ON THE ORIGIN OF NOISE STORM CONTINUUM DEPRESSIONS

During solar flares sometimes there is observed a drop-down of a prefl are noise storm continuum. It reveals a strong causal connection betwe en 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 h ybrid plasma instability at omega(L) = root omega(p)(2) + omega(B)(2) for inducing the noise storm continuum emission. The source should act above the second gyroresonance absorption level: 2 omega(B) < omega(L ). Under such conditions additional heating of the plasma in the radio continuum source due to the flare results in an essential strengtheni ng of the gyroresonance absorption of plasma waves visible as a contin uum depression. Further, plasma heating leads to a certain increase of the frequency of the excited plasma waves. After some time it is larg er than the cut-off frequency omega(L) > omega(cut). Now scattering of plasma waves starts to be possible not only into the ordinary mode (a s before the flare) but also into the extraordinary mode. This explain s the strange decay of the degree of circular polarization after the c ontinuum intensity decrease (cf. Aurass et al. 1990).