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Author name code: danilovic
ADS astronomy entries on 2022-09-14
author:"Danilovic, Sanja"
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Title: Modeling of Chromospheric Features and Dynamics in Solar Plage
Authors: Danilovic, Sanja
2022arXiv220803744D Altcode:
The chromosphere is a dynamic and complex layer where all the relevant
physical processes happen on very small spatio-temporal scales. A
few spectral lines that can be used as chromospheric diagnostics
give us convoluted information that is hard to interpret without
realistic theoretical models. What are the key ingredients that these
models need to contain? The magnetic field has a paramount effect
on chromospheric structuring. This is obvious from the ubiquitous
presence of chromospheric dynamic fibrilar structures visible on the
solar disk and at the limb. The numerical experiments presented in this
manuscript illustrate the present state of modeling. They showcase
to what extent our models reproduce various chromospheric features
and their dynamics. The publication describes the effect different
ingredients have on chromospheric models and provides a recipe for
building one-to-one models. Combining these models with observations
will provide insight into the physical processes that take place in
the solar atmosphere.
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Title: Rapid Blue- and Red-shifted Excursions in H$\alpha$ line
profiles synthesized from realistic 3D MHD simulations
Authors: Danilovic, S.; Bjørgen, J. P.; Leenaarts, J.; Rempel, M.
2022arXiv220813749D Altcode:
Rapid blue- and red-shifted events (RBEs/RREs) may have an important
role in mass-loading and heating the solar corona, but their nature
and origin are still debatable. We aim to model these features to
learn more about their properties, formation and origin. A realistic
three-dimensional (3D) magneto-hydrodynamic (MHD) model of a solar
plage region is created. Synthetic H$\alpha$ spectra are generated
and the spectral signatures of these features are identified. The
magnetic field lines associated with these events are traced and the
underlying dynamic is studied. The model reproduces well many properties
of RBEs and RREs, such as spatial distribution, lateral movement,
length and lifetimes. Synthetic H$\alpha$ line profiles, similarly to
observed ones, show strong blue- or red-shift and asymmetries. These
line profiles are caused by the vertical component of velocity with
magnitudes larger than $30-40$ km/s that appear mostly in the height
range of $2-4$ Mm. By tracing magnetic field lines, we show that the
vertical velocity that causes the appearance of RBE/RREs to appear is
always associated with the component of velocity perpendicular to the
magnetic field line. The study confirms the hypothesis that RBEs and
RREs are signs of Alfv{é}nic waves with, in some cases, a significant
contribution from slow magneto-acoustic mode.
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Title: Active region chromospheric magnetic fields. Observational
inference versus magnetohydrostatic modelling
Authors: Vissers, G. J. M.; Danilovic, S.; Zhu, X.; Leenaarts, J.;
Díaz Baso, C. J.; da Silva Santos, J. M.; de la Cruz Rodríguez,
J.; Wiegelmann, T.
2022A&A...662A..88V Altcode: 2021arXiv210902943V
Context. A proper estimate of the chromospheric magnetic fields is
thought to improve modelling of both active region and coronal mass
ejection evolution. However, because the chromospheric field is not
regularly obtained for sufficiently large fields of view, estimates
thereof are commonly obtained through data-driven models or field
extrapolations, based on photospheric boundary conditions alone and
involving pre-processing that may reduce details and dynamic range in
the magnetograms. <BR /> Aims: We investigate the similarity between
the chromospheric magnetic field that is directly inferred from
observations and the field obtained from a magnetohydrostatic (MHS)
extrapolation based on a high-resolution photospheric magnetogram. <BR
/> Methods: Based on Swedish 1-m Solar Telescope Fe I 6173 Å and
Ca II 8542 Å observations of NOAA active region 12723, we employed
the spatially regularised weak-field approximation (WFA) to derive
the vector magnetic field in the chromosphere from Ca II, as well as
non-local thermodynamic equilibrium (non-LTE) inversions of Fe I and Ca
II to infer a model atmosphere for selected regions. Milne-Eddington
inversions of Fe I serve as photospheric boundary conditions for the
MHS model that delivers the three-dimensional field, gas pressure,
and density self-consistently. <BR /> Results: For the line-of-sight
component, the MHS chromospheric field generally agrees with the
non-LTE inversions and WFA, but tends to be weaker by 16% on average
than these when larger in magnitude than 300 G. The observationally
inferred transverse component is systematically stronger, up to an
order of magnitude in magnetically weaker regions, but the qualitative
distribution with height is similar to the MHS results. For either
field component, the MHS chromospheric field lacks the fine structure
derived from the inversions. Furthermore, the MHS model does not
recover the magnetic imprint from a set of high fibrils connecting
the main polarities. <BR /> Conclusions: The MHS extrapolation and
WFA provide a qualitatively similar chromospheric field, where the
azimuth of the former is better aligned with Ca II 8542 Å fibrils than
that of the WFA, especially outside strong-field concentrations. The
amount of structure as well as the transverse field strengths are,
however, underestimated by the MHS extrapolation. This underscores the
importance of considering a chromospheric magnetic field constraint in
data-driven modelling of active regions, particularly in the context
of space weather predictions.
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Title: Heating of the solar chromosphere through current dissipation
Authors: da Silva Santos, J. M.; Danilovic, S.; Leenaarts, J.; de
la Cruz Rodríguez, J.; Zhu, X.; White, S. M.; Vissers, G. J. M.;
Rempel, M.
2022A&A...661A..59D Altcode: 2022arXiv220203955D
Context. The solar chromosphere is heated to temperatures higher than
predicted by radiative equilibrium. This excess heating is greater
in active regions where the magnetic field is stronger. <BR />
Aims: We aim to investigate the magnetic topology associated with an
area of enhanced millimeter (mm) brightness temperatures in a solar
active region mapped by the Atacama Large Millimeter/submillimeter
Array (ALMA) using spectropolarimetric co-observations with the 1-m
Swedish Solar Telescope (SST). <BR /> Methods: We used Milne-Eddington
inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions,
and a magnetohydrostatic extrapolation to obtain constraints on the
three-dimensional (3D) stratification of temperature, magnetic field,
and radiative energy losses. We compared the observations to a snapshot
of a magnetohydrodynamics simulation and investigate the formation
of the thermal continuum at 3 mm using contribution functions. <BR />
Results: We find enhanced heating rates in the upper chromosphere of up
to ∼5 kW m<SUP>−2</SUP>, where small-scale emerging loops interact
with the overlying magnetic canopy leading to current sheets as shown
by the magnetic field extrapolation. Our estimates are about a factor
of two higher than canonical values, but they are limited by the ALMA
spatial resolution (∼1.2″). Band 3 brightness temperatures reach
about ∼10<SUP>4</SUP> K in the region, and the transverse magnetic
field strength inferred from the non-LTE inversions is on the order
of ∼500 G in the chromosphere. <BR /> Conclusions: We are able to
quantitatively reproduce many of the observed features including the
integrated radiative losses in our numerical simulation. We conclude
that the heating is caused by dissipation in current sheets. However,
the simulation shows a complex stratification in the flux emergence
region where distinct layers may contribute significantly to the
emission in the mm continuum. <P />The movie is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243191/olm">https://www.aanda.org</A>
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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). 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, 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: What drives peacock jets?
Authors: Danilovic, Sanja
2021AGUFMSH43A..04D Altcode:
What drives peacock jets? Peacock jets are remarkable features
that appear above light bridges or generally in newly emerging
regions. Observed in chromospheric lines like Halpha and Ca II K,
they show fine structures that resemble extended peacock tails. The
size and dynamics of these features can vary, but the common property
is a clear intrusion into a strong magnetic field region e.g. umbra or
pore. By using MURaM code and simulating the observed magnetic field
configuration, we report here on the first realistic model of such a
feature. We follow its evolution from the formation till the destruction
and discuss the acceleration and collimation mechanisms. We compare
synthetic chromospheric and coronal observables with real observations
and discuss similarities and differences.
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Title: Non-LTE inversions of a confined X2.2 flare. I. The vector
magnetic field in the photosphere and chromosphere
Authors: Vissers, G. J. M.; Danilovic, S.; de la Cruz Rodríguez,
J.; Leenaarts, J.; Morosin, R.; Díaz Baso, C. J.; Reid, A.; Pomoell,
J.; Price, D. J.; Inoue, S.
2021A&A...645A...1V Altcode: 2020arXiv200901537V
Context. Obtaining an accurate measurement of magnetic field vector
in the solar atmosphere is essential for studying changes in field
topology during flares and reliably modelling space weather. <BR />
Aims: We tackle this problem by applying various inversion methods to a
confined X2.2 flare that occurred in NOAA AR 12673 on 6 September 2017
and comparing the photospheric and chromospheric magnetic field vector
with the results of two numerical models of this event. <BR /> Methods:
We obtained the photospheric magnetic field from Milne-Eddington
and (non-)local thermal equilibrium (non-LTE) inversions of Hinode
SOT/SP Fe I 6301.5 Å and 6302.5 Å. The chromospheric field was
obtained from a spatially regularised weak-field approximation (WFA)
and non-LTE inversions of Ca II 8542 Å observed with CRISP at the
Swedish 1 m Solar Telescope. We investigated the field strengths
and photosphere-to-chromosphere shear in the field vector. <BR />
Results: The LTE- and non-LTE-inferred photospheric magnetic field
components are strongly correlated across several optical depths in
the atmosphere, with a tendency towards a stronger field and higher
temperatures in the non-LTE inversions. For the chromospheric field,
the non-LTE inversions correlate well with the spatially regularised
WFA, especially in terms of the line-of-sight field strength and field
vector orientation. The photosphere exhibits coherent strong-field
patches of over 4.5 kG, co-located with similar concentrations
exceeding 3 kG in the chromosphere. The obtained field strengths
are up to two to three times higher than in the numerical models,
while the photosphere-to-chromosphere shear close to the polarity
inversion line is more concentrated and structured. <BR /> Conclusions:
In the photosphere, the assumption of LTE for Fe I line formation
does not yield significantly different magnetic field results in
comparison to the non-LTE case, while Milne-Eddington inversions
fail to reproduce the magnetic field vector orientation where Fe
I is in emission. In the chromosphere, the non-LTE-inferred field
is excellently approximated by the spatially regularised WFA. Our
inversions confirm the locations of flux rope footpoints that have
been predicted by numerical models. However, pre-processing and lower
spatial resolution lead to weaker and smoother field in the models than
what our data indicate. This highlights the need for higher spatial
resolution in the models to better constrain pre-eruptive flux ropes.
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Title: Probing chromospheric heating with millimeter interferometry
Authors: da Silva Santos, J. M.; de la Cruz Rodriguez, J.; White,
S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.; Danilovic, S.
2020AGUFMSH0010001D Altcode:
Observations at visible and ultraviolet wavelengths have shown that
solar active regions host different kinds of small-scale, transient,
bright structures that are believed to be heating events resulting
from the release of magnetic energy in the low atmosphere of the Sun,
especially at the early stages of flux emergence. It is of great
scientific interest to be able to accurately infer temperatures and
formation heights of the most localized events, which are still
matter of debate, in the hope of learning about the evolution of
active regions where occasionally more energetic phenomena lead to
much larger outbursts that propagate across the Solar System. The
millimeter (mm) continuum is a new complementary diagnostic for
chromospheric heating that is now available thanks to the Atacama
Large Millimeter/submillimeter Array (ALMA). <P />We report on the
first ALMA 3 mm observations of small-scale heating events in a
solar active region. In contrast with the low-amplitude brightness
temperature variations in the quiet-Sun, the interferometric maps show
that the active region consists of long, warm, fibril-like structures
that connect magnetic concentrations of opposite polarity and often
flare up along with compact, flickering mm-bursts -- reminiscent of
ultraviolet bursts -- with brightness temperatures of up to 14000 K at
1.2" scales. These events also show simultaneous EUV emission observed
by the Solar Dynamics Observatory (SDO). We find a weak correlation
between the photospheric bright patches and the 3 mm continuum
brightness and, in particular, we do not detect any mm counterpart of
Ellerman bombs which confirms that they are photospheric phenomena. <P
/>Our observations and modelling highlight the diagnostic capabilities
of ALMA for local heating in solar active regions and emphasize the
need for coordinated observations with IRIS and DKIST in the future.
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Title: Tracing the origin of spicules in 3D radiative MHD models
Authors: Danilovic, S.
2020AGUFMSH004..07D Altcode:
Are the flux emergence and ambipolar diffusion essential for the
production of solar spicules? What is the role of vorticity and magnetic
reconnection? Are RBEs and RREs traces of plasma flows? We try to give
answers to these questions by analyzing 3D model of a solar plage.
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Title: Physical properties of bright Ca II K fibrils in the solar
chromosphere
Authors: Kianfar, Sepideh; Leenaarts, Jorrit; Danilovic, Sanja;
de la Cruz Rodríguez, Jaime; Díaz Baso, Carlos José
2020A&A...637A...1K Altcode: 2020arXiv200311302K
Context. Broad-band images of the solar chromosphere in the Ca
II H&K line cores around active regions are covered with fine
bright elongated structures called bright fibrils. The mechanisms
that form these structures and cause them to appear bright are still
unknown. <BR /> Aims: We aim to investigate the physical properties,
such as temperature, line-of-sight velocity, and microturbulence,
in the atmosphere that produces bright fibrils and to compare those
to the properties of their surrounding atmosphere. <BR /> Methods:
We used simultaneous observations of a plage region in Fe I 6301-2
Å, Ca II 8542 Å, Ca II K, and Hα acquired by the CRISP and CHROMIS
instruments on the Swedish 1 m Solar Telescope. We manually selected
a sample of 282 Ca II K bright fibrils. We compared the appearance
of the fibrils in our sample to the Ca II 8542 Å and Hα data. We
performed non-local thermodynamic equilibrium inversions using the
inversion code STiC on the Fe I 6301-2 Å, Ca II 8542 Å, and Ca II
K lines to infer the physical properties of the atmosphere. <BR />
Results: The line profiles in bright fibrils have a higher intensity in
their K<SUB>2</SUB> peaks compared to profiles formed in the surrounding
atmosphere. The inversion results show that the atmosphere in fibrils is
on average -100 K hotter at an optical depth log(τ<SUB>500 nm</SUB>)
= -4.3 compared to their surroundings. The line-of-sight velocity
at chromospheric heights in the fibrils does not show any preference
towards upflows or downflows. The microturbulence in the fibrils is on
average 0.5 km s<SUP>-1</SUP> higher compared to their surroundings. Our
results suggest that the fibrils have a limited extent in height, and
they should be viewed as hot threads pervading the chromosphere. <P
/>Movies associated to Figs. 9, 11, and 15 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202037572/olm">https://www.aanda.org</A>
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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.
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Title: Three-dimensional modeling of chromospheric spectral lines
in a simulated active region
Authors: Bjørgen, Johan P.; Leenaarts, Jorrit; Rempel, Matthias;
Cheung, Mark C. M.; Danilovic, Sanja; de la Cruz Rodríguez, Jaime;
Sukhorukov, Andrii V.
2019A&A...631A..33B Altcode: 2019arXiv190601098B
Context. Because of the complex physics that governs the formation of
chromospheric lines, interpretation of solar chromospheric observations
is difficult. The origin and characteristics of many chromospheric
features are, because of this, unresolved. <BR /> Aims: We focus on
studying two prominent features: long fibrils and flare ribbons. To
model these features, we use a 3D magnetohydrodynamic simulation of
an active region, which self-consistently reproduces both of these
features. <BR /> Methods: We modeled the Hα, Mg II k, Ca II K,
and Ca II 8542 Å lines using the 3D non-LTE radiative transfer
code Multi3D. To obtain non-LTE electron densities, we solved the
statistical equilibrium equations for hydrogen simultaneously with the
charge conservation equation. We treated the Ca II K and Mg II k lines
with partially coherent scattering. <BR /> Results: This simulation
reproduces long fibrils that span between the opposite-polarity
sunspots and go up to 4 Mm in height. They can be traced in all lines
owing to density corrugation. In contrast to previous studies, Hα,
Mg II h&k, and Ca II H&K are formed at similar height in this
model. Although some of the high fibrils are also visible in the Ca II
8542 Å line, this line tends to sample loops and shocks lower in the
chromosphere. Magnetic field lines are aligned with the Hα fibrils,
but the latter holds to a lesser extent for the Ca II 8542 Å line. The
simulation shows structures in the Hα line core that look like flare
ribbons. The emission in the ribbons is caused by a dense chromosphere
and a transition region at high column mass. The ribbons are visible in
all chromospheric lines, but least prominent in Ca II 8542 Å line. In
some pixels, broad asymmetric profiles with a single emission peak
are produced similar to the profiles observed in flare ribbons. They
are caused by a deep onset of the chromospheric temperature rise
and large velocity gradients. <BR /> Conclusions: The simulation
produces long fibrils similar to what is seen in observations. It
also produces structures similar to flare ribbons despite the lack
of nonthermal electrons in the simulation. The latter suggests that
thermal conduction might be a significant agent in transporting flare
energy to the chromosphere in addition to nonthermal electrons.
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Title: Solar image denoising with convolutional neural networks
Authors: Díaz Baso, C. J.; de la Cruz Rodríguez, J.; Danilovic, S.
2019A&A...629A..99D Altcode: 2019arXiv190802815D
The topology and dynamics of the solar chromosphere are greatly
affected by the presence of magnetic fields. The magnetic field
can be inferred by analyzing polarimetric observations of spectral
lines. Polarimetric signals induced by chromospheric magnetic fields
are, however, particularly weak, and in most cases very close to
the detection limit of current instrumentation. Because of this,
there are only few observational studies that have successfully
reconstructed the three components of the magnetic field vector in the
chromosphere. Traditionally, the signal-to-noise ratio of observations
has been improved by performing time-averages or spatial averages, but
in both cases, some information is lost. More advanced techniques, like
principal-component analysis, have also been employed to take advantage
of the sparsity of the observations in the spectral direction. In the
present study, we use the spatial coherence of the observations to
reduce the noise using deep-learning techniques. We designed a neural
network that is capable of recovering weak signals under a complex
noise corruption (including instrumental artifacts and non-linear
post-processing). The training of the network is carried out without
a priori knowledge of the clean signals, or an explicit statistical
characterization of the noise or other corruption. We only use the same
observations as our generative model. The performance of this method
is demonstrated on both synthetic experiments and real data. We show
examples of the improvement in typical signals obtained in current
telescopes such as the Swedish 1 m Solar Telescope. The presented
method can recover weak signals equally well no matter what spectral
line or spectral sampling is used. It is especially suitable for cases
when the wavelength sampling is scarce.
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Title: STiC: A multiatom non-LTE PRD inversion code for full-Stokes
solar observations
Authors: de la Cruz Rodríguez, J.; Leenaarts, J.; Danilovic, S.;
Uitenbroek, H.
2019A&A...623A..74D Altcode: 2018arXiv181008441D
The inference of the underlying state of the plasma in the solar
chromosphere remains extremely challenging because of the nonlocal
character of the observed radiation and plasma conditions in this
layer. Inversion methods allow us to derive a model atmosphere that
can reproduce the observed spectra by undertaking several physical
assumptions. The most advanced approaches involve a depth-stratified
model atmosphere described by temperature, line-of-sight velocity,
turbulent velocity, the three components of the magntic field vector,
and gas and electron pressure. The parameters of the radiative transfer
equation are computed from a solid ground of physical principles. In
order to apply these techniques to spectral lines that sample the
chromosphere, nonlocal thermodynamical equilibrium effects must be
included in the calculations. We developed a new inversion code STiC
(STockholm inversion Code) to study spectral lines that sample the
upper chromosphere. The code is based on the RH forward synthesis code,
which we modified to make the inversions faster and more stable. For
the first time, STiC facilitates the processing of lines from multiple
atoms in non-LTE, also including partial redistribution effects (PRD)
in angle and frequency of scattered photons. Furthermore, we include
a regularization strategy that allows for model atmospheres with a
complex depth stratification, without introducing artifacts in the
reconstructed physical parameters, which are usually manifested in
the form of oscillatory behavior. This approach takes steps toward
a node-less inversion, in which the value of the physical parameters
at each grid point can be considered a free parameter. In this paper
we discuss the implementation of the aforementioned techniques, the
description of the model atmosphere, and the optimizations that we
applied to the code. We carry out some numerical experiments to show
the performance of the code and the regularization techniques that we
implemented. We made STiC publicly available to the community.
---------------------------------------------------------
Title: Chromospheric condensations and magnetic field in a C3.6-class
flare studied via He I D<SUB>3</SUB> spectro-polarimetry
Authors: Libbrecht, Tine; de la Cruz Rodríguez, Jaime; Danilovic,
Sanja; Leenaarts, Jorrit; Pazira, Hiva
2019A&A...621A..35L Altcode: 2018arXiv180606880L
Context. Magnetic reconnection during flares takes place in the
corona, but a substantial part of flare energy is deposited in
the chromosphere. However, high-resolution spectro-polarimetric
chromospheric observations of flares are very rare. The most used
observables are Ca II 8542 Å and He I 10830 Å. <BR /> Aims:
We aim to study the chromosphere during a C3.6 class flare via
spectro-polarimetric observations of the He I D<SUB>3</SUB> line. <BR
/> Methods: We present the first SST/CRISP spectro-polarimetric
observations of He I D<SUB>3</SUB>. We analyzed the data using the
inversion code HAZEL, and estimate the line-of-sight velocity and
the magnetic field vector. <BR /> Results: Strong He I D<SUB>3</SUB>
emission at the flare footpoints, as well as strong He I D<SUB>3</SUB>
absorption profiles tracing the flaring loops are observed during the
flare. The He I D<SUB>3</SUB> traveling emission kernels at the flare
footpoints exhibit strong chromospheric condensations of up to ∼60
km s<SUP>-1</SUP> at their leading edge. Our observations suggest that
such condensations result in shocking the deep chromosphere, causing
broad and modestly blueshifted He I D<SUB>3</SUB> profiles indicating
subsequent upflows. A strong and rather vertical magnetic field of up
to ∼2500 G is measured in the flare footpoints, confirming that the He
I D<SUB>3</SUB> line is likely formed in the deep chromosphere at those
locations. We provide chromospheric line-of-sight velocity and magnetic
field maps obtained via He I D<SUB>3</SUB> inversions. We propose a
fan-spine configuration as the flare magnetic field topology. <BR />
Conclusions: The He I D<SUB>3</SUB> line is an excellent diagnostic to
study the chromosphere during flares. The impact of strong condensations
on the deep chromosphere has been observed. Detailed maps of the flare
dynamics and the magnetic field are obtained.
---------------------------------------------------------
Title: Solar Ultraviolet Bursts
Authors: Young, Peter R.; Tian, Hui; Peter, Hardi; Rutten, Robert J.;
Nelson, Chris J.; Huang, Zhenghua; Schmieder, Brigitte; Vissers, Gregal
J. M.; Toriumi, Shin; Rouppe van der Voort, Luc H. M.; Madjarska, Maria
S.; Danilovic, Sanja; Berlicki, Arkadiusz; Chitta, L. P.; Cheung, Mark
C. M.; Madsen, Chad; Reardon, Kevin P.; Katsukawa, Yukio; Heinzel, Petr
2018SSRv..214..120Y Altcode: 2018arXiv180505850Y
The term "ultraviolet (UV) burst" is introduced to describe small,
intense, transient brightenings in ultraviolet images of solar active
regions. We inventorize their properties and provide a definition
based on image sequences in transition-region lines. Coronal signatures
are rare, and most bursts are associated with small-scale, canceling
opposite-polarity fields in the photosphere that occur in emerging flux
regions, moving magnetic features in sunspot moats, and sunspot light
bridges. We also compare UV bursts with similar transition-region
phenomena found previously in solar ultraviolet spectrometry and
with similar phenomena at optical wavelengths, in particular Ellerman
bombs. Akin to the latter, UV bursts are probably small-scale magnetic
reconnection events occurring in the low atmosphere, at photospheric
and/or chromospheric heights. Their intense emission in lines with
optically thin formation gives unique diagnostic opportunities
for studying the physics of magnetic reconnection in the low solar
atmosphere. This paper is a review report from an International Space
Science Institute team that met in 2016-2017.
---------------------------------------------------------
Title: STiC: Stockholm inversion code
Authors: de la Cruz Rodríguez, J.; Leenaarts, J.; Danilovic, S.;
Uitenbroek, H.
2018ascl.soft10014D Altcode:
STiC is a MPI-parallel non-LTE inversion code for observed full-Stokes
observations. The code processes lines from multiple atoms in non-LTE,
including partial redistribution effects of scattered photons in
angle and frequency of scattered photons (PRD), and can be used with
model atmospheres that have a complex depth stratification without
introducing artifacts.
---------------------------------------------------------
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: Simulating Ellerman bomb-like events
Authors: Danilovic, S.
2017A&A...601A.122D Altcode: 2017arXiv170102112D
Context. Ellerman bombs (EB) seem to be a part of a whole spectrum
of phenomena that might have the same underlying physical mechanism:
magnetic reconnection. <BR /> Aims: The aim of this study is to
investigate whether the proposed mechanism, applied to the circumstances
of EBs, produces the observed characteristics. <BR /> Methods:
To this end, comprehensive three-dimensional magnetohydrodynamic
(3D MHD) simulations were used. Two different cases are presented:
the quiet Sun and an active region. <BR /> Results: Both runs confirm
that EB-like brightenings coincide with hot and dense plasma, which is
in agreement with predictions of 1D and 2D modellings. The simulated
EB-like phenomena assume the observed flame-like form which depends on
the complexity of the ongoing reconnection and the viewing angle. At
the layers sampled by Hα-wings, near temperature minimum and below,
the magnetic field topology seems always to be the same. The field
lines there trace the base of the current sheet and the reconnected
Ω-loops. <BR /> Conclusions: The EB features are caused by reconnection
of strong-field patches of opposite polarity in the regions where the
surface flows are the strongest. The weakest cases among them can be
reproduced quantitatively by the current simulations.
---------------------------------------------------------
Title: High-frequency Oscillations in Small Magnetic Elements Observed
with Sunrise/SuFI
Authors: Jafarzadeh, S.; Solanki, S. K.; Stangalini, M.; Steiner,
O.; Cameron, R. H.; Danilovic, S.
2017ApJS..229...10J Altcode: 2016arXiv161109302J
We characterize waves in small magnetic elements and investigate
their propagation in the lower solar atmosphere from observations at
high spatial and temporal resolution. We use the wavelet transform to
analyze oscillations of both horizontal displacement and intensity
in magnetic bright points found in the 300 nm and the Ca II H 396.8
nm passbands of the filter imager on board the Sunrise balloon-borne
solar observatory. Phase differences between the oscillations at the
two atmospheric layers corresponding to the two passbands reveal
upward propagating waves at high frequencies (up to 30 mHz). Weak
signatures of standing as well as downward propagating waves are also
obtained. Both compressible and incompressible (kink) waves are found
in the small-scale magnetic features. The two types of waves have
different, though overlapping, period distributions. Two independent
estimates give a height difference of approximately 450 ± 100 km
between the two atmospheric layers sampled by the employed spectral
bands. This value, together with the determined short travel times of
the transverse and longitudinal waves provide us with phase speeds of 29
± 2 km s<SUP>-1</SUP> and 31 ± 2 km s<SUP>-1</SUP>, respectively. We
speculate that these phase speeds may not reflect the true propagation
speeds of the waves. Thus, effects such as the refraction of fast
longitudinal waves may contribute to an overestimate of the phase speed.
---------------------------------------------------------
Title: The Second Flight of the Sunrise Balloon-borne Solar
Observatory: Overview of Instrument Updates, the Flight, the Data,
and First Results
Authors: Solanki, S. K.; Riethmüller, T. L.; Barthol, P.; Danilovic,
S.; Deutsch, W.; Doerr, H. -P.; Feller, A.; Gandorfer, A.; Germerott,
D.; Gizon, L.; Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.;
Lagg, A.; Meller, R.; Tomasch, G.; van Noort, M.; Blanco Rodríguez,
J.; Gasent Blesa, J. L.; Balaguer Jiménez, M.; Del Toro Iniesta,
J. C.; López Jiménez, A. C.; Orozco Suarez, D.; Berkefeld, T.;
Halbgewachs, C.; Schmidt, W.; Álvarez-Herrero, A.; Sabau-Graziati,
L.; Pérez Grande, I.; Martínez Pillet, V.; Card, G.; Centeno, R.;
Knölker, M.; Lecinski, A.
2017ApJS..229....2S Altcode: 2017arXiv170101555S
The Sunrise balloon-borne solar observatory, consisting of a 1 m
aperture telescope that provides a stabilized image to a UV filter
imager and an imaging vector polarimeter, carried out its second science
flight in 2013 June. It provided observations of parts of active regions
at high spatial resolution, including the first high-resolution images
in the Mg II k line. The obtained data are of very high quality, with
the best UV images reaching the diffraction limit of the telescope
at 3000 Å after Multi-Frame Blind Deconvolution reconstruction
accounting for phase-diversity information. Here a brief update is
given of the instruments and the data reduction techniques, which
includes an inversion of the polarimetric data. Mainly those aspects
that evolved compared with the first flight are described. A tabular
overview of the observations is given. In addition, an example time
series of a part of the emerging active region NOAA AR 11768 observed
relatively close to disk center is described and discussed in some
detail. The observations cover the pores in the trailing polarity of
the active region, as well as the polarity inversion line where flux
emergence was ongoing and a small flare-like brightening occurred in
the course of the time series. The pores are found to contain magnetic
field strengths ranging up to 2500 G, and while large pores are clearly
darker and cooler than the quiet Sun in all layers of the photosphere,
the temperature and brightness of small pores approach or even exceed
those of the quiet Sun in the upper photosphere.
---------------------------------------------------------
Title: Photospheric Response to an Ellerman Bomb-like Event—An
Analogy of Sunrise/IMaX Observations and MHD Simulations
Authors: Danilovic, S.; Solanki, S. K.; Barthol, P.; Gandorfer,
A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.;
Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.;
Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229....5D Altcode: 2016arXiv160903817D
Ellerman Bombs are signatures of magnetic reconnection, which is an
important physical process in the solar atmosphere. How and where they
occur is a subject of debate. In this paper, we analyze Sunrise/IMaX
data, along with 3D MHD simulations that aim to reproduce the exact
scenario proposed for the formation of these features. Although
the observed event seems to be more dynamic and violent than the
simulated one, simulations clearly confirm the basic scenario for the
production of EBs. The simulations also reveal the full complexity of
the underlying process. The simulated observations show that the Fe I
525.02 nm line gives no information on the height where reconnection
takes place. It can only give clues about the heating in the aftermath
of the reconnection. However, the information on the magnetic field
vector and velocity at this spatial resolution is extremely valuable
because it shows what numerical models miss and how they can be
improved.
---------------------------------------------------------
Title: Are Internetwork Magnetic Fields in the Solar Photosphere
Horizontal or Vertical?
Authors: Lites, B. W.; Rempel, M.; Borrero, J. M.; Danilovic, S.
2017ApJ...835...14L Altcode:
Using many observations obtained during 2007 with the
Spectro-Polarimeter of the Hinode Solar Optical Telescope, we explore
the angular distribution of magnetic fields in the quiet internetwork
regions of the solar photosphere. Our work follows from the insight of
Stenflo, who examined only linear polarization signals in photospheric
lines, thereby avoiding complications of the analysis arising from the
differing responses to linear and circular polarization. We identify
and isolate regions of a strong polarization signal that occupy only
a few percent of the observed quiet Sun area yet contribute most to
the net linear polarization signal. The center-to-limb variation of
the orientation of linear polarization in these strong signal regions
indicates that the associated magnetic fields have a dominant vertical
orientation. In contrast, the great majority of the solar disk is
occupied by much weaker linear polarization signals. The orientation of
the linear polarization in these regions demonstrates that the field
orientation is dominantly horizontal throughout the photosphere. We
also apply our analysis to Stokes profiles synthesized from the
numerical MHD simulations of Rempel as viewed at various oblique
angles. The analysis of the synthetic data closely follows that of
the observations, lending confidence to using the simulations as a
guide for understanding the physical origins of the center-to-limb
variation of linear polarization in the quiet Sun area.
---------------------------------------------------------
Title: Observed and simulated power spectra of kinetic and magnetic
energy retrieved with 2D inversions
Authors: Danilovic, S.; Rempel, M.; van Noort, M.; Cameron, R.
2016A&A...594A.103D Altcode: 2016arXiv160706242D
Context. Information on the origin of internetwork magnetic field is
hidden at the smallest spatial scales. <BR /> Aims: We try to retrieve
the power spectra with certainty to the highest spatial frequencies
allowed by current instrumentation. <BR /> Methods: To accomplish this,
we use a 2D inversion code that is able to recover information up to
the instrumental diffraction limit. <BR /> Results: The retrieved power
spectra have shallow slopes that extend further down to much smaller
scales than has been found before. They do not seem to show any power
law. The observed slopes at subgranular scales agree with those obtained
from recent local dynamo simulations. Small differences are found for
the vertical component of kinetic energy that suggest that observations
suffer from an instrumental effect that is not taken into account. <BR
/> Conclusions: Local dynamo simulations quantitatively reproduce the
observed magnetic energy power spectra on the scales of granulation
down to the resolution limit of Hinode/SP, within the error bars
inflicted by the method used and the instrumental effects replicated.
---------------------------------------------------------
Title: Internetwork magnetic field as revealed by two-dimensional
inversions
Authors: Danilovic, S.; van Noort, M.; Rempel, M.
2016A&A...593A..93D Altcode: 2016arXiv160700772D
Context. Properties of magnetic field in the internetwork regions
are still fairly unknown because of rather weak spectropolarimetric
signals. <BR /> Aims: We address the matter by using the two-dimensional
(2D) inversion code, which is able to retrieve the information on
smallest spatial scales up to the diffraction limit, while being less
susceptible to noise than most of the previous methods used. <BR />
Methods: Performance of the code and the impact of various effects
on the retrieved field distribution is tested first on the realistic
magneto-hydrodynamic (MHD) simulations. The best inversion scenario
is then applied to the real data obtained by Spectropolarimeter (SP)
on board Hinode. <BR /> Results: Tests on simulations show that: (1)
the best choice of node position ensures a decent retrieval of all
parameters; (2) the code performs well for different configurations
of magnetic field; (3) slightly different noise levels or slightly
different defocus included in the spatial point spread function
(PSF) produces no significant effect on the results; and (4)
temporal integration shifts the field distribution to a stronger,
more horizontally inclined field. <BR /> Conclusions: Although the
contribution of the weak field is slightly overestimated owing to noise,
2D inversions are able to recover well the overall distribution of the
magnetic field strength. Application of the 2D inversion code on the
Hinode SP internetwork observations reveals a monotonic field strength
distribution. The mean field strength at optical depth unity is ~
130 G. At higher layers, field strength drops as the field becomes
more horizontal. Regarding the distribution of the field inclination,
tests show that we cannot directly retrieve it with the observations
and tools at hand, however, the obtained distributions are consistent
with those expected from simulations with a quasi-isotropic field
inclination after accounting for observational effects.
---------------------------------------------------------
Title: Variation of the Mn I 539.4 nm line with the solar cycle
Authors: Danilovic, S.; Solanki, S. K.; Livingston, W.; Krivova, N.;
Vince, I.
2016A&A...587A..33D Altcode: 2015arXiv151101286D
Context. As a part of the long-term program at Kitt Peak National
Observatory (KPNO), the Mn I 539.4 nm line has been observed for
nearly three solar cycles using the McMath telescope and the 13.5 m
spectrograph in double-pass mode. These full-disk spectrophotometric
observations revealed an unusually strong change of this line's
parameters over the solar cycle. <BR /> Aims: Optical pumping by the Mg
II k line was originally proposed to explain these variations. More
recent studies have proposed that this is not required and that
the magnetic variability (I.e., the changes in solar atmospheric
structure due to faculae) might explain these changes. Magnetic
variability is also the mechanism that drives the changes in total
solar irradiance variations (TSI). With this work we investigate this
proposition quantitatively by using the same model that was earlier
successfully employed to reconstruct the irradiance. <BR /> Methods:
We reconstructed the changes in the line parameters using the model
SATIRE-S, which takes only variations of the daily surface distribution
of the magnetic field into account. We applied exactly the same model
atmospheres and value of the free parameter as were used in previous
solar irradiance reconstructions to now model the variation in the Mn
I 539.4 nm line profile and in neighboring Fe I lines. We compared
the results of the theoretical model with KPNO observations. <BR />
Results: The changes in the Mn I 539.4 nm line and a neighbouring Fe
I 539.52 nm line over approximately three solar cycles are reproduced
well by the model without additionally tweaking the model parameters,
if changes made to the instrument setup are taken into account. The
model slightly overestimates the change for the strong Fe I 539.32 nm
line. <BR /> Conclusions: Our result confirms that optical pumping
of the Mn II 539.4 nm line by Mg II k is not the main cause of its
solar cycle change. It also provides independent confirmation of solar
irradiance models which are based on the assumption that irradiance
variations are caused by the evolution of the solar surface magnetic
flux. The result obtained here also supports the spectral irradiance
variations computed by these models.
---------------------------------------------------------
Title: Simulated magnetic flows in the solar photosphere
Authors: Danilovic, S.; Cameron, R. H.; Solanki, S. K.
2015A&A...574A..28D Altcode: 2014arXiv1408.6159D
Context. Recent Sunrise/IMaX observations have revealed supersonic
magnetic flows. <BR /> Aims: Our aim is to determine the origin of
these flows by using realistic magnetohydrodynamics simulations. <BR
/> Methods: We simulated cancellation and emergence of magnetic
flux through the solar photosphere. Our first numerical experiment
started with a magnetic field of both polarities. To simulate
emergence into a region with pre-existing field, we introduced a
large-scale horizontally uniform sheet of a horizontal field. We
followed the subsequent evolution and created synthetic polarimetric
observations, including known instrumental effects of the Sunrise/IMaX
and Hinode/SP instruments. We compared the simulated and observed
spectropolarimetric signals. <BR /> Results: Strongly blue- and
redshifted Stokes V signals are produced in locations where strong
line-of-sight velocities coincide with the strong line-of-sight
component of the magnetic field. The size and strength of simulated
events is smaller than observed, and they are mostly associated with
downflows, contrary to observations. In a few cases where they appear
above a granule, single blue-lobed Stokes V are produced by strong
gradients in magnetic field and velocity. No change of magnetic field
sign is detected along the line of sight in these instances. More
high-speed magnetised flows occurred when an emergence was simulated
than when no horizontal field was added. <BR /> Conclusions: The
simulations indicate that the observed events result from magnetic flux
emergences in which reconnection may take place, but does not seem to
be necessary. <P />The movies are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201423779/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Centre-to-limb properties of small, photospheric quiet-Sun jets
Authors: Rubio da Costa, F.; Solanki, S. K.; Danilovic, S.; Hizberger,
J.; Martínez-Pillet, V.
2015A&A...574A..95R Altcode: 2014arXiv1412.1620R
Context. Strongly Doppler-shifted Stokes V profiles have been detected
in the quiet Sun with the IMaX instrument on-board the SUNRISE
stratospheric balloon-borne telescope. High velocities are required
to produce such signals, hence these events have been interpreted as
jets, although other sources are also possible. <BR /> Aims: We aim
to characterize the variation of the main properties of these events
(occurrence rate, lifetime, size, and velocities) with their position on
the solar disk between disk centre and the solar limb. <BR /> Methods:
These events were identified in SUNRISE/IMaX data according to the same
objective criteria at all available positions on the solar disk. Their
properties were determined using standard techniques. <BR /> Results:
Our study yielded a number of new insights into this phenomenon. Most
importantly, the number density of these events is independent of
the heliocentric angle, meaning that the investigated supersonic
flows are nearly isotropically distributed. Size and lifetime are
also nearly independent of the heliocentric angle, while their
intensity contrast increases towards the solar limb. The Stokes V
jets are associated with upflow velocities deduced from Stokes I,
which are stronger towards the limb. Their intensity decreases with
time, while their line-of-sight velocity does not display a clear
temporal evolution. Their association with linear polarization signals
decreases towards the limb. <BR /> Conclusions: The density of events
appears to be independent of heliocentric angle, establishing that they
are directed nearly randomly. If these events are jets triggered by
magnetic reconnection between emerging magnetic flux and the ambient
field, then our results suggest that there is no preferred geometry
for the reconnection process.
---------------------------------------------------------
Title: Inclinations of small quiet-Sun magnetic features based on
a new geometric approach
Authors: Jafarzadeh, S.; Solanki, S. K.; Lagg, A.; Bellot Rubio,
L. R.; van Noort, M.; Feller, A.; Danilovic, S.
2014A&A...569A.105J Altcode: 2014arXiv1408.2443J
Context. High levels of horizontal magnetic flux have been reported
in the quiet-Sun internetwork, often based on Stokes profile
inversions. <BR /> Aims: Here we introduce a new method for deducing
the inclination of magnetic elements and use it to test magnetic field
inclinations from inversions. <BR /> Methods: We determine accurate
positions of a set of small, bright magnetic elements in high spatial
resolution images sampling different photospheric heights obtained by
the Sunrise balloon-borne solar observatory. Together with estimates
of the formation heights of the employed spectral bands, these provide
us with the inclinations of the magnetic features. We also compute
the magnetic inclination angle of the same magnetic features from the
inversion of simultaneously recorded Stokes parameters. <BR /> Results:
Our new, geometric method returns nearly vertical fields (average
inclination of around 14° with a relatively narrow distribution
having a standard deviation of 6°). In strong contrast to this, the
traditionally used inversions give almost horizontal fields (average
inclination of 75 ± 8°) for the same small magnetic features,
whose linearly polarised Stokes profiles are adversely affected by
noise. We show that for such magnetic features inversions overestimate
the flux in horizontal magnetic fields by an order of magnitude. <BR />
Conclusions: The almost vertical field of bright magnetic features from
our geometric method is clearly incompatible with the nearly horizontal
magnetic fields obtained from the inversions. This indicates that the
amount of magnetic flux in horizontal fields deduced from inversions is
overestimated in the presence of weak Stokes signals, in particular if
Stokes Q and U are close to or under the noise level. Inversions should
be used with great caution when applied to data with no clear Stokes Q
and no U signal. By combining the proposed method with inversions we are
not just improving the inclination, but also the field strength. This
technique allows us to analyse features that are not reliably treated
by inversions, thus greatly extending our capability to study the
complete magnetic field of the quiet Sun.
---------------------------------------------------------
Title: Interpreting the Helioseismic and Magnetic Imager (HMI)
Multi-Height Velocity Measurements
Authors: Nagashima, Kaori; Löptien, Björn; Gizon, Laurent; Birch,
Aaron C.; Cameron, Robert; Couvidat, Sebastien; Danilovic, Sanja;
Fleck, Bernhard; Stein, Robert
2014SoPh..289.3457N Altcode: 2014arXiv1404.3569N; 2014SoPh..tmp...84N
The Solar Dynamics Observatory/Helioseismic and Magnetic Imager
(SDO/HMI) filtergrams, taken at six wavelengths around the Fe I 6173.3
Å line, contain information about the line-of-sight velocity over
a range of heights in the solar atmosphere. Multi-height velocity
inferences from these observations can be exploited to study wave
motions and energy transport in the atmosphere. Using realistic
convection-simulation datasets provided by the STAGGER and MURaM
codes, we generate synthetic filtergrams and explore several methods
for estimating Dopplergrams. We investigate at which height each
synthetic Dopplergram correlates most strongly with the vertical
velocity in the model atmospheres. On the basis of the investigation,
we propose two Dopplergrams other than the standard HMI-algorithm
Dopplergram produced from HMI filtergrams: a line-center Dopplergram
and an average-wing Dopplergram. These two Dopplergrams correlate most
strongly with vertical velocities at the heights of 30 - 40 km above
(line center) and 30 - 40 km below (average wing) the effective height
of the HMI-algorithm Dopplergram. Therefore, we can obtain velocity
information from two layers separated by about a half of a scale height
in the atmosphere, at best. The phase shifts between these multi-height
Dopplergrams from observational data as well as those from the simulated
data are also consistent with the height-difference estimates in the
frequency range above the photospheric acoustic-cutoff frequency.
---------------------------------------------------------
Title: Vigorous convection in a sunspot granular light bridge
Authors: Lagg, Andreas; Solanki, Sami K.; van Noort, Michiel;
Danilovic, Sanja
2014A&A...568A..60L Altcode: 2014arXiv1407.1202L
Context. Light bridges are the most prominent manifestation of
convection in sunspots. The brightest representatives are granular
light bridges composed of features that appear to be similar to
granules. <BR /> Aims: An in-depth study of the convective motions,
temperature stratification, and magnetic field vector in and around
light bridge granules is presented with the aim of identifying
similarities and differences to typical quiet-Sun granules. <BR />
Methods: Spectropolarimetric data from the Hinode Solar Optical
Telescope were analyzed using a spatially coupled inversion technique
to retrieve the stratified atmospheric parameters of light bridge and
quiet-Sun granules. <BR /> Results: Central hot upflows surrounded by
cooler fast downflows reaching 10 km s<SUP>-1</SUP> clearly establish
the convective nature of the light bridge granules. The inner part
of these granules in the near surface layers is field free and is
covered by a cusp-like magnetic field configuration. We observe
hints of field reversals at the location of the fast downflows. The
quiet-Sun granules in the vicinity of the sunspot are covered by a
low-lying canopy field extending radially outward from the spot. <BR
/> Conclusions: The similarities between quiet-Sun and light bridge
granules point to the deep anchoring of granular light bridges in
the underlying convection zone. The fast, supersonic downflows are
most likely a result of a combination of invigorated convection
in the light bridge granule due to radiative cooling into the
neighboring umbra and the fact that we sample deeper layers, since the
downflows are immediately adjacent to the slanted walls of the Wilson
depression. <P />The two movies are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424071/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Comparison between Mg II k and Ca II H Images Recorded by
SUNRISE/SuFI
Authors: Danilovic, S.; Hirzberger, J.; Riethmüller, T. L.; Solanki,
S. K.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Gizon, L.; Knölker,
M.; Schmidt, W.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.
2014ApJ...784...20D Altcode:
We present a comparison of high-resolution images of the solar surface
taken in the Mg II k and Ca II H channels of the Filter Imager on the
balloon-borne solar observatory SUNRISE. The Mg and Ca lines are sampled
with 0.48 nm and 0.11 nm wide filters, respectively. The two channels
show remarkable qualitative and quantitative similarities in the quiet
Sun, in an active region plage and during a small flare. However, the Mg
filtergrams display 1.4-1.7 times higher intensity contrast and appear
more smeared and smoothed in the quiet Sun. In addition, the fibrils
in a plage are wider. Although the exposure time is 100 times longer
for Mg images, the evidence suggests that these differences cannot be
explained only with instrumental effects or the evolution of the solar
scene. The differences at least partially arise because of different
line-formation heights, the stronger response of Mg k emission peaks
to the higher temperatures, and the larger height range sampled by
the broad Mg filter used here. This is evidently manifested during
the flare when a surge in Mg evolves differently than in Ca.
---------------------------------------------------------
Title: Point spread function of SDO/HMI and the effects of stray
light correction on the apparent properties of solar surface phenomena
Authors: Yeo, K. L.; Feller, A.; Solanki, S. K.; Couvidat, S.;
Danilovic, S.; Krivova, N. A.
2014A&A...561A..22Y Altcode: 2013arXiv1310.4972Y
<BR /> Aims: We present a point spread function (PSF) for the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) and discuss the effects of its removal on the
apparent properties of solar surface phenomena in HMI data. <BR />
Methods: The PSF was retrieved from observations of Venus in transit
by matching it to the convolution of a model of the Venusian disc and
solar background with a guess PSF. We described the PSF as the sum of
five Gaussian functions, the amplitudes of which vary sinusoidally with
azimuth. This relatively complex functional form was required by the
data. Observations recorded near in time to the transit of Venus were
corrected for instrumental scattered light by the deconvolution with the
PSF. We also examined the variation in the shape of the solar aureole in
daily data, as an indication of PSF changes over time. <BR /> Results:
Granulation contrast in restored HMI data is greatly enhanced relative
to the original data and exhibit reasonable agreement with numerical
simulations. Image restoration enhanced the apparent intensity and
pixel averaged magnetic field strength of photospheric magnetic features
significantly. For small-scale magnetic features, restoration enhanced
intensity contrast in the continuum and core of the Fe I 6173 Å line
by a factor of 1.3, and the magnetogram signal by a factor of 1.7. For
sunspots and pores, the enhancement varied strongly within and between
features, being more acute for smaller features. Magnetic features are
also rendered smaller, as signal smeared onto the surrounding quiet
Sun is recovered. Image restoration increased the apparent amount of
magnetic flux above the noise floor by a factor of about 1.2, most
of the gain coming from the quiet Sun. Line-of-sight velocity due to
granulation and supergranulation is enhanced by a factor of 1.4 to 2.1,
depending on position on the solar disc. The shape of the solar aureole
varied, with time and between the two CCDs. There are also indications
that the PSF varies across the FOV. However, all these variations were
found to be relatively small, such that a single PSF can be applied to
HMI data from both CCDs, over the period examined without introducing
significant error. <BR /> Conclusions: Restoring HMI observations
with the PSF presented here returns a reasonable estimate of the stray
light-free intensity contrast. Image restoration affects the measured
radiant, magnetic and dynamic properties of solar surface phenomena
sufficiently to significantly impact interpretation.
---------------------------------------------------------
Title: First High-resolution Images of the Sun in the 2796 Å Mg II
k Line
Authors: Riethmüller, T. L.; Solanki, S. K.; Hirzberger, J.;
Danilovic, S.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Gizon, L.;
Knölker, M.; Schmidt, W.; Del Toro Iniesta, J. C.
2013ApJ...776L..13R Altcode: 2013arXiv1309.5213R
We present the first high-resolution solar images in the Mg II k 2796
Å line. The images, taken through a 4.8 Å broad interference filter,
were obtained during the second science flight of Sunrise in 2013 June
by the Sunrise Filter Imager (SuFI) instrument. The Mg II k images
display structures that look qualitatively very similar to images taken
in the core of Ca II H. The Mg II images exhibit reversed granulation
(or shock waves) in the internetwork regions of the quiet Sun, at
intensity contrasts that are similar to those found in Ca II H. Very
prominent in Mg II are bright points, both in the quiet Sun and in plage
regions, particularly near the disk center. These are much brighter than
at other wavelengths sampled at similar resolution. Furthermore, Mg II k
images also show fibril structures associated with plage regions. Again,
the fibrils are similar to those seen in Ca II H images, but tend to
be more pronounced, particularly in weak plage.
---------------------------------------------------------
Title: On the structure and dynamics of Ellerman bombs. Detailed
study of three events and modelling of Hα
Authors: Bello González, N.; Danilovic, S.; Kneer, F.
2013A&A...557A.102B Altcode:
<BR /> Aims: We study the structure and dynamics of three Ellerman
bombs (EBs) observed in an evolving active region. <BR /> Methods: The
active region NOAA 11271 was observed with the Vacuum Tower Telescope
at Observatorio del Teide/Tenerife on August 18, 2011. We used the
two-dimensional Triple Etalon SOlar Spectrometer (TESOS) to obtain time
sequences of the active region and of EBs in Hα at a cadence of 15
s. Simultaneously, we obtained full Stokes profiles with the Tenerife
Infrared Polarimeter (TIP II) in the two magnetically sensitive Fe i
infrared lines (IR) at 1.56 μ, scanning spatial sections of the area
with cadences of 28-46 s. The Hα data were reconstructed with speckle
methods to study the evolution of the atmospheric stratification. Two
methods were used to extract magnetic field information from the IR
Stokes profiles: 1) fitting of the (Q,U,V) profiles by Gaussians; and
2) applying the Milne-Eddington approximation, assuming two separate
magnetic structures in the resolution element and fitting by trial and
error some profiles from the EB areas. Data from SDO-HMI and -AIA were
also used. We performed two-dimensional (2D) non-LTE radiative transfer
calculations of Hα in parameterised models of EBs. <BR /> Results:
The three EBs studied in detail occurred in a complex active region near
sunspots. They were very bright with a factor of 1.5-2.8 brighter than
the nearby area. They lived for 1/2 h and longer. They were related to
broadband faculae, but the latter were not the brightest features in the
field of view. The EBs occurred in magnetic field configurations with
opposite polarity close together. One EB was located at the outskirts
of a penumbra of a complex sunspot and showed repeated "flaring" in
SDO-AIA data. Another was close to a strong field patch and moved into
this during the end of its lifetime. The third EB showed clear changes
of field structure during the time it was observed. We obtained from
the 2D modelling that heating and increase in Hα opacity are likely
to occur at heights of 300-800 km. Line shifts and asymmetries can
well be reproduced by velocities at these heights and also at much
larger heights. <BR /> Conclusions: The three EBs occurred at sites
with magnetic fields of opposite polarity, which were likely the cause
of the Hα brightening upon reconnection.
---------------------------------------------------------
Title: On the relation between continuum brightness and magnetic
field in solar active regions
Authors: Danilovic, S.; Röhrbein, D.; Cameron, R. H.; Schüssler, M.
2013A&A...550A.118D Altcode:
Context. Variations of solar irradiance are mainly determined
by the changing coverage of the visible solar disk with magnetic
flux concentrations. The relationship between brightness and field
strength is an important ingredient for models and reconstructions of
irradiance variations. <BR /> Aims: We assess the effect of limited
observational resolution on the relationship between brightness
and magnetic field by comparing comprehensive MHD simulations with
observational results. <BR /> Methods: Simulations of magnetoconvection
representing the near-surface layers of a plage region were used to
determine maps of the continuum brightness and Stokes profiles for the
Fe i line at 630.22 nm. After convolving with instrumental profiles,
synthetic observations of the magnetic field were generated by applying
a Stokes inversion code. We compare the resulting relation between
brightness and apparent vertical magnetic field to the corresponding
outcome derived from real observations of a plage region with the
Hinode satellite. <BR /> Results: Consideration of the image smearing
effects due to the limited resolution of the observations transform the
largely monotonic relation between brightness and field strength at the
original resolution of the simulations into a profile with a maximum
at intermediate field strength, which is in good agreement with the
observations. <BR /> Conclusions: Considering the effect of limited
observational resolution renders the relation between brightness and
magnetic field from comprehensive MHD simulations consistent with
observational results. This is a necessary prerequisite for the
utilization of simulations for models and reconstruction of solar
irradiance variations.
---------------------------------------------------------
Title: Structure and dynamics of isolated internetwork Ca II H bright
points observed by SUNRISE
Authors: Jafarzadeh, S.; Solanki, S. K.; Feller, A.; Lagg, A.;
Pietarila, A.; Danilovic, S.; Riethmüller, T. L.; Martínez Pillet, V.
2013A&A...549A.116J Altcode: 2012arXiv1211.4836J
<BR /> Aims: We aim to improve our picture of the low chromosphere in
the quiet-Sun internetwork by investigating the intensity, horizontal
velocity, size and lifetime variations of small bright points (BPs;
diameter smaller than 0.3 arcsec) observed in the Ca II H 3968 Å
passband along with their magnetic field parameters, derived from
photospheric magnetograms. <BR /> Methods: Several high-quality
time series of disc-centre, quiet-Sun observations from the Sunrise
balloon-borne solar telescope, with spatial resolution of around 100
km on the solar surface, have been analysed to study the dynamics
of BPs observed in the Ca II H passband and their dependence on the
photospheric vector magnetogram signal. <BR /> Results: Parameters such
as horizontal velocity, diameter, intensity and lifetime histograms of
the isolated internetwork and magnetic Ca II H BPs were determined. Mean
values were found to be 2.2 km s<SUP>-1</SUP>, 0.2 arcsec (≈150 km),
1.48 ⟨ I<SUB>Ca</SUB> ⟩ and 673 s, respectively. Interestingly, the
brightness and the horizontal velocity of BPs are anti-correlated. Large
excursions (pulses) in horizontal velocity, up to 15 km s<SUP>-1</SUP>,
are present in the trajectories of most BPs. These could excite kink
waves travelling into the chromosphere and possibly the corona, which we
estimate to carry an energy flux of 310 W m<SUP>-2</SUP>, sufficient to
heat the upper layers, although only marginally. <BR /> Conclusions:
The stable observing conditions of Sunrise and our technique for
identifying and tracking BPs have allowed us to determine reliable
parameters of these features in the internetwork. Thus we find, e.g.,
that they are considerably longer lived than previously thought. The
large velocities are also reliable, and may excite kink waves. Although
these wave are (marginally) energetic enough to heat the quiet corona,
we expect a large additional contribution from larger magnetic elements
populating the network and partly also the internetwork.
---------------------------------------------------------
Title: First Results from the SUNRISE Mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.;
Gandorfer, A.; Hirzberger, J.; Jafarzadeh, S.; Lagg, A.; Riethmüller,
T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; González,
M. J. M.; Pillet, V. M.; Khomenko, E.; Yelles Chaouche, L.; Iniesta,
J. C. d. T.; Domingo, V.; Palacios, J.; Knölker, M.; González,
N. B.; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt,
W.; Steiner, O.; Title, A. M.
2012ASPC..455..143S Altcode:
The SUNRISE balloon-borne solar observatory consists of a 1m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system, and further infrastructure. The first
science flight of SUNRISE yielded high-quality data that reveal the
structure, dynamics, and evolution of solar convection, oscillations,
and magnetic fields at a resolution of around 100 km in the quiet
Sun. Here we describe very briefly the mission and the first results
obtained from the SUNRISE data, which include a number of discoveries.
---------------------------------------------------------
Title: Observational signatures of simulated reconnection in solar
photosphere
Authors: Danilovic, S.
2012decs.confE..61D Altcode:
Recent IMaX/Sunrise observations reveal many short-lived high
velocity flows that appear at very small scales everywhere in
the quiet Sun. The flows usually appear close to the patches of
opposite magnetic polarities. In some cases, the inversion result show
localised temperature increase and strong downflows. Here, we confirm
the hypothesis that some of the observed events are in fact produced
by the reconnection of the emerging with the preexisting field. We
concentrate on individual reconnection events in the realistic 3D MHD
simulations and describe observational signatures that are likely to
arise. The comparison of simulated with observed cases suggests that
there might be currents sheets forming very low in the atmosphere. The
retrieved temperatures and velocities imply that the observed events
are much more energetic than the simulated cases.
---------------------------------------------------------
Title: The Sun at high resolution: first results from the Sunrise
mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller,
A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.;
Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez;
Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.;
Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.;
Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M.
2011IAUS..273..226S Altcode:
The Sunrise balloon-borne solar observatory consists of a 1m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system and further infrastructure. The first
science flight of Sunrise yielded high-quality data that reveal the
structure, dynamics and evolution of solar convection, oscillations
and magnetic fields at a resolution of around 100 km in the quiet
Sun. Here we describe very briefly the mission and the first results
obtained from the Sunrise data, which include a number of discoveries.
---------------------------------------------------------
Title: Transport of Magnetic Flux from the Canopy to the Internetwork
Authors: Pietarila, A.; Cameron, R. H.; Danilovic, S.; Solanki, S. K.
2011ApJ...729..136P Altcode: 2011arXiv1102.1397P
Recent observations have revealed that 8% of linear polarization
patches in the internetwork (INW) quiet Sun are fully embedded in
downflows. These are not easily explained with the typical scenarios for
the source of INW fields which rely on flux emergence from below. Using
radiative MHD simulations, we explore a scenario where magnetic flux
is transported from the magnetic canopy overlying the INW into the
photosphere by means of downward plumes associated with convective
overshoot. We find that if a canopy-like magnetic field is present in
the simulation, the transport of flux from the canopy is an important
process for seeding the photospheric layers of the INW with magnetic
field. We propose that this mechanism is relevant for the Sun as well,
and it could naturally explain the observed INW linear polarization
patches entirely embedded in downflows.
---------------------------------------------------------
Title: SUNRISE: Instrument, Mission, Data, and First Results
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.;
Gandorfer, A.; Hirzberger, J.; Riethmüller, T. L.; Schüssler, M.;
Bonet, J. A.; Martínez Pillet, V.; del Toro Iniesta, J. C.; Domingo,
V.; Palacios, J.; Knölker, M.; Bello González, N.; Berkefeld, T.;
Franz, M.; Schmidt, W.; Title, A. M.
2010ApJ...723L.127S Altcode: 2010arXiv1008.3460S
The SUNRISE balloon-borne solar observatory consists of a 1 m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system, and further infrastructure. The first
science flight of SUNRISE yielded high-quality data that revealed the
structure, dynamics, and evolution of solar convection, oscillations,
and magnetic fields at a resolution of around 100 km in the quiet
Sun. After a brief description of instruments and data, the first
qualitative results are presented. In contrast to earlier observations,
we clearly see granulation at 214 nm. Images in Ca II H display narrow,
short-lived dark intergranular lanes between the bright edges of
granules. The very small-scale, mixed-polarity internetwork fields
are found to be highly dynamic. A significant increase in detectable
magnetic flux is found after phase-diversity-related reconstruction
of polarization maps, indicating that the polarities are mixed right
down to the spatial resolution limit and probably beyond.
---------------------------------------------------------
Title: Transverse Component of the Magnetic Field in the Solar
Photosphere Observed by SUNRISE
Authors: Danilovic, S.; Beeck, B.; Pietarila, A.; Schüssler, M.;
Solanki, S. K.; Martínez Pillet, V.; Bonet, J. A.; del Toro Iniesta,
J. C.; Domingo, V.; Barthol, P.; Berkefeld, T.; Gandorfer, A.;
Knölker, M.; Schmidt, W.; Title, A. M.
2010ApJ...723L.149D Altcode: 2010arXiv1008.1535D
We present the first observations of the transverse component of
a photospheric magnetic field acquired by the imaging magnetograph
SUNRISE/IMaX. Using an automated detection method, we obtain statistical
properties of 4536 features with significant linear polarization
signal. We obtain a rate of occurrence of 7 × 10<SUP>-4</SUP>
s<SUP>-1</SUP> arcsec<SUP>-2</SUP>, which is 1-2 orders of magnitude
larger than the values reported by previous studies. We show that
these features have no characteristic size or lifetime. They appear
preferentially at granule boundaries with most of them being caught
in downflow lanes at some point. Only a small percentage are entirely
and constantly embedded in upflows (16%) or downflows (8%).
---------------------------------------------------------
Title: Probing quiet Sun magnetism using MURaM simulations and
Hinode/SP results: support for a local dynamo
Authors: Danilovic, S.; Schüssler, M.; Solanki, S. K.
2010A&A...513A...1D Altcode: 2010arXiv1001.2183D
Context. Owing to the limited spatial resolution and the weak
polarization signal coming from the quietest regions on the Sun, the
organization of the magnetic field on the smallest scales is largely
unknown. <BR /> Aims: We obtain information about the magnetic flux
present in the quiet Sun by comparing radiative MHD simulations with
observations, with particular emphasis on the role of surface dynamo
action. <BR /> Methods: We synthesized Stokes profiles on the basis of
the MHD simulation results. The profiles are degraded by taking the
properties of the spectropolarimeter (SP) into account onboard the
Hinode satellite. We used simulation runs with different magnetic
Reynolds numbers (R<SUB>m</SUB>) and observations at different
heliocentric angles with different levels of noise. <BR /> Results:
Simulations with an imposed mixed-polarity field and R<SUB>m</SUB>
below the threshold for dynamo action reproduce the observed vertical
flux density, but do not display a high enough horizontal flux
density. Surface dynamo simulations at the highest R<SUB>m</SUB>
feasible at the moment yield a ratio of the horizontal and vertical
flux density consistent with observational results, but the overall
amplitudes are too low. Based on the properties of the local dynamo
simulations, a tentative scaling of the magnetic field strength
by a factor 2-3 reproduces the signal observed in the internetwork
regions. <BR /> Conclusions: We find agreement with observations at
different heliocentric angles. The mean field strength in internetwork
implied by our analysis is roughly 170 G at the optical depth unity. Our
study shows that surface dynamo could be responsible for most of the
magnetic flux in the quiet Sun outside the network, given that the
extrapolation to higher R<SUB>m</SUB> is valid.
---------------------------------------------------------
Title: The small-scale solar surface dynamo
Authors: Pietarila Graham, Jonathan; Danilovic, Sanja; Schuessler,
Manfred
2010arXiv1003.0347P Altcode:
The existence of a turbulent small-scale solar surface dynamo is likely,
considering existing numerical and laboratory experiments, as well as
comparisons of a small-scale dynamo in MURaM simulations with Hinode
observations. We find the observed peaked probability distribution
function (PDF) from Stokes-V magnetograms is consistent with a monotonic
PDF of the actual vertical field strength. The cancellation function
of the vertical flux density from a Hinode SP observation is found to
follow a self-similar power law over two decades in length scales down
to the ~200 km resolution limit. This provides observational evidence
that the scales of magnetic structuring in the photosphere extend
at least down to 20 km. From the power law, we determine a lower
bound for the true quiet-Sun mean vertical unsigned flux density of
~43 G, consistent with our numerically-based estimates that 80% or
more of the vertical unsigned flux should be invisible to Stokes-V
observations at a resolution of 200 km owing to cancellation. Our
estimates significantly reduce the order-of-magnitude discrepancy
between Zeeman- and Hanle-based estimates.
---------------------------------------------------------
Title: Magnetic field intensification: comparison of 3D MHD
simulations with Hinode/SP results
Authors: Danilovic, S.; Schüssler, M.; Solanki, S. K.
2010A&A...509A..76D Altcode: 2009arXiv0910.1211D
Context. Recent spectro-polarimetric observations have provided detailed
measurements of magnetic field, velocity and intensity during events of
magnetic field intensification in the solar photosphere. <BR /> Aims:
By comparing with synthetic observations derived from MHD simulations,
we investigate the physical processes underlying the observations,
as well as verify the simulations and the interpretation of the
observations. <BR /> Methods: We consider the temporal evolution of
the relevant physical quantities for three cases of magnetic field
intensification in a numerical simulation. In order to compare with
observations, we calculate Stokes profiles and take into account the
spectral and spatial resolution of the spectropolarimeter (SP) on
board Hinode. We determine the evolution of the intensity, magnetic
flux density and zero-crossing velocity derived from the synthetic
Stokes parameters, using the same methods as applied to the Hinode/SP
observations to derive magnetic field and velocity information from
the spectro-polarimetric data. <BR /> Results: The three events
considered show a similar evolution: advection of magnetic flux to a
granular vertex, development of a strong downflow, evacuation of the
magnetic feature, increase of the field strength and the appearance
of the bright point. The magnetic features formed have diameters of
0.1-0.2´´. The downflow velocities reach maximum values of 5-10
km s<SUP>-1</SUP> at τ = 1. In the largest feature, the downflow
reaches supersonic speed in the lower photosphere. In the same case,
a supersonic upflow develops approximately 200 s after the formation of
the flux concentration. We find that synthetic and real observations
are qualitatively consistent and, for one of the cases considered,
also agree very well quantitatively. The effect of finite resolution
(spatial smearing) is most pronounced in the case of small features,
for which the synthetic Hinode/SP observations miss the bright point
formation and also the high-velocity downflows during the formation
of the smaller magnetic features. <BR /> Conclusions: The observed
events are consistent with the process of field intensification by
flux advection, radiative cooling, and evacuation by strong downflow
found in MHD simulations. The quantitative agreement of synthetic and
real observations indicates the validity of both the simulations and
the interpretations of the spectro-polarimetric observations.
---------------------------------------------------------
Title: Relation between the Sunrise photospheric magnetic field and
the Ca II H bright features
Authors: Jafarzadeh, Shahin; Hirzberger, J.; Feller, A.; Lagg, A.;
Solanki, S. K.; Pietarila, A.; Danilovic, S.; Riethmueller, T.;
Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knülker, M.; Martínez
Pillet, V.; Schmidt, W.; Schüssler, M.; Title, A.
2010cosp...38.2856J Altcode: 2010cosp.meet.2856J
Recent observations from the Sunrise balloon-borne solar telescope
have enabled us to reach an unprecedented high spatial resolution
on the solar surface with the near-ultraviolet photo-spheric and
chromospheric images as well as the magnetograms. We use these high
resolution observations to investigate the structure of the solar
upper photosphere and lower chromosphere as well as their temporal
evolutions. We study the relation between the inter-granular Ca II
397 nm bright structures in images obtained by the Sunrise Filter
Imager (SuFI) and their corresponding photospheric vector magnetic
field computed from the Imaging Magnetogram eXperiment (IMaX)
observations. The targets under study are in a quiet Sun region and
close to disc-centre.
---------------------------------------------------------
Title: The Small-Scale Solar Surface Dynamo (Keynote)
Authors: Pietarila Graham, J.; Danilovic, S.; Schüssler, M.
2009ASPC..415...43P Altcode:
The existence of a turbulent small-scale solar surface dynamo is likely,
considering existing numerical and laboratory experiments, as well as
comparisons of a small-scale dynamo in MURaM simulations with Hinode
observations. We find the observed peaked probability distribution
function (PDF) from Stokes-V magnetograms is consistent with a monotonic
PDF of the actual vertical field strength. The cancellation function
of the vertical flux density from a Hinode SP observation is found
to follow a self-similar power law over two decades in length scales
down to the ≈200 km resolution limit. This provides observational
evidence that the scales of magnetic structuring in the photosphere
extend at least down to 20 km. From the power law, we determine a
lower bound for the true quiet-Sun mean vertical unsigned flux density
of ≈43 G, consistent with our numerically-based estimates that 80%
or more of the vertical unsigned flux should be invisible to Stokes-V
observations at a resolution of 200 km owing to cancellation. Our
estimates significantly reduce the order-of-magnitude discrepancy
between Zeeman- and Hanle-based estimates.
---------------------------------------------------------
Title: Turbulent Magnetic Fields in the Quiet Sun: Implications of
Hinode Observations and Small-Scale Dynamo Simulations
Authors: Pietarila Graham, Jonathan; Danilovic, Sanja; Schüssler,
Manfred
2009ApJ...693.1728P Altcode: 2008arXiv0812.2125P
Using turbulent MHD simulations (magnetic Reynolds numbers up to
≈8000) and Hinode observations, we study effects of turbulence on
measuring the solar magnetic field outside active regions. First,
from synthetic Stokes V profiles for the Fe I lines at 6301 and
6302 Å, we show that a peaked probability distribution function
(PDF) for observationally derived field estimates is consistent
with a monotonic PDF for actual vertical field strengths. Hence, the
prevalence of weak fields is greater than would be naively inferred from
observations. Second, we employ the fractal self-similar geometry of the
turbulent solar magnetic field to derive two estimates (numerical and
observational) of the true mean vertical unsigned flux density. We also
find observational evidence that the scales of magnetic structuring in
the photosphere extend at least down to an order of magnitude smaller
than 200 km: the self-similar power-law scaling in the signed measure
from a Hinode magnetogram ranges (over two decades in length scales
and including the granulation scale) down to the ≈200 km resolution
limit. From the self-similar scaling, we determine a lower bound for
the true quiet-Sun mean vertical unsigned flux density of ~50 G. This
is consistent with our numerically based estimates that 80% or more of
the vertical unsigned flux should be invisible to Stokes V observations
at a resolution of 200 km owing to the cancellation of signal from
opposite magnetic polarities. Our estimates significantly reduce
the order-of-magnitude discrepancy between Zeeman- and Hanle-based
estimates.
---------------------------------------------------------
Title: Magnetic fine structure in the solar photosphere: observations
and MHD simulations
Authors: Danilović, Sanja
2009PhDT.......244D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: How Well Do Zeeman Measurements Reflect the Turbulent Solar
Magnetic Field?
Authors: Pietarila, J. Graham; Danilovic, S.; Schüssler, M.
2008ESPM...12.3.13P Altcode:
We employ the turbulent nature of the magnetic field in the solar
photosphere to constrain interpretations of Zeeman-polarimetry-based
observations. Using higher Reynolds number, more turbulent, simulations
of the solar convection zone than have previously been reported,
we compare the distribution and cancellation statistics of the
magnetic field itself with the statistics derived from simulated
Stokes profiles. A favorable comparison between the cancellation
statistics of the observables and the field allow for a prediction
at the approximated magnetic Reynolds number of the sun, ReM = 1e6,
and a comparison with Hinode observations. The difference between the
probability distribution functions (PDFs) from simulations, Zeeman-based
polarimetry, and interpretations of the Hanle effect are also examined
and a possible explanation is suggested.
---------------------------------------------------------
Title: The intensity contrast of solar granulation: comparing Hinode
SP results with MHD simulations
Authors: Danilovic, S.; Gandorfer, A.; Lagg, A.; Schüssler, M.;
Solanki, S. K.; Vögler, A.; Katsukawa, Y.; Tsuneta, S.
2008A&A...484L..17D Altcode: 2008arXiv0804.4230D
Context: The contrast of granulation is an important quantity
characterizing solar surface convection. <BR />Aims: We compare the
intensity contrast at 630 nm, observed using the Spectro-Polarimeter
(SP) aboard the Hinode satellite, with the 3D radiative MHD simulations
of Vögler & Schüssler (2007, A&A, 465, L43). <BR />Methods:
A synthetic image from the simulation is degraded using a theoretical
point-spread function of the optical system, and by considering other
important effects. <BR />Results: The telescope aperture and the
obscuration by the secondary mirror and its attachment spider, reduce
the simulated contrast from 14.4% to 8.5%. A slight effective defocus
of the instrument brings the simulated contrast down to 7.5%, close to
the observed value of 7.0%. <BR />Conclusions: A proper consideration
of the effects of the optical system and a slight defocus, lead to
sufficient degradation of the synthetic image from the MHD simulation,
such that the contrast reaches almost the observed value. The remaining
small discrepancy can be ascribed to straylight and slight imperfections
of the instrument, which are difficult to model. Hence, Hinode SP data
are consistent with a granulation contrast which is predicted by 3D
radiation MHD simulations.
---------------------------------------------------------
Title: Magnetic source of the solar cycle variation of the Mn I
539.4 nm line
Authors: Danilović, S.; Solanki, S. K.; Livingston, W.; Krivova,
N.; Vince, I.
2007msfa.conf..189D Altcode:
As a part of the long term program at KPNO, the Mn I 539.4 nm line has
been observed for nearly three solar cycles using the McMath telescope
and the 13.5 m spectrograph in double pass mode. These full-disk
spectrophotometric observations revealed an unusually large amplitude
change of its parameters over the solar cycle and its correlation with
Ca II K intensity. One of the proposed explanations for this phenomenon
is the optical pumping by the Mg II k line. With this work we would like
to show that this may not be the main mechanism behind the change. We
reconstructed the changes of the line parameters using a model that
takes into account only changes of the daily surface distributions of
magnetic field. This model has already been used to successfully model
total solar irradiance. We now apply it for modelling the Mn I line,
as well as its neighboring Fe I line using exactly the same value
of the free parameter as used for the reconstruction of total solar
irradiance. We reproduce well the Mn I and Fe I line changes over the
cycle purely with LTE modelling. This indicates that optical pumping
of the Mn I line by Mg II k is not the main cause of its solar cycle
change and sets an independent constraint on solar irradiance models.
---------------------------------------------------------
Title: Formation of Neutral Manganese Lines Potentially Suitable
for Plasma Diagnostics
Authors: Vitas, N.; Danilović, S.; Atanacković-Vukmanović, O.;
Vince, Ištvan
2005ESASP.600E..73V Altcode: 2005dysu.confE..73V; 2005ESPM...11...73V
No abstract at ADS
---------------------------------------------------------
Title: Time Series Analysis of Long Term Full Disk Observations Of
The Mn I 539.4 nm Solar Line
Authors: Danilovic, S.; Vince, I.; Vitas, N.; Jovanovic, P.
2005SerAJ.170...79D Altcode:
The equivalent width and central depth data of Mn 539.4 nm solar
spectral line, observed in the period from 1979 to 1992 at Kitt
Peak Observatory, was analyzed in pursuit for periodic changes. As
the observations are highly unevenly sampled, test if the periods
really exist in the observed data was needed. Two different methods
for spectral analysis were applied to synthesized data sampled in the
same way as observations. Comparation of these results with results
obtained from the observed data showed that the parameters display
at least three periodic changes with the periods of: 11-years,
quasi-biannual and 27-days.
---------------------------------------------------------
Title: Variability of the Mn I 539.4 nm solar spectral line parameters
with solar activity.
Authors: Danilovic, S.; Vince, I.
2005MmSAI..76..949D Altcode:
The time analysis of Mn I 539.4 nm parameters, observed in solar
flux spectrum, shows high degree of correlation with sunspot area
and Mg II c/w index time spectra. The more pronounced correlation
between equivalent width of this line with Mg II index implies that
bright magnetic elements are important source of the line parameters
variation. We synthesized line profiles, taking into account the
coverage of solar surface with different features, at various solar
activity levels and compared them with observations. The result shows
very good correspondence between modelled and observed change.
---------------------------------------------------------
Title: Sensitivity of the MnI 539.47 nm Spectral Line to Solar
Activity
Authors: Danilovic, S.; Vince, I.
2004SerAJ.169...47D Altcode: 2005SerAJ.169...47D
CCD observations of the MnI 539.47 nm spectral line in the solar flux
made at the Astronomical Observatory in Belgrade between 1994 and 2003
have been reduced and certain line parameters -the equivalent width,
full width at half maximum and central depth have been derived. The
variation of these parameters with the solar activity has been
evaluated. The relative variation of the equivalent width from minimum
to maximum of solar activity was found to be 1.4%, while the variation
of the central depth is 2.3%. The full width at half maximum remains
practically constant.
