Ion pitch-angle scattering by Alfvén waves

Abstract

Freshly created ions can be picked up by a moving plasma without relying on collisions. It is well known that such an ion pickup process can be accomplished via the interaction with Alfvén waves. However, it should be stressed that in general ion pickup is attributed to two distinctly different sub-processes, namely, pitch-angle diffusion and pitch-angle scattering. In this article their difference is discussed and furthermore, some new results from a recent theoretical study are reported. It is found that under some conditions the usual quasilinear theory which describes the pitch-angle diffusion process cannot be justified even when the turbulence level is low. Another significant finding is that in the presence of strong Alfvén turbulence, thermal ions can be intensely heated by a nonlinear damping of the waves, which does not depend upon the usual ion cyclotron resonance.

A Source of Energetic Particles Associated with Solar Flares

“…As a matter of fact, the pickup process itself was not thoroughly comprehended until quite recently. For example, Li et al (1997) discussed the case of a relative velocity between the background plasma and newborn ions much greater than the speed and also examined the case in which high Alfven levels of intrinsic turbulence were present, making Alfvenic the usual gyroresonance unnecessary. Such a physical parameter regime has not been discussed in the usual ion pickup theories.…”

Density Depletion in a Coronal Flux Tube Associated With Solar Radio Emission

“…This process is extensively discussed in the literature of ion pickup process (e.g., early works are cited in articles written by Yoon and Wu, 1991;Li et al, 1997;Wang and Lin, 2003). An important consequence of this process is that a major portion of the kinetic energy associated with the beam is transformed into random motion in the wave frame.…”

Phase-matching enhanced ion heating by nonresonant Alfvén waves

“…9 Chen et al 10,11 investigated stochastic ion heating by obliquely propagating large-amplitude Alfvén waves and found that significant perpendicular heating of ions can occur at a fractional frequency. Using a test particle approach, Wu et al 12 found that initially unaffected ions can be picked up by propagating Alfvén waves and Li et al 13 numerically verified the ion-pickup with a self-consistent model. Guo et al 14 demonstrated analytically and numerically that ions can be heated by low-frequency linearly polarized obliquely propagating shear Alfvén waves (SAWs).…”

“…Theoretical, experimental, and numerical investigations of Alfvén wave resonance heating have been widely reported. [7][8][9][10][11][12][13][14] proposed that Alfvén waves can heat the ions by nonlinear interaction. Hasegawa and Chen 8 found that Alfvén wave phase mixing caused by the non-uniformity in plasmas can heat the plasma.…”