Observation of O+ Characteristics During the Terrestrial Alfv\'en Wing State Induced by the April 2023 Coronal Mass Ejection

TL;DR

This study uses MMS observations to analyze oxygen ion behavior during a 2023 solar event, revealing bi-directional streaming, sources, and escape mechanisms of O+ ions in the Earth's magnetosphere during Alfvén wing formation.

Contribution

First detailed analysis of O+ ions at the contact region of unshocked solar wind and magnetosphere during an Alfvén wing event, highlighting their sources and escape processes.

Findings

O+ ions show energies from hundreds of eV to ~30 keV.

Bi-directional streaming along closed field lines observed.

Escaping O+ ions detected in the solar wind.

Abstract

We report Magnetospheric Multiscale observations of oxygen ions (O+) during a coronal mass ejection in April 2023 when the solar wind was sub-Alfv\'enic and Alfv\'en wings formed. For the first time, O+ characteristics are studied at the contact region between the unshocked solar wind and the magnetosphere. The O+ ions show energies between 100s eV and ~30 keV. The possible sources are the ring current, the warm plasma cloak, and the ionosphere. The O+ ions exhibit bi-directional streaming along newly-formed closed field lines (CFLs), and dominantly anti-parallel on earlier-formed CFLs. Escaping O+ ions in the unshocked solar wind are observed. During the recovery phase, the O+ pitch-angle distribution associated with flux tubes shows dispersion, indicating potential loss to the solar wind. Our results show escaping as well as trapped O+ ions in the region where a magnetic cloud, an Alfv\'en wing, and magnetospheric field lines are mixed.