Studies on hard X-ray emission from solar flares and on cyclotron radiation from a cold magnetoplasma

This thesis proposes an interpretation of hard X-ray emission from solar flares and presents a theoretical study on the generation of cyclotron radiation by a source in a cold magnetically active plasma. Hard X-ray observations by the ESRO TD-1A satellite are analyzed, the solar flares of May 18 and August 4, 1972, are examined in detail, and a set of consistent parameters are derived for the flare of May 18 from observations of hard-X rays and simultaneously emitted centimeter radio waves. A model for the flare of August 4 is suggested in which a coronal magnetic trap is filled with fast electrons by some acceleration process, trap eigenmode oscillations are excited, and expansion occurs. It is proposed that electrons in ordinary solar flares are accelerated by resonant interactions with Langmuir waves generated in thin current sheets and propagating laterally from the sheets. In the cyclotron study, the radiation flux is defined on the basis of Poynting's vector, and an expression for the harmonic frequency is found which differs nontrivially from the one commonly used.