M. Siarkowski

Corona(e) of AR Lacertae. II. The spatial structure

The X-ray light curves in the 0.4-1.5 keV and 2-7 keV bands of the RS CVn binary AR Lacertae observed on 1993 June 1-3 over one full orbital cycle with the ASCA satellite have been used to map the spatial structure of AR Lacs coronae. We find that both stars are X-ray active, that the corona of the K-type secondary star appears to be hotter than that of the G-type primary star, that X-ray emission is concentrated on the sides of the stars facing each other, and that there are compact and well-localized regions of enhanced X-ray emission with heights much smaller than the stellar radii. In one class of solutions there are additional extended regions with dimensions similar to or greater then the radii of the underlying stars which may be structures that interconnect the two stars. There are also other acceptable models without extended structures, however our analysis indicates that solutions with extended sources are more probable. Also, about 50% of the X-ray emission is unmodulated and could come from either an extended halo region, from the poles of the larger K star, or from other symmetric or uneclipsed structures in the orbital plane. We compare the coronal structures inferred from the ASCA observations with those inferred using the same technique from an EXOSAT observation of AR Lac made in 1984 and find that there are substantial differences between the coronal structures at these two epochs. For the solution with extended material in the orbital plane, we have derived the rough physical parameters for the X-ray-emitting plasma, using the derived information on the spatial sizes of the various spatial components together with information about the emission measure and temperatures obtained from a simple spectral analysis of the ASCA data.

FLARES AND THEIR UNDERLYING MAGNETIC COMPLEXITY

SphinX (Solar PHotometer IN X-rays), a full-disk-integrated spectrometer, observed 137 flare-like/transient events with active region (AR) 11024 being the only AR on disk. The Hinode X-Ray Telescope (XRT) and Solar Optical Telescope observe 67 of these events and identified their location from 12:00 UT on July 3 through 24:00 UT 2009 July 7. We find that the predominant mechanisms for flares observed by XRT are (1) flux cancellation and (2) the shearing of underlying magnetic elements. Point- and cusp-like flare morphologies seen by XRT all occur in a magnetic environment where one polarity is impeded by the opposite polarity and vice versa, forcing the flux cancellation process. The shearing is either caused by flux emergence at the center of the AR and separation of polarities along a neutral line or by individual magnetic elements having a rotational motion. Both mechanisms are observed to contribute to single- and multiple-loop flares. We observe that most loop flares occur along a large portion of a polarity inversion line. Point- and cusp-like flares become more infrequent as the AR becomes organized with separation of the positive and negative polarities. SphinX, which allows us to identify when these flares occur, provides us with a statistically significant temperature and emission scaling law for A and B class flares: EM = 6.1 × 1033 T 1.9±0.1.

PLASMA HEATING IN THE VERY EARLY PHASE OF SOLAR FLARES

In this Letter, we analyze soft X-ray (SXR) and hard X-ray (HXR) emission of the 2002 September 20 M1.8 GOES class solar flare observed by the RHESSI and GOES satellites. In this flare event, SXR emission precedes the onset of the main bulk HXR emission by approx5 minutes. This suggests that an additional heating mechanism may be at work at the early beginning of the flare. However, RHESSI spectra indicate a presence of the non-thermal electrons also before the impulsive phase. So, we assumed that a dominant energy transport mechanism during the rise phase of solar flares is electron-beam-driven evaporation. We used non-thermal electron beams derived from RHESSI spectra as the heating source in a hydrodynamic model of the analyzed flare. We showed that energy delivered by non-thermal electron beams is sufficient to heat the flare loop to temperatures in which it emits SXR closely following the GOES 1-8 A light curve. We also analyze the number of non-thermal electrons, the low-energy cutoff, electron spectral indices, and the changes of these parameters with time.

Relationship between non-thermal electron energy spectra and GOES classes

Aims. We investigate influence of variations in the energy spectrum of non-thermal electrons on the resulting GOES classes of solar flares. Methods. Twelve observed flares with various soft-to-hard X-ray emission ratios were modeled using different non-thermal electron energy distributions. Initial values of the flare physical parameters including geometrical properties were estimated using observations. Results. We found that, for a fixed total energy of non-thermal electrons in a flare, the resulting GOES class of the flare can change significantly by varying the spectral index and low energy cut-off of the non-thermal electron distribution. Thus, the GOES class of a flare depends not only on the total non-thermal electrons energy but also on the electron beam parameters. For example, we were able to convert a M2.7 class solar flare into a merely C1.4 class one and a B8.1 class event into a C2.6 class flare. The results of our work also suggest that the level of correlation between the cumulative time integral of HXR and SXR fluxes can depend on the considered HXR energy range.

SphinX Measurements of the 2009 Solar Minimum X-Ray Emission

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 × 1047 cm–3 and 1.1 × 1048 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 Suns 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.

PLASMA HEATING IN THE VERY EARLY AND DECAY PHASES OF SOLAR FLARES

In this paper, we analyze the energy budgets of two single-loop solar flares under the assumption that non-thermal electrons (NTEs) are the only source of plasma heating during all phases of both events. The flares were observed by RHESSI and GOES on 2002 September 20 and 2002 March 17, respectively. For both investigated flares we derived the energy fluxes contained in NTE beams from the RHESSI observational data constrained by observed GOES light curves. We showed that energy delivered by NTEs was fully sufficient to fulfill the energy budgets of the plasma during the pre-heating and impulsive phases of both flares as well as during the decay phase of one of them. We concluded that in the case of the investigated flares there was no need to use any additional ad hoc heating mechanisms other than heating by NTEs.

Soft X-ray variability over the present minimum of solar activity as observed by SphinX

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.

Temporal variations of the CaXIX spectra in solar flares

Aims. The standard model of solar flares comprises a bulk expansion and a rise of abruptly heated plasma (chromospheric evaporation). Emission from plasma ascending along loops rooted in the visible solar disk often should be dominated, at least temporally, by a blue-shifted emission. However, there is only a very limited number of published observations of solar flares having spectra in which the blue-shifted component dominates the stationary one. In this work we compare observed X-ray spectra of three solar flares recorded during their impulsive phases and relevant synthetic spectra calculated using one-dimensional hydro-dynamic numerical model of these flares. The main aim of the work is to explain why many flares do not show blue-shifted spectra. Methods. We synthetised time series of Bragg Crystal Spectrometer (BCS) spectra of three solar flares at various moments of their evolution from the beginning of the impulsive phases to beyond maxima of the X-ray emission using a 1D numerical model of the solar flares and standard software to calculate BCS synthetic spectra of the flaring plasma. The models of the flares were calculated using observed energy distributions of the non-thermal electron beams injected into the loops, initial values of the main physical parameters of the plasma confined in the loops and geometrical properties of the loops estimated using available observational data. The synthesized BCS spectra of the flares were compared with the relevant observed BCS spectra. Results. Taking into account the geometrical dependences of the line-of-sight velocities of the plasma moving along the flaring loop inclined toward the solar surface as well as a distribution of the investigated flares over the solar disk, we conclude that the stationary component of the spectrum should be observed for almost all flares during their early phases of evolution. On the contrary, the blue-shifted component of the spectrum could not be detected in flares having plasma rising along the flaring loop even with high velocity due to the geometric dependences only. Our simulations based on realistic heating rates of plasma by non-thermal electrons indicate that the upper chromosphere is heated by non-thermal electrons a few seconds before the beginning of noticeable high-velocity bulk motion, and before this time plasma emits the stationary component of the spectrum only. After the start of the upward flow, the blue-shifted component temporally dominates the synthetic spectra of the investigated flares in their early phases.

Determination of differential emission measure from X-ray solar spectra registered by RESIK aboard CORONAS-F

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.

Diagnostic of the temperature and differential emission measure (DEM) based on Hinode /XRT data

We discuss here various methodologies and an optimal strategy of the temperature and emission measure diagnostics based on Hinode X-Ray Telescope data. As an example of our results we present the determination of the temperature distribution of the X-rays emitting plasma using a filters ratio method and three various methods of the calculation of the differential emission measure (DEM). We have found that all these methods give results similar to the two filters ratio method. Additionally, all methods of the DEM calculation gave similar solutions. We can state that the majority of the pairs of the Hinode filters allows one to derive the temperature and emission measure in the isothermal plasma approximation using standard diagnostics based on the two filters ratio method. In cases of strong flares one can also expect good conformity of the results obtained using a Withbroe – Sylwester, genetic algorithm and least-squares methods of the DEM evaluation.