Transient Heliosheath Modulation

TL;DR

Voyager 1 observations of the heliosheath reveal transient cosmic ray intensity drops caused by boundary flapping and radial flows near the heliopause, challenging steady-state models of cosmic ray modulation.

Contribution

This paper introduces a transient modulation model involving boundary flapping and radial flows to explain cosmic ray intensity variations near the heliopause.

Findings

Four episodes of rapid cosmic ray intensity change observed.

Radial flows at sound speed and slab turbulence control modulation.

Boundary boundary flapping and magnetic field gradients explain intensity drops.

Abstract

Voyager 1 has explored the solar wind-interstellar medium interaction region between the terminal shock and heliopause following the intensity distribution of galactic cosmic ray protons above 200 MeV energy. Before this component reached the galactic level at 121.7 AU, 4 episodes of rapid intensity change occured similar to the Forbush Decreases found near the sun, rather than the expected result of models related to those describing Long Term Modulation in the inner solar system. Because the mean solar wind flow is both expected and observed to be perpendicular to the radial direction close to the heliopause, explanation is given in terms of transient radial flows related to possible heliopause boundary flapping. It is necessary that radial flows are at the sound speed found for conditions downstream of the teminal shock and that the relevant perpendicular cosmic ray diffusion is controlled by 'slab' field fluctuations accounting for 20 percent or less of the total power in field variance. However, additional radial drift motion related to possible north to south gradients in the magnetic field may allow the inclusion of some diffusion according to 2-D turbulence theory. The required field gradients may arise due to variation in the field carried by the solar plasma deflected away from the solar equatorial plane. Modulation amounting to a total 30 percent drop in galactic intensity requires explanation by a combination of several transient episodes.