The disappearance of anomalous protons at Voyager 1 and Voyager 2 in the outer heliosphere between 1998 and 2002

Abstract

In 1998 and 1999 at V1 and V2 in the outer heliosphere between 55 and 75 AU, large intensities of anomalous H were observed with a peak in the differential energy spectrum at ∼25 MeV. Subsequently, in 2000 and 2001 when these spacecraft were 5–10 AU further out, these intensities were greatly reduced by 11-year modulation effects, so that by the beginning of 2002 anomalous H had completely disappeared beneath the E^(1.0) spectrum of galactic H at low energies. The modulation between 1998 and 2002 produced intensity changes of galactic H similar to those observed at the Earth in the same time period except that the changes at V1 and V2 were much smaller. The intensity change at V2 was also larger than that at V1. For anomalous H, however, the intensity changes at V1 and V2 were a factor of 10–20 times larger than those for galactic H, so that by the end of 2001 the anomalous H intensities were less than 1% of their 1998 values. Also, during one time period in late 2001 there was a large reduction of anomalous H at both V1 and V2 with no corresponding change in galactic H. These different intensity changes between galactic and anomalous H provide an insight into particle transport, drift, and acceleration processes in the region near the heliospheric termination shock, which is located just beyond V1. Radial gradient comparisons in 1998 and also at the times of reduced anomalous H intensities imply that most of the intensity reduction of anomalous H apparently occurred at ∼90 AU or beyond. This could indicate source changes in the anomalous H spectra but most likely is a measure of changes in the "connection" between V1 and V2 and the source region, such as might be caused by changing drift patterns and particle entry into the heliosphere.

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