Acceleration of electrons at type II shock fronts and production of shock‐accelerated type III bursts

Abstract

We present evidence of electron acceleration by type II‐burst‐emitting shocks in the corona. Some of the electrons travel outward along open magnetic field lines and produce “shock‐accelerated type III bursts” (or SA type III bursts) along their paths. The SA type III bursts are evident in dynamic spectra that cover part or all of the range from metric to kilometric wavelengths. The unique feature of our observations is the complete or near‐complete frequency coverage from about 2 GHz to < 0.1 MHz, that is, ≲ 1.01 Ro to 1 AU. A sample of eight events is presented. All would be classified as “shock accelerated events” at hectometric wavelengths, as first defined by Cane et al. [1981]. Our complete spectra frequently show several to many type III‐like bursts emanating from near the type II burst toward low frequencies, with no trace of emission at frequencies higher than that of the type II burst. The drift rates of these SA type III bursts are similar to those of normal type III bursts, and the exciting electrons have speeds of order 0.1c to 0.2c, or energies of 3–10 keV and higher. Their intensity at hectometer wavelengths is similar to that of normal type III bursts. They often persist to the lowest frequencies observable, near the local plasma frequency at 1 AU. In most of the events examined, there were no microwave bursts from the low corona whose intensity profiles were similar to the hectometric profiles. We therefore conclude that these SA type III bursts originate in type II shocks and are caused by energetic electrons accelerated at the shocks. Of the eight events analyzed, three contain only SA type III bursts. For the remainder, normal type III bursts predominate initially, followed by SA type III bursts later in the event. We emphasize the need for spectra with near‐continuous coverage, especially from decametric to kilometric wavelengths, to identify SA type III bursts unambiguously and to distinguish between the contributions of normal and SA type III bursts.