Observations of a new foreshock region upstream of a foreshock bubble's shock

Abstract

Earth's foreshock is a region within the solar wind upstream of Earth's bow shock filled with backstreaming solar wind particles reflected at the shock. Within this region, when the interplanetary field is approximately radial, foreshock bubbles (FBs) can be formed when the backstreaming particles interact with approaching discontinuities embedded in the solar wind. Foreshock bubbles can grow to 5–10 RE in scale, well upstream of the bow shock. Having a high concentration of thermalized upstream ions and slow, or even sunward, speeds within them, these transient phenomena deflect the solar wind by forming a new shock ahead of them. Although FBs eventually succumb to solar wind dynamic pressure and crash onto Earth's bow shock and magnetopause, they may last long enough to allow solar wind reflection at their own shocks, which forms a new FB foreshock region upstream of them. The FB shock may be of different obliquity than the parent bow shock providing new and diverse opportunities for particle acceleration. Using a case study from Time History of Events and Macroscale Interactions during Substorms, we demonstrate that ions and electrons are reflected at the FB shock, where they acquire energies consistent with shock acceleration theory. These are the first definitive observations of a new ion and electron foreshock region upstream of the FB shock with implications for shock acceleration in general.