Energy spectra of 50‐keV to 20‐MeV protons accelerated at corotating interaction regions at Ulysses

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Abstract
We analyze here the energy spectra of 50‐keV to 20‐MeV protons accelerated at corotating shocks observed at Ulysses in the southern heliosphere during 1992 and 1993. In general, our results are inconsistent with the predictions of two models based on diffusive shock acceleration theory (Fisk and Lee [1980] and Jones and Ellison [1991]). The main results are the following: (1) The relationships between the spectral indices and the shock compression ratio show significant departures from those predicted by the two models; the observed spectra are substantially harder than predicted; (2) the spectral indices at the reverse shocks depend strongly on the heliographic latitude of Ulysses; the hardest spectra are associated with strong quasi‐perpendicular shocks observed between 20°S and 30°S; (3) the spectral indices at the reverse shocks are anticorrelated with the upstream plasma velocity V′u in the de Hoffman‐Teller frame of the shock; and (4) the spectral indices at the forward shocks show no dependence on either the shock parameters or the heliographic latitude. The latitudinal dependence of the reverse shock spectra is probably related to the tilt (∼25°) of the heliospheric current sheet with respect to the solar equatorial plane during 1992 and 1993. The negative correlation between the spectral indices at the reverse shocks and V′u indicates that the gradient drift mechanism plays a crucial role in accelerating particles at corotating interaction regions (CIRs). The differences in our observations at the forward and reverse shocks may be due to the presence of a more energetic seed population and an enhancement in the level of magnetic field fluctuations upstream of the trailing edges of the CIRs.