On the Energy Dependence of Galactic Cosmic Ray Anisotropies in the Very Local Interstellar Medium

Kavli Affiliate: John Richardson

| First 5 Authors: Romina Nikoukar, Matthew E. Hill, Lawrence Brown, Stamatios M. Krimigis, Robert B. Decker

| Summary:

We report on the energy dependence of galactic cosmic rays (GCRs) in the very
local interstellar medium (VLISM) as measured by the Low Energy Charged
Particle (LECP) instrument on the Voyager 1 (V1) spacecraft. The LECP
instrument includes a dual-ended solid state detector particle telescope
mechanically scanning through 360 deg across eight equally-spaced angular
sectors. As reported previously, LECP measurements showed a dramatic increase
in GCR intensities for all sectors of the >=211 MeV count rate (CH31) at the V1
heliopause (HP) crossing in 2012, however, since then the count rate data have
demonstrated systematic episodes of intensity decrease for particles around
90{deg} pitch angle. To shed light on the energy dependence of these GCR
anisotropies over a wide range of energies, we use V1 LECP count rate and pulse
height analyzer (PHA) data from >=211 MeV channel together with lower energy
LECP channels. Our analysis shows that while GCR anisotropies are present over
a wide range of energies, there is a decreasing trend in the amplitude of
second-order anisotropy with increasing energy during anisotropy episodes. A
stronger pitch-angle scattering at the higher velocities is argued as a
potential cause for this energy dependence. A possible cause for this velocity
dependence arising from weak rigidity dependence of the scattering mean free
path and resulting velocity-dominated scattering rate is discussed. This
interpretation is consistent with a recently reported lack of corresponding GCR
electron anisotropies.

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