The mechanism that generates sporadic (nonthermal) solar radio emission, which is of great interest to radio astronomy and sclar physics, is rather obscure. Recently the authors considered this problem [1,2,3] and this report gives their summary.It is convenient to classify the mechanisms as (a) incoherent or coherent, or (b) involving an isotropic plasma or a magnetoactive one. In addition, it is convenient to consider first the conditions of propagation in, and emission of electromagnetic waves from, the solar corona. In the isotropic coronal plasma, electron streams generate only plasma waves; however, we are interested in transverse waves escaping from the corona. In a homogeneous plasma, the transformation of plasma waves into trans verse, e.g. radio, waves takes place only owing to scattering on fluctuations of the electron concentration 8N = dN' -f dN"'. Here dN' represents 574 available at https://www.cambridge.org/core/terms. https://doi PROPAGATION AND EMISSION OF ELECTROMAGNETIC WAVES

We have searched for X-ray emission from a sample of five nearby (6-20 pc), strongly magnetic (10-200 MG), relatively cool (6000-14000 K), single white dwarfs, two of which may possess coronae. We detect one star (GR 290) at better than 99% confidence and give upper limits from Einstein Observatory IPG data for four others. The detected luminosities and limits are in the range 1.4-12.5 X 10 27 ergs s _1 .

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