No abstract

In this paper, we study stellar light curves from the Transiting Exoplanet Survey Satellite (TESS) for the presence of stellar flares. The main aim is to detect stellar flares using 2 minute cadence data and to perform a statistical analysis. To find and analyze stellar flares, we prepared the automatic software WARPFINDER. We implemented three methods described in this paper: trend, difference, and profile fitting. Automated searches for flares was accompanied by visual inspection. Using our software we analyzed the 2 minute cadence light curves of 330,000 stars located in the first 39 sectors of TESS observations. As a result, we detected over 25,000 stars showing flare activity with the total number of more than 140,000 flares. This means that about 7.7% of all the analyzed objects are flaring stars. The estimated flare energies range between 1031 and 1036 erg. We prepared a preliminary preview of the statistical distribution of parameters such as the flare duration, amplitude, and energy, and compared it with previous results. The relationship between stellar activity and spectral type, temperature, and mass was also statistically analyzed. Based on the scaling laws, we estimated the average values of the magnetic field strength and length of the flare loops. In our work, we used both single (about 60%), and double (about 40%) flare profiles to fit the observational data. The components of the double profile are supposed to be related to the direct heating of the photosphere by nonthermal electrons and back-warming processes.

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.

In this paper we analyze soft and hard X-ray emission of the 2002 September 20 M1.8 GOES class solar flare observed by RHESSI and GOES satellites. In this flare event, soft X-ray emission precedes the onset of the main bulk hard X-ray emission by ∼5 min. This suggests that an additional heating mechanism may be at work at the early beginning of the flare. However RHESSI spectra indicate presence of the non-thermal electrons also before impulsive phase. So, we assumed that a dominant energy transport mechanism during 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 soft X-ray closely following the GOES 1-8 Å lightcurve. We also analyze the number of non-thermal electrons, the low energy cut-off, electron spectral indices and the changes of these parameters with time.

In this paper, the energy budgets of two single-loop like flares observed in X-ray are analysed 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 February 20 th , 2002 and June 2 nd , 2002, respectively. Using a 1D hydrodynamic code for both flares the energy deposited in the chromosphere was derived applying RHESSI observational data. The use of the Fokker-Planck formalism permits the calculation of distributions of the non-thermal electrons in flaring loops, thus spatial distributions of the X-ray non-thermal emissions and integral fluxes for the selected energy ranges which were compared with the observed ones. Additionally, a comparative analysis of the spatial distributions of the signals in the RHESSI images was conducted for the footpoints and for the entire flare loops in selected energy ranges with these quantities fluxes obtained from the models. The best compatibility of the model and observations was obtained for the June 2 nd , 2002 event in the 0.5-4 Å GOES range and total fluxes in the 6-12 keV, 12-25 keV, 20-25 keV and 50-100 keV energy bands. Results of photometry of the individual flaring structures in a high energy range shows that the best compliance occurred for the June 2 nd , 2002 flare, where the synthesized emissions were 30% or more higher than the observed emissions. For the February 20 th , 2002 flare, synthesized emission is about 4 times lower than the observed one. However, in the low energy range the best conformity was obtained for the February 20 th , 2002 flare, where emission from the model is about 11% lower than the observed one. The larger inconsistency occurs for the June 2 nd , 2002 solar flare, where synthesized emission is about 12 times greater or even more than the observed emission. Some part of these differences may be caused by inevitable flaws of the applied methodology, like by an assumption that the model of the flare is symmetric and there are no differences in the emissions originating from the feet of the flares loop and by relative simplicity of the applied numerical 1D code and procedures. No doubt a significant refinement of the applied numerical models and more sophisticated implementation of the various physical mechanisms involved are required to achieve