The paper describes an investigation aimed at finding out whether solar radio bursts of spectral type III are due to disturbances which travel out through the corona with velocities exceeding O�lc, as predicted by the well-known hypothesis that the emissions are due to plasma oscillations. If the proposition is correct, emissions at different frequencies would be generated at different levels in the corona-the lower the frequency the higher the source. This property is tested by simultaneous directional observations at a number of frequencies between 40 and 70 Mc/s, using a swept-frequency interferometer.
(Astrophys. Letters). The measured amount of band-splitting, Δf, in the spectra of nine harmonic type II bursts is illustrated in Figure 1. Here, as in previous, smaller samples (Roberts, 1959; Maxwell and Thompson, 1962; Weiss, 1965) Δf is found to increase with frequency, f.
Observational results are given concerning the relative positions on the Sun's disk of the fundamental and second-harmonic emissions of solar radio bursts of spectral types II and III. Contrary to simple theory, the results indicate that it is common for the harmonic emission in type II bursts to arrive from directions corresponding to much lower heights in the solar atmosphere than the fundamental. The results for type III bursts are inconclusive but suggest the same trend.
We present observations of a coronal hole made with the EUV spectroheliometer of the Harvard College aboard Skylab and with 'high resolution' (2-4') radio telescopes at Cnlgoora and Fleurs Australia and Bonn, West Germany. We attempt to derive the density and temperature distributions in the transition region and inner corona from the combined observations. No one 'standard' model can explain both sets of observations; characteristically, models based on EUV data yield higher radio brightnesses than are observed, while models based on radio data yield lower EUV line intensities than are observed. The discrepancy is essentially that the electron density inferred from the EUV data is about three times that inferred from the radio data, After examining several possible modifications of the standard models we suggest that the discrepancy would disappear if the abundances of the heavier elements were increased by about a factor of 10. Such increases could result from differential diffusion in the large temperature gradient of the transition region. We conclude therefore that models which incorporate thermal diffusion, as well as mass outflow and departures from ionization equilibrium, offer the greatest hope of reconciling the EUV and radio observations of coronal holes.
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