Directivity of high-energy emission from solar flares: solar-maximum mission observations
The data base consisting of flares detected by the gamma-ray spectrometer (GRS) on board the Solar Maximum Mission (SMM) satellite is used to study the directivity of high-energy radiation. A number of observations strongly indicate that the high-energy emission from flares is anisotropic. They are the following: the fraction of events detected at energies > 300 keV near the limb is higher than is expected for isotropically emitting flares; there is a statistically significant center-to-limb variation in the 300-keV to 1-MeV spectra of flares detected by the SMM GRS; the 25-200 keV hard x-ray spectra measured during the impulsive phase by the SMM GRS show a center-to-limb variation; and nearly all of the events detected at > 10 MeV are located near the limb. Then these observations are best explained by models in which nonthermal electrons radiate in a thick-target emission region lower in the atmosphere. To produce the observed center-to-limb variations, one needs an electron distribution that has an intensity which increases with angle from the outward normal. Candidate distributions are downwardly directed Gaussian beams and pancake distributions that peak in directions parallel to the photosphere. During the impulsive phase of the flare this nonthermal component seems to be visible down to energies less than 40 keV. These results imply that the procedure normally used to deduce the properties of flare-accelerated electrons from hard x-ray and gamma-ray observations an substantially underestimate the spectral hardness and number of high-energy electrons. Another consequence is that, on the average, disk flares should appear to be richer in nuclear emission than limb flares.
- Research Organization:
- New Hampshire Univ., Durham (USA). Dept. of Physics
- OSTI ID:
- 6575733
- Report Number(s):
- AD-A-199499/5/XAB
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ATMOSPHERES
DISTRIBUTION
ELECTRON SPECTRA
EMISSION
GAMMA SPECTROMETERS
HARDNESS
MEASURING INSTRUMENTS
MECHANICAL PROPERTIES
PHOTON EMISSION
PHOTOSPHERE
SOLAR ACTIVITY
SOLAR ATMOSPHERE
SOLAR FLARES
SPECTRA
SPECTROMETERS