A Source of Energetic Particles Associated with Solar Flares

Abstract

Energetic particles produced at a solar Ñare event are an issue of great theoretical interest. A scenario involving magnetic Ðeld reconnection and newborn ions produced by ionization of neutral atoms in the lower corona is proposed by Wu. A series of two-dimensional hybrid simulations has been carried out to investigate this scenario further, and the results are presented in this paper. It is shown that under certain conditions the proposed process can produce protons with energies in the range of 10È100 MeV with a timescale of 10~4 to 10~3 s in a reconnection layer via the so-called ion pickup process. The acceleration process is sensitive to the ambient magnetic Ðeld. The maximum energy attainable is also discussed in terms of altitudes above the photosphere on the basis of a certain magnetic Ðeld model.

Lower Hybrid Drive in Solar Magnetic Reconnection Regions: Implications for Electron Acceleration and Solar Heating

“…The LH drive process primarily involves the pickup ring being broadened, mostly to small v ⊥ although with a small tail to larger v ⊥ due to stochastic acceleration (McBride et al 1972;Omelchenko et al 1989;McClements et al 1993;Shapiro et al 1998), but does not affect v . This is a significant difference from the situation studied by Wu and collaborators (Wu 1996;Wu et al 1998;Wang et al 2001), in which growth of Alfvénic waves by pickup ions leads to scattering onto a shell in velocity space and so to significant increases in v i . Instead, LH drive with pickup ions leads to perpendicular ion acceleration only to about 4 times the flow energy, to a perpendicular energy ≈2m i v 2 flow ≈ 2m i V 2 A .…”

“…This situation leads naturally to ion ring distributions (Wu 1996;Wu et al 1998;Wang et al 2001): in the rest frame of the outflowing protons there is no convection electric field but a newborn proton starts off at v = v ⊥ = −v flow + v 0 and so gyrates with gyrospeed v g = v ⊥ = |−v flow + v 0 |, thereby being found on a ring in perpendicular (to B) velocity space with ring speed…”

“…Attention is focused first on electron acceleration in chromospheric and transition region phenomena which show evidence for reconnection flows, then on low-lying solar active regions, and then on higher coronal events at heights of order 0.1 solar radii that may be relevant to type III bursts. The ion energisation produced in LH drive is compared with the scenario of Wu and colleagues (Wu 1996;Wu et al 1998;Wang et al 2001) in Section 3.4.…”

“…Recently, however, Wu (1996), Wu et al (1998), and Wang et al (2001) considered collisional charge-exchange of neutral hydrogen with protons in the outflow regions of solar magnetic reconnection sites. They showed that this 'pickup' process naturally forms ion ring distributions, as described in the next section.…”

“…They showed that this 'pickup' process naturally forms ion ring distributions, as described in the next section. Wang et al (2001) assumed low plasma β (equal to the ratio of thermal to magnetic pressures) 10 −2 and discussed the growth of MHD waves and associated scattering of the ring ions onto a shell in velocity space. They claimed reasonable agreement with the acceleration timescales and ion energies inferred from observations.…”