Type III solar radio noise bursts at hectometer wavelengths

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Abstract A study has been made of type III solar radio noise bursts that appear in the recordings of the swept-frequency receivers in the Alouettes I and II satellites. These bursts extend down in frequency to substantially less than 1 MHz and, in general, have characteristics that are quite similar to those observed at decametric wavelengths except that their duration is increased at the lower frequencies. The electron temperatures in the corona where the bursts originate have been deduced from their decay rates, and these are shown to decrease much more rapidly with distance from the Sun than expected for the solar wind. These findings, together with the observed frequency drift rates of the bursts, lead to an electron density model for the coronal irregularities extending into the interplanetary regions whose values exceed the solar wind densities by about an order of magnitude. The average source drift velocities are found to be about 0.35c, in substantial agreement with results obtained at metric and decametric wavelengths. These results are discussed with particular reference to the coronal streamers out to solar distances of about 1 4 of an Astronomical Unit, and to the probable noise generation processes for the type III bursts.