Considerations limiting cyclotron-resonant damping of cascading interplanetary turbulence and why the ‘slab’ approximation fails

Abstract

In attempting to understand the dissipation of MHD scale fluctuation energy in the solar wind, the challenge is to harness kinetic theory (1, 2) effects in a way that is consistent with the presence of an active spectral cascade in a collisionless plasma. Recent observational studies (3, 4) have begun the task of sorting out the constraints that spacecraft observations place on dissipation range dynamical processes. Here we examine some implications of inertial- and dissipation-range correlation and spectral analyses extracted from 33 intervals of WIND magnetic field data (4). When field polarity and signatures of cross helicity and magnetic helicity are examined most of the data sets suggest some role of resonant dissipative processes involving thermal protons. Here we seek an explanation for this effect by postulating that an active spectral cascade into the dissipation range is balanced by a combination of resonant and nonresonant kinetic dissipation mechanisms. By solving a pair of rate equations, and employing constraints from the data, this theory suggests that the ratio of the two methods of dissipation is of order unity. With an additional assumption that mixed cross helicity corresponds to random directional sweeping, the theory approximates the relationship between magnetic and cross helicities seen in the WIND datasets. Although highly simplified, this approach appears to account for several observed features, and explains why complete absorption, and the corresponding pure signature in the magnetic helicity spectrum, is usually not observed. The results of the theory are consistent with magnetic fluctuations having oblique wave vectors, which is strongly supported by the inability of models based on parallel-propagating waves to adequately predict the onset of the dissipation range.