Author name code: loukitcheva ADS astronomy entries on 2022-09-14 author:"Loukitcheva, Maria A." ------------------------------------------------------------------------ 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. Bibcode: 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. 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. Bibcode: 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. Title: Measuring magnetic field with Atacama Large Millimeter/Submillimeter Array Authors: Loukitcheva, Maria Bibcode: 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. Title: ALMA detection of dark chromospheric holes Authors: White, Stephen M.; Loukitcheva, Maria A.; Solanki, Sami K. Bibcode: 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. Title: ALMA Detection of Dark Chromospheric Holes in the Quiet Sun Authors: Loukitcheva, Maria A.; White, Stephen M.; Solanki, Sami K. Bibcode: 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. Title: First solar observations with ALMA Authors: Loukitcheva, Maria Bibcode: 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. Title: Force-free Field Reconstructions Enhanced by Chromospheric Magnetic Field Data Authors: Fleishman, Gregory; Mysh'yakov, Ivan; Stupishin, Alexey; Loukitcheva, Maria; Anfinogentov, Sergey Bibcode: 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. 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. Bibcode: 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 ≥108 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. Title: Erratum: “A First Comparison of Millimeter Continuum and Mg II Ultraviolet Line Emission from the Solar Chromosphere” (2017, ApJL, 845, L19) Authors: Bastian, T. S.; Chintzoglou, G.; De Pontieu, B.; Shimojo, M.; Schmit, D.; Leenaarts, J.; Loukitcheva, M. Bibcode: 2018ApJ...860L..16B Altcode: No abstract at ADS 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Solar ALMA Observations: Constraining the Chromosphere above Sunspots Authors: Loukitcheva, Maria A.; Iwai, Kazumasa; Solanki, Sami K.; White, Stephen M.; Shimojo, Masumi Bibcode: 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. 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. Bibcode: 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 TB and the Mg II h line radiation temperature Trad 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 TB (1.25 mm) and mean Trad (Mg II). Partitioning the maps into “sunspot,” “quiet areas,” and “plage regions” we find the relation between the IRIS Mg II h line Trad and the ALMA TB 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. 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 Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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″, 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. 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. Bibcode: 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. Bibcode: 2017A&A...601A..43L Altcode: 2017arXiv170206018L
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).
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.
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.
Conclusions: Consequently, high-resolution and high-precision observations of circularly polarized radiation at millimeter wavelengths can be a powerful tool for producing chromospheric longitudinal magnetograms. Title: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 2015A&A...575A..15L Altcode: 2015arXiv150102898L
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.
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.
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.
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. Bibcode: 2014A&A...561A.133L Altcode: 2014arXiv1403.3436L
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.
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.
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.
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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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.
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.
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. Bibcode: 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.

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.

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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Title: Acoustic shock waves in the solar chromosphere from millimeter observations Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Tb 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.

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. Bibcode: 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. Title: The solar chromosphere as seen in high-resolution millimeter observations Authors: Loukitcheva, M. A.; Solanki, S. K.; White, S. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.