Particle acceleration at corotating reverse shocks in the southern hemisphere: Ulysses results

Abstract

We have correlated the ∼1 MeV proton intensity (J) measured at corotating reverse shocks detected by Ulysses in the southern hemisphere with two parameters that characterise the strength of the shock. Between 6°-29°S J is essentially independent of the shock strength, whereas between 29°-41°S J is well correlated with the same. The poor correlation from 6°S to 29°S is attributed to variations in the background intensity caused by changes in solar activity. Our results also show that J is independent of the shock normal angle at all latitudes. We compensate for the effects of a fluctuating background by assuming that reverse shocks associated with two adjacent compression regions observed during a ∼26 day period accelerate particles out of the same seed population. The relation between the ratio of the proton intensities and the ratios of the shock parameters for such reverse shock-pairs indicates that the shock acceleration efficiency increases with the shock strength but is independent of the normal angle.