Temporal and spectral variations of anomalous oxygen nuclei measured by Voyager 1 and Voyager 2 in the outer heliosphere

Abstract

We have studied the temporal and spectral variations of anomalous oxygen nuclei at the Voyager 1 (V1) and 2 (V2) spacecraft in the outer heliosphere from 1990 to the present time in 2006 when V1 is now beyond the heliospheric termination shock. During this time period, the intensities increased from their lowest values in 1990–1991 up to a maximum in 1998–1999 and then decreased rapidly in 2000–2001 in time coincidence with the change in solar magnetic polarity from positive to negative. During the time period after 2001, significant changes in intensities and spectra are observed relative to the earlier period of positive solar magnetic polarity before 2001. It is found that the intensities of O above ∼10 MeV/nuc at V1 after ∼2002.0 were higher relative to the same galactic cosmic ray He intensity between 150–380 MeV/nuc than in the earlier time period. As a result, by 2006 these intensities were a factor ∼3 times those measured at the intensity maximum in 1998–1999 in the previous polarity cycle. The changes observed at V2 followed a similar pattern, but the relative intensity changes of O were a factor ∼2 times greater than those observed at V1. Also, above ∼10 MeV/nuc, the intensity changes at V1 and V2 were nearly energy independent, and the spectra at all times before and after the solar magnetic polarity change and at all modulation levels remained ∼E−3.0±0.2, possibly characteristic of a “source” spectrum. When V1 crossed the termination shock, no noticeable spectral or intensity changes were observed.