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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 Spectrometer Telescope for Imaging X-rays (STIX) is one of the instruments installed onboard Solar Orbiter mission which will be launched in February 2020. After 1.5 years of cruise phase it will start to gather scientific data from the orbit with perihelion distance about 0.28 au. It means that STIX will operate also during the next solar minimum. In the paper we estimate flux measured by the instrument during periods of low solar activity. For this purpose we used solar observations which were recorded by the Solar Photometer in X-rays (SphinX) during the last minimum of solar activity. The estimation was obtained for instruments overlapping energy range from 4 to 15 keV. Presented results indicate that STIX instrument will provide efficient imaging the solar emission even during low level of solar activity (B1 GOES class).

The SphinX X-ray spectrophotometer on the CORONAS-PHOTON mission observed the 1 – 15 keV X-ray spectrum of the spatially integrated solar corona during the deep minimum of 2009, when solar activity was exceptionally low. Its sensitivity for energies $> 1.2~\mbox{keV}$ was higher than that of any other solar X-ray spectrometer in orbit at the time, including the detectors on the Geostationary Operational Environmental Satellites (GOES). Using much improved instrumental data than was used previously, we analysed SphinX spectra in 576 intervals for which there was no discernible activity (NA), 40 intervals when there were X-ray brightenings (B), and 16 intervals when there were micro-flares with peak emission less than GOES A1 (F). An instrumental background spectrum, formed over 34 hours of spacecraft night-time periods and including electronic noise and particle radiation, was subtracted from the solar spectra. Theoretical spectra were used to deduce temperatures on an isothermal assumption for the NA, B, and F intervals (1.69, 1.81, and 1.86 MK, respectively). Differential emission measure (DEM) analysis for the same spectra revealed a “cooler” component ($\log T = 6.2$ or $T \approx 1.6~\mbox{MK}$) in each case, but with a second hotter component having a less well-defined peak temperature varying from $\approx 2.5$ to $\approx 3.5~\mbox{MK}$ ($\log T = 6.4$ and 6.55) and an emission measure between two and three orders smaller than that of the cooler component. These results are similar to those obtained at times just after solar minimum with the EVE instrument. A very hot component that might indicate the signature of nano-flare heating of the corona is not evident in SphinX data.

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.

The SphinX X-ray spectrophotometer on the CORONAS-PHOTON spacecraft measured soft X-ray emission in the 1-15 keV energy range during the deep solar minimum of 2009 with a sensitivity much greater than GOES. Several intervals are identified when the X-ray flux was exceptionally low, and the flux and solar X-ray luminosity are estimated. Spectral fits to the emission at these times give temperatures of 1.7-1.9 MK and emission measures between 4 x 10^47 cm^-3 and 1.1 x 10^48 cm^-3. Comparing SphinX emission with that from the Hinode X-ray Telescope, we deduce that most of the emission is from general coronal structures rather than confined features like bright points. For one of 27 intervals of exceptionally low activity identified in the SphinX data, the Sun's X-ray luminosity in an energy range roughly extrapolated to that of ROSAT (0.1-2.4 keV) was less than most nearby K and M dwarfs., Astrophysical Journal, in press. 14 pp, 3 figures