Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980a%26a....92..260w&link_type=abstract
Astronomy and Astrophysics, Vol. 92, P. 260, 1980
Astronomy and Astrophysics
Astrophysics
14
Scientific paper
Nine simple impulsive microwave bursts were investigated for time delays in their times of maximum emission at high and low microwave frequencies. In seven of them the high frequency time profile preceded the low frequency time profile by 8-49 s. One event (1972 May 18 at 16:17 UT) which was well observed in the frequency range 1.4-15.4 GHz (time resolution ≍ 1 s) and in the hard X-ray energy range 28-102 keV (time resolution = 1.2 s) was investigated in detail:
A strong anticorrelation between emission measure EM and temperature T (EM ∝ T-3.2) was found which indicates an expanding burst source. The 1.4 GHz burst maximum is delayed by 9 s relative to the 15.4 GHz maximum. The times of maximum of the hard X-rays seem to coincide with the times of maximum of the high frequency microwave bursts but precede the low frequency microwave burst maxima. The time delays are interpreted as evidence for a collisionless conduction front (Brown et al., 1979) confining the hot plasma (T ≍ 108 °K) and propagating at the ion- sound speed from lower to higher levels in a flux tube. Using the fact that the microwave radiation is severely attenuated by free- free absorption and that the electron density decreases towards the observer the heights of the emitting volumes for each frequency were calculated. From the differences of the emission heights the time delays could be derived by assuming a mean ion-sound speed of 900 km s-1. The frequency dependent rise time of the microwave bursts is given by the time it takes the conduction front to travel through the emitting layer. For typical solar burst plasmas the radiation is strongly self-absorbed by the gyrosynchrotron process inside the hot source. This enables us to estimate the layer thickness for each frequency and thus the rise time.
Magun Andreas
Schochlin W. A.
Wiehl H. J.
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