Galactic Cosmic Ray Pitch-Angle Anisotropies Beyond the Edge of the Hot Heliosheath: Voyager-1 Observations and Interpretations

Immediately upon the emergence on 24 August 2012 of Voyager-1 (VGR1) from the hot heliosheath into the 'depletion region' (devoid of hot heliosheath ions), enhanced intensities of galactic cosmic rays (GCR) displayed an unmistakable anisotropy in their pitch-angle distributions (p.a.d.). We have analyzed GCR proton p.a.d. from that crossing to the present time using 3-day averages from an integral-energy (E>211MeV) channel (CH31) of the Low Energy Charged Particle (LECP) instrument. Even though CH31 is a double-ended particle telescope, the fact that the two opposing heads have significantly different geometry factors (GL/GH=0.6 for the lower and higher factors) allows us to extract the p.a.d. when we make one key assumption - that the GCRs are gyrotropic. This is expected, because any anisotropy transverse to the magnetic field should be small in the depletion region: negligible gradients across the magnetic field, and negligible Compton-Getting effect (plasma velocity only ~10 km/s). Quite generally, we can write the energy-integrated (>211 MeV) GCR pitch-cosine distribution as the sum of its even and odd components: J(mu)=Jeve(mu)+Jodd(mu). Then when we form the sum and difference of the Ch31 rates for opposing scan-motor positions, Rave is proportional to Jeve(mu) and Rdif to Jodd(mu). Fortuitously, the measured VGR1 magnetic field direction lies almost in the LECP scan plane, allowing us to conclude that 1) the parallel streaming anisotropy for GCRs is dlnJodd/dmu=0.0015+/-0.0015 (a measurement consistent with zero), and that 2) our estimates of Jeve(mu) for pitch cosines near (1,0.7,0.0) imply time-dependent depletions in the intensities localized in pitch angle near 90 deg (mu=0). We can show that such p.a.d. would be observed whenever VGR1 is located between (at least) two compressions of the magnetic field on a field line, both (distant) ends of which contain equal isotropic GCR intensities (J0). We believe that such field compressions could be produced by the propagation of pressure enhancements of heated solar wind pickup ions from the termination shock, through the hot heliosheath, and into the otherwise quiet magnetic field in the depletion region. These compressions would, by their very nature, be time dependent, thus explaining the time dependence of our observed p.a.d. anisotropies in Jeve(mu). In any case (regardless of their interpretation), the observed time-dependent p.a.d. of the GCRs demonstrate that the depletion region is still under some control of solar processes and that VGR1 has not yet entered an undisturbed interstellar medium.