Excess heating of ⁴He²⁺ and O⁶⁺ relative to H⁺ downstream of interplanetary shocks

Abstract

Using observations made by the solar wind ion composition spectrometer on Ulysses during its trip in the ecliptic plane to Jupiter, we examined the temperature ratios T(⁴He²⁺)/T(H⁺) and T(O⁶⁺)/T(H⁺) across 15 shocks, including forward and reverse shock pairs as well as isolated shocks. The mean value over the whole set of downstream data studied was 7+/-3 for the T(⁴He²⁺)/T(H⁺). Appreciable differentiated heating downstream of a shock occurred for an upstream ratio T(⁴He²⁺)/T(H⁺)<4.5. Downstream in the shock regions, characterized by a quasisteady plasma flux, values of T(⁴He²⁺)/T(H⁺) and T(O⁶⁺)/T(H⁺) are observed in the range 4.6 to 10.8 and 19 to 48, respectively. Downstream of the shock, extra-heated ions heavier than H⁺ added a mean 13+/-6% to the ions' plasma pressure. (This is in addition to the pressure calculated using the common assumption of a temperature scaling with the ion masses in heliospheric collisionless plasma.) The extra heating of ⁴He²⁺ relative to H⁺ was larger for a smaller plasma β, agreeing with the prediction of the magnetohydrodynamic calculations by Zhao et al. [1991], although they appeared to underestimate the amount of added heating by a factor of 1.25 to 2.