Variation of the Interplanetary Shocks in the Inner Heliosphere

Abstract

Observations of the solar wind plasma and interplanetary magnetic field in the ecliptic plane by the Voyager 1 and 2 spacecraft are utilized to study the variation of the interplanetary shocks (with magnetosonic Mach number >1) encountered in the inner heliosphere, between ∼1 and ∼15 au from the Sun. The overwhelming majority (∼76%) of the shocks are fast forward (FF) moving at an average shock speed (V sh) of ∼458 km s-1 away from the Sun, and only ∼24% are fast reverse (FR) shocks with an average V sh of ∼323 km s-1 toward the Sun. About ∼89% of the shocks are quasi-perpendicular with an average shock angle (θ Bn) of ∼72° relative to the ambient magnetic field, and only ∼11% are quasi-parallel with an average θ Bn of ∼32°. While the interplanetary coronal mass ejections are the major (∼89%) driver of the FF shocks, a significant part (∼41%) of the FR shocks are associated with the corotating interaction regions. The shock occurrence rate, with a peak at the heliocentric distance (R h ) of ∼5 au, exhibits a clear decrease with the increasing R h . While the shock strength and propagation angle do not exhibit any systematic change with R h , V sh increases with the increasing R h at a rate of ∼11 km s-1 au-1. © 2021. The American Astronomical Society. All rights reserved.