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During nearly three years of operation, STIX aboard Solar Orbiter observed thousands of flares. Many of them were simultaneously observed by Solar X-Ray Monitor (XSM) on board Indian Chandrayaan-2 circling the Moon. We present results of a multi-wavelength study for one selected flare of M GOES class as seen from 1.a.u. STIX data provided the opportunity for a detailed analysis of hard X-ray emission in several energy bands including the light curves, reconstructed hard X-ray images and spectra. The differential emission measure diagnostics of the flaring plasma have been carried out based on interpretation of the XSM X-ray spectra. Using the differential evolution (DE) approach we have determined the “full” model of emitting source including the temperature, emission measure and elemental abundances as determined simultaneously throughout the flare. We discuss patterns of elemental composition history for individual plasma temperature components.

Using methods of differential evolution (DE), we determined the coronal elemental abundances and the differential emission measure (DEM) distributions for the plasma flaring on 2003 January 21. The analyses have been made based on RESIK X-ray spectra. DE belongs to the family of evolutionary algorithms. DE is conceptually simple and easy to implement, so it has been applied to solve many problems in science and engineering. In this study we apply this method in a new context: simultaneous determination of plasma composition and DEM. In order to increase the confidence of the results obtained using DE, we tested the use of its algorithms by comparing the DE synthesized with respective spectra observed by RESIK. Extensive discussion of the DE method used and the obtained physical characteristics of flaring plasma is presented.

The calcium abundance in flare plasmas is estimated using X-ray spectra from the Solar Maximum Mission Bent Crystal Spectrometer (BCS) during the decays of 194 flares (Geostationary Operational Environmental Satellite, GOES, classifications from B6.4 to X13) occurring between 1980 and 1989. Previous work by Sylwester et al. found that the abundance varied from flare to flare. That analysis is improved on here using updated instrument parameters and by including all calcium lines viewed by the BCS instead of only the resonance line, so greatly enhancing the photon count statistics. The abundance variations are confirmed with the average abundance, A(Ca) (expressed logarithmically with A(H) = 12), equal to 6.77 ± 0.20 for 194 flares (141 of which are new in this study). This range corresponds to factors of between 1.7 and 7.2 larger than the photospheric abundance, and so our results are in line with a “first ionization potential” (FIP) effect whereby low-FIP elements like Ca (FIP = 6.11 eV) have enhanced coronal abundances. The Ca flare abundance is uncorrelated with solar activity indices, but weak correlations are suggested with GOES flare class and duration (larger A(Ca) for smaller and shorter flares). The ponderomotive force theory of Laming explaining the FIP effect gives a range of parameters within which our estimates of A(Ca) agree with the theory. However, this then gives rise to disagreements with previous estimates of the flare silicon and sulfur abundances, although those of argon and iron are in good agreement. Small adjustments of the theory may thus be necessary.

The Polish X-ray spectrometer RESIK observed the spectra in four wavelength bands from 3.3 A to 6.1 A. This spectral range contains many emission lines of H- and He-like ions for Si, S, Ar and K formed in the high temperature of solar coronal plasma. Analysis of measured spectra gives a possibility to study the differential emission measure distributions (DEM) in the temperature range between 1 MK and 30 MK. We present the analysis of DEM distributions of the multi-peaked C9.8 flare observed by RESIK on 9 January 2003 adopting the model of elementary flare profile (EFP). The model allows to distinguish the individual flare components based on the observed light curves in selected spectral bands.

The Bent Crystal Spectrometer (BCS) on the NASA Solar Maximum Mission spacecraft observed the X-ray spectra of numerous solar flares during the periods 1980 February to November and 1984~--~1989. The instrument, the first of its kind to use curved crystal technology, observed the resonance lines of He-like Ca (\caxix) and Fe (\fexxv) and neighboring satellite lines, allowing the study of the rapid evolution of flare plasma temperature, turbulence, mass motions etc. To date there has not been a solar X-ray spectrometer with comparable spectral and time resolution, while subsequent solar cycles have delivered far fewer and less intense flares. The BCS data archive thus offers an unparalleled resource for flare studies. A recent re-assessment of the BCS calibration and its operations is extended here by using data during a spacecraft scan in the course of a flare on 1980 November~6 that highlights small deformations in the crystal curvature of the important channel~1 (viewing lines of \caxix\ and satellites). The results explain long-standing anomalies in spectral line ratios which have been widely discussed in the past. We also provide an in-flight estimation of the BCS collimator field of view which improves the absolute intensity calibration of the BCS. The BCS channel~1 background is shown to be entirely due to solar continuum radiation, confirming earlier analyses implying a time-variable flare abundance of Ca. We suggest that BCS high-resolution \caxix\ and \fexxv\ line spectra be used as templates for the analysis of X-ray spectra of non-solar sources., Comment: To be published, Astrophysical Journal

In 2009, the Russian Complex Orbital Observations Near-Earth of Activity of the Sun (CORONAS-Photon) spacecraft was launched, carrying the Polish Solar PHotometer In X-rays (SphinX). The SphinX was most sensitive in the spectral range 1.2 – 15 keV, thus an excellent opportunity appeared for comparison with the low-energy end of Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spectra. Common spectral measurements with these instruments cover the range where most of the flare energy is accumulated. We have chosen four consecutive small solar events observed on 4 July 2009 at 13:43 UT, 13:48 UT, 13:52 UT, and 13:55 UT (RHESSI flare peak times) and used them to compare the data and results from the two instruments. Moreover, we included Geostationary Operational Environmental Satellite (GOES) records in our analysis. In practice, the range of comparison performed for SphinX and RHESSI is limited roughly to 3 – 6 keV. RHESSI fluxes measured with a use of one, four, and nine detectors in the 3 – 4 keV energy band agree with SphinX measurements. However, we observed that SphinX spectral irradiances are three times higher than those of RHESSI in the 4 – 6 keV energy band. This effect contributes to the difference in obtained emission measures, but the derived temperatures of plasma components are similar. RHESSI spectra were fitted using a model with two thermal components. We have found that the RHESSI hot component is in agreement with GOES, and the RHESSI hotter component fits the SphinX flaring component well. Moreover, we calculated the so-called thermodynamic measure and the total thermal energy content in the four microflares that we studied. The results obtained show that SphinX is a very sensitive complementary observatory for RHESSI and GOES.

We propose a new model for description of solar flare lightcurve profile observed in soft X-rays. The method assumes that single-peaked ‘regular’ flares seen in lightcurves can be fitted with the elementary time profile being a convolution of Gaussian and exponential functions. More complex, multi-peaked flares can be decomposed as a sum of elementary profiles. During flare lightcurve fitting process a linear background is determined as well. In our study we allow the background shape over the event to change linearly with time. Presented approach originally was dedicated to the soft X-ray small flares recorded by Polish spectrophotometer SphinX during the phase of very deep solar minimum of activity, between 23rd and 24th Solar Cycles. However, the method can and will be used to interpret the lightcurves as obtained by the other soft X-ray broad-band spectrometers at the time of both low and higher solar activity level. In the paper we introduce the model and present examples of fits to SphinX and GOES 1-8 Å channel observations as well.

A new methodology is given to determine basic parameters of flares from their X-ray light curves. Algorithms are developed from the analysis of small X-ray flares occurring during the deep solar minimum of 2009, between Solar Cycles 23 and 24, observed by the Polish Solar Photometer in X-rays (SphinX) on the Complex Orbital Observations Near-Earth of Activity of the Sun-Photon (CORONAS-Photon) spacecraft. One is a semi-automatic flare detection procedure that gives start, peak, and end times for single (“elementary”) flare events under the assumption that the light curve is a simple convolution of a Gaussian and exponential decay functions. More complex flares with multiple peaks can generally be described by a sum of such elementary flares. Flare time profiles in the two energy ranges of SphinX (1.16 – 1.51 keV, 1.51 – 15 keV) are used to derive temperature and emission measure as a function of time during each flare. The result is a comprehensive catalogue – the SphinX Flare Catalogue – which contains 1600 flares or flare-like events and is made available for general use. The methods described here can be applied to observations made by Geosynchronous Operational Environmental Satellites (GOES), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and other broad-band spectrometers.

We present results of multitemperature analysis of GOES C7.2 class flare SOL2003-03-29T10:15. This event occurred close to the centre of the solar disk and had two maxima in soft X-rays. We have performed analysis of physical parameters characterizing evolution of conditions in the flaring plasma. The temperature diagnostics have been carried out using the differential emission measure (DEM) approach based on the soft X-ray spectra collected by RESIK Bragg spectrometer. Analysis of data obtained by RHESSI provided opportunity to estimate the volume and thus calculating the density and thermal energy content of hot flaring plasma.

Solar Photometer in X-rays (SphinX) is an instrument designed to observe the Sun in X-rays in the energy range 0.85–15.00 keV. SphinX is incorporated within the Russian TESIS X and EUV telescope complex aboard the CORONAS-Photon satellite which was launched on January 30, 2009 at 13:30 UT from the Plesetsk Cosmodrome, northern Russia. Since February, 2009 SphinX has been measuring solar X-ray radiation nearly continuously. The principle of SphinX operation and the content of the instrument data archives is studied. Issues related to dissemination of SphinX calibration, data, repository mirrors locations, types of data and metadata are discussed. Variability of soft X-ray solar flux is studied using data collected by SphinX over entire mission duration.

The differential emission measure (DEM) describes the temperature distribution of the emitting plasma. The DEM distribution allows one to study the physical conditions and the energy of flares in detail (including the mean temperature and the total emission measure). In this paper, we analyze the time changes of the DEM distributions for a selected flare, which has been observed with the RESIK instrument. To calculate the differential emission measure, we used the Withbroe-Sylwester (W-S) iterative algorithm corresponding to the maximum likelihood procedure. The required emission functions were calculated with the CHIANTI package. We calculated the DEM for four available estimates of the ionization equilibrium and coronal composition of plasma.

The RESIK instrument is an X-ray spectrometer with bent crystals onboard the CORONAS-F sat- ellite. It was used to observe the spectra of solar flares, active regions, and quiet corona. During the period of the instrument's operation, many spectra were collected in four energy channels covering the wavelength range from 3.2 to 6.1 A. For the present analysis, we selected solar flares of various X-ray classes (B, C, and M in the GOES notation), which were observed during moderate level of solar activity (from January to March 2003). The analysis of the RESIK spectra fulfilled with different techniques allowed us to determine the temperature, emission measure, and temperature distribution of the differential emission measure, as well as to examine their time variability.

RESIK is the X-ray bent crystal spectrometer on the CORONAS-F satellite. Between 2002 and 2003, RESIK collected numerous spectra of active regions and flares in the wavelength range from 3.37 to 6.09 A. This range includes many strong emission lines due to transitions 1s 2 –1s np and 1s–np, in He-like and H-like ions, respectively; the n = 2 and 3 lines are routinely observed for Si, S and Ar ions. For some flares RESIK has observed enhanced emission in spectral features coinciding with lines due to transitions for n up to 9 or 10. Identifications of these features, not previously observed in astrophysical spectra, are presented in this paper. Their observed intensities are compared with those from theory.

We have investigated problems concerning the discrepancy between estimates of absolute calcium abundances (A Ca ) in solar flares as derived from soft X-ray high resolution spectra as obtained from Yohkoh and Solar Maximum Mission ( SMM ) Bragg spectrometers. The analysis of these spectra allows for determination of absolute calcium abundance since both Ca xix resonance line and neighbouring continuum are reliably measured in these experiments. The abundance determinations are possible by means of fitting the synthetic spectra to the observed ones. In previous work, we attributed part (around 50%) of the observed disagreement to a possible instrumental problem in Yohkoh . In the present research, we consider in more details possible causes of the other part of the discrepancy. We first investigate whether the estimates of (A Ca ) depend substantially on the extension of the observed short wavelength wing of the Ca xix resonance line. This problem is important in the case of the analysis of Yohkoh spectra, since the extension of the spectrum towards the short wavelengths depends on the position of the flare along North—South direction on the Sun. After verifying the robustness of results of the spectral fitting technique, we have considered possible physical scenario which may cause the observed discrepancy. The hypothesis tested was that the Ca abundance in flares is correlated with brightness in the source — namely that it is a few times higher in the centre of the source (flaring kernel) than outside. We have found that postulated model may account for a part of the noted discrepancy as the SMM spectrometer has been equipped with the 6 × 6 arcmin collimator and no collimator has been used on Yohkoh .

Previous estimates of the solar flare abundances of Si, S, Cl, Ar, and K from the RESIK X-ray crystal spectrometer on board the CORONAS-F spacecraft were made on the assumption of isothermal X-ray emission. We investigate the effect on these estimates by relaxing this assumption and instead determining the differential emission measure (DEM) or thermal structure of the emitting plasma by re-analyzing RESIK data for a GOES class M1.0 flare on 2002 November~14 (SOL2002-11-14T22:26) for which there was good data coverage. The analysis method uses a maximum-likelihood (Withbroe--Sylwester) routine for evaluating the DEM. In a first step, called here AbuOpt, an optimized set of abundances of Si, S, Ar, and K is found that is consistent with the observed spectra. With these abundances, the differential emission measure evolution during the flare is found. The abundance optimization leads to revised abundances of silicon and sulfur in the flare plasma: $A({\rm S}) = 6.94 \pm 0.06$ and $A({\rm Si}) = 7.56 \pm 0.08$ (on a logarithmic scale with $A({\rm H}) = 12$). Previously determined abundances of Ar, K, and Cl from an isothermal assumption are still the preferred values. During the flare's maximum phase, the X-ray-emitting plasma has a basically two-temperature structure, with the cooler plasma with approximately constant temperature (3--6~MK) and a hotter plasma with temperature $16-21$~MK. Using imaging data from the RHESSI hard X-ray spacecraft, the emission volume of the hot plasma is deduced from which lower limits of the electron density $N_e$ and the thermal content of the plasma are given., 7 figures

In February 2009, during recent deepest solar minimum, Polish Solar Photometer in X-rays (SphinX) begun observations of the Sun in the energy range of 1.2–15 keV. SphinX was almost 100 times more sensitive than GOES X-ray Sensors. The silicon PIN diode detectors used in the experiment were carefully calibrated on the ground using Synchrotron Radiation Source BESSY II. The SphinX energy range overlaps with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) energy range. The instrument provided us with observations of hundreds of very small flares and X-ray brightenings. We have chosen a group of solar flares observed simultaneously with GOES, SphinX and RHESSI and performed spectroscopic analysis of observations wherever possible. The analysis of thermal part of the spectra showed that SphinX is a very sensitive complementary observatory for RHESSI and GOES.