Journal Article

Field‐Aligned Current Structures During the Terrestrial Magnetosphere's Transformation Into Alfvén Wings and Recovery

Genestreti, K. J., Shuster, J. R., Chen, Y., Rice, R. C., Ardakani, A. S., Gurram, H., Gershman, D. J., Chen, L.‐J., Beedle, J. M. H., Burkholder, B. L., Torbert, R. B.

2024

Overview

On 24 April 2023, a Coronal Mass Ejection event caused the solar wind to become sub‐Alfvénic, leading to the development of an Alfvén Wing configuration in the Earth's magnetosphere. Alfvén Wings have previously been observed as cavities of low flow around moons in Jupiter's and Saturn's magnetospheres, but the observing spacecraft did not have the ability to directly measure the Alfvén Wings' current structures. Through in situ measurements made by the Magnetospheric Multiscale spacecraft, the 24 April event provides us with the first direct measurements of current structures during an Alfvén Wing configuration. These structures are observed to be significantly more anti‐field‐aligned and electron‐driven than the typical diamagnetic magnetopause current, indicating the disruption caused to the magnetosphere current system by the Alfvén Wing formation. The magnetopause current is then observed to recover more of its typical, perpendicular structure during the magnetosphere's recovery from the Alfvén Wing formation. Plain Language Summary The solar wind applies pressure on the Earth's magnetic field, distorting it from a dipole into its compressed dayside and stretched tail configuration. However, this typical structure can be disrupted by eruptive solar events such as Coronal Mass Ejections (CMEs), which may cause the solar wind's pressure to drop low enough that it is no longer able to push the magnetosphere back to form a single unified tail. When this occurs, the tail splits into two separate structures, called Alfvén Wings. While this configuration is rare at Earth, it is common from interactions of the outer planets' magnetosphere's with their moons, where Alfvén Wing configurations have been studied and modeled. However, because the observing spacecraft lacked the necessary instrumentation, we have not yet directly observed the Alfvén Wing current structures. On 24 April 2023, a CME event led to the creation of an Alfvén Wing formation in the Earth's magnetosphere. We observed this event using the Magnetospheric Multiscale spacecraft, which enabled us to make the first direct observations of Alfvén Wing current structures. These currents were found to be mainly parallel to the local magnetic field, in contrast to typical magnetopause currents. Key Points On 24 April 2023, the Magnetospheric Multiscale (MMS) spacecraft observed an Alfvén Wing formation along the dawn‐flank of Earth's magnetosphere MMS's observations represent the first in situ measurements of Alfvén Wing current structures The current structures are found to be primarily anti‐field‐aligned, electron‐driven, and filamentary

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Alfven Wing currents / Alfven wings / Chapman‐Ferraro current / Configurations / Coronal mass ejection / Diamagnetism / Dipoles / Earth / Earth magnetosphere / field‐aligned currents / Geomagnetic field / In situ measurement / Instrumentation / Jupiter / Jupiter atmosphere / Jupiter probes / Low flow / Magnetic field / Magnetic fields / Magnetopause / magnetopause current / Magnetopause currents / Moon / Outer planets explorers / Planetary magnetospheres / Recovery / Solar corona / Solar magnetic field / Solar wind / Spacecraft / Structures / Tails / Wings