4.2 SEPs: electrons

The fluxes of non-thermal electrons as products of solar transient processes are very variable and hard to predict (for a general review see Lin, 1985Jump To The Next Citation Point). Apparently, there is a variety of processes to produce these electrons: 1) The energy spectra extend smoothly to the suprathermal range, down to 2 keV, indicating that these events originate high in the corona (Potter et al., 1980). 2) Electrons accelerated at flare sites with a very wide energy spectrum that cause the radio type III bursts as discussed above (Alvarez et al., 1972). 3) Electrons accelerated at the propagating interplanetary shock waves, both upward and downward, leading to the characteristic “herringbone” pattern of radio spectra shown in Figure 19View Image (Mann and Klassen, 2005).

At low energies below a few tens of keV, the observed electron fluxes may be a mixture of all the mentioned sources. That is probably the reason why spectra in this energy range are so variable and often show several breaks and kinks (see, e.g. Lin, 1985). At higher energies, above a few MeV, the situation changes. Evenson et al. (1984) found that most events with high fluxes of 5 to 50 MeV electrons appear to have been produced by flares which also produced observable fluxes of Gamma-rays. These events are never accompanied by strong interplanetary shocks which therefore are not a source of electron acceleration above a few MeV. It is concluded that the variable nature of the interplanetary particle events must reflect some fundamental but variable property of the shocks – possibly their direction of propagation.

For the very large events, electrons with energies up to 100 MeV have been observed (Simnett, 1974Moses et al., 1989). The energy spectra are usually rather hard, i.e., with power law indices of 3 and higher. For some events, spectral steepening above a few MeV is observed (Lin et al., 1982). It is not clear yet whether the energy spectrum extends beyond 100 MeV.

There are several open questions concerning the origin and propagation of energetic electrons from flares and shocks. This is illustrated by the recent ongoing controversy about “delayed injection”. The well-known type III burst producing electrons with their typical energies below 30 keV are also directly observed near the Earth. However, some near-relativistic electron events (energies above 30 keV) were released up to 30 minutes later in comparison to the type III radio emission onset. For the origin of this delay three explanations are being debated: 1.) The delayed electrons come from coronal shocks, like the herringbones at type II radio bursts (Krucker et al., 1999Klassen et al., 2002Haggerty and Roelof, 2002). 2.) The delayed electrons originate at reconnection site behind the shock front, and the CME shock itself plays only a minor role (Laitinen et al., 2000Klein et al., 2001Maia and Pick, 2004). 3.) All electrons originate from a single population, and the delay is due to propagation effects across magnetic field lines (Cane, 2003). Klassen et al. (2005) analyzed the October 28, 2003 flare and found three separate stages of injection. After some detailed analysis and discussion they state: “… the association of the two delayed injections with solar events is not well understood.”

In this context, another potentially relevant issue should at least be mentioned. Several authors recently addressed the appearance of a local minimum and maximum of the local Alfvén velocity in the middle of the corona and at 4 Rs, respectively. That would have effects on the formation of shocks in the corona and interplanetary space as well as the temporal behavior of the associated energetic particle events (Gopalswamy et al., 2001aMann et al., 19992003Vrsnak et al., 2004Warmuth and Mann, 2005).

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