Physics
Scientific paper
Apr 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt..........s&link_type=abstract
PhDT
Physics
1
Electron Cyclotron Maser, Nonthermal-Sun, Radio Radiation-Waves, Gyrosynchrotron, Synchrotron
Scientific paper
The subject of this research is decimetric and microwave emission from solar flares. The research deals with gyrosynchrotron emission from a population of fast electrons. Two frequency ranges are investigated: low multiples (1-3) of the cyclotron frequency nu_B , and high multiples ( 6 nu_B and above) of the cyclotron frequency. In the high frequency range a simple geometrical model of a flare loop is investigated. We show that our simple model reproduces observed microwave fluxes and spectra, and present a fit for the microwave spectrum and time evolution of the flare of June 4th 1991. We conclude that the flare spectrum can be explained with a simple model, and that even the time evolution can be fit by changing a single parameter at a time. We also investigate how the observed flare spectrum changes for different locations of the flare on the sun, a question not dealt with previously. The shape of the spectrum is quantified by the ratio of the flux at 10 GHz to the flux at 4 GHz, and this ratio is shown to change by an order of magnitude as the same flare model is moved from the Limb to the Disk center. In the low frequency range we investigate amplification through the Electron Cyclotron Maser (ECM) mechanism in the range 0.5 < nu_p / nu_B < 2, where nu_p is the plasma frequency. The ECM process is widely assumed to produce solar microwave millisecond spike emission. Most previous studies concentrated on the region nu_p / nu_B<1, and claim that spikes are produced there. We show that maser emission should be produced at the foot-points of the flare, where nu_p / nu_B > 1. Previous studies claim emission at 70 degrees to the magnetic field,or more, and emission at the first or second harmonic. We show that spike emission from the foot-points should appear at angles of 30-70 degrees to the magnetic field, and at frequencies near the second cyclotron harmonic, usually at 2.06 nu_B. We also find that ECM emission depends on the temperature and that the unobservable Z-mode quenchs the maser for temperatures above 10(7) Kelvin. These conclusions can be checked by observations of high spatial resolution, especially if performed simultaneously with observations from which the temperature can be estimated.
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