Recent Developments on the Galactic Cosmic-Ray Anisotropies Observed by Voyager 1 in the Local Interstellar Medium

Voyager 1 continues to observe unexpected time-varying enhancements and reductions in galactic cosmic-ray intensities in the local interstellar medium (LISM). Between 2012 and 2017, three large-scale depletions lasting from 100 to 630 days were observed by the Low Energy Charged Particle instrument in its > 211 MeV proton channel and by the Cosmic Ray Subsystem (CRS) in its omnidirectional (>20 MeV) and bi-directional (>70 MeV) proton rates. These episodes are characterized by a reduction of particles with pitch angles near 90-deg., while other directions reveal general uniformity. The latest analysis is presented, linking these episodes to adiabatic cooling behind LISM shock transients.

Kim et al. 2017 connected Voyager 1's LISM shock transients to periods of solar activity at 1 AU using input from OMNI data. Additionally, McComas et al. 2017, 2018 used a combination of IBEX simulation and data (with OMNI input) to predict that an intensification of solar wind dynamic pressure at 1 AU during the second half of 2014 should cause a major change to the structure of the heliosphere and lead to enhanced ENA emissions toward the nose "most likely in the 2017-2018 time frame". In early 2018, Voyager 1 observed the onset of a fourth galactic cosmic-ray anisotropy episode, following another transient event. Although Voyager 1 is 20 AU beyond where it crossed the heliopause on August 25, 2012, the current level of anisotropy is comparable to the magnitude of the 2013 anisotropy episode and continues to develop without showing signs of recovery. This latest event is examined in light of the predicted large-scale time-dependent changes to the heliopause.