Energy-Dependent Timing of Thermal Emission in Solar Flares

We report solar flare plasma to be multi-thermal in nature based on the theoretical model and study of the energy-dependent timing of thermal emission in ten M-class flares. We employ high-resolution X-ray spectra observed by the Si detector of the "Solar X-ray Spectrometer" (SOXS). The SOXS onboard the Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May 2003. Firstly we model the spectral evolution of the X-ray line and continuum emission flux F(\epsilon) from the flare by integrating a series of isothermal plasma flux. We find that multi-temperature integrated flux F(\epsilon) is a power-law function of \epsilon with a spectral index (\gamma) \approx -4.65. Next, based on spectral-temporal evolution of the flares we find that the emission in the energy range E= 4 - 15 keV is dominated by temperatures of T= 12 - 50 MK, while the multi-thermal power-law DEM index (\gamma) varies in the range of -4.4 and -5.7. The temporal evolution of the X-ray flux F(\epsilon,t) assuming a multi-temperature plasma governed by thermal conduction cooling reveals that the temperature-dependent cooling time varies between 296 and 4640 s and the electron density (n_e) varies in the range of n_e= (1.77-29.3)*10^10 cm-3. Employing temporal evolution technique in the current study as an alternative method for separating thermal from non-thermal components in the energy spectra, we measure the break-energy point ranging between 14 and 21\pm1.0 keV.Comment: Solar Physics, in pres

Science from "Solar X-ray spectrometer (SOXS)" - proposed payload onboard Indian satellite

It is proposed to fly a high spectral and temporal resolution "Solar X-ray Spectrometer (SOXS)" onboard Indian satellite to understand the mechanisms of energy release and particle acceleration in solar flares. The SOXS will provide the disk integrated flux in the energy range 2 keV-10 MeV. The proposed SOXS will consist of two detector modules - SOXS Low Energy Detector (SLD) and SOXS High Energy Detector (SHD). The proposed instrument will enable us to measure precisely the low energy cut-off below 60 keV to estimate the total energy release in the flare. It is proposed that high spectral and temporal resolution efficiencies of our detectors will reveal, perhaps for the first time, the observed break below 60 keV in the characteristic double power-law shape of hard X-ray spectrum. Whether electrons and protons are accelerated simultaneously may be also answered by correlating high temporal spectra of SLD and SHD. The high temporal and sub-keV resolution spectra from SLD will be capable to investigate the nature of micro/nano flares considered responsible to heat the chromosphere and corona. It is proposed to use the observations from this space borne instrument, along with extensive simultaneous ground based high spatial and time resolution observations in optical and radio wavebands for better understanding of the flare phenomena

Spectropolarimetery of umbral fine structures from Hinode: Evidence for magnetoconvection

We present spectropolarimetric analysis of umbral dots and a light bridge fragment that show dark lanes in G-band images. Umbral dots show upflow as well as associated positive Stokes V area asymmetry in their central parts. Larger umbral dots show down flow patches in their surrounding parts that are associated with negative Stokes V area asymmetry. Umbral dots show weaker magnetic field in central part and higher magnetic field in peripheral area. Umbral fine structures are much better visible in total circularly polarized light than in continuum intensity. Umbral dots show a temperature deficit above dark lanes. The magnetic field inclination show a cusp structure above umbral dots and a light bridge fragment. We compare our observational findings with 3D magnetohydrodynamic simulations.Comment: Accepted for publication in MNRAS, 6 pages, 6 figure

Probing the Role of Magnetic-Field Variations in NOAA AR 8038 in Producing Solar Flare and CME on 12 May 1997

We carried out a multi-wavelength study of a CME and a medium-size 1B/C1.3 flare occurring on 12 May 1997. We present the investigation of magnetic-field variations in the NOAA Active Region 8038 which was observed on the Sun during 7--16 May 1997. Analyses of H{\alpha} filtergrams and MDI/SOHO magnetograms revealed continual but discrete surge activity, and emergence and cancellation of flux in this active region. The movie of these magnetograms revealed two important results that the major opposite polarities of pre-existing region as well as in the emerging flux region (EFR) were approaching towards each other and moving magnetic features (MMF) were ejecting out from the major north polarity at a quasi-periodicity of about ten hrs during 10--13 May 1997. These activities were probably caused by the magnetic reconnection in the lower atmosphere driven by photospheric convergence motions, which were evident in magnetograms. The magnetic field variations such as flux, gradient, and sunspot rotation revealed that free energy was slowly being stored in the corona. The slow low-layer magnetic reconnection may be responsible for this storage and the formation of a sigmoidal core field or a flux rope leading to the eventual eruption. The occurrence of EUV brightenings in the sigmoidal core field prior to the rise of a flux rope suggests that the eruption was triggered by the inner tether-cutting reconnection, but not the external breakout reconnection. An impulsive acceleration revealed from fast separation of the H{\alpha} ribbons of the first 150 seconds suggests the CME accelerated in the inner corona, which is consistent with the temporal profile of the reconnection electric field. In conclusion, we propose a qualitative model in view of framework of a solar eruption involving, mass ejections, filament eruption, CME, and subsequent flare.Comment: 8 figures, accepted for publication in Solar Physic

Multi-wavelength observations of an unusual impulsive flare associated with CME

We present the results of a detailed analysis of multi-wavelength observations of a very impulsive solar flare 1B/M6.7, which occurred on 10 March, 2001 in NOAA AR 9368 (N27 W42). The observations show that the flare is very impulsive with very hard spectrum in HXR that reveal non-thermal emission was most dominant. On the other hand this flare also produced type II radio burst and coronal mass ejections (CME), which are not general characteristics for impulsive flares. In H$\alpha$ we observed the bright mass ejecta (BME) followed by drak mass ejecta (DME). Based on the consistence of the onset times and direction of BME and CME, we conclude that these two phenomena are closely associated. It is inferred that the energy build-up took place due to photospheric reconnection between emerging positive parasitic polarity and predominant negative polarity, which resulted as a consequence of flux cancellation. The shear increased to $>80^o$ due to further emergence of positive parasitic polarity causing strongly enhanced cancellation of flux. It appears that such enhanced magnetic flux cancellation in a strongly sheared region triggered the impulsive flare.Comment: 14 pages, 8 Figures, Accepted for the publication in Solar Physic