fl127.gopalswamy03 Posted: 06-Jun-93 Updated: 26-Dec-93, 17-Sep-94, 29-Jun-95 Events specified: Flare on 22-Apr-93 at 14:05
N. Gopalswamy, M. R. Kundu, J. R. Lemen, N. Nitta, K. T. Strong and anyone who is interested in X-ray - radio comparisons is welcome
The M1.5 flare of 22 April, 1993 at 14:05 UT (optical importance SB) was fully observed by the Very Large array (VLA) in the B configuration at 333 MHz and 1.4 GHz. The flare was quite impulsive with a sharp rise and a simple exponential decay. The microwave spectrum seems to peak around 10 GHz. The VLA images of the flare at 1.4 GHz show that the flare emission starts as a single source and becomes double as the flare progresses; the latter source (in the N direction) becomes dominant throughout the flare. This source is very compact, with a FWHM of only 26" at 1.4 GHz.
The SXT images taken in the early phase show the existence of two loops aligned in the E-W direction and several discrete bright points suggestive of both footpoints and tops of the loops. Later the eastern loop became dominant. There is only one PFI image in quiet mode prior to the transition to flare mode, but the VLA observation started well before the onset, and the initial brightening at 1.4 GHz may be more directly associated with the primary enrgy release than the X-ray brightening. We will compare the temporal and spatial characteristics of the flare in radio and X-ray wavelength domains.
Update 29-Jun-95
This project has been completed with a paper that will appear in ApJ (Dec 1995). The abstract follows.
VLA and Yohkoh Observations of an M1.5 Flare
N. Gopalswamy, J.-P. Raulin, M. R. Kundu, N. Nitta, J. R. Lemen, R. Hermann, D. Zarro and T. Kosugi
A major solar flare (X-ray importance M1.5 and optical importance SB) was fully observed by the Very Large Array and the Yohkoh mission on 1993 April 22. Both thermal and nonthermal emissions were observed in radio. In soft X-rays, the flare was confined to a compact region in an arcade. In hard X-rays, there were two prominent footpoints, coincident in projection with the soft X-ray footpoints and located on either side of the magnetic neutral line inferred from photospheric magnetograms. The Yohkoh BCS data provided important context information which was helpful in cross-checking the qualitative agreement between radio and X-ray data. The microwave spectrum peaked around 10 GHz and showed Razin suppression in the beginning. Later on, the low frequency spectral index dropped to a value of 2 suggesting thermal emission. The VLA images of the flare at 1.5 GHz show that the flare emission started as a single source above one footpoint; later on, the emission centroid moved towards the soft X-ray structure to finally become co-spatial with the latter. The two locations of the 20 cm source corresponded to nonthermal (footpoint source) and thermal (source co-spatial with the soft X-ray structure) emissions. We performed temperature and emission measure analysis of the X-ray data (SXT, BCS and HXT) and used them as input to determine the expected radio emission. While there is morphological agreement between the radio and soft X-ray structures in the thermal phase, the 20 cm brightness temperature shows quantitative agreement with temperature derived from the BCS data. We were able to identify three emission mechanisms contributing to the 20 cm radio emission at different times without any ad hoc assumption regarding emission mechanisms. Razin-suppressed nonthermal gyroresonance emission, plasma emission and thermal free-free emission seem to be operating and are found to be consistent with the plasma parameters derived from the X-ray data. The magnetic field structure in the flaring region showed differences before and after the flare as traced by soft X-ray structures in the flaring region and confirmed by 20 cm radio images. The superhot component with a temperature of 32 MK was observed in hard X-ray images and in light curves during the impulsive phase of the flare with possible radio signatures at 20 cm wavelength. We derived the physical parameters of the flaring plasma, the magnetic field and the characteristics of nonthermal particles in the flaring region.
Update 17-Sep-94
Analysis of this project has been completed recently and we are preapring a paper publication in a refereed journal. We presented preliminary results of this project recently in the AAS-SPD/AGU meeting (May 1994) at Baltimore, USA. We obtained some very interesting results thanks to the combination of VLA, HXT and SXT data. Notable results are the radio observations of chromospheric evaporation and shift in the hard x-ray emitting footpoint. Once a complete draft is created, we will mail a summary of the paper.
Update 26-Dec-93
Add Jean-Pierre Raulin (UMD) in the collaboration.
Based on a study of soft X-ray light curves, we have found the connectivities of soft X-ray patches. There are at least three loops involved in the flare. Hard X-ray double sources are likely to represent footpoints of one of the loops. The VLA 20 cm source sits on one of the patches in the initial phase, but it moves to east. Soft X-ray emission is very weak at the utltimate 20 cm source centroid. Comparing the 20 cm light curves with RSTN data, we confirmed that missing flux at the VLA 20 cm source is negligible, and that the optically thick part of the spectrum has a very steep slope (alpha ~ 8). More detailed analysis is under way.