The Systematic Increase of the Electron Density and Other Related Effects as Voyager 1 Moves Outward into the Interstellar Space
Abstract
Voyager 1, which is now at a heliospheric radial distance of 143 AU (Astronomical Units), crossed the heliopause into interstellar space on 25 August 2012 at 122 AU. Since then the plasma wave instrument (PWS) has detected seven distinct electron plasma oscillation events, the frequencies of which give measurements of the local electron density. The plasma oscillations are believed to be excited by electron beams from shocks propagating outward from energetic disturbances at the Sun. Two new such events have occurred within the past year, from days 213 to 249 in 2017, and from days 150 to 157 in 2018. Sometimes these events are directly associated with shocks that are detected locally by jumps in the magnetic field, and in other cases are believed to be produced by magnetic field-aligned electron beams from shocks too distant to be detected locally. Each of the PWS electron plasma oscillation events appears to be correlated with extended periods of reduced intensity for Galactic Cosmic Rays (GCRs) with near 90-degree pitch angles. The electron densities measured by the PWS are observed to have increased systematically as the spacecraft has moved outward into the interstellar medium, from about 0.06 cm-3 near the heliopause at 123 AU, to about 0.14 cm-3 in the most recent events at about 140 AU. The reasons for this positive density gradient in the nearby interstellar plasma density are unknown. Suggestions have included a pile-up effect in the interstellar plasma flow around the heliosphere, the formation of a depletion layer due to a magnetic anisotropy-driven instability that pitch-angle scatters the plasma along the upstream boundary of the heliosphere, and shock-related dynamical pressure effects driven by the solar-cycle dependent occurrence and intensity of shock waves propagating outward from the Sun.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMSH13C2939G
- Keywords:
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- 2124 Heliopause and solar wind termination;
- INTERPLANETARY PHYSICSDE: 2126 Heliosphere/interstellar medium interactions;
- INTERPLANETARY PHYSICSDE: 2152 Pickup ions;
- INTERPLANETARY PHYSICSDE: 7845 Particle acceleration;
- SPACE PLASMA PHYSICS