Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...291..990z&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 291, no. 3, p. 990-1000
Astronomy and Astrophysics
Astrophysics
6
Magnetohydrodynamic Stability, Magnetohydrodynamic Waves, Solar Flares, Solar Radio Emission, Plasma Frequencies, Plasma Radiation, Polarization (Waves), Whistlers
Scientific paper
During solar flares sometimes there is observed a drop-down of a preflare noise storm continuum. It reveals a strong casual connection between the flaring active region and the source region of the noise storm. We use the facts given in a recently published case study (Aurass et al. 1993) to deduce arguments confirming the importance of the upper hybrid plasma instability at omegaL = square root of(omegap exp 2+omegaB exp 2) for inducing the noise storm continuum emission. The source should act above the second gyroresonance absorption level: 2 omegaB is less than omegaL. Under such conditions additional heating of the plasma in the radio continuum source due to the flare results in an essential strengthening of the gyroresonance absorption plasma waves visible as a continuum depression. Further, plasma heating leads to a certain increase of the frequency of the excited plasma waves. After some time it is larger than the cut-off frequency omegaL is greater than omegacut. Now scattering of plasma waves starts to be possible not only into the ordinary mode (as before the flare) but also into the extraordinary mode. This explains the strange decay of the degree of circular polarization after the continuum intensity decrease (ef. Aurass et al. 1990).
Aurass Henry
Kruger Andrew
Mann Gottfried
Zaitsev Valerii V.
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