5.5 Particle acceleration

Particle acceleration associated with a flare has intensively been investigated (Ramaty and Murphy, 1987; Miller et al., 1997; Tsuneta and Naito, 1998; Aschwanden, 2002). Detailed processes of particle acceleration are beyond the scope of MHD, where the kinetic process involving charged particles become important. A typical length scale characterizing the kinetic process is the ion Larmor radius or ion inertial length, both of which are of the order meter in the corona. This is much smaller than the typical size of a flare, making it difficult to incorporate particle acceleration into the model developed for the global structure and overall evolution of a flare.

Here, we estimate the convective electric field associated with magnetic reconnection, which possibly plays an important role in accelerating charged particles during a flare:

vi vjet- E ∼ c Bi ∼ c Bjet (M )( B )2( n )−1∕2 ∼ 3 × 103 ---A ------ --10-jet-−3 V m −1. (42 ) 0.1 100 G 10 cm
This electric field decreases significantly in a gradual phase of a flare because of reduced vi, which explains why particle acceleration is less efficient in a gradual phase than an impulsive phase. This also explains why LDE flares have only a gradual phase and not an impulsive phase in which the signature of particle acceleration such as strong HXR emissions is observed.

High-energy electrons produced by strong convective electric field could contribute to forming foot-point as well as loop-top HXR sources, as illustrated in Figure 42View Image. Some of the high-energy electrons are not trapped near the Sun; instead they travel outward through the corona along open field lines. These electrons drive the plasma oscillation in the corona, which is observed as the type III radio bursts.

In some big flares (GOES X-class flares), solar neutron events (SNE) can be observed on the ground of the Earth. The neutrons are produced by the interaction of relativistic ions accelerated at the flare site and atomic nuclei (Watanabe et al., 2006). It is to be noted that when a large amount of electrons are accelerated simultaneously to produce a strong electron beam, a return current (a reverse current) may be generated around the electron beam to cause atmospheric heating. The effect of the return current have been studied by many authors (e.g., Karlický, 2008).

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