujillo Bueno, J.; Asensio
Ramos, A.; Štěpán, J.; Belluzzi, L.; Manso Sainz, R.; De Pontieu,
B.; Ichimoto, K.; Carlsson, M.; Casini, R.; Goto, M.
2016ApJ...832..141K Altcode:
High-cadence observations by the slit-jaw (SJ) optics system of the
sounding rocket experiment known as the Chromospheric Lyman Alpha
Spectropolarimeter (CLASP) reveal ubiquitous intensity disturbances
that recurrently propagate in either the chromosphere or the transition
region or both at a speed much higher than the speed of sound. The
CLASP/SJ instrument provides a time series of two-dimensional images
taken with broadband filters centered on the Lyα line at a 0.6 s
cadence. The multiple fast-propagating intensity disturbances appear in
the quiet Sun and in an active region, and they are clearly detected in
at least 20 areas in a field of view of 527″ × 527″ during the 5
minute observing time. The apparent speeds of the intensity disturbances
range from 150 to 350 km s<SUP>-1</SUP>, and they are comparable
to the local Alfvén speed in the transition region. The intensity
disturbances tend to propagate along bright elongated structures away
from areas with strong photospheric magnetic fields. This suggests
that the observed fast-propagating intensity disturbances are related
to the magnetic canopy structures. The maximum distance traveled by
the intensity disturbances is about 10″, and the widths are a few
arcseconds, which are almost determined by a pixel size of 1.″03. The
timescale of each intensity pulse is shorter than 30 s. One possible
explanation for the fast-propagating intensity disturbances observed
by CLASP is magnetohydrodynamic fast-mode waves.
---------------------------------------------------------
Title: On the Misalignment between Chromospheric Features and the
Magnetic Field on the Sun
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats;
Hansteen, Viggo
2016ApJ...831L...1M Altcode: 2016arXiv160702551M
Observations of the upper chromosphere show an enormous amount of
intricate fine structure. Much of this comes in the form of linear
features, which are most often assumed to be well aligned with the
direction of the magnetic field in the low plasma β regime that is
thought to dominate the upper chromosphere. We use advanced radiative
magnetohydrodynamic simulations, including the effects of ion-neutral
interactions (using the generalized Ohm’s law) in the partially
ionized chromosphere, to show that the magnetic field is often not well
aligned with chromospheric features. This occurs where the ambipolar
diffusion is large, I.e., ions and neutral populations decouple as
the ion-neutral collision frequency drops, allowing the field to
slip through the neutral population; where currents perpendicular to
the field are strong; and where thermodynamic timescales are longer
than or similar to those of ambipolar diffusion. We find this often
happens in dynamic spicule or fibril-like features at the top of the
chromosphere. This has important consequences for field extrapolation
methods, which increasingly use such upper chromospheric features
to help constrain the chromospheric magnetic field: our results
invalidate the underlying assumption that these features are aligned
with the field. In addition, our results cast doubt on results from
1D hydrodynamic models, which assume that plasma remains on the same
field lines. Finally, our simulations show that ambipolar diffusion
significantly alters the amount of free energy available in the coronal
part of our simulated volume, which is likely to have consequences
for studies of flare initiation.
---------------------------------------------------------
Title: The cause of spatial structure in solar He I 1083 nm multiplet
images
Authors: Leenaarts, Jorrit; Golding, Thomas; Carlsson, Mats; Libbrecht,
Tine; Joshi, Jayant
2016A&A...594A.104L Altcode: 2016arXiv160800838L
Context. The He I 1083 nm is a powerful diagnostic for inferring
properties of the upper solar chromosphere, in particular for the
magnetic field. The basic formation of the line in one-dimensional
models is well understood, but the influence of the complex
three-dimensional structure of the chromosphere and corona has however
never been investigated. This structure must play an essential role
because images taken in He I 1083 nm show structures with widths
down to 100 km. <BR /> Aims: We aim to understand the effect of
the three-dimensional temperature and density structure in the
solar atmosphere on the formation of the He I 1083 nm line. <BR />
Methods: We solved the non-LTE radiative transfer problem assuming
statistical equilibrium for a simple nine-level helium atom that
nevertheless captures all essential physics. As a model atmosphere we
used a snapshot from a 3D radiation-MHD simulation computed with the
Bifrost code. Ionising radiation from the corona was self-consistently
taken into account. <BR /> Results: The emergent intensity in the He
I 1083 nm is set by the source function and the opacity in the upper
chromosphere. The former is dominated by scattering of photospheric
radiation and does not vary much with spatial location. The latter
is determined by the photonionisation rate in the He I ground state
continuum, as well as the electron density in the chromosphere. The
spatial variation of the flux of ionising radiation is caused
by the spatially-structured emissivity of the ionising photons
from material at T ≈ 100 kK in the transition region. The hotter
coronal material produces more ionising photons, but the resulting
radiation field is smooth and does not lead to small-scale variation
of the UV flux. The corrugation of the transition region further
increases the spatial variation of the amount of UV radiation in the
chromosphere. Finally we find that variations in the chromospheric
electron density also cause strong variation in He I 1083 nm
opacity. We compare our findings to observations using SST, IRIS and
SDO/AIA data. <P />A movie associated to Fig. 4 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201628490/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Chromospheric and Coronal Wave Generation in a Magnetic
Flux Sheath
Authors: Kato, Yoshiaki; Steiner, Oskar; Hansteen, Viggo; Gudiksen,
Boris; Wedemeyer, Sven; Carlsson, Mats
2016ApJ...827....7K Altcode: 2016arXiv160608826K
Using radiation magnetohydrodynamic simulations of the solar
atmospheric layers from the upper convection zone to the lower corona,
we investigate the self-consistent excitation of slow magneto-acoustic
body waves (slow modes) in a magnetic flux concentration. We
find that the convective downdrafts in the close surroundings of
a two-dimensional flux slab “pump” the plasma inside it in
the downward direction. This action produces a downflow inside the
flux slab, which encompasses ever higher layers, causing an upwardly
propagating rarefaction wave. The slow mode, excited by the adiabatic
compression of the downflow near the optical surface, travels along the
magnetic field in the upward direction at the tube speed. It develops
into a shock wave at chromospheric heights, where it dissipates,
lifts the transition region, and produces an offspring in the form
of a compressive wave that propagates further into the corona. In the
wake of downflows and propagating shock waves, the atmosphere inside
the flux slab in the chromosphere and higher tends to oscillate with a
period of ν ≈ 4 mHz. We conclude that this process of “magnetic
pumping” is a most plausible mechanism for the direct generation
of longitudinal chromospheric and coronal compressive waves within
magnetic flux concentrations, and it may provide an important heat
source in the chromosphere. It may also be responsible for certain
types of dynamic fibrils.
---------------------------------------------------------
Title: The SPICE Spectral Imager on Solar Orbiter: Linking the Sun
to the Heliosphere
Authors: Fludra, Andrzej; Haberreiter, Margit; Peter, Hardi; Vial,
Jean-Claude; Harrison, Richard; Parenti, Susanna; Innes, Davina;
Schmutz, Werner; Buchlin, Eric; Chamberlin, Phillip; Thompson,
William; Gabriel, Alan; Morris, Nigel; Caldwell, Martin; Auchere,
Frederic; Curdt, Werner; Teriaca, Luca; Hassler, Donald M.; DeForest,
Craig; Hansteen, Viggo; Carlsson, Mats; Philippon, Anne; Janvier, Miho;
Wimmer-Schweingruber, Robert; Griffin, Douglas; Davila, Joseph; Giunta,
Alessandra; Waltham, Nick; Eccleston, Paul; Gottwald, Alexander;
Klein, Roman; Hanley, John; Walls, Buddy; Howe, Chris; Schuehle, Udo
2016cosp...41E.607F Altcode:
The SPICE (Spectral Imaging of the Coronal Environment) instrument is
one of the key remote sensing instruments onboard the upcoming Solar
Orbiter Mission. SPICE has been designed to contribute to the science
goals of the mission by investigating the source regions of outflows
and ejection processes which link the solar surface and corona to the
heliosphere. In particular, SPICE will provide quantitative information
on the physical state and composition of the solar atmosphere
plasma. For example, SPICE will access relative abundances of ions to
study the origin and the spatial/temporal variations of the 'First
Ionization Potential effect', which are key signatures to trace the
solar wind and plasma ejections paths within the heliosphere. Here we
will present the instrument and its performance capability to attain the
scientific requirements. We will also discuss how different observation
modes can be chosen to obtain the best science results during the
different orbits of the mission. To maximize the scientific return of
the instrument, the SPICE team is working to optimize the instrument
operations, and to facilitate the data access and their exploitation.
---------------------------------------------------------
Title: Solar abundances with the SPICE spectral imager on Solar
Orbiter
Authors: Giunta, Alessandra; Haberreiter, Margit; Peter, Hardi;
Vial, Jean-Claude; Harrison, Richard; Parenti, Susanna; Innes, Davina;
Schmutz, Werner; Buchlin, Eric; Chamberlin, Phillip; Thompson, William;
Bocchialini, Karine; Gabriel, Alan; Morris, Nigel; Caldwell, Martin;
Auchere, Frederic; Curdt, Werner; Teriaca, Luca; Hassler, Donald M.;
DeForest, Craig; Hansteen, Viggo; Carlsson, Mats; Philippon, Anne;
Janvier, Miho; Wimmer-Schweingruber, Robert; Griffin, Douglas; Baudin,
Frederic; Davila, Joseph; Fludra, Andrzej; Waltham, Nick; Eccleston,
Paul; Gottwald, Alexander; Klein, Roman; Hanley, John; Walls, Buddy;
Howe, Chris; Schuehle, Udo; Gyo, Manfred; Pfiffner, Dany
2016cosp...41E.681G Altcode:
Elemental composition of the solar atmosphere and in particular
abundance bias of low and high First Ionization Potential (FIP)
elements are a key tracer of the source regions of the solar wind. These
abundances and their spatio-temporal variations, as well as the other
plasma parameters , will be derived by the SPICE (Spectral Imaging
of the Coronal Environment) EUV spectral imager on the upcoming
Solar Orbiter mission. SPICE is designed to provide spectroheliograms
(spectral images) using a core set of emission lines arising from ions
of both low-FIP and high-FIP elements. These lines are formed over
a wide range of temperatures, enabling the analysis of the different
layers of the solar atmosphere. SPICE will use these spectroheliograms
to produce dynamic composition maps of the solar atmosphere to be
compared to in-situ measurements of the solar wind composition of
the same elements (i.e. O, Ne, Mg, Fe). This will provide a tool to
study the connectivity between the spacecraft (the Heliosphere) and
the Sun. We will discuss the SPICE capabilities for such composition
measurements.
---------------------------------------------------------
Title: Emergence of Granular-sized Magnetic Bubbles Through the
Solar Atmosphere. III. The Path to the Transition Region
Authors: Ortiz, Ada; Hansteen, Viggo H.; Bellot Rubio, Luis Ramón;
de la Cruz Rodríguez, Jaime; De Pontieu, Bart; Carlsson, Mats;
Rouppe van der Voort, Luc
2016ApJ...825...93O Altcode: 2016arXiv160400302O
We study, for the first time, the ascent of granular-sized magnetic
bubbles from the solar photosphere through the chromosphere into the
transition region and above. Such events occurred in a flux emerging
region in NOAA 11850 on 2013 September 25. During that time, the
first co-observing campaign between the Swedish 1-m Solar Telescope
(SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft
was carried out. Simultaneous observations of the chromospheric Hα
656.28 nm and Ca II 854.2 nm lines, plus the photospheric Fe I 630.25
nm line, were made with the CRISP spectropolarimeter at the Spitzer
Space Telescope (SST) reaching a spatial resolution of 0.″14. At
the same time, IRIS was performing a four-step dense raster of the
emerging flux region, taking slit jaw images at 133 (C II, transition
region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper
chromosphere), and 283.2 nm (Mg II k, wing, photosphere). Spectroscopy
of several lines was performed by the IRIS spectrograph in the far-
and near-ultraviolet, of which we have used the Si IV 140.3 and the
Mg II k 279.6 nm lines. Coronal images from the Atmospheric Imaging
Assembly of the Solar Dynamics Observatory were used to investigate
the possible coronal signatures of the flux emergence events. The
photospheric and chromospheric properties of small-scale emerging
magnetic bubbles have been described in detail in Ortiz et al. Here
we are able to follow such structures up to the transition region. We
describe the properties, including temporal delays, of the observed
flux emergence in all layers. We believe this may be an important
mechanism of transporting energy and magnetic flux from subsurface
layers to the transition region and corona.
---------------------------------------------------------
Title: The Appearance of Spicules in High Resolution Observations
of Ca II H and Hα
Authors: Pereira, Tiago M. D.; Rouppe van der Voort, Luc; Carlsson,
Mats
2016ApJ...824...65P Altcode: 2016arXiv160403116P
Solar spicules are chromospheric fibrils that appear everywhere on
the Sun, yet their origin is not understood. Using high resolution
observations of spicules obtained with the Swedish 1 m Solar
Telescope, we aim to understand how spicules appear in filtergrams
and Dopplergrams, how they compare in Ca II H and Hα filtergrams,
and what can make them appear and disappear. We find that spicules
display a rich and detailed spatial structure, and show a distribution
of transverse velocities that, when aligned with the line of sight,
can make them appear at different Hα wing positions. They become more
abundant at positions closer to the line core, reflecting a distribution
of Doppler shifts and widths. In Hα width maps they stand out as
bright features both on disk and off limb, reflecting their large
Doppler motions and possibly higher temperatures than in the typical
Hα formation region. Spicule lifetimes measured from narrowband images
at only a few positions will be an underestimate because Doppler shifts
can make them disappear prematurely from such images; for such cases,
width maps are a more robust tool. In Hα and Ca II H filtergrams,
off-limb spicules essentially have the same properties, appearance,
and evolution. We find that the sudden appearance of spicules can be
explained by Doppler shifts from their transverse motions, and does
not require other convoluted explanations.
---------------------------------------------------------
Title: Physics & Diagnostics of the Drivers of Solar Eruptions
Authors: Cheung, Mark; Rempel, Matthias D.; Martinez-Sykora, Juan;
Testa, Paola; Hansteen, Viggo H.; Viktorovna Malanushenko, Anna;
Sainz Dalda, Alberto; DeRosa, Marc L.; De Pontieu, Bart; Carlsson,
Mats; Chen, Feng; McIntosh, Scott W.; Gudiksen, Boris
2016SPD....47.0607C Altcode:
We provide an update on our NASA Heliophysics Grand Challenges Research
(HGCR) project on the ‘Physics & Diagnostics of the Drivers of
Solar Eruptions’. This presentation will focus on results from a
data-inspired, 3D radiative MHD model of a solar flare. The model
flare results from the interaction of newly emerging flux with a
pre-existing active region. Synthetic observables from the model
reproduce observational features compatible with actual flares. These
include signatures of coronal magnetic reconnection, chromospheric
evaporation, EUV flare arcades, sweeping motion of flare ribbons
and sunquakes.
---------------------------------------------------------
Title: Spectro-polarimetric observation in UV with CLASP to probe
the chromosphere and transition region
Authors: Kano, Ryouhei; Ishikawa, Ryohko; Winebarger, Amy R.; Auchère,
Frédéric; Trujillo Bueno, Javier; Narukage, Noriyuki; Kobayashi,
Ken; Bando, Takamasa; Katsukawa, Yukio; Kubo, Masahito; Ishikawa,
Shin-Nosuke; Giono, Gabriel; Hara, Hirohisa; Suematsu, Yoshinori;
Shimizu, Toshifumi; Sakao, Taro; Tsuneta, Saku; Ichimoto, Kiyoshi;
Goto, Motoshi; Cirtain, Jonathan W.; De Pontieu, Bart; Casini, Roberto;
Manso Sainz, Rafael; Asensio Ramos, Andres; Stepan, Jiri; Belluzzi,
Luca; Carlsson, Mats
2016SPD....4710107K Altcode:
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a NASA
sounding-rocket experiment that was performed in White Sands in
the US on September 3, 2015. During its 5-minute ballistic flight,
CLASP successfully made the first spectro-polarimetric observation in
the Lyman-alpha line (121.57 nm) originating in the chromosphere and
transition region. Since the Lyman-alpha polarization is sensitive
to magnetic field of 10-100 G by the Hanle effect, we aim to infer
the magnetic field information in such upper solar atmosphere with
this experiment.The obtained CLASP data showed that the Lyman-alpha
scattering polarization is about a few percent in the wings and
the order of 0.1% in the core near the solar limb, as it had been
theoretically predicted, and that both polarization signals have a
conspicuous spatio-temporal variability. CLASP also observed another
upper-chromospheric line, Si III (120.65 nm), whose critical field
strength for the Hanle effect is 290 G, and showed a measurable
scattering polarization of a few % in this line. The polarization
properties of the Si III line could facilitate the interpretation of
the scattering polarization observed in the Lyman-alpha line.In this
presentation, we would like to show how the upper chromosphere and
transition region are seen in the polarization of these UV lines and
discuss the possible source of these complicated polarization signals.
---------------------------------------------------------
Title: Division G Commission 36: Theory of Stellar Atmospheres
Authors: Puls, Joachim; Hubeny, Ivan; Asplund, Martin; Allard, France;
Allende Prieto, Carlos; Ayres, Thomas R.; Carlsson, Mats; Gustafsson,
Bengt; Kudritzki, Rolf-Peter; Ryabchikova, Tatiana A.
2016IAUTA..29..453P Altcode:
Different from previous triennial reports, this report covers the
activities of IAU Commission 36 `Theory of Stellar Atmospheres'
over the past six years†, and will be the last report from the
`old' Commission 36. After the General Assembly in Honolulu (August
2015), a new Commission `Stellar and Planetary Atmospheres' (C.G5,
under Division G, `Stars and Stellar Physics') has come into life,
and will continue our work devoted to the outer envelopes of stars,
as well as extend it to the atmospheres of planets (see Sect. 4).
---------------------------------------------------------
Title: Modeling Repeatedly Flaring δ Sunspots
Authors: Chatterjee, Piyali; Hansteen, Viggo; Carlsson, Mats
2016PhRvL.116j1101C Altcode: 2016arXiv160100749C
Active regions (ARs) appearing on the surface of the Sun are classified
into α , β , γ , and δ by the rules of the Mount Wilson Observatory,
California on the basis of their topological complexity. Amongst these,
the δ sunspots are known to be superactive and produce the most
x-ray flares. Here, we present results from a simulation of the Sun
by mimicking the upper layers and the corona, but starting at a more
primitive stage than any earlier treatment. We find that this initial
state consisting of only a thin subphotospheric magnetic sheet breaks
into multiple flux tubes which evolve into a colliding-merging system
of spots of opposite polarity upon surface emergence, similar to those
often seen on the Sun. The simulation goes on to produce many exotic δ
sunspot associated phenomena: repeated flaring in the range of typical
solar flare energy release and ejective helical flux ropes with embedded
cool-dense plasma filaments resembling solar coronal mass ejections.
---------------------------------------------------------
Title: Non-equilibrium Helium Ionization in an MHD Simulation of
the Solar Atmosphere
Authors: Golding, Thomas Peter; Leenaarts, Jorrit; Carlsson, Mats
2016ApJ...817..125G Altcode: 2015arXiv151204738G
The ionization state of the gas in the dynamic solar chromosphere can
depart strongly from the instantaneous statistical equilibrium commonly
assumed in numerical modeling. We improve on earlier simulations of
the solar atmosphere that only included non-equilibrium hydrogen
ionization by performing a 2D radiation-magnetohydrodynamics
simulation featuring non-equilibrium ionization of both hydrogen
and helium. The simulation includes the effect of hydrogen Lyα and
the EUV radiation from the corona on the ionization and heating
of the atmosphere. Details on code implementation are given. We
obtain helium ion fractions that are far from their equilibrium
values. Comparison with models with local thermodynamic equilibrium
(LTE) ionization shows that non-equilibrium helium ionization leads to
higher temperatures in wavefronts and lower temperatures in the gas
between shocks. Assuming LTE ionization results in a thermostat-like
behavior with matter accumulating around the temperatures where the
LTE ionization fractions change rapidly. Comparison of DEM curves
computed from our models shows that non-equilibrium ionization leads
to more radiating material in the temperature range 11-18 kK, compared
to models with LTE helium ionization. We conclude that non-equilibrium
helium ionization is important for the dynamics and thermal structure
of the upper chromosphere and transition region. It might also help
resolve the problem that intensities of chromospheric lines computed
from current models are smaller than those observed.
---------------------------------------------------------
Title: A publicly available simulation of an enhanced network region
of the Sun
Authors: Carlsson, Mats; Hansteen, Viggo H.; Gudiksen, Boris V.;
Leenaarts, Jorrit; De Pontieu, Bart
2016A&A...585A...4C Altcode: 2015arXiv151007581C
Context. The solar chromosphere is the interface between the
solar surface and the solar corona. Modelling of this region is
difficult because it represents the transition from optically
thick to thin radiation escape, from gas-pressure domination to
magnetic-pressure domination, from a neutral to an ionised state,
from MHD to plasma physics, and from near-equilibrium (LTE) to
non-equilibrium conditions. <BR /> Aims: Our aim is to provide the
community with realistic simulations of the magnetic solar outer
atmosphere. This will enable detailed comparison of existing and
upcoming observations with synthetic observables from the simulations,
thereby elucidating the complex interactions of magnetic fields and
plasma that are crucial for our understanding of the dynamic outer
atmosphere. <BR /> Methods: We used the radiation magnetohydrodynamics
code Bifrost to perform simulations of a computational volume
with a magnetic field topology similar to an enhanced network
area on the Sun. <BR /> Results: The full simulation cubes are
made available from the Hinode Science Data Centre Europe. The
general properties of the simulation are discussed, and limitations
are discussed. <P />The Hinode Science Data Centre Europe (<A
href="http://www.sdc.uio.no/search/simulations">http://www.sdc.uio.no/search/simulations</A>).
---------------------------------------------------------
Title: ALMA Observations of the Sun in Cycle 4 and Beyond
Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.;
Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.;
Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson,
M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.;
Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov,
V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões,
P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig,
A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
2016arXiv160100587W Altcode:
This document was created by the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) in preparation of
the first regular observations of the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA), which are anticipated to start
in ALMA Cycle 4 in October 2016. The science cases presented here
demonstrate that a large number of scientifically highly interesting
observations could be made already with the still limited solar
observing modes foreseen for Cycle 4 and that ALMA has the potential
to make important contributions to answering long-standing scientific
questions in solar physics. With the proposal deadline for ALMA Cycle
4 in April 2016 and the Commissioning and Science Verification campaign
in December 2015 in sight, several of the SSALMON Expert Teams composed
strategic documents in which they outlined potential solar observations
that could be feasible given the anticipated technical capabilities
in Cycle 4. These documents have been combined and supplemented
with an analysis, resulting in recommendations for solar observing
with ALMA in Cycle 4. In addition, the detailed science cases also
demonstrate the scientific priorities of the solar physics community
and which capabilities are wanted for the next observing cycles. The
work on this White Paper effort was coordinated in close cooperation
with the two international solar ALMA development studies led by
T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be
further updated until the beginning of Cycle 4 in October 2016. In
particular, we plan to adjust the technical capabilities of the solar
observing modes once finally decided and to further demonstrate the
feasibility and scientific potential of the included science cases by
means of numerical simulations of the solar atmosphere and corresponding
simulated ALMA observations.
---------------------------------------------------------
Title: Probing the Sun with ALMA: Observations and Simulations
Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M.; Carlsson, M.
2015ASPC..499..349L Altcode: 2015arXiv150805686L
ALMA will open a new chapter in the study of the Sun by providing a leap
in spatial resolution and sensitivity compared to currently available
mm wavelength observations. In preparation of ALMA, we have carried out
a large number of observational tests and state-of-the-art radiation
MHD simulations. Here we review the best available observations of
the Sun at millimeter wavelengths.Using state of the art radiation
MHD simulations of the solar atmosphere we demonstrate the huge
potential of ALMA observations for uncovering the nature of the solar
chromosphere. We show that ALMA will not only provide a reliable probe
of the thermal structure and dynamics of the chromosphere, it will also
open up a powerful new diagnostic of magnetic field at chromospheric
heights, a fundamentally important, but so far poorly known parameter.
---------------------------------------------------------
Title: New Insights into White-Light Flare Emission from
Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
Authors: Kowalski, Adam F.; Hawley, S. L.; Carlsson, M.; Allred,
J. C.; Uitenbroek, H.; Osten, R. A.; Holman, G.
2015SoPh..290.3487K Altcode: 2015SoPh..tmp...61K; 2015arXiv150307057K
The heating mechanism at high densities during M-dwarf flares is
poorly understood. Spectra of M-dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum
components during the impulsive phase: 1) an energetically dominant
blackbody component with a color temperature of T ≈10<SUP>4</SUP>K
in the blue-optical, 2) a smaller amount of Balmer continuum emission
in the near-ultraviolet at λ ≤3 646 Å, and 3) an apparent
pseudo-continuum of blended high-order Balmer lines between λ =3
646 Å and λ ≈3 900 Å. These properties are not reproduced by
models that employ a typical "solar-type" flare heating level of
≤10<SUP>11</SUP>ergcm−<SUP>2</SUP>s−<SUP>1</SUP> in nonthermal
electrons, and therefore our understanding of these spectra is
limited to a phenomenological three-component interpretation. We
present a new 1D radiative-hydrodynamic model of an M-dwarf flare
from precipitating nonthermal electrons with a high energy flux of
10<SUP>13</SUP>ergcm−<SUP>2</SUP>s−<SUP>1</SUP>. The simulation
produces bright near-ultraviolet and optical continuum emission from a
dense (n >10<SUP>15</SUP>cm−<SUP>3</SUP>), hot (T ≈12 000 -13 500
K) chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T ≈10<SUP>4</SUP>K
blackbody-like continuum component and a low Balmer jump ratio result
from optically thick Balmer (∞ →n =2 ) and Paschen recombination
(∞ →n =3 ) radiation, and thus the properties of the flux spectrum
are caused by blue (λ ≈4 300 Å) light escaping over a larger
physical depth range than by red (λ ≈6 700 Å) and near-ultraviolet
(λ ≈3 500 Å) light. To model the near-ultraviolet pseudo-continuum
previously attributed to overlapping Balmer lines, we include the
extra Balmer continuum opacity from Landau-Zener transitions that
result from merged, high-order energy levels of hydrogen in a dense,
partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated
during dMe and solar flares.
---------------------------------------------------------
Title: Heating of the Solar Chromosphere and Connections to the
Transition Region and Corona
Authors: Carlsson, M.
2015AGUFMSH23D..01C Altcode:
We present observations of the chromosphere and transition region
from IRIS and the SST. These observations are interpreted using 1D
semi-empirical numerical experiments, 1D radiaton-MHD simulations and 3D
"realistic" radiation-MHD simulations spanning the solar atmosphere
from the convection zone to the corona. A picture is emerging of
the energy balance in the solar chromosphere and connections to the
transition region and corona.
---------------------------------------------------------
Title: Impact of the Ion-Neutral Interaction Effects in the Solar
Chromosphere
Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.;
Carlsson, M.
2015AGUFMSH31B2411M Altcode:
The complexity of the chromosphere is due to various regime changes
that take place across it. Consequently, the interpretation of
chromospheric observations is a challenging task. It is thus crucial
to combine these observations with advanced radiative-MHD numerical
modeling. Because the photosphere, chromosphere and transition region
are partially ionized, the interaction between ionized and neutral
particles has important consequences on the magneto-thermodynamics
of these regions. We implemented the effects of partial ionization
using generalized Ohm's law in the Bifrost code (Gudiksen et al. 2011)
which solves the full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along magnetic field lines. We perform
2.5D simulations which combines large and small scales structures. This
leads to a highly dynamic chromosphere with large variety of physical
processes which have not been reproduced with smaller simulations. The
implementation of partial ionization effects impact our modeled
radiative-MHD atmosphere, such as producing chromospheric heating and
diffusion of photospheric magnetic field into the upper-chromosphere. We
will also focus on which observables of these processes can be revealed
with chromospheric observations.
---------------------------------------------------------
Title: Hα Line Profile Asymmetries and the Chromospheric Flare
Velocity Field
Authors: Kuridze, D.; Mathioudakis, M.; Simões, P. J. A.; Rouppe van
der Voort, L.; Carlsson, M.; Jafarzadeh, S.; Allred, J. C.; Kowalski,
A. F.; Kennedy, M.; Fletcher, L.; Graham, D.; Keenan, F. P.
2015ApJ...813..125K Altcode: 2015arXiv151001877K
The asymmetries observed in the line profiles of solar flares can
provide important diagnostics of the properties and dynamics of the
flaring atmosphere. In this paper the evolution of the Hα and Ca ii
λ8542 lines are studied using high spatial, temporal, and spectral
resolution ground-based observations of an M1.1 flare obtained with
the Swedish 1 m Solar Telescope. The temporal evolution of the Hα
line profiles from the flare kernel shows excess emission in the red
wing (red asymmetry) before flare maximum and excess in the blue wing
(blue asymmetry) after maximum. However, the Ca ii λ8542 line does
not follow the same pattern, showing only a weak red asymmetry during
the flare. RADYN simulations are used to synthesize spectral line
profiles for the flaring atmosphere, and good agreement is found
with the observations. We show that the red asymmetry observed in
Hα is not necessarily associated with plasma downflows, and the blue
asymmetry may not be related to plasma upflows. Indeed, we conclude
that the steep velocity gradients in the flaring chromosphere modify
the wavelength of the central reversal in the Hα line profile. The
shift in the wavelength of maximum opacity to shorter and longer
wavelengths generates the red and blue asymmetries, respectively.
---------------------------------------------------------
Title: The Formation of Iris Diagnostics. VIII. Iris Observations
in the C II 133.5 nm Multiplet.
Authors: Rathore, Bhavna; Pereira, Tiago M. D.; Carlsson, Mats;
De Pontieu, Bart
2015ApJ...814...70R Altcode: 2015arXiv151004845R
The C ii 133.5 nm multiplet has been observed by NASA’s Interface
Region Imaging Spectrograph (IRIS) in unprecedented spatial
resolution. The aims of this work are to characterize these new
observations of the C ii lines, place them in context with previous
work, and to identify any additional value the C ii lines bring when
compared with other spectral lines. We make use of wide, long exposure
IRIS rasters covering the quiet Sun and an active region. Line
properties such as velocity shift and width are extracted from
individual spectra and analyzed. The lines have a variety of shapes
(mostly single-peak or double-peak), are strongest in active regions
and weaker in the quiet Sun. The ratio between the 133.4 and 133.5 nm
components is always less than 1.8, indicating that their radiation
is optically thick in all locations. Maps of the C ii line widths are
a powerful new diagnostic of chromospheric structures, and their line
shifts are a robust velocity diagnostic. Compared with earlier quiet Sun
observations, we find similar absolute intensities and mean line widths,
but smaller redshifts; this difference can perhaps be attributed to
differences in spectral resolution and spatial coverage. The C ii
intensity maps are somewhat similar to those of transition region
lines, but also share some features with chromospheric maps such as
those from the Mg ii k line, indicating that they are formed between
the upper chromosphere and transition region. C ii intensity, width,
and velocity maps can therefore be used to gather additional information
about the upper chromosphere.
---------------------------------------------------------
Title: Combined Modeling of Acceleration, Transport, and Hydrodynamic
Response in Solar Flares. II. Inclusion of Radiative Transfer
with RADYN
Authors: Rubio da Costa, Fatima; Liu, Wei; Petrosian, Vahé; Carlsson,
Mats
2015ApJ...813..133R Altcode: 2015arXiv150501549R
Solar flares involve complex processes that are coupled and span a
wide range of temporal, spatial, and energy scales. Modeling such
processes self-consistently has been a challenge in the past. Here we
present results from simulations that couple particle kinetics with
hydrodynamics (HD) of the atmospheric plasma. We combine the Stanford
unified Fokker-Planck code that models particle acceleration and
transport with the RADYN HD code that models the atmospheric response
to collisional heating by accelerated electrons through detailed
radiative transfer calculations. We perform simulations using two
different electron spectra, one an ad hoc power law and the other
predicted by the model of stochastic acceleration by turbulence or
plasma waves. Surprisingly, the later model, even with energy flux \ll
{10}<SUP>10</SUP> {erg} {{{s}}}<SUP>-1</SUP> {{cm}}<SUP>-2</SUP>, can
cause “explosive” chromospheric evaporation and drive stronger
up- and downflows (and HD shocks). This is partly because our
acceleration model, like many others, produces a spectrum consisting
of a quasi-thermal component plus a power-law tail. We synthesize
emission-line profiles covering different heights in the lower
atmosphere, including Hα 6563 Å, He ii 304 Å, Ca ii K 3934 Å, and
Si iv 1393 Å. One interesting result is the unusual high temperature
(up to a few times 10<SUP>5</SUP> K) of the formation site of He ii
304 Å, which is expected owing to photoionization-recombination under
flare conditions, compared to those in the quiet Sun dominated by
collisional excitation. When compared with observations, our results
can constrain the properties of nonthermal electrons and thus the
poorly understood particle acceleration mechanism.
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. VII. The Formation of the
OI 135.56 NM Line in the Solar Atmosphere
Authors: Lin, Hsiao-Hsuan; Carlsson, Mats
2015ApJ...813...34L Altcode: 2015arXiv150906605L
The O i 135.56 nm line is covered by NASA's Interface Region Imaging
Spectrograph (IRIS) small explorer mission which studies how the
solar atmosphere is energized. We study here the formation and
diagnostic potential of this line by means of non-local thermodynamic
equilibrium modeling employing both 1D semi-empirical and 3D radiation
magnetohydrodynamic models. We study the basic formation mechanisms
and derive a quintessential model atom that incorporates essential
atomic physics for the formation of the O i 135.56 nm line. This
atomic model has 16 levels and describes recombination cascades
through highly excited levels by effective recombination rates. The
ionization balance O i/O ii is set by the hydrogen ionization balance
through charge exchange reactions. The emission in the O i 135.56 nm
line is dominated by a recombination cascade and the line is optically
thin. The Doppler shift of the maximum emission correlates strongly with
the vertical velocity in its line forming region, which is typically
located at 1.0-1.5 Mm height. The total intensity of the line emission
is correlated with the square of the electron density. Since the O i
135.56 nm line is optically thin, the width of the emission line is a
very good diagnostic of non-thermal velocities. We conclude that the
O i 135.56 nm line is an excellent probe of the middle chromosphere,
and compliments other powerful chromospheric diagnostics of IRIS such
as the Mg ii h & k lines and the C ii lines around 133.5 nm.
---------------------------------------------------------
Title: Evidence for a Transition Region Response to Penumbral
Microjets in Sunspots
Authors: Vissers, G. J. M.; Rouppe van der Voort, L. H. M.; Carlsson,
M.
2015ApJ...811L..33V Altcode: 2015arXiv150901402V
Penumbral microjets (PMJs) are short-lived, fine-structured, and
bright jets that are generally observed in chromospheric imaging of the
penumbra of sunspots. Here we investigate their potential transition
region signature by combining observations with the Swedish 1-m Solar
Telescope in the Ca ii H and Ca ii 8542 Å lines with ultraviolet
imaging and spectroscopy obtained with the Interface Region Imaging
Spectrograph (IRIS), which includes the C ii 1334/1335 Å, Si iv
1394/1403 Å, and Mg ii h & k 2803/2796 Å lines. We find a clear
corresponding signal in the IRIS Mg ii k, C ii, and Si iv slit-jaw
images, typically offset spatially from the Ca ii signature in the
direction along the jets: from base to top, the PMJs are predominantly
visible in Ca ii, Mg ii k, and C ii/Si iv, suggesting progressive
heating to transition region temperatures along the jet extent. Hence,
these results support the suggestion from earlier studies that PMJs
may heat to transition region temperatures.
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. VI. The Diagnostic Potential
of the C II Lines at 133.5 nm in the Solar Atmosphere
Authors: Rathore, Bhavna; Carlsson, Mats; Leenaarts, Jorrit; De
Pontieu, Bart
2015ApJ...811...81R Altcode: 2015arXiv150804423R
We use 3D radiation magnetohydrodynamic models to investigate how the
thermodynamic quantities in the simulation are encoded in observable
quantities, thus exploring the diagnostic potential of the C ii 133.5
nm lines. We find that the line core intensity is correlated with the
temperature at the formation height but the correlation is rather weak,
especially when the lines are strong. The line core Doppler shift is a
good measure of the line-of-sight velocity at the formation height. The
line width is both dependent on the width of the absorption profile
(thermal and non-thermal width) and an opacity broadening factor of
1.2-4 due to the optically thick line formation with a larger broadening
for double peak profiles. The C ii 133.5 nm lines can be formed both
higher and lower than the core of the Mg ii k line depending on the
amount of plasma in the 14-50 kK temperature range. More plasma in
this temperature range gives a higher C ii 133.5 nm formation height
relative to the Mg ii k line core. The synthetic line profiles have been
compared with Interface Region Imaging Spectrograph observations. The
derived parameters from the simulated line profiles cover the parameter
range seen in observations but, on average, the synthetic profiles are
too narrow. We interpret this discrepancy as a combination of a lack
of plasma at chromospheric temperatures in the simulation box and too
small non-thermal velocities. The large differences in the distribution
of properties between the synthetic profiles and the observed ones
show that the C ii 133.5 nm lines are powerful diagnostics of the
upper chromosphere and lower transition region.
---------------------------------------------------------
Title: Numerical Simulations of Coronal Heating through Footpoint
Braiding
Authors: Hansteen, V.; Guerreiro, N.; De Pontieu, B.; Carlsson, M.
2015ApJ...811..106H Altcode: 2015arXiv150807234H
Advanced three-dimensional (3D) radiative MHD simulations now reproduce
many properties of the outer solar atmosphere. When including a domain
from the convection zone into the corona, a hot chromosphere and corona
are self-consistently maintained. Here we study two realistic models,
with different simulated areas, magnetic field strength and topology,
and numerical resolution. These are compared in order to characterize
the heating in the 3D-MHD simulations which self-consistently
maintains the structure of the atmosphere. We analyze the heating
at both large and small scales and find that heating is episodic and
highly structured in space, but occurs along loop-shaped structures,
and moves along with the magnetic field. On large scales we find that
the heating per particle is maximal near the transition region and that
widely distributed opposite-polarity field in the photosphere leads
to a greater heating scale height in the corona. On smaller scales,
heating is concentrated in current sheets, the thicknesses of which are
set by the numerical resolution. Some current sheets fragment in time,
this process occurring more readily in the higher-resolution model
leading to spatially highly intermittent heating. The large-scale
heating structures are found to fade in less than about five minutes,
while the smaller, local, heating shows timescales of the order of two
minutes in one model and one minutes in the other, higher-resolution,
model.
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. V. A Quintessential Model
Atom of C II and General Formation Properties of the C II Lines at
133.5 nm
Authors: Rathore, Bhavna; Carlsson, Mats
2015ApJ...811...80R Altcode: 2015arXiv150804365R
The C ii 133.5 {nm} lines are important observables for the NASA/SMEX
mission Interface Region Imaging Spectrograph. To make three-dimensional
(3D) non-LTE radiative transfer computationally feasible, it is crucial
to have a model atom with as few levels as possible while retaining
the main physical processes. We here develop such a model atom and we
study the general formation properties of the C ii lines. We find that a
nine-level model atom of C i-C iii with the transitions treated assuming
complete frequency redistribution (CRD) suffices to describe the C ii
133.5 {nm} lines. 3D scattering effects are important for the intensity
in the core of the line. The lines are formed in the optically thick
regime. The core intensity is formed in layers where the temperature is
about 10 kK at the base of the transition region. The lines are 1.2-4
times wider than the atomic absorption profile due to the formation in
the optically thick regime. The smaller opacity broadening happens for
single peak intensity profiles where the chromospheric temperature is
low with a steep source function increase into the transition region,
the larger broadening happens when there is a temperature increase
from the photosphere to the low chromosphere leading to a local source
function maximum and a double peak intensity profile with a central
reversal. Assuming optically thin formation with the standard coronal
approximation leads to several errors: neglecting photoionization
severly underestimates the amount of C ii at temperatures below 16 kK,
erroneously shifts the formation from 10 kK to 25 kK, and leads to
too low intensities.
---------------------------------------------------------
Title: Ellerman Bombs at High Resolution. III. Simultaneous
Observations with IRIS and SST
Authors: Vissers, G. J. M.; Rouppe van der Voort, L. H. M.; Rutten,
R. J.; Carlsson, M.; De Pontieu, B.
2015ApJ...812...11V Altcode: 2015arXiv150700435V
Ellerman bombs (EBs) are transient brightenings of the extended wings
of the solar Balmer lines in emerging active regions. We describe
their properties in the ultraviolet lines sampled by the Interface
Region Imaging Spectrograph (IRIS), using simultaneous imaging
spectroscopy in Hα with the Swedish 1-m Solar Telescope (SST) and
ultraviolet images from the Solar Dynamics Observatory for Ellerman
bomb detection and identification. We select multiple co-observed
EBs for detailed analysis. The IRIS spectra strengthen the view that
EBs mark reconnection between bipolar kilogauss fluxtubes with the
reconnection and the resulting bi-directional jet located within the
solar photosphere and shielded by overlying chromospheric fibrils in
the cores of strong lines. The spectra suggest that the reconnecting
photospheric gas underneath is heated sufficiently to momentarily reach
stages of ionization normally assigned to the transition region and the
corona. We also analyze similar outburst phenomena that we classify as
small flaring arch filaments and ascribe to reconnection at a higher
location. They have different morphologies and produce hot arches in
million-Kelvin diagnostics.
---------------------------------------------------------
Title: Observed Variability of the Solar Mg II h Spectral Line
Authors: Schmit, D.; Bryans, P.; De Pontieu, B.; McIntosh, S.;
Leenaarts, J.; Carlsson, M.
2015ApJ...811..127S Altcode: 2015arXiv150804714S
The Mg ii h&k doublet are two of the primary spectral lines observed
by the Sun-pointing Interface Region Imaging Spectrograph (IRIS). These
lines are tracers of the magnetic and thermal environment that spans
from the photosphere to the upper chromosphere. We use a double-Gaussian
model to fit the Mg ii h profile for a full-Sun mosaic data set taken
on 2014 August 24. We use the ensemble of high-quality profile fits to
conduct a statistical study on the variability of the line profile as
it relates the magnetic structure, dynamics, and center-to-limb viewing
angle. The average internetwork profile contains a deeply reversed
core and is weakly asymmetric at h2. In the internetwork, we find a
strong correlation between h3 wavelength and profile asymmetry as well
as h1 width and h2 width. The average reversal depth of the h3 core
is inversely related to the magnetic field. Plage and sunspots exhibit
many profiles that do not contain a reversal. These profiles also occur
infrequently in the internetwork. We see indications of magnetically
aligned structures in plage and network in statistics associated with
the line core, but these structures are not clear or extended in the
internetwork. The center-to-limb variations are compared to predictions
of semi-empirical model atmospheres. We measure a pronounced limb
darkening in the line core that is not predicted by the model. The
aim of this work is to provide a comprehensive measurement baseline
and preliminary analysis on the observed structure and formation of
the Mg ii profiles observed by IRIS.
---------------------------------------------------------
Title: Numerical Modeling of the Solar Chromosphere and Corona:
What Has Been Done? What Should Be Done?
Authors: Hansteen, V.; Carlsson, M.; Gudiksen, B.
2015ASPC..498..141H Altcode:
A number of increasingly sophisticated numerical simulations spanning
the solar atmosphere from below the photosphere in the convection
zone to far above in the corona have shed considerable insight into
the role of the magnetic field in the structure and energetics of the
Sun's outer layers. This development is strengthened by the wealth of
observational data now coming on-line from both ground and space based
observatories. In this talk we will concentrate on the successes and
failures of the modeling effort thus far and discuss the inclusion of
various effects not traditionally considered in the MHD description
such as time dependent ionization, non-LTE radiative transfer, and
generalized Ohm's law.
---------------------------------------------------------
Title: What Do IRIS Observations of Mg II k Tell Us about the Solar
Plage Chromosphere?
Authors: Carlsson, Mats; Leenaarts, Jorrit; De Pontieu, Bart
2015ApJ...809L..30C Altcode: 2015arXiv150804888C
We analyze observations from the Interface Region Imaging Spectrograph
of the Mg ii k line, the Mg ii UV subordinate lines, and the O i
135.6 {nm} line to better understand the solar plage chromosphere. We
also make comparisons with observations from the Swedish 1-m Solar
Telescope of the Hα line, the Ca ii 8542 line, and Solar Dynamics
Observatory/Atmospheric Imaging Assembly observations of the coronal
19.3 {nm} line. To understand the observed Mg ii profiles, we compare
these observations to the results of numerical experiments. The
single-peaked or flat-topped Mg ii k profiles found in plage imply a
transition region at a high column mass and a hot and dense chromosphere
of about 6500 K. This scenario is supported by the observed large-scale
correlation between moss brightness and filled-in profiles with very
little or absent self-reversal. The large wing width found in plage
also implies a hot and dense chromosphere with a steep chromospheric
temperature rise. The absence of emission in the Mg ii subordinate
lines constrain the chromospheric temperature and the height of the
temperature rise while the width of the O i 135.6 {nm} line sets a
limit to the non-thermal velocities to around 7 km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: A Unified Computational Model for Solar and Stellar Flares
Authors: Allred, Joel C.; Kowalski, Adam F.; Carlsson, Mats
2015ApJ...809..104A Altcode: 2015arXiv150704375A
We present a unified computational framework that can be used to
describe impulsive flares on the Sun and on dMe stars. The models
assume that the flare impulsive phase is caused by a beam of charged
particles that is accelerated in the corona and propagates downward
depositing energy and momentum along the way. This rapidly heats
the lower stellar atmosphere causing it to explosively expand and
dramatically brighten. Our models consist of flux tubes that extend from
the sub-photosphere into the corona. We simulate how flare-accelerated
charged particles propagate down one-dimensional flux tubes and heat the
stellar atmosphere using the Fokker-Planck kinetic theory. Detailed
radiative transfer is included so that model predictions can be
directly compared with observations. The flux of flare-accelerated
particles drives return currents which additionally heat the stellar
atmosphere. These effects are also included in our models. We examine
the impact of the flare-accelerated particle beams on model solar and
dMe stellar atmospheres and perform parameter studies varying the
injected particle energy spectra. We find the atmospheric response
is strongly dependent on the accelerated particle cutoff energy and
spectral index.
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. IV. The Mg II Triplet
Lines as a New Diagnostic for Lower Chromospheric Heating
Authors: Pereira, Tiago M. D.; Carlsson, Mats; De Pontieu, Bart;
Hansteen, Viggo
2015ApJ...806...14P Altcode: 2015arXiv150401733P
A triplet of subordinate lines of Mg ii exists in the region around
the h&k lines. In solar spectra these lines are seen mostly
in absorption, but in some cases can become emission lines. The
aim of this work is to study the formation of this triplet, and
investigate any diagnostic value they can bring. Using 3D radiative
magnetohydrodynamic simulations of quiet Sun and flaring flux emergence,
we synthesize spectra and investigate how spectral features respond
to the underlying atmosphere. We find that emission in the lines
is rare and is typically caused by a steep temperature increase in
the lower chromosphere (above 1500 K, with electron densities above
10<SUP>17</SUP> m<SUP>-3</SUP>). In both simulations the lines are
sensitive to temperature increases taking place at column masses ≳5
· 10<SUP>-4</SUP> g cm<SUP>-2</SUP>. Additional information can
also be inferred from the peak-to-wing ratio and shape of the line
profiles. Using observations from NASA's Interface Region Imaging
Spectrograph we find both absorption and emission line profiles with
similar shapes to the synthetic spectra, which suggests that these lines
represent a useful diagnostic that complements the Mg ii h&k lines.
---------------------------------------------------------
Title: IRIS observations and 3D `realistic' MHD models of the solar
chromosphere
Authors: Hansteen, V.; Carlsson, M.; Gudiksen, B.
2015hsa8.conf...19H Altcode:
The Interface Region Imaging Spectrograph (IRIS) is a NASA “Small
Explorer” mission. It was launched in late June 2013 and since then
it has obtained spectra and images from the outer solar atmosphere at
unprecedented spatial and temporal resolution. Its primary goal is to
probe the photosphere-corona interface: the source region of outer
atmosphere heating and dynamics and a region that has an extremely
complicated interplay between plasma, radiation and magnetic field. The
scientific justification for IRIS hinges on the capabilities of 3D
magnetohydrodynamic models to allow the confident interpretation of
observed data. The interplay between observations and modeling is
discussed, illustrated with examples from recent IRIS observations.
---------------------------------------------------------
Title: First High-resolution Spectroscopic Observations of an Erupting
Prominence Within a Coronal Mass Ejection by the Interface Region
Imaging Spectrograph (IRIS)
Authors: Liu, Wei; De Pontieu, Bart; Vial, Jean-Claude; Title, Alan
M.; Carlsson, Mats; Uitenbroek, Han; Okamoto, Takenori J.; Berger,
Thomas E.; Antolin, Patrick
2015ApJ...803...85L Altcode: 2015arXiv150204738L
Spectroscopic observations of prominence eruptions associated with
coronal mass ejections (CMEs), although relatively rare, can provide
valuable plasma and three-dimensional geometry diagnostics. We report
the first observations by the Interface Region Imaging Spectrograph
mission of a spectacular fast CME/prominence eruption associated with
an equivalent X1.6 flare on 2014 May 9. The maximum plane-of-sky and
Doppler velocities of the eruption are 1200 and 460 km s<SUP>-1</SUP>,
respectively. There are two eruption components separated by ∼200
km s<SUP>-1</SUP> in Doppler velocity: a primary, bright component
and a secondary, faint component, suggesting a hollow, rather than
solid, cone-shaped distribution of material. The eruption involves
a left-handed helical structure undergoing counterclockwise (viewed
top-down) unwinding motion. There is a temporal evolution from upward
eruption to downward fallback with less-than-free-fall speeds and
decreasing nonthermal line widths. We find a wide range of Mg ii k/h
line intensity ratios (less than ∼2 expected for optically-thin
thermal emission): the lowest ever reported median value of 1.17
found in the fallback material, a comparably high value of 1.63 in
nearby coronal rain, and intermediate values of 1.53 and 1.41 in
the two eruption components. The fallback material exhibits a strong
(\gt 5σ ) linear correlation between the k/h ratio and the Doppler
velocity as well as the line intensity. We demonstrate that Doppler
dimming of scattered chromospheric emission by the erupted material
can potentially explain such characteristics.
---------------------------------------------------------
Title: Three-dimensional Radiative Transfer Simulations of
the Scattering Polarization of the Hydrogen Lyα Line in a
Magnetohydrodynamic Model of the Chromosphere-Corona Transition Region
Authors: Štěpán, J.; Trujillo Bueno, J.; Leenaarts, J.; Carlsson, M.
2015ApJ...803...65S Altcode: 2015arXiv150106382S
Probing the magnetism of the upper solar chromosphere requires measuring
and modeling the scattering polarization produced by anisotropic
radiation pumping in UV spectral lines. Here we apply PORTA (a novel
radiative transfer code) to investigate the hydrogen Lyα line in
a three-dimensional model of the solar atmosphere resulting from a
state of the art magnetohydrodynamic (MHD) simulation. At full spatial
resolution the linear polarization signals are very significant all
over the solar disk, with a large fraction of the field of view (FOV)
showing line-center amplitudes well above the 1% level. Via the Hanle
effect the line-center polarization signals are sensitive to the
magnetic field of the model's transition region, even when its mean
field strength is only 15 G. The breaking of the axial symmetry of the
radiation field produces significant forward-scattering polarization
in Lyα, without the need of an inclined magnetic field. Interestingly,
the Hanle effect tends to decrease such forward-scattering polarization
signals in most of the points of the FOV. When the spatial resolution is
degraded, the line-center polarization of Lyα drops below the 1% level,
reaching values similar to those previously found in one-dimensional
(1D) semi-empirical models (i.e., up to about 0.5 %). The center to
limb variation (CLV) of the spatially averaged polarization signals
is qualitatively similar to that found in 1D models, with the largest
line-center amplitudes at μ =cos θ ≈ 0.4 (θ being the heliocentric
angle). These results are important, both for designing the needed
space-based instrumentation and for a reliable interpretation of future
observations of the Lyα polarization.
---------------------------------------------------------
Title: The role of partial ionization effects in the chromosphere
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo;
Carlsson, Mats
2015RSPTA.37340268M Altcode: 2015arXiv150302723M
The energy for the coronal heating must be provided from the
convection zone. However, the amount and the method by which this
energy is transferred into the corona depend on the properties of the
lower atmosphere and the corona itself. We review: (i) how the energy
could be built in the lower solar atmosphere, (ii) how this energy is
transferred through the solar atmosphere, and (iii) how the energy is
finally dissipated in the chromosphere and/or corona. Any mechanism of
energy transport has to deal with the various physical processes in the
lower atmosphere. We will focus on a physical process that seems to
be highly important in the chromosphere and not deeply studied until
recently: the ion-neutral interaction effects in the chromosphere. We
review the relevance and the role of the partial ionization in the
chromosphere and show that this process actually impacts considerably
the outer solar atmosphere. We include analysis of our 2.5D radiative
magnetohydrodynamic simulations with the Bifrost code (Gudiksen et
al. 2011 Astron. Astrophys. 531, A154 (doi:10.1051/0004-6361/201116520))
including the partial ionization effects on the chromosphere
and corona and thermal conduction along magnetic field lines. The
photosphere, chromosphere and transition region are partially ionized
and the interaction between ionized particles and neutral particles
has important consequences on the magneto-thermodynamics of these
layers. The partial ionization effects are treated using generalized
Ohm's law, i.e. we consider the Hall term and the ambipolar diffusion
(Pedersen dissipation) in the induction equation. The interaction
between the different species affects the modelled atmosphere as
follows: (i) the ambipolar diffusion dissipates magnetic energy and
increases the minimum temperature in the chromosphere and (ii) the
upper chromosphere may get heated and expanded over a greater range
of heights. These processes reveal appreciable differences between
the modelled atmospheres of simulations with and without ion-neutral
interaction effects.
---------------------------------------------------------
Title: On Fibrils and Field Lines: the Nature of Hα Fibrils in the
Solar Chromosphere
Authors: Leenaarts, Jorrit; Carlsson, Mats; Rouppe van der Voort, Luc
2015ApJ...802..136L Altcode: 2015arXiv150200295L
Observations of the solar chromosphere in the line core of the Hα
line show dark elongated structures called fibrils that show swaying
motion. We performed a three-dimensional radiation-MHD simulation of a
network region and computed synthetic Hα images from this simulation
to investigate the relation between fibrils and the magnetic field
lines in the chromosphere. The periods, amplitudes, and phase speeds
of the simulated fibrils are consistent with observations. We find that
some fibrils trace out the same field line along the fibril’s length,
while other fibrils sample different field lines at different locations
along their length. Fibrils sample the same field lines on a timescale
of ∼200 s. This is shorter than their own lifetime. Fibril-threading
field lines carry slow-mode waves, as well as transverse waves
propagating with the Alfvén speed. Transverse waves propagating
in opposite directions cause an interference pattern with complex
apparent phase speeds. The relationship between fibrils and field lines
is governed by constant migration and swaying of the field lines,
their mass loading and draining, and their visibility in Hα. Field
lines are visible where they lie close to the optical depth unity
surface. The location of the latter is at a height at which the column
mass reaches a certain fixed value. The visibility of the field line
is thus determined by its own mass density and by the mass density of
the material above it. Using the swaying motion of fibrils as a tracer
of chromospheric transverse oscillations must be done with caution.
---------------------------------------------------------
Title: Internetwork Chromospheric Bright Grains Observed With IRIS
and SST
Authors: Martínez-Sykora, Juan; Rouppe van der Voort, Luc; Carlsson,
Mats; De Pontieu, Bart; Pereira, Tiago M. D.; Boerner, Paul; Hurlburt,
Neal; Kleint, Lucia; Lemen, James; Tarbell, Ted D.; Title, Alan;
Wuelser, Jean-Pierre; Hansteen, Viggo H.; Golub, Leon; McKillop, Sean;
Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Jaeggli,
Sarah; Kankelborg, Charles
2015ApJ...803...44M Altcode: 2015arXiv150203490M
The Interface Region Imaging Spectrograph (IRIS) reveals small-scale
rapid brightenings in the form of bright grains all over coronal holes
and the quiet Sun. These bright grains are seen with the IRIS 1330,
1400, and 2796 Å slit-jaw filters. We combine coordinated observations
with IRIS and from the ground with the Swedish 1 m Solar Telescope
(SST) which allows us to have chromospheric (Ca ii 8542 Å, Ca ii H
3968 Å, Hα, and Mg ii k 2796 Å) and transition region (C ii 1334 Å,
Si iv 1403 Å) spectral imaging, and single-wavelength Stokes maps
in Fe i 6302 Å at high spatial (0\buildrel{\prime\prime}\over{.}
33), temporal, and spectral resolution. We conclude that the IRIS
slit-jaw grains are the counterpart of so-called acoustic grains,
i.e., resulting from chromospheric acoustic waves in a non-magnetic
environment. We compare slit-jaw images (SJIs) with spectra from the
IRIS spectrograph. We conclude that the grain intensity in the 2796
Å slit-jaw filter comes from both the Mg ii k core and wings. The
signal in the C ii and Si iv lines is too weak to explain the presence
of grains in the 1300 and 1400 Å SJIs and we conclude that the grain
signal in these passbands comes mostly from the continuum. Although
weak, the characteristic shock signatures of acoustic grains can often
be detected in IRIS C ii spectra. For some grains, a spectral signature
can be found in IRIS Si iv. This suggests that upward propagating
acoustic waves sometimes reach all the way up to the transition region.
---------------------------------------------------------
Title: A Unified Computational Model for Solar and Stellar Flares
Authors: Allred, Joel; Kowalski, Adam; Carlsson, Mats
2015TESS....130207A Altcode:
We describe a unified computational framework which can be used to model
impulsive flares on the Sun and on dMe stars. The models are constructed
assuming that the flare impulsive phase is caused by a beam of charged
particles (primarily electrons and protons) that is accelerated in the
corona and propagates downward depositing energy and momentum along
the way. This rapidly heats the lower stellar atmosphere causing it to
explosively expand and emission to dramatically brighten. Our models
consist of flux tubes that extend from the sub-photosphere into the
corona. We simulate how these flare-accelerated particles propagate
down one dimensional flux tubes and heat the stellar atmosphere
using Fokker-Planck kinetic theory. Detailed radiative transfer is
included so that model predictions can be directly compared with
observations. The flux of flare-accelerated particles drives return
currents which additionally heat the stellar atmosphere, and these
effects are also included in our models. We examine the impact of
the flare-accelerated particle beams on model solar and dMe stellar
atmospheres and perform parameter studies varying the injected particle
energy spectra. We find the atmospheric response is strongly dependent
on the accelerated particle cutoff energy and spectral index.
---------------------------------------------------------
Title: Millimeter radiation from a 3D model of the solar
atmosphere. I. Diagnosing chromospheric thermal structure
Authors: Loukitcheva, M.; Solanki, S. K.; Carlsson, M.; White, S. M.
2015A&A...575A..15L Altcode: 2015arXiv150102898L
<BR /> Aims: We use advanced 3D non-local thermodynamic equilibrium
radiative magnetohydrodynamic simulations of the solar atmosphere to
carry out detailed tests of chromospheric diagnostics at millimeter and
submillimeter wavelengths. <BR /> Methods: We focused on the diagnostics
of the thermal structure of the chromosphere in the wavelength bands
from 0.4 mm up to 9.6 mm that can be accessed with the Atacama Large
Millimeter/Submillimeter Array (ALMA) and investigated how these
diagnostics are affected by the instrumental resolution. <BR /> Results:
We find that the formation height range of the millimeter radiation
depends on the location in the simulation domain and is related to the
underlying magnetic structure. Nonetheless, the brightness temperature
is a reasonable measure of the gas temperature at the effective
formation height at a given location on the solar surface. There is
considerable scatter in this relationship, but this is significantly
reduced when very weak magnetic fields are avoided. Our results
indicate that although instrumental smearing reduces the correlation
between brightness and temperature, millimeter brightness can still
be used to reliably diagnose electron temperature up to a resolution
of 1”. If the resolution is more degraded, then the value of the
diagnostic diminishes rapidly. <BR /> Conclusions: We conclude that
millimeter brightness can image the chromospheric thermal structure
at the height at which the radiation is formed. Thus multiwavelength
observations with ALMA with a narrow step in wavelength should provide
sufficient information for a tomographic imaging of the chromosphere.
---------------------------------------------------------
Title: Homologous Helical Jets: Observations By IRIS, SDO, and Hinode
and Magnetic Modeling With Data-Driven Simulations
Authors: Cheung, Mark C. M.; De Pontieu, B.; Tarbell, T. D.; Fu, Y.;
Tian, H.; Testa, P.; Reeves, K. K.; Martínez-Sykora, J.; Boerner,
P.; Wülser, J. P.; Lemen, J.; Title, A. M.; Hurlburt, N.; Kleint,
L.; Kankelborg, C.; Jaeggli, S.; Golub, L.; McKillop, S.; Saar, S.;
Carlsson, M.; Hansteen, V.
2015ApJ...801...83C Altcode: 2015arXiv150101593C
We report on observations of recurrent jets by instruments on board
the Interface Region Imaging Spectrograph, Solar Dynamics Observatory
(SDO), and Hinode spacecraft. Over a 4 hr period on 2013 July 21,
recurrent coronal jets were observed to emanate from NOAA Active Region
11793. Far-ultraviolet spectra probing plasma at transition region
temperatures show evidence of oppositely directed flows with components
reaching Doppler velocities of ±100 km s<SUP>-1</SUP>. Raster Doppler
maps using a Si iv transition region line show all four jets to have
helical motion of the same sense. Simultaneous observations of the
region by SDO and Hinode show that the jets emanate from a source
region comprising a pore embedded in the interior of a supergranule. The
parasitic pore has opposite polarity flux compared to the surrounding
network field. This leads to a spine-fan magnetic topology in the
coronal field that is amenable to jet formation. Time-dependent
data-driven simulations are used to investigate the underlying drivers
for the jets. These numerical experiments show that the emergence of
current-carrying magnetic field in the vicinity of the pore supplies
the magnetic twist needed for recurrent helical jet formation.
---------------------------------------------------------
Title: Heating Signatures in the Disk Counterparts of Solar Spicules
in Interface Region Imaging Spectrograph Observations
Authors: Rouppe van der Voort, L.; De Pontieu, B.; Pereira, T. M. D.;
Carlsson, M.; Hansteen, V.
2015ApJ...799L...3R Altcode: 2014arXiv1412.4531R
We use coordinated observations with the Interface Region Imaging
Spectrograph (IRIS) and the Swedish 1 m Solar Telescope to identify
the disk counterpart of type II spicules in upper-chromospheric and
transition region (TR) diagnostics. These disk counterparts were
earlier identified through short-lived asymmetries in chromospheric
spectral lines: rapid blue- or red-shifted excursions (RBEs or RREs). We
find clear signatures of RBEs and RREs in Mg II h & k, often with
excursions of the central h3 and k3 absorption features in concert with
asymmetries in co-temporal and co-spatial Hα spectral profiles. We find
spectral signatures for RBEs and RREs in C II 1335 and 1336 Å and Si
IV 1394 and 1403 Å spectral lines and interpret this as a sign that
type II spicules are heated to at least TR temperatures, supporting
other recent work. These C II and Si IV spectral signals are weaker
for a smaller network region than for more extended network regions in
our data. A number of bright features around extended network regions
observed in IRIS slit-jaw imagery SJI 1330 and 1400, recently identified
as network jets, can be clearly connected to Hα RBEs and/or RREs in
our coordinated data. We speculate that at least part of the diffuse
halo around network regions in the IRIS SJI 1330 and 1400 images can
be attributed to type II spicules with insufficient opacity in the C
II and Si IV lines to stand out as single features in these passbands.
---------------------------------------------------------
Title: IRIS diagnostics of non-thermal particles in coronal loops
heated by nanoflares
Authors: Testa, P.; De Pontieu, B.; Allred, J. C.; Carlsson, M.;
Reale, F.; Daw, A. N.
2014AGUFMSH53D..08T Altcode:
The variability of emission of the "moss", i.e., the upper transition
region (TR) layer of high pressure loops in active regions, provides
stringent constraints on the characteristics of heating events. We
will discuss the new coronal heating diagnostics provided by the
Interface Region Imaging Spectrograph (IRIS) together with SDO/AIA. IRIS
provides imaging and spectral observations of the solar chromosphere
and transition region, at high spatial (0.166 arcsec/pix) and temporal
(down to ~1s) resolution at FUV and NUV wavelengths. We discuss how
simultaneous IRIS and AIA observations, together with loop modeling
(with the RADYN code) including chromosphere, transition region and
corona, allow us to study impulsive heating events (nanoflares) and the
energy transport mechanism between the corona and the lower atmospheric
layers (thermal conduction vs. beams of non-thermal particles). We will
show how the modeling of rapid moss brightenings provides diagnostics
for the presence and properties of non-thermal particles in nanoflares,
which are below the detectability threshold of hard X-ray observations.
---------------------------------------------------------
Title: Observables of Ion-Neutral Interaction Effects in the Solar
Chromosphere
Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.;
Pereira, T. M. D.; Leenaarts, J.; Carlsson, M.
2014AGUFMSH51C4176M Altcode:
The chromosphere and transition region constitute the interface
between the solar surface and the corona and modulate the flow of
mass and energy into the upper atmosphere. IRIS was launched in 2013
to study the chromosphere and transition region. The complexity of the
chromosphere is due to various regime changes that take place across it,
like: Hydrogen goes from predominantly neutral to predominantly ionized;
the plasma behavior changes from collisional to collision-less; it goes
from gas-pressure dominated to magnetically driven, etc. Consequently,
the interpretation of chromospheric observations in general and those
from IRIS, in particular, is a challenging task. It is thus crucial
to combine IRIS observations with advanced radiative-MHD numerical
modeling. Because the photosphere, chromosphere and transition region
are partially ionized, the interaction between ionized and neutral
particles has important consequences on the magneto-thermodynamics of
these regions. We implemented the effects of partial ionization using
generalized Ohm's law in the Bifrost code (Gudiksen et al. 2011) which
solves the full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along magnetic field lines. The
implementation of partial ionization effects impact our modeled
radiative-MHD atmosphere, such as producing chromospheric heating and
diffusion of photospheric magnetic field into the upper-chromosphere. We
will focus on which observables of these processes can be revealed
with IRIS.
---------------------------------------------------------
Title: Probing the solar interface region
Authors: De Pontieu, Bart; Title, Alan; Carlsson, Mats
2014Sci...346..315D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Hot explosions in the cool atmosphere of the Sun
Authors: Peter, H.; Tian, H.; Curdt, W.; Schmit, D.; Innes, D.;
De Pontieu, B.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.;
Tarbell, T. D.; Wuelser, J. P.; Martínez-Sykora, Juan; Kleint,
L.; Golub, L.; McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.;
Kankelborg, C.; Jaeggli, S.; Carlsson, M.; Hansteen, V.
2014Sci...346C.315P Altcode: 2014arXiv1410.5842P
The solar atmosphere was traditionally represented with a simple
one-dimensional model. Over the past few decades, this paradigm shifted
for the chromosphere and corona that constitute the outer atmosphere,
which is now considered a dynamic structured envelope. Recent
observations by the Interface Region Imaging Spectrograph (IRIS) reveal
that it is difficult to determine what is up and down, even in the cool
6000-kelvin photosphere just above the solar surface: This region hosts
pockets of hot plasma transiently heated to almost 100,000 kelvin. The
energy to heat and accelerate the plasma requires a considerable
fraction of the energy from flares, the largest solar disruptions. These
IRIS observations not only confirm that the photosphere is more complex
than conventionally thought, but also provide insight into the energy
conversion in the process of magnetic reconnection.
---------------------------------------------------------
Title: A Sounding Rocket Experiment for the Chromospheric Lyman-Alpha
Spectro-Polarimeter (CLASP)
Authors: Kubo, M.; Kano, R.; Kobayashi, K.; Bando, T.; Narukage, N.;
Ishikawa, R.; Tsuneta, S.; Katsukawa, Y.; Ishikawa, S.; Suematsu, Y.;
Hara, H.; Shimizu, T.; Sakao, T.; Ichimoto, K.; Goto, M.; Holloway,
T.; Winebarger, A.; Cirtain, J.; De Pontieu, B.; Casini, R.; Auchère,
F.; Trujillo Bueno, J.; Manso Sainz, R.; Belluzzi, L.; Asensio Ramos,
A.; Štěpán, J.; Carlsson, M.
2014ASPC..489..307K Altcode:
A sounding-rocket experiment called the Chromospheric Lyman-Alpha
Spectro-Polarimeter (CLASP) is presently under development to measure
the linear polarization profiles in the hydrogen Lyman-alpha (Lyα)
line at 121.567 nm. CLASP is a vacuum-UV (VUV) spectropolarimeter to aim
for first detection of the linear polarizations caused by scattering
processes and the Hanle effect in the Lyα line with high accuracy
(0.1%). This is a fist step for exploration of magnetic fields in
the upper chromosphere and transition region of the Sun. Accurate
measurements of the linear polarization signals caused by scattering
processes and the Hanle effect in strong UV lines like Lyα are
essential to explore with future solar telescopes the strength
and structures of the magnetic field in the upper chromosphere and
transition region of the Sun. The CLASP proposal has been accepted by
NASA in 2012, and the flight is planned in 2015.
---------------------------------------------------------
Title: The unresolved fine structure resolved: IRIS observations of
the solar transition region
Authors: Hansteen, V.; De Pontieu, B.; Carlsson, M.; Lemen, J.; Title,
A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Pereira,
T. M. D.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar,
S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.;
Martínez-Sykora, J.
2014Sci...346E.315H Altcode: 2014arXiv1412.3611H
The heating of the outer solar atmospheric layers, i.e., the transition
region and corona, to high temperatures is a long-standing problem
in solar (and stellar) physics. Solutions have been hampered by an
incomplete understanding of the magnetically controlled structure of
these regions. The high spatial and temporal resolution observations
with the Interface Region Imaging Spectrograph (IRIS) at the solar
limb reveal a plethora of short, low-lying loops or loop segments
at transition-region temperatures that vary rapidly, on the time
scales of minutes. We argue that the existence of these loops solves
a long-standing observational mystery. At the same time, based on
comparison with numerical models, this detection sheds light on a
critical piece of the coronal heating puzzle.
---------------------------------------------------------
Title: Evidence of nonthermal particles in coronal loops heated
impulsively by nanoflares
Authors: Testa, P.; De Pontieu, B.; Allred, J.; Carlsson, M.; Reale,
F.; Daw, A.; Hansteen, V.; Martinez-Sykora, J.; Liu, W.; DeLuca, E. E.;
Golub, L.; McKillop, S.; Reeves, K.; Saar, S.; Tian, H.; Lemen, J.;
Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.;
Kleint, L.; Kankelborg, C.; Jaeggli, S.
2014Sci...346B.315T Altcode: 2014arXiv1410.6130T
The physical processes causing energy exchange between the Sun’s
hot corona and its cool lower atmosphere remain poorly understood. The
chromosphere and transition region (TR) form an interface region between
the surface and the corona that is highly sensitive to the coronal
heating mechanism. High-resolution observations with the Interface
Region Imaging Spectrograph (IRIS) reveal rapid variability (~20 to
60 seconds) of intensity and velocity on small spatial scales (≲500
kilometers) at the footpoints of hot and dynamic coronal loops. The
observations are consistent with numerical simulations of heating by
beams of nonthermal electrons, which are generated in small impulsive
(≲30 seconds) heating events called “coronal nanoflares.” The
accelerated electrons deposit a sizable fraction of their energy
(≲10<SUP>25 </SUP>erg) in the chromosphere and TR. Our analysis
provides tight constraints on the properties of such electron beams
and new diagnostics for their presence in the nonflaring corona.
---------------------------------------------------------
Title: Prevalence of small-scale jets from the networks of the solar
transition region and chromosphere
Authors: Tian, H.; DeLuca, E. E.; Cranmer, S. R.; De Pontieu, B.;
Peter, H.; Martínez-Sykora, J.; Golub, L.; McKillop, S.; Reeves,
K. K.; Miralles, M. P.; McCauley, P.; Saar, S.; Testa, P.; Weber,
M.; Murphy, N.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.;
Tarbell, T. D.; Wuelser, J. P.; Kleint, L.; Kankelborg, C.; Jaeggli,
S.; Carlsson, M.; Hansteen, V.; McIntosh, S. W.
2014Sci...346A.315T Altcode: 2014arXiv1410.6143T
As the interface between the Sun’s photosphere and corona, the
chromosphere and transition region play a key role in the formation and
acceleration of the solar wind. Observations from the Interface Region
Imaging Spectrograph reveal the prevalence of intermittent small-scale
jets with speeds of 80 to 250 kilometers per second from the narrow
bright network lanes of this interface region. These jets have lifetimes
of 20 to 80 seconds and widths of ≤300 kilometers. They originate from
small-scale bright regions, often preceded by footpoint brightenings
and accompanied by transverse waves with amplitudes of ~20 kilometers
per second. Many jets reach temperatures of at least ~10<SUP>5</SUP>
kelvin and constitute an important element of the transition region
structures. They are likely an intermittent but persistent source of
mass and energy for the solar wind.
---------------------------------------------------------
Title: On the prevalence of small-scale twist in the solar
chromosphere and transition region
Authors: De Pontieu, B.; Rouppe van der Voort, L.; McIntosh, S. W.;
Pereira, T. M. D.; Carlsson, M.; Hansteen, V.; Skogsrud, H.; Lemen,
J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser,
J. P.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar,
S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.;
Martinez-Sykora, J.
2014Sci...346D.315D Altcode: 2014arXiv1410.6862D
The solar chromosphere and transition region (TR) form an interface
between the Sun’s surface and its hot outer atmosphere. There,
most of the nonthermal energy that powers the solar atmosphere
is transformed into heat, although the detailed mechanism remains
elusive. High-resolution (0.33-arc second) observations with NASA’s
Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere
and TR that are replete with twist or torsional motions on sub-arc
second scales, occurring in active regions, quiet Sun regions, and
coronal holes alike. We coordinated observations with the Swedish
1-meter Solar Telescope (SST) to quantify these twisting motions and
their association with rapid heating to at least TR temperatures. This
view of the interface region provides insight into what heats the low
solar atmosphere.
---------------------------------------------------------
Title: An Interface Region Imaging Spectrograph First View on Solar
Spicules
Authors: Pereira, T. M. D.; De Pontieu, B.; Carlsson, M.; Hansteen,
V.; Tarbell, T. D.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt,
N.; Wülser, J. P.; Martínez-Sykora, J.; Kleint, L.; Golub, L.;
McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.; Tian, H.; Jaeggli,
S.; Kankelborg, C.
2014ApJ...792L..15P Altcode: 2014arXiv1407.6360P
Solar spicules have eluded modelers and observers for decades. Since
the discovery of the more energetic type II, spicules have become
a heated topic but their contribution to the energy balance of the
low solar atmosphere remains unknown. Here we give a first glimpse of
what quiet-Sun spicules look like when observed with NASA's recently
launched Interface Region Imaging Spectrograph (IRIS). Using IRIS
spectra and filtergrams that sample the chromosphere and transition
region, we compare the properties and evolution of spicules as
observed in a coordinated campaign with Hinode and the Atmospheric
Imaging Assembly. Our IRIS observations allow us to follow the thermal
evolution of type II spicules and finally confirm that the fading
of Ca II H spicules appears to be caused by rapid heating to higher
temperatures. The IRIS spicules do not fade but continue evolving,
reaching higher and falling back down after 500-800 s. Ca II H type
II spicules are thus the initial stages of violent and hotter events
that mostly remain invisible in Ca II H filtergrams. These events
have very different properties from type I spicules, which show lower
velocities and no fading from chromospheric passbands. The IRIS spectra
of spicules show the same signature as their proposed disk counterparts,
reinforcing earlier work. Spectroheliograms from spectral rasters also
confirm that quiet-Sun spicules originate in bushes from the magnetic
network. Our results suggest that type II spicules are indeed the
site of vigorous heating (to at least transition region temperatures)
along extensive parts of the upward moving spicular plasma.
---------------------------------------------------------
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Kushner, G. D.;
Akin, D. J.; Allard, B.; Berger, T.; Boerner, P.; Cheung, M.; Chou,
C.; Drake, J. F.; Duncan, D. W.; Freeland, S.; Heyman, G. F.; Hoffman,
C.; Hurlburt, N. E.; Lindgren, R. W.; Mathur, D.; Rehse, R.; Sabolish,
D.; Seguin, R.; Schrijver, C. J.; Tarbell, T. D.; Wülser, J. -P.;
Wolfson, C. J.; Yanari, C.; Mudge, J.; Nguyen-Phuc, N.; Timmons,
R.; van Bezooijen, R.; Weingrod, I.; Brookner, R.; Butcher, G.;
Dougherty, B.; Eder, J.; Knagenhjelm, V.; Larsen, S.; Mansir, D.;
Phan, L.; Boyle, P.; Cheimets, P. N.; DeLuca, E. E.; Golub, L.;
Gates, R.; Hertz, E.; McKillop, S.; Park, S.; Perry, T.; Podgorski,
W. A.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Weber, M.; Dunn, C.;
Eccles, S.; Jaeggli, S. A.; Kankelborg, C. C.; Mashburn, K.; Pust, N.;
Springer, L.; Carvalho, R.; Kleint, L.; Marmie, J.; Mazmanian, E.;
Pereira, T. M. D.; Sawyer, S.; Strong, J.; Worden, S. P.; Carlsson,
M.; Hansteen, V. H.; Leenaarts, J.; Wiesmann, M.; Aloise, J.; Chu,
K. -C.; Bush, R. I.; Scherrer, P. H.; Brekke, P.; Martinez-Sykora,
J.; Lites, B. W.; McIntosh, S. W.; Uitenbroek, H.; Okamoto, T. J.;
Gummin, M. A.; Auker, G.; Jerram, P.; Pool, P.; Waltham, N.
2014SoPh..289.2733D Altcode: 2014arXiv1401.2491D; 2014SoPh..tmp...25D
The Interface Region Imaging Spectrograph (IRIS) small explorer
spacecraft provides simultaneous spectra and images of the photosphere,
chromosphere, transition region, and corona with 0.33 - 0.4 arcsec
spatial resolution, two-second temporal resolution, and 1 km
s<SUP>−1</SUP> velocity resolution over a field-of-view of up to
175 arcsec × 175 arcsec. IRIS was launched into a Sun-synchronous
orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a
19-cm UV telescope that feeds a slit-based dual-bandpass imaging
spectrograph. IRIS obtains spectra in passbands from 1332 - 1358 Å,
1389 - 1407 Å, and 2783 - 2834 Å, including bright spectral lines
formed in the chromosphere (Mg II h 2803 Å and Mg II k 2796 Å) and
transition region (C II 1334/1335 Å and Si IV 1394/1403 Å). Slit-jaw
images in four different passbands (C II 1330, Si IV 1400, Mg II k
2796, and Mg II wing 2830 Å) can be taken simultaneously with spectral
rasters that sample regions up to 130 arcsec × 175 arcsec at a variety
of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to
emission from plasma at temperatures between 5000 K and 10 MK and will
advance our understanding of the flow of mass and energy through an
interface region, formed by the chromosphere and transition region,
between the photosphere and corona. This highly structured and dynamic
region not only acts as the conduit of all mass and energy feeding
into the corona and solar wind, it also requires an order of magnitude
more energy to heat than the corona and solar wind combined. The
IRIS investigation includes a strong numerical modeling component
based on advanced radiative-MHD codes to facilitate interpretation of
observations of this complex region. Approximately eight Gbytes of data
(after compression) are acquired by IRIS each day and made available
for unrestricted use within a few days of the observation.
---------------------------------------------------------
Title: Detection of Supersonic Downflows and Associated Heating
Events in the Transition Region above Sunspots
Authors: Kleint, L.; Antolin, P.; Tian, H.; Judge, P.; Testa, P.;
De Pontieu, B.; Martínez-Sykora, J.; Reeves, K. K.; Wuelser, J. P.;
McKillop, S.; Saar, S.; Carlsson, M.; Boerner, P.; Hurlburt, N.; Lemen,
J.; Tarbell, T. D.; Title, A.; Golub, L.; Hansteen, V.; Jaeggli, S.;
Kankelborg, C.
2014ApJ...789L..42K Altcode: 2014arXiv1406.6816K
Interface Region Imaging Spectrograph data allow us to study the solar
transition region (TR) with an unprecedented spatial resolution of
0.”33. On 2013 August 30, we observed bursts of high Doppler shifts
suggesting strong supersonic downflows of up to 200 km s<SUP>-1</SUP>
and weaker, slightly slower upflows in the spectral lines Mg II h
and k, C II 1336, Si IV 1394 Å, and 1403 Å, that are correlated
with brightenings in the slitjaw images (SJIs). The bursty behavior
lasts throughout the 2 hr observation, with average burst durations
of about 20 s. The locations of these short-lived events appear to
be the umbral and penumbral footpoints of EUV loops. Fast apparent
downflows are observed along these loops in the SJIs and in the
Atmospheric Imaging Assembly, suggesting that the loops are thermally
unstable. We interpret the observations as cool material falling
from coronal heights, and especially coronal rain produced along the
thermally unstable loops, which leads to an increase of intensity
at the loop footpoints, probably indicating an increase of density
and temperature in the TR. The rain speeds are on the higher end of
previously reported speeds for this phenomenon, and possibly higher
than the free-fall velocity along the loops. On other observing days,
similar bright dots are sometimes aligned into ribbons, resembling
small flare ribbons. These observations provide a first insight into
small-scale heating events in sunspots in the TR.
---------------------------------------------------------
Title: F-CHROMA.Flare Chromospheres: Observations, Models and Archives
Authors: Cauzzi, Gianna; Fletcher, Lyndsay; Mathioudakis, Mihalis;
Carlsson, Mats; Heinzel, Petr; Berlicki, Arek; Zuccarello, Francesca
2014AAS...22412339C Altcode:
F-CHROMA is a collaborative project newly funded under the EU-Framework
Programme 7 "FP7-SPACE-2013-1", involving seven different European
research Institutes and Universities. The goal of F-CHROMA is to
substantially advance our understanding of the physics of energy
dissipation and radiation in the flaring solar atmosphere, with a
particular focus on the flares' chromosphere. A major outcome of the
F-CHROMA project will be the creation of an archive of chromospheric
flare observations and models to be made available to the community
for further research.In this poster we describe the structure and
milestones of the project, the different activities planned, as well
as early results. Emphasis will be given to the dissemination efforts
of the project to make results of these activities available to and
usable by the community.
---------------------------------------------------------
Title: IRIS Observations of Twist in the Low Solar Atmosphere
Authors: De Pontieu, Bart; Rouppe van der Voort, Luc; Pereira,
Tiago M. D.; Skogsrud, Haakon; McIntosh, Scott W.; Carlsson, Mats;
Hansteen, Viggo
2014AAS...22431302D Altcode:
The Interface Region Imaging Spectrograph (IRIS) small explorer
was launched in June 2013. IRIS’s high-resolution (0.33 arcsec),
high-cadence (2s) images and spectra reveal a solar chromosphere and
transition region that is riddled with twist. This is evidenced by the
presence of ubiquitous torsional motions on very small (subarcsec)
spatial scales. These motions occur in active regions, quiet Sun
and coronal holes on a variety of structures such as spicules at
the limb, rapid-blue/red-shifted events (RBEs and RREs) as well as
low-lying loops. We use IRIS data and observations from the Swedish
Solar Telescope (SST) in La Palma, Spain to describe these motions
quantitatively, study their propagation, and illustrate how such
strong twisting motions are often associated with significant and
rapid heating to at least transition region temperatures.
---------------------------------------------------------
Title: Diagnostics of coronal heating and mechanisms of energy
transport from IRIS and AIA observations of active region moss
Authors: Testa, Paola; De Pontieu, Bart; Allred, Joel C.; Carlsson,
Mats; Reale, Fabio; Daw, Adrian N.; Hansteen, Viggo
2014AAS...22431305T Altcode:
The variability of emission of the "moss", i.e., the upper transition
region (TR) layer of high pressure loops in active regions provides
stringent constraints on the characteristics of heating events. The
Interface Region Imaging Spectrograph (IRIS), launched in June
2013, provides imaging and spectral observations at high spatial
(0.166 arcsec/pix), and temporal (down to ~1s) resolution at FUV
and NUV wavelengths, and together with the high spatial and temporal
resolution observations of SDO/AIA, can provide important insights
into the coronal heating mechanisms. We present here an analysis of
the temporal variability properties of moss regions at the footpoints
of hot active region core loops undergoing heating, as observed by IRIS
and AIA, covering emission from the corona to the transition region and
the chromosphere. We model the observations using dynamic loop models
(the Palermo-Harvard code, and RADYN, which also includes the effects of
non-thermal particles) and discuss the implications on energy transport
mechanisms (thermal conduction vs beams of non-thermal particles).
---------------------------------------------------------
Title: IRIS observations of the transition region above sunspots:
oscillations and moving penumbral dots
Authors: Tian, Hui; DeLuca, Ed; Weber, Mark A.; McKillop, Sean;
Reeves, Kathy; Kleint, Lucia; Martinez-Sykora, Juan; De Pontieu,
Bart; Carlsson, Mats
2014AAS...22431306T Altcode:
NASA's IRIS mission is providing high-cadence and high-resolution
observations of the solar transition region and chromosphere. We
present results from IRIS observation of the transition region above
sunspots. The major findings can be summarized as following: (1) The C
II and Mg II line profiles are almost Gaussian in the sunspot umbra and
clearly exhibit a deep reversal at the line center in the plage region,
suggesting a greatly reduced opacity in the sunspot atmosphere. (2)
Strongly nonlinear sunspot oscillations can be clearly identified
in not only the slit jaw images of 2796Å, 1400Å and 1330Å, but
also in spectra of the bright Mg II, C II and Si IV lines. The Si
iv oscillation lags those of C ii and Mg ii by 3 and 12 seconds,
respectively. The temporal evolution of the line core is dominated by
the following behavior: a rapid excursion to the blue side, accompanied
by an intensity increase, followed by a linear decrease of the velocity
to the red side. The maximum intensity slightly lags the maximum blue
shift in Si iv , whereas the intensity enhancement slightly precedes the
maximum blue shift in Mg ii . We find a positive correlation between
the maximum velocity and deceleration. These results are consistent
with numerical simulations of upward propagating magneto-acoustic
shock waves. We also demonstrate that the strongly nonlinear line
width oscillation, reported both previously and here, is spurious. (3)
Many small-scale bright dots are present in the penumbral filaments and
light bridges in SJI 1330Å and 1400Å images obtained in high-cadence
observations. They are usually smaller than 1" and often just a couple
of pixels wide. Some bright dots show apparent movement with a speed of
20-60 km/s(either outward or inward). The lifetime of these penumbral
dots is mostly less than 1 min. The most obvious feature of the Si IV
profiles in the bright dots is the enhanced line width. Besides that,
the profile looks normal and no obvious fast flows are detected. The
bright dots in the light bridges even show oscillation patterns. It's
not clear whether these oscillations are triggered by the umbral
oscillations or not.
---------------------------------------------------------
Title: Hybrid Kinetic and Radiative Hydrodynamic Simulations of
Solar Flares and Comparison With Multiwavelength Observations
Authors: Rubio Da Costa, Fatima; Petrosian, Vahe; Liu, Wei; Carlsson,
Mats; Kleint, Lucia
2014AAS...22440906R Altcode:
We present a unified simulation which combines two physical processes:
how the particles are accelerated and the energy is transported along
a coronal loop, and how the atmosphere responds. The “flare”
code from Stanford University (Petrosian et al, 2001) models the
stochastic acceleration and transport of particles and radiation of
solar flares. It includes pitch angle diffusion and energy loss, and
computes collisional heating to the background plasma and bremsstrahlung
emission along the loop. The radiative hydrodynamic RADYN Code
(Carlsson et al, 1992, 1996; Allred et al, 2005) computes the energy
transport by the injected non-thermal electrons at the top of a 1D
coronal loop. Recently, we have combined the two codes by updating the
non-thermal heating in the RADYN code from the "flare" code, allowing us
to develop a self-consistent simulation. In addition, we can now model
more realistically the multi-wavelength emission of solar flares and
compare it with observations, e.g., at optical wavelengths from IBIS at
the Dunn Solar Telescope and in X-rays from RHESSI. The high resolution
UV observations from the recently launched IRIS imaging spectrograph
will be particularly useful in this regard. These will allow us to
compare numerically modeled and observed emissions of solar flares in
several lines using more robust simulations than possible before.
---------------------------------------------------------
Title: High-resolution Observations of the Shock Wave Behavior for
Sunspot Oscillations with the Interface Region Imaging Spectrograph
Authors: Tian, H.; DeLuca, E.; Reeves, K. K.; McKillop, S.; De Pontieu,
B.; Martínez-Sykora, J.; Carlsson, M.; Hansteen, V.; Kleint, L.;
Cheung, M.; Golub, L.; Saar, S.; Testa, P.; Weber, M.; Lemen, J.;
Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.;
Kankelborg, C.; Jaeggli, S.; McIntosh, S. W.
2014ApJ...786..137T Altcode: 2014arXiv1404.6291T
We present the first results of sunspot oscillations from observations
by the Interface Region Imaging Spectrograph. The strongly nonlinear
oscillation is identified in both the slit-jaw images and the
spectra of several emission lines formed in the transition region and
chromosphere. We first apply a single Gaussian fit to the profiles of
the Mg II 2796.35 Å, C II 1335.71 Å, and Si IV 1393.76 Å lines in the
sunspot. The intensity change is ~30%. The Doppler shift oscillation
reveals a sawtooth pattern with an amplitude of ~10 km s<SUP>-1</SUP>
in Si IV. The Si IV oscillation lags those of C II and Mg II by ~3 and
~12 s, respectively. The line width suddenly increases as the Doppler
shift changes from redshift to blueshift. However, we demonstrate
that this increase is caused by the superposition of two emission
components. We then perform detailed analysis of the line profiles at
a few selected locations on the slit. The temporal evolution of the
line core is dominated by the following behavior: a rapid excursion
to the blue side, accompanied by an intensity increase, followed by a
linear decrease of the velocity to the red side. The maximum intensity
slightly lags the maximum blueshift in Si IV, whereas the intensity
enhancement slightly precedes the maximum blueshift in Mg II. We find
a positive correlation between the maximum velocity and deceleration,
a result that is consistent with numerical simulations of upward
propagating magnetoacoustic shock waves.
---------------------------------------------------------
Title: Detailed and Simplified Nonequilibrium Helium Ionization in
the Solar Atmosphere
Authors: Golding, Thomas Peter; Carlsson, Mats; Leenaarts, Jorrit
2014ApJ...784...30G Altcode: 2014arXiv1401.7562G
Helium ionization plays an important role in the energy balance of the
upper chromosphere and transition region. Helium spectral lines are also
often used as diagnostics of these regions. We carry out one-dimensional
radiation-hydrodynamics simulations of the solar atmosphere and
find that the helium ionization is set mostly by photoionization and
direct collisional ionization, counteracted by radiative recombination
cascades. By introducing an additional recombination rate mimicking
the recombination cascades, we construct a simplified three-level
helium model atom consisting of only the ground states. This model
atom is suitable for modeling nonequilibrium helium ionization in
three-dimensional numerical models. We perform a brief investigation of
the formation of the He I 10830 and He II 304 spectral lines. Both lines
show nonequilibrium features that are not recovered with statistical
equilibrium models, and caution should therefore be exercised when
such models are used as a basis for interpretating observations.
---------------------------------------------------------
Title: The Effect of Isotopic Splitting on the Bisector and Inversions
of the Solar Ca II 854.2 nm Line
Authors: Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Kochukhov,
Oleg; Carlsson, Mats
2014ApJ...784L..17L Altcode: 2014arXiv1401.5019L
The Ca II 854.2 nm spectral line is a common diagnostic of the solar
chromosphere. The average line profile shows an asymmetric core,
and its bisector shows a characteristic inverse-C shape. The line
actually consists of six components with slightly different wavelengths
depending on the isotope of calcium. This isotopic splitting of the
line has been taken into account in studies of non-solar stars, but
never for the Sun. We performed non-LTE radiative transfer computations
from three models of the solar atmosphere and show that the line-core
asymmetry and inverse C-shape of the bisector of the 854.2 nm line
can be explained by isotopic splitting. We confirm this finding by
analyzing observations and showing that the line asymmetry is present
irrespective of conditions in the solar atmosphere. Finally, we show
that inversions based on the Ca II 854.2 nm line should take the
isotopic splitting into account, otherwise the inferred atmospheres
will contain erroneous velocity gradients and temperatures.
---------------------------------------------------------
Title: Diagnostics of coronal heating and mechanisms of energy
transport from IRIS and AIA observations of active region moss
Authors: Testa, Paola; Reale, Fabio; De Pontieu, Bart; Hansteen,
Viggo; Carlsson, Mats; Allred, Joel; Daw, Adrian
2014cosp...40E3323T Altcode:
The variability of emission of the "moss", i.e., the upper transition
region (TR) layer of high pressure loops in active regions provides
stringent constraints on the characteristics of heating events. The
Interface Region Imaging Spectrograph (IRIS), launched in June
2013, provides imaging and spectral observations at high spatial
(0.166 arcsec/pix), and temporal (down to ~1s) resolution at FUV
and NUV wavelengths, and together with the high spatial and temporal
resolution observations of SDO/AIA, can provide important insights
into the coronal heating mechanisms. We present here an analysis of
the temporal variability properties of moss regions at the footpoints
of hot active region core loops undergoing heating, as observed by IRIS
and AIA, covering emission from the corona to the transition region and
the chromosphere. We model the observations using dynamic loop models
(the Palermo-Harvard code, and RADYN, which also includes the effects of
non-thermal particles) and discuss the implications on energy transport
mechanisms (thermal conduction vs beams of non-thermal particles).
---------------------------------------------------------
Title: Measuring energy flux of magneto-acoustic wave in the magnetic
elements by using IRIS
Authors: Kato, Yoshiaki; De Pontieu, Bart; Martinez-Sykora, Juan;
Hansteen, Viggo; Pereira, Tiago; Leenaarts, Jorritt; Carlsson, Mats
2014cosp...40E1423K Altcode:
NASA's Interface Region Imaging Spectrograph (IRIS) has opened a new
window to explore the chromospheric/coronal waves that potentially
energize the solar atmosphere. By using an imaging spectrograph covering
the Si IV and Mg II h&k lines as well as a slit-jaw imager centered
at Si IV and Mg II k onboard IRIS, we can determine the nature of
propagating magneto-acoustic waves just below and in the transition
region. In this study, we compute the vertically emergent intensity of
the Si IV and Mg II h&k lines from a time series of snapshots of
a magnetic element in a two-dimensional Radiative MHD simulation from
the Bifrost code. We investigate the synthetic line profiles to detect
the slow magneto-acoustic body wave (slow mode) which becomes a slow
shock at the lower chromosphere in the magnetic element. We find that
the Doppler shift of the line core gives the velocity amplitude of the
longitudinal magneto-acoustic body wave. The contribution function of
the line core indicates that the formation of Mg II h&k lines is
associated with the propagating shocks and therefore the time evolution
of the line core intensity represents the propagating shocks projected
on the optical surface. We will report on measurement of the energy
flux of slow modes in the magnetic elements by using IRIS observations.
---------------------------------------------------------
Title: The Atmospheric Response to High Fluxes of Nonthermal Electrons
during M Dwarf Flares
Authors: Kowalski, Adam; Allred, J. C.; Carlsson, M.; Hawley, S. L.;
Holman, G. D.; Mathioudakis, M.; Osten, R. A.; Uitenbroek, H.
2014AAS...22315117K Altcode:
Flares are thought to be the result of magnetic fields in the stellar
corona that undergo reconnection and accelerate charged particles
into the lower atmosphere. Spectra of M dwarf flares in the optical
and near-ultraviolet wavelength regimes can be used to constrain the
heating mechanism of the lower stellar atmosphere. These observations
show several ubiquitous properties of the continuum emission, which
is not reproduced by models that use typical “solar-type” heating
functions. We present results from a grid of new flare models using the
RADYN code, which simultaneously calculates the radiative transfer and
hydrodynamics on short timescales. We explore the atmospheric response
to a short ~2 second burst of a very high heating rate from nonthermal
electrons using a solar-type heating function, and we propose a new
“M dwarf-type” heating variation that explains a range of observed
spectral properties, such as ~10,000 K blackbody emission and a smooth
continuum across the Balmer jump wavelength (3646A).
---------------------------------------------------------
Title: Comparison between IRIS Data and Numerical Models
Authors: Carlsson, Mats; De Pontieu, Bart; Hansteen, Viggo; Pereira,
Tiago; Leenaarts, Jorritt
2014cosp...40E.458C Altcode:
The enigmatic chromosphere is the transition between the solar surface
and the eruptive outer solar atmosphere. The chromosphere harbours
and constrains the mass and energy loading processes that define the
heating of the corona, the acceleration and the composition of the solar
wind, and the energetics and triggering of solar outbursts (filament
eruptions, flares, coronal mass ejections). The chromosphere is arguably
the most difficult and least understood domain of solar physics. All
at once it represents the transition from optically thick to thin
radiation escape, from gas-pressure domination to magnetic-pressure
domination, from neutral to ionised state, from MHD to plasma physics,
and from near-equilibrium ("LTE") to non-equilibrium conditions. IRIS
provides a leap in observational capability of the chromospheric
plasma with an unprecedented combination of high spatial, temporal
and spectral resolution in lines with diagnostic information all the
way from the photosphere to the upper transition region. To fully
extract this information it is necessary to combine the observations
with numerical simulations that include a realistic description of the
complicated physics of the chromosphere. In this talk, we will present
such realistic simulations, spanning the solar atmosphere from the
convection zone to the corona, and synthetic observations calculated
from the simulations. These synthetic observations are compared with
observations from IRIS.
---------------------------------------------------------
Title: Impact of the Partial Ionization in the solar atmosphere
using 2.5D Radiative MHD Simulations
Authors: Martinez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo;
Carlsson, Mats
2014cosp...40E2019M Altcode:
The chromosphere/transition region constitute the interface between
the solar surface and the corona and modulate the flow of mass and
energy into the upper atmosphere. IRIS was launched in 2013 to study the
chromosphere and transition region. The complexity of the chromosphere
is due to various regime changes that take place across it, like:
Hydrogen goes from predominantly neutral to predominantly ionized;
the plasma behavior changes from collisional to collision-less; it goes
from gas-pressure dominated to magnetically driven, etc. Consequently,
the interpretation of chromospheric observations in general and those
from IRIS, in particular, is a challenging task. It is thus crucial
to combine IRIS observations with advanced radiative-MHD numerical
modeling. Because the photosphere, chromosphere and transition region
are partially ionized, the interaction between ionized and neutral
particles has important consequences on the magneto-thermodynamics
of these regions. We implemented the effects of partial ionization
using generalized Ohm's law in the Bifrost code (Gudiksen et al. 2011)
which includes full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along magnetic field lines. I will
describe the importance and impact of taking into account partial
ionization effects in the modeled radiative-MHD atmosphere, such as
chromospheric heating, photospheric magnetic field diffused into the
upper-chromosphere which expands into the upper atmosphere filling
the corona with mass, magnetic flux, energy and current, etc.
---------------------------------------------------------
Title: Effects of flux emergence in the outer solar
atmosphere. Observational advances
Authors: Ortiz Carbonell, Ada; De Pontieu, Bart; Bellot Rubio, L. R.;
Hansteen, Viggo; Rouppe van der Voort, Luc; Carlsson, Mats
2014cosp...40E2387O Altcode:
We study granular sized magnetic flux emergence events that occur in
a flux emergence region in NOAA 11850 on September 25, 2013. During
that time, the first co-observing campaign between the Swedish 1 m
Solar Telescope and the IRIS spacecraft was carried out. Simultaneous
observations of the Halpha 656.28 nm and Ca II 854.2 nm chromospheric
lines, and the Fe I 630.25 nm photospheric line, were made with
the CRISP/SST spectropolarimeter reaching a spatial resolution of
0."14. At the same time, IRIS was performing a four-step dense raster
of the said emerging flux region, taking slit-jaw images at 133 (C II
transiti on region), 140 (Si IV, transition region), 279.6 (Mg II k,
core, upper chromosphere), and 283.2 nm (Mg II k, wing, photosphere),
obtaining thus the highest resolution images ever taken of the upper
chromosphere and transition region. The photospheric and chromospheric
properties of the emerging magnetic flux bubbles have been described
in detail in Ortiz et al. (2014). However, in the current work we are
able to follow such lower atmosphere observations of flux emergence
up to the transition region with unprecedented spatial and temporal
resolution. We describe the properties (size, time delays, lifetime,
velocities, temperature) of the observed signatures of flux emergence
in the transition region. We believe this may be an important mechanism
of transporting energy and magnetic flux to the upper layers of the
solar atmosphere, namely the transition region and corona, at least
in cases when active regions are formed by flux emerging through the
photosphere. * Ortiz et al. (2014) ApJ 781, 126
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. III. Near-ultraviolet
Spectra and Images
Authors: Pereira, T. M. D.; Leenaarts, J.; De Pontieu, B.; Carlsson,
M.; Uitenbroek, H.
2013ApJ...778..143P Altcode: 2013arXiv1310.1926P
The Mg II h&k lines are the prime chromospheric diagnostics
of NASA's Interface Region Imaging Spectrograph (IRIS). In the
previous papers of this series, we used a realistic three-dimensional
radiative magnetohydrodynamics model to calculate the h&k lines
in detail and investigated how their spectral features relate to the
underlying atmosphere. In this work, we employ the same approach to
investigate how the h&k diagnostics fare when taking into account
the finite resolution of IRIS and different noise levels. In addition,
we investigate the diagnostic potential of several other photospheric
lines and near-continuum regions present in the near-ultraviolet
(NUV) window of IRIS and study the formation of the NUV slit-jaw
images. We find that the instrumental resolution of IRIS has a small
effect on the quality of the h&k diagnostics; the relations
between the spectral features and atmospheric properties are mostly
unchanged. The peak separation is the most affected diagnostic, but
mainly due to limitations of the simulation. The effects of noise
start to be noticeable at a signal-to-noise ratio (S/N) of 20, but we
show that with noise filtering one can obtain reliable diagnostics at
least down to a S/N of 5. The many photospheric lines present in the
NUV window provide velocity information for at least eight distinct
photospheric heights. Using line-free regions in the h&k far wings,
we derive good estimates of photospheric temperature for at least
three heights. Both of these diagnostics, in particular the latter,
can be obtained even at S/Ns as low as 5.
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. II. The Formation of the
Mg II h&k Lines in the Solar Atmosphere
Authors: Leenaarts, J.; Pereira, T. M. D.; Carlsson, M.; Uitenbroek,
H.; De Pontieu, B.
2013ApJ...772...90L Altcode: 2013arXiv1306.0671L
NASA's Interface Region Imaging Spectrograph (IRIS) small explorer
mission will study how the solar atmosphere is energized. IRIS
contains an imaging spectrograph that covers the Mg II h&k lines
as well as a slit-jaw imager centered at Mg II k. Understanding
the observations requires forward modeling of Mg II h&k line
formation from three-dimensional (3D) radiation-magnetohydrodynamic
(RMHD) models. This paper is the second in a series where we undertake
this modeling. We compute the vertically emergent h&k intensity
from a snapshot of a dynamic 3D RMHD model of the solar atmosphere,
and investigate which diagnostic information about the atmosphere is
contained in the synthetic line profiles. We find that the Doppler
shift of the central line depression correlates strongly with the
vertical velocity at optical depth unity, which is typically located
less than 200 km below the transition region (TR). By combining the
Doppler shifts of the h and k lines we can retrieve the sign of the
velocity gradient just below the TR. The intensity in the central line
depression is anti-correlated with the formation height, especially
in subfields of a few square Mm. This intensity could thus be used to
measure the spatial variation of the height of the TR. The intensity
in the line-core emission peaks correlates with the temperature at
its formation height, especially for strong emission peaks. The peaks
can thus be exploited as a temperature diagnostic. The wavelength
difference between the blue and red peaks provides a diagnostic of the
velocity gradients in the upper chromosphere. The intensity ratio of
the blue and red peaks correlates strongly with the average velocity
in the upper chromosphere. We conclude that the Mg II h&k lines
are excellent probes of the very upper chromosphere just below the
TR, a height regime that is impossible to probe with other spectral
lines. They also provide decent temperature and velocity diagnostics
of the middle chromosphere.
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. I. A Quintessential
Model Atom of Mg II and General Formation Properties of the Mg II
h&k Lines
Authors: Leenaarts, J.; Pereira, T. M. D.; Carlsson, M.; Uitenbroek,
H.; De Pontieu, B.
2013ApJ...772...89L Altcode: 2013arXiv1306.0668L
NASA's Interface Region Imaging Spectrograph (IRIS) space mission will
study how the solar atmosphere is energized. IRIS contains an imaging
spectrograph that covers the Mg II h&k lines as well as a slit-jaw
imager centered at Mg II k. Understanding the observations will require
forward modeling of Mg II h&k line formation from three-dimensional
(3D) radiation-MHD models. This paper is the first in a series where we
undertake this forward modeling. We discuss the atomic physics pertinent
to h&k line formation, present a quintessential model atom that can
be used in radiative transfer computations, and discuss the effect of
partial redistribution (PRD) and 3D radiative transfer on the emergent
line profiles. We conclude that Mg II h&k can be modeled accurately
with a four-level plus continuum Mg II model atom. Ideally radiative
transfer computations should be done in 3D including PRD effects. In
practice this is currently not possible. A reasonable compromise is to
use one-dimensional PRD computations to model the line profile up to
and including the central emission peaks, and use 3D transfer assuming
complete redistribution to model the central depression.
---------------------------------------------------------
Title: Chromospheric Lyman Alpha SpectroPolarimeter: CLASP
Authors: Kobayashi, Ken; Kano, R.; Trujillo Bueno, J.; Winebarger,
A. R.; Cirtain, J. W.; Bando, T.; De Pontieu, B.; Ishikawa, R.;
Katsukawa, Y.; Kubo, M.; Narukage, N.; Sakao, T.; Tsuneta, S.;
Auchère, F.; Asensio Ramos, A.; Belluzzi, L.; Carlsson, M.; Casini,
R.; Hara, H.; Ichimoto, K.; Manso Sainz, R.; Shimizu, T.; Stepan,
J.; Suematsu, Y.; Holloway, T.
2013SPD....44..142K Altcode:
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a VUV
spectropolarimeter optimized for measuring the linear polarization of
the Lyman-alpha line (121.6 nm). The Lyman-alpha line is predicted to
show linear polarization caused by atomic scattering in the chromosphere
and modified by the magnetic field through the Hanle effect. The
Hanle effect is sensitive to weaker magnetic fields than Zeeman
effect, and is not canceled by opposing fields, making it sensitive
to tangled or unresolved magnetic field structures. These factors make
the Hanle effect a valuable tool for probing the magnetic field in the
chromosphere above the quiet sun. To meet this goal, CLASP is designed
to measure linear polarization with 0.1% polarization sensitivity
at 0.01 nm spectral resolution and 10" spatial resolution. CLASP is
scheduled to be launched in 2015.
---------------------------------------------------------
Title: A Detailed Comparison between the Observed and Synthesized
Properties of a Simulated Type II Spicule
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Leenaarts, Jorrit;
Pereira, Tiago M. D.; Carlsson, Mats; Hansteen, Viggo; Stern, Julie
V.; Tian, Hui; McIntosh, Scott W.; Rouppe van der Voort, Luc
2013ApJ...771...66M Altcode: 2013arXiv1305.2397M
We have performed a three-dimensional radiative MHD simulation of the
solar atmosphere. This simulation shows a jet-like feature that shows
similarities to the type II spicules observed for the first time with
Hinode's Solar Optical Telescope. Rapid blueshifted events (RBEs) on the
solar disk are associated with these spicules. Observational results
suggest they may contribute significantly in supplying the corona
with hot plasma. We perform a detailed comparison of the properties
of the simulated jet with those of type II spicules (observed with
Hinode) and RBEs (with ground-based instruments). We analyze a wide
variety of synthetic emission and absorption lines from the simulations
including chromospheric (Ca II 8542 Å, Ca II H, and Hα) to transition
region and coronal temperatures (10,000 K to several million K). We
compare their synthetic intensities, line profiles, Doppler shifts,
line widths, and asymmetries with observations from Hinode/SOT and
EIS, SOHO/SUMER, the Swedish 1 m Solar Telescope, and SDO/AIA. Many
properties of the synthetic observables resemble the observations,
and we describe in detail the physical processes that lead to these
observables. Detailed analysis of the synthetic observables provides
insight into how observations should be analyzed to derive information
about physical variables in such a dynamic event. For example, we
find that line-of-sight superposition in the optically thin atmosphere
requires the combination of Doppler shifts and spectral line asymmetry
to determine the velocity in the jet. In our simulated type II spicule,
the lifetime of the asymmetry of the transition region lines is shorter
than that of the coronal lines. Other properties differ from the
observations, especially in the chromospheric lines. The mass density
of the part of the spicule with a chromospheric temperature is too low
to produce significant opacity in chromospheric lines. The synthetic
Ca II 8542 Å and Hα profiles therefore do not show signal resembling
RBEs. These and other discrepancies are described in detail, and we
discuss which mechanisms and physical processes may need to be included
in the MHD simulations to mimic the thermodynamic processes of the
chromosphere and corona, in particular to reproduce type II spicules.
---------------------------------------------------------
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, Bart; Title, A. M.; Lemen, J.; Wuelser, J.;
Tarbell, T. D.; Schrijver, C. J.; Golub, L.; Kankelborg, C.; Carlsson,
M.; Hansteen, V. H.; Worden, S.; IRIS Team
2013SPD....44...03D Altcode:
The solar chromosphere and transition region (TR) form a highly
structured and dynamic interface region between the photosphere and
the corona. This region not only acts as the conduit of all mass and
energy feeding into the corona and solar wind, it also requires an
order of magnitude more energy to heat than the corona. Nevertheless,
the chromosphere remains poorly understood, because of the complexity
of the required observational and analytical tools: the interface
region is highly complex with transitions from optically thick to
optically thin radiation, from pressure to magnetic field domination,
and large density and temperature contrasts on small spatial scales. The
Interface Region Imaging Spectrograph (IRIS) was selected for a NASA
SMEX mission in 2009 and is scheduled to launch on 26-June-2013 (with
first light scheduled for mid July). IRIS addresses critical questions:
(1) Which types of non-thermal energy dominate in the chromosphere and
beyond? (2) How does the chromosphere regulate mass and energy supply
to the corona and heliosphere? (3) How do magnetic flux and matter
rise through the lower atmosphere, and what role does flux emergence
play in flares and mass ejections? These questions are addressed with
a high-resolution near and far UV imaging spectrometer sensitive to
emission from plasma at temperatures between 5,000 K and 10 MK. IRIS
has a field-of-view of 120 arcsec, a spatial resolution of 0.4 arcsec,
and velocity resolution of 0.5 km/s. The IRIS investigation includes
a strong numerical modeling component based on advanced radiative MHD
codes to facilitate interpretation of observations. We describe the
IRIS instrumentation and numerical modeling, and present the plans for
observations, calibration and data distribution. We will highlight some
of the issues that IRIS observations can help resolve. More information
can be found at http://iris.lmsal.com
---------------------------------------------------------
Title: A new approach to model particle acceleration and energy
transfer in solar flares
Authors: Rubio Da Costa, Fatima; Zuccarello, F.; Fletcher, L.;
Labrosse, N.; Kasparova, J.; Prosecký, T.; Carlsson, M.; Petrosian,
V.; Liu, W.
2013SPD....4440401R Altcode:
Motivated by available observations of two different flares in Lyα and
Hα, we model the conditions of the solar atmosphere using a radiation
hydrodynamics code (RADYN, Carlsson & Stein, 1992) and analyze the
energy transport carried by a beam of non-thermal electrons injected
at the top of a 1D coronal loop. The numerical Lyα and Hα intensities
match with the observations. The electron energy distribution is assumed
to follow a power law of the form (E/E<SUP>c</SUP> )<SUB>-δ</SUB> for
energies greater than a cutoff value of E<SUP>c</SUP>. Abbett &
Hawley (1999) and Allred et al. (2005) assumed that the non-thermal
electrons flux injected at the top of a flaring loop, the cut-off energy
and the power law index are constant over time. An improvement was
achieved by Allred & Hawley (2006), who modified the RADYN code
in such a way that the input parameters were time dependent. Their
inputs were based on observations of a flare obtained with RHESSI. By
combining RADYN with the “flare” code from Stanford University
which models the acceleration and transport of particles and radiation
of solar flares in non-LTE regime, we can calculate the non-thermal
electrons flux, the cut-off energy and the power law index at every
simulated time step. The atmospheric parameters calculated by RADYN
could in turn be used as updated inputs for "flare", providing several
advantages over the results from Liu et al. (2009), who combined the
particle acceleration code with a 1-D hydrodynamic code, improving
the atmospheric conditions.
---------------------------------------------------------
Title: Numerical Simulations of Spicule Acceleration
Authors: Guerreiro, N.; Carlsson, M.; Hansteen, V.
2013ApJ...766..128G Altcode:
Observations in the Hα line of hydrogen and the H and K lines of singly
ionized calcium on the solar limb reveal the existence of structures
with jet-like behavior, usually designated as spicules. The driving
mechanism for such structures remains poorly understood. Sterling
et al. shed some light on the problem mimicking reconnection events
in the chromosphere with a one-dimensional code by injecting energy
with different spatial and temporal distributions and tracing the
thermodynamic evolution of the upper chromospheric plasma. They found
three different classes of jets resulting from these injections. We
follow their approach but improve the physical description by including
non-LTE cooling in strong spectral lines and non-equilibrium hydrogen
ionization. Increased cooling and conversion of injected energy into
hydrogen ionization energy instead of thermal energy both lead to weaker
jets and smaller final extent of the spicules compared with Sterling
et al. In our simulations we find different behavior depending on
the timescale for hydrogen ionization/recombination. Radiation-driven
ionization fronts also form.
---------------------------------------------------------
Title: Heating of the Magnetic Chromosphere: Observational Constraints
from Ca II λ8542 Spectra
Authors: de la Cruz Rodríguez, J.; De Pontieu, B.; Carlsson, M.;
Rouppe van der Voort, L. H. M.
2013ApJ...764L..11D Altcode: 2013arXiv1301.3141D
The heating of the Sun's chromosphere remains poorly understood. While
progress has been made on understanding what drives the quiet-Sun
internetwork chromosphere, chromospheric heating in strong magnetic
field regions continues to present a difficult challenge, mostly
because of a lack of observational constraints. We use high-resolution
spectropolarimetric data from the Swedish 1 m Solar Telescope to
identify the location and spatio-temporal properties of heating in
the magnetic chromosphere. In particular, we report the existence of
raised-core spectral line profiles in the Ca II λ8542 line. These
profiles are characterized by the absence of an absorption line
core, showing a quasi-flat profile between λ ≈ ±0.5 Å, and are
abundant close to magnetic bright points and plage. Comparison with
three-dimensional MHD simulations indicates that such profiles occur
when the line of sight goes through an "elevated temperature canopy"
associated with the expansion with height of the magnetic field of
flux concentrations. This temperature canopy in the simulations is
caused by ohmic dissipation where there are strong magnetic field
gradients. The raised-core profiles are thus indicators of locations
of increased chromospheric heating. We characterize the location and
temporal and spatial properties of such profiles in our observations,
thus providing much stricter constraints on theoretical models of
chromospheric heating mechanisms than before.
---------------------------------------------------------
Title: The Effects of Spatio-temporal Resolution on Deduced Spicule
Properties
Authors: Pereira, Tiago M. D.; De Pontieu, Bart; Carlsson, Mats
2013ApJ...764...69P Altcode: 2012arXiv1212.2969P
Spicules have been observed on the Sun for more than a century,
typically in chromospheric lines such as Hα and Ca II H. Recent
work has shown that so-called "type II" spicules may have a role in
providing mass to the corona and the solar wind. In chromospheric
filtergrams these spicules are not seen to fall back down, and they
are shorter lived and more dynamic than the spicules that have been
classically reported in ground-based observations. Observations of
type II spicules with Hinode show fundamentally different properties
from what was previously measured. In earlier work we showed that these
dynamic type II spicules are the most common type, a view that was not
properly identified by early observations. The aim of this work is to
investigate the effects of spatio-temporal resolution in the classical
spicule measurements. Making use of Hinode data degraded to match
the observing conditions of older ground-based studies, we measure
the properties of spicules with a semi-automated algorithm. These
results are then compared to measurements using the original Hinode
data. We find that degrading the data has a significant effect on the
measured properties of spicules. Most importantly, the results from
the degraded data agree well with older studies (e.g., mean spicule
duration more than 5 minutes, and upward apparent velocities of about
25 km s<SUP>-1</SUP>). These results illustrate how the combination
of spicule superposition, low spatial resolution and cadence affect
the measured properties of spicules, and that previous measurements
can be misleading.
---------------------------------------------------------
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Wuelser, J.;
Tarbell, T. D.; Schrijver, C.; Golub, L.; Kankelborg, C. C.; Hansteen,
V. H.; Carlsson, M.
2012AGUFMSH33D2256D Altcode:
The solar chromosphere and transition region (TR) form a highly
structured and dynamic interface region between the photosphere and
the corona. This region not only acts as the conduit of all mass and
energy feeding into the corona and solar wind, it also requires an
order of magnitude more energy to heat than the corona. Nevertheless,
the chromosphere remains poorly understood, because of the complexity
of the required observational and analytical tools: the interface
region is highly complex with transitions from optically thick to
optically thin radiation, from pressure to magnetic field domination,
and large density and temperature contrasts on small spatial scales. The
Interface Region Imaging Spectrograph (IRIS) was selected for a NASA
SMEX mission in 2009 and is scheduled to launch in early 2013. IRIS
addresses critical questions: (1) Which types of non-thermal energy
dominate in the chromosphere and beyond? (2) How does the chromosphere
regulate mass and energy supply to the corona and heliosphere? (3)
How do magnetic flux and matter rise through the lower atmosphere, and
what role does flux emergence play in flares and mass ejections? These
questions are addressed with a high-resolution near and far UV imaging
spectrometer sensitive to emission from plasma at temperatures between
5,000 K and 10 MK. IRIS has a field-of-view of 120 arcsec, a spatial
resolution of 0.4 arcsec, and velocity resolution of 0.5 km/s. The
IRIS investigation includes a strong numerical modeling component
based on advanced radiative MHD codes to facilitate interpretation of
observations. We will describe the IRIS instrumentation and numerical
modeling, and present the status of the IRIS observatory development. We
will highlight some of the issues that IRIS observations can help
resolve.
---------------------------------------------------------
Title: Chromospheric Magnetic Fields: Observations, Simulations and
their Interpretation
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; Carlsson, M.;
Leenaarts, J.
2012ASPC..463...15D Altcode: 2012arXiv1203.4577D
The magnetic field of the quiet-Sun chromosphere remains a mystery
for solar physicists. The reduced number of chromospheric lines are
intrinsically hard to model and only a few of them are magnetically
sensitive. In this work, we use a 3D numerical simulation of the outer
layers of the solar atmosphere, to asses the reliability of non-LTE
inversions, in this case applied to the Ca II λ8542 Å line. We show
that NLTE inversions provide realistic estimates of physical quantities
from synthetic observations.
---------------------------------------------------------
Title: 2nd ATST-EAST Workshop in Solar Physics: Magnetic Fields from
the Photosphere to the Corona
Authors: Rimmele, T. R.; Tritschler, A.; Wöger, F.; Collados Vera,
M.; Socas-Navarro, H.; Schlichenmaier, R.; Carlsson, M.; Berger, T.;
Cadavid, A.; Gilbert, P. R.; Goode, P. R.; Knölker, M.
2012ASPC..463.....R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Quantifying Spicules
Authors: Pereira, Tiago M. D.; De Pontieu, Bart; Carlsson, Mats
2012ApJ...759...18P Altcode: 2012arXiv1208.4404P
Understanding the dynamic solar chromosphere is fundamental in solar
physics. Spicules are an important feature of the chromosphere,
connecting the photosphere to the corona, potentially mediating
the transfer of energy and mass. The aim of this work is to study
the properties of spicules over different regions of the Sun. Our
goal is to investigate if there is more than one type of spicule,
and how spicules behave in the quiet Sun, coronal holes, and active
regions. We make use of high cadence and high spatial resolution Ca
II H observations taken by Hinode/Solar Optical Telescope. Making use
of a semi-automated detection algorithm, we self-consistently track
and measure the properties of 519 spicules over different regions. We
find clear evidence of two types of spicules. Type I spicules show
a rise and fall and have typical lifetimes of 150-400 s and maximum
ascending velocities of 15-40 km s<SUP>-1</SUP>, while type II spicules
have shorter lifetimes of 50-150 s, faster velocities of 30-110 km
s<SUP>-1</SUP>, and are not seen to fall down, but rather fade at
around their maximum length. Type II spicules are the most common, seen
in the quiet Sun and coronal holes. Type I spicules are seen mostly
in active regions. There are regional differences between quiet-Sun
and coronal hole spicules, likely attributable to the different field
configurations. The properties of type II spicules are consistent with
published results of rapid blueshifted events (RBEs), supporting the
hypothesis that RBEs are their disk counterparts. For type I spicules
we find the relations between their properties to be consistent with
a magnetoacoustic shock wave driver, and with dynamic fibrils as their
disk counterpart. The driver of type II spicules remains unclear from
limb observations.
---------------------------------------------------------
Title: Investigating the Reliability of Coronal Emission Measure
Distribution Diagnostics using Three-dimensional Radiative
Magnetohydrodynamic Simulations
Authors: Testa, Paola; De Pontieu, Bart; Martínez-Sykora, Juan;
Hansteen, Viggo; Carlsson, Mats
2012ApJ...758...54T Altcode: 2012arXiv1208.4286T
Determining the temperature distribution of coronal plasmas can provide
stringent constraints on coronal heating. Current observations with
the Extreme ultraviolet Imaging Spectrograph (EIS) on board Hinode
and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory provide diagnostics of the emission measure distribution
(EMD) of the coronal plasma. Here we test the reliability of temperature
diagnostics using three-dimensional radiative MHD simulations. We
produce synthetic observables from the models and apply the Monte
Carlo Markov chain EMD diagnostic. By comparing the derived EMDs with
the "true" distributions from the model, we assess the limitations
of the diagnostics as a function of the plasma parameters and the
signal-to-noise ratio of the data. We find that EMDs derived from
EIS synthetic data reproduce some general characteristics of the true
distributions, but usually show differences from the true EMDs that
are much larger than the estimated uncertainties suggest, especially
when structures with significantly different density overlap along
the line of sight. When using AIA synthetic data the derived EMDs
reproduce the true EMDs much less accurately, especially for broad
EMDs. The differences between the two instruments are due to the:
(1) smaller number of constraints provided by AIA data and (2) broad
temperature response function of the AIA channels which provide looser
constraints to the temperature distribution. Our results suggest that
EMDs derived from current observatories may often show significant
discrepancies from the true EMDs, rendering their interpretation
fraught with uncertainty. These inherent limitations to the method
should be carefully considered when using these distributions to
constrain coronal heating.
---------------------------------------------------------
Title: The Hanle Effect of Lyα in a Magnetohydrodynamic Model of
the Solar Transition Region
Authors: Štěpán, J.; Trujillo Bueno, J.; Carlsson, M.; Leenaarts, J.
2012ApJ...758L..43S Altcode: 2012arXiv1208.4929S
In order to understand the heating of the solar corona it is crucial
to obtain empirical information on the magnetic field in its lower
boundary (the transition region). To this end, we need to measure
and model the linear polarization produced by scattering processes in
strong UV lines, such as the hydrogen Lyα line. The interpretation
of the observed Stokes profiles will require taking into account that
the outer solar atmosphere is highly structured and dynamic, and that
the height of the transition region may well vary from one place in
the atmosphere to another. Here, we report on the Lyα scattering
polarization signals we have calculated in a realistic model of an
enhanced network region, resulting from a state-of-the-art radiation
magnetohydrodynamic simulation. This model is characterized by spatially
complex variations of the physical quantities at transition region
heights. The results of our investigation lead us to emphasize that
scattering processes in the upper solar chromosphere should indeed
produce measurable linear polarization in Lyα. More importantly,
we show that via the Hanle effect the model's magnetic field produces
significant changes in the emergent Q/I and U/I profiles. Therefore, we
argue that by measuring the polarization signals produced by scattering
processes and the Hanle effect in Lyα and contrasting them with those
computed in increasingly realistic atmospheric models, we should be
able to decipher the magnetic, thermal, and dynamic structure of the
upper chromosphere and transition region of the Sun.
---------------------------------------------------------
Title: Chromospheric Lyman-alpha spectro-polarimeter (CLASP)
Authors: Kano, Ryouhei; Bando, Takamasa; Narukage, Noriyuki; Ishikawa,
Ryoko; Tsuneta, Saku; Katsukawa, Yukio; Kubo, Masahito; Ishikawa,
Shin-nosuke; Hara, Hirohisa; Shimizu, Toshifumi; Suematsu, Yoshinori;
Ichimoto, Kiyoshi; Sakao, Taro; Goto, Motoshi; Kato, Yoshiaki; Imada,
Shinsuke; Kobayashi, Ken; Holloway, Todd; Winebarger, Amy; Cirtain,
Jonathan; De Pontieu, Bart; Casini, Roberto; Trujillo Bueno, Javier;
Štepán, Jiří; Manso Sainz, Rafael; Belluzzi, Luca; Asensio Ramos,
Andres; Auchère, Frédéric; Carlsson, Mats
2012SPIE.8443E..4FK Altcode:
One of the biggest challenges in heliophysics is to decipher the
magnetic structure of the solar chromosphere. The importance of
measuring the chromospheric magnetic field is due to both the key role
the chromosphere plays in energizing and structuring the outer solar
atmosphere and the inability of extrapolation of photospheric fields to
adequately describe this key boundary region. Over the last few years,
significant progress has been made in the spectral line formation
of UV lines as well as the MHD modeling of the solar atmosphere. It
is found that the Hanle effect in the Lyman-alpha line (121.567 nm)
is a most promising diagnostic tool for weaker magnetic fields in
the chromosphere and transition region. Based on this groundbreaking
research, we propose the Chromospheric Lyman-Alpha Spectro-Polarimeter
(CLASP) to NASA as a sounding rocket experiment, for making the first
measurement of the linear polarization produced by scattering processes
and the Hanle effect in the Lyman-alpha line (121.567 nm), and making
the first exploration of the magnetic field in the upper chromosphere
and transition region of the Sun. The CLASP instrument consists
of a Cassegrain telescope, a rotating 1/2-wave plate, a dual-beam
spectrograph assembly with a grating working as a beam splitter, and
an identical pair of reflective polarization analyzers each equipped
with a CCD camera. We propose to launch CLASP in December 2014.
---------------------------------------------------------
Title: Implications for Coronal Heating from Coronal Rain
Authors: Antolin, P.; Shibata, K.; Carlsson, M.; Rouppe van der Voort,
L.; Vissers, G.; Hansteen, V.
2012ASPC..454..171A Altcode:
Coronal rain is a phenomenon above active regions in which cool plasma
condensations fall down from coronal heights. Numerical simulations of
loops have shown that such condensations can naturally form in the case
of footpoint concentrated heating through the “catastrophic cooling”
mechanism. In this work we analize high resolution limb observations in
Ca II H and Hα of coronal rain performed by Hinode/SOT and by Crisp of
SST and derive statistical properties. We further investigate the link
between coronal rain and the coronal heating mechanisms by performing
1.5-D MHD simulations of a loop subject to footpoint heating and to
Alfvén waves generated in the photosphere. It is found that if a loop
is heated predominantly from Alfvén waves coronal rain is inhibited
due to the characteristic uniform heating they produce. Hence coronal
rain can point both to the spatial distribution of the heating and to
the agent of the heating itself, thus acting as a marker for coronal
heating mechanisms.
---------------------------------------------------------
Title: Non-local thermodynamic equilibrium inversions from a 3D
magnetohydrodynamic chromospheric model
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; Carlsson, M.;
Leenaarts, J.
2012A&A...543A..34D Altcode: 2012arXiv1205.3171D
Context. The structure of the solar chromosphere is believed to
be governed by magnetic fields, even in quiet-Sun regions that
have a relatively weak photospheric field. During the past decade
inversion methods have emerged as powerful tools for analyzing the
chromosphere of active regions. The applicability of inversions to
infer the stratification of the physical conditions in a dynamic 3D
solar chromosphere has not yet been studied in detail. <BR /> Aims:
This study aims to establish the diagnostic capabilities of non-local
thermodynamical equilibrium (NLTE) inversion techniques of Stokes
profiles induced by the Zeeman effect in the Ca ii λ8542 Å line. <BR
/> Methods: We computed the Ca ii atomic level populations in a snapshot
from a 3D radiation-MHD simulation of the quiet solar atmosphere in
non-LTE using the 3D radiative transfer code Multi3d. These populations
were used to compute synthetic full-Stokes profiles in the Ca ii
λ8542 Å line using 1.5D radiative transfer and the inversion code
Nicole. The profiles were then spectrally degraded to account for
finite filter width, and Gaussian noise was added to account for
finite photon flux. These profiles were inverted using Nicole and
the results were compared with the original model atmosphere. <BR />
Results: Our NLTE inversions applied to quiet-Sun synthetic observations
provide reasonably good estimates of the chromospheric magnetic field,
line-of-sight velocities and somewhat less accurate, but still very
useful, estimates of the temperature. Three-dimensional scattering
of photons cause cool pockets in the chromosphere to be invisible in
the line profile and consequently they are also not recovered by the
inversions. To successfully detect Stokes linear polarization in this
quiet snapshot, a noise level below 10<SUP>-3.5</SUP> is necessary.
---------------------------------------------------------
Title: Ubiquitous Torsional Motions in Type II Spicules
Authors: De Pontieu, B.; Carlsson, M.; Rouppe van der Voort, L. H. M.;
Rutten, R. J.; Hansteen, V. H.; Watanabe, H.
2012ApJ...752L..12D Altcode: 2012arXiv1205.5006D
Spicules are long, thin, highly dynamic features that jut out
ubiquitously from the solar limb. They dominate the interface between
the chromosphere and corona and may provide significant mass and energy
to the corona. We use high-quality observations with the Swedish 1
m Solar Telescope to establish that so-called type II spicules are
characterized by the simultaneous action of three different types of
motion: (1) field-aligned flows of order 50-100 km s<SUP>-1</SUP>,
(2) swaying motions of order 15-20 km s<SUP>-1</SUP>, and (3) torsional
motions of order 25-30 km s<SUP>-1</SUP>. The first two modes have been
studied in detail before, but not the torsional motions. Our analysis
of many near-limb and off-limb spectra and narrowband images using
multiple spectral lines yields strong evidence that most, if not all,
type II spicules undergo large torsional modulation and that these
motions, like spicule swaying, represent Alfvénic waves propagating
outward at several hundred km s<SUP>-1</SUP>. The combined action
of the different motions explains the similar morphology of spicule
bushes in the outer red and blue wings of chromospheric lines, and
needs to be taken into account when interpreting Doppler motions to
derive estimates for field-aligned flows in spicules and determining
the Alfvénic wave energy in the solar atmosphere. Our results also
suggest that large torsional motion is an ingredient in the production
of type II spicules and that spicules play an important role in the
transport of helicity through the solar atmosphere.
---------------------------------------------------------
Title: A Sharp Look at Coronal Rain with Hinode/SOT and SST/CRISP
Authors: Antolin, P.; Carlsson, M.; Rouppe van der Voort, L.;
Verwichte, E.; Vissers, G.
2012ASPC..455..253A Altcode: 2012arXiv1202.0787A
The tropical wisdom that when it is hot and dense we can expect
rain might also apply to the Sun. Indeed, observations and numerical
simulations have showed that strong heating at footpoints of loops,
as is the case for active regions, puts their coronae out of thermal
equilibrium, which can lead to a phenomenon known as catastrophic
cooling. Following local pressure loss in the corona, hot plasma
locally condenses in these loops and dramatically cools down to
chromospheric temperatures. These blobs become bright in Hα and
Ca ii H in time scales of minutes, and their dynamics seem to be
subject more to internal pressure changes in the loop rather than to
gravity. They thus become trackers of the magnetic field, which results
in the spectacular coronal rain that is observed falling down coronal
loops. In this work we report on high resolution observations of coronal
rain with the Solar Optical Telescope (SOT) on Hinode and CRISP at
the Swedish Solar Telescope (SST). A statistical study is performed in
which properties such as velocities and accelerations of coronal rain
are derived. We show how this phenomenon can constitute a diagnostic
tool for the internal physical conditions inside loops. Furthermore, we
analyze transverse oscillations of strand-like condensations composing
coronal rain falling in a loop, and discuss the possible nature of the
wave. This points to the important role that coronal rain can play in
the fields of coronal heating and coronal seismology.
---------------------------------------------------------
Title: 4th Hinode Science Meeting: Unsolved Problems and Recent
Insights
Authors: Bellot Rubio, L.; Reale, F.; Carlsson, M.
2012ASPC..455.....B Altcode: 2012ASPC..455.....R
No abstract at ADS
---------------------------------------------------------
Title: The Chromospheric Lyman-Alpha SpectroPolarimeter: CLASP
Authors: Kobayashi, K.; Kano, R.; Trujillo-Bueno, J.; Asensio Ramos,
A.; Bando, T.; Belluzzi, L.; Carlsson, M.; De Pontieu, R. C. B.; Hara,
H.; Ichimoto, K.; Ishikawa, R.; Katsukawa, Y.; Kubo, M.; Manso Sainz,
R.; Narukage, N.; Sakao, T.; Stepan, J.; Suematsu, Y.; Tsuneta, S.;
Watanabe, H.; Winebarger, A.
2012ASPC..456..233K Altcode:
The magnetic field plays a crucial role in the chromosphere and the
transition region, and our poor empirical knowledge of the magnetic
field in the upper chromosphere and transition region is a major
impediment to advancing the understanding of the solar atmosphere. The
Hanle effect promises to be a valuable alternative to Zeeman effect
as a method of measuring the magnetic field in the chromosphere and
transition region; it is sensitive to weaker magnetic fields, and
also sensitive to tangled, unresolved field structures. <P />CLASP
is a sounding rocket experiment that aims to observe the Hanle effect
polarization of the Lyman α (1215.67Å) line in the solar chromosphere
and transition region, and prove the usefulness of this technique in
placing constraints on the magnetic field strength and orientation
in the low plasma-β region of the solar atmosphere. The Ly-α line
has been chosen because it is a chromospheric/transition-region line,
and because the Hanle effect polarization of this line is predicted to
be sensitive to 10-250 Gauss, encompassing the range of interest. The
CLASP instrument is designed to measure linear polarization in the
Ly-α line with a polarization sensitivity of 0.1%. The instrument is
currently funded for development. The optical design of the instrument
has been finalized, and an extensive series of component-level tests
are underway to validate the design.
---------------------------------------------------------
Title: How Low-Quality Observations Affect Spicule Properties
Authors: Pereira, Tiago M. D.; De Pontieu, B.; Carlsson, M.
2012AAS...22020306P Altcode:
Spicules have been observed on the sun for more than 80 years, in
several chromospheric lines such as H-alpha and Ca II H. Recent work
has shown that spicules have the potential to drive the solar wind
and heat the chromosphere, making them a hotly contested topic in
solar physics. Despite the wealth of observations available, their
properties are still a matter of debate. Difficulties in measuring
their properties arise because spicules occur on short spatial and
temporal scales, and are very abundant (superimposed) at the limb. Most
of the older observations lacked either the spatial resolution or
cadence necessary to measure spicules. This changed with Hinode/SOT,
which has provided seeing-free observations with high cadence and
spatial resolution. Using SOT observations, we find that in the
quiet sun most spicules are shorter lived and can move much faster
than previously measured. In this work we try to reconcile the recent
results with results from older observations. We degrade SOT data to
match the cadence and resolution of older data sets, and apply the same
semi-automated method to detect and measure the properties of spicules
to both the original and degraded data. We find that degrading the data
has a significant effect on the measured properties of spicules. Most
importantly, the results from the degraded data agree very well
with older studies (e.g. mean spicule duration more than 5 minutes,
and upward velocities of about 25 km/s). These results illustrate how
the combination of spicule superposition, low spatial resolution, and
cadence affect the measured properties of spicules, and that previous
measurements can thus be unreliable.
---------------------------------------------------------
Title: The Formation of the Hα Line in the Solar Chromosphere
Authors: Leenaarts, J.; Carlsson, M.; Rouppe van der Voort, L.
2012ApJ...749..136L Altcode: 2012arXiv1202.1926L
We use state-of-the-art radiation-MHD simulations and three-dimensional
(3D) non-LTE radiative transfer computations to investigate Hα
line formation in the solar chromosphere and apply the results of
this investigation to develop the potential of Hα as a diagnostic
of the chromosphere. We show that one can accurately model Hα line
formation assuming statistical equilibrium and complete frequency
redistribution provided the computation of the model atmosphere
included non-equilibrium ionization of hydrogen and the Lyα and
Lyβ line profiles are described by Doppler profiles. We find that
3D radiative transfer is essential in modeling hydrogen lines due
to the low photon destruction probability in Hα. The Hα opacity
in the upper chromosphere is mainly sensitive to the mass density
and only weakly sensitive to the temperature. We find that the Hα
line-core intensity is correlated with the average formation height:
The larger the average formation height is, the lower the intensity
will be. The line-core width is a measure of the gas temperature
in the line-forming region. The fibril-like dark structures seen in
Hα line-core images computed from our model atmosphere are tracing
magnetic field lines. These structures are caused by field-aligned
ridges of enhanced chromospheric mass density that raise their average
formation height, and therefore make them appear dark against their
deeper-formed surroundings. We compare with observations, and find that
the simulated line-core widths are very similar to the observed ones,
without the need for additional microturbulence.
---------------------------------------------------------
Title: State-of-the-art of non-LTE diagnostics: observations and
simulations
Authors: Carlsson, Mats
2012decs.confE..32C Altcode:
Advanced MHD simulations combined with non-LTE diagnostics are
revolutionizing our view of chromospheric dynamics and heating. We will
discuss how well synthetic observables compare with observations,
how diagnostic codes can be used to derive physical information
about the atmosphere, what is missing in current calculations and the
consequences of current assumptions. Examples will focus on often used
or to-be-used diagnostics of chromospheric lines, such as Ca II 8542,
H-alpha and Mg II h/k.
---------------------------------------------------------
Title: Using 3D MHD realistic simulations of the solar corona to
test plasma diagnostics
Authors: Testa, P.; De Pontieu, B.; Martinez-Sykora, J.; Hansteen,
V.; Carlsson, M.
2012decs.confE..27T Altcode:
We synthesize coronal images and spectra from advanced 3D MHD
simulations obtained from the state-of-the art Bifrost code, and
explore how well they reproduce coronal observations with SDO/AIA and
Hinode/EIS. We apply standard diagnostic techniques (e.g., density, and
temperature diagnostics) to the synthetic observations and investigate
how accurately the derived physical information matches the plasma
parameters of the model. We discuss the limitations of the diagnostics
and their implications.
---------------------------------------------------------
Title: NLTE inversions from a 3D MHD Chromospheric simulation
Authors: de la Cruz Rodriguez, J.; Socas-Navarro, H.; Carlsson, M.;
Leenaarts, J.
2012decs.confE..80D Altcode:
The structure of the solar chromosphere is believed to be governed
by magnetic fields, even in quiet Sun regions with a relatively weak
field. Measuring the magnetic field of the solar chromosphere is an
outstanding challenge for observers. Inversion codes allow for detailed
interpretation of full-Stokes data from spectral lines formed in the
chromosphere. However, the applicability of non-LTE inversions to infer
physical conditions in the dynamic 3D solar chromosphere, has not yet
been studied in detail. In this study, we use a snapshot from a 3D MHD
simulation of quiet-sun, extending from the photosphere to the corona,
to asses the reliability of non-LTE inversions to infer chromospheric
quantities, especially the magnetic field.
---------------------------------------------------------
Title: The formation of the Halpha line in the solar chromosphere
Authors: Leenaarts, J.; Carlsson; M.; Rouppe van der Voort, Rouppe, L.
2012decs.confE..14L Altcode:
We use state-of-the-art radiation-MHD simulations and 3D non-LTE
radiative transfer computations to investigate Halpha line formation
in the solar chromosphere. We find that 3D radiative transfer is
essential in modeling hydrogen lines due to the low photon destruction
probability in Halpha. The Halpha opacity in the upper chromosphere
is mainly sensitive to the mass density and only weakly sensitive to
temperature. We find that the Halpha line-core intensity is correlated
with the average formation height: the lower the intensity, the larger
the average formation height. The line-core width is a measure of
the gas temperature in the line-forming region. The fibril-like dark
structures seen in Halpha line-core images computed from our model
atmosphere are tracing magnetic field lines. These structures are
caused by field-aligned ridges of enhanced chromospheric mass density
that raise their average formation height, and therefore makes them
appear dark against their deeper-formed surroundings.
---------------------------------------------------------
Title: Potential for diagnostics with IRIS and Mg II lines
Authors: Pereira, Tiago M. D.; Carlsson, Mats; Leenaarts, Jorrit;
Uitenbroek, Han; De Pontieu, Bart; Martinez-Sykora, Juan
2012decs.confE..13P Altcode:
The IRIS mission will open up a new window into the solar chromosphere
and transition region. An important diagnostic that IRIS will bring
is the Mg II H and K lines. Radiation from these lines is believed
to be come from a wide range of formation depths, from the higher
photosphere to the onset of the transition region. With a complex
formation mechanism, Mg II H and K suffer from departures from LTE
and partial redistribution (PRD). In this preliminary analysis we will
look into the potential for diagnostics of Mg II H and K. Using a new
parallel version of the RH code we synthesised Mg II H and K spectra
from 3D rMHD simulations of the solar atmosphere. We will discuss
the relevance of several approximations on the final observables,
and will compare the Mg II H and K filtergrams with those of Ca II H,
a robust chromospheric diagnostic line widely used with Hinode/SOT/BFI.
---------------------------------------------------------
Title: The generation of shock waves traveling from the photosphere
to the transition region within network magnetic elements
Authors: Kato, Y.; Hansteen, V.; Steiner, O.; Carlsson, M.
2012decs.confE..54K Altcode:
We investigate the generation of shock waves near the photosphere by
convective downdrafts in the immediate surroundings of the magnetic
flux concentration, using radiation magnetohydrodynamic (RMHD) 2D
simulations of the solar atmosphere. The simulations comprise the layers
from the upper convection zone to the lower corona. We call this the
"magnetic pumping process". We find that the generated slow modes via
magnetic pumping travel upward along the magnetic flux concentration,
developing into a shock wave in chromospheric heights. The waves
continue to propagate further up through the transition region and into
the corona. In the course of propagation through the transition layer,
a small fraction of the longitudinal slow mode is converted into a
transverse wave mode. We report on how much energy is deposited by
propagating shock waves through the transition region and we discuss
the the dissipation process above the photosphere within the magnetic
flux concentration..
---------------------------------------------------------
Title: Approximations for radiative cooling and heating in the
solar chromosphere
Authors: Carlsson, M.; Leenaarts, J.
2012A&A...539A..39C Altcode: 2012arXiv1202.2996C
Context. The radiative energy balance in the solar chromosphere is
dominated by strong spectral lines that are formed out of LTE. It
is computationally prohibitive to solve the full equations of
radiative transfer and statistical equilibrium in 3D time dependent
MHD simulations. <BR /> Aims: We look for simple recipes to compute
the radiative energy balance in the dominant lines under solar
chromospheric conditions. <BR /> Methods: We use detailed calculations
in time-dependent and 2D MHD snapshots to derive empirical formulae
for the radiative cooling and heating. <BR /> Results: The radiative
cooling in neutral hydrogen lines and the Lyman continuum, the H
and K and intrared triplet lines of singly ionized calcium and the h
and k lines of singly ionized magnesium can be written as a product
of an optically thin emission (dependent on temperature), an escape
probability (dependent on column mass) and an ionization fraction
(dependent on temperature). In the cool pockets of the chromosphere
the same transitions contribute to the heating of the gas and similar
formulae can be derived for these processes. We finally derive a simple
recipe for the radiative heating of the chromosphere from incoming
coronal radiation. We compare our recipes with the detailed results
and comment on the accuracy and applicability of the recipes.
---------------------------------------------------------
Title: The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP)j
Authors: Kobayashi, K.; Tsuneta, S.; Trujillo Bueno, J.; Bando, T.;
Belluzzi, L.; Casini, R.; Carlsson, M.; Cirtain, J. W.; De Pontieu,
B.; Hara, H.; Ichimoto, K.; Ishikawa, R.; Kano, R.; Katsukawa, Y.;
Kim, T.; Kubo, M.; Manso Sainz, R.; Narukage, N.; Asensio Ramos,
A.; Robinson, B.; Sakao, T.; Shimizu, T.; Stepan, J.; Suematsu, Y.;
Watanabe, H.; West, E.; Winebarger, A. R.
2011AGUFM.P14C..05K Altcode:
We present an overview of the Chromospheric Lyman-Alpha
SpectroPolarimeter (CLASP) program. CLASP is a proposed sounding rocket
experiment currently under development as collaboration between Japan,
USA and Spain. The aim is to achieve the first measurement of magnetic
field in the upper chromosphere and transition region of the Sun
through the detection and measurement of Hanle effect polarization
of the Lyman alpha line. The Hanle effect (i.e. the magnetic field
induced modification of the linear polarization due to scattering
processes in spectral lines) is believed to be a powerful tool for
measuring the magnetic field in the upper chromosphere, as it is more
sensitive to weaker magnetic fields than the Zeeman effect, and also
sensitive to magnetic fields tangled at spatial scales too small to be
resolved. The Lyman-alpha (121.567 nm) line has been chosen because
it is a chromospheric/transition-region line, and because the Hanle
effect polarization of the Lyman-alpha line is predicted to be sensitive
to 10-250 Gauss, encompassing the range of interest. Hanle effect is
predicted to be observable as linear polarization or depolarization,
depending on the geometry, with a fractional polarization amplitude
varying between 0.1% and 1% depending on the strength and orientation of
the magnetic field. This quantification of the chromospheric magnetic
field requires a highly sensitive polarization measurement. The
CLASP instrument consists of a large aperture (287 mm) Cassegrain
telescope mated to a polarizing beamsplitter and a matched pair
of grating spectrographs. The polarizing beamsplitter consists
of a continuously rotating waveplate and a linear beamsplitter,
allowing simultaneous measurement of orthogonal polarizations and
in-flight self-calibration. Development of the instrument is underway,
and prototypes of all optical components have been tested using a
synchrotron beamline. The experiment is proposed for flight in 2014.
---------------------------------------------------------
Title: Quantifying spicules
Authors: Pereira, T. M.; De Pontieu, B.; Carlsson, M.
2011AGUFMSH34B..01P Altcode:
Understanding the dynamic solar chromosphere is of paramount importance
in solar physics. Spicules are an important feature of the chromosphere,
connecting the photosphere to the corona, potentially mediating the
transfer of energy and mass. While it is generally accepted that
there is more than one type of spicule, their quick motions, small
spatial scales, and short lifetimes have prevented a systematic study
of their properties over different solar regions. In the present work
we undertake such a study, using Ca H filtergrams from Hinode/SOT and
a semi-automated method to detect and track the spicules. Looking at
different magnetic field configurations (quiet Sun, coronal holes,
active regions), we discuss how the properties of the spicules change,
how the two spicule populations (type I and type II) are connected,
and how spicules are related to other chromospheric phenomena such as
dynamic fibrils.
---------------------------------------------------------
Title: Wave Propagation and Jet Formation in the Chromosphere
Authors: Heggland, L.; Hansteen, V. H.; De Pontieu, B.; Carlsson, M.
2011ApJ...743..142H Altcode: 2011arXiv1112.0037H
We present the results of numerical simulations of wave propagation
and jet formation in solar atmosphere models with different magnetic
field configurations. The presence in the chromosphere of waves with
periods longer than the acoustic cutoff period has been ascribed to
either strong inclined magnetic fields, or changes in the radiative
relaxation time. Our simulations include a sophisticated treatment
of radiative losses, as well as fields with different strengths
and inclinations. Using Fourier and wavelet analysis techniques,
we investigate the periodicity of the waves that travel through the
chromosphere. We find that the velocity signal is dominated by waves
with periods around 5 minutes in regions of strong, inclined field,
including at the edges of strong flux tubes where the field expands,
whereas 3 minute waves dominate in regions of weak or vertically
oriented fields. Our results show that the field inclination is very
important for long-period wave propagation, whereas variations in the
radiative relaxation time have little effect. Furthermore, we find
that atmospheric conditions can vary significantly on timescales of
a few minutes, meaning that a Fourier analysis of wave propagation
can be misleading. Wavelet techniques take variations with time into
account and are more suitable analysis tools. Finally, we investigate
the properties of jets formed by the propagating waves once they reach
the transition region, and find systematic differences between the
jets in inclined-field regions and those in vertical field regions,
in agreement with observations of dynamic fibrils.
---------------------------------------------------------
Title: Generation and propagation of Alfvenic waves in spicules
Authors: De Pontieu, B.; Okamoto, T. J.; Rouppe van der Voort, L.;
Hansteen, V. H.; Carlsson, M.
2011AGUFMSH13B1956D Altcode:
Both spicules and Alfven waves have recently been implicated in
playing a role in the heating of the outer atmosphere. Yet we do
not know how spicules or Alfven waves are generated. Here we focus
on the properties of Alfvenic waves in spicules and their role in
forming spicules. We use high-resolution observations taken with the
Solar Optical Telescope onboard Hinode, and with the CRISP Fabry-Perot
Interferometer at the Swedish Solar Telescope (SST) in La Palma to study
the generation and propagation of Alfvenic waves in spicules and their
disk counterparts. Using automated detection algorithms to identify
propagating waves in limb spicules, we find evidence for both up-
and downward propagating as well as standing waves. Our data suggests
significant reflection of waves in and around spicules and provides
constraints for theoretical models of spicules and wave propagation
through the chromosphere. We also show observational evidence (using
SST data) of the generation of Alfven waves and the role they play in
forming spicules.
---------------------------------------------------------
Title: Testing coronal plasma diagnostics using 3D MHD models of
the solar atmosphere
Authors: Testa, P.; Martinez-Sykora, J.; Hansteen, V. H.; De Pontieu,
B.; Carlsson, M.
2011AGUFMSH53C..06T Altcode:
We synthesize coronal images and spectra from advanced 3D radiative
MHD simulations obtained from the state-of-the-art Bifrost code, and
explore how well they reproduce coronal observations with SDO/AIA
and Hinode/EIS and XRT. We apply standard diagnostic techniques
(e.g., density, temperature, abundance diagnostics) to the synthetic
observations and investigate how accurately the derived physical
information matches the plasma parameters of the model. We discuss
the limitations of the diagnostics and their implications.
---------------------------------------------------------
Title: Overview of Chromospheric Lyman-Alpha SpectroPolarimeter
(CLASP)
Authors: Narukage, Noriyuki; Tsuneta, Saku; Bando, Takamasa; Kano,
Ryouhei; Kubo, Masahito; Ishikawa, Ryoko; Hara, Hirohisa; Suematsu,
Yoshinori; Katsukawa, Yukio; Watanabe, Hiroko; Ichimoto, Kiyoshi;
Sakao, Taro; Shimizu, Toshifumi; Kobayashi, Ken; Robinson, Brian; Kim,
Tony; Winebarger, Amy; West, Edward; Cirtain, Jonathan; De Pontieu,
Bart; Casini, Roberto; Trujillo Bueno, Javier; Stepan, Jiri; Manso
Sainz, Rafael; Belluzzi, Luca; Asensio Ramos, Andres; Carlsson, Mats
2011SPIE.8148E..0HN Altcode: 2011SPIE.8148E..16N
The solar chromosphere is an important boundary, through which all of
the plasma, magnetic fields and energy in the corona and solar wind
are supplied. Since the Zeeman splitting is typically smaller than
the Doppler line broadening in the chromosphere and transition region,
it is not effective to explore weak magnetic fields. However, this is
not the case for the Hanle effect, when we have an instrument with
high polarization sensitivity (~ 0.1%). "Chromospheric Lyman- Alpha
SpectroPolarimeter (CLASP)" is the sounding rocket experiment to detect
linear polarization produced by the Hanle effect in Lyman-alpha line
(121.567 nm) and to make the first direct measurement of magnetic
fields in the upper chromosphere and lower transition region. To
achieve the high sensitivity of ~ 0.1% within a rocket flight (5
minutes) in Lyman-alpha line, which is easily absorbed by materials,
we design the optical system mainly with reflections. The CLASP
consists of a classical Cassegrain telescope, a polarimeter and a
spectrometer. The polarimeter consists of a rotating 1/2-wave plate
and two reflecting polarization analyzers. One of the analyzer also
works as a polarization beam splitter to give us two orthogonal linear
polarizations simultaneously. The CLASP is planned to be launched in
2014 summer.
---------------------------------------------------------
Title: Alfvénic waves with sufficient energy to power the quiet
solar corona and fast solar wind
Authors: McIntosh, Scott W.; de Pontieu, Bart; Carlsson, Mats;
Hansteen, Viggo; Boerner, Paul; Goossens, Marcel
2011Natur.475..477M Altcode:
Energy is required to heat the outer solar atmosphere to millions of
degrees (refs 1, 2) and to accelerate the solar wind to hundreds of
kilometres per second (refs 2-6). Alfvén waves (travelling oscillations
of ions and magnetic field) have been invoked as a possible mechanism
to transport magneto-convective energy upwards along the Sun's magnetic
field lines into the corona. Previous observations of Alfvénic waves
in the corona revealed amplitudes far too small (0.5kms<SUP>-1</SUP>)
to supply the energy flux (100-200Wm<SUP>-2</SUP>) required to
drive the fast solar wind or balance the radiative losses of the
quiet corona. Here we report observations of the transition region
(between the chromosphere and the corona) and of the corona that
reveal how Alfvénic motions permeate the dynamic and finely structured
outer solar atmosphere. The ubiquitous outward-propagating Alfvénic
motions observed have amplitudes of the order of 20kms<SUP>-1</SUP> and
periods of the order of 100-500s throughout the quiescent atmosphere
(compatible with recent investigations), and are energetic enough to
accelerate the fast solar wind and heat the quiet corona.
---------------------------------------------------------
Title: Quiet-Sun imaging asymmetries in Na I D<SUB>1</SUB> compared
with other strong Fraunhofer lines
Authors: Rutten, R. J.; Leenaarts, J.; Rouppe van der Voort, L. H. M.;
de Wijn, A. G.; Carlsson, M.; Hansteen, V.
2011A&A...531A..17R Altcode: 2011arXiv1104.4307R
Imaging spectroscopy of the solar atmosphere using the Na I
D<SUB>1</SUB> line yields marked asymmetry between the blue and
red line wings: sampling a quiet-Sun area in the blue wing displays
reversed granulation, whereas sampling in the red wing displays normal
granulation. The Mg I b<SUB>2</SUB> line of comparable strength does
not show this asymmetry, nor does the stronger Ca II 8542 Å line. We
demonstrate the phenomenon with near-simultaneous spectral images in
Na I D<SUB>1</SUB>, Mg I b<SUB>2</SUB>, and Ca II 8542 Å from the
Swedish 1-m Solar Telescope. We then explain it with line-formation
insights from classical 1D modeling and with a 3D magnetohydrodynamical
simulation combined with NLTE spectral line synthesis that permits
detailed comparison with the observations in a common format. The
cause of the imaging asymmetry is the combination of correlations
between intensity and Dopplershift modulation in granular overshoot
and the sensitivity to these of the steep profile flanks of the Na
I D<SUB>1</SUB> line. The Mg I b<SUB>2</SUB> line has similar core
formation but much wider wings due to larger opacity buildup and
damping in the photosphere. Both lines obtain marked core asymmetry
from photospheric shocks in or near strong magnetic concentrations,
less from higher-up internetwork shocks that produce similar asymmetry
in the spatially averaged Ca II 8542 Å profile.
---------------------------------------------------------
Title: The stellar atmosphere simulation code Bifrost. Code
description and validation
Authors: Gudiksen, B. V.; Carlsson, M.; Hansteen, V. H.; Hayek, W.;
Leenaarts, J.; Martínez-Sykora, J.
2011A&A...531A.154G Altcode: 2011arXiv1105.6306G
Context. Numerical simulations of stellar convection and photospheres
have been developed to the point where detailed shapes of observed
spectral lines can be explained. Stellar atmospheres are very complex,
and very different physical regimes are present in the convection zone,
photosphere, chromosphere, transition region and corona. To understand
the details of the atmosphere it is necessary to simulate the whole
atmosphere since the different layers interact strongly. These physical
regimes are very diverse and it takes a highly efficient massively
parallel numerical code to solve the associated equations. <BR /> Aims:
The design, implementation and validation of the massively parallel
numerical code Bifrost for simulating stellar atmospheres from the
convection zone to the corona. <BR /> Methods: The code is subjected
to a number of validation tests, among them the Sod shock tube test,
the Orzag-Tang colliding shock test, boundary condition tests and
tests of how the code treats magnetic field advection, chromospheric
radiation, radiative transfer in an isothermal scattering atmosphere,
hydrogen ionization and thermal conduction. Results.Bifrost completes
the tests with good results and shows near linear efficiency scaling
to thousands of computing cores.
---------------------------------------------------------
Title: On the minimum temperature of the quiet solar chromosphere
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Gudiksen, B. V.
2011A&A...530A.124L Altcode: 2011arXiv1104.5081L
<BR /> Aims: We aim to provide an estimate of the minimum temperature
of the quiet solar chromosphere. <BR /> Methods: We perform a 2D
radiation-MHD simulation spanning the upper convection zone to the
lower corona. The simulation includes non-LTE radiative transfer
and an equation-of-state that includes non-equilibrium ionization
of hydrogen and non-equilibrium H<SUB>2</SUB> molecule formation. We
analyze the reliability of the various assumptions made in our model
in order to assess the realism of the simulation. <BR /> Results:
Our simulation contains pockets of cool gas with down to 1660 K from
1 Mm up to 3.2 Mm height. It overestimates the radiative heating,
and contains non-physical heating below 1660 K. Therefore we conclude
that cool pockets in the quiet solar chromosphere might have even
lower temperatures than in the simulation, provided that there exist
areas in the chromosphere without significant magnetic heating. We
suggest off-limb molecular spectroscopy to look for such cool pockets
and 3D simulations including a local dynamo and a magnetic carpet to
investigate Joule heating in the quiet chromosphere.
---------------------------------------------------------
Title: Helioseismic Studies With Multi-wavelength Data From HMI And
AIA Onboard SDO
Authors: Hill, Frank; Jain, K.; Tripathy, S.; Kholikov, S.; Gonzalez
Hernandez, I.; Leibacher, J.; Howe, R.; Baudin, F.; Carlsson, M.;
Chaplin, W.; Tarbell, T.
2011SPD....42.2111H Altcode: 2011BAAS..43S.2111H
The successful launch of the Solar Dynamics Observatory (SDO) in
February 2010 opens important, new possibilities for helioseismic
exploration of the solar interior and atmosphere using multi-wavelength
observations from multiple instruments. In order to better understand
the solar interior and atmosphere, as well as the physics of the
helioseismic modes and waves themselves, we exploit the potential of
the Atmospheric Imaging Assembly (AIA) 1600 and 1700 Angstrom continuum
measurements and the contemporaneous Helioseismic and Magnetic Imager
(HMI) Fe I 6173.3 Angstrom velocity and intensity observations. Standard
techniques of helioseismology e.g Sun-as-a-star, spherical harmonic
analysis, ring diagrams, and time- distance analysis are applied to
obtain acoustic mode parameters and other characteristics. Here we
present our preliminary results, and interpret these in the context
of the differences in the heights of formation of the lines.
---------------------------------------------------------
Title: A Sounding Rocket Experiment for Spectropolarimetric
Observations with the Ly<SUB>α</SUB> Line at 121.6 nm (CLASP)
Authors: Ishikawa, R.; Bando, T.; Fujimura, D.; Hara, H.; Kano,
R.; Kobiki, T.; Narukage, N.; Tsuneta, S.; Ueda, K.; Wantanabe,
H.; Kobayashi, K.; Trujillo Bueno, J.; Manso Sainz, R.; Stepan, J.;
de Pontieu, B.; Carlsson, M.; Casini, R.
2011ASPC..437..287I Altcode:
A team consisting of Japan, USA, Spain, and Norway is developing a
high-throughput Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP),
which is proposed to fly with a NASA sounding rocket in 2014. CLASP will
explore the magnetism of the upper solar chromosphere and transition
region via the Hanle effect of the Ly<SUB>α</SUB> line for the first
time. This experiment requires spectropolarimetric observations with
high polarimetric sensitivity (∼0.1%) and wavelength resolution
(0.1 Å). The final spatial resolution (slit width) is being discussed
taking into account the required high signal-to-noise ratio. We have
demonstrated the performance of the Ly<SUB>α</SUB> polarimeter by
extensively using the Ultraviolet Synchrotron ORbital Radiation Facility
(UVSOR) at the Institute for Molecular Sciences. In this contribution,
we report these measurements at UVSOR together with the current status
of the CLASP project.
---------------------------------------------------------
Title: Non-equilibrium calcium ionisation in the solar atmosphere
Authors: Wedemeyer-Böhm, S.; Carlsson, M.
2011A&A...528A...1W Altcode: 2011arXiv1101.2211W
Context. The chromosphere of the Sun is a temporally and spatially
very varying medium for which the assumption of ionisation equilibrium
is questionable. <BR /> Aims: Our aim is to determine the dominant
processes and timescales for the ionisation equilibrium of calcium under
solar chromospheric conditions. <BR /> Methods: The study is based on
numerical simulations with the RADYN code, which combines hydrodynamics
with a detailed solution of the radiative transfer equation. The
calculations include a detailed non-equilibrium treatment of hydrogen,
calcium, and helium. Next to an hour long simulation sequence,
additional simulations are produced, for which the stratification is
slightly perturbed so that a ionisation relaxation timescale can be
determined. The simulations are characterised by upwards propagating
shock waves, which cause strong temperature fluctuations and variations
of the (non-equilibrium) ionisation degree of calcium. <BR /> Results:
The passage of a hot shock front leads to a strong net ionisation
of Ca II, rapidly followed by net recombination. The relaxation
timescale of the calcium ionisation state is found to be of the
order of a few seconds at the top of the photosphere and 10 to 30 s
in the upper chromosphere. At heights around 1 Mm, we find typical
values around 60 s and in extreme cases up to ~150 s. Generally,
the timescales are significantly reduced in the wakes of ubiquitous
hot shock fronts. The timescales can be reliably determined from a
simple analysis of the eigenvalues of the transition rate matrix. The
timescales are dominated by the radiative recombination from Ca III
into the metastable Ca II energy levels of the 4d <SUP>2</SUP>D
term. These transitions depend strongly on the density of free
electrons and therefore on the (non-equilibrium) ionisation degree of
hydrogen, which is the main electron donor. <BR /> Conclusions: The
ionisation/recombination timescales derived here are too long for the
assumption of an instantaneous ionisation equilibrium to be valid and,
on the other hand, are not long enough to warrant an assumption of a
constant ionisation fraction. Fortunately, the ionisation degree of Ca
ii remains small in the height range, where the cores of the H, K, and
the infrared triplet lines are formed. We conclude that the difference
due to a detailed treatment of Ca ionisation has only negligible impact
on the modelling of spectral lines of Ca ii and the plasma properties
under the conditions in the quiet solar chromosphere.
---------------------------------------------------------
Title: Solar velocity references from 3D HD photospheric models
Authors: de la Cruz Rodríguez, J.; Kiselman, D.; Carlsson, M.
2011A&A...528A.113D Altcode: 2011arXiv1101.2671D
Context. The measurement of Doppler velocities in spectroscopic solar
observations requires a reference for the local frame of rest. The
rotational and radial velocities of the Earth and the rotation of
the Sun introduce velocity offsets in the observations. Normally,
good references for velocities are missing (e.g. telluric lines),
especially in filter-based spectropolarimetric observations. <BR />
Aims: We determine an absolute reference for line-of-sight velocities
measured from solar observations for any heliocentric angle, calibrating
the convective line shift of spatially-averaged profiles on quiet sun
from a 3D hydrodynamical simulation. This method works whenever there
is quiet sun in the field-of-view, and it has the advantage of being
relatively insensitive to uncertainties in the atomic data. <BR />
Methods: We carry out radiative transfer computations in LTE for
selected C i and Fe i lines, whereas the Ca ii infrared lines are
synthesized in non-LTE. Radiative transfer calculations are done with
a modified version of Multi, using the snapshots of a non-magnetic
3D hydrodynamical simulation of the photosphere. <BR /> Results:
The resulting synthetic profiles show the expected C-shaped bisector
at disk center. The degree of asymmetry and the line shifts, however,
show a clear dependence on the heliocentric angle and the properties of
the lines. The profiles at μ = 1 are compared with observed profiles
to prove their reliability, and they are tested against errors induced
by the LTE calculations, inaccuracies in the atomic data and the 3D
simulation. <BR /> Conclusions: Theoretical quiet-sun profiles of lines
commonly used by solar observers are provided to the community. Those
can be used as absolute references for line-of-sight velocities. The
limb effect is produced by the projection of the 3D atmosphere along the
line of sight. Non-LTE effects on Fe i lines are found to have a small
impact on the convective shifts of the lines, reinforcing the usability
of the LTE approximation in this case. We estimate the precision of the
disk-center line shifts to be approximately 50 m s<SUP>-1</SUP>, but
the off-center profiles remain to be tested against observations. <P
/>The spectral profiles are only available in electronic form at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/528/A113">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/528/A113</A>
---------------------------------------------------------
Title: VizieR Online Data Catalog: Synthetic lines in the Sun (de
la Cruz Rodríguez+, 2011)
Authors: de la Cruz Rodríguez, J.; Kiselman, D.; Carlsson, M.
2011yCat..35289113D Altcode:
We have computed synthetic spectra from a realistic 3D numerical
simulation of the solar photosphere. We provide the spatially
averaged spectra for selected lines that are commonly used on solar
applications. These data can be used to calibrate Doppler velocity
measurements in the solar photosphere. The calculations are carried
out along the solar disk from heliocentric angle mu=1.0 to mu=0.3. <P
/>(11 data files).
---------------------------------------------------------
Title: The Origins of Hot Plasma in the Solar Corona
Authors: De Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Tarbell, T. D.; Boerner, P.; Martinez-Sykora, J.; Schrijver,
C. J.; Title, A. M.
2011Sci...331...55D Altcode:
The Sun's outer atmosphere, or corona, is heated to millions of degrees,
considerably hotter than its surface or photosphere. Explanations for
this enigma typically invoke the deposition in the corona of nonthermal
energy generated by magnetoconvection. However, the coronal heating
mechanism remains unknown. We used observations from the Solar Dynamics
Observatory and the Hinode solar physics mission to reveal a ubiquitous
coronal mass supply in which chromospheric plasma in fountainlike jets
or spicules is accelerated upward into the corona, with much of the
plasma heated to temperatures between ~0.02 and 0.1 million kelvin (MK)
and a small but sufficient fraction to temperatures above 1 MK. These
observations provide constraints on the coronal heating mechanism(s)
and highlight the importance of the interface region between photosphere
and corona.
---------------------------------------------------------
Title: Ubiquitous Alfvenic Motions in Quiet Sun, Coronal Hole and
Active Region Corona
Authors: McIntosh, S. W.; de Pontieu, B.; Carlsson, M.; Hansteen,
V. H.; Sdo/Aia Mission Team
2010AGUFMSH14A..01M Altcode:
We use observations with AIA onboard SDO and report the discovery of
ubiquitous Alfvenic oscillations in the corona of quiet Sun, active
regions and coronal holes. These Alfvenic oscillations have significant
power, and seem to be connected to the chromospheric Alfvenic
oscillations previously reported with Hinode. We use Monte Carlo
simulations to determine the strength and periods of the waves. Using
unique joint observations of Hinode, the Solar Dynamics Observatory, and
HAO's CoMP instrument we study the excitation of transverse oscillations
as a function of space, time, and temperature. We will discuss the
energetic impact and diagnostic capabilities of this ever-present
process and how it can be used to build a more self-consistent picture
of energy transport into the inner heliosphere. Transverse Oscillations
Observed Above the Solar North Pole in the He II 304Å (bottom) and Fe
IX 171Å (top) channels. Studying the progression of such points with
altitude yields important information about wave propagation into the
magnetically open corona.
---------------------------------------------------------
Title: The Chromospheric Lyman Alpha SpectroPolarimeter (CLASP)
Authors: Kobayashi, K.; Tsuneta, S.; Trujillo Bueno, J.; Cirtain,
J. W.; Bando, T.; Kano, R.; Hara, H.; Fujimura, D.; Ueda, K.; Ishikawa,
R.; Watanabe, H.; Ichimoto, K.; Sakao, T.; de Pontieu, B.; Carlsson,
M.; Casini, R.
2010AGUFMSH11B1632K Altcode:
Magnetic fields in the solar chromosphere play a key role in the
energy transfer and dynamics of the solar atmosphere. Yet a direct
observation of the chromospheric magnetic field remains one of the
greatest challenges in solar physics. While some advances have been
made for observing the Zeeman effect in strong chromospheric lines,
the effect is small and difficult to detect outside sunspots. The
Hanle effect offers a promising alternative; it is sensitive to weaker
magnetic fields (e.g., 5-500 G for Ly-Alpha), and while its magnitude
saturates at stronger magnetic fields, the linear polarization signals
remain sensitive to the magnetic field orientation. The Hanle effect
is not only limited to off-limb observations. Because the chromosphere
is illuminated by an anisotropic radiation field, the Ly-Alpha line is
predicted to show linear polarization for on-disk, near-limb regions,
and magnetic field is predicted to cause a measurable depolarization. At
disk center, the Ly-Alpha radiation is predicted to be negligible
in the absence of magnetic field, and linearly polarized to an order
of 0.3% in the presence of an inclined magnetic field. The proposed
CLASP sounding rocket instrument is designed to detect 0.3% linear
polarization of the Ly-Alpha line at 1.5 arcsecond spatial resolution
(0.7’’ pixel size) and 10 pm spectral resolution. The instrument
consists of a 30 cm aperture Cassegrain telescope and a dual-beam
spectropolarimeter. The telescope employs a “cold mirror’’ design
that uses multilayer coatings to reflect only the target wavelength
range into the spectropolarimeter. The polarization analyzer consists of
a rotating waveplate and a polarizing beamsplitter that comprises MgF2
plates placed at Brewster’s Angle. Each output beam of the polarizing
beamsplitter, representing two orthogonal linear polarizations, is
dispersed and focused using a separate spherical varied-line-space
grating, and imaged with a separate 512x512 CCD camera. Prototypes
of key optical components have been fabricated and tested. Instrument
design is being finalized, and the experiment will be proposed for a
2014 flight aboard a NASA sounding rocket.
---------------------------------------------------------
Title: The role of the chromosphere in filling the corona with hot
plasma (Invited)
Authors: de Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Tarbell, T. D.; Boerner, P.; Martinez-Sykora, J.; Schrijver,
C. J.; Title, A. M.
2010AGUFMSH21C..03D Altcode:
We use coordinated observations from the Solar Dynamics Observatory
(SDO), Hinode and the Swedish Solar Telescope (SST) to show how
plasma is heated to coronal temperatures from its source in the
chromosphere. Our observations reveal a ubiquitous mass supply
for the solar corona in which chromospheric plasma is accelerated
upward into the corona with much of the plasma heated to transition
region temperatures, and a small, but significant fraction heated
to temperatures in excess of 1 million K. Our observations show,
for the first time, how chromospheric spicules, fountain-like jets
that have long been considered potential candidates for coronal
heating, are directly associated with heating of plasma to coronal
temperatures. These results provide strong physical constraints on
the mechanism(s) responsible for coronal heating and do not seem
compatible with current models. The association with chromospheric
spicules highlights the importance of the interface region between
the photosphere and corona to gain a full understanding of the coronal
heating problem.
---------------------------------------------------------
Title: Radiative Hydrodynamic Simulations of Acoustic Waves in
Sunspots
Authors: Bard, S.; Carlsson, M.
2010ApJ...722..888B Altcode:
We investigate the formation and evolution of the Ca II H line in a
sunspot. The aim of our study is to establish the mechanisms underlying
the formation of the frequently observed brightenings of small regions
of sunspot umbrae known as "umbral flashes." We perform fully consistent
NLTE radiation hydrodynamic simulations of the propagation of acoustic
waves in sunspot umbrae and conclude that umbral flashes result from
increased emission of the local solar material during the passage
of acoustic waves originating in the photosphere and steepening to
shock in the chromosphere. To quantify the significance of possible
physical mechanisms that contribute to the formation of umbral flashes,
we perform a set of simulations on a grid formed by different wave
power spectra, different inbound coronal radiation, and different
parameterized chromospheric heating. Our simulations show that the
waves with frequencies in the range 4.5-7.0 mHz are critical to the
formation of the observed blueshifts of umbral flashes while waves with
frequencies below 4.5 mHz do not play a role despite their dominance
in the photosphere. The observed emission in the Ca II H core between
flashes only occurs in the simulations that include significant inbound
coronal radiation and/or extra non-radiative chromospheric heating in
addition to shock dissipation.
---------------------------------------------------------
Title: On Redshifts and Blueshifts in the Transition Region and Corona
Authors: Hansteen, V. H.; Hara, H.; De Pontieu, B.; Carlsson, M.
2010ApJ...718.1070H Altcode: 2010arXiv1001.4769H
Emission lines formed in the transition region (TR) of the Sun have long
been known to show pervasive redshifts. Despite a variety of proposed
explanations, these TR downflows (and the slight upflows in the low
corona) remain poorly understood. We present results from comprehensive
three-dimensional MHD models that span the upper convection zone up to
the corona, 15 Mm above the photosphere. The TR and coronal heating
in these models is caused by the stressing of the magnetic field by
photospheric and convection "zone dynamics," but also in some models by
the injection of emerging magnetic flux. We show that rapid, episodic
heating, at low heights of the upper chromospheric plasma to coronal
temperatures naturally produces downflows in TR lines, and slight
upflows in low coronal lines, with similar amplitudes to those observed
with EUV/UV spectrographs. We find that TR redshifts naturally arise
in episodically heated models where the average volumetric heating
scale height lies between that of the chromospheric pressure scale
height of 200 km and the coronal scale height of 50 Mm.
---------------------------------------------------------
Title: On the Solar Chromosphere Observed at the LIMB with Hinode
Authors: Judge, Philip G.; Carlsson, Mats
2010ApJ...719..469J Altcode: 2010arXiv1004.1398J
Broadband images in the Ca II H line, from the Broadband Filter Imager
(BFI) instrument on the Hinode spacecraft, show emission from spicules
emerging from and visible right down to the observed limb. Surprisingly,
little absorption of spicule light is seen along their lengths. We
present formal solutions to the transfer equation for given (ad hoc)
source functions, including a stratified chromosphere from which
spicules emanate. The model parameters are broadly compatible with
earlier studies of spicules. The visibility of Ca II spicules down
to the limb in Hinode data seems to require that spicule emission
be Doppler shifted relative to the stratified atmosphere, either by
supersonic turbulent or organized spicular motion. The non-spicule
component of the chromosphere is almost invisible in the broadband BFI
data, but we predict that it will be clearly visible in high spectral
resolution data. Broadband Ca II H limb images give the false impression
that the chromosphere is dominated by spicules. Our analysis serves
as a reminder that the absence of a signature can be as significant
as its presence.
---------------------------------------------------------
Title: Radiative transfer with scattering for domain-decomposed 3D
MHD simulations of cool stellar atmospheres. Numerical methods and
application to the quiet, non-magnetic, surface of a solar-type star
Authors: Hayek, W.; Asplund, M.; Carlsson, M.; Trampedach, R.; Collet,
R.; Gudiksen, B. V.; Hansteen, V. H.; Leenaarts, J.
2010A&A...517A..49H Altcode: 2010arXiv1007.2760H
<BR /> Aims: We present the implementation of a radiative
transfer solver with coherent scattering in the new BIFROST
code for radiative magneto-hydrodynamical (MHD) simulations of
stellar surface convection. The code is fully parallelized using
MPI domain decomposition, which allows for large grid sizes and
improved resolution of hydrodynamical structures. We apply the code
to simulate the surface granulation in a solar-type star, ignoring
magnetic fields, and investigate the importance of coherent scattering
for the atmospheric structure. <BR /> Methods: A scattering term
is added to the radiative transfer equation, requiring an iterative
computation of the radiation field. We use a short-characteristics-based
Gauss-Seidel acceleration scheme to compute radiative flux divergences
for the energy equation. The effects of coherent scattering are
tested by comparing the temperature stratification of three 3D
time-dependent hydrodynamical atmosphere models of a solar-type star:
without scattering, with continuum scattering only, and with both
continuum and line scattering. <BR /> Results: We show that continuum
scattering does not have a significant impact on the photospheric
temperature structure for a star like the Sun. Including scattering in
line-blanketing, however, leads to a decrease of temperatures by about
350 K below log<SUB>10</SUB> τ<SUB>5000</SUB> ⪉ -4. The effect is
opposite to that of 1D hydrostatic models in radiative equilibrium,
where scattering reduces the cooling effect of strong LTE lines in
the higher layers of the photosphere. Coherent line scattering also
changes the temperature distribution in the high atmosphere, where
we observe stronger fluctuations compared to a treatment of lines as
true absorbers.
---------------------------------------------------------
Title: On the Origin of High-Frequency "Acoustic” Power in
Photospheric and Chromospheric Velocity Power Spectra
Authors: Fleck, Bernard; Straus, T.; Carlsson, M.; Jefferies, S. M.;
Severino, G.; Tarbell, T. D.
2010AAS...21640309F Altcode: 2010BAAS...41..879F
In a recent paper (Fleck et al., 2010) we compared observed Dopplergram
time series from Hinode with results from 3-D numerical simulations
based on the Oslo "Stagger” and CO5BOLD codes. Given the rapid falloff
of atmospheric modulation transfer functions at high frequencies due
to the extended widths of typical velocity response functions, one
would expect the high-frequency tail of Doppler power spectra to drop
significantly below those of actual velocities at the corresponding
heights in the simulations. Surprisingly, our analysis of power spectra
of Doppler shifts of simulated line profiles did not reveal such a
steep falloff at high frequencies. Instead, they are comparable to
(and in some cases even larger than) those of the actual velocities,
making estimates of the energy flux of high frequency acoustic
waves questionable, in particular those that apply atmospheric MTF
corrections. In this work we study the cause of this unexpected
behavior in detail, with particular emphasis on the role of rapidly
changing velocity response functions in a dynamic atmosphere with
strong vertical velocity gradients.
---------------------------------------------------------
Title: New Frontiers in Solar Chromospheric Physics
Authors: Carlsson, Mats P. O.
2010AAS...21620701C Altcode:
The enigmatic chromosphere is the transition between the solar surface
and the eruptive outer solar atmosphere. The chromosphere harbours
and constrains the mass and energy loading processes that define the
heating of the corona, the acceleration and the composition of the solar
wind, and the energetics and triggering of solar outbursts (filament
eruptions, flares, coronal mass ejections). <P />Magnetic fields
break through the solar surface in a hierarchy of magnetic elements
ranging from Earth-sized sunspots down to tiny concentrations that are
barely resolved in the highest-resolution photospheric images. In the
chromosphere they combine in intricate, highly dynamic, and continuously
evolving fibrilar patterns. Movements of the photospheric field-line
footpoints drive, guide, and control the flows of energy and mass
into the corona, and trigger energy-releasing magnetic reconnection
through relentless topological rearrangement. The conversion from
convectively driven footpoint motion to outer-atmosphere outflows and
loading takes place in the dynamic, fine-structured chromosphere. <P
/>The chromosphere is arguably the most difficult and least understood
domain of solar physics. All at once it represents the transition from
optically thick to thin radiation escape, from gas-pressure domination
to magnetic-pressure domination, from neutral to ionised state, from MHD
to plasma physics, and from near-equilibrium ("LTE") to non-equilibrium
conditions. <P />A number of important facilities for observing the
solar chromosphere have recently come on line (e.g. the Hinode satellite
and ground-based Fabry-Perot interferometers) or will become operational
in the near future (e.g. SDO and IRIS). The overwhelming complexity of
the chromosphere makes it necessary to have numerical simulations for
the interpretation of the observations. Such realistic simulations,
spanning the solar atmosphere from the convection zone to the corona,
are now becoming feasible. <P />This presentation will introduce the
fascinating aspects of chromospheric physics and review recent results
from both observations and numerical simulations.
---------------------------------------------------------
Title: Radiative Hydrodynamic Simulation of the Continuum Emission
in Solar White-Light Flares
Authors: Cheng, J. X.; Ding, M. D.; Carlsson, Mats
2010ApJ...711..185C Altcode:
It is believed that solar white-light flares (WLFs) originate in the
lower chromosphere and upper photosphere. In particular, some recently
observed WLFs show a large continuum enhancement at 1.56 μm where the
opacity reaches its minimum. Therefore, it is important to clarify
how the energy is transferred to the lower layers responsible for
the production of WLFs. Based on radiative hydrodynamic simulations,
we study the role of non-thermal electron beams in increasing the
continuum emission. We vary the parameters of the electron beam and
disk positions and compare the results with observations. The electron
beam heated model can explain most of the observational white-light
enhancements. For the most energetic WLFs observed so far, however, a
very large electron beam flux and a high low-energy cutoff, which are
possibly beyond the parameter space in our simulations, are required
in order to reproduce the observed white-light emission.
---------------------------------------------------------
Title: The Quiet Solar Atmosphere Observed and Simulated in Na
I D<SUB>1</SUB>
Authors: Leenaarts, J.; Rutten, R. J.; Reardon, K.; Carlsson, M.;
Hansteen, V.
2010ApJ...709.1362L Altcode: 2009arXiv0912.2206L
The Na I D<SUB>1</SUB> line in the solar spectrum is sometimes
attributed to the solar chromosphere. We study its formation in
quiet-Sun network and internetwork. We first present high-resolution
profile-resolved images taken in this line with the imaging
spectrometer Interferometric Bidimensional Spectrometer at the Dunn
Solar Telescope and compare these to simultaneous chromospheric images
taken in Ca II 8542 Å and Hα. We then model Na I D<SUB>1</SUB>
formation by performing three-dimensional (3D) non-local
thermodynamic equilibrium profile synthesis for a snapshot from a
3D radiation-magnetohydrodynamics simulation. We find that most Na I
D<SUB>1</SUB> brightness is not chromospheric but samples the magnetic
concentrations that make up the quiet-Sun network in the photosphere,
well below the height where they merge into chromospheric canopies,
with aureoles from 3D resonance scattering. The line core is sensitive
to magneto-acoustic shocks in and near magnetic concentrations, where
shocks occur deeper than elsewhere, and may provide evidence of heating
deep within magnetic concentrations.
---------------------------------------------------------
Title: On red-shifts in the transition region and corona .
Authors: Hansteen, V. H.; Hara, H.; de Pontieu, B.; Carlsson, M.
2010MmSAI..81..729H Altcode:
We present evidence that transition region red-shifts are naturally
produced in episodically heated models where the average volumetric
heating scale height lies between that of the chromospheric
pressure scale height of 200 km and the coronal scale height of
50 Mm. In order to do so we present results from 3d MHD models
spanning the upper convection zone up to the corona, 15 Mm above the
photosphere. Transition region and coronal heating in these models
is due both the stressing of the magnetic field by photospheric and
convection `zone dynamics, but also in some models by the injection
of emerging magnetic flux.
---------------------------------------------------------
Title: High frequency waves in the solar atmosphere?.
Authors: Fleck, B.; Straus, T.; Carlsson, M.; Jefferies, S. M.;
Severino, G.; Tarbell, T. D.
2010MmSAI..81..777F Altcode: 2010arXiv1002.3285F
The present study addresses the following questions: How representative
of the actual velocities in the solar atmosphere are the Doppler
shifts of spectral lines? How reliable is the velocity signal derived
from narrowband filtergrams? How well defined is the height of the
measured Doppler signal? Why do phase difference spectra always pull
to 0<SUP>o</SUP> phase lag at high frequencies? Can we actually observe
high frequency waves (P< 70 s)? What is the atmospheric MTF of high
frequency waves? How reliably can we determine the energy flux of high
frequency waves? We address these questions by comparing observations
obtained with Hinode/NFI with results from two 3D numerical simulations
(Oslo Stagger and CO<SUP>5</SUP>BOLD). Our results suggest that the
observed high frequency Doppler velocity signal is caused by rapid
height variations of the velocity response function in an atmosphere
with strong velocity gradients and cannot be interpreted as evidence of
propagating high frequency acoustic waves. Estimates of the energy flux
of high frequency waves should be treated with caution, in particular
those that apply atmospheric MTF corrections.
---------------------------------------------------------
Title: On the detection of fast moving upflows in the quiet solar
photosphere.
Authors: Straus, Th.; Fleck, B.; Jefferies, S. M.; Carlsson, M.;
Tarbell, T. D.
2010MmSAI..81..751S Altcode: 2010arXiv1002.3305S
In our studies of the dynamics and energetics of the solar atmosphere,
we have detected, in high-quality observations from Hinode SOT/NFI,
ubiquitous small-scale upflows which move horizontally with supersonic
velocities in the quiet Sun. We present the properties of these fast
moving upflows (FMUs) and discuss different interpretations.
---------------------------------------------------------
Title: Chromospheric heating and structure as determined from high
resolution 3D simulations .
Authors: Carlsson, M.; Hansteen, V. H.; Gudiksen, B. V.
2010MmSAI..81..582C Altcode: 2010arXiv1001.1546C
We have performed 3D radiation MHD simulations extending from the
convection zone to the corona covering a box 16 Mm<SUP>3</SUP> at 32
km spatial resolution. The simulations show very fine structure in
the chromosphere with acoustic shocks interacting with the magnetic
field. Magnetic flux concentrations have a temperature lower than the
surroundings in the photosphere but higher in the low chromosphere. The
heating is there mostly through ohmic dissipation preferentially at
the edges of the flux concentrations. The magnetic field is often
wound up around the flux concentrations. When acoustic waves travel
up along the field this topology leads to swirling motions seen in
chromospheric diagnostic lines such as the calcium infrared triplet.
---------------------------------------------------------
Title: The Interface Region Imaging Spectrograph (IRIS) Small Explorer
Authors: de Pontieu, B.; Title, A. M.; Schryver, C. J.; Lemen, J. R.;
Golub, L.; Kankelborg, C. C.; Carlsson, M.
2009AGUFMSH33B1499D Altcode:
The Interface Region Imaging Spectrograph (IRIS) was recently selected
as a small explorer mission by NASA. The primary goal of IRIS is to
understand how the solar atmosphere is energized. The IRIS investigation
combines advanced numerical modeling with a high resolution 20 cm UV
imaging spectrograph that will obtain spectra covering temperatures
from 4,500 to 10 MK in three wavelength ranges (1332-1358 Angstrom,
1390-1406 Angstrom and 2785-2835 Angstrom) and simultaneous images
covering temperatures from 4,500 K to 65,000 K. IRIS will obtain UV
spectra and images with high resolution in space (1/3 arcsec) and time
(1s) focused on the chromosphere and transition region of the Sun, a
complex dynamic interface region between the photosphere and corona. In
this region, all but a few percent of the non-radiative energy leaving
the Sun is converted into heat and radiation. IRIS fills a crucial gap
in our ability to advance Sun-Earth connection studies by tracing the
flow of energy and plasma through this foundation of the corona and
heliosphere. The IRIS investigation is led by PI Alan Title (LMSAL)
with major participation by the Harvard Smithsonian Astrophysical
Observatory, Montana State University, NASA Ames Research Center,
Stanford University and the University of Oslo (Norway). IRIS is
scheduled for launch in late 2012, and will have a nominal two year
mission lifetime.
---------------------------------------------------------
Title: MULTI3D: A Domain-Decomposed 3D Radiative Transfer Code
Authors: Leenaarts, J.; Carlsson, M.
2009ASPC..415...87L Altcode:
We present MULTI3D, a 3D radiative transfer code currently under
development. It is optimized for computing NLTE problems based on
(radiation-)MHD models of stellar atmospheres. MULTI3D is based on
MULTI and includes most of the physics present in that code. MULTI3D was
first written as a serial code by Botnen (1997) and has recently been
upgraded to an MPI-parallelized, domain-decomposed version. The code
has so far successfully been run on up to 64 processors, solving the
NLTE radiative transfer for a six-level Ca II atom with 400 frequency
points in an atmosphere of 256 × 128 × 108 grid points.
---------------------------------------------------------
Title: On-disk Counterparts of Type II Spicules in the Ca II 854.2
nm and Hα Lines
Authors: Rouppe van der Voort, L.; Leenaarts, J.; de Pontieu, B.;
Carlsson, M.; Vissers, G.
2009ApJ...705..272R Altcode: 2009arXiv0909.2115R
Recently, a second type of spicules was discovered at the solar
limb with the Solar Optical Telescope onboard the Japanese Hinode
spacecraft. These previously unrecognized type II spicules are thin
chromospheric jets that are shorter lived (10-60 s) and that show much
higher apparent upward velocities (of order 50-100 km s<SUP>-1</SUP>)
than the classical spicules. Since they have been implicated in
providing hot plasma to coronal loops, their formation, evolution,
and properties are important ingredients for a better understanding
of the mass and energy balance of the low solar atmosphere. Here, we
report on the discovery of the disk counterparts of type II spicules
using spectral imaging data in the Ca II 854.2 nm and Hα lines with
the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope in
La Palma. We find rapid blueward excursions in the line profiles of
both chromospheric lines that correspond to thin, jet-like features
that show apparent velocities of order 50 km s<SUP>-1</SUP>. These
blueward excursions seem to form a separate absorbing component with
Doppler shifts of order 20 and 50 km s<SUP>-1</SUP> for the Ca II 854.2
nm and Hα line, respectively. We show that the appearance, lifetimes,
longitudinal and transverse velocities, and occurrence rate of these
rapid blue excursions on the disk are very similar to those of the type
II spicules at the limb. A detailed study of the spectral line profiles
in these events suggests that plasma is accelerated along the jet,
and plasma is being heated throughout the short lifetime of the event.
---------------------------------------------------------
Title: Twisted Flux Tube Emergence from the Convection Zone to the
Corona. II. Later States
Authors: Martínez-Sykora, Juan; Hansteen, Viggo; Carlsson, Mats
2009ApJ...702..129M Altcode: 2009arXiv0906.5464M
Three-dimensional numerical simulations of magnetic flux emergence
are carried out in a computational domain spanning the upper layers
of the convection zone to the lower corona. We use the Oslo Staggered
Code to solve the full magnetohydrodynamic equations with non-gray
and non-local thermodynamic equilibrium radiative transfer and thermal
conduction along the magnetic field lines. In this paper, we concentrate
on the later stages of the simulations and study the evolution of the
structure of the rising flux in the upper chromosphere and corona, the
interaction between the emerging flux and the weak coronal magnetic
field initially present, and the associated dynamics. The flux tube
injected at the bottom boundary rises to the photosphere where it
largely remains. However, some parts of the flux tube become unstable
and expand in patches into the upper chromosphere. The flux rapidly
expands toward the corona, pushing the coronal and transition region
material aside, and lifting and maintaining the transition region at
heights greater than 5 Mm above the photosphere for extensive periods
of time. The pre-existing magnetic field in the corona and transition
region is perturbed by the incoming flux and reoriented by a series of
high Joule heating events. Low-density structures form in the corona,
while at later times a high-density filamentary structure appears in
the lower part of the expanding flux. The dynamics of these and other
structures is discussed. While Joule heating due to the expanding flux
is episodic, it increases in relative strength as fresh magnetic field
rises and becomes energetically important in the upper chromosphere and
corona at later times. Chromospheric, transition region, and coronal
lines are computed and their response to the perturbation caused by
the expanding emerging flux is discussed.
---------------------------------------------------------
Title: Spicule-Like Structures Observed in Three-Dimensional Realistic
Magnetohydrodynamic Simulations
Authors: Martínez-Sykora, Juan; Hansteen, Viggo; De Pontieu, Bart;
Carlsson, Mats
2009ApJ...701.1569M Altcode: 2009arXiv0906.4446M
We analyze features that resemble type I spicules in two different
three-dimensional numerical simulations in which we include horizontal
magnetic flux emergence in a computational domain spanning the
upper layers of the convection zone to the lower corona. The two
simulations differ mainly in the pre-existing ambient magnetic field
strength and in the properties of the inserted flux tube. We use the
Oslo Staggered Code to solve the full magnetohydrodynamic equations
with nongray and non-LTE radiative transfer and thermal conduction
along the magnetic field lines. We find a multitude of features that
show a spatiotemporal evolution that is similar to that observed in
type I spicules, which are characterized by parabolic height versus
time profiles, and are dominated by rapid upward motion at speeds
of 10-30 km s<SUP>-1</SUP>, followed by downward motion at similar
velocities. We measured the parameters of the parabolic profile of the
spicules and find similar correlations between the parameters as those
found in observations. The values for height (or length) and duration
of the spicules found in the simulations are more limited in range than
those in the observations. The spicules found in the simulation with
higher pre-existing ambient field have shorter length and smaller
velocities. From the simulations, it appears that these kinds of
spicules can, in principle, be driven by a variety of mechanisms that
include p-modes, collapsing granules, magnetic energy release in the
photosphere and lower chromosphere, and convective buffeting of flux
concentrations.
---------------------------------------------------------
Title: Neutral oxygen spectral line formation revisited with new
collisional data: large departures from LTE at low metallicity
Authors: Fabbian, D.; Asplund, M.; Barklem, P. S.; Carlsson, M.;
Kiselman, D.
2009A&A...500.1221F Altcode: 2009arXiv0902.4472F
Aims: A detailed study is presented, including estimates of the
impact on elemental abundance analysis, of the non-local thermodynamic
equilibrium (non-LTE) formation of the high-excitation neutral oxygen
777 nm triplet in model atmospheres representative of stars with
spectral types F to K. <BR />Methods: We have applied the statistical
equilibrium code MULTI to a number of plane-parallel MARCS atmospheric
models covering late-type stars (4500 ≤ T_eff ≤ 6500 K, 2 ≤ log
g ≤ 5 [cgs], and -3.5 ≤ [Fe/H] ≤ 0). The atomic model employed
includes, in particular, recent quantum-mechanical electron collision
data. <BR />Results: We confirm that the O i triplet lines form under
non-LTE conditions in late-type stars, suffering negative abundance
corrections with respect to LTE. At solar metallicity, the non-LTE
effect, mainly attributed in previous studies to photon losses in the
triplet itself, is also driven by an additional significant contribution
from line opacity. At low metallicity, the very pronounced departures
from LTE are due to overpopulation of the lower level (3s ^5S^o) of the
transition. Large line opacity stems from triplet-quintet intersystem
electron collisions, a form of coupling previously not considered or
seriously underestimated. The non-LTE effects generally become severe
for models (both giants and dwarfs) with higher T_eff. Interestingly,
in metal-poor turn-off stars, the negative non-LTE abundance corrections
tend to rapidly become more severe towards lower metallicity. When
neglecting H collisions, they amount to as much as |Δlog ɛ_O|
~ 0.9 dex and ~1.2 dex, respectively at [Fe/H] = -3 and [Fe/H]
= -3.5. Even when such collisions are included, the LTE abundance
remains a serious overestimate, correspondingly by |Δlog ɛ_O| ~ 0.5
dex and ~0.9 dex at such low metallicities. Although the poorly known
inelastic hydrogen collisions thus remain an important uncertainty,
the large metallicity-dependent non-LTE effects seem to point to
a resulting “low” (compared to LTE) [O/Fe] in metal-poor halo
stars. <BR />Conclusions: Our results may be important in solving
the long-standing [O/Fe] debate. When applying the derived non-LTE
corrections, the LTE oxygen abundance inferred from the 777 nm permitted
triplet will be decreased substantially at low metallicity. If the
classical Drawin formula is employed for O+H collisions, the derived
[O/Fe] trend becomes almost flat below [Fe/H] ~ -1, in better agreement
with recent literature estimates generally obtained from other oxygen
abundance indicators. A value of [O/Fe] ⪉ +0.5 may therefore be
appropriate, as suggested by standard theoretical models of type II
supernovae nucleosynthetic yields. If neglecting impacts with H atoms
instead, [O/Fe] decreases towards lower [Fe/H], which would open new
questions. Our tests using ATLAS model atmospheres show that, though
non-LTE corrections for metal-poor dwarfs are smaller (by ~0.2 dex
when adopting efficient H collisions) than in the MARCS case, our
main conclusions are preserved, and that the LTE approach tends to
seriously overestimate the O abundance at low metallicity. However,
in order to finally reach consistency between oxygen abundances from
the different available spectral features, it is of high priority to
reduce the large uncertainty regarding H collisions, to undertake a
full investigation of the interplay of non-LTE and 3D effects, and to
clarify the issue of the temperature scale at low metallicity.
---------------------------------------------------------
Title: The Temperature Diagnostic Properties of the Mg I 457.1 nm Line
Authors: Langangen, Øystein; Carlsson, Mats
2009ApJ...696.1892L Altcode: 2009arXiv0903.1492L
We analyze the important formation processes for the Mg I 457.1 nm
line. This line is an intercombination line and the source function
is close to the local thermodynamic equilibrium (LTE) value. The
strong coupling to the local temperature and the relatively high
population of the lower level (the ground state of Mg I) makes this
line an ideal candidate for temperature diagnostics in the lower
chromosphere/temperature minimum region. Linking the temperature probed
to an absolute physical height is nontrivial because of non-LTE (NLTE)
ionization. We analyze the NLTE effects and find that photoionization
from the lower energy levels together with cascading collisional
recombination dominates the ionization balance. Taking properly into
account the line blanketing in the UV is essential for obtaining the
right photoionization rates. The identification of the main NLTE effects
in the line allows us to construct a "quintessential" model atom, ideal
for computationally demanding tasks, e.g., full three-dimensional
and/or time-dependent radiative transfer. Furthermore, we analyze
the diagnostic potential to temperature of this line in solarlike
atmospheres, by synthesizing the line from a series of parameterized
atmospheric models. These models have been constructed with fixed
effective temperature, but with a variable heat term in the energy
equation to obtain a chromospheric temperature rise at different
heights. We conclude that the line has a significant potential in
the diagnostics of the lower chromosphere temperature structure,
especially for cooler atmospheres, such as sunspots.
---------------------------------------------------------
Title: Three-Dimensional Non-LTE Radiative Transfer Computation of
the CA 8542 Infrared Line From a Radiation-MHD Simulation
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Rouppe van der
Voort, L.
2009ApJ...694L.128L Altcode: 2009arXiv0903.0791L
The interpretation of imagery of the solar chromosphere in the
widely used Ca II 854.2 nm infrared line is hampered by its complex,
three-dimensional, and non-LTE formation. Forward modeling is
required to aid understanding. We use a three-dimensional non-LTE
radiative transfer code to compute synthetic Ca II 854.2 nm images
from a radiation-MHD simulation of the solar atmosphere spanning
from the convection zone to the corona. We compare the simulation
with observations obtained with the CRISP filter at the Swedish 1 m
Solar Telescope. We find that the simulation reproduces dark patches
in the blue line wing caused by Doppler shifts, brightenings in the
line core caused by upward-propagating shocks, and thin dark elongated
structures in the line core that form the interface between upward and
downward gas motion in the chromosphere. The synthetic line core is
narrower than the observed one, indicating that the Sun exhibits both
more vigorous large-scale dynamics as well as small scale motions that
are not resolved within the simulation, presumably owing to a lack of
spatial resolution.
---------------------------------------------------------
Title: Hydrodynamics and radiative transfer of 3D model atmospheres.
Current status, limitations, and how to make headway
Authors: Carlsson, Mats
2009MmSAI..80..606C Altcode:
3D MHD models are important tools for advancing our understanding of
stellar atmospheres. A major computational challenge is the treatment
of radiative transfer; both to get a realistic treatment of the
energy transfer in the 3D modelling and for the diagnostic problem of
calculating the emergent spectrum in more detail from such models. The
current status, limitations and future directions of 3D MHD atmospheric
modelling and the treatment of radiative transfer are here discussed.
---------------------------------------------------------
Title: Advancing our understanding of the chromosphere
Authors: Hansteen, V. H.; Carlsson, M.; de Pontieu, B.
2008AGUFMSH51C..01H Altcode:
Recent progress has shown the solar chromosphere to be fundamentally
dynamic, where non-linear techniques must be used to understand
its nature. It is also the region where the magnetic field grows
to dominate the plasma and where the coupling between radiation and
matter becomes becomes quite tenuous. Understanding the workings of the
chromosphere is vital if one is to understand the flow of energy between
the solar surface and its outer atmosphere and wind. Recent numerical
developments have shown that it is feasible to model the chromosphere,
even to the extent that newly available high resolution observations
sometimes can be reproduced in detail. We will discuss the challenges
facing numerical chromospheric models and the observations needed to
validate or refute them.
---------------------------------------------------------
Title: Dynamics of the upper chromosphere
Authors: de Pontieu, B.; Carlsson, M.; McIntosh, S.; Hansteen, V.;
Tarbell, T.
2008AGUFMSH51C..05D Altcode:
In the past few years, high-resolution observations with ground-based
telescopes and the Broadband Filter Imager (BFI) and Narrowband
Filter Imager (NFI) of the Solar Optical Telescope onboard Hinode
have revolutionized our view of the dynamics and energetics of
the chromosphere. We review some of these results, including the
discovery of two different types of spicules and the finding that the
chromosphere is riddled with strong Alfvenic waves. We describe how
these observations, when combined with advanced numerical simulations,
can help address important unresolved issues regarding the connection
between the photosphere and corona, such as the role of waves and
of reconnection in driving the dynamics and energetics of the upper
chromosphere, and how chromospheric dynamics impact the transition
region and corona.
---------------------------------------------------------
Title: 3D radiative transfer in stellar atmospheres
Authors: Carlsson, M.
2008PhST..133a4012C Altcode:
Three-dimensional (3D) radiative transfer in stellar atmospheres is
reviewed with special emphasis on the atmospheres of cool stars and
applications. A short review of methods in 3D radiative transfer shows
that mature methods exist, both for taking into account radiation as an
energy transport mechanism in 3D (magneto-) hydrodynamical simulations
of stellar atmospheres and for the diagnostic problem of calculating the
emergent spectrum in more detail from such models, both assuming local
thermodynamic equilibrium (LTE) and in non-LTE. Such methods have been
implemented in several codes, and examples of applications are given.
---------------------------------------------------------
Title: What do Spicules Tell us About the Chromosphere?
Authors: de Pontieu, B.; Carlsson, M.; McIntosh, S.; Hansteen, V.;
Tarbell, T.
2008ESPM...12.2.15D Altcode:
In the past few years, high-resolution observations with ground-based
telescopes and the Broadband Filter Imager (BFI) and Narrowband
Filter Imager (NFI) of the Solar Optical Telescope onboard
Hinode have revolutionized our view of spicules and their role in
the chromosphere. We review some of these results, including the
discovery of two different types of spicules with different dynamics
and formation mechanisms, as well as the finding that the chromosphere
is riddled with strong Alfvenic waves. <P />In an effort to determine
the formation mechanism of spicules and their impact on the outer
atmosphere, we further focus on the thermal evolution and velocities
developed by spicules. We use Dopplergrams made in the Na D 589.6 nm,
H-alpha 656.3 nm and Mg B 517.3 nm passbands, as well as filtergrams in
the Ca H 396.8 nm passband to study the spatio-temporal relationship
between the various spicular features. We compare those findings with
synthesized images based on line profiles computed from high-resolution
3D MHD numerical simulations from the University of Oslo. We also use
the Dopplergram data to investigate the velocities that develop in
the two types of spicules that were reported previously. We perform
statistical analysis of apparent velocities in the plane of the sky
and line-of-sight velocities derived from Dopplergrams to disentangle
the superposition of Alfvenic wave amplitudes and field-aligned
flows. We study these properties for a variety of magnetic field
configurations (coronal holes, quiet Sun, active region). Finally,
we focus on the formation mechanism of spicules by analyzing spicular
features in Dopplergrams on the disk that were taken simultaneously
with SP magnetograms.
---------------------------------------------------------
Title: Constructing Computationally Tractable Models of Si I for
the 1082.7 nm Transition
Authors: Bard, S.; Carlsson, M.
2008ApJ...682.1376B Altcode:
The Si I 1082.7 nm line is often observed together with the He
I 1083.0 nm. The silicon line is assumed to give a measure of the
photospheric driver of the chromospheric dynamics observed in the He
I 1083.0 nm line. To exploit the diagnostic potential of the silicon
line it is of interest to study its line formation. We here analyze
physical processes in the formation of the Si I 1082.7 nm line and
find that it is formed in non-LTE (NLTE). Its line core intensity is
lower than the corresponding LTE value because the source function is
below the Planck function due to photon losses in the line itself. In
order to make possible full NLTE dynamic calculations of this line
we develop a general method for reducing the size of atomic models
used in NLTE calculations by representing several physically similar
energy levels as one mean level. Our procedure preserves all the
important physical properties of the atom that have a bearing on the
observable spectrographic properties of the line under study. We
apply our procedure to the Si I atom for the 1082.7 nm transition
under solar conditions, and we are able to reduce the size of the
atomic model from 238 levels and 3152 transitions to 23 levels and
171 transitions with no significant change in the calculated emergent
intensity of the 1082.7 nm line. We calculate the response functions
for the Si I 1082.7 nm line both in the quiet Sun and above sunspot
umbrae. We find that the line-center Doppler shift has a mean response
height to velocity perturbations of 541 km in a quiet-Sun atmosphere
and 308 km in a sunspot atmosphere. These numbers depend on the model
atmosphere employed.
---------------------------------------------------------
Title: Search for High Velocities in the Disk Counterpart of Type
II Spicules
Authors: Langangen, Ø.; De Pontieu, B.; Carlsson, M.; Hansteen,
V. H.; Cauzzi, G.; Reardon, K.
2008ApJ...679L.167L Altcode: 2008arXiv0804.3256L
Recently, De Pontieu and coworkers discovered a class of spicules
that evolve more rapidly than previously known spicules, with rapid
apparent motions of 50-150 km s<SUP>-1</SUP>, thickness of a few 100
km, and lifetimes of order 10-60 s. These so-called type II spicules
have been difficult to study because of limited spatiotemporal and
thermal resolution. Here we use the IBIS instrument to search for the
high velocities in the disk counterpart of type II spicules. We have
detected rapidly evolving events, with lifetimes that are less than a
minute and often equal to the cadence of the instrument (19 s). These
events are characterized by a Doppler shift that only appears in the
blue wing of the Ca II IR line. Furthermore, the spatial extent,
lifetime, and location near network all suggest a link to type II
spicules. However, the magnitude of the measured Doppler velocity is
significantly lower than the apparent motions seen at the limb. We
use Monte Carlo simulations to show that this discrepancy can be
explained by a forward model in which the visibility on the disk of
the high-velocity flows in these events is limited by a combination
of line-of-sight projection and reduced opacity in upward propelled
plasma, especially in reconnection driven jets that are powered by a
roughly constant energy supply.
---------------------------------------------------------
Title: Twisted Flux Tube Emergence From the Convection Zone to
the Corona
Authors: Martínez-Sykora, Juan; Hansteen, Viggo; Carlsson, Mats
2008ApJ...679..871M Altcode: 2007arXiv0712.3854M
Three-dimensional numerical simulations of a horizontal magnetic flux
tube emergence with different twist are carried out in a computational
domain spanning the upper layers of the convection zone to the lower
corona. We use the Oslo Stagger Code to solve the full MHD equations
with non-gray, non-LTE radiative transfer and thermal conduction along
the magnetic lines. A magnetic flux tube is input at the bottom boundary
into a weakly magnetized atmosphere. The photospheric and chromospheric
response is described with magnetograms and synthetic continuum as
well as Ca II H line images and velocity field distributions. In the
photosphere the granular size increases when the flux tube approaches
from below, as has been reported previously in the literature. In
the convective overshoot region, some 200 km above the photosphere,
adiabatic expansion produces cooling, darker regions with the structure
of granulation cells. We also find evidence of collapsed granulation
at the boundaries of the rising flux tube. Once the flux tube has
crossed the photosphere, bright points related to concentrated magnetic
field, vorticity, high vertical velocities, and heating by compressed
material are found at heights up to 500 km above the photosphere. At
greater heights, in the magnetized chromosphere, the rising flux tube
produces a large, cool, magnetized bubble that tends to expel the
usual chromospheric oscillations. In addition, the rising flux tube
dramatically increases the chromospheric scale height, pushing the
transition region and corona aside, such that the chromosphere extends
up to 6 Mm above the photosphere. We find flux tube emergence through
the photosphere to the lower corona to be a relatively slow process,
taking of order 1 hr.
---------------------------------------------------------
Title: Velocities and thermal evolution of chromospheric spicules
Authors: de Pontieu, B.; McIntosh, S. W.; Tarbell, T.; Carlsson,
M. P.; Hansteen, V. H.
2008AGUSMSP53A..06D Altcode:
We use the Broadband Filter Imager (BFI) and Narrowband Filter
Imager (NFI) of the Solar Optical Telescope on Hinode to study the
thermal evolution and velocities developed by chromospheric plasma in
spicules. We use Dopplergrams made in the Na D 589.6 nm, Hα 656.3 nm
and Mg B 517.3 nm passbands, as well as filtergrams in the Ca H 396.8 nm
passband to study the spatio-temporal relationship between the various
spicular features. We compare those findings with synthesized images
based on line profiles computed from high-resolution 3D MHD numerical
simulations from the University of Oslo. We also use the Dopplergram
data to investigate the velocities that develop in the two types of
spicules that were reported previously. We perform statistical analysis
of apparent velocities in the plane of the sky and line-of-sight
velocities derived from Dopplergrams to disentangle the superposition
of Alfvenic wave amplitudes and field-aligned flows. We study these
properties for a variety of magnetic field configurations (coronal
holes, quiet Sun, active region). Finally, we focus on the formation
mechanism of spicules by analyzing spicular features in Dopplergrams
on the disk that were taken simultaneously with SP magnetograms. This
work was supported by NASA contract NNM07AA01C. The Hinode mission is
operated by ISAS/JAXA, NAOJ, NASA, STFC, ESA and NSC.
---------------------------------------------------------
Title: Chromospheric Flows in the Vicinity of Magnetic Features in
the Quiet Sun Observed with Hinode SOT
Authors: Tarbell, T.; de Pontieu, B.; Carlsson, M.; Hansteen, V.;
McIntosh, S.; Ichimoto, K.
2008AGUSMSP41B..02T Altcode:
The Narrowband Filter Imager of the Solar Optical Telescope on Hinode
can measure Doppler shifts and line-of- sight magnetic fields in two
lines with contributions from the low chromosphere: Na D 589.6 nm and
Mg b 517.3 nm. The SOT Spectro-Polarimeter also measures very accurate
vector magnetic fields and Doppler velocities in the photosphere. These
observations have diffraction-limited spatial resolution and superb
stability. We present examples of these measurements in quiet sun
at various disk positions. In addition to the expected granulation
and f- and p-modes, conspicuous longer-lived downflows are seen near
strong network flux elements. Transient upflows are also detected,
presumably the base of flows seen in spicules at the limb and H-alpha
mottles on the disk. Velocity features associated with emerging and
cancelling magnetic features are also described. The observations are
compared with synthesized images made from line profiles computed from
the University of Oslo 3-D MHD simulations. This work was supported by
NASA contract NNM07AA01C. The Hinode mission is operated by ISAS/JAXA,
NAOJ, NASA, STFC, ESA and NSC.
---------------------------------------------------------
Title: Spectroscopic Measurements of Dynamic Fibrils in the Ca II
λ8662 Line
Authors: Langangen, Øystein; Carlsson, Mats; Rouppe van der Voort,
Luc; Hansteen, Viggo; De Pontieu, Bart
2008ApJ...673.1194L Altcode: 2007arXiv0710.0247L
We present high spatial resolution spectroscopic measurements of dynamic
fibrils (DFs) in the Ca II λ8662 line. These data show clear Doppler
shifts in the identified DFs, which demonstrates that at least a subset
of DFs are actual mass motions in the chromosphere. A statistical
analysis of 26 DFs reveals a strong and statistically significant
correlation between the maximal velocity and the deceleration. The
range of the velocities and the decelerations are substantially lower,
about a factor 2, in our spectroscopic observations compared to the
earlier results based on proper motion in narrowband images. There
are fundamental differences in the different observational methods;
when DFs are observed spectroscopically, the measured Doppler shifts
are a result of the atmospheric velocity, weighted with the response
function to velocity over an extended height. When the proper motion
of DFs is observed in narrowband images, the movement of the top
of the DF is observed. This point is sharply defined because of the
high contrast between the DF and the surroundings. The observational
differences between the two methods are examined by several numerical
experiments using both numerical simulations and a time series of
narrowband Hα images. With basis in the simulations we conclude that
the lower maximal velocity is explained by the low formation height of
the Ca IR line. We conclude that the present observations support the
earlier result that DFs are driven by magnetoacoustic shocks excited
by convective flows and p-modes.
---------------------------------------------------------
Title: Chromospheric Alfvénic Waves Strong Enough to Power the
Solar Wind
Authors: De Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Tarbell, T. D.; Schrijver, C. J.; Title, A. M.; Shine, R. A.;
Tsuneta, S.; Katsukawa, Y.; Ichimoto, K.; Suematsu, Y.; Shimizu, T.;
Nagata, S.
2007Sci...318.1574D Altcode:
Alfvén waves have been invoked as a possible mechanism for the heating
of the Sun's outer atmosphere, or corona, to millions of degrees and
for the acceleration of the solar wind to hundreds of kilometers per
second. However, Alfvén waves of sufficient strength have not been
unambiguously observed in the solar atmosphere. We used images of
high temporal and spatial resolution obtained with the Solar Optical
Telescope onboard the Japanese Hinode satellite to reveal that the
chromosphere, the region sandwiched between the solar surface and
the corona, is permeated by Alfvén waves with strong amplitudes on
the order of 10 to 25 kilometers per second and periods of 100 to
500 seconds. Estimates of the energy flux carried by these waves and
comparisons with advanced radiative magnetohydrodynamic simulations
indicate that such Alfvén waves are energetic enough to accelerate
the solar wind and possibly to heat the quiet corona.
---------------------------------------------------------
Title: Hinode and the Corona's Lower Boundary: Spicules and Alfven
Waves
Authors: de Pontieu, B.; McIntosh, S. W.; Hansteen, V.; Carlsson, M. P.
2007AGUFMSH52C..08D Altcode:
The lower boundary of the corona, or chromosphere, requires of order
100 times more energy than the corona itself, and provides the mass
to fill coronal loops. Yet the chromosphere and its coupling to the
corona is often overlooked. Recently, observations with the Solar
Optical Telescope (SOT) onboard Hinode and ground-based telescopes
combined with advanced numerical simulations have provided us with
unprecedented views and a better understanding of the (spicular)
dynamics of the chromosphere and how the lower boundary couples to
the corona and solar wind. We analyze high-resolution, high-cadence
Ca II and Hα observations of the solar chromosphere and find that the
dynamics of the magnetized chromosphere are dominated by at least two
different types of spicules. We show that the first type involves up-
and downward motion that is driven by shock waves that form when global
oscillations and convective flows leak into the chromosphere along
magnetic field lines on on 3-7 minute timescales. The second type of
spicules is much more dynamic: they form rapidly (in ~10s), are very
thin (<200km wide), have lifetimes of 10-150s (at any one height) and
seem to be rapidly heated to (at least) transition region temperatures,
sending material through the chromosphere at speeds of order 50-150
km/s. The properties of Type II spicules suggest a formation process
that is a consequence of magnetic reconnection. We discuss the impact of
both spicules types on the coronal mass and energy balance. Our analysis
of Hinode data also indicates that the chromosphere is permeated by
strong Alfvén waves. Both types of spicules are observed to carry
these Alfvén waves, which have significant amplitudes of order 20 km/s,
transverse displacements of order 500-1,000 km and periods of 150-400
s. Estimates of the energy flux carried by these Alfvén waves and
comparisons to advanced radiative MHD simulations indicate that these
waves most likely play a significant role in the acceleration of the
solar wind, and possibly the heating of the quiet Sun corona. We will
discuss the implications of these waves on the energy balance of the
lower atmosphere.
---------------------------------------------------------
Title: On Connecting the Dynamics of the Chromosphere and Transition
Region with Hinode SOT and EIS
Authors: Hansteen, Viggo H.; de Pontieu, Bart; Carlsson, Mats;
McIntosh, Scott; Watanabe, Tetsuya; Warren, Harry P.; Harra, Louise K.;
Hara, Hirohisa; Tarbell, Theodore D.; Shine, Dick; Title, Alan M.;
Schrijver, Carolus J.; Tsuneta, Saku; Katsukawa, Yukio; Ichimoto,
Kiyoshi; Suematsu, Yoshinori; Shimizu, Toshifumi
2007PASJ...59S.699H Altcode: 2007arXiv0711.0487H
We use coordinated Hinode SOT/EIS observations that include
high-resolution magnetograms, chromospheric, and transition region
(TR) imaging, and TR/coronal spectra in a first test to study how
the dynamics of the TR are driven by the highly dynamic photospheric
magnetic fields and the ubiquitous chromospheric waves. Initial
analysis shows that these connections are quite subtle and require a
combination of techniques including magnetic field extrapolations,
frequency-filtered time-series, and comparisons with synthetic
chromospheric and TR images from advanced 3D numerical simulations. As a
first result, we find signatures of magnetic flux emergence as well as
3 and 5mHz wave power above regions of enhanced photospheric magnetic
field in both chromospheric, transition region, and coronal emission.
---------------------------------------------------------
Title: Can High Frequency Acoustic Waves Heat the Quiet Sun
Chromosphere?
Authors: Carlsson, Mats; Hansteen, Viggo H.; de Pontieu, Bart;
McIntosh, Scott; Tarbell, Theodore D.; Shine, Dick; Tsuneta, Saku;
Katsukawa, Yukio; Ichimoto, Kiyoshi; Suematsu, Yoshinori; Shimizu,
Toshifumi; Nagata, Shin'ichi
2007PASJ...59S.663C Altcode: 2007arXiv0709.3462C
We use Hinode/SOT Ca II H-line and blue continuum broadband observations
to study the presence and power of high frequency acoustic waves at
high spatial resolution. We find that there is no dominant power at
small spatial scales; the integrated power using the full resolution of
Hinode (0.05” pixels, 0.16” resolution) is larger than the power in
the data degraded to 0.5” pixels (TRACE pixel size) by only a factor
of 1.2. At 20 mHz the ratio is 1.6. Combining this result with the
estimates of the acoustic flux based on TRACE data of Fossum &
Carlsson (2006), we conclude that the total energy flux in acoustic
waves of frequency 5-40 mHz entering the internetwork chromosphere of
the quiet Sun is less than 800 W m$^{-2}$, inadequate to balance the
radiative losses in a static chromosphere by a factor of five.
---------------------------------------------------------
Title: A Tale of Two Spicules: The Impact of Spicules on the Magnetic
Chromosphere
Authors: de Pontieu, Bart; McIntosh, Scott; Hansteen, Viggo H.;
Carlsson, Mats; Schrijver, Carolus J.; Tarbell, Theodore D.; Title,
Alan M.; Shine, Richard A.; Suematsu, Yoshinori; Tsuneta, Saku;
Katsukawa, Yukio; Ichimoto, Kiyoshi; Shimizu, Toshifumi; Nagata,
Shin'ichi
2007PASJ...59S.655D Altcode: 2007arXiv0710.2934D
We use high-resolution observations of the Sun in CaIIH (3968Å)
from the Solar Optical Telescope on Hinode to show that there are
at least two types of spicules that dominate the structure of the
magnetic solar chromosphere. Both types are tied to the relentless
magnetoconvective driving in the photosphere, but have very different
dynamic properties. “Type-I” spicules are driven by shock waves
that form when global oscillations and convective flows leak into
the upper atmosphere along magnetic field lines on 3--7minute
timescales. “Type-II” spicules are much more dynamic: they form
rapidly (in ∼ 10s), are very thin (≤ 200 km wide), have lifetimes
of 10-150s (at any one height), and seem to be rapidly heated to
(at least) transition region temperatures, sending material through
the chromosphere at speeds of order 50--150kms<SUP>-1</SUP>. The
properties of Type II spicules suggest a formation process that is
a consequence of magnetic reconnection, typically in the vicinity
of magnetic flux concentrations in plage and network. Both types of
spicules are observed to carry Alfvén waves with significant amplitudes
of order 20kms<SUP>-1</SUP>.
---------------------------------------------------------
Title: Non-equilibrium hydrogen ionization in 2D simulations of the
solar atmosphere
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Rutten, R. J.
2007A&A...473..625L Altcode: 2007arXiv0709.3751L
Context: The ionization of hydrogen in the solar chromosphere and
transition region does not obey LTE or instantaneous statistical
equilibrium because the timescale is long compared with important
hydrodynamical timescales, especially of magneto-acoustic shocks. Since
the pressure, temperature, and electron density depend sensitively on
hydrogen ionization, numerical simulation of the solar atmosphere
requires non-equilibrium treatment of all pertinent hydrogen
transitions. The same holds for any diagnostic application employing
hydrogen lines. <BR />Aims: To demonstrate the importance and to
quantify the effects of non-equilibrium hydrogen ionization, both
on the dynamical structure of the solar atmosphere and on hydrogen
line formation, in particular Hα. <BR />Methods: We implement an
algorithm to compute non-equilibrium hydrogen ionization and its
coupling into the MHD equations within an existing radiation MHD code,
and perform a two-dimensional simulation of the solar atmosphere from
the convection zone to the corona. <BR />Results: Analysis of the
simulation results and comparison to a companion simulation assuming
LTE shows that: a) non-equilibrium computation delivers much smaller
variations of the chromospheric hydrogen ionization than for LTE. The
ionization is smaller within shocks but subsequently remains high in
the cool intershock phases. As a result, the chromospheric temperature
variations are much larger than for LTE because in non-equilibrium,
hydrogen ionization is a less effective internal energy buffer. The
actual shock temperatures are therefore higher and the intershock
temperatures lower. b) The chromospheric populations of the hydrogen
n = 2 level, which governs the opacity of Hα, are coupled to the
ion populations. They are set by the high temperature in shocks
and subsequently remain high in the cool intershock phases. c)
The temperature structure and the hydrogen level populations differ
much between the chromosphere above photospheric magnetic elements
and above quiet internetwork. d) The hydrogen n = 2 population and
column density are persistently high in dynamic fibrils, suggesting
that these obtain their visibility from being optically thick in Hα
also at low temperature. <P />Movie and Appendix A are only available
in electronic form at http://www.aanda.org
---------------------------------------------------------
Title: High-Resolution Observations and Numerical Simulations of
Chromospheric Fibrils and Mottles
Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2007ASPC..368...65D Altcode:
With the recent advent of the Swedish 1-m Solar Telescope (SST),
advanced image processing techniques, as well as numerical simulations
that provide a more realistic view of the chromosphere, a comprehensive
understanding of chromospheric jets such as spicules, mottles and
fibrils is now within reach. In this paper, we briefly summarize results
from a recent analysis of dynamic fibrils, short-lived jet-like features
that dominate the chromosphere (as imaged in Hα) above and about active
region plage. Using extremely high-resolution observations obtained
at the SST, and advanced numerical 2D radiative MHD simulations, we
show that fibrils are most likely formed by chromospheric shock waves
that occur when convective flows and global oscillations leak into the
chromosphere along the field lines of magnetic flux concentrations. <P
/>In addition, we present some preliminary observations of quiet Sun
jets or mottles. We find that the mechanism that produces fibrils
in active regions is most likely also at work in quiet Sun regions,
although it is modified by the weaker magnetic field and the presence of
more mixed-polarity. A comparison with numerical simulations suggests
that the weaker magnetic field in quiet Sun allows for significantly
stronger (than in active regions) transverse motions that are
superposed on the field-aligned, shock-driven motions. This leads
to a more dynamic, and much more complex environment than in active
region plage. In addition, our observations of the mixed polarity
environment in quiet Sun regions suggest that other mechanisms, such
as reconnection, may well play a significant role in the formation of
some quiet Sun jets. Simultaneous high-resolution magnetograms (such
as those provided by Hinode), as well as numerical simulations that
take into account a whole variety of different magnetic configurations,
will be necessary to determine the relative importance in quiet Sun of,
respectively, the fibril-mechanism and reconnection.
---------------------------------------------------------
Title: Non-equilibrium Hydrogen Ionization in the Solar Atmosphere
Authors: Leenaarts, J.; Wedemeyer-Böhm, S.; Carlsson, M.; Hansteen,
V. H.
2007ASPC..368..103L Altcode:
The assumption of statistical equilibrium for atomic level populations
of hydrogen does not hold under the conditions of the chromosphere
due to the low density and the short dynamic timescale. In order to
calculate the hydrogen ionization balance and the electron density one
has to solve the time-dependent rate equations. We present results
from 2D and 3D radiation-magneto-hydrodynamics simulations of the
solar atmosphere incorporating the time-dependent rate equations
for hydrogen. Both the hydrogen ionization degree and the electron
density in our models are much more constant than LTE and statistical
equilibrium theory predict. These simulations provide multi-dimensional
model atmospheres with realistic electron densities and hydrogen level
populations that can be used in detailed radiative transfer modeling.
---------------------------------------------------------
Title: Chromospheric Spectrometry at High Spatial Resolution
Authors: Langangen, Ø.; Carlsson, M.; Rouppe van der Voort, L.;
Hansteen, V. H.; de Pontieu, B.
2007ASPC..368..145L Altcode:
In this summary we present spectrograms and images of the
chromosphere obtained in a co-observation campaign with the SST and
the DOT. The data are used to identify and measure the Doppler shifts
of dynamic fibrils. Quantitative comparison with the results of <P
/>\citep{ol-2006ApJ...647L..73H} requires compensation for several
observational issues.
---------------------------------------------------------
Title: 3D Numerical Models of the Chromosphere, Transition Region,
and Corona
Authors: Hansteen, V. H.; Carlsson, M.; Gudiksen, B.
2007ASPC..368..107H Altcode: 2007arXiv0704.1511H
A major goal in solar physics has during the last five decades
been to find how energy flux generated in the solar convection zone
is transported and dissipated in the outer solar layers. Progress
in this field has been slow and painstaking. However, advances in
computer hardware and numerical methods, vastly increased observational
capabilities and growing physical insight seem finally to be leading
towards understanding. Here we present exploratory numerical MHD models
that span the entire solar atmosphere from the upper convection zone
to the lower corona. These models include non-grey, non-LTE radiative
transport in the photosphere and chromosphere, optically thin radiative
losses as well as magnetic field-aligned heat conduction in the
transition region and corona.
---------------------------------------------------------
Title: Modeling the Solar Chromosphere
Authors: Carlsson, M.
2007ASPC..368...49C Altcode: 2007arXiv0704.1509C
Spectral diagnostic features formed in the solar chromosphere are few
and difficult to interpret --- they are neither formed in the optically
thin regime nor in local thermodynamic equilibrium (LTE). <P />To probe
the state of the chromosphere, both from observations and theory, it
is therefore necessary with modeling. <P />I discuss both traditional
semi-empirical modeling, numerical experiments illustrating important
ingredients necessary for a self-consistent theoretical modeling of
the solar chromosphere and the first results of such models.
---------------------------------------------------------
Title: Observational Evidence For The Ubiquity Of Strong Alfven
Waves In The Magnetized Chromosphere
Authors: De Pontieu, Bart; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Schrijver, C. J.; Tarbell, T. D.; Title, A.; SOT Team
2007AAS...210.9415D Altcode: 2007BAAS...39R.219D
Hinode/SOT Ca II broadband observations show that Alfven waves with
significant amplitudes of order 10-20 km/s and periods of 150-300 s are
extremely ubiquitous in the magnetized middle to upper chromosphere. Our
observations focus on spicules at the limb, and straw-like features
associated with network and plage on the disk. We find that the
weak straw-like features and most spicules all undergo significant
transverse motions that are driven by Alfven waves. These waves are
seen to propagate both up- and downward, and may carry an energy flux
that is significant compared to both the local, coronal and solar wind
energy balance. We will provide estimates of the energy flux carried
by these waves, and will compare our observations with Alfven waves
that are observed in 3D numerical simulations that include advanced
radiative transfer treatment for the chromosphere. <P />This work was
supported by NASA contract NNM07AA01C.
---------------------------------------------------------
Title: Connecting The Dynamics Of The Chromosphere And Transition
Region With Hinode/sot And Eis
Authors: Hansteen, Viggo H.; McIntosh, S.; De Pontieu, B.; Carlsson,
M.; SOT Team
2007AAS...210.9430H Altcode: 2007BAAS...39..222H
We will use coordinated Hinode SOT/EIS observations that include
high-resolution magnetograms, chromospheric and TR imaging and
TR/coronal spectra to study how the dynamics of the TR are driven by
the higly dynamic photospheric magnetic fields and the ubiquitous
chromospheric waves. Using travel time analysis, magnetic field
extrapolations, frequency filtered timeseries and comparisons with
synthetic chromospheric and TR images from advanced 3D numerical
simulations, we will study and establish how the dynamics of the
photosphere, chromosphere and TR are connected.
---------------------------------------------------------
Title: Magnetoacoustic Shocks as a Driver of Quiet-Sun Mottles
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Hansteen,
V. H.; Carlsson, M.; van Noort, M.
2007ApJ...660L.169R Altcode: 2007astro.ph..3535R
We present high spatial and high temporal resolution observations of
the quiet Sun in Hα obtained with the Swedish 1-m Solar Telescope on
La Palma. We observe that many mottles, jetlike features in the quiet
Sun, display clear up- and downward motions along their main axis. In
addition, many mottles show vigorous transverse displacements. Unique
identification of the mottles throughout their lifetime is much harder
than for their active region counterpart, dynamic fibrils. This is
because many seem to lack a sharply defined edge at their top, and
significant fading often occurs throughout their lifetime. For those
mottles that can be reliably tracked, we find that the mottle tops
often undergo parabolic paths. We find a linear correlation between
the deceleration these mottles undergo and the maximum velocity they
reach, similar to what was found earlier for dynamic fibrils. Combined
with an analysis of oscillatory properties, we conclude that at least
part of the quiet-Sun mottles are driven by magnetoacoustic shocks. In
addition, the mixed-polarity environment and vigorous dynamics suggest
that reconnection may play a significant role in the formation of some
quiet-Sun jets.
---------------------------------------------------------
Title: A Tale of Two Spicules
Authors: McIntosh, Scott W.; De Pontieu, B.; Carlsson, M.; Hansteen,
V. H.; Schrijver, C. J.; Tarbell, T. D.; Title, A. M.; SOT Team
2007AAS...210.9414M Altcode: 2007BAAS...39..219M
Hinode/SOT Ca II broadband images and movies show that there are
several different types of spicules at the limb. These different
types are distinguished by dynamics on different timescales. The
first type involves up- and downward motion on timescales of 3-5
minutes. The dynamics of these spicules are very similar to those of
fibrils and mottles as observed on the disk. Recent work suggests that
these are driven by slow-mode magnetoacoustic shocks that form when
convective flows and global oscillations leak into the chromosphere
along magnetic flux tubes. The second type is much more dynamic with
typical lifetimes of 10-60 s. These spicules are characterized by
sudden appearance and disappearance that may be indicative of rapid
heating to TR temperatures. We will describe the properties of these
spicules in various magnetic environments (coronal hole, quiet Sun,
active region) and study the possible role of reconnection in driving
the second type of spicules. In addition, we will perform detailed
comparisons of these different types of jets with synthetic Ca images
derived from advanced 3D numerical simulations that encompass the
convection zone up through the corona.
---------------------------------------------------------
Title: Can High Frequency Acoustic Waves Heat the Quiet Sun
Chromosphere?
Authors: Carlsson, Mats P. O.; De Pontieu, B.; Tarbell, T.; Hansteen,
V. H.; McIntosh, S.; SOT Team
2007AAS...210.6306C Altcode: 2007BAAS...39..172C
We use Hinode/SOT Ca II, G-band and blue continuum broadband
observations to study the presence and power of high frequency acoustic
waves at high spatial resolution. Previous observations with TRACE,
which were limited by the 1 arcsec resolution, and 1D numerical
simulations (Fossum & Carlsson, 2005) have been used to constrain
the possible role of high frequency waves in the heating of the quiet
Sun chromosphere. We will use the higher spatial resolution Hinode
data and comparisons with both 1D and 3D numerical models to study
the amount of high frequency power at smaller scales, and whether that
power is sufficient to heat the quiet Sun chromosphere.
---------------------------------------------------------
Title: Observations and Simulations of Fibrils and Mottles
Authors: De Pontieu, Bart; Hansteen, Viggo H.; Rouppe van der Voort,
Luc; van Noort, Michiel; Carlsson, Mats
2007astro.ph..2081D Altcode:
With the recent advent of the Swedish 1-m Solar Telescope (SST),
advanced image processing techniques, as well as numerical simulations
that provide a more realistic view of the chromosphere, a comprehensive
understanding of chromospheric jets such as spicules, mottles and
fibrils is now within reach. In this paper, we briefly summarize
results from a recent analysis of dynamic fibrils, short-lived
jet-like features that dominate the chromosphere (as imaged in
H-alpha) above and about active region plage. Using extremely
high-resolution observations obtained at the SST, and advanced
numerical 2D radiative MHD simulations, we show that fibrils are most
likely formed by chromospheric shock waves that occur when convective
flows and global oscillations leak into the chromosphere along the
field lines of magnetic flux concentrations. In addition, we present
some preliminary observations of quiet Sun jets or mottles. We find
that the mechanism that produces fibrils in active regions is most
likely also at work in quiet Sun regions, although it is modified by
the weaker magnetic field and the presence of more mixed-polarity. A
comparison with numerical simulations suggests that the weaker magnetic
field in quiet Sun allows for significantly stronger (than in active
regions) transverse motions that are superposed on the field-aligned,
shock-driven motions. This leads to a more dynamic, and much more
complex environment than in active region plage. In addition, our
observations of the mixed polarity environment in quiet Sun regions
suggest that other mechanisms, such as reconnection, may well play a
significant role in the formation of some quiet Sun jets.
---------------------------------------------------------
Title: Velocities Measured in Small-Scale Solar Magnetic Elements
Authors: Langangen, Øystein; Carlsson, Mats; Rouppe van der Voort,
Luc; Stein, R. F.
2007ApJ...655..615L Altcode: 2006astro.ph.11741L
We have obtained high-resolution spectrograms of small-scale magnetic
structures with the Swedish 1-m Solar Telescope. We present Doppler
measurements at 0.2" spatial resolution of bright points, ribbons,
and flowers, and their immediate surroundings, in the C I λ5380.3 line
(formed in the deep photosphere) and the two Fe I lines at 5379.6 and
5386.3 Å. The velocity inside the flowers and ribbons are measured to
be almost zero, while we observe downflows at the edges. These downflows
are increasing with decreasing height. We also analyze realistic
magnetoconvective simulations to obtain a better understanding of the
interpretation of the observed signal. We calculate how the Doppler
signal depends on the velocity field in various structures. Both the
smearing effect of the nonnegligible width of this velocity response
function along the line of sight and of the smearing from the telescope
and atmospheric point-spread function are discussed. These studies lead
us to the conclusion that the velocity inside the magnetic elements
is really upflow of order 1-2 km s<SUP>-1</SUP>, while the downflows
at the edges really are much stronger than observed, of order 1.5-3.3
km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: High-Resolution Observations and Modeling of Dynamic Fibrils
Authors: De Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2007ApJ...655..624D Altcode: 2007astro.ph..1786D
We present unprecedented high-resolution Hα observations, obtained
with the Swedish 1 m Solar Telescope, that, for the first time,
spatially and temporally resolve dynamic fibrils in active regions on
the Sun. These jetlike features are similar to mottles or spicules in
quiet Sun. We find that most of these fibrils follow almost perfect
parabolic paths in their ascent and descent. We measure the properties
of the parabolic paths taken by 257 fibrils and present an overview
of the deceleration, maximum velocity, maximum length, and duration,
as well as their widths and the thickness of a bright ring that often
occurs above dynamic fibrils. We find that the observed deceleration
of the projected path is typically only a fraction of solar gravity
and incompatible with a ballistic path at solar gravity. We report on
significant differences of fibril properties between those occurring
above a dense plage region and those above a less dense plage region
where the magnetic field seems more inclined from the vertical. We
compare these findings to advanced numerical two-dimensional radiative
MHD simulations and find that fibrils are most likely formed by
chromospheric shock waves that occur when convective flows and global
oscillations leak into the chromosphere along the field lines of
magnetic flux concentrations. Detailed comparison of observed and
simulated fibril properties shows striking similarities of the values
for deceleration, maximum velocity, maximum length, and duration. We
compare our results with observations of mottles and find that a
similar mechanism is most likely at work in the quiet Sun.
---------------------------------------------------------
Title: High Spatial Resolution Observations of Solar Magnetic
Structures
Authors: Rouppe van der Voort, L.; van Noort, M.; Carlsson, M.;
Hansteen, V.
2006ASPC..354...37R Altcode:
We present observations of the dynamic evolution of photospheric
magnetic structures in the G-band, continuum, magnetograms and
Dopplergrams. The observations were obtained with the Swedish one-m
Solar Telescope on La Palma. Using adaptive optics and the Multi-Object
Multi-Frame Blind Deconvolution image restoration technique, we obtained
several datasets at close to the diffraction limit of the telescope
(0.1 arcsec) over long periods of time. We show examples of the
dynamical evolution of different magnetic structures: the advection
of individual bright points by the granular flow, the formation and
fragmentation of flux sheets, and the continuous transition between
micro-pores, elongated ribbons and more circular “flowers”. Narrow
sheets with downdrafts are found right at the edges of magnetic field
concentrations.
---------------------------------------------------------
Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks
Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2006AGUFMSH23B0359D Altcode:
With the recent advent of the Swedish 1-m Solar Telescope (SST),
advanced image processing techniques, as well as numerical simulations
that provide a more realistic view of the chromosphere, a comprehensive
understanding of chromospheric jets such as spicules, mottles and
fibrils is now within reach. We will present results from a recent
analysis of dynamic fibrils, short-lived jet-like features that
dominate the chromosphere (as imaged in Hα) above and about active
region plage. These jets are similar to mottles and spicules in quiet
Sun. Our analysis is based on a time series of extremely high-resolution
(120 km) images taken in Hα linecenter at 1 second cadence, obtained by
the Oslo group at the SST in October 2005. The 78 min long time series
for the first time, spatially and temporally resolves dynamic fibrils
in active regions. Our analysis shows that most of the fibrils follow
almost perfect parabolic paths in their ascent and descent. We measure
the properties of the parabolic paths taken by 257 different dynamic
fibrils, and find that the observed deceleration of the projected
path is typically only a fraction of solar gravity, and incompatible
with a ballistic path at solar gravity. We report on significant
differences of measured fibril properties between those occurring in
association with a dense plage region, and those above a less dense
plage region where the magnetic field seems more inclined away from
the vertical. We compare these observational findings to advanced
numerical 2D radiative MHD simulations, and find that fibrils are most
likely formed by chromospheric shock waves that occur when convective
flows and global oscillations leak into the chromosphere along the
field lines of magnetic flux concentrations. Detailed comparison
of the properties of fibrils found in our observations and those in
our numerical simulations shows striking similarities of the values
for deceleration, maximum velocity, maximum length and duration. The
numerical simulations also reproduce the correlations we observe between
various fibrils properties, as well as the regional differences, taking
into account the different magnetic configuration for the various
regions. We compare our results with observations of mottles and find
that a similar mechanism is most likely at work in the quiet Sun.
---------------------------------------------------------
Title: Chromospheric Modeling
Authors: Carlsson, M.
2006ASPC..354..291C Altcode:
We discuss chromospheric modeling -- both semi-empirical models and
theoretical models. The development of new computational schemes,
new approximate treatments of time dependent hydrogen ionization and
more powerful computers have paved the way for comprehensive models in
three dimensions extending from the convection zone to the corona. The
first results of such models have just appeared.
---------------------------------------------------------
Title: The non-LTE line formation of neutral carbon in late-type stars
Authors: Fabbian, D.; Asplund, M.; Carlsson, M.; Kiselman, D.
2006A&A...458..899F Altcode: 2006astro.ph..8284F
Aims.We investigate the non-Local Thermodynamic Equilibrium (non-LTE)
line formation of neutral carbon in late-type stars in order to remove
some of the potential systematic errors in stellar abundance analyses
employing C i features.<BR /> Methods: .The statistical equilibrium
code MULTI was used on a grid of plane-parallel 1D MARCS atmospheric
models.<BR /> Results: .Within the parameter space explored, the
high-excitation C i lines studied are stronger in non-LTE due to
the combined effect of line source function drop and increased line
opacity due to overpopulation of the lower level for the transitions
considered; the relative importance of the two effects depends on
the particular combination of T<SUB>{eff</SUB>}, log g, [Fe/H] and
[C/Fe] and on the analysed C i line. As a consequence, the non-LTE
abundance corrections are negative and can be substantially so,
for example ∼ -0.4 dex in halo turn-off stars at [Fe/H]∼ -3. The
magnitude of the non-LTE corrections is rather insensitive to whether
inelastic H collisions are included or not.<BR /> Conclusions: .Our
results have implications on studies of nucleosynthetic processes and
on Galactic chemical evolution models. When applying our calculated
corrections to recent observational data, the upturn in [C/O] at low
metallicity might still be present (thus apparently still necessitating
contributions from massive Pop. III stars for the carbon production),
but at a lower level and possibly with a rather shallow trend of ∼
-0.2 dex/dex below [O/H]∼ -1.
---------------------------------------------------------
Title: Rapid Temporal Variability of Faculae: High-Resolution
Observations and Modeling
Authors: De Pontieu, B.; Carlsson, M.; Stein, R.; Rouppe van der Voort,
L.; Löfdahl, M.; van Noort, M.; Nordlund, Å.; Scharmer, G.
2006ApJ...646.1405D Altcode:
We present high-resolution G-band observations (obtained with the
Swedish 1 m Solar Telescope) of the rapid temporal variability of
faculae, which occurs on granular timescales. By combining these
observations with magnetoconvection simulations of a plage region, we
show that much of this variability is not intrinsic to the magnetic
field concentrations that are associated with faculae, but rather
a phenomenon associated with the normal evolution and splitting of
granules. We also show examples of facular variability caused by
changes in the magnetic field, with movies of dynamic behavior of
the striations that dominate much of the facular appearance at 0.1"
resolution. Examples of these dynamics include merging, splitting,
rapid motion, apparent fluting, and possibly swaying.
---------------------------------------------------------
Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks
Authors: Hansteen, V. H.; De Pontieu, B.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2006ApJ...647L..73H Altcode: 2006astro.ph..7332H
The formation of jets such as dynamic fibrils, mottles, and spicules
in the solar chromosphere is one of the most important, but also
most poorly understood, phenomena of the Sun's magnetized outer
atmosphere. We use extremely high resolution observations from the
Swedish 1 m Solar Telescope combined with advanced numerical modeling
to show that in active regions these jets are a natural consequence of
upwardly propagating slow-mode magnetoacoustic shocks. These shocks
form when waves generated by convective flows and global p-mode
oscillations in the lower lying photosphere leak upward into the
magnetized chromosphere. We find excellent agreement between observed
and simulated jet velocities, decelerations, lifetimes, and lengths. Our
findings suggest that previous observations of quiet-Sun spicules and
mottles may also be interpreted in light of a shock-driven mechanism.
---------------------------------------------------------
Title: Determination of the Acoustic Wave Flux in the Lower Solar
Chromosphere
Authors: Fossum, Astrid; Carlsson, Mats
2006ApJ...646..579F Altcode:
High-cadence observations of the quiet Sun have been obtained with
the Transition Region And Coronal Explorer (TRACE) satellite in two
UV passbands around 1600 and 1700 Å. The observational program has
been optimized for the detection of high-frequency acoustic waves:
the cadence is strictly regular, there is no data compression, and the
exposure time is optimized for quiet-Sun regions. Significant intensity
variations are detected up to 40 mHz frequency. Non-LTE radiation
hydrodynamics simulations are performed in order to calculate the
acoustic energy flux that the observed intensity variations correspond
to. The derived acoustic energy flux spectrum at the formation height
of the UV continua (about 400 km) is decreasing with frequency. For
frequencies above 40 mHz we derive an upper limit. The integrated
acoustic energy flux is 255 W m<SUP>-2</SUP> in the frequency range
5-50 mHz. This is lower than what is needed to balance the radiative
losses from the quiet-Sun chromosphere by at least a factor of 10. The
major uncertainty in the analysis is the possibility of high-frequency
power with spatial scales smaller than the resolution element of
TRACE. We make estimates of this effect and find it unlikely that
it is larger than a factor of 2. In the convection zone, where the
waves are generated, the energy spectrum is rather flat and may have
a peak at high frequencies. We also show that the sensitivity of the
observations to high-frequency waves is directly given by the Fourier
transform of the response function.
---------------------------------------------------------
Title: Radiative Hydrodynamic Models of Optical and Ultraviolet
Emission from M Dwarf Flares
Authors: Allred, Joel C.; Hawley, Suzanne L.; Abbett, William P.;
Carlsson, Mats
2006ApJ...644..484A Altcode: 2006astro.ph..3195A
We report on radiative hydrodynamic simulations of M dwarf stellar
flares and compare the model predictions to observations of several
flares. The flares were simulated by calculating the hydrodynamic
response of a model M dwarf atmosphere to a beam of nonthermal
electrons. Radiative back-warming through numerous soft X-ray,
extreme-ultraviolet, and ultraviolet transitions are also included. The
equations of radiative transfer and statistical equilibrium are treated
in non-LTE for many transitions of hydrogen, helium, and the Ca II
ion, allowing the calculation of detailed line profiles and continuum
radiation. Two simulations were carried out, with electron beam fluxes
corresponding to moderate and strong beam heating. In both cases we
find that the dynamics can be naturally divided into two phases: an
initial gentle phase in which hydrogen and helium radiate away much
of the beam energy and an explosive phase characterized by large
hydrodynamic waves. During the initial phase, lower chromospheric
material is evaporated into higher regions of the atmosphere, causing
many lines and continua to brighten dramatically. The He II 304 line
is especially enhanced, becoming the brightest line in the flaring
spectrum. The hydrogen Balmer lines also become much brighter and show
very broad line widths, in agreement with observations. We compare
our predicted Balmer decrements to decrements calculated for several
flare observations and find the predictions to be in general agreement
with the observations. During the explosive phase both condensation and
evaporation waves are produced. The moderate flare simulation predicts
a peak evaporation wave of ~130 km s<SUP>-1</SUP> and a condensation
wave of ~30 km s<SUP>-1</SUP>. The velocity of the condensation wave
matches velocities observed in several transition region lines. The
optical continuum also greatly intensifies, reaching a peak increase
of 130% (at 6000 Å) for the strong flare, but does not match observed
white-light spectra.
---------------------------------------------------------
Title: A comparison of solar proxy-magnetometry diagnostics
Authors: Leenaarts, J.; Rutten, R. J.; Carlsson, M.; Uitenbroek, H.
2006A&A...452L..15L Altcode:
Aims.We test various proxy-magnetometry diagnostics, i.e., brightness
signatures of small-scale magnetic elements, for studying magnetic
field structures in the solar photosphere.<BR /> Methods: .Images are
numerically synthesized from a 3D solar magneto-convection simulation
for, respectively, the G band at 430.5 nm, the CN band at 388.3 nm,
and the blue wings of the H α, H β, Ca ii H, and Ca ii 854.2 nm
lines.<BR /> Results: .Both visual comparison and scatter diagrams of
the computed intensity versus the magnetic field strength show that,
in particular for somewhat spatially extended magnetic elements, the
blue H α wing presents the best proxy-magnetometry diagnostic, followed
by the blue wing of H β. The latter yields higher diffraction-limit
resolution.<BR /> Conclusions: .We recommend using the blue H α wing
to locate and track small-scale photospheric magnetic elements through
their brightness appearance.
---------------------------------------------------------
Title: Simulation of Quiet-Sun Waves in the Ca II Infrared Triplet
Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T.; Carlsson, M.;
Stein, R. F.
2006ApJ...640.1142P Altcode: 2005astro.ph.10744P
The Ca II infrared triplet lines around 8540 Å are good candidates
for observing chromospheric magnetism. Model spectra of these lines
are obtained by combining a radiation hydrodynamic simulation with a
Stokes synthesis code. The simulation shows interesting time-varying
behavior of the Stokes V profiles as waves propagate through the
formation region of the lines. Disappearing and reappearing lobes
in the Stokes V profiles as well as profile asymmetries are closely
related to the atmospheric velocity gradients.
---------------------------------------------------------
Title: DOT tomography of the solar atmosphere. VI. Magnetic elements
as bright points in the blue wing of Hα
Authors: Leenaarts, J.; Rutten, R. J.; Sütterlin, P.; Carlsson, M.;
Uitenbroek, H.
2006A&A...449.1209L Altcode:
High-resolution solar images taken in the blue wing of the Balmer H
α line with the Dutch Open Telescope show intergranular magnetic
elements as strikingly bright features, similar to, but with
appreciably larger contrast over the surrounding granulation than
their more familiar manifestation as G-band bright points. Part of
this prominent appearance is due to low granular contrast, without
granule/lane brightness reversal as, e.g., in the wings of Ca II H
& K. We use 1D and 2D radiative transfer modeling and 3D solar
convection and magnetoconvection simulations to reproduce and explain
the H α wing images. We find that the blue H α wing obeys near-LTE
line formation. It appears particularly bright in magnetic elements
through low temperature gradients. The granulation observed in the blue
wing of H α has low contrast because of the lack of H α opacity in the
upper photosphere, Doppler cancellation, and large opacity sensitivity
to temperature working against source function sensitivity. We conclude
that the blue H α wing represents a promising proxy magnetometer to
locate and track isolated intermittent magnetic elements, a better one
than the G band and the wings of Ca II H & K although less sharp
at given aperture.
---------------------------------------------------------
Title: Numerical modelling of MHD waves in the solar chromosphere
Authors: Carlsson, Mats; Bogdan, Thomas J.
2006RSPTA.364..395C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Ci Non-LTE Spectral Line Formation in Late-Type Stars
Authors: Fabbian, D.; Asplund, M.; Carlsson, M.
2006cams.book...52F Altcode:
We present the results from our non-LTE investigation for neutral
carbon, which was carried out to remove potential systematic errors
in stellar abundance analyses. The calculations were performed for
late-type stars and give substantial negative non-LTE abundance
corrections. When applied to observations of extremely metal-poor
stars, which within the LTE framework seem to suggest a possible [C/O]
uprise at low metallicities (Akerman et al. 2004), these improvements
will have important implications, enabling us to understand if the
standard chemical evolution model is adequate, with no need to invoke
signatures by Pop. III stars for the carbon nucleosynthesis.
---------------------------------------------------------
Title: Simulated Solar Plages
Authors: Stein, R. F.; Carlsson, M.; de Pontieu, B.; Scharmer, G.;
Nordlund, Å.; Benson, D.
2006apri.meet...30S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Are High Frequency Acoustic Waves Sufficient to Heat the
Solar Chromosphere?
Authors: Fossum, A.; Carlsson, M.
2005ESASP.600E..17F Altcode: 2005ESPM...11...17F; 2005dysu.confE..17F
No abstract at ADS
---------------------------------------------------------
Title: The Diagnostic Potential of the MG i 4571.1 Å Line
Authors: Langangen, Ø.; Carlsson, M.; Rouppe van der Voort, L.
2005ESASP.600E..65L Altcode: 2005ESPM...11...65L; 2005dysu.confE..65L
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric Waves
Authors: Carlsson, M.; Hansteen, V.
2005ESASP.600E..16C Altcode: 2005dysu.confE..16C; 2005ESPM...11...16C
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric Waves
Authors: Carlsson, M.; Hansteen, V.
2005ESASP.596E..39C Altcode: 2005ccmf.confE..39C
No abstract at ADS
---------------------------------------------------------
Title: Small Scale Magnetic Elements as Bright Points in the Blue
Hα Wing
Authors: Leenaarts, J.; Sütterlin, P.; Rutten, R. J.; Carlsson, M.;
Uitenbroek, H.
2005ESASP.596E..15L Altcode: 2005ccmf.confE..15L
No abstract at ADS
---------------------------------------------------------
Title: Radiative Hydrodynamic Models of the Optical and Ultraviolet
Emission from Solar Flares
Authors: Allred, Joel C.; Hawley, Suzanne L.; Abbett, William P.;
Carlsson, Mats
2005ApJ...630..573A Altcode: 2005astro.ph..7335A
We report on radiative hydrodynamic simulations of moderate and strong
solar flares. The flares were simulated by calculating the atmospheric
response to a beam of nonthermal electrons injected at the apex of a
one-dimensional closed coronal loop and include heating from thermal
soft X-ray, extreme ultraviolet, and ultraviolet (XEUV) emission. The
equations of radiative transfer and statistical equilibrium were
treated in non-LTE and solved for numerous transitions of hydrogen,
helium, and Ca II, allowing the calculation of detailed line profiles
and continuum emission. This work improves on previous simulations
by incorporating more realistic nonthermal electron beam models and
includes a more rigorous model of thermal XEUV heating. We find that
XEUV back-warming contributes less than 10% of the heating, even in
strong flares. The simulations show elevated coronal and transition
region densities resulting in dramatic increases in line and continuum
emission in both the UV and optical regions. The optical continuum
reaches a peak increase of several percent, which is consistent with
enhancements observed in solar white-light flares. For a moderate flare
(~M class), the dynamics are characterized by a long gentle phase of
near balance between flare heating and radiative cooling, followed
by an explosive phase with beam heating dominating over cooling and
characterized by strong hydrodynamic waves. For a strong flare (~X
class), the gentle phase is much shorter, and we speculate that for even
stronger flares the gentle phase may be essentially nonexistent. During
the explosive phase, synthetic profiles for lines formed in the upper
chromosphere and transition region show blueshifts corresponding to
a plasma velocity of ~120 km s<SUP>-1</SUP>, and lines formed in the
lower chromosphere show redshifts of ~40 km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: High-frequency acoustic waves are not sufficient to heat the
solar chromosphere
Authors: Fossum, Astrid; Carlsson, Mats
2005Natur.435..919F Altcode:
One of the main unanswered questions in solar physics is why the Sun's
outer atmosphere is hotter than its surface. Theory predicts abundant
production of high-frequency (10-50mHz) acoustic waves in subsurface
layers of the Sun, and such waves are believed by many to constitute the
dominant heating mechanism of the chromosphere (the lower part of the
outer solar atmosphere) in non-magnetic regions. Such high-frequency
waves are difficult to detect because of high-frequency disturbances
in Earth's atmosphere (seeing) and other factors. Here we report the
detection of high-frequency waves, and we use numerical simulations
to show that the acoustic energy flux of these waves is too low, by a
factor of at least ten, to balance the radiative losses in the solar
chromosphere. Acoustic waves therefore cannot constitute the dominant
heating mechanism of the solar chromosphere.
---------------------------------------------------------
Title: Response Functions of the Ultraviolet Filters of TRACE and
the Detectability of High-Frequency Acoustic Waves
Authors: Fossum, Astrid; Carlsson, Mats
2005ApJ...625..556F Altcode:
We have used detailed non-LTE radiation hydrodynamic simulations to
investigate the detectability of high-frequency acoustic waves with
the Transition Region And Coronal Explorer (TRACE). A broad spectrum
of acoustic waves are fed into the computational domain at the lower
boundary of the model atmosphere, and TRACE UV continuum intensities
are calculated by folding the derived intensities with the TRACE filter
transmission functions for the 1700 and 1600 filters. Power spectra,
phase diagrams, and intensity response functions are calculated, and
intensity formation heights are derived. The simulations show that the
average response height of the 1700 and 1600 passbands are 360 and 430
km, with widths of 325 and 185 km. The width of the TRACE intensity
response functions reduces the power of the intensity oscillations
considerably, but if waves are present with power enough to be of
importance for the energy balance of the chromosphere, they should be
detectable at least up to 40 mHz in the absence of instrumental noise,
especially in the 1600 passband. The phase difference between the
synthesized 1600 and 1700 TRACE intensities follows the curve expected
for propagating acoustic waves up to 15 mHz. For higher frequencies
the phase difference decreases and approaches zero before the coherence
drops down, similar to the observed behavior. This is explained by the
double-peaked nature of the response function for the 1700 intensities.
---------------------------------------------------------
Title: Multi-dimensional Radiation MHD as a Tool to Understanding
Waves in Realistic Magnetic Topologies of the Solar Chromosphere
Authors: Carlsson, M.
2005AGUSMSH12A..01C Altcode:
Advances in computing capabilities have made it possible to perform
multi-dimensional radiation MHD simulations with a realistic description
of the physics in the photosphere. In the chromosphere non-LTE becomes
important and the choice is between realistic physics in one dimension
and approximations to the non-local radiation in multi-dimensions. We
will here discuss such simulations from a methodological vantage
point. We will discuss how we can make such simulations to study how
waves are excited in the solar convection zone, how they propagate
through the chromosphere and transition region and undergo mode
conversion, refraction and reflection in various magnetic field
topologies. We will also discuss the limitations of observational
diagnostics with respect to wide response functions, non-locality and
non-equilibrium effects.
---------------------------------------------------------
Title: Solar magnetic elements at 0.1 arcsec resolution. II. Dynamical
evolution
Authors: Rouppe van der Voort, L. H. M.; Hansteen, V. H.; Carlsson,
M.; Fossum, A.; Marthinussen, E.; van Noort, M. J.; Berger, T. E.
2005A&A...435..327R Altcode:
Small magnetic structures can be seen in G-band filtergrams as isolated
bright points, strings of bright points and dark micro-pores. At a
resolution of 0.1 arcsec, new forms of magnetic structures are found
in strong field areas: elongated “ribbons” and more circular
“flowers”. In this paper we study the temporal evolution of
these small scale magnetic structures. In strong-field regions the
time-evolution is more that of a magnetic fluid than that of collections
of flux-tubes that keep their identity. We find that the granular flow
concentrates the magnetic field into flux sheets that are visible as
thin bright features in the filtergrams. Weak upflows are found in
the flux sheets and downflows in the immediate surroundings. The flux
sheets often become unstable to a fluting instability and the edges
buckle. The sheets tend to break up into strings of bright points,
still with weak upflows in the magnetic elements and zero velocity or
downflows between them. Where there are larger flux concentrations
we find ribbons, flowers and micro-pores. There is a continuous
transition between these forms and they evolve from one form to
another. The appearance is mostly determined by the horizontal size
- larger structures are dark (micro-pores), narrower structures are
ribbon shaped and the flowers are the smallest in extent. All these
structures have darker inner parts and a bright edge. The plasma is
found to be at rest in the ribbons, with small concentrations of weak
upflow sites. Narrow sheets with downdrafts are found right at the
edges of the magnetic field concentrations.
---------------------------------------------------------
Title: Detectability of high frequency acoustic waves with TRACE
Authors: Fossum, A.; Carlsson, M.
2005ASSL..320..239F Altcode: 2005smp..conf..239F
No abstract at ADS
---------------------------------------------------------
Title: CI non LTE spectral line formation in late-type stars
Authors: Fabbian, D.; Asplund, M.; Carlsson, M.; Kiselman, D.
2005IAUS..228..255F Altcode: 2005astro.ph..8063F
We present non-Local Thermodynamic Equilibrium (non-LTE) calculations
for neutral carbon spectral line formation, carried out for a grid
of model atmospheres covering the range of late-type stars. The
results of our detailed calculations suggest that the carbon
non-LTE corrections in these stars are higher than usually adopted,
remaining substantial even at low metallicity. For the most metal-poor
stars in the sample of Akerman et al (2004), the effects are of the
order of Δlogɛ<SUB>C</SUB> ≃ -0.35ldots-0.45 (when neglecting H
collisions). Applying our results to those observations, the apparent
[C/O] upturn seen in their LTE analysis is no longer present, thus
revealing no need to invoke contributions from Pop. III stars to the
carbon nucleosynthesis.
---------------------------------------------------------
Title: Chromospheric Heating and Dynamics
Authors: Carlsson, M.; Stein, R. F.
2004ASPC..325..243C Altcode:
We review observations of the dynamics and energetics of the solar
chromosphere. The observations are interpreted with the help of
detailed radiation hydrodynamic modelling. It is concluded that
acoustic waves play an important role for the dynamics and energetics
of the chromosphere but additional heating is necessary, even for the
internetwork regions. This additional heating is strongly correlated
with the observed magnetic field strength.
---------------------------------------------------------
Title: Solar magnetic elements at 0.1 arcsec resolution. General
appearance and magnetic structure
Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Löfdahl,
M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.;
Title, A.; Scharmer, G.
2004A&A...428..613B Altcode:
New observations of solar magnetic elements in a remnant active region
plage near disk center are presented. The observations were obtained at
the recently commissioned Swedish 1-m Solar Telescope on La Palma. We
examine a single 430.5 nm G-band filtergram that resolves ∼70 km
(0.1 arcsec) structures and find new forms of magnetic structures
in this particular region. A cotemporal Ca II H-line image is used
to examine the low-chromosphere of network elements. A cotemporal Fe
I 630.25 nm magnetogram that resolves structures as small as 120 km
(0.18 arcsec) FWHM with a flux sensitivity of approximately 130 Mx
cm<SUP>-2</SUP> quantifies the magnetic structure of the region. A
Ni I 676.8 nm Dopplergram establishes relative velocity patterns
associated with the network features with an accuracy of about 300 m
s<SUP>-1</SUP>. We find that magnetic flux in this region as seen in
both the magnetogram and the G-band image is typically structured into
larger, amorphous, “ribbons” which are not resolved into individual
flux tubes. The measured magnetic flux density in the ribbon structures
ranges from 300 to 1500 Mx cm<SUP>-2</SUP>, the higher values occurring
at localized concentrations embedded within the ribbons. The Dopplergram
indicates relative downflows associated with all magnetic elements
with some indication that higher downflows occur adjacent to the peak
magnetic flux location. The mean absolute magnetic flux density of the
remnant plage network is about 130 Mx cm<SUP>-2</SUP>; in the lowest
flux regions of the field-of-view, the mean absolute flux density is
approximately 60 Mx cm<SUP>-2</SUP>. Within these quiet regions we do
not find evidence of pervasive kilo-gauss strength magnetic elements
as seen in recent high resolution internetwork studies. In general,
the observations confirm recent 3-dimensional numerical simulations
which show that the magnetic field in high-density regions such as
plage is concentrated in complex structures that are not generally
composed of discrete magnetic flux tubes. <P />Appendices are only
available in electronic form at http://www.edpsciences.org
---------------------------------------------------------
Title: Mode Conversion in Magneto-Atmospheres
Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V.; Heggland, L.;
Leer, E.; McMurry, A. D.; Stein, R. F.
2004AGUFMSH13A1162B Altcode:
Numerical simulations of wave propagation in a simple magneto-atmosphere
are employed to illustrate the complex nature of wave transformation
and conversion taking place in solar and stellar atmospheres. An
isothermal atmosphere threaded by a potential poloidal magnetic
field, and a superposed uniform toroidal field, is treated in a local
cartesian approximation. Spatial variations are restricted to the
two poloidal dimensions, but the toroidal field ensures that all
three MHD waves are present in the simulation. As in our previous
purely two-dimensional simulations (Bogdan et al. ApJ 599, 626-60,
2003), mode mixing and transformation take place at surfaces where
the magnetic and thermal pressures are equal. In the present case,
the upward propagating acoustic-gravity (MAG) wave is converted into
roughly equal parts transmitted fast, intermediate (Alfven), and
slow magneto-acoustic-gravity waves in passing through this mixing
layer. Unlike the fast and slow waves, the Alfven wave is weakly
damped, and is able to deposit its energy and momentum in the upper
chromosphere and corona. The fast and slow MAG waves are decoupled
on either side of mixing layer owing to their disparate propagation
speeds. Under certain fortuitous circumstances, the Alfven wave also
decouples from the fast and slow MAG waves.
---------------------------------------------------------
Title: Observational Manifestations of Solar Magnetoconvection:
Center-to-Limb Variation
Authors: Carlsson, Mats; Stein, Robert F.; Nordlund, Åke; Scharmer,
Göran B.
2004ApJ...610L.137C Altcode: 2004astro.ph..6160C
We present the first center-to-limb G-band images synthesized from
high-resolution simulations of solar magnetoconvection. Toward the
limb the simulations show “hilly” granulation with dark bands on
the far side, bright granulation walls, and striated faculae, similar
to observations. At disk center G-band bright points are flanked
by dark lanes. The increased brightness in magnetic elements is due
to their lower density compared with the surrounding intergranular
medium. One thus sees deeper layers where the temperature is higher. At
a given geometric height, the magnetic elements are cooler than the
surrounding medium. In the G band, the contrast is further increased
by the destruction of CH in the low-density magnetic elements. The
optical depth unity surface is very corrugated. Bright granules have
their continuum optical depth unity 80 km above the mean surface,
the magnetic elements 200-300 km below. The horizontal temperature
gradient is especially large next to flux concentrations. When viewed
at an angle, the deep magnetic elements' optical surface is hidden by
the granules and the bright points are no longer visible, except where
the “magnetic valleys” are aligned with the line of sight. Toward
the limb, the low density in the strong magnetic elements causes unit
line-of-sight optical depth to occur deeper in the granule walls behind
than for rays not going through magnetic elements, and variations
in the field strength produce a striated appearance in the bright
granule walls.
---------------------------------------------------------
Title: A Comparison of the Outer Atmosphere of the “Flat Activity”
Star τ Ceti (G8 V) with the Sun (G2 V) and α Centauri A (G2 V)
Authors: Judge, Philip G.; Saar, Steven H.; Carlsson, Mats; Ayres,
Thomas R.
2004ApJ...609..392J Altcode:
Driven by the desire to understand the roles of acoustic and
magnetic mechanisms in heating the outer atmospheres of Sun-like
stars, we compare solar UV spectra with archival STIS spectra from
the Hubble Space Telescope of α Cen A (G2 V) and new STIS spectra
of the extremely inactive dwarf star τ Cet (G8 V, V=3.5). The
activity of τ Cet shows occasional rotational modulations but no
long-term cyclic variation. It may be in a phase analogous to the
solar Maunder minimum. Solar disk center intensities from both the
HRTS instrument and the SUMER instrument on SOHO were converted to
Sun-as-a-star fluxes by using center-to-limb data from Dammasch and
colleagues. The derived solar flux spectrum represents conditions
near the minimum of the solar magnetic activity cycle. We find that
the τ Cet line profiles differ systematically from those of the Sun
and α Cen A. While lines formed in the middle chromospheres appear
similar, the entire emission from the upper chromosphere to the middle
transition region of τ Cet has lower flux densities by factors of
~2, the line widths are significantly narrower, and, uniquely, the
transition region lines are not significantly redshifted. The soft
X-ray surface flux of τ Cet, measured between 0.1 and 2.4 keV, is
~9×10<SUP>3</SUP> ergs cm<SUP>-2</SUP> s<SUP>-1</SUP>, several times
smaller than the median solar value. We also find that the UV spectrum
of α Cen serves as a proxy for a Sun-as-a-star spectrum when the Sun
is in an intermediate phase of its activity cycle but that its coronal
emission may be somewhat smaller. We discuss the implications of these
results for magnetic fields and heating mechanisms in the stars and
speculate that τ Cet's UV spectrum might represent the solar spectrum
during a grand minimum phase.
---------------------------------------------------------
Title: Solar Physics at Evergreen
Authors: Zita, E. J.; Bogdan, T. J.; Carlsson, M.; Judge, P.; Heller,
N.; Johnson, M.; Petty, S.
2004APS..NWS.C1005Z Altcode:
We have recently established a solar physics research program at
The Evergreen State College. Famed for its cloudy skies, the Pacific
Northwest is an ideal location for solar physics research activities
that do not require local observations. Collaborators from the High
Altitude Observatory (HAO) at the National Center for Atmospheric
Research have shared solar data from satellite-borne instruments such
as TRACE and SUMER. HAO colleagues also share data from computer
simulations of magneto-hydrodynamics (MHD) in the chromosphere,
generated by the Institute for Theoretical Astrophysics (ITA) at
the University of Oslo. Evergreen students and faculty learned to
analyze data from satellites and simulations, in Boulder and Oslo, and
established an infrastructure for continuing our analyses in Olympia. We
are investigating the role of magnetic waves in heating the solar
atmosphere. Comparing data from satellites and simulations shows that
acoustic oscillations from the photosphere cannot effectively propagate
into the chromosphere, but that magnetic waves can carry energy up
toward the hot, thin corona. We find that acoustic waves can change into
magnetic waves, especially near the magnetic "canopy," a region where
the sound speed is comparable to magnetic wave speeds. Understanding
MHD wave transformations and their role in energy transport can help
answer outstanding questions about the anomalous heating of the solar
atmosphere. Ref: Waves in the magnetized solar atmosphere II: Waves
from localized sources in magnetic flux concentrations. Bogdan et al.,
2003, ApJ 597
---------------------------------------------------------
Title: Millimeter observations and chromospheric dynamics
Authors: Loukitcheva, M.; Solanki, S. K.; Carlsson, M.; Stein, R. F.
2004A&A...419..747L Altcode:
The intensities of submillimeter and millimeter continua, which are
formed in LTE and depend linearly on temperature, may be able to provide
a test of models of the Solar chromosphere. We have taken a collection
of submillimeter and millimeter wave observed brightness temperatures
T<SUB>b</SUB> of the quiet Sun from the literature and compared it
with brightness temperatures computed from the standard static models
of Fontenla, Avrett and Loeser (FAL) and the dynamic simulations of
Carlsson & Stein (CS). The analysis of the dynamic simulations
of Carlsson & Stein reveals that radio emission at millimeter
wavelengths is extremely sensitive to dynamic processes in the
chromosphere, if these are spatially and temporally resolved. The most
striking result is that the dynamic picture of the solar internetwork
chromosphere is consistent with currently available millimeter and
submillimeter brightness observations. The spectrum obtained by
averaging over the spectra from all time-steps of CS simulations
provides a good fit to observed temporally and spatially averaged
millimeter data in spite of the absence of a permanent temperature
rise at low chromospheric heights in the simulations. This does not by
itself rule out the presence of a chromospheric temperature rise as
present in the FAL models, since a combination of such models also
reproduces the (low resolution) data relatively well. Millimeter
observations indicate that using radio techniques it is possible
to extend observations of the solar oscillatory component to the
heights above those previously observed in the photospheric and low
chromospheric spectral lines and submillimeter continuum. For more
precise diagnostics of chromospheric dynamics, high temporal and spatial
resolution interferometric observations in the millimeter-wavelength
region would be particularly useful. <P />Table \ref{tab:table} is
only available in electronic form at http://www.edpsciences.org
---------------------------------------------------------
Title: G-band Images from MHD Convection Simulations
Authors: Stein, R. F.; Carlsson, M.; Nordlund, A.; Scharmer, G.
2004AAS...204.8804S Altcode: 2004BAAS...36..820S
High resolution magneto-convection simulations are used to calculate
G-band and G-continuum images at various angles. Towards the limb
the simulations show "hilly" granulation, bright granulation walls,
intergranular striations and "sticking out" G-band bright features
similar to observations. The increased brightness in magnetic
elements is due to their lower density compared with the surrounding
intergranular medium, so that one sees deeper layers where the
temperature is higher. At a given geometric height, the magnetic
elements are not hotter than the surrounding medium. In the G-band,
the contrast is further increased by the destruction of CH in the
low density magnetic elements. The optical depth unity surface is
very corrugated. Bright granules have their continuum optical depth
unity 80 km above the mean surface, the magnetic elements 200-300 km
below. At large angles, the deep lying magnetic elements are hidden
by the granules and the bright points are no longer visible. Where
the "magnetic valleys" are aligned with the line of sight, they are
visible as elongated structures seemingly "sticking out". Even when
the deep hot surface is hidden, the low density in the strong magnetic
elements causes unit line-of-sight optical depth to occur deeper in
the granule walls behind then for rays not going through magnetic
elements. Flux concentrations in intergranular lanes therefore cause
a striped intensity pattern. This work is funded by NSF grants AST
0205500 and ATM 99881112 and NASA grants NAG 5 12450 and NNGO4GB92G.
---------------------------------------------------------
Title: Observations of solar magnetic elements with 0.1" resolution
Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Lofdahl,
M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.;
Title, A. M.; Scharmer, G.
2004AAS...204.2005B Altcode: 2004BAAS...36..686B
New observations of solar magnetic elements in a remnant active region
plage near disk center are presented. The observations were taken with
the Swedish 1-meter Solar Telescope on La Palma. We examine a single
430.5 nm G-band filtergram that resolves ∼70 km (0.”1) structures
and find new forms of magnetic structures in this particular region. A
simultaneous Ca II H-line image is used to examine the low-chromosphere
of network elements. A simultaneous Fe I 630.25 nm magnetogram
that resolves structures as small as 120 km (0.”18) FWHM with a
flux sensitivity of approximately 130 Mx cm<SUP>-2</SUP> quantifies
the magnetic structure of the region. A Ni I 676.8 nm Dopplergram
establishes relative velocity patterns associated with the network
features with an accuracy of about 300 m s<SUP>-1</SUP>. Magnetic flux
in this region as seen in both the magnetogram and the G-band image
is typically structured into larger, amorphous, “ribbons” with a
wide range of flux density values, rather than isolated kilogauss
flux tubes. We also present filtergrams and magnetograms of magnetic
elements at the solar limb showing that solar faculae are resolved
into bright granular walls that appear to project 350 to 500 km above
the photosphere.
---------------------------------------------------------
Title: Detectability of High Frequency Acoustic Waves with TRACE
Authors: Fossum, A.; Carlsson, M.
2004ESASP.547..125F Altcode: 2004soho...13..125F
High frequency acoustic waves have been proposed as a mechanism to heat
the Solar chromosphere in internetwork regions. Such waves are difficult
to detect using ground based observations because of seeing. Space
based Solar observatories like SOHO and TRACE are not hampered by such
high frequency disturbances caused by the Earth's atmosphere. We have
used detailed NLTE radiation hydrodynamic simulations to investigate
the detectability of high frequency acoustic waves with TRACE. A broad
spectrum of acoustic waves are fed into the computational domain at
the lower boundary of the model atmosphere and TRACE UV continuum
intensities are calculated by folding the derived intensities with the
TRACE filter functions for the 1600 and 1700 filters. Power spectra,
phase diagrams and intensity response functions are calculated and
intensity formation heights are derived. The simulations show that
the width of the TRACE intensity response functions sets an upper
frequency limit of 40 mHz for the detection of high frequency waves
even in the absence of instrumental noise. Furthermore, Poisson noise
for typical TRACE exposure times in a 30 minute time-series limits
the detectability to below 25 mHz.
---------------------------------------------------------
Title: Observations at 0{. ^{primeprime}}1 Resolution of the Dynamic
Evolution of Magnetic Elements
Authors: Carlsson, Mats; Rouppe van der Voort, Luc; Hansteen, Viggo
2004IAUS..223..207C Altcode: 2005IAUS..223..207C
We present observations of the dynamic evolution of photospheric
magnetic elements in the G-band, magnetograms and Dopplergrams. The
observations were obtained with the Swedish 1m Solar Telescope on La
Palma at close to the diffraction limit of 0{. ^{primeprime}}1. In
the most quiet regions we observe individual bright points in the
G-band with corresponding magnetic signal in the magnetograms. Where
the filling factor of the magnetic field is larger, the bright points
interact when advected by the granular and super-granular flow-fields,
flux sheets form and fragment. The plage region of the decaying active
region is filled with more complex topologies like ribbon structures
with darker interior and bright, knotted edges. These change into
flower-like shape when small in extent and into micro-pores when
the flux region is larger in extent. The magnetic elements in the
plage region are associated with upflows with strong downflows in the
immediate vicinity in the low-field region.
---------------------------------------------------------
Title: Radiative hydrodynamic simulations of acoustic waves in
sunspots
Authors: Bard, S.; Carlsson, M.
2004IAUS..223..445B Altcode: 2005IAUS..223..445B
We describe fully consistent NLTE radiation hydrodynamic simulations
of the propagation of acoustic waves in sunspot umbrae. In contrast
to the case of the quiet internetwork Sun, we find that the observed
behavior of the resonance lines of singly ionized calcium (H and K
lines) cannot be explained without adding non-acoustic heating. The
best agreement with observations is obtained with an extra heating
term as a function of height that is constant per volume.
---------------------------------------------------------
Title: High resolution limb images synthesized from 3D MHD simulations
Authors: Carlsson, Mats; Stein, Robert F.; Nordlund, Åke; Scharmer,
Göran B.
2004IAUS..223..233C Altcode: 2005IAUS..223..233C
We present the first center-to-limb G-band images synthesized from
high resolution simulations of solar magneto-convection. Towards the
limb the simulations show "hilly" granulation with dark bands on the
far side, bright granulation walls and striated faculae, similar
to observations. At disk center G-band bright points are flanked
by dark lanes. The increased brightness in magnetic elements is due
to their lower density compared with the surrounding intergranular
medium. One thus sees deeper layers where the temperature is higher. At
a given geometric height, the magnetic elements are cooler than the
surrounding medium. In the G-band, the contrast is further increased
by the destruction of CH in the low density magnetic elements. The
optical depth unity surface is very corrugated. Bright granules have
their continuum optical depth unity 80 km above the mean surface,
the magnetic elements 200-300 km below. The horizontal temperature
gradient is especially large next to flux concentrations. When viewed
at an angle, the deep magnetic elements optical surface is hidden by
the granules and the bright points are no longer visible, except where
the "magnetic valleys" are aligned with the line of sight. Towards
the limb, the low density in the strong magnetic elements causes
unit line-of-sight optical depth to occur deeper in the granule
walls behind than for rays not going through magnetic elements and
variations in the field strength produce a striated appearance in the
bright granule walls.
---------------------------------------------------------
Title: Theory and Simulations of Solar Atmosphere Dynamics
Authors: Stein, R. F.; Bogdan, T. J.; Carlsson, M.; Hansteen, V.;
McMurry, A.; Rosenthal, C. S.; Nordlund, Å.
2004ESASP.547...93S Altcode: 2004soho...13...93S
Numerical simulations are used to study the generation and propagation
of waves in the solar atmosphere. Solar p-mode oscillations are excited
by turbulent pressure work and entropy fluctuations (non-adiabatic gas
pressure work) near the solar surface. Interactions between short and
long period waves and radiative energy transfer control the formation of
shocks. The magnetic structure of the atmosphere induces coupling among
various MHD wave modes, with intense coupling and wave transformation
at the beta equal one surface, which likely is the location of the
so-called "magnetic canopy".
---------------------------------------------------------
Title: Waves in the Magnetized Solar Atmosphere. II. Waves from
Localized Sources in Magnetic Flux Concentrations
Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V. H.; McMurry, A.;
Rosenthal, C. S.; Johnson, M.; Petty-Powell, S.; Zita, E. J.; Stein,
R. F.; McIntosh, S. W.; Nordlund, Å.
2003ApJ...599..626B Altcode:
Numerical simulations of wave propagation in a two-dimensional
stratified magneto-atmosphere are presented for conditions that
are representative of the solar photosphere and chromosphere. Both
the emergent magnetic flux and the extent of the wave source are
spatially localized at the lower photospheric boundary of the
simulation. The calculations show that the coupling between the
fast and slow magneto-acoustic-gravity (MAG) waves is confined to
thin quasi-one-dimensional atmospheric layers where the sound speed
and the Alfvén velocity are comparable in magnitude. Away from this
wave conversion zone, which we call the magnetic canopy, the two MAG
waves are effectively decoupled because either the magnetic pressure
(B<SUP>2</SUP>/8π) or the plasma pressure (p=Nk<SUB>B</SUB>T)
dominates over the other. The character of the fluctuations observed
in the magneto-atmosphere depend sensitively on the relative location
and orientation of the magnetic canopy with respect to the wave source
and the observation point. Several distinct wave trains may converge
on and simultaneously pass through a given location. Their coherent
superposition presents a bewildering variety of Doppler and intensity
time series because (1) some waves come directly from the source while
others emerge from the magnetic canopy following mode conversion, (2)
the propagation directions of the individual wave trains are neither
co-aligned with each other nor with the observer's line of sight, and
(3) the wave trains may be either fast or slow MAG waves that exhibit
different characteristics depending on whether they are observed in
high-β or low-β plasmas (β≡8πp/B<SUP>2</SUP>). Through the
analysis of four numerical experiments a coherent and physically
intuitive picture emerges of how fast and slow MAG waves interact
within two-dimensional magneto-atmospheres.
---------------------------------------------------------
Title: MHD Waves in Magnetic Flux Concentrations
Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V.; Zita, E. J.;
Stein, R. F.; McIntosh, S. W.
2003AGUFMSH42B0535B Altcode:
Results from 2D MHD simulations of waves in a stratified isothermal
atmosphere will be presented and analyzed. The waves are generated
by a localized piston source situated on the lower, photospheric,
boundary of the computational domain. A combination of fast and slow
magneto-atmospheric waves propagates with little mutual interaction
until they encounter the surface where the sound speed and the Alfven
speed are comparable in magnitude. The waves couple strongly in this
region and emerge with different amplitudes and phases. Owing to
this mode mixing and the large variation in the Alfven speed in the
magneto-atmosphere, the fluctuations observed at a given location are
often a superposition of both fast and slow waves which have traversed
different paths and have undergone different transformations during
their journies.
---------------------------------------------------------
Title: On the Origin of the Basal Emission from Stellar Atmospheres:
Analysis of Solar C II Lines
Authors: Judge, Philip G.; Carlsson, Mats; Stein, Robert F.
2003ApJ...597.1158J Altcode:
Combining a variety of data with radiation hydrodynamic simulations,
we examine the heating of the Sun's internetwork chromosphere
and the hypothesis that the chromospheric “basal” emission
arises because of acoustic wave dissipation. We focus on the
2s2p<SUP>2</SUP><SUP>2</SUP>D-2s<SUP>2</SUP>2p<SUP>2</SUP>P<SUP>o</SUP>
multiplet of C II near 1335 Å, whose basal level of chromospheric
emission has been reliably determined for stars and the Sun by
Schrijver and colleagues. By accounting for center-to-limb variations
and the different spectral bandpasses of the instruments used, we find
that Schrijver's C II solar basal intensity substantially exceeds
stellar values, and that it can be identified with intensities seen
in typical internetwork regions with the SUMER instrument on the SOHO
spacecraft. Some time-series data sets of internetwork regions are
then examined and compared with simulations made specifically for a
typical observational data set, with vertical velocities at the lower
boundaries fixed from observations with the MDI instrument on SOHO. The
simulations can qualitatively account for the observed internetwork UV
continuum fluctuations seen with SUMER, formed 0.6-0.85 Mm above the
photosphere. However, they fail to capture almost any property of the
observed internetwork C II multiplet, which is formed substantially
higher. The time-averaged simulations can account for between 1/7
and 1/4 of the C II basal intensities; they predict oscillatory power
between 5 and 10 mHz, whereas internetwork observations are dominated by
low-frequency (<2 mHz) power of solar origin. The average simulated
C II intensities, which have a large contribution from the transition
region heated by conduction down from a coronal upper boundary,
fall short even of the smaller stellar basal intensities by a factor
of >=2. Together with known properties of weak, internetwork
photospheric magnetic fields, we conclude that the internetwork
upper chromosphere is probably dominated by magnetic heating. Thus,
the solar basal (and internetwork) intensities of the C II 1335 Å
multiplet originate from magnetic, and not acoustic, mechanisms,
in contradiction to the commonly accepted picture
---------------------------------------------------------
Title: Dynamic Modelling of the Outer Atmosphere of α Tau
Authors: McMurry, A. D.; Carlsson, M.; Stein, R. F.
2003csss...12..323M Altcode:
Using one-dimensional radiation-hydrodynamics simulations a model of
the outer atmosphere of α Tau is created. The reaction of the model
to acoustic waves is explored. It is found that high frequency waves
are radiatively damped out in the photosphere. The lower frequency
waves above the Hydrodynamic acoustic cutoff frequency do produce some
chromospheric heating.
---------------------------------------------------------
Title: Nonequilibrium CO Chemistry in the Solar Atmosphere
Authors: Asensio Ramos, A.; Trujillo Bueno, J.; Carlsson, M.;
Cernicharo, J.
2003ApJ...588L..61A Altcode: 2003astro.ph..3460R; 2003astro.ph..3460A
Investigating the reliability of the assumption of instantaneous
chemical equilibrium (ICE) for calculating the CO number density in
the solar atmosphere is of crucial importance for the resolution of
the long-standing controversy over the existence of “cool clouds”
in the chromosphere and for determining whether the cool gas owes its
existence to CO radiative cooling or to a hydrodynamical process. Here
we report the first results of such an investigation in which we
have carried out time-dependent gas-phase chemistry calculations
in radiation hydrodynamical simulations of solar chromospheric
dynamics. We show that while the ICE approximation turns out to be
suitable for modeling the observed infrared CO lines at the solar disk
center, it may substantially overestimate the “heights of formation”
of strong CO lines synthesized close to the edge of the solar disk,
especially concerning vigorous dynamic cases resulting from relatively
strong photospheric disturbances. This happens because during the
cool phases of the hydrodynamical simulations, the CO number density
in the outer atmospheric regions is smaller than what is stipulated
by the ICE approximation, resulting in decreased CO opacity in the
solar chromosphere. As a result, the cool CO-bearing gas that produces
the observed molecular lines must be located at atmospheric heights
not greater than ~700 km. We conclude that taking into account the
nonequilibrium chemistry improves the agreement with the available
on-disk and off-limb observations but that the hydrodynamical simulation
model has to be even cooler than anticipated by the ICE approximation,
and this has to be the case at the “new” (i.e., deeper) formation
regions of the rovibrational CO lines.
---------------------------------------------------------
Title: Multi-level 3D non-LTE computations of lithium lines in the
metal-poor halo stars HD 140283 and HD 84937
Authors: Asplund, M.; Carlsson, M.; Botnen, A. V.
2003A&A...399L..31A Altcode: 2003astro.ph..2406A
The lithium abundances in metal-poor halo stars are of importance
for cosmology, galaxy evolution and stellar structure. In an attempt
to study possible systematic errors in the derived Li abundances,
the line formation of Li I lines has been investigated by means of
realistic 3D hydrodynamical model atmospheres of halo stars and
3D non-LTE radiative transfer calculations. These are the first
detailed 3D non-LTE computations reported employing a multi-level
atomic model showing that such problems are now computationally
tractable. The detailed computations reveal that the Li I population
has a strong influence from the radiation field rather than the local
gas temperature, indicating that the low derived Li abundances found by
Asplund et al. (\cite{Asplund1999}) are an artifact of their assumption
of LTE. Relative to 3D LTE, the detailed calculations show pronounced
over-ionization. In terms of abundances the 3D non-LTE values are
within 0.05 dex of the 1D non-LTE results for the particular cases of
HD 140283 and HD 84937, which is a consequence of the dominance of the
radiation in determining the population density of Li I. Although 3D
non-LTE can be expected to give results rather close (~+/- 0.1 dex)
to 1D non-LTE for this reason, there may be systematic trends with
metallicity and effective temperature.
---------------------------------------------------------
Title: 3D NLTE Radiative Transfer - Current Status and Future
Prospects
Authors: Carlsson, M.
2003IAUS..210...51C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Towards 3D NLTE Radiation Magneto-Hydrodynamics
Authors: Carlsson, M.; Stein, R. F.
2003ASPC..288..505C Altcode: 2003sam..conf..505C
The problem of 3D Radiation Magneto-Hydrodynamics is too complex to
solve numerically in the general case; approximations are needed to
bring the numerical complexity to tractable levels. These approximations
are problem dependent. We will use the case of the Solar chromosphere
to illustrate these issues. The implementation of a 1D Radiation
Hydrodynamics code with a rather detailed and realistic treatment
of the coupling between radiation and matter is described. Scaling
properties and parallelization issues are discussed. Various
strategies and on-going work for the implementation of a 3D Radiation
Magneto-Hydrodynamics code are described.
---------------------------------------------------------
Title: Modelling Acoustic Shocks in Outer Atmospheres of Cool
Giant Stars
Authors: McMurry, A. D.; Carlsson, M.; Stein, R. F.
2003IAUS..210P..B7M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Wave processes in the solar upper atmosphere
Authors: Carlsson, Mats; Stein, Robert F.
2002ESASP.505..293C Altcode: 2002IAUCo.188..293C; 2002solm.conf..293C
The existence of a wide variety of wave-like phenomena are inferred
from observations of the solar upper atmosphere. Acoustic waves play
an important role for the dynamics and energetics of the chromosphere
but additional heating seems necessary even for the internetwork
regions. We show that it is unlikely that this extra heating is due
to high frequency acoustic waves due to the lack of their preferential
excitation and the strong radiative damping of high frequency waves in
the photosphere. When acoustic waves reach the height where the magnetic
pressure equals the gas pressure they will undergo mode conversion,
refraction and reflection. We discuss these processes and show that
the critical quantity is the angle between the magnetic field and the
velocity polarization; at angles smaller than 30 degrees much of the
acoustic, fast mode from the photosphere is transmitted as an acoustic,
slow mode propagating along the field lines. At larger angles, most of
the energy is refracted/reflected and returns as a fast mode creating
an interference pattern between the upward and downward propagating
waves. In 3D, this interference between waves at small angles creates
patterns with large horizontal phase speeds, especially close to
magnetic field concentrations. When damping from shock dissipation and
radiation is taken into account, the waves in the low-mid chromosphere
have mostly the character of upward propagating acoustic waves and it
is only close to the reflecting layer we get similar amplitudes for
the upward propagating and refracted/reflected waves.
---------------------------------------------------------
Title: Waves in magnetic flux concentrations: The critical role of
mode mixing and interference
Authors: Bogdan, T. J.; Rosenthal, C. S.; Carlsson, M.; Hansteen, V.;
McMurry, A.; Zita, E. J.; Johnson, M.; Petty-Powell, S.; McIntosh,
S. W.; Nordlund, Å.; Stein, R. F.; Dorch, S. B. F.
2002AN....323..196B Altcode:
Time-dependent numerical simulations of nonlinear wave propagation
in a two-dimensional (slab) magnetic field geometry show wave mixing
and interference to be important aspects of oscillatory phenomena in
starspots and sunspots. Discrete sources located within the umbra
generate both fast and slow MHD waves. The latter are compressive
acoustic waves which are guided along the magnetic field lines and
steepen into N-waves with increasing height in the spot atmosphere. The
former are less compressive, and accelerate rapidly upward through the
overlying low-beta portion of the umbral photosphere and chromosphere
(beta equiv 8pi p/ B<SUP>2</SUP>). As the fast wave fronts impinge
upon the beta ~ 1 penumbral “magnetic canopy" from above, they
interfere with the outward-propagating field-guided slow waves, and
they also mode convert to (non-magnetic) acoustic-gravity waves as
they penetrate into the weak magnetic field region which lies between
the penumbral canopy and the base of the surrounding photosphere. In
a three-dimensional situation, one expects additional generation,
mixing and interference with the remaining torsional Alfvén waves.
---------------------------------------------------------
Title: Dynamics in the solar chromosphere as a function of the
magnetic field topology
Authors: Karlsen, N.; Carlsson, M.
2002ESASP.508..303K Altcode: 2002soho...11..303K
We have looked at the coupling between the magnetic field and
chromospheric dynamics. Observations with the SUMER spectrograph of the
continuum radiation at 1319 Å have been correlated with simultaneous
MDI magnetograms and dopplergrams in high resolution mode. We have used
7 different observing runs for our analysis, all from 1996. The absolute
value of the magnetic field crossing the SUMER slit lies in the range
0-100 gauss. We observe a correlation between continuum intensity and
magnetic field strength all the way to the sensitivity limit of MDI
(about 2 G as 3σ in the mean value). Relative intensity fluctuations
at frequencies corresponding to propagating acoustic waves (>4.5
mHz) have smaller amplitudes with increasing radiation temperature
(or magnetic field strength). The absolute intensity fluctuations show
an increase with increasing radiation temperature. These findings are
consistent with a picture where a basic intensity level is set by a
magnetic heating process even in the darkest internetwork areas with
superimposed intensity variations caused by acoustic waves.
---------------------------------------------------------
Title: Dynamic Hydrogen Ionization
Authors: Carlsson, Mats; Stein, R. F.
2002ApJ...572..626C Altcode: 2002astro.ph..2313C
We investigate the ionization of hydrogen in a dynamic solar
atmosphere. The simulations include a detailed non-LTE treatment
of hydrogen, calcium, and helium but lack other important
elements. Furthermore, the omission of magnetic fields and the
one-dimensional approach make the modeling unrealistic in the upper
chromosphere and higher. We discuss these limitations and show
that the main results remain valid for any reasonable chromospheric
conditions. As in the static case, we find that the ionization of
hydrogen in the chromosphere is dominated by collisional excitation in
the Lyα transition followed by photoionization by Balmer continuum
photons-the Lyman continuum does not play any significant role. In
the transition region, collisional ionization from the ground
state becomes the primary process. We show that the timescale for
ionization/recombination can be estimated from the eigenvalues of a
modified rate matrix where the optically thick Lyman transitions that
are in detailed balance have been excluded. We find that the timescale
for ionization/recombination is dominated by the slow collisional
leakage from the ground state to the first excited state. Throughout the
chromosphere the timescale is long (10<SUP>3</SUP>-10<SUP>5</SUP> s),
except in shocks where the increased temperature and density shorten
the timescale for ionization/recombination, especially in the upper
chromosphere. Because the relaxation timescale is much longer than
dynamic timescales, hydrogen ionization does not have time to reach
its equilibrium value and its fluctuations are much smaller than the
variation of its statistical equilibrium value appropriate for the
instantaneous conditions. Because the ionization and recombination
rates increase with increasing temperature and density, ionization
in shocks is more rapid than recombination behind them. Therefore,
the ionization state tends to represent the higher temperature of the
shocks, and the mean electron density is up to a factor of 6 higher
than the electron density calculated in statistical equilibrium from
the mean atmosphere. The simulations show that a static picture and
a dynamic picture of the chromosphere are fundamentally different
and that time variations are crucial for our understanding of the
chromosphere itself and the spectral features formed there.
---------------------------------------------------------
Title: Dynamics and energetics of the solar chromosphere
Authors: Carlsson, Mats; Stein, Robert F.
2002ESASP.508..245C Altcode: 2002soho...11..245C
We present a summary of results from a number of observational programs
carried out with the SUMER instrument on board SOHO. Most datasets
show clear quasi-periodic dynamic behavior ("grains") in contiunuum
intensities with frequencies 3-10 mHz. Corresponding grains are seen
in intensities and velocities in neutral lines, normally with phase
differences consistent with upward propagating sound-waves. We compare
the observations with 1D radiation hydrodynamic simulations using
MDI Doppler-shifts to set the lower boundary. For continua formed in
the mid-chromosphere we find that the simulations give a good match
to the intensity fluctuations but that the minimum intensity is too
low. We find that high frequency acoustic waves (missing from the
current simulations) are unlikely to give the extra heating necessary
because of the strong radiative damping (90-99%) of such waves in
the photosphere. In continua formed in the low chromosphere the mean
intensity is similar in the simulations and the observations but
the simulated fluctuations are too large. The reported findings are
consistent with a picture where a basic intensity level is set by a
magnetic heating process even in the darkest internetwork areas with
superimposed intensity variations caused by acoustic waves.
---------------------------------------------------------
Title: Waves in the Magnetized Solar Atmosphere. I. Basic Processes
and Internetwork Oscillations
Authors: Rosenthal, C. S.; Bogdan, T. J.; Carlsson, M.; Dorch,
S. B. F.; Hansteen, V.; McIntosh, S. W.; McMurry, A.; Nordlund, Å.;
Stein, R. F.
2002ApJ...564..508R Altcode:
We have modeled numerically the propagation of waves through magnetic
structures in a stratified atmosphere. We first simulate the propagation
of waves through a number of simple, exemplary field geometries in
order to obtain a better insight into the effect of differing field
structures on the wave speeds, amplitudes, polarizations, direction
of propagation, etc., with a view to understanding the wide variety of
wavelike and oscillatory processes observed in the solar atmosphere. As
a particular example, we then apply the method to oscillations in the
chromospheric network and internetwork. We find that in regions where
the field is significantly inclined to the vertical, refraction by
the rapidly increasing phase speed of the fast modes results in total
internal reflection of the waves at a surface whose altitude is highly
variable. We conjecture a relationship between this phenomenon and the
observed spatiotemporal intermittancy of the oscillations. By contrast,
in regions where the field is close to vertical, the waves continue
to propagate upward, channeled along the field lines but otherwise
largely unaffected by the field.
---------------------------------------------------------
Title: Men-Invasion of Space
Authors: Carlsson, M. E.
2002iaf..confE.649C Altcode:
Last year I was given the fantastic opportunity by ESA to go to the IAF
congress in Toulouse. This was one of the most exciting experiences of
my life. I loved every minute of it, listening to the brilliant men from
different areas of space research, telling us wonderfully interesting
things about space. After a couple of sessions I understood there were
very few women standing up there and giving presentations. Except
of course, the Russian women translators and the students that were
invited. This struck me and I started to think about it and wondered
why? This encouraged me to investigate. I realized that I had to
turn to the children, the ones that are going to take over after
us, our new faces in space. When we were children we all wanted to
become astronauts, both boys and girls. But after a couple of years
it changed, why? So I started my research by asking questions to the
children in different ages concerning their lives and upbringing. I was
curious to find out when and how their personal goals changed in life,
compared to the men. Could it depend on what our parents did when we
were growing up or maybe where we were raised geographically? Perhaps
the encouragement of our teachers in our first years in school or
our social background, genes, manners and customs. When we have the
answers we also must ask ourselves how we can make women choose a
more engineering and scientific profile. Should we try to convert
them to engineers or should we use their other special abilities and
try to integrate them into the space program for example as medical
doctors? I think that ESA and other space organizations should use
their university students in a new outreach project. The students
should visit their local schools and inform the children about space
and the wonderful opportunities it can give us. This would give the
boys, but mostly the girls, a chance to discover space.
---------------------------------------------------------
Title: Wave Propagation in a Magnetized Atmosphere
Authors: Bogdan, T. J.; Rosenthal, C. S.; Carlsson, M.; McIntosh,
S.; Dorch, S.; Hansteen, V.; McMurry, A.; Nordlund, Å; Stein, R. F.
2001AGUSM..SH41A01B Altcode:
Numerical simulations of MHD wave propagation in plane-parallel
atmospheres threaded by non-trivial potential magnetic fields will be
presented, and their implications for understanding distinctions between
intranetwork and internetwork oscillations will be discussed. Our
findings basically confirm the conjecture of McIntosh et al. (2001,
ApJ 548, L237), that the two-dimensional surface where the Alfvén
and sound speeds coincide (i.e., where the plasma-β , the ratio of
gas to magnetic pressure, is of order unity) plays a fundamental
role in mediating the conversion between the fast-, intermediate-
(Alfvén), and slow-Magneto-Atmospheric-Gravity (MAG) waves. For
example, upward-propagating acoustic waves generated at the base of
the internetwork photosphere suffer significant downward reflection
when they encounter this β ≈ 1 surface. Close to the network, this
surface descends from the upper chromosphere and low corona (which
pertains in the internetwork cell interiors) down into the photosphere,
and so chromospheric oscillation `shadows' are predicted to surround
the network. In the network, strong vertical magnetic fields further
depress the β ≈ 1 surface below the surface layers where the
(magnetic field-aligned) acoustic waves (i.e., slow MAG-waves) are
generated. For frequencies in excess of the cutoff frequency, these
acoustic waves suffer little reflection from the overlying atmosphere
and they steepen as they progress upward.
---------------------------------------------------------
Title: An Observational Manifestation of Magnetoatmospheric Waves
in Internetwork Regions of the Chromosphere and Transition Region
Authors: McIntosh, S. W.; Bogdan, T. J.; Cally, P. S.; Carlsson, M.;
Hansteen, V. H.; Judge, P. G.; Lites, B. W.; Peter, H.; Rosenthal,
C. S.; Tarbell, T. D.
2001ApJ...548L.237M Altcode:
We discuss an observational signature of magnetoatmospheric waves in
the chromosphere and transition region away from network magnetic
fields. We demonstrate that when the observed quantity, line or
continuum emission, is formed under high-β conditions, where β is
the ratio of the plasma and magnetic pressures, we see fluctuations in
intensity and line-of-sight (LOS) Doppler velocity consistent with the
passage of the magnetoatmospheric waves. Conversely, if the observations
form under low-β conditions, the intensity fluctuation is suppressed,
but we retain the LOS Doppler velocity fluctuations. We speculate that
mode conversion in the β~1 region is responsible for this change in
the observational manifestation of the magnetoatmospheric waves.
---------------------------------------------------------
Title: Waves in the Magnetised Solar Atmosphere
Authors: Rosenthal, C. S.; Carlsson, M.; Hansteen, V.; McMurry,
A.; Bogdan, T. J.; McIntosh, S.; Nordlund, A.; Stein, R. F.; Dorch,
S. B. F.
2001IAUS..203..170R Altcode:
We have simulated the propagation of magneto-acoustic disturbances
through various magneto-hydrostatic structures constructed to mimic
the solar magnetic field. As waves propagate from regions of strong
to weak magnetic field and vice-versa different types of wave modes
(transverse and longitudinal) are coupled. In closed-field geometries
we see the trapping of wave energy within loop-like structures. In
open-field regions we see wave energy preferentially focussed away
from strong-field regions. We discuss these oscillations in terms
of various wave processes seen on the Sun - umbral oscillations,
penumbral running waves, internetwork oscillations etc.
---------------------------------------------------------
Title: Solar Chromospheric Oscillations
Authors: Carlsson, M.
2000eaa..bookE2261C Altcode:
By the term solar chromospheric oscillations we here refer to any
periodic or quasi-periodic dynamic phenomena seen in the solar
CHROMOSPHERE. We thus include both propagating and standing waves....
---------------------------------------------------------
Title: Rapid intensity and velocity variations in solar transition
region lines
Authors: Hansteen, V. H.; Betta, R.; Carlsson, M.
2000A&A...360..742H Altcode:
We have obtained short exposure (3 s) time series of strong upper
chromospheric and transition region emission lines from the quiet Sun
with the SUMER instrument onboard SOHO during two 1 hour periods in
1996. With a Nyqvist frequency of 167 mHz and relatively high count
rates the dataset is uniquely suited for searching for high frequency
variations in intensity and Doppler velocity. From Monte-Carlo
experiments taking into account the photon-counting statistics we
estimate our detection limit to correspond to a wave-packet of four
periods coherent over 3<SUP>”</SUP> with a Doppler-shift amplitude
of 2.5km s<SUP>-1</SUP> in the darkest internetwork areas observed in
C III. In the network the detection limit is estimated to be 1.5km
s<SUP>-1</SUP>. Above 50 mHz we detect wave-packet amplitudes above
3km s<SUP>-1</SUP> less than 0.5% of the time. Between 20 and 50 mHz
we detect some wave-packets with a typical duration of four periods
and amplitudes up to 8km s<SUP>-1</SUP>. At any given internetwork
location these wave-packets are present 1% of the time. In the
10-20 mHz range we see amplitudes above 3km s<SUP>-1</SUP> 12% of
the time. At lower frequencies our dataset is consistent with other
SUMER datasets reported in the literature. The chromospheric 3-7 mHz
signal is discernible in the line emission. In the internetwork this
is the dominant oscillation frequency but higher frequencies (7-10
mHz) are often present and appear coherent in Doppler velocity over
large spatial regions (≍ 40"). Wavelet analysis implies that these
oscillations have typical durations of 1000s. The network emission also
shows a 5 mHz signal but is dominated by low frequency variations (of
< 4 mHz) in both intensity and velocity. The oscillations show less
power in intensity than in velocity. We find that while both red and
blue shifted emission is observed, the transition region lines are on
average red shifted between 5-10km s<SUP>-1</SUP> in the network. A net
red shift is also found in the internetwork emission but it is smaller
(< 4km s<SUP>-1</SUP>). The line widths do not differ much between
the internetwork and network, the non-thermal line widths increase
with increasing temperature of line formation from 30km s<SUP>-1</SUP>
for the C II 1334 Å line to 45km s<SUP>-1</SUP> for the O VI 1032
Å line. By constructing scatterplots of velocity versus intensity we
find that in the network a mean redshift is correlated with a high mean
intensity. In the internetwork regions we do not find any correlation
between the intensity and the Doppler velocity.
---------------------------------------------------------
Title: The Dynamic Solar Chromosphere
Authors: Carlsson, M.; Stein, R. F.
2000SPD....31.1102C Altcode: 2000BAAS...32..843C
The natural state of the Solar chromosphere is dynamic. Any
photospheric disturbance will grow and naturally form shocks over
the twenty scale-heights in density between the photosphere and
the corona. Observations in the resonance lines from singly ionized
calcium and in the ultraviolet region of the spectrum observed with
the Solar and Heliospheric Observatory satellite also show a dynamic
chromosphere. The dynamic picture is further supported by numerical
simulations. Static and dynamic pictures of the chromosphere are
fundamentally different. The simulations also show that time variations
are crucial for our understanding of the chromosphere itself and for
the spectral features formed there.
---------------------------------------------------------
Title: Chromospheric and Transition Region Internetwork Oscillations:
A Signature of Upward-propagating Waves
Authors: Wikstøl, Ø.; Hansteen, V. H.; Carlsson, M.; Judge, P. G.
2000ApJ...531.1150W Altcode:
We analyze spectral time series obtained on 1997 April 25 with
the SUMER instrument on SOHO. Line and continuum data near 1037
Å were acquired at a cadence of 16 s. This spectral region was
chosen because it contains strong emission lines of C II, formed
in the upper chromosphere/lower transition region; O VI, formed
in the upper transition region; and neighboring continuum emission
formed in the middle chromosphere. The time series reveal oscillatory
behavior. Subsonic (3-5 km s<SUP>-1</SUP> amplitude) Doppler velocity
oscillations in the C II and O VI lines, with periods between 120
and 200 s, are prominent. They are seen as large-scale coherent
oscillations, typically of 3-7 Mm length scale, occasionally approaching
15 Mm, visible most clearly in internetwork regions. The Doppler
velocity oscillations are related to oscillations seen in the continuum
intensity, which precede upward velocity in C II by 40-60 s. The C II
Doppler shift precedes the O VI Doppler shift by 3-10 s. Oscillations
are also present in the line intensities, but the intensity amplitudes
associated with the oscillations are small. The continuum intensity
precedes the C II intensity by 30-50 s. Phase difference analysis
shows that there is a preponderance of upward-propagating waves in the
upper chromosphere that drive an oscillation in the transition region
plasma, thus extending the evidence for upward-propagating waves from
the photosphere up to the base of the corona.
---------------------------------------------------------
Title: Oxygen Line Formation in 3D Hydrodynamical Model Atmospheres
Authors: Asplund, M.; Carlsson, M.; Garcia Perez, A. E.; Kiselman, D.
2000IAUJD...8E...8A Altcode: 2000astro.ph.11043A
The new generation of realistic 3-dimensional, time-dependent,
hydrodynamical model atmospheres have been applied to the line
formation of {O}{I}, [{O}{I}] and OH lines. Additionally detailed 3D
non-LTE calculations have been performed for {O}{I} in order to study
the influence of temperature inhomogeneities on the line formation
process. Implications in terms of the evolution of oxygen abundance
with metallicity will be discussed, partly based on new VLT/UVES
observations of metal-poor stars.
---------------------------------------------------------
Title: Chromospheric Dynamics as seen by SUMER
Authors: Carlsson, M.
1999ESASP.448..183C Altcode: 1999ESPM....9..183C; 1999mfsp.conf..183C
No abstract at ADS
---------------------------------------------------------
Title: Transition Region Oscillations
Authors: Hansteen, V. H.; Wisktol, O.; Carlsson, M.; Judge, P. G.
1999ESASP.446..351H Altcode: 1999soho....8..351H
We analyze time series data obtained April 25th 1997 with the SUMER
instrument on SOHO. Line and continuum data near 1037 were acquired at
a cadence of 16s. This spectral region was chosen because it contains
strong emission lines of C II, formed in the upper chromosphere/lower
transition region, O VI, formed in the upper transition region, as well
as neighboring continuum emission formed in the middle chromosphere. The
dataset reveals oscillatory behavior containing valuable information
on the physical structure of the chromosphere and transition
region. Prominent in the data are subsonic (3-5 km/s amplitude)
velocity oscillations with periods between 120 and 200 sec. They
are seen as large scale coherent oscillations, typically of 3--7 Mm
length scale but sometimes approaching 15Mm, visible most clearly
in internetwork regions. The oscillations are present in C II and O
VI velocities, as well as in the continuum intensity. The continuum
intensity precedes upward velocity in the C II line by 40-60s and the C
III velocity precedes the O VI velocity by 3-10s. The oscillations are
also present in the intensities of the two lines, but the intensity
amplitudes associated with the oscillations are small. We find that
the continuum intensity precedes the C II intensity by 30-50s. These
phase shifts indicate that there are upward propagating waves in the
upper chromosphere that drive an oscillation in the transition region
plasma. The oscillations seem to be present in most internetwork areas
at any time, thus they are the dominant resolvable dynamical feature
of the internetwork chromosphere and transition region.
---------------------------------------------------------
Title: Wave Modes in a Chromospheric Cavity
Authors: Carlsson, M.; Stein, R. S.
1999ASPC..184..206C Altcode:
Observations show enhanced power near 5.3 mHz and 7 mHz and phase
jumps near 7 mHz. Interpretations have varied from standing waves,
resonant wakes, shock merging and source interference. We investigate
these processes using a non-LTE radiation hydrodynamics code. It
is shown that the enhanced power at certain frequencies is caused
by partial reflections from steep temperature gradients in the
photosphere and the transition region. Due to a phase-speed that is
very dependent on frequency close to the acoustic cut-off frequency,
the lowest mode frequencies are rather insensitive to the location of
the upper reflecting layer. Temperature jumps across shocks introduce
additional reflection and modify the mode structure. A corrugated
and moving transition region washes out the higher frequency modes
but does not affect the lowest modes much. The modal structure is
superimposed on a wave field that is dominated by propagating waves
in the lower chromosphere.
---------------------------------------------------------
Title: The dynamic solar chromosphere and the ionization of hydrogen
Authors: Carlsson, Mats; Stein, Robert F.
1999AIPC..471...23C Altcode: 1999sowi.conf...23C
Basic physical considerations, observations and numerical simulations
show that the solar chromosphere is very dynamic. The enhanced
chromospheric emission, which corresponds to an outwardly increasing
semiempirical temperature structure, can even be produced by wave
motion without any increase in the mean gas temperature. Hence,
the sun may not have a classical chromosphere in magnetic field free
internetwork regions. This dynamic picture of the solar internetwork
chromosphere is consistent with ground based observations of the Call
resonance lines and with observations from the SOHO satellite. The
simulations also show that a static picture and a dynamic picture of
the chromosphere are fundamentally different and that time variations
are crucial for our understanding of the chromosphere itself and
the spectral features formed there. Whether the dynamic nature of
the chromosphere is important for solar wind models depends on their
sensitivity to chromospheric conditions. Contrary to some claims in
the literature, the ionization of hydrogen in the upper chromosphere
is dominated by collisional excitation in the Lyman line followed by
photo-ionization by Balmer continuum photons-the Lyman continuum does
not play any significant role. In the transition region, collisional
ionization takes over as the major process. Ionization/recombination
time-scales can be on the order of hundreds of seconds causing the
ionization balance in the chromosphere to be significantly out of
equilibrium with higher ionization than the equilibrium value. The
hydrogen ionization zone is also considerably thicker than is claimed
from dimension analyses; the ionization fraction goes from 1% to 40%
over a height range of 600 km.
---------------------------------------------------------
Title: Multi3D, 3D Non-LTE Radiative Transfer
Authors: Botnen, A.; Carlsson, M.
1999ASSL..240..379B Altcode: 1999numa.conf..379B
No abstract at ADS
---------------------------------------------------------
Title: The new chromosphere
Authors: Carlsson, M.; Stein, R. F.
1998IAUS..185..435C Altcode:
Numerical simulations have shown that enhanced chromospheric emission,
which corresponds to an outwardly increasing semiempirical temperature
structure, can be produced by wave motion without any increase in
the mean gas temperture. Hence, the sun may not have a classical
chromosphere in magnetic field free internetwork regions. This dynamic
picture of the solar internetwork chromosphere is also consistent
with ground based observations of the CaII resonance lines and of CO
absorption lines and with observations from the SOHO satellite. The
simulations also show that a static picture and a dynamic picture of
the chromosphere are fundamentally different and that time variations
are crucial for our understanding of the chromosphere itself and for
the spectral features formed there.
---------------------------------------------------------
Title: SUMER observations in transition region lines
Authors: Betta, R.; Hansteen, V.; Carlsson, M.; Wilhelm, K.
1998MmSAI..69..699B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Radiative Transfer and Radiation Hydrodynamics
Authors: Carlsson, Mats
1998LNP...507..163C Altcode: 1998sspt.conf..163C
Radiation plays an important role, firstly in determining the structure
of stars through the dominant role radiation plays in the energy balance
(in some objects also in the momentum balance), secondly because we
diagnose astrophysical plasma through the emitted electromagnetic
radiation.
---------------------------------------------------------
Title: SUMER Observations of the Quiet Solar Atmosphere: The Network
Chromosphere and Lower Transition Region
Authors: Judge, Philip; Carlsson, Mats; Wilhelm, Klaus
1997ApJ...490L.195J Altcode:
We examine spectral properties of the network chromosphere and lower
transition region from the SUMER instrument on the SOHO spacecraft,
using time-series data sets discussed in an accompanying Letter by
Carlsson, Judge, & Wilhelm. The data were obtained early in the
mission with no tracking of solar features and so cannot generally
be used to examine intrinsic variations in features on timescales
in excess of 383 s. Upon examination of the temporal variations and
some preliminary power spectrum analysis, we find the following:
(1) Transition region lines show more redshift in network regions
than in internetwork regions and also a correlation between line
intensity brightenings and increased redshift. (2) The internetwork
“Ca II grain” phenomenon is not seen in He I λ584 or in lines
of Si III and C III. (3) Very rapid changes are seen in the network
for transition region lines with no obvious correspondence with the
underlying chromosphere. (4) He I λ584 line profiles show very slow
time variations. (5) Small-amplitude (2-5 km s<SUP>-1</SUP>) coherent
oscillations of 5"-10" scale length and ~130 s period are seen in
Doppler shifts of Si III between regions of bright network elements. (6)
Essentially all blueshifts or redshifts are substantially less than line
widths. We conclude that upward-propagating acoustic shock waves do not
contribute significantly to the heating of the lower transition region,
and that ionization equilibrium is likely to fail for the interpretation
of certain emission lines. The spatial coherency of the Si III velocity
oscillations indicates that the quiet Sun's magnetic field topology
is more uniform than emission-line intensity data alone might suggest.
---------------------------------------------------------
Title: Dynamic Behavior of the Solar Atmosphere
Authors: Stein, R. F.; Carlsson, M.
1997ASSL..225..261S Altcode: 1997scor.proc..261S
We have studied the dynamics of acoustic and MHD waves in the
solar atmosphere using a one-dimensional, non-LTE, radiation
magneto-hydrodynamic code, with 6 level model atoms for hydrogen and
singly ionized calcium. We drive waves by a piston through an initial
atmosphere in radiative equilibrium. We report on the effects of
radiative energy loss on the waves, the effects of shocks on line
formation, and the behavior of typical diagnostics in a dynamic
atmosphere.
---------------------------------------------------------
Title: SUMER Observations Confirm the Dynamic Nature of the Quiet
Solar Outer Atmosphere: The Internetwork Chromosphere
Authors: Carlsson, Mats; Judge, P. G.; Wilhelm, K.
1997ApJ...486L..63C Altcode: 1997astro.ph..6226C
On 1996 March 12, during the commissioning phase of the SOHO mission, we
obtained observations of the quiet-Sun with the SUMER instrument. The
observations were sequences of 15-20 s exposures of ultraviolet
emission-line profiles and of the neighboring continua. These data
contain signatures of the dynamics of the solar chromosphere that are
uniquely useful because of wavelength coverage, moderate signal-to-noise
ratios, and image stability. <P />We focus on data for the internetwork
chromosphere. The dominant observed phenomenon is an oscillatory
behavior that is analogous to the 3 minute oscillations seen in CaII
lines. The oscillations appear to be coherent over 3"-8" diameter
areas. At any time they occur over about 50% of the area studied,
and they appear as large perturbations in the intensities of lines
and continua. The oscillations are most clearly seen in intensity
variations in the ultraviolet (λ > 912 Å) continua, and they are
also seen in the intensities and velocities of chromospheric lines of
CI, NI, and OI. Intensity brightenings are accompanied by blueshifts
of typically 5 km s<SUP>-1</SUP>. Phase differences between continuum
and line intensities also indicate the presence of upward propagating
waves. The detailed behavior is different between different lines,
sometimes showing phase lags. The 3 minute intensity oscillations
are occasionally seen in second spectra (CII λ1335) but never in
third spectra (CIII and SiIII). Third spectra and HeI λ584 show
oscillations in velocity that are not simply related to the 3 minute
oscillations. The continuum intensity variations are consistent with
recent simulations of chromospheric dynamics (Carlsson and Stein),
while the line observations indicate that important ingredients are
missing at higher layers in the simulations. <P />The data show that
time variations are crucial for our understanding of the chromosphere
itself and for the spectral features formed there--the quiet-Sun's
chromosphere is very dynamic and not “quiet.” The implications of
these data should be considered when planning chromospheric work with
instruments such as those on SOHO.
---------------------------------------------------------
Title: Formation of Solar Calcium H and K Bright Grains
Authors: Carlsson, Mats; Stein, Robert F.
1997ApJ...481..500C Altcode:
We have simulated the generation of Ca II H<SUB>2V</SUB> bright
grains by acoustic shocks. We employ a one-dimensional, non-LTE
radiation-hydrodynamic code, with six-level model atoms for hydrogen
and singly ionized calcium. We drive acoustic waves through a stratified
radiative equilibrium atmosphere by a piston, whose velocity is chosen
to match the Doppler shift observed in the Fe I 396.68 nm line in
the H line wing, formed at about 260 km above τ<SUB>500</SUB> =
1. <P />The simulations closely match the observed behavior of Ca
II H<SUB>2V</SUB> bright grains down to the level of individual
grains. The bright grains are produced by shocks near 1 Mm above
τ<SUB>500</SUB> = 1. Shocks in the mid-chromosphere produce a large
source function (and therefore high emissivity) because the density
is high enough for collisions to couple the Ca II populations to the
local conditions. The asymmetry of the line profile is due to velocity
gradients near 1 Mm. Material motion Doppler-shifts the frequency at
which atoms emit and absorb photons, so the maximum opacity is located
at--and the absorption profile is symmetric about--the local fluid
velocity, which is shifted to the blue behind shocks. The optical
depth depends upon the velocity structure higher up. Shocks propagate
generally into downflowing material, so there is little matter above
to absorb the Doppler-shifted radiation. The corresponding red peak
is absent because of small opacity at the source function maximum and
large optical depth due to overlying material. The bright grains are
produced primarily by waves from the photosphere that are slightly
above the acoustic cutoff frequency. The precise time and strength
of a grain depend upon the interference between these waves near the
acoustic cutoff frequency and higher frequency waves. When waves near
the acoustic cutoff frequency are weak, then higher frequency waves
may produce grains. The “5 minute” trapped p-mode oscillations are
not the source of the grains, although they can slightly modify the
behavior of higher frequency waves.
---------------------------------------------------------
Title: Chromospheric Dynamics - What Can Be Learnt from Numerical
Simulations
Authors: Carlsson, M.; Stein, R. F.
1997LNP...489..159C Altcode: 1997shpp.conf..159C
Observations of the solar chromosphere are often interpreted using
methods derived from static modeling (e.g., the Vernazza et al. 1981
model atmospheres and work based on such models) or linear theory
(e.g., phase relations). Recent numerical simulations have shown that
such an analysis can be very misleading. It is found that enhanced
chromospheric emission, which corresponds to an outwardly increasing
semi-empirical temperature structure, can be produced by wave motions
without any increase in the mean gas temperature. Thus, despite
long held beliefs, the Sun may not have a classical chromosphere
in magnetic field free internetwork regions. This dynamic picture
is consistent with observations in CO lines and the calcium H and
K bright grains. More opaque lines, on the other hand, seem to show
emission all of the time. This indicates the existence of a hotter,
magnetic, component that increases in importance with height.
---------------------------------------------------------
Title: Shock Signature in Sunspots
Authors: Bard, S.; Carlsson, M.
1997ESASP.404..189B Altcode: 1997cswn.conf..189B
No abstract at ADS
---------------------------------------------------------
Title: Numerical Simulations Can Lead to New Insights
Authors: Stein, Robert F.; Carlsson, Mats; Nordlund, Ake
1997ASPC..123...72S Altcode: 1997taca.conf...72S
No abstract at ADS
---------------------------------------------------------
Title: The non-magnetic solar chromosphere.
Authors: Carlsson, M.; Stein, R. F.
1997smf..conf...59C Altcode:
The authors summarize recent results form self-consistent non-LTE
radiation hydrodynamics simulations of the propagation of acoustic
waves through the non-magnetic solar chromosphere. References to more
detailed write-ups of the work are given.
---------------------------------------------------------
Title: Intensity and Velocity Variations in Transition Region Lines
Observed with SUMER
Authors: Betta, R.; Hansteen, V.; Carlsson, M.; Wilhelm, K.
1997ESASP.404..205B Altcode: 1997cswn.conf..205B
No abstract at ADS
---------------------------------------------------------
Title: The NLTE formation of neutral-boron lines in cool stars.
Authors: Kiselman, D.; Carlsson, M.
1996A&A...311..680K Altcode: 1996astro.ph..1144K
We study the formation of BI lines in a grid of cool stellar model
atmospheres without the assumption of local thermodynamic equilibrium
(LTE). The non-LTE modelling includes the effect of other lines
blending with the BI resonance lines. Except for the cases where the
BI lines are very strong, the departures from LTE relevant for the
resonance lines can be described as an overionisation effect and an
optical-pumping effect. This causes the lines to be weaker than in LTE
so that an abundance analysis assuming LTE will underestimate stellar
boron abundances. We present non-LTE abundance corrections useful to
improve on abundances derived from the Bi250nm and 209nm lines under
the LTE assumption. Application of the results on literature data
indicates that the B/Fe ratio in metal-poor stars is constant.
---------------------------------------------------------
Title: Solar chromospheric dynamics - Results from numerical
simulations
Authors: Carlsson, M.; Stein, R. F.
1996ASPC..109..119C Altcode: 1996csss....9..119C
No abstract at ADS
---------------------------------------------------------
Title: Does a Nonmagnetic Solar Chromosphere Exist?
Authors: Carlsson, Mats; Stein, Robert F.
1995ApJ...440L..29C Altcode: 1994astro.ph.11036C
Enhanced chromospheric emission which corresponds to an outwardly
increasing semiempirical temperature structure can be produced by
wave motion without any increase in the mean gas temperture. Hence,
the sun may not have a classical chromosphere in magnetic field
free internetwork regions. Other significant differences between the
properties of dynamic and static atmospheres should be considered when
analyzing chromospheric observations.
---------------------------------------------------------
Title: Stellar Atmospheres
Authors: Carlsson, M.
1995fras.conf..181C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Infrared lines as probes of solar magnetic features. VIII. MgI
12μm diagnostics of sunspots.
Authors: Bruls, J. H. M. J.; Solanki, S. K.; Rutten, R. J.; Carlsson,
M.
1995A&A...293..225B Altcode:
Due to their large Zeeman sensitivity, the MgI lines at 12μm are
important diagnostics of solar magnetism. The formation of their
central emission features is now understood, enabling quantitative
modeling and diagnostic application of these lines. We supply the
first systematic analysis of solar MgI 12μm Stokes profiles employing
detailed line-profile synthesis. We compute Stokes profiles of MgI
12.32μm for the quiet Sun, for sunspot penumbrae and for the extended
("superpenumbral") magnetic canopies surrounding sunspots. We use these
computations to analyze recent MgI 12μm observations by Hewagama
et al. (1993). Our results are the following: (1) -Saha-Boltzmann
temperature sensitivity explains that the emission peaks are stronger in
penumbrae than in the quiet Sun, and that they disappear in umbrae. (2)
-The formation heights of the emission features are approximately the
same in penumbrae and in the quiet Sun, namely τ_500_=~10^-3^. (3)
-The simple Seares formula allows relatively accurate determinations
of field strength and magnetic inclination. (4) -The observed excess
broadening of the σ-component peaks compared with the π component
in penumbrae is well explained by primarily horizontal, smooth radial
variation of the magnetic field strength. Additional small-scale
variations are less than {DELTA}B =~200G. (5) -The vertical field
gradients dB/dz in penumbrae range from 0.7G/km to 3G/km; the larger
gradients occur near the umbra, the smaller ones near the outer edge of
the penumbra. (6) -The MgI 12μm lines are well-suited to measure the
base heights of superpenumbral magnetic canopies. These heights range
between 300km and 500km above τ_500_=1 out to twice the sunspot radius,
in excellent agreement with determinations from other infrared lines.
---------------------------------------------------------
Title: No Magnetic Field - No Chromosphere (Abstract only)
Authors: Carlsson, M.; Stein, R.
1995itsa.conf..325C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Non-LTE Effects on Be and B Abundance Determinations in
Cool Stars
Authors: Kiselman, D.; Carlsson, M.
1995lea..conf..372K Altcode: 1994astro.ph..9064K
We discuss the nature of non-LTE effects affecting abundance analysis
of cool stars. The departures from LTE of importance for the B I
lines in solar-type stars are described and some new results are
presented. Boron abundances derived under the LTE assumption have
significant systematic errors, especially for metal-poor stars. For
beryllium, current results suggest that departures from LTE will not
affect abundance analysis significantly.
---------------------------------------------------------
Title: The Formation of the Solar He II 1640.4 Angstrom Emission Line
Authors: Wahlstrom, Cathrine; Carlsson, Mats
1994ApJ...433..417W Altcode:
We explain the formation of the solar He II 1640.4 A Balmer-alpha
emission line. Non-LTE (non-local thermodynamic equilibrium) modeling is
performed including the effects of an incoming coronal radiation field,
the overlapping emission profile of He II 304 A with the He I continuum
and the pumping effect of hydrogen Lyman-alpha. We find that the line
is formed by the photoionization from the ground state of He II, mainly
by the incident coronal radiation, followed by cascade recombination
into the n = 3 levels. The peak of the contribution functions is at
low temperatures, 7000-10,000 K, with a very small contribution from
collisional excitation at a temperature of 70,000 K. We obtain good
agreement with the observed line widths, the observed fine structure
and the relative strength of the fine structure components with the
blue peak about 10% stronger than the red peak. It is shown that these
results are insensitive to the assumptions of geometry in the line
forming regions and to the filling factor. The relative intensities
only depend on an optically thin 1640.4 A line and optically thick
He II Lyman lines. The observed asymmetry also imposes an upper limit
to the density of 30 times the density at 8000 K in a plane parallel
model. It is shown that the total intensity is very sensitive to the
assumed geometry and filling factors. This sensitivity is parameterized
in terms of a multiplicative factor to the assumed coronal radiation
field. In a plane-parallel model the total intensity comes out a factor
of 4 too low with the adopted coronal radiation field but already an
enhancement factor of 2 of the incident radiation field gives a total
intensity in agreement with observations. Note that this enhancement
factor is only a quantification of the disagreement with observations.
---------------------------------------------------------
Title: The non-LTE formation of Li I lines in cool stars
Authors: Carlsson, M.; Rutten, R. J.; Bruls, J. H. M. J.; Shchukina,
N. G.
1994A&A...288..860C Altcode:
We study the non-LTE (non local thermodynamic equilibrium)
formation of Li I lines in the spectra of cool stars for a grid of
radiative-equilibrium model atmospheres with variation in effective
temperature, gravity, metallicity and lithium abundance. We analyze
the mechanisms by which departures from LTE (local thermodynamic
equilibrium) arise for Li I lines, first for the young sun (prior to
its lithium depletion) and then across the cool-star grid. There are
various mechanisms which compete in their effects on emergent Li I
line strengths. Their neglect produces errors in lithium abundance
determinations that vary in sign as well as size, both across the
stellar grid and between different Li I lines (Figs). The errors are
appreciable for all cooler stars and largest for cool lithium-rich
metal-poor giants. They reverse sign between lithium-rich stars and
lithium-poor stars for the λ=670.8nm resonance line, but not for the
λ=610.4nm subordinate line. The non-LTE corrections are large enough
that they should be taken into account in ongoing debates on lithium
synthesis and depletion. We provide convenient numerical approximations
of our results (Table 1) to this purpose. We end the paper with some
examples in which non-LTE corrections change the slope of published
relationships.
---------------------------------------------------------
Title: The Formation of Infrared Rydberg Lines
Authors: Rutten, R. J.; Carlsson, M.
1994IAUS..154..309R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Computation of Infrared Hydrogen Lines
Authors: Carlsson, M.; Rutten, R. J.
1994IAUS..154..341C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Non-LTE Formation of Li I Lines from Cool Stars
Authors: Carlsson, M.; Rutten, R. J.; Bruls, J. H. M. J.; Shchukina,
N. G.
1994ASPC...64..270C Altcode: 1994csss....8..270C
No abstract at ADS
---------------------------------------------------------
Title: Radiation shock dynamics in the solar chromosphere - results
of numerical simulations
Authors: Carlsson, M.; Stein, R. F.
1994chdy.conf...47C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric dynamics
Authors: Carlsson, Mats
1994chdy.conf.....C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Calcium II phase relations and chromospheric dynamics
Authors: Skartlien, R.; Carlsson, M.; Stein, R. F.
1994chdy.conf...79S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: MgI 12 μm diagnostics of sunspot penumbrae
Authors: Bruls, J. H. M. J.; Solanki, S. K.; Rutten, R. J.; Carlsson,
M.
1994smf..conf..191B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On i Lines in the Sun and Stars. I. Understanding the
Resonance Lines
Authors: Carlsson, M.; Judge, P. G.
1993ApJ...402..344C Altcode:
The use of O I lines as spectral diagnostics of conditions in the
chromospheres of the sun and cool stars is addressed, focusing on
the resonance lines. The important processes influencing these lines
are identified and an attempt it made to understand the detailed line
transfer calculations in terms of simpler analytical models. How the
lines respond to changes in uncertain atomic parameters is illustrated,
and it is shown how the O I line flux densities depend simply and
sensitively on the radiative transfer solution for hydrogen. Approximate
analytical formulas are derived for the O I line flux densities in
terms of the hydrogen number densities.
---------------------------------------------------------
Title: Non-LTE radiating shocks and the formation of Ca II lines in
the solar chromosphere.
Authors: Carlsson, M.; Stein, R. F.
1993wpst.conf...21C Altcode:
The authors present self-consistent solutions of the time dependent
one-dimensional equations of non-LTE radiation-hydrodynamics in solar
chromospheric conditions. The vertical propagation of acoustic waves
is calculated.
---------------------------------------------------------
Title: Non-LTE Radiating Acoustic Shocks and CA II K2V Bright Points
Authors: Carlsson, Mats; Stein, Robert F.
1992ApJ...397L..59C Altcode:
We present, for the first time, a self-consistent solution of the
time-dependent 1D equations of non-LTE radiation hydrodynamics in
solar chromospheric conditions. The vertical propagation of sinusoidal
acoustic waves with periods of 30, 180, and 300 s is calculated. We
find that departures from LTE and ionization recombination determine
the temperature profiles of the shocks that develop. In LTE almost all
the thermal energy goes into ionization, so the temperature rise is very
small. In non-LTE, the finite transition rates delay the ionization to
behind the shock front. The compression thus goes into thermal energy at
the shock front leading to a high temperature amplitude. Further behind
the shock front, the delayed ionization removes energy from the thermal
pool, which reduces the temperature, producing a temperature spike. The
180 s waves reproduce the observed temporal changes in the calcium K
line profiles quite well. The observed wing brightening pattern, the
violet/red peak asymmetry and the observed line center behavior are
all well reproduced. The short-period waves and the 5 minute period
waves fail especially in reproducing the observed behavior of the wings.
---------------------------------------------------------
Title: Solar hydrogen lines in the infrared
Authors: Carlsson, M.; Rutten, R. J.
1992A&A...259L..53C Altcode:
We study recently observed H I lines in the infrared solar spectrum,
employing detailed NLTE modeling to explain their formation and to
evaluate their diagnostic merits. The solar infrared H I lines vary much
in character, depending on opacity and wavelength; our computations
reproduce the observations closely. The line wings are primarily set
by Stark broadening due to metal ions and protons; the line cores are
sensitive to NLTE population departure divergence which is driven by
Balmer-continuum photoionization. The formation heights of the H I
lines range from the deep photosphere for near-infrared line wings
to the chromosphere for line cores with wavelengths greater than 10
microns; these features provide valuable diagnostics of the thermal
structure of the solar atmosphere.
---------------------------------------------------------
Title: The calcium infrared triplet lines in stellar spectra.
Authors: Jorgensen, U. G.; Carlsson, M.; Johnson, H. R.
1992A&A...254..258J Altcode:
Observations of the infrared triplet lines of ionized calcium are often
used as diagnostics of surface gravity among the stars contributing
to the integrated light of distant galaxies. We have calculated the
equivalent widths of these lines for a series of models with a broad
range of input parameters to test their sensitivity to surface gravity,
temperature, and calcium abundance. Over a larger range of parameter
space than in earlier investigations, we find the sensitivity to be more
complex than previously thought. We derive theoretical relations between
equivalent width and log(g) for different values of T<SUB>eff</SUB> and
metallicity and compare our results with observations. By actual NLTE
calculations, we show that departures from LTE are small. Comparison
of our results with observations of stars in the Galaxy suggests that
the Ca/Fe ratio is an increasing function of metallicity.
---------------------------------------------------------
Title: The formation of the MG I emission features near 12 microns
Authors: Carlsson, M.; Rutten, R. J.; Shchukina, N. G.
1992A&A...253..567C Altcode:
The formation of two Mg I 12-micron emission features in the
solar spectrum, the existence of which was reported by Murcray et
al. (1981), is explained using plane-parallel nonlocal thermodynamic
equilibrium modeling with a radiative-equilibrium model atmosphere
without chromosphere. It is shown that these emissions are a natural
consequence of population depletion by line photon losses followed by
population replenishment from the ionic reservoir in the highly excited
levels. The results confirm the suggestion by Lemke and Holweger (1987)
that the 12-micron lines are formed in the photosphere and disprove
the claim by Zirin and Popp (1989) that the temperature minimum occurs
much deeper than in standard models of the solar atmosphere.
---------------------------------------------------------
Title: Formation of the MG 112 TTM Lines
Authors: Carlsson, M.; Rutten, R. J.; Shchukina, N. G.
1992ASPC...26..518C Altcode: 1992csss....7..518C
No abstract at ADS
---------------------------------------------------------
Title: CA II K2V Bright Grains Formed by Acoustic Waves
Authors: Carlsson, M.; Stein, R.
1992ASPC...26..515C Altcode: 1992csss....7..515C
No abstract at ADS
---------------------------------------------------------
Title: The MULTI Non-LTE Program (Invited Review)
Authors: Carlsson, M.
1992ASPC...26..499C Altcode: 1992csss....7..499C
No abstract at ADS
---------------------------------------------------------
Title: Formation of the KI 7699A Line in Sunspots
Authors: Caccin, B.; Carlsson, M.; Gomez, M. T.; Severino, G.
1991ASIC..341..415C Altcode: 1991sabc.conf..415C
No abstract at ADS
---------------------------------------------------------
Title: Global and Local Methods for One-Dimensional Problems -
Implementation Aspects and Cpu-Time and Memory Scalings
Authors: Carlsson, M.
1991ASIC..341...39C Altcode: 1991sabc.conf...39C
No abstract at ADS
---------------------------------------------------------
Title: Shock Amplification by Radiation (With 1 Figure)
Authors: Carlsson, M.; Stein, R.
1991mcch.conf..366C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Deviation from the Wilson-Bappu relationship in faint red
dwarf stars.
Authors: Elgaroy, O.; Engvold, O.; Carlsson, M.
1990A&A...234..308E Altcode:
New observations of the Mg II h and k lines on red dwarf stars have been
carried out with the International Ultraviolet Explorer observatory. It
is found that several of the dwarf stars in the sample have Mg II h
and k lines which are narrower than expected from the Wilson-Bappu
relationship, that is established for stars brighter than M(v) about
7. The deviations may depend on the Mg II h and k line opacity. Evidence
is presented in favor of the suggestion that the optical thickness of
the Mg II lines in faint red dwarfs depends on the level of activity
of the stars. Calculations based on models for the sun and YZ CMi
(Mv = 12.3) support the observational results.
---------------------------------------------------------
Title: The Formation of the Mg I 12-Micron Emission Lines
Authors: Carlsson, M.; Rutten, R. J.; Shchukina, N. G.
1990PDHO....7..260C Altcode: 1990dysu.conf..260C; 1990ESPM....6..260C
Contents: The Mg I 12 μm line, LTE or NLTE, chromospheric formation,
photospheric formation, collisional NLTE; departure diffusion.
---------------------------------------------------------
Title: Non-LTE radiative hydrodynamic interactions in the solar
chromosphere.
Authors: Carlsson, M.; Stein, R.
1990ppst.conf..177C Altcode:
Strong, optically thick lines from iron and from ionized calcium and
magnesium dominate the radiative losses of the solar chromosphere. This
radiative loss cannot be approximated in the optically thin limit or
by a grey approximation. In order to properly calculate the effects
of waves in the chromosphere it is necessary to solve simultaneously
the equations of hydrodynamics, radiative transfer and statistical
equilibrium. Efficient methods in radiative transfer are here being
combined with a treatment of the dynamical equations capable of
resolving shocks. The authors present the first results showing that
radiative hydrodynamic interactions may have a significant effect on
the heating by acoustic waves.
---------------------------------------------------------
Title: Waves and radiation in stellar atmospheres.
Authors: Carlsson, M.
1990ppst.conf..115C Altcode:
The author discusses the problem of calculating the interactions between
radiation and hydrodynamics in various density regimes. The possible
approximations are outlined and diagnostic problems are discussed. The
interactions between waves and radiation in the intermediate density
regime (applicable to e.g. the solar chromosphere) are reviewed in
some detail.
---------------------------------------------------------
Title: A 1-D code for radiation hydrodynamics problems.
Authors: Carlsson, M.
1989ftsa.conf...93C Altcode:
In order to properly calculate the effects of waves in the chromosphere
it is necessary to solve simultaneously the equation of hydrodynamics,
radiative transfer and statistical equilibrium. Efficient methods
in radiative transfer are beeing combined with methods to treat the
dynamical equations capable of resolving shocks. There is no complete
code finished yet and this report is more of a progress report
centered on methodology rather than a presentation of physically
relevant results.
---------------------------------------------------------
Title: A Computer Program for Solving Multi-Level Non-Lte Radiative
Transfer Problems in Moving or Static Atmospheres
Authors: Carlsson, M.
1988ASSL..138..273C Altcode: 1988pffl.proc..273C; 1988IAUCo..94..273C
A summary of the characteristics of a non-LTE computer program is
presented. The code can be used to solve non-LTE radiative transfer
problems in semi-infinite, plane-parallel one-dimensional atmospheres
with a prescribed velocity field. The model atom can contain many
atomic levels and several ionization stages. The lines are assumed to
be formed with complete redistribution over the line profile which is
assumed to be a Voigt function.
---------------------------------------------------------
Title: Stokes Profile Analysis and Vector Magnetic
Fields. III. Extended Temperature Minima of Sunspot Umbrae as Inferred
from Stokes Profiles of MG i lambda 4571
Authors: Lites, B. W.; Skumanich, A.; Rees, D. E.; Murphy, G. A.;
Carlsson, M.
1987ApJ...318..930L Altcode:
Observed Stokes profiles of Mg I 4571 A are analyzed as a diagnostic
of the magnetic field and thermal structure at the temperature minimum
of sunspot umbrae. Multilevel non-LTE transfer calculations of the Mg
I-II-III excitation and ionization balance in model umbral atmospheres
show: (1) Mg I to be far less ionized in sunspot umbrae than in the
quiet sun, leading to greatly enhanced opacity in 4571 A, and (2) LTE
excitation of 4571 A. Existing umbral models predict emission cores of
the Stokes I profile due to the chromospheric temperature rise. This
feature is not present in observed umbral profiles. Moreover, such
an emission reversal causes similar anomalous features in the Stokes
Q, U, V profiles, which are also not observed. Umbral atmospheres
with extended temperature minima are suggested. Implications for
chromospheric heating mechanisms and the utility of this line for
solar vector magnetic field measurements are discussed.
---------------------------------------------------------
Title: Radiative transfer : an operator perturbation method and the
application to mesoturbulence
Authors: Carlsson, Mats Per-Olof
1987PhDT.......250C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A New Sunspot Umbral Model and Its Variation with the Solar
Cycle
Authors: Maltby, P.; Avrett, E. H.; Carlsson, M.; Kjeldseth-Moe, O.;
Kurucz, R. L.; Loeser, R.
1986ApJ...306..284M Altcode:
Semiempirical model atmospheres are presented for the darkest parts of
large sunspot umbrae, regions have called umbral cores. The approach is
based on general-purpose computational procedures that are applicable
to different types of stellar atmospheres. It is shown that recent
umbral intensity measurements of the spectral energy distribution may
be accounted for by an umbral core atmospheric model that varies with
time during the solar cycle; the observed center-limb variation can be
accounted for by the properties of the model. Three umbral core models
are presented, corresponding to the early, middle, and late phases of
the solar cycle. These three models also may be regarded as having the
properties of dark, average, and bright umbral cores respectively. The
effects of atomic, opacity, and abundance data uncertainties on
the model calculations are briefly discussed. For comparison, a new
reference model for the average quiet solar photosphere is given.
---------------------------------------------------------
Title: The outer atmosphere of the carbon star TX Piscium.
Authors: Eriksson, K.; Gustafsson, B.; Johnson, H. R.; Querci, F.;
Querci, M.; Baumert, J. H.; Carlsson, M.; Olofsson, H.
1986A&A...161..305E Altcode:
A high-resolution LWP IUE spectrum of the bright N-type carbon star TX
Psc demonstrates that the Mg II h and k emission profiles are strongly
affected by absorption from Mg II, Mn I, probably Fe I, and possibly
from molecules. The indication that the absorbing matter has a column
density of not less than 10 to the 20th H atoms or molecules per sq
cm is consistent with absorption in a slowly expanding envelope. The
integrated Mg II line flux is found to be much greater than in 1981,
and the radio CO (J = 1 - 0) line from the circumstellar shell is
detected. Results for a column density of not larger than 10 to the 22nd
H2 molecules/sq cm, and a radial velocity close to that of the star,
are in agreement with those obtained from UV data. Some dust emission
from carbon grains is suggested by the far infrared flux distribution,
and a mass-loss rate estimation for the star of 10 to the -6th to 10
to the -8th solar masses is obtained.
---------------------------------------------------------
Title: Extended Temperature Minima of Sunspot Umbrae
Authors: Lites, B. W.; Skumanich, A.; Rees, D. E.; Murphy, G. A.;
Carlsson, M.
1986BAAS...18..662L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A computer program for solving multi-level non-LTE radiative
transferproblems in moving or static atmospheres.
Authors: Carlsson, M.
1986UppOR..33.....C Altcode:
A computer program for solving multi-level non-LTE radiative transfer
problems is presented. The method used is that developed by Scharmer
(1981) and Scharmer & Carlsson (1985a, 1985b). The code can be
used to solve non-LTE radiative transfer problems in semi-infinite,
plane-parallel one-dimensional atmospheres with a prescribed macroscopic
velocity field. The model atom can contain many atomic levels and
several ionization stages. The lines are assumed to be formed with
complete redistribution over the profile function which is assumed
to be a Voigt function. <P />Some aspects of the numerical solution
of radiative transfer problems are also discussed; in particular the
choice of a starting approximation, instability problems due to the
optically thin radiation field and the accuracy of methods for solving
the transfer equation with a known source function.
---------------------------------------------------------
Title: Radiative Transfer and Turbulent Atmospheres
Authors: Carlsson, M.
1985tphr.conf...67C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A new approach to multi-level non-LTE radiative transfer
problems.
Authors: Scharmer, G. B.; Carlsson, M.
1985JCoPh..59...56S Altcode:
A new approach to the numerical solution of multilevel, non-LTE
problems is described. The standard non-LTE multilevel problem is
formulated, and the statistical equilibrium equations and the radiative
transfer equation are linearized. It is shown how to precondition the
statistical equilibrium equations and the radiative transfer equation
in a way which enables the solution of problems with strong numerical
cancellation which arise from 'passive' scatterings at large optical
depth. Simplifications in the numerical representation of radiative
transfer processes are introduced which lead to rapid methods for
setting up and solving the statistical equilibrium equations. Some
calculations which have been made to test the convergence properties
of the present scheme are described, and some generalizations of the
present methods are briefly outlined.
---------------------------------------------------------
Title: A proposal for the LEST control system.
Authors: Engvold, O.; Andersen, T. E.; Carlsson, M.; Jensen, J. R.;
Klim, K.
1985LFTR...15.....E Altcode:
Contents: Trends in development of computers and peripherals. Trends
in telescope control and operation. Control system requirements for
LEST. Proposed LEST control system. Remote control of LEST. Cost
estimate. Estimated total cost of control and data reduction system.
---------------------------------------------------------
Title: Radiative transfer and turbulent atmospheres.
Authors: Carlsson, M.
1985MPARp.212...67C Altcode:
The paper is divided into two parts - in the first part some trends
in radiative transfer methodology towards three dimensional modeling
are reviewed. Special emphasis is given to operator perturbation
techniques and to the impact of increased parallelism in new computer
architecture. The second part illustrates some effects of velocity
fields on the radiative transfer in spectral lines.
---------------------------------------------------------
Title: Effects of meso-scale velocity fields on the solar Ca II
spectral lines.
Authors: Carlsson, M.; Scharmer, G. B.
1985cdm..proc..137C Altcode:
The effects of random mesoscale velocity fields on the solar Ca II
K-line and the infrared 8542 A line are discussed, and average profiles
and radiative cooling functions are compared with those obtained from
a combination of microturbulence and macroturbulence. Profiles in
the mesoscale regime could not be reproduced with any combination of
micro- and macroturbulence owing to the fact that mesoscale velocity
fields give core intensities that are much higher than those of static
atmospheres. Calculations of radiative cooling in the two lines were
different when mesoscale velocity fields were taken into account;
this difference was due mainly to a redistribution of the radiative
cooling between the lines.
---------------------------------------------------------
Title: A new method for solving multi-level non-LTE problems.
Authors: Scharmer, G. B.; Carlsson, M.
1985ASIC..152..189S Altcode: 1985pssl.proc..189S
A new scheme for solving multi-level non-LTE problems is described. This
method uses an approximate operator for the relation between the
intensity and the source function. This operator results in a
matrix equation for the population numbers which has a simple and
characteristic structure. Solutions are obtained such that the results
are "exact", irrespective of the choice of the approximate operator.
---------------------------------------------------------
Title: The outer atmosphere of the carbon star TX Piscium.
Authors: Eriksson, K.; Gustafsson, B.; Johnson, H. R.; Querci, F.;
Querci, M.; Baumert, J. H.; Carlsson, M.; Olofsson, H.
1985BETSP...2....5E Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The lightcurve and phase relation of the asteroid 133 Cyrene
Authors: Harris, A. W.; Carlsson, M.; Young, J. W.; Lagerkvist, C. I.
1984Icar...58..377H Altcode:
The asteroid 133 Cyrene was observed photometrically on 17 nights
during oppositions in 1979 and 1980. The synodic period of rotation
was found to be 12. <SUP>h</SUP>708 ± 0. <SUP>h</SUP>001 with an
amplitude of ∼0. <SUP>m</SUP>30 during both oppositions. At large
phase angles, the phase relation is quite ordinary ( β<SUB>v</SUB>
≈ 0.025 mag/degree); however, the low phase angle observations reveal
a dramatic opposition brightening, ∼0.2 mag/degree near zero phase
angle. The absolute magnitude, V(1,0), extrapolated with the above
linear phase coefficient, is 8.40. The following color indicates were
also measured: B- V = 0.90, U-B = 0.51.
---------------------------------------------------------
Title: Physical studies of asteroids. XI - Photoelectric observations
of the asteroids 2, 161, 216 and 276
Authors: Carlsson, M.; Lagerkvist, C. -I.
1983A&AS...53..157C Altcode:
Lightcurves and UBV-colours are presented for the asteroids 2 Pallas,
161 Athor, 216 Kleopatra and 276 Adelheid. For asteroid 216 a lightcurve
amplitude of l<SUP>m</SUP>.1 was observed.
---------------------------------------------------------
Title: Physical studies of asteroids. IV - Photoelectric observations
of the asteroids 47, 95, 431
Authors: Carlsson, M.; Lagerkvist, C. -I.
1981A&AS...45....1C Altcode:
Photoelectric lightcurves are presented. The synodic period of
rotation of asteroid 95 was found to be 0<SUP>d</SUP>.3620 ±
0<SUP>d</SUP>.0005. The asteroids 47 and 431 showed no significant
brightness variation throughout the observing runs of 8 hours.
---------------------------------------------------------
Title: Physical studies of asteroids. I - Photoelectric observations
of the asteroids 38, 218, 268, 344, 485, 683, 690 and 792
Authors: Carlsson, M.; Lagerkvist, C. -I.
1981A&AS...44...15C Altcode:
Lightcurves, rotation periods, maximum amplitudes, absolute magnitudes
and UBV colours are presented. The following synodic periods of rotation
were derived : Asteroid 218: 0<SUP>d</SUP>.268 ± 0<SUP>d</SUP>.006,
asteroid 485: 0<SUP>d</SUP>.7331 ± 0<SUP>d</SUP>.0002, asteroid
683: 0<SUP>d</SUP>.1801 ± 0<SUP>d</SUP>.0004, asteroid 792:
0.<SUP>d</SUP>3823 ± 0<SUP>d</SUP>.0004.
---------------------------------------------------------
Title: Positions of asteroids obtained with the Schmidt telescope
at the Uppsala Southern Station
Authors: Carlsson, M.; Hahn, G.; Lagerkvist, C. -I.
1980A&AS...41..117C Altcode:
Photographic positions of asteroids obtained at the Uppsala Southern
Station are presented. Positions are presented for asteroids marked
X or XX in Ephemerides of Minor Planets (1978), ordinary main-belt
asteroids, Trojans and unnumbered asteroids. The IRIS plate measuring
machine (Aslund et al., 1975) was used, and the means of the absolute
differences between the catalog positions of the AGK3 stars and the
observed positions were found to be 0.46 and 0.43 arc sec in right
ascension (RA) and declination, respectively. For the SAO stars the
corresponding values were 0.76 and 0.84 arc sec in RA and declination.