Adiabatic heating in impulsive solar flares
The dynamic X-ray spectra from 28 to 254 keV of two simple, impulsive solar flares are examined together with H..cap alpha.., microwave, and meter-wave radio observations. When the effects of the photospheric albedo are taken into account, the X-ray spectra of both events are characteristic of thermal bremsstrahlung from single-temperature plasmas with 10 keV< or =T< or =60 keV. The symmetry between rise and fall, reported by Crannell et al., is found to hold not only for the intensity but also for the temperature and emission measure. The relationship between temperature and emission measure is that of an adiabatic compression followed by adiabatic expansion; the adiabatic index of 5/3 indicates that the electron distribution remains isotropic. Observations in H..cap alpha.. provide further evidence for compressive energy transfer, in that both flares show explosive expansion after the end of the intense x-ray spikes. Under the assumption that the X-ray and microwave emissions are produced in the same thermal plasma, the projected areas of the two flares are determined, and the volumes are estimated. Emission measures and areas for these events are found to be n/sub e//sup 2/Vapprox. =10/sup 45/cm/sup -3/ and Aapprox. =10/sup 18/cm/sup 2/, respectively. In the case of homogeneous, spherically symmetric sources, densities of n/sub e/approx. =10/sup 9/cm/sup -3/ are obtained. The strong self-absorption of gyrosynchrotron radiation in a hot plasma is indicative of relatively low magnetic field strengths Bapprox. =100 gauss. The resulting high-beta plasmas inside the source regions are shown to be consistent with the single-temperature hypothesis. The nonthermal end phase observed in one event can be explained by the ejection of hot flare plasma or by a second-stage acceleration process.
- Research Organization:
- NASA Goddard Space Flight Center, Greenbelt, Maryland
- OSTI ID:
- 6651181
- Journal Information:
- Astrophys. J.; (United States), Vol. 223:3
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GENERAL PHYSICS
SOLAR FLARES
ADIABATIC PROCESSES
SOLAR RADIATION
ENERGY TRANSFER
COMPRESSION
EXPANSION
PLASMA
SOLAR RADIOWAVE RADIATION
SOLAR X-RAY BURSTS
X-RAY SPECTRA
ELECTROMAGNETIC RADIATION
RADIATIONS
RADIOWAVE RADIATION
SOLAR ACTIVITY
SPECTRA
640104* - Astrophysics & Cosmology- Solar Phenomena