3.8 Type III radio bursts

Major fractions of the flare-accelerated electrons and protons escape into space (to be discussed in more detail in the next Section 3.9), guided by the magnetic field lines that are carried out into the heliosphere by the evolving solar wind. Injections of electrons in the keV energy range are accompanied by radio wave emission with frequencies from MHz down to few kHz. These so-called type III radio bursts (Wild, 1950) are generated in a two-step process (see also Figure 15View Image from Gurnett et al., 1980Jump To The Next Citation Point): 1. electrons accelerated in solar flares to energies of some keV are streaming away from the Sun and excite plasma oscillations locally, all the way from the corona into the distant heliosphere, the frequency fp being determined by the local electron density ne:
√ --- fp = 9 × ne,
with fp in kHz and ne in cm3. 2. These plasma oscillations (sometimes also called Langmuir waves) are converted to escaping electromagnetic radiation (of the same frequency or its harmonic) by non-linear wave-wave interactions. The resulting radio waves can be picked up by appropriate receivers in space. Because of the outward travel of the electrons through the radial density gradient the wave frequency gradually decreases with time, and their onset times are gradually delayed. That leads to the characteristic frequency variation of type III bursts as shown in Figure 16View Image from Kellogg (1980Jump To The Next Citation Point). In many cases, there is an overlaid strong and more spiky signal at very low frequencies observed. It is caused by the plasma oscillations excited locally upon arrival of the electrons at the position of the observing spacecraft. In the case shown in Figure 16View Image, the local plasma density of 7.5 cm3 would correspond to a local plasma frequency of 24.6 kHz, which is indeed at the intensity maximum of the observed plasma oscillation spectrum. Note that the onset times of the oscillations and the waves are almost identical.
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Figure 15: A representative radial profile of the electron plasma frequency in the solar wind illustrating the generation of electron plasma oscillations and the subsequent electromagnetic radiation at the plasmafrequency and its harmonic. From Gurnett et al. (1980).
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Figure 16: Signal amplitude for a number of radio wave channels during the type III radio burst on December 7, 1977, as observed from Helios 2. From Kellogg (1980).

Modern antenna systems onboard spaceprobes allow determination of the direction of the source of the type III radiation. The frequency itself is a measure of the source’s plasma density which, in conjunction with an assumed density model, is a measure of the radial distance of the source. This way, the location of any particular radio source along the path of the type III electrons can be determined. Indeed, the actual shape of the Parker spiral along which these electrons have to move could be experimentally verified (Reiner et al., 1995). Simultaneous radio measurements from two distant spacecraft allow rather precise triangulation of the radiation sources and a stereoscopic view of the electron beam path, even without any assumption of a density profile (Baumback et al., 1976Gurnett et al., 1978).

The type III electrons have a rather wide energy spectrum. Thus, their travel times to an in situ observer located at some distance from the Sun will vary considerably. That is indeed observed, as is shown in the bottom panel of Figure 17View Image (from Lin, 2005Jump To The Next Citation Point): the 27 keV electrons arrive about 20 minutes later than the 517 keV electrons.

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Figure 17: Example of a flare hard X-ray burst observed by RHESSI with corresponding solar type III radio burst and energetic electrons (and Langmuir waves) observed in situ by the WIND spacecraft (Krucker and Lin, 2002). Top panel: GOES soft X-rays; second panel: Spectrogram of RHESSI X-rays from 3 to 250 keV; third and fourth panels: radio emission observed by the WIND WAVES instrument; fifth panel: Electrons from 20 to 400 keV observed by WIND 3-DP instrument. From Lin (2005).

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