Galactic Cosmic Rays measured by UVS on Voyager 1 and the end of the
  modulation: Is the upwind heliopause a collapsed charge-exchange layer?
  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.
