Two-dimensional velocity of the magnetic structure observed on 11 July 2017 by the Magnetospheric Multiscale spacecraft

Abstract

In magnetic reconnection, plasma flows toward the magnetic X line (a magnetic null in the reconnection plane, in which it appears as an X point) with an inflow velocity and is accelerated and ejected in an orthogonal direction with an outflow velocity because of the large curvature of the magnetic field in the vicinity of the X line (e.g., Sonnerup, 1979;Vasyliunas, 1975). To determine these velocities, one needs to determine the frame of reference in which the X line is stationary. Thus an important part of the process of understanding a magnetic reconnection event is to determine the velocity of the magnetic structure relative to the observing spacecraft. Although on large scales, plasma may be "frozen in" to the magnetic field, at least in directions perpendicular to the magnetic field, this is typically not the case on small scales close to the X line, especially in the region known as the electron diffusion region (Hesse et al., 2011(Hesse et al., , 2014. Shi et al. (2019) has reviewed methods to determine a coordinate system and magnetic structure velocity. Methods to determine the velocity include calculating the deHoffmann-Teller frame in which the electric field is approximately zero, various types of timing analysis, various reconstruction methods, and the Spatial-Temporal Difference (STD) method (Shi et al., 2006). STD has been used by Denton et al. (2016aDenton et al. ( , 2016b and Yao et al. (2016Yao et al. ( , 2018 to determine the time-dependent velocity of a magnetic structure in the normal direction. Alm et al. ( 2017) used STD to calculate the time-dependent two-dimensional velocity of the spacecraft moving through a structure of ion-scale magnetopause flux ropes. Manuzzo et al. (2019)