Global Structure of the Cislunar Magnetotail and Its Evolution During Substorms

Abstract

So far, the structure and dynamics of the terrestrial magnetotail have largely been investigated within $31R_E$ ($R_E$ is the Earth's radius) because of the lack of in-situ observations beyond that distance, as well as limitations of earlier empirical models. Here we reconstruct the global structure of the cislunar tail within $65R_E$ using 2011–2023 data from the ARTEMIS mission, the 2010 THEMIS-ARTEMIS transition orbits and other missions, such as IMP-8 and Geotail. The reconstruction is made using a new-generation data mining-based empirical algorithm with minimal ad hoc assumptions on the structure of the equatorial current sheet and its evolution during storms and substorms. It is found that the cislunar tail has a regular structure, stable in the substorm growth phase and regularly changing during the expansion and recovery phases. Substorms likely involve the formation of an X-line around $40R_E$ and magnetic flux accumulation earthward of it, which persists in the growth phase and flattens out after the substorm onset. Consistent with earlier reconstructions of the shorter tail region, the present ones reveal the near-Earth dipolarization of the magnetic field earthward of $\sim 17R_E$, which is a part of the whole cislunar tail flux redistribution. Thin current sheets embedded into a thicker current halo extend up to 40 $R_E$ and may exist at lunar distances, consistent with in-situ ARTEMIS observations, near O-lines of plasmoids/flux ropes.