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Determination of Differential Emission Measure from Solar Extreme Ultraviolet Images
摘要： The Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) has been providing high-cadence, high-resolution, full-disk UV-visible/extreme ultraviolet (EUV) images since 2010, with the best time coverage among all the solar missions. A number of codes have been developed to extract plasma differential emission measures (DEMs) from AIA images. Although widely used, they cannot effectively constrain the DEM at ?aring temperatures with AIA data alone. This often results in much higher X-ray ?uxes than observed. One way to solve the problem is by adding more constraint from other data sets (such as soft X-ray images and ?uxes). However, the spatial information of plasma DEMs are lost in many cases. In this Letter, we present a different approach to constrain the DEMs. We tested the sparse inversion code and show that the default settings reproduce X-ray ?uxes that could be too high. Based on the tests with both simulated and observed AIA data, we provided recommended settings of basis functions and tolerances. The new DEM solutions derived from AIA images alone are much more consistent with (thermal) X-ray observations, and provide valuable information by mapping the thermal plasma from ～0.3 to ～30 MK. Such improvement is a key step in understanding the nature of individual X-ray sources, and particularly important for studies of ?are initiation.
关键词： magnetic reconnection,methods: data analysis,Sun: X-rays, gamma rays,Sun: ?ares,Sun: corona,Sun: UV radiation
更新于2025-09-23 15:23:52
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Thermal structure of a hot non-flaring corona from Hinode/EIS
摘要： Aims. In previous studies, a very hot plasma component has been diagnosed in solar active regions through the images in three different narrow-band channels of Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). This diagnostic from extreme ultraviolet (EUV) imaging data has also been supported by the matching morphology of emission in the hot Ca XVII line, as observed with Extreme-Ultraviolet Imaging Spectrometer (EIS) on board Hinode. This evidence is debated because of the unknown distribution of the emission measure along the line of sight. Here we investigate in detail the thermal distribution of one such region using EUV spectroscopic data. Methods. In an active region observed with SDO/AIA, Hinode/EIS, and X-ray telescope (XRT), we select a sub-region with a very hot plasma component and another cooler sub-region for comparison. The average spectrum is extracted for both, and 14 intense lines are selected for analysis that probe the 5.5 < log T < 7 temperature range uniformly. From these lines, the emission measure distributions are reconstructed with the Markov-chain Monte Carlo method. Results are cross-checked in comparison with the two sub-regions, with a different inversion method, with the morphology of the images, and with the addition of fluxes measured with narrow, and broadband imagers. Results. We find that, whereas the cool region has a flat and featureless distribution that drops at temperature log T ≥ 6.3, the distribution of the hot region shows a well-defined peak at log T = 6.6 and gradually decreasing trends on both sides, thus supporting the very hot nature of the hot component diagnosed with imagers. The other cross-checks are consistent with this result. Conclusions. This study provides a completion of the analysis of active region components, and the resulting scenario supports the presence of a minor very hot plasma component in the core, with temperatures log T > 6.6.
关键词： techniques: imaging spectroscopy,Sun: UV radiation,Sun: corona,techniques: spectroscopic,Sun: X-rays, gamma rays
更新于2025-09-23 15:23:52
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UV optical measurements of the Nozomi spacecraft interpreted with a two-component LIC-flow model <i>(Corrigendum)</i>
摘要： We found a typographical error regarding the uncertainty of the helium flow direction in our published paper Nakagawa et al. (2008). The helium flow direction should be (258.7 ± 8.0°, 3.4 ± 8.0°) instead of (258.7 ± 3.4°). As a consequence, the description for the helium flow direction in page 39 has to be replaced by “Residuals of simulated intensities with respect to observations attained minimum values for a downwind direction of (78.78 ± 8.0°, ?3.48 ± 8.0°), corresponding to an upwind direction of (258.78 ± 8.0°, 3.48 ± 8.0°).” All conclusions of the original paper remain unaffected. H. Nakagawa is grateful to Priscilla Frisch for pointing out this error.
关键词： ISM: atoms,ultraviolet: ISM,Sun: UV radiation,addenda,errata
更新于2025-09-23 15:23:52
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The Spectral Content of <i>SDO</i> /AIA 1600 and 1700 ? Filters from Flare and Plage Observations
摘要： The strong enhancement of the ultraviolet emission during solar flares is usually taken as an indication of plasma heating in the lower solar atmosphere caused by the deposition of the energy released during these events. Images taken with broadband ultraviolet filters by the Transition Region and Coronal Explorer and Atmospheric Imaging Assembly (AIA; 1600 and 1700 ?) have revealed the morphology and evolution of flare ribbons in great detail. However, the spectral content of these images is still largely unknown. Without knowledge of the spectral contribution to these UV filters, the use of these rich imaging data sets is severely limited. Aiming to solve this issue, we estimate the spectral contributions of the AIA UV flare and plage images using high-resolution spectra in the range 1300–1900 ? from the Skylab NRL SO82B spectrograph. We find that the flare excess emission in AIA 1600 ? is dominated by the C IV 1550 ? doublet (26%), Si I continua (20%), with smaller contributions from many other chromospheric lines such as C I 1561 and 1656 ? multiplets, He II 1640 ?, and Si II 1526 and 1533 ?. For the AIA 1700 ? band, the C I 1656 ? multiplet is the main contributor (38%), followed by He II 1640 (17%), and accompanied by a multitude of other, weaker chromospheric lines, with minimal contribution from the continuum. Our results can be generalized to state that the AIA UV flare excess emission is of chromospheric origin, while plage emission is dominated by photospheric continuum emission in both channels.
关键词： Sun: UV radiation,Sun: flares,Sun: atmosphere,Sun: chromosphere,Sun: photosphere
更新于2025-09-23 15:22:29
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Proper horizontal photospheric flows in a filament channel
摘要： Context. An extended ?lament in the central part of the active region NOAA 11106 crossed the central meridian on Sept. 17, 2010 in the southern hemisphere. It has been observed in Hα with the THEMIS telescope in the Canary Islands and in 304 ? with the EUV imager (AIA) onboard the Solar Dynamic Observatory (SDO). Counterstreaming along the Hα threads and bright moving blobs (jets) along the 304 ? ?lament channel were observed during 10 h before the ?lament erupted at 17:03 UT. Aims. The aim of the paper is to understand the coupling between magnetic ?eld and convection in ?lament channels and relate the horizontal photospheric motions to the activity of the ?lament. Methods. An analysis of the proper photospheric motions using SDO/HMI continuum images with the new version of the coherent structure tracking (CST) algorithm developed to track granules, as well as the large scale photospheric ?ows, was performed for three hours. Using corks, we derived the passive scalar points and produced a map of the cork distribution in the ?lament channel. Averaging the velocity vectors in the southern hemisphere in each latitude in steps of 3.5 arcsec, we de?ned a pro?le of the di?erential rotation. Results. Supergranules are clearly identi?ed in the ?lament channel. Diverging ?ows inside the supergranules are similar in and out of the ?lament channel. Converging ?ows corresponding to the accumulation of corks are identi?ed well around the Hα ?lament feet and at the edges of the EUV ?lament channel. At these convergence points, the horizontal photospheric velocity may reach 1 km s?1, but with a mean velocity of 0.35 km s?1. In some locations, horizontal ?ows crossing the channel are detected, indicating eventually large scale vorticity. Conclusions. The coupling between convection and magnetic ?eld in the photosphere is relatively strong. The ?lament experienced the convection motions through its anchorage points with the photosphere, which are magnetized areas (ends, feet, lateral extensions of the EUV ?lament channel). From a large scale point-of-view, the di?erential rotation induced a shear of 0.1 km s?1 in the ?lament. From a small scale point-of-view, any convective motions favored the interaction of the parasitic polarities responsible for the anchorages of the ?lament to the photosphere with the surrounding network and may explain the activity of the ?lament.
关键词： prominences – Sun: photosphere – Sun: UV radiation – Sun: magnetic ?elds,Sun: ?laments
更新于2025-09-23 15:22:29
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CONFINED FLARES IN SOLAR ACTIVE REGION 12192 FROM 2014 OCTOBER 18 TO 29
摘要： Using the observations from the Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory, we investigate 6 X-class and 29 M-class ?ares occurring in solar active region (AR) 12192 from October 18 to 29. Among them, 30 (including 6 X- and 24 M-class) ?ares originated from the AR core, and the other 5 M-?ares appeared at the AR periphery. Four of the X-?ares exhibited similar ?aring structures, indicating they were homologous ?ares with an analogous triggering mechanism. The possible scenario is that photospheric motions of emerged magnetic ?uxes lead to shearing of the associated coronal magnetic ?eld, which then yields a tether-cutting favorable con?guration. Among the ?ve periphery M-?ares, four were associated with jet activities. The HMI vertical magnetic ?eld data show that the photospheric ?uxes of opposite magnetic polarities emerged, converged, and canceled with each other at the footpoints of the jets before the ?ares. Only one M-?are from the AR periphery was followed by a coronal mass ejection (CME). From October 20 to 26, the mean decay index of the horizontal background ?eld within the height range of 40–105 Mm is below the typical threshold for torus instability onset. This suggests that a strong con?nement from the overlying magnetic ?eld might be responsible for the poor CME production of AR 12192.
关键词： Sun: coronal mass ejections (CMEs),Sun: ?ares,Sun: activity,Sun: UV radiation
更新于2025-09-23 15:22:29
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First Detection of Solar Flare Emission in Mid-ultraviolet Balmer Continuum
摘要： We present the ?rst detection of solar ?are emission at mid-ultraviolet wavelengths around 2000?? by the channel 2 of the Large-Yield RAdiometer (LYRA) on board the PRoject for OnBoard Autonomy 2 mission. The ?are (SOL20170906) was also observed in the channel 1 of LYRA centered at the H I Lyα line at 1216??, showing a clear non-thermal pro?le in both channels. The ?are radiation in channel 2 is consistent with the hydrogen Balmer continuum emission produced by an optically thin chromospheric slab heated up to 10,000 K. Simultaneous observations in channels 1 and 2 allow the separation of the line emission (primarily from the Lyα line) from the Balmer continuum emission. Together with the recent detection of the Balmer continuum emission in the near-ultraviolet by the Interface Region Imaging Spectrometer, the LYRA observations strengthen the interpretation of broadband ?are emission as the hydrogen recombination continua originating in the chromosphere.
关键词： Sun: ?ares,Sun: chromosphere,Sun: UV radiation
更新于2025-09-23 15:21:01
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SOLAR/SOLSPEC mission on ISS: In-flight performance for SSI measurements in the UV
摘要： Context. The SOLar SPECtrum (SOLSPEC) experiment is part of the Solar Monitoring Observatory (SOLAR) payload, and has been externally mounted on the Columbus module of the International Space Station (ISS) since 2008. SOLAR/SOLSPEC combines three absolutely calibrated double monochromators with concave gratings for measuring the solar spectral irradiance (SSI) from 166 nm to 3088 nm. This physical quantity is a key input for studies of climatology, planetary atmospheres, and solar physics. Aims. A general description of the instrument is given, including in-?ight operations and performance of the ultraviolet (UV) channel from 175 nm to 340 nm. Methods. We developed a range of processing and correction methods, which are described in detail. For example, methods for correcting thermal behavior e?ects, instrument linearity, and especially the accuracy of the wavelength and absolute radiometric scales have been validated by modeling the standard uncertainties. Results. The deliverable is a quiet Sun UV reference solar spectrum as measured by SOLAR/SOLSPEC during the minimum of solar activity prior to cycle 24. Comparisons with other instruments measuring SSI are also presented.
关键词： instrumentation: detectors,space vehicles: instruments,Sun: UV radiation,Sun: general,techniques: spectroscopic
更新于2025-09-23 15:19:57
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SI iv Resonance Line Emission during Solar Flares: Non-LTE, Nonequilibrium, Radiation Transfer Simulations
摘要： The Interface Region Imaging Spectrograph routinely observes the Si IV resonance lines. When analyzing quiescent observations of these lines, it has typically been assumed that they form under optically thin conditions. This is likely valid for the quiescent Sun, but this assumption has also been applied to the more extreme flaring scenario. We used 36 electron-beam-driven radiation hydrodynamic solar flare simulations, computed using the RADYN code, to probe the validity of this assumption. Using these simulated atmospheres, we solved the radiation transfer equations to obtain the non-LTE, nonequilibrium populations, line profiles, and opacities for a model silicon atom, including charge exchange processes. This was achieved using the 'minority species' version of RADYN. The inclusion of charge exchange resulted in a substantial fraction of Si IV at cooler temperatures than those predicted by ionization equilibrium. All simulations with an injected energy flux F > 5 × 10^10 erg cm^{-2} s^{-1} resulted in optical depth effects on the Si IV emission, with differences in both intensity and line shape compared to the optically thin calculation. Weaker flares (down to F ≈ 5 × 10^9 erg cm^{-2} s^{-1}) also resulted in Si IV emission forming under optically thick conditions, depending on the other beam parameters. When opacity was significant, the atmospheres generally had column masses in excess of 5 × 10^{-6} g cm^{-2} over the temperature range 40–100 kK, and the Si IV formation temperatures were between 30 and 60 kK. We urge caution when analyzing Si IV flare observations, or when computing synthetic emission without performing a full radiation transfer calculation.
关键词： Sun: UV radiation,line: formation,radiative transfer,Sun: flares,Sun: transition region,methods: numerical
更新于2025-09-19 17:15:36
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IRIS observations of the Mg ii h and k lines during a solar flare
摘要： The bulk of the radiative output of a solar flare is emitted from the chromosphere, which produces enhancements in the optical and UV continuum, and in many lines, both optically thick and thin. We have, until very recently, lacked observations of two of the strongest of these lines: the Mg ii h and k resonance lines. We present a detailed study of the response of these lines to a solar flare. The spatial and temporal behaviour of the integrated intensities, k/h line ratios, line of sight velocities, line widths and line asymmetries were investigated during an M class flare (SOL2014-02-13T01:40). Very intense, spatially localised energy input at the outer edge of the ribbon is observed, resulting in redshifts equivalent to velocities of ～15–26 km s?1, line broadenings, and a blue asymmetry in the most intense sources. The characteristic central reversal feature that is ubiquitous in quiet Sun observations is absent in flaring profiles, indicating that the source function increases with height during the flare. Despite the absence of the central reversal feature, the k/h line ratio indicates that the lines remain optically thick during the flare. Subordinate lines in the Mg ii passband are observed to be in emission in flaring sources, brightening and cooling with similar timescales to the resonance lines. This work represents a first analysis of potential diagnostic information of the flaring atmosphere using these lines, and provides observations to which synthetic spectra from advanced radiative transfer codes can be compared.
关键词： Sun: chromosphere,Sun: flares,Sun: UV radiation
更新于2025-09-19 17:15:36