Local Reacceleration of Galactic Cosmic Rays at the Heliosphere’s Termination Shock

Abstract

The measured intensities in the ecliptic plane of the GCR He (265 MeV/n) and ACR Oxygen (7 - 17 MeV/n) at 70 AU over the solar minimum (1996-1998) of cycle 22 are significantly less than that observed over the 1987 solar minimum of cycle 21 at 42 AU. The GCR He at 42 AU is 36% greater than that at 70 AU while ACR O is 5x greater. The different intensity levels and their very different time histories in the opposite phases of the heliomagnetic cycle represent a challenge to modulation theory and must in part be a drift related effect. Kota and Jokipii [1] have modelled the transport of GCRs and ACRs in a 1D model that includes a termination shock and a heliosheath region and in a 2D heliosphere that includes not only drifts but also the effects of an extended heliosheath region. They find that near the plane of the ecliptic in qA<0 cycles there is a significant enhancement of GCRs produced by local reacceleration in the heliosheath. This is accompanied by a much larger increase in the ACR O intensity, consistent with the Pioneer/Voyager observations.