Ion Nongyrotropy in Solar Wind Discontinuities

Magnetic field fluctuations in the solar wind are essentially Alfvénic with a good correlation between plasma and magnetic field variations. One of the most investigated types of such fluctuations is (rotational) discontinuities, rapid rotations of the solar wind magnetic field, usually accompanied by velocity jumps, Δvl, comparable to Alfvén speed jumps, ΔvA. Although models of stationary discontinuities predict $| {\rm{\Delta }}{v}_{l}| =| {\rm{\Delta }}{v}_{{\rm{A}}}| $, observations often show $| {\rm{\Delta }}{v}_{{\rm{A}}}| /| {\rm{\Delta }}{v}_{l}| \gt 1$. This difference has previously been interpreted as: (1) a possible contribution of anisotropy that decreases ΔvA, or (2) a discontinuity non-stationarity due to residual magnetic energy. We propose an alternate interpretation: an ion nonadiabatic interaction with intense (thin) discontinuities that shapes the nongyrotropic ion distribution to include a nondiagonal term of the pressure tensor, with a cross-discontinuity gradient decreasing ΔvA. Using several examples of ARTEMIS observations of intense solar wind discontinuities, we demonstrate the existence of an ion population that contributes to such a nondiagonal pressure component with spatial profile and amplitude sufficient to significantly decrease ΔvA. The observed pressure nongyrotropy (a finite nondiagonal pressure component) balances the discontinuity configuration and can explain the $| {\rm{\Delta }}{v}_{{\rm{A}}}| /| {\rm{\Delta }}{v}_{l}| \gt 1$ paradox for intense discontinuities.