Plasma Oscillations and Radio Noise from the Disturbed Sun

Abstract

Many investigators have suggested that plasma oscillations in the solar corona may be the source of large bursts of radio noise in the meter wavelength region. Two aspects of this problem are considered in this report: (a) the excitation of plasma oscillations by directed beams of charged particles, and (b) the conversion of energy in the longitudinal plasma oscillations to transverse electromagnetic waves by means of random inhomogeneities in electron density. It appears unlikely that charged particles whose velocity is much less than the r.m.s. thermal velocity of the coronal electrons will excite plasma oscillations. Charged particles whose velocity is much greater than the r.m.s. thermal velocity excite oscillations in a band of frequencies, including frequencies above the local plasma frequency. However, qualitative arguments indicate that the noise should be concentrated in a narrow band of frequencies slightly below the local plasma frequency. Thus it is impossible to explain the Type II (slow) bursts in the manner assumed and unlikely that the Type III (fast) bursts are explainable in this manner. The transfer of energy is studied in detail and it is shown that only waves whose phase velocity is less than the directed beam of charged particles receive energy from the beam. It is shown that plasma oscillations radiate a small fraction of their energy if the electron density is not uniform. In particular, random fluctuations in density, of the amount expected in thermal equilibrium, cause about 10-5 of the plasma-oscillation energy to be radiated; the remainder is dissipated by short-range collisions. Larger fluctuations than this are likely, and hence more energy should be radiated.