The transverse motions of the sources of solar radio bursts

Abstract

Observations of the spectrum of solar radio bursts at meter wavelengths have indicated the desirability of measuring positions on the sun's disk not only as a function of time but also as a function of frequency. With this objective in view, we are now using a swept-frequency interferometer to determine the east-west disk coordinate of the transient solar sources at time intervals of £ second and freqency intervals of about 5Mc/s within the frequency range 40 to 70 Mc/s. The accuracy to which the centroid of the source is located is about ±1 minute of arc. In its initial form [1], the interferometer contained two aerials separated by a distance of 1 km. As a result of preliminary tests, two major additions have been made: (1) a second interferometer of much smaller spacing (ikm) has been added to resolve the usual ambiguities associated with two-aerial interferometry, and (2) an automatic system of lobe-switching and phase-calibration has been incor porated to facilitate the reduction of the complex data recorded.In its modified form, the equipment was put into operation on 1958 June 4, and the observations described here were taken during the ensuing 5 weeks. TYPE III BURSTSThe first problem to be investigated concerns the physical interpretation of the short-lived bursts of spectral type III. These bursts, which often occur in groups near the beginning of solar flares, are characterized by a rapid drift across the spectrum of the frequency of maximum intensity: the sweep from 70 to 40 Mc/s, for instance, occurs in about 1 to 3 seconds. We wished to test by direct means the suggestion [2,3] that the frequency drift is due to the excitation of plasma oscillations of frequencies that decrease with time as the exciting agency travels outward through regions of decreasing electron density in the solar corona. Assuming the usual estimates of electron den sity in the corona, the velocity of the exciting agency is calculated to be between 3 x 10 4 and 10 5 km/second. The method was to search for transverse motions of the sources across the sun's disk: if the theory is correct the transverse velocity should often be comparable with the inferred radial velocities, especially in the case of disturbances near the limb.Using the swept-frequency interferometer, the positions of a number of 176 available at https://www.cambridge.org/core/terms. https://doi

Solar Radio Bursts of Spectral Type II

“…Such a procedure will not yield reliable results, since the coronal density varies with time, particularly above sunspots. Indeed, Wild, Sheridan, and Trent (1959) have recently measured the direction of arrival of the different frequencies in type II bursts and have shown that, while the results qualitatively support the plasma hypothesis, they require the scale of the corona to be several times greater than in the conventional models. However, in the absence of detailed knowledge of the electron densities in the corona, it appears worth while to discuss the drift rates in terms of an averaged model, whilst bearing in mind that the results may be correct to order of magnitude only.…”

“…However, as already indicated, the velocities inferred from the Baumbach-Allen model are almost certainly low by a factor ",,2, so that the agreement is considerably better than is suggested by these figures. Indeed, it may not be too optimistic to hope that the direct measurements of the motion of the sources of type II bursts now being made by Wild, Sheridan, and Trent (1959) may make it possible to associate a particular magnetic storm with a particular type II burst.…”

“…(See e.g. Wild, Murray, and Rowe 1954;Ginzburg andZhelezniakov 1958, 1959;Wild, Sheridan, and Trent 1959. ) II.…”

A review of solar type III radio bursts

“…Closely related to the type III burst are type V bursts, classified due to their long durations (minutes) and wide spectra (Wild et al, 1959). The type V emission appears as a continuation of a type III burst in the dynamic spectra.…”

The Type IV Solar Radio Burst at Metre Wavelengths

“…The sample of events selected for analysis contains only one burst exhibiting rapid initial movement, but the absence of any systematic variation of position with frequency is characteristic also of the "moving" type IV bursts (see also Wild, Sheridan, and Trent 1959).…”