explanation blue bibcodes open ADS page with paths to full text
Author name code: loukitcheva
ADS astronomy entries on 2022-09-14
author:"Loukitcheva, Maria A."
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Title: Hot Jets in the Solar Corona: Creating a Catalogue of Events
Based on Multi-Instrumental Observations
Authors: Kaltman, T. I.; Stupishin, A. G.; Anfinogentov, S. A.;
Nakariakov, V. M.; Loukitcheva, M. A.; Shendrik, A. V.
2021Ge&Ae..61.1083K Altcode:
We present a catalogue of solar coronal plasma jets with a temperature
above 0.5 MK, which includes primary information about the events,
parameters of the diagnosed jets, as well as related eruptive
phenomena. The catalogue (https://solar.sao.ru/coronal-jets-catalog/)
contains data obtained using the spaceborne EUV high-precision
telescope SDO/AIA and ground-based radio telescopes and spectrometers,
including RATAN-600, SRH and NoRH. For a number of events data on
the reconstructed magnetic field is also presented. The purpose
of the catalogue is to provide summary information on coronal jets
for further statistical analysis, determination of characteristic
parameters of jets, and for in-depth study of the individual events
by all interested researchers.
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Title: Features of Radio-Brightness Distribution over the Solar Disk
at Millimeter Waves: Models and Observations
Authors: Nagnibeda, V. G.; Topchilo, N. A.; Loukitcheva, M. A.;
Rakhimov, I. A.
2021Ge&Ae..61.1150N Altcode:
Millimeter emission of the quiet Sun is generated entirely in
the chromosphere and therefore can serve as a convenient tool
for chromospheric plasma diagnostics. This paper presents model
calculations of the radio-brightness distribution over the solar
disk to test two chosen versions of a modern, realistic, spatially
inhomogeneous, three-dimensional model of the chromosphere based on the
Bifrost code (Carlsson et al., 2016). Comparison of the calculated and
observed data demonstrates agreement: the disk brightness (on average,
without small-scale fluctuations reflecting the inhomogeneity of
the chromosphere) remains constant up to distances of around 0.95
of the solar radius from the disk center. The model brightness at
the limb does not exceed twice the brightness of the disk center,
with no significant brightening immediately behind the limb. At the
same time, the model values of the radio radius, which characterize
the height of the chromosphere, turn out to be much smaller than the
observed values available in the literature. This discrepancy (an
underestimated value of the radio radius) may be due the fact that a
number of physical processes are not taken into account in 3D models,
e.g., the LTE assumption (Martínez-Sykora et al., 2020). Conversely,
the observed values of the radio radius may be overestimated, as
evidenced by our recent eclipse measurements in 2020.
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Title: Measuring magnetic field with Atacama Large
Millimeter/Submillimeter Array
Authors: Loukitcheva, Maria
2020FrASS...7...45L Altcode:
This article reviews the use of magnetic bremsstrahlung at short
radio wavelengths to measure solar magnetic fields. The vertical
component of the chromospheric magnetic field can be deduced from the
observed polarization and brightness temperature spectrum at millimeter
wavelengths. State-of-the-art 3D radiative magnetohydrodynamic (R-MHD)
simulations of the quiet solar atmosphere were used to synthesize
observational deliverables at the wavelengths of the Atacama Large
Millimeter/Submillimeter Array (ALMA) and to test the applicability of
the method. The article provides selected observational examples of the
successful application of the method and presents an overview of the
recent developments and potential of the magnetic field measurements
with ALMA.
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Title: ALMA detection of dark chromospheric holes
Authors: White, Stephen M.; Loukitcheva, Maria A.; Solanki, Sami K.
2019AAS...23421704W Altcode:
Atacama Large Millimeter/submillimeter Array (ALMA) observations of a
quiet-Sun region at a wavelength of 3 mm are compared with available
chromospheric observations in the UV and visible as well as with
photospheric magnetograms. The ALMA images clearly reveal the presence
of distinctive cold areas in the millimeter maps having temperatures of
around 60% of the normal quiet Sun at 3 mm, which are not seen in the
other data. We speculate that ALMA is sensing cool chromospheric gas,
whose presence had earlier been inferred from infrared CO spectra.
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Title: ALMA Detection of Dark Chromospheric Holes in the Quiet Sun
Authors: Loukitcheva, Maria A.; White, Stephen M.; Solanki, Sami K.
2019ApJ...877L..26L Altcode: 2019arXiv190506763L
We present Atacama Large Millimeter/submillimeter Array (ALMA)
observations of a quiet-Sun region at a wavelength of 3 mm, obtained
during the first solar ALMA cycle on 2017 April 27, and compare them
with available chromospheric observations in the UV and visible as
well as with photospheric magnetograms. ALMA images clearly reveal the
presence of distinct particularly dark/cool areas in the millimeter
maps with temperatures as low as 60% of the normal quiet Sun at 3 mm,
which are not seen in the other data. We speculate that ALMA is sensing
cool chromospheric gas, whose presence had earlier been inferred from
infrared CO spectra.
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Title: First solar observations with ALMA
Authors: Loukitcheva, Maria
2019AdSpR..63.1396L Altcode: 2018arXiv180900430L
The Atacama Large Millimeter-Submillimeter Array (ALMA) has opened a
new window for studying the Sun via high-resolution high-sensitivity
imaging at millimeter wavelengths. In this contribution I review the
capabilities of the instrument for solar observing and describe the
extensive effort taken to bring the possibility of solar observing with
ALMA to the scientific community. The first solar ALMA observations were
carried out during 2014 and 2015 in two ALMA bands, Band 3 (λ = 3 mm)
and Band 6 (λ = 1.3 mm), in single-dish and interferometric modes,
using single pointing and mosaicing observing techniques, with spatial
resolution up to ∼ 2″ and ∼ 1″ in the two bands, respectively. I
overview several recently published studies which made use of the first
solar ALMA observations, describe current status of solar observing
with ALMA and briefly discuss the future capabilities of the instrument.
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Title: Force-free Field Reconstructions Enhanced by Chromospheric
Magnetic Field Data
Authors: Fleishman, Gregory; Mysh'yakov, Ivan; Stupishin, Alexey;
Loukitcheva, Maria; Anfinogentov, Sergey
2019ApJ...870..101F Altcode: 2018arXiv181102093F
A 3D picture of the coronal magnetic field remains an outstanding
problem in solar physics, particularly in active regions. Nonlinear
force-free field reconstructions that employ routinely available
full-disk photospheric vector magnetograms represent state-of-the-art
coronal magnetic field modeling. Such reconstructions, however,
suffer from an inconsistency between a force-free coronal magnetic
field and a non-force-free photospheric boundary condition, from
which the coronal reconstruction is performed. In this study we focus
on integrating the additional chromospheric and/or coronal magnetic
field data with the vector photospheric magnetograms with the goal of
improving the reliability of the magnetic field reconstructions. We
develop a corresponding modification of the available optimization
codes described in Fleishman et al. and test their performance
using a full-fledged magnetohydrodynamics model obtained from the
Bifrost code by performing a “voxel-by-voxel” comparison between
the reconstructed and the model magnetic fields. We demonstrate that
adding even an incomplete set of chromospheric magnetic field data can
measurably improve the reconstruction of the coronal magnetic field
and greatly improve reconstructions of the magnetic connectivity and
of the coronal electric current.
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Title: The Coronal Volume of Energetic Particles in Solar Flares as
Revealed by Microwave Imaging
Authors: Fleishman, Gregory D.; Loukitcheva, Maria A.; Kopnina,
Varvara Yu.; Nita, Gelu M.; Gary, Dale E.
2018ApJ...867...81F Altcode: 2018arXiv180904753F
The spectrum of gyrosynchrotron emission from solar flares generally
peaks in the microwave range. Its optically thin, high-frequency
component, above the spectral peak, is often used for diagnostics
of the nonthermal electrons and the magnetic field in the radio
source. Under favorable conditions, its low-frequency counterpart
brings additional, complementary information about these parameters
as well as thermal plasma diagnostics, either through gyrosynchrotron
self-absorption, free-free absorption by the thermal plasma, or the
suppression of emission through the so-called Razin effect. However,
their effect on the low-frequency spectrum is often masked by spatial
nonuniformity. To disentangle the various contributions to low-frequency
gyrosynchrotron emission, a combination of spectral and imaging data
is needed. To this end, we have investigated Owens Valley Solar Array
(OVSA) multi-frequency images for 26 solar bursts observed jointly with
RHESSI during the first half of 2002. For each, we examined dynamic
spectra, time- and frequency-synthesis maps, RHESSI images with overlaid
OVSA contours, and a few representative single-frequency snapshot OVSA
images. We focus on the frequency dependence of microwave source sizes
derived from the OVSA images and their effect on the low-frequency
microwave spectral slope. We succeed in categorizing 18 analyzed
events into several groups. Four events demonstrate clear evidence of
being dominated by gyrosynchrotron self-absorption, with an inferred
brightness temperature of ≥10<SUP>8</SUP> K. The low-frequency
spectra in the remaining events are affected to varying degrees by
Razin suppression. We find that many radio sources are rather large
at low frequencies, which can have important implications for solar
energetic particle production and escape.
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Title: Erratum: “A First Comparison of Millimeter Continuum and
Mg II Ultraviolet Line Emission from the Solar Chromosphere”
(<A href="http://doi.org/10.3847/2041-8213/aa844c">2017, ApJL,
845, L19</A>)
Authors: Bastian, T. S.; Chintzoglou, G.; De Pontieu, B.; Shimojo,
M.; Schmit, D.; Leenaarts, J.; Loukitcheva, M.
2018ApJ...860L..16B Altcode:
No abstract at ADS
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Title: Exploring the Sun with ALMA
Authors: Bastian, T. S.; Bárta, M.; Brajša, R.; Chen, B.; Pontieu,
B. D.; Gary, D. E.; Fleishman, G. D.; Hales, A. S.; Iwai, K.; Hudson,
H.; Kim, S.; Kobelski, A.; Loukitcheva, M.; Shimojo, M.; Skokić,
I.; Wedemeyer, S.; White, S. M.; Yan, Y.
2018Msngr.171...25B Altcode:
The Atacama Large Millimeter/submillimeter Array (ALMA) Observatory
opens a new window onto the Universe. The ability to perform continuum
imaging and spectroscopy of astrophysical phenomena at millimetre and
submillimetre wavelengths with unprecedented sensitivity opens up new
avenues for the study of cosmology and the evolution of galaxies, the
formation of stars and planets, and astrochemistry. ALMA also allows
fundamentally new observations to be made of objects much closer
to home, including the Sun. The Sun has long served as a touchstone
for our understanding of astrophysical processes, from the nature of
stellar interiors, to magnetic dynamos, non-radiative heating, stellar
mass loss, and energetic phenomena such as solar flares. ALMA offers
new insights into all of these processes.
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Title: Dressing the Coronal Magnetic Extrapolations of Active Regions
with a Parameterized Thermal Structure
Authors: Nita, Gelu M.; Viall, Nicholeen M.; Klimchuk, James A.;
Loukitcheva, Maria A.; Gary, Dale E.; Kuznetsov, Alexey A.; Fleishman,
Gregory D.
2018ApJ...853...66N Altcode:
The study of time-dependent solar active region (AR) morphology and
its relation to eruptive events requires analysis of imaging data
obtained in multiple wavelength domains with differing spatial and
time resolution, ideally in combination with 3D physical models. To
facilitate this goal, we have undertaken a major enhancement of our
IDL-based simulation tool, GX_Simulator, previously developed for
modeling microwave and X-ray emission from flaring loops, to allow it
to simulate quiescent emission from solar ARs. The framework includes
new tools for building the atmospheric model and enhanced routines
for calculating emission that include new wavelengths. In this paper,
we use our upgraded tool to model and analyze an AR and compare the
synthetic emission maps with observations. We conclude that the modeled
magneto-thermal structure is a reasonably good approximation of the
real one.
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Title: ALMA Discovery of Solar Umbral Brightness Enhancement at λ =
3 mm
Authors: Iwai, K.; Loukitcheva, M.; Shimojo, M.; Solanki, S. K.;
White, S. M.
2017AGUFMSH43A2806I Altcode:
We report the discovery of a brightness enhancement in the center of
a large sunspot umbra at a wavelength of 3 mm using the Atacama Large
Millimeter/sub-millimeter Array (ALMA). Sunspots are among the most
prominent features on the solar surface, but many of their aspects are
surprisingly poorly understood. We analyzed a λ = 3 mm (100 GHz) mosaic
image obtained by ALMA that includes a large sunspot within the active
region AR12470, on 2015 December 16. The 3 mm map has a 300”×300”
field of view and 4.9”×2.2” spatial resolution, which is the highest
spatial resolution map of an entire sunspot in this frequency range. We
find a gradient of 3 mm brightness from a high value in the outer
penumbra to a low value in the inner penumbra/outer umbra. Within the
inner umbra, there is a marked increase in 3 mm brightness temperature,
which we call an umbral brightness enhancement. This enhanced emission
corresponds to a temperature excess of 800 K relative to the surrounding
inner penumbral region and coincides with excess brightness in the
1330 and 1400 Å slit-jaw images of the Interface Region Imaging
Spectrograph (IRIS), adjacent to a partial lightbridge. This λ = 3 mm
brightness enhancement may be an intrinsic feature of the sunspot umbra
at chromospheric heights, such as a manifestation of umbral flashes, or
it could be related to a coronal plume, since the brightness enhancement
was coincident with the footpoint of a coronal loop observed at 171 Å.
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Title: Casting the Coronal Magnetic Field Reconstructions with
Magnetic Field Constraints above the Photosphere in 3D Using MHD
Bifrost Model
Authors: Fleishman, G. D.; Anfinogentov, S.; Loukitcheva, M.;
Mysh'yakov, I.; Stupishin, A.
2017AGUFMSH13A2461F Altcode:
Measuring and modeling coronal magnetic field, especially above active
regions (ARs), remains one of the central problems of solar physics
given that the solar coronal magnetism is the key driver of all solar
activity. Nowadays the coronal magnetic field is often modelled using
methods of nonlinear force-free field reconstruction, whose accuracy
has not yet been comprehensively assessed. Given that the coronal
magnetic probing is routinely unavailable, only morphological tests
have been applied to evaluate performance of the reconstruction methods
and a few direct tests using available semi-analytical force-free
field solution. Here we report a detailed casting of various tools
used for the nonlinear force-free field reconstruction, such as
disambiguation methods, photospheric field preprocessing methods,
and volume reconstruction methods in a 3D domain using a 3D snapshot
of the publicly available full-fledged radiative MHD model. We take
advantage of the fact that from the realistic MHD model we know the
magnetic field vector distribution in the entire 3D domain, which
enables us to perform "voxel-by-voxel" comparison of the restored
magnetic field and the true magnetic field in the 3D model volume. Our
tests show that the available disambiguation methods often fail at
the quiet sun areas, where the magnetic structure is dominated by
small-scale magnetic elements, while they work really well at the AR
photosphere and (even better) chromosphere. The preprocessing of the
photospheric magnetic field, although does produce a more force-free
boundary condition, also results in some effective `elevation' of
the magnetic field components. The effective `elevation' height turns
out to be different for the longitudinal and transverse components of
the magnetic field, which results in a systematic error in absolute
heights in the reconstructed magnetic data cube. The extrapolation
performed starting from actual AR photospheric magnetogram (i.e.,
without preprocessing) are free from this systematic error, while have
other metrics either comparable or only marginally worse than those
estimated for extrapolations from the preprocessed magnetograms. This
finding favors the use of extrapolations from the original photospheric
magnetogram without preprocessing.
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Title: Solar ALMA Observations: Constraining the Chromosphere above
Sunspots
Authors: Loukitcheva, Maria A.; Iwai, Kazumasa; Solanki, Sami K.;
White, Stephen M.; Shimojo, Masumi
2017ApJ...850...35L Altcode: 2017arXiv171003812L
We present the first high-resolution Atacama Large
Millimeter/Submillimeter Array (ALMA) observations of a sunspot at
wavelengths of 1.3 and 3 mm, obtained during the solar ALMA Science
Verification campaign in 2015, and compare them with the predictions
of semi-empirical sunspot umbral/penumbral atmosphere models. For
the first time, millimeter observations of sunspots have resolved
umbral/penumbral brightness structure at the chromospheric heights,
where the emission at these wavelengths is formed. We find that the
sunspot umbra exhibits a radically different appearance at 1.3 and 3
mm, whereas the penumbral brightness structure is similar at the two
wavelengths. The inner part of the umbra is ∼600 K brighter than the
surrounding quiet Sun (QS) at 3 mm and is ∼700 K cooler than the QS
at 1.3 mm, being the coolest part of sunspot at this wavelength. On
average, the brightness of the penumbra at 3 mm is comparable to
the QS brightness, while at 1.3 mm it is ∼1000 K brighter than
the QS. Penumbral brightness increases toward the outer boundary in
both ALMA bands. Among the tested umbral models, that of Severino et
al. provides the best fit to the observational data, including both
the ALMA data analyzed in this study and data from earlier works. No
penumbral model among those considered here gives a satisfactory fit
to the currently available measurements. ALMA observations at multiple
millimeter wavelengths can be used for testing existing sunspot models,
and serve as an important input to constrain new empirical models.
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Title: A First Comparison of Millimeter Continuum and Mg II
Ultraviolet Line Emission from the Solar Chromosphere
Authors: Bastian, T. S.; Chintzoglou, G.; De Pontieu, B.; Shimojo,
M.; Schmit, D.; Leenaarts, J.; Loukitcheva, M.
2017ApJ...845L..19B Altcode: 2017arXiv170604532B
We present joint observations of the Sun by the Atacama Large
Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging
Spectrograph (IRIS). Both millimeter/submillimeter-λ continuum emission
and ultraviolet (UV) line emission originate from the solar chromosphere
and both have the potential to serve as powerful and complementary
diagnostics of physical conditions in this enigmatic region of the solar
atmosphere. The observations were made of a solar active region on 2015
December 18 as part of the ALMA science verification effort. A map of
the Sun’s continuum emission was obtained by ALMA at a wavelength of
1.25 mm (239 GHz). A contemporaneous map was obtained by IRIS in the
Mg II h doublet line at 2803.5 Å. While a clear correlation between
the 1.25 mm brightness temperature T<SUB>B</SUB> and the Mg II h
line radiation temperature T<SUB>rad</SUB> is observed, the slope
is <1, perhaps as a result of the fact that these diagnostics
are sensitive to different parts of the chromosphere and that the
Mg II h line source function includes a scattering component. There
is a significant difference (35%) between the mean T<SUB>B</SUB>
(1.25 mm) and mean T<SUB>rad</SUB> (Mg II). Partitioning the maps
into “sunspot,” “quiet areas,” and “plage regions” we
find the relation between the IRIS Mg II h line T<SUB>rad</SUB> and
the ALMA T<SUB>B</SUB> region-dependent. We suggest this may be the
result of regional dependences of the formation heights of the IRIS
and ALMA diagnostics and/or the increased degree of coupling between
the UV source function and the local gas temperature in the hotter,
denser gas in plage regions.
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Title: Solar Commissioning Observations of the Sun with ALMA
Authors: White, Stephen M.; Shimojo, Masumi; Bastian, Timothy S.;
Iwai, Kazumasa; Hales, Antonio; Brajsa, Roman; Skokic, Ivica; Kim,
Sujin; Hudson, Hugh S.; Loukitcheva, Maria; Wedemeyer, Sven
2017SPD....4820402W Altcode:
PI-led science observations have commenced with the Atacama
Large Millimeter-submillimeter Array (ALMA) following an extensive
commissioning effort. This talk will summarize that effort and discuss
some of the scientific results derived from the commissioning data. As
the solar cycle declines, ALMA observations will mainly address
chromospheric science topics. Examples of data obtained during
commissioning, both from the interferometer and from single-dish
observations, will be presented. The temperatures of the layers that
ALMA is most sensitive to have been determined for the two frequency
bands currently used for solar observations. Curious behavior in a
sunspot umbra and an observations of a small chromospheric ejection
will be discussed.
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Title: Observing the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA): High-Resolution Interferometric
Imaging
Authors: Shimojo, M.; Bastian, T. S.; Hales, A. S.; White, S. M.;
Iwai, K.; Hills, R. E.; Hirota, A.; Phillips, N. M.; Sawada, T.;
Yagoubov, P.; Siringo, G.; Asayama, S.; Sugimoto, M.; Brajša, R.;
Skokić, I.; Bárta, M.; Kim, S.; de Gregorio-Monsalvo, I.; Corder,
S. A.; Hudson, H. S.; Wedemeyer, S.; Gary, D. E.; De Pontieu, B.;
Loukitcheva, M.; Fleishman, G. D.; Chen, B.; Kobelski, A.; Yan, Y.
2017SoPh..292...87S Altcode: 2017arXiv170403236S
Observations of the Sun at millimeter and submillimeter wavelengths
offer a unique probe into the structure, dynamics, and heating of the
chromosphere; the structure of sunspots; the formation and eruption
of prominences and filaments; and energetic phenomena such as jets
and flares. High-resolution observations of the Sun at millimeter and
submillimeter wavelengths are challenging due to the intense, extended,
low-contrast, and dynamic nature of emission from the quiet Sun,
and the extremely intense and variable nature of emissions associated
with energetic phenomena. The Atacama Large Millimeter/submillimeter
Array (ALMA) was designed with solar observations in mind. The
requirements for solar observations are significantly different from
observations of sidereal sources and special measures are necessary
to successfully carry out this type of observations. We describe the
commissioning efforts that enable the use of two frequency bands,
the 3-mm band (Band 3) and the 1.25-mm band (Band 6), for continuum
interferometric-imaging observations of the Sun with ALMA. Examples of
high-resolution synthesized images obtained using the newly commissioned
modes during the solar-commissioning campaign held in December 2015
are presented. Although only 30 of the eventual 66 ALMA antennas
were used for the campaign, the solar images synthesized from the
ALMA commissioning data reveal new features of the solar atmosphere
that demonstrate the potential power of ALMA solar observations. The
ongoing expansion of ALMA and solar-commissioning efforts will continue
to enable new and unique solar observing capabilities.
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Title: Observing the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
Authors: White, S. M.; Iwai, K.; Phillips, N. M.; Hills, R. E.; Hirota,
A.; Yagoubov, P.; Siringo, G.; Shimojo, M.; Bastian, T. S.; Hales,
A. S.; Sawada, T.; Asayama, S.; Sugimoto, M.; Marson, R. G.; Kawasaki,
W.; Muller, E.; Nakazato, T.; Sugimoto, K.; Brajša, R.; Skokić, I.;
Bárta, M.; Kim, S.; Remijan, A. J.; de Gregorio, I.; Corder, S. A.;
Hudson, H. S.; Loukitcheva, M.; Chen, B.; De Pontieu, B.; Fleishmann,
G. D.; Gary, D. E.; Kobelski, A.; Wedemeyer, S.; Yan, Y.
2017SoPh..292...88W Altcode: 2017arXiv170504766W
The Atacama Large Millimeter/submillimeter Array (ALMA) radio
telescope has commenced science observations of the Sun starting
in late 2016. Since the Sun is much larger than the field of view
of individual ALMA dishes, the ALMA interferometer is unable to
measure the background level of solar emission when observing the
solar disk. The absolute temperature scale is a critical measurement
for much of ALMA solar science, including the understanding of energy
transfer through the solar atmosphere, the properties of prominences,
and the study of shock heating in the chromosphere. In order to provide
an absolute temperature scale, ALMA solar observing will take advantage
of the remarkable fast-scanning capabilities of the ALMA 12 m dishes
to make single-dish maps of the full Sun. This article reports on the
results of an extensive commissioning effort to optimize the mapping
procedure, and it describes the nature of the resulting data. Amplitude
calibration is discussed in detail: a path that uses the two loads in
the ALMA calibration system as well as sky measurements is described
and applied to commissioning data. Inspection of a large number of
single-dish datasets shows significant variation in the resulting
temperatures, and based on the temperature distributions, we derive
quiet-Sun values at disk center of 7300 K at λ =3 mm and 5900 K at
λ =1.3 mm. These values have statistical uncertainties of about 100
K, but systematic uncertainties in the temperature scale that may be
significantly larger. Example images are presented from two periods
with very different levels of solar activity. At a resolution of about
25<SUP>″</SUP>, the 1.3 mm wavelength images show temperatures on
the disk that vary over about a 2000 K range. Active regions and plages
are among the hotter features, while a large sunspot umbra shows up as
a depression, and filament channels are relatively cool. Prominences
above the solar limb are a common feature of the single-dish images.
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Title: ALMA Discovery of Solar Umbral Brightness Enhancement at λ =
3 mm
Authors: Iwai, Kazumasa; Loukitcheva, Maria; Shimojo, Masumi; Solanki,
Sami K.; White, Stephen M.
2017ApJ...841L..20I Altcode: 2017arXiv170503147I
We report the discovery of a brightness enhancement in the center
of a large sunspot umbra at a wavelength of 3 mm using the Atacama
Large Millimeter/sub-millimeter Array (ALMA). Sunspots are among
the most prominent features on the solar surface, but many of their
aspects are surprisingly poorly understood. We analyzed a λ = 3 mm
(100 GHz) mosaic image obtained by ALMA that includes a large sunspot
within the active region AR12470, on 2015 December 16. The 3 mm map
has a 300″ × 300″ field of view and 4.″9 × 2.″2 spatial
resolution, which is the highest spatial resolution map of an entire
sunspot in this frequency range. We find a gradient of 3 mm brightness
from a high value in the outer penumbra to a low value in the inner
penumbra/outer umbra. Within the inner umbra, there is a marked increase
in 3 mm brightness temperature, which we call an umbral brightness
enhancement. This enhanced emission corresponds to a temperature
excess of 800 K relative to the surrounding inner penumbral region
and coincides with excess brightness in the 1330 and 1400 Å slit-jaw
images of the Interface Region Imaging Spectrograph (IRIS), adjacent
to a partial lightbridge. This λ = 3 mm brightness enhancement may be
an intrinsic feature of the sunspot umbra at chromospheric heights,
such as a manifestation of umbral flashes, or it could be related to
a coronal plume, since the brightness enhancement was coincident with
the footpoint of a coronal loop observed at 171 Å.
---------------------------------------------------------
Title: Millimeter radiation from a 3D model of the solar
atmosphere. II. Chromospheric magnetic field
Authors: Loukitcheva, M.; White, S. M.; Solanki, S. K.; Fleishman,
G. D.; Carlsson, M.
2017A&A...601A..43L Altcode: 2017arXiv170206018L
<BR /> Aims: We use state-of-the-art, three-dimensional non-local
thermodynamic equilibrium (non-LTE) radiative magnetohydrodynamic
simulations of the quiet solar atmosphere to carry out detailed tests
of chromospheric magnetic field diagnostics from free-free radiation at
millimeter and submillimeter wavelengths (mm/submm). <BR /> Methods:
The vertical component of the magnetic field was deduced from the
mm/submm brightness spectra and the degree of circular polarization
synthesized at millimeter frequencies. We used the frequency bands
observed by the Atacama Large Millimeter/Submillimeter Array (ALMA)
as a convenient reference. The magnetic field maps obtained describe
the longitudinal magnetic field at the effective formation heights of
the relevant wavelengths in the solar chromosphere. <BR /> Results:
The comparison of the deduced and model chromospheric magnetic fields
at the spatial resolution of both the model and current observations
demonstrates a good correlation, but has a tendency to underestimate
the model field. The systematic discrepancy of about 10% is probably
due to averaging of the restored field over the heights contributing
to the radiation, weighted by the strength of the contribution. On
the whole, the method of probing the longitudinal component of the
magnetic field with free-free emission at mm/submm wavelengths
is found to be applicable to measurements of the weak quiet-Sun
magnetic fields. However, successful exploitation of this technique
requires very accurate measurements of the polarization properties
(primary beam and receiver polarization response) of the antennas,
which will be the principal factor that determines the level to which
chromospheric magnetic fields can be measured. <BR /> Conclusions:
Consequently, high-resolution and high-precision observations of
circularly polarized radiation at millimeter wavelengths can be a
powerful tool for producing chromospheric longitudinal magnetograms.
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Title: Casting the Coronal Magnetic Field Reconstruction Tools in
3D Using the MHD Bifrost Model
Authors: Fleishman, Gregory D.; Anfinogentov, Sergey; Loukitcheva,
Maria; Mysh'yakov, Ivan; Stupishin, Alexey
2017ApJ...839...30F Altcode: 2017arXiv170306360F
Quantifying the coronal magnetic field remains a central problem
in solar physics. Nowadays, the coronal magnetic field is often
modeled using nonlinear force-free field (NLFFF) reconstructions,
whose accuracy has not yet been comprehensively assessed. Here we
perform a detailed casting of the NLFFF reconstruction tools, such
as π-disambiguation, photospheric field preprocessing, and volume
reconstruction methods, using a 3D snapshot of the publicly available
full-fledged radiative MHD model. Specifically, from the MHD model, we
know the magnetic field vector in the entire 3D domain, which enables
us to perform a “voxel-by-voxel” comparison of the restored and the
true magnetic fields in the 3D model volume. Our tests show that the
available π-disambiguation methods often fail in the quiet-Sun areas
dominated by small-scale magnetic elements, while they work well in
the active region (AR) photosphere and (even better) chromosphere. The
preprocessing of the photospheric magnetic field, although it does
produce a more force-free boundary condition, also results in some
effective “elevation” of the magnetic field components. This
“elevation” height is different for the longitudinal and transverse
components, which results in a systematic error in absolute heights
in the reconstructed magnetic data cube. The extrapolations performed
starting from the actual AR photospheric magnetogram are free from this
systematic error, while other metrics are comparable with those for
extrapolations from the preprocessed magnetograms. This finding favors
the use of extrapolations from the original photospheric magnetogram
without preprocessing. Our tests further suggest that extrapolations
from a force-free chromospheric boundary produce measurably better
results than those from a photospheric boundary.
---------------------------------------------------------
Title: Solar Observations with the Atacama Large
Millimeter/submillimeter Array (ALMA)
Authors: Kobelski, A.; Bastian, T. S.; Bárta, M.; Brajša, R.; Chen,
B.; De Pontieu, B.; Fleishman, G.; Gary, D.; Hales, A.; Hills, R.;
Hudson, H.; Hurford, G.; Loukitcheva, M.; Iwai, K.; Krucker, S.;
Shimojo, M.; Skokić, I.; Wedemeyer, S.; White, S.; Yan, Y.; ALMA
Solar Development Team
2016ASPC..504..327K Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a
joint North American, European, and East Asian project that opens
the mm-sub mm wavelength part of the electromagnetic spectrum for
general astrophysical exploration, providing high-resolution imaging
in frequency bands currently ranging from 84 GHz to 950 GHz (300
microns to 3 mm). It is located in the Atacama desert in northern
Chile at an elevation of 5000 m. Despite being a general purpose
instrument, provisions have been made to enable solar observations
with ALMA. Radiation emitted at ALMA wavelengths originates mostly
from the chromosphere, which plays an important role in the transport
of matter and energy, and the in heating the outer layers of the solar
atmosphere. Despite decades of research, the solar chromosphere remains
a significant challenge: both to observe, owing to the complicated
formation mechanisms of currently available diagnostics; and to
understand, as a result of the complex nature of the structure and
dynamics of the chromosphere. ALMA has the potential to change the
scene substantially as it serves as a nearly linear thermometer at
high spatial and temporal resolution, enabling us to study the complex
interaction of magnetic fields and shock waves and yet-to-be-discovered
dynamical processes. Moreover, ALMA will play an important role in
the study of energetic emissions associated with solar flares at
sub-THz frequencies.
---------------------------------------------------------
Title: Solar Science with the Atacama Large Millimeter/Submillimeter
Array—A New View of Our Sun
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.;
Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu,
B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin,
P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz,
A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.;
Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary,
D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van
der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.;
Selhorst, C. L.; Barta, M.
2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new
powerful tool for observing the Sun at high spatial, temporal, and
spectral resolution. These capabilities can address a broad range
of fundamental scientific questions in solar physics. The radiation
observed by ALMA originates mostly from the chromosphere—a complex
and dynamic region between the photosphere and corona, which plays a
crucial role in the transport of energy and matter and, ultimately,
the heating of the outer layers of the solar atmosphere. Based on
first solar test observations, strategies for regular solar campaigns
are currently being developed. State-of-the-art numerical simulations
of the solar atmosphere and modeling of instrumental effects can help
constrain and optimize future observing modes for ALMA. Here we present
a short technical description of ALMA and an overview of past efforts
and future possibilities for solar observations at submillimeter and
millimeter wavelengths. In addition, selected numerical simulations
and observations at other wavelengths demonstrate ALMA's scientific
potential for studying the Sun for a large range of science cases.
---------------------------------------------------------
Title: ALMA Observations of the Sun in Cycle 4 and Beyond
Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.;
Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.;
Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson,
M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.;
Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov,
V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões,
P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig,
A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
2016arXiv160100587W Altcode:
This document was created by the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) in preparation of
the first regular observations of the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA), which are anticipated to start
in ALMA Cycle 4 in October 2016. The science cases presented here
demonstrate that a large number of scientifically highly interesting
observations could be made already with the still limited solar
observing modes foreseen for Cycle 4 and that ALMA has the potential
to make important contributions to answering long-standing scientific
questions in solar physics. With the proposal deadline for ALMA Cycle
4 in April 2016 and the Commissioning and Science Verification campaign
in December 2015 in sight, several of the SSALMON Expert Teams composed
strategic documents in which they outlined potential solar observations
that could be feasible given the anticipated technical capabilities
in Cycle 4. These documents have been combined and supplemented
with an analysis, resulting in recommendations for solar observing
with ALMA in Cycle 4. In addition, the detailed science cases also
demonstrate the scientific priorities of the solar physics community
and which capabilities are wanted for the next observing cycles. The
work on this White Paper effort was coordinated in close cooperation
with the two international solar ALMA development studies led by
T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be
further updated until the beginning of Cycle 4 in October 2016. In
particular, we plan to adjust the technical capabilities of the solar
observing modes once finally decided and to further demonstrate the
feasibility and scientific potential of the included science cases by
means of numerical simulations of the solar atmosphere and corresponding
simulated ALMA observations.
---------------------------------------------------------
Title: SSALMON - The Solar Simulations for the Atacama Large
Millimeter Observatory Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson,
H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu,
B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.;
Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.;
Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier,
A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M.
2015AdSpR..56.2679W Altcode: 2015arXiv150205601W
The Solar Simulations for the Atacama Large Millimeter Observatory
Network (SSALMON) was initiated in 2014 in connection with two ALMA
development studies. The Atacama Large Millimeter/submillimeter Array
(ALMA) is a powerful new tool, which can also observe the Sun at
high spatial, temporal, and spectral resolution. The international
SSALMONetwork aims at co-ordinating the further development of solar
observing modes for ALMA and at promoting scientific opportunities
for solar physics with particular focus on numerical simulations,
which can provide important constraints for the observing modes and
can aid the interpretation of future observations. The radiation
detected by ALMA originates mostly in the solar chromosphere - a
complex and dynamic layer between the photosphere and corona, which
plays an important role in the transport of energy and matter and the
heating of the outer layers of the solar atmosphere. Potential targets
include active regions, prominences, quiet Sun regions, flares. Here,
we give a brief overview over the network and potential science cases
for future solar observations with ALMA.
---------------------------------------------------------
Title: Probing the Sun with ALMA: Observations and Simulations
Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M.; Carlsson, M.
2015ASPC..499..349L Altcode: 2015arXiv150805686L
ALMA will open a new chapter in the study of the Sun by providing a leap
in spatial resolution and sensitivity compared to currently available
mm wavelength observations. In preparation of ALMA, we have carried out
a large number of observational tests and state-of-the-art radiation
MHD simulations. Here we review the best available observations of
the Sun at millimeter wavelengths.Using state of the art radiation
MHD simulations of the solar atmosphere we demonstrate the huge
potential of ALMA observations for uncovering the nature of the solar
chromosphere. We show that ALMA will not only provide a reliable probe
of the thermal structure and dynamics of the chromosphere, it will also
open up a powerful new diagnostic of magnetic field at chromospheric
heights, a fundamentally important, but so far poorly known parameter.
---------------------------------------------------------
Title: Solar ALMA: Observation-Based Simulations of the mm and sub-mm
Emissions from Active Regions
Authors: Fleishman, G.; Loukitcheva, M.; Nita, G.
2015ASPC..499..351F Altcode: 2015arXiv150608395F
We developed an efficient algorithm integrated in our 3D modeling
tool, GX Simulator (Nita et al. 2015), allowing quick computation of
the synthetic intensity and polarization maps of solar active regions
(AR) in the ALMA spectral range.
---------------------------------------------------------
Title: Solar ALMA observations - A revolutionizing new view at our
host star
Authors: Wedemeyer, Sven; Brajsa, Roman; Bastian, Timothy S.; Barta,
Miroslav; Hales, Antonio; Yagoubov, Pavel; Hudson, Hugh; Loukitcheva,
Maria; Fleishman, Gregory
2015IAUGA..2256732W Altcode:
Observations of the Sun with the Atacama Large Millimeter/submillimeter
Array (ALMA) have a large potential for revolutionizing our
understanding of our host star with far reaching implications
for stars in general. The radiation emitted at ALMA wavelengths
originates mostly from the chromosphere - a complex and dynamic layer
between the photosphere and the corona, which plays an important
role in the transport of energy and matter and the heating of the
outer layers of the solar atmosphere.Despite decades of intensive
research, the chromosphere is still elusive and challenging to
observe owing to the complicated formation mechanisms of currently
available diagnostics. ALMA will change the scene substantially as
it serves as a nearly linear thermometer at high spatial, temporal,
and spectral resolution, enabling us to study the complex interaction
of magnetic fields and shock waves and yet-to-be-discovered dynamical
processes. Furthermore, radio recombination and molecular lines
may have great diagnostic potential but need to be investigated
first. These unprecedented capabilities promise important new findings
for a large range of topics in solar physics including the structure,
dynamics and energy balance of quiet Sun regions, active regions and
sunspots, flares and prominences. As a part of ongoing development
studies, an international network has been initiated, which aims at
defining and preparing key solar science with ALMA through simulation
studies: SSALMON -- Solar Simulations for the Atacama Large Millimeter
Observatory Network (http://ssalmon.uio.no). Here, we give an overview
of potential science cases.
---------------------------------------------------------
Title: Synthetic 3D modeling of active regions and simulation of
their multi-wavelength emission
Authors: Nita, Gelu M.; Fleishman, Gregory; Kuznetsov, Alexey A.;
Loukitcheva, Maria A.; Viall, Nicholeen M.; Klimchuk, James A.; Gary,
Dale E.
2015TESS....131204N Altcode:
To facilitate the study of solar active regions, we have created a
synthetic modeling framework that combines 3D magnetic structures
obtained from magnetic extrapolations with simplified 1D thermal
models of the chromosphere, transition region, and corona. To handle,
visualize, and use such synthetic data cubes to compute multi-wavelength
emission maps and compare them with observations, we have
undertaken a major enhancement of our simulation tools, GX_Simulator
(ftp://sohoftp.nascom.nasa.gov/solarsoft/packages/gx_simulator/),
developed earlier for modeling emission from flaring loops. The greatly
enhanced, object-based architecture, which now runs on Windows, Mac,
and UNIX platform, offers important new capabilities that include the
ability to either import 3D density and temperature distribution models,
or to assign to each individual voxel numerically defined coronal
or chromospheric temperature and densities, or coronal Differential
Emission Measure distributions. Due to these new capabilities, the
GX_Simulator can now apply parametric heating models involving average
properties of the magnetic field lines crossing a given voxel volume,
as well as compute and investigate the spatial and spectral properties
of radio (to be compared with VLA or EOVSA data), (sub-)millimeter
(ALMA), EUV (AIA/SDO), and X-ray (RHESSI) emission calculated from the
model. The application integrates shared-object libraries containing
fast free-free, gyrosynchrotron, and gyroresonance emission codes
developed in FORTRAN and C++, and soft and hard X-ray and EUV codes
developed in IDL. We use this tool to model and analyze an active
region and compare the synthetic emission maps obtained in different
wavelengths with observations.This work was partially supported
by NSF grants AGS-1250374, AGS-1262772, NASA grant NNX14AC87G, the
Marie Curie International Research Staff Exchange Scheme "Radiosun"
(PEOPLE-2011-IRSES-295272), RFBR grants 14-02-91157, 15-02-01089,
15-02-03717, 15-02-03835, 15-02-08028.
---------------------------------------------------------
Title: Millimeter radiation from a 3D model of the solar
atmosphere. I. Diagnosing chromospheric thermal structure
Authors: Loukitcheva, M.; Solanki, S. K.; Carlsson, M.; White, S. M.
2015A&A...575A..15L Altcode: 2015arXiv150102898L
<BR /> Aims: We use advanced 3D non-local thermodynamic equilibrium
radiative magnetohydrodynamic simulations of the solar atmosphere to
carry out detailed tests of chromospheric diagnostics at millimeter and
submillimeter wavelengths. <BR /> Methods: We focused on the diagnostics
of the thermal structure of the chromosphere in the wavelength bands
from 0.4 mm up to 9.6 mm that can be accessed with the Atacama Large
Millimeter/Submillimeter Array (ALMA) and investigated how these
diagnostics are affected by the instrumental resolution. <BR /> Results:
We find that the formation height range of the millimeter radiation
depends on the location in the simulation domain and is related to the
underlying magnetic structure. Nonetheless, the brightness temperature
is a reasonable measure of the gas temperature at the effective
formation height at a given location on the solar surface. There is
considerable scatter in this relationship, but this is significantly
reduced when very weak magnetic fields are avoided. Our results
indicate that although instrumental smearing reduces the correlation
between brightness and temperature, millimeter brightness can still
be used to reliably diagnose electron temperature up to a resolution
of 1”. If the resolution is more degraded, then the value of the
diagnostic diminishes rapidly. <BR /> Conclusions: We conclude that
millimeter brightness can image the chromospheric thermal structure
at the height at which the radiation is formed. Thus multiwavelength
observations with ALMA with a narrow step in wavelength should provide
sufficient information for a tomographic imaging of the chromosphere.
---------------------------------------------------------
Title: The chromosphere above sunspots at millimeter wavelengths
Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M.
2014A&A...561A.133L Altcode: 2014arXiv1403.3436L
<BR /> Aims: The aim of this paper is to demonstrate that millimeter
wave data can be used to distinguish between various atmospheric models
of sunspots, whose temperature structure in the upper photosphere
and chromosphere has been the source of some controversy. <BR />
Methods: We use observations of the temperature contrast (relative
to the quiet Sun) above a sunspot umbra at 3.5 mm obtained with
the Berkeley-Illinois-Maryland Array (BIMA), complemented by submm
observations from Lindsey & Kopp (1995) and 2 cm observations
with the Very Large Array. These are compared with the umbral contrast
calculated from various atmospheric models of sunspots. <BR /> Results:
Current mm and submm observational data suggest that the brightness
observed at these wavelengths is low compared to the most widely used
sunspot models. These data impose strong constraints on the temperature
and density stratifications of the sunspot umbral atmosphere, in
particular on the location and depth of the temperature minimum and the
location of the transition region. <BR /> Conclusions: A successful
model that is in agreement with millimeter umbral brightness should
have an extended and deep temperature minimum (below 3000 K). Better
spatial resolution as well as better wavelength coverage are needed
for a more complete determination of the chromospheric temperature
stratification above sunspot umbrae.
---------------------------------------------------------
Title: Measuring chromospheric magnetic field with ALMA
Authors: Loukitcheva, Maria
2014cosp...40E1885L Altcode:
Polarized radio emission at millimeter wavelengths provides
an alternative method for measuring magnetic field in the solar
chromosphere. We use simulated millimeter brightness calculated from
recent 3D MHD models of the quiet-Sun regions to estimate the magnetic
field at the chromospheric heights. We discuss the results of this
investigation in the light of the future solar observations with the
Atacama Large Millimeter/Submillimeter Array (ALMA).
---------------------------------------------------------
Title: Diagnostic of chromospheric thermal structure based on
millimeter/submillimeter radiation
Authors: Loukitcheva, Maria
2014cosp...40E1886L Altcode:
We study formation of millimeter and submillimeter continua using
most advanced 3D MHD simulations of the solar atmosphere. To estimate
the accuracy of the diagnostic of chromospheric thermal structure
we compare simulated mm/submm brightness with the model temperature
distributions. We also investigate effect of spatial smearing of the
model brightness in the light of future interferometric observations
with the Atacama Large Millimeter/Submillimeter Array (ALMA).
---------------------------------------------------------
Title: Diagnostics of solar chromosphere plasma based on observations
of millimeter radiation
Authors: Nagnibeda, V. G.; Loukitcheva, M. A.
2012CP....398...33N Altcode:
In this paper we review the current knowledge of the solar chromosphere
from its observations at millimeter wavelengths. We present the
observational spectrum of the quiet Sun millimeter wave brightness
temperature and its comparison with brightness temperatures computed
from the standard static models and the dynamic simulations. Reporting
on the findings regarding the structure and dynamics of the solar
chromosphere from the best available interferometric data obtained
at 3.5 mm we demonstrate a great potential of the mm observations for
the study of the solar chromosphere.
---------------------------------------------------------
Title: Chromosphere above sunspots as seen at millimeter wavelengths
Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen M.
2011IAUS..273..408L Altcode:
Millimeter emission is known to be a sensitive diagnostic of temperature
and density in the solar chromosphere. In this work we use millimeter
wave data to distinguish between various atmospheric models of sunspots,
whose temperature structure in the upper photosphere and chromosphere
has been the source of some controversy. From mm brightness simulations
we expect a radio umbra to change its appearance from dark to bright
(compared to the Quiet Sun) at a given wavelength in the millimeter
spectrum (depending on the exact temperature in the model used). Thereby
the millimeter brightness observed above an umbra at several wavelengths
imposes strong constraints on temperature and density stratification of
the sunspot atmosphere, in particular on the location and depth of the
temperature minimum and the location of the transition region. Current
mm/submm observational data suggest that brightness observed at
short wavelengths is unexpectedly low compared to the most widely
used sunspot models such as of Maltby et al. (1986). A successful
model that is in agreement with millimeter umbral brightness should
have an extended and deep temperature minimum (below 3000 K), such as
in the models of Severino et al. (1994). However, we are not able to
resolve the umbra cleanly with the presently available observations
and better resolution as well as better wavelength coverage are
needed for accurate diagnostics of umbral brightness at millimeter
wavelengths. This adds one more scientific objective for the Atacama
Large Millimeter/Submillimeter Array (ALMA).
---------------------------------------------------------
Title: Observations of the solar chromosphere at millimeter
wavelengths
Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M.
2010MmSAI..81..592L Altcode:
Millimeter wavelengths provide a powerful tool to study the thermal
structure of the solar chromosphere and its response to dynamic
processes. We present initial studies of chromospheric fine structure
and its dynamics obtained from observations of the quiet Sun with
BIMA at 3.5 mm with a resolution of 12 arcsec. The two-dimensional
millimeter maps of the solar chromosphere reveal brightness features
corresponding to supergranular network boundaries and bright points
within cells. Significant intensity oscillations with frequencies of
1.5 - 8.0 mHz with a tendency toward shorter-period oscillations in
the internetwork and longer periods in network regions are found in
the data. However, higher spatial resolution is required for a more
detailed study. We discuss the requirements imposed on observations in
the millimeter domain that might provide an insight into the fundamental
questions of solar physics. We also review the capabilities of the
current and future millimeter-wave interferometers, including the
CARMA and ALMA arrays.
---------------------------------------------------------
Title: On the relation between photospheric magnetic field and
chromospheric emission in the quiet Sun
Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen M.
2009IAUS..259..185L Altcode:
In this contribution we present an observational study of the
interaction of the photosphere with different chromospheric layers. We
study the correlations between emissions at varying temperature from the
temperature minimum region (UV continuum at 1600 Å from TRACE) through
the low chromosphere (CaII K-line from BBSO) to the middle chromosphere
(continuum at 3.5 mm from BIMA) and photospheric magnetic field from
MDI/SOHO. For the first time millimeter observational data are included
in such analysis. <P />We report a high degree of correlation between
considered emissions formed at different heights in the chromosphere. A
power law is found to be a good representation for the relationship
between photospheric magnetic field and chromospheric emissions at
all considered wavelengths. Our analysis shows that the dependence
of chromospheric intensities on magnetic field is different for the
network and internetwork regions. In the network a power law provides
the best fit with the exponent being close to 0.5-0.6, while almost
no dependence of chromospheric intensity on magnetic flux is found for
the cell interiors. The obtained results support the idea of different
heating mechanisms acting in the network (magnetic) and cell interiors
(acoustic).
---------------------------------------------------------
Title: The relationship between chromospheric emissions and magnetic
field strength
Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M.
2009A&A...497..273L Altcode: 2009arXiv0910.1985L
Aims: We analyze observational data from 4 instruments to study the
correlations between chromospheric emission, spanning the heights
from the temperature minimum region to the middle chromosphere,
and photospheric magnetic field. <BR />Methods: The data consist of
radio images at 3.5 mm from the Berkeley-Illinois-Maryland Array
(BIMA), UV images at 1600 Å from TRACE, Ca II K-line filtergrams
from BBSO, and MDI/SOHO longitudinal photospheric magnetograms. For
the first time interferometric millimeter data with the highest
currently available resolution are included in such an analysis. We
determine various parameters of the intensity maps and correlate the
intensities with each other and with the magnetic field. <BR />Results:
The chromospheric diagnostics studied here show a pronounced similarity
in their brightness structures and map out the underlying photospheric
magnetic field relatively well. We find a power law to be a good
representation of the relationship between photospheric magnetic field
and emission from chromospheric diagnostics at all wavelengths. The
dependence of chromospheric brightness on magnetic field is found to
be different for network and internetwork regions.
---------------------------------------------------------
Title: The Solar Chromosphere at Millimeter Wavelengths
Authors: Loukitcheva, M.; Solanki, S. K.; White, S.
2008ESPM...12.2.18L Altcode:
The solar chromosphere remains the least understood layer of the
solar atmosphere. There is yet no answer to the question concerning
its structure. Is it better described by the classical picture of a
steady temperature rise as a function of height, with superposed weak
oscillations, or does the temperature keep dropping outwards, with hot
shocks producing strong localized heating? Observations in the UV and
the IR give contrasting results, since they only sample either the
hot or the cool parts of the chromosphere. Computations carried out
with sophisticated dynamic models of the solar chromosphere demonstrate
that millimeter emission is extremely sensitive to dynamic processes in
the chromosphere and the appropriate wavelengths to look for dynamic
signatures are in the range 0.8-5.0 mm. The models also suggest
that high resolution observations at mm wavelengths have the unique
property of reacting to both the hot and the cool gas, and thus have the
potential of distinguishing between models. <P />In this contribution
we use high-resolution millimeter-interferometer observations, obtained
with the Berkeley-Illinois-Maryland Array at 3.5 mm (resolution of
12 arcsec), as a diagnostic tool to study the thermal structure of
the solar chromosphere and its response to dynamic processes. <P
/>Our initial results obtained from the observations of the quiet
Sun reveal brightness features corresponding to supergranular network
boundaries and bright points within the cells. We found significant
intensity oscillations with frequencies of 1.5-8 mHz with a tendency
toward short-period oscillations in internetwork and longer periods in
network regions. However higher spatial resolution is required for a
clean separation between the brightness features and for an adequate
comparison with the output of the comprehensive dynamic simulations.
---------------------------------------------------------
Title: ALMA as the ideal probe of the solar chromosphere
Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen
2008Ap&SS.313..197L Altcode: 2007arXiv0704.0023L; 2007Ap&SS.tmp..371L
The very nature of the solar chromosphere, its structuring and
dynamics, remains far from being properly understood, in spite of
intensive research. Here we point out the potential of chromospheric
observations at millimeter wavelengths to resolve this long-standing
problem. Computations carried out with a sophisticated dynamic model
of the solar chromosphere due to Carlsson and Stein demonstrate that
millimeter emission is extremely sensitive to dynamic processes in
the chromosphere and the appropriate wavelengths to look for dynamic
signatures are in the range 0.8 5.0 mm. The model also suggests that
high resolution observations at mm wavelengths, as will be provided
by ALMA, will have the unique property of reacting to both the hot
and the cool gas, and thus will have the potential of distinguishing
between rival models of the solar atmosphere. Thus, initial results
obtained from the observations of the quiet Sun at 3.5 mm with the
BIMA array (resolution of 12″) reveal significant oscillations with
amplitudes of 50 150 K and frequencies of 1.5 8 mHz with a tendency
toward short-period oscillations in internetwork and longer periods
in network regions. However higher spatial resolution, such as that
provided by ALMA, is required for a clean separation between the
features within the solar atmosphere and for an adequate comparison
with the output of the comprehensive dynamic simulations.
---------------------------------------------------------
Title: High-resolution millimeter-interferometer observations of
the solar chromosphere
Authors: White, S. M.; Loukitcheva, M.; Solanki, S. K.
2006A&A...456..697W Altcode:
The use of millimeter-interferometer data for the study of chromospheric
structure and dynamics is tested using 85 GHz observations with the
10-element Berkeley-Illinois-Maryland Array (BIMA). Interferometer
data have the advantage over single-dish data that they allow both high
spatial resolution and dense temporal sampling simultaneously. However,
snapshot imaging of the quiet solar atmosphere with a small number of
dishes is challenging. We demonstrate that techniques are available to
carry out this task successfully using maximum entropy deconvolution
from a default image constructed from the entire observation: one of
our results is that the solar chromosphere at millimeter wavelengths
exhibits features that are long-lasting and the map of the entire
observation is significant provided that atmospheric phase errors do
not prevent deconvolution. We compare observations of quiet Sun, active
region and coronal hole targets. The interferometer is not sensitive
to the disk emission and the positivity constraint of the maximum
entropy algorithm used forces the zero level in the images to be at
the temperature of the coolest feature in each field. The brightest
features in the images are typically 1000-1500 K above the zero level,
with a snapshot noise level of order 100 K. We use extensive tests to
determine whether oscillation power can be recovered from sequences of
snapshot images and show that individual sources can be down to quite
weak levels at locations in the image where significant flux is present;
oscillation power located in cool regions of the image is not well
recovered due to the deconvolution method used and may be redistributed
to brighter regions of the millimeter image. We then investigate
whether the data do show oscillation power using uninterrupted 30-min
scans of the target regions. Intensity oscillations with significant
power in the frequency range 1.5-8.0 mHz are found in the quiet-Sun
and active region targets. For the quiet-Sun region we compare the
oscillation properties of network boundaries and cell interiors
(internetwork) in the spatially-resolved time series. In agreement
with investigations at other wavelengths, in the millimeter data the
power in the network tends to be at periods of 5 min and longer while
power in the internetwork is present also at shorter (3-min) periods.
---------------------------------------------------------
Title: The dynamics of the solar chromosphere: comparison of model
predictions with millimeter-interferometer observations
Authors: Loukitcheva, M.; Solanki, S. K.; White, S.
2006A&A...456..713L Altcode:
We analyze the millimeter intensity spectrum expected from the dynamic
model of Carlsson & Stein together with the interferometric
observations of the quiet Sun obtained at a wavelength of 3.5 mm with
the Berkeley-Illinois-Maryland Array. The observational data products
(Fourier and wavelet spectra, brightness histograms) are compared
with the corresponding products obtained for the Carlsson &
Stein (CS) models. We estimate how the limited spatial resolution
of the observations influences the comparison with the predictions
of chromospheric dynamic models and discuss the limitations of a
one-dimensional non-magnetic modeling approach. In addition, we test the
effect of the integration time of the BIMA observations on the dynamic
signatures. The dependence of the observed brightness variations on
spatial resolution is studied by employing artificial image degradation
and approximating the obtained dependence by power laws. We are able
to establish a correspondence between the CS model predictions and
the observational data under assumptions on the horizontal coherence
length of the oscillations. The reconstructed brightness rms values
indicate that, assuming the coherence length of oscillating elements
to be of order of 1”, the oscillation power in the observations
recorded with 10” resolution agrees within a factor of 2 with the
power predicted by the CS model. We argue that millimeter continuum
observations promise to be an important diagnostic of chromospheric
structure and dynamics. Based on the analysis carried out in this work,
the appropriate wavelengths to look for dynamic signatures are in the
range 0.8-5.0 mm. Further millimeter interferometric observations with
longer sequences and higher spatial resolution are highly desirable
along with the development of realistic three-dimensional radiation
magnetohydrodynamic simulations.
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Title: Acoustic shock waves in the solar chromosphere from millimeter
observations
Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen
2006IAUS..233..104L Altcode:
We argue that millimeter continuum observations promise to be an
important diagnostic of chromospheric dynamics and the appropriate
wavelengths to look for dynamic signatures are in the range 0.8-5.0
mm. We have analyzed the millimeter intensity spectrum expected from
the dynamic model of the solar non-magnetic atmosphere of Carlsson
& Stein (1992, 1995, 1997, 2002, hereafter CS) together with the
interferometric observations of the quiet Sun obtained at a wavelength
of 3.5 mm with the Berkeley-Illinois-Maryland Array. Model radio
emission at millimeter wavelengths is found to be extremely sensitive
to dynamic processes in the chromosphere, if these are spatially and
temporally resolved. The estimated millimeter brightness temperatures
are time-dependent, following changes in the atmospheric parameters,
and result in clear signatures of waves with a period of 180 s seen
in the radio intensity as a function of time. At the same time,
the interferometric observations of the internetwork regions reveal
significant oscillations with amplitudes of 50-150 K in the frequency
range 1.5-8 mHz. We give an estimate of the influence of the limited
available spatial resolution of observations on the comparison with the
predictions of dynamic models. We are able to establish a correspondence
between the CS model predictions and the observational data if we
assume that the horizontal coherence length of the oscillations is on
the order of 1 arcsec.
---------------------------------------------------------
Title: Chromospheric Dynamics: Model Predictions and Comparison
With Observations
Authors: Loukitcheva, M.; Solanki, S. K.; White, S.
2006apri.meet...38L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Millimeter-Interferometer Observations of Oscillations in
the Solar Chromosphere
Authors: White, S. M.; Loukitcheva, M.; Solanki, S. K.
2005AGUSMSH12A..03W Altcode:
The use of millimeter-interferometer data for the study of chromospheric
structure and dynamics is tested using 85 GHz observations with the
10-element Berkeley-Illinois-Maryland Array (BIMA). Interferometer
data have the advantage over single-dish data that they allow both high
spatial resolution and dense temporal sampling simultaneously. However,
snapshot imaging of the quiet solar atmosphere with a small number of
dishes is challenging. We demonstrate that techniques are available to
carry out this task successfully. We compare observations of quiet Sun,
active region and coronal hole targets using images with 10 arcsec
resolution at 15 second cadence. The brightest features in the images
are typically 1500 K above the background level, with a snapshot noise
level of order 100 K. We use extensive tests to determine whether
oscillation power can be recovered from sequences of snapshot images
and show that they can down to quite weak levels. Intensity oscillations
with significant power in the frequency range 1.5-8.0 mHz are found in
the quiet-Sun and active region targets. For the quiet-Sun region we
use the spatially-resolved time series to investigate the properties
of network boundaries and cell interiors (internetwork) separately. In
agreement with investigations at other wavelengths, the millimeter
data show that power in the network tends to be at periods of 5 minutes
and longer while power in the internetwork is present also at shorter
(3-minute) periods.
---------------------------------------------------------
Title: Millimeter observations and chromospheric dynamics
Authors: Loukitcheva, M.; Solanki, S. K.; Carlsson, M.; Stein, R. F.
2004A&A...419..747L Altcode:
The intensities of submillimeter and millimeter continua, which are
formed in LTE and depend linearly on temperature, may be able to provide
a test of models of the Solar chromosphere. We have taken a collection
of submillimeter and millimeter wave observed brightness temperatures
T<SUB>b</SUB> of the quiet Sun from the literature and compared it
with brightness temperatures computed from the standard static models
of Fontenla, Avrett and Loeser (FAL) and the dynamic simulations of
Carlsson & Stein (CS). The analysis of the dynamic simulations
of Carlsson & Stein reveals that radio emission at millimeter
wavelengths is extremely sensitive to dynamic processes in the
chromosphere, if these are spatially and temporally resolved. The most
striking result is that the dynamic picture of the solar internetwork
chromosphere is consistent with currently available millimeter and
submillimeter brightness observations. The spectrum obtained by
averaging over the spectra from all time-steps of CS simulations
provides a good fit to observed temporally and spatially averaged
millimeter data in spite of the absence of a permanent temperature
rise at low chromospheric heights in the simulations. This does not by
itself rule out the presence of a chromospheric temperature rise as
present in the FAL models, since a combination of such models also
reproduces the (low resolution) data relatively well. Millimeter
observations indicate that using radio techniques it is possible
to extend observations of the solar oscillatory component to the
heights above those previously observed in the photospheric and low
chromospheric spectral lines and submillimeter continuum. For more
precise diagnostics of chromospheric dynamics, high temporal and spatial
resolution interferometric observations in the millimeter-wavelength
region would be particularly useful. <P />Table \ref{tab:table} is
only available in electronic form at http://www.edpsciences.org
---------------------------------------------------------
Title: On the relationship between chromospheric oscillations of
radio brightness at 1.76 cm with periods from minutes to hours and
magnetic field changes
Authors: Loukitcheva, M. A.; Gelfreikh, G. B.; Nagnibeda, V. G.
2004IAUS..223..281L Altcode: 2005IAUS..223..281L
We present the results of Fourier and wavelet time series analysis for
the high-cadence observations of an active region NOAA 8011, obtained
with the Nobeyama Radioheliograph (NoRH) at the wavelength of 1.76 cm
on 17.01.1997. Oscillations in brightness are found to be present with
periods in the range from minutes to hours. The relationship between
the active region oscillations in the microwave total intensity (I)
and circular polarization (V) emission and changes of the magnetic
field, deduced from the high-resolution magnetograms from MDI/SoHO,
is investigated. We concentrate on the identification of different
oscillation modes, the temporal behavior of the oscillations and the
spatial distribution of different oscillating frequencies.
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Title: The solar chromosphere as seen in high-resolution millimeter
observations
Authors: Loukitcheva, M. A.; Solanki, S. K.; White, S.
2004IAUS..223..643L Altcode: 2005IAUS..223..643L
We report on chromospheric oscillations studied with 15 sec
cadence observations obtained at a wavelength of 3.5 mm with the
Berkeley-Illinois-Maryland Array (BIMA). Different solar structures,
including active and quiet-Sun regions, are analyzed. We compare the
high-resolution millimeter 2-D images with Ca II K line images from Big
Bear Solar Observatory. Using Fourier and wavelet time series analysis
techniques we find signatures of intensity oscillations with periods
in the range of 2-7 minutes. We discuss the results in the context of
the dynamical model of the solar atmosphere by Carlsson & Stein
(1995, 1997).
---------------------------------------------------------
Title: Some results of the chromospheric magnetic field study based
on millimeter wave observations of active regions
Authors: Loukitcheva, M. A.; Mandrini, C. H.; Stenborg, G. A.
2002ESASP.505..481L Altcode: 2002solm.conf..481L; 2002IAUCo.188..481L
Solar radio observations combined with unique methods of radio
diagnostics of solar plasma are capable to provide an estimate of the
magnetic field above the photospheric level. In the paper we present
some results of the study of magnetic fields in the solar chromosphere
of active regions associated with sunspots and plages through
observations of their free-free and gyroresonance microwave emission
in intensity and polarization obtained with Nobeyama radioheliograph
at 17.6 mm and 8 mm wavelengths. The contributions to millimeter and
short centimeter emission of active regions from the free-free and
gyroradiation processes are discussed. Two-dimensional intensity maps
and radiomagnetograms with 10 and 5 arcsec resolution are compared
with the SXT/Yohkoh observations and with the chromospheric fields
obtained by extrapolating the KP/NSO observed photospheric field to
the chromosphere using a potential model of the magnetic field.
---------------------------------------------------------
Title: Radio Emission of Solar Chromosphere at Millimeter Wavelengths
Authors: Loukitcheva, M. A.; Nagnibeda, V. G.
2000ESASP.463..363L Altcode: 2000sctc.proc..363L
No abstract at ADS
---------------------------------------------------------
Title: Chromosphere Active Region Plasma Diagnostics Based On
Observations Of Millimeter Radiation
Authors: Loukitcheva, M.; Nagnibeda, V.
1999ESASP.446..451L Altcode: 1999soho....8..451L
In this paper we present the results of millimeter radiation
calculations for different elements of chromospheric and transition
region structures of the quiet Sun and S-component - elements of
chromosphere network, sunspot groups and plages. The calculations
were done on the basis of standard optical and UV models ( models
by Vernazza et al. (1981,VAL), their modifications by Fontenla et
al. (1993,FAL)). We also considered the sunspot model by Lites and
Skumanich (1982,LS), S-component model by Staude et al.(1984) and
modification of VAL and FAL models by Bocchialini and Vial - models
NET and CELL. We compare these model calculations with observed
characteristics of components of millimeter Solar radiation for the
quiet Sun and S-component obtained with the radiotelescope RT-7.5
MGTU (wavelength 3.4 mm) and radioheliograph Nobeyama (wavelength
17.6 mm). From observations we derived spectral characteristics of
millimeter sources and active region source structure. The comparison
has shown that observed radio data are clearly in dissagrement with
all the considered models. Finally, we propose further improvement
of chromospheric and transition region models based on optical and UV
observations in order to use for modelling information obtained from
radio data.
---------------------------------------------------------
Title: Spectral features of millimetre emission of solar active
regions.
Authors: Nagnibeda, V.; Loukitcheva, M.
1998joso.proc...99N Altcode:
From solar maps at 3.4 mm wavelength obtained with RT 7.5 radio
telescope of Moscow State Technical University and solar maps at 17.6 mm
wavelength from Japan Nobeyama heliograph, spectral characteristics of
sources of slowly varying component have been studied. It has been shown
that two components in millimetre radio emission of an active region can
be resolved: sunspot emission and plage emission. Their contribution
into total flux density of the active region are different at 3.4 mm
and 17.6 mm wavelengths. At the wavelength of 3.4 mm the brightness
temperature of a source over sunspot group does not differ much from
the brightness temperature of the whole active region. Disagreement
between the observational data obtained at mm domain and the standard
chromosphere models is discussed.
