The Positions and Movements of the Sources of Solar Radio Bursts of Spectral Type II

Abstract

SummaryThe east-west position coordinates of the sources of 22 type II radio bursts, measured in the range 40-70 Mc/s using a swept-frequency interferometer, are analysed and discussed, in conjunction with dynamic spectra obtained in the frequency range 15-210 Mc/s. Many bursts are multiple and consist of a number of separate bursts excited by disturbances ejected in different directions from the vicinity of an optical flare, which may be equally complex.A statistical analysis of position data for type II and type III bursts, which reveals a remarkable similarity in the general behaviour of the two types of bursts, confirms the hypothesis of generation of the radio emissions by plasma oscillations. Other topics discussed are the relative intensities of fundamental and second harmonic bands of the same burst, the relative positions of the sources of fundamental and second harmonic bands of identical frequency, and the relative positions of the sources of the two ridges of a split band.Many type II bursts exhibit temporal changes of position at a single frequency. These are indicative of tangential movements in the solar atmosphere, with speeds of the order of 1000-2000 km/s. The spectra of these bursts with movement on the disk are characterized by little or no drift of frequency with time, and by broad bandwidth.A mean radial distribution of electron density in the disturbed corona is derived. This distribution does not differ greatly from that found optically for the average coronal streamer.

The Type IV Solar Radio Burst at Metre Wavelengths

“…If it is assumed that on the average the.type IV source lies radially above the flare, the mean height of the stable positions is given directly, in terms of the photospheric radius R o , by the slope of the relation between burst position and flare position. The mean height is estimated to be 2·11 ±O· 26R o ' which seems indistinguishable from the mean height, 1·97 ±O' 28R o , derived from a similar analysis of the positions of the fundamental bands of type II· bursts at the same frequencies (Weiss 1963). The latter are presumed to lie near the plasma levels and so it seems reasonable to conclude that, on the average, the stable positions of the sources of type IV bursts are located close to (possibly slightly above) the mean plasma level for the small range of frequencies involved (45-65 Mc/s).…”

“…The error in the mean position for a given burst, which may be estimated from over 100 individual measurements, is correspondingly less, except when the intenSity of the burst is very low. Ionospheric wedge refraction effects, which are a potential threat to the accuracy of the position measurements (Wild, Sheridan, and Neylan 1959;Weiss 1963) are easily recognized. Nevertheless, it is not possible to be certain that the effects of ionospheric refraction, whether regular or irregular, have been completely removed.…”

The Nature and Velocity of the Sources of Type II Solar Radio Bursts

“…Here, however, we wish to emphasize that the use of an average radial* electron density gradient can lead to large errors in the velocities for individual disturbances, as there are undoubtedly large fluctuations in coronal density between the streamers, where the type II sources are presumed to propagate, and in many cases the sources do not propagate even approximately radially (Weiss 1963). It is also emphasized that there are no reliable radio or optical measurements of electron * "Radial" is used throughout this paper in t,he "heliocentric" rather than the "geocentric" sense.…”

“…Velocities derived from two density models (10BA and 2N) have been examined. Of these models the 2N, which leads to substantially higher velocities at greater heights, is preferred because of support by other radio observations of source heights (Weiss 1963).…”

“…The relation between derived and true velocities for non-radial propagation is examined in Section lI(e). For our density model we choose, firstly, densities twice those given by Newkirk (1961) for the axis of an average optical coronal streamer (1956)(1957)(1958); the agreement of other radio measurements with this "2N" model has been discussed by the author elsewhere (Weiss 1963). The appropriate formulae are:…”