Preferential Low-Latitude Acceleration and Transport of Low-Energy Anomalous Cosmic Rays
Abstract
During the last decade the encounters by the Voyager 1 and 2 spacecraft with the termination shock and continuing cruise through the heliosheath have upset the classical anomalous cosmic ray (ACR) paradigm, which has interstellar neutral atoms being ionized, picked up by the solar wind, and accelerated at essentially all regions of the termination shock. Observations show that ACRs are not accelerated at the termination shock, at least not at the locations of the Voyager encounters. ACR transport is also supposed to be dominated by drift motion arising from the curvature and gradients of the global heliospheric magnetic field; the well-known drift pattern of cosmic rays down along the poles and out along the heliospheric current sheet during the so called A > 0 solar cycles (and reverse pattern during A < 0) is considered a hallmark of the classic theory. Additionally the observed striking similarity of the spectral slope of suprathermal ions across wide regions and conditions in the heliosphere has forced a general rethinking of the role of diffusive shock acceleration. We present the results of two independent lines of inquiry. First, recent theoretical and particle-in-cell simulation work has combined determination of the physical cause for the special spectral form with a proposed mechanism for the source of ACRs—explained by the conversion of magnetic energy into kinetic energy due to the annihilation of magnetic flux near the heliopause, at low latitudes. Second, observational study of ACR transport during the A > 0 cycle shows—for low-energy ions of a given species (actually for low rigidities, less than 2 GV, when all species are considered)—no evidence for a strong positive latitudinal intensity gradient, as predicted. Quite the opposite is true. For the lowest rigidities these gradients are negative, as large as -15%/degree. This and other work suggests that the low-latitude region is the region where ACR activity is most significant. We will compare the theoretical and observational results and discuss the implications for the surprising dearth of ACRs observed during the termination shock encounters, and the subsequent variations in the heliosheath.
- Publication:
-
38th COSPAR Scientific Assembly
- Pub Date:
- 2010
- Bibcode:
- 2010cosp...38.1666H