Galactic cosmic rays measured by UVS on Voyager 1 and the end of the modulation

Abstract

The detectors of the UltraViolet Spectrographs (UVS) on Voyager 1/2 are recording a background that was earlier assigned to disintegrations in the RTG. We show that it arises instead from Galactic Cosmic Rays (GCRs). We show the 1992-2013 GCR flux measured by UVS on V1 and, by comparing with data from the GCR dedicated detectors, we estimate the energy range responsible for this UVS signal, around 300 MeV, and the response of UVS to the GCR anisotropy. After the abrupt jumps of May and August 2012 the count rate has been fluctuating only slightly around a constant value, but comparing with data from the LECP and the CRS instruments shows that those small variations are only responses to a varying anisotropy and not to a flux change. Taking advantage of the similarity in energy range to one of the products of the CRS instrument suite, we use the ratio between the two independent signals as a proxy for the temporal evolution of the GCR spectral slope around the 300 MeV range. We show that this slope has remained unchanged since Aug 2012 and find strong evidence that it will no longer vary, implying the end of the modulation at those energies and that V1 at this date is near or past the heliopause. The origin of this narrow and stagnating inner heliosheath is still unclear, and we discuss the potential effects of low solar wind speed episodes and subsequent self-amplified charge-exchange with interstellar neutrals, as a source of deceleration and collapse. We suggest that the quasi-static region encountered by V1 may be related to such effects, triggered by the strong post solar-maximum variability. This did not happen for V2 due to its trajectory at an angle further from the heliosphere axis and a later termination shock crossing. The existence on the upwind side of a mixing layer formed by charge transfer instead of a pure plasma contact discontinuity could explain various observations.