Sudden Intensity Increases and Radial Gradient Changes of Cosmic Ray MeV Electrons and Protons Observed at Voyager 1 beyond 111 AU in the Heliosheath

TL;DR

Voyager 1 observed sudden increases in MeV electrons and protons beyond 111 AU, indicating entry into a different propagation region in the heliosheath, with significant changes in intensity gradients and implications for cosmic ray modulation.

Contribution

This study reports the first detailed observation of abrupt intensity and gradient changes in cosmic ray electrons and protons beyond 111 AU, revealing new insights into heliosheath propagation conditions.

Findings

Sudden electron intensity increase of ~20% at 111.2 AU

Radial gradient for electrons decreased from ~19%/AU to ~8%/AU

Second intensity increase observed at 116.6 AU for electrons (~25%) and protons (~5%)

Abstract

Voyager 1 has entered regions of different propagation conditions for energetic cosmic rays in the outer heliosheath beginning at a distance of about 111 AU from the Sun. This conclusion is based on the fact that the low energy 6-14 MeV galactic electron intensity suddenly increased by ~20% over a time period \leg 10 days and the electron radial intensity gradient abruptly decreased from ~19%/AU to ~8%/AU at 2009.7 at a radial distance of 111.2 AU. A sudden radial gradient change was also observed at this time for >200 MeV protons. The gradients were constant during the time period before and after the electron increase. At about 2011.2 at a distance of 116.6 AU a second abrupt intensity increase was observed, this time for both electrons and protons. The increase for electrons was ~25% and occurred over a time period ~15 days or less. For >200 MeV protons the increase at this time was ~5% (unusually large) and occurred over a longer time period ~50 days. Between about 2011.2 and 2011.6, radial intensity gradients ~18%/AU and 3%/AU were observed for electrons and protons, respectively. These gradients were very similar to those observed for these particles before the 1st sudden increase at 2009.7. These large positive gradients observed after 2011.2 indicate that V1, although it has entered a different propagation region, is still within the overall heliospheric modulating region at least up to a time ~2011.6 (118.0 AU). In this paper we will discuss these events in more detail and consider possibilities for their explanation that have recently been suggested.