The spatial distribution of upstream ion events from the Earth's bow shock measured by ACE, Wind, and STEREO

Abstract

Using simultaneous measurements of >40 keV upstream ions observed at ACE, Wind, and STEREO‐A during the solar minimum period of 2007, day 1 through 2007, day 181, we investigate their spatial distributions by calculating their occurrence probabilities as a function of lateral and radial separation between L1 and STEREO‐A. Our main results are given as follows: (1) STEREO‐A observed upstream events even when it was separated from Earth by ∼1750 RE and ∼3800 RE in the radial and lateral directions, respectively. (2) The occurrence probability (∼20–30%) for measuring simultaneous upstream events at L1 and STEREO‐A was far greater than that expected from accidental coincidences. (3) The occurrence rate of simultaneous upstream events at L1 and STEREO‐A is significantly higher inside rarefaction regions of high‐speed solar wind flows (>500 km s−1) that follow corotating compression regions and when there exist antisunward propagating Alfvén waves. These new results confirm the global nature of the source region and place limits on the spatial size of the interplanetary structures that could either accelerate the ions in the first place or at the very least provide them with easier access by facilitating their scatter‐free transport from the Earth's foreshock into the far upstream regions traversed by STEREO‐A. We suggest that the existence of large amplitude Alfvén waves with spatial scales of the order of 0.03 AU that are embedded in and get convected past the Earth by high‐speed solar wind streams plays a critical and necessary role in the occurrence of upstream ion events near the Earth.