Possible Source of Spectral Index -5 in Suprathermal Solar Wind

Abstract:

A persistent feature of suprathermal tail component of the solar wind (SW) distribution function is a -5 power law spectral index (Gloeckler et al., 2003). Fisk and Gloeckler (2006) have proposed that this index is a consequence of stochastic acceleration due to compressional turbulence in the SW. In this talk, we call attention to the fact that the -5 index also describes the collision rate of two particles in which particle 1 is a cold beam and particle 2 is Maxwellian plasma. Longmire (1963) has shown that in collisions of two particles via the Coulomb potential, the beam spreads in velocity space and the velocity distribution approaches equilibrium with a collision rate proportional to ∆v⁻⁵, where ∆v is the change of velocity of particle 1 as a result of scattering by particle 2. The two could be purely coincidental but since the -5 spectral index is a fundamentally important observational constraint for understanding the dynamics of the SW and the origin of the suprathermal component, we consider the possibility that this unique index may come from collisions of SW beam with the ambient plasma.