Statistical Survey of Magnetic Forces Associated With Earthward Bursty Bulk Flows Measured by MMS 2017-2021

Abstract

We investigate the magnetic forces (the magnetic pressure gradient force, the curvature force, and their sum the j × B-force) associated with earthward bursty bulk flows (BBFs) using Magnetospheric Multiscale (MMS) data from five tail seasons (2017-2021). For the first time, the magnetic forces are inferred downtail of XGSM = −20 R_E and in the GSM XY and YZ planes. The results suggest that BBFs tend to be accelerated earthward by the magnetic pressure gradient force tailward of ∼19 R_E and decelerated closer to that distance in the 2017-2018 data. The force magnitudes increase with distance. This is in accordance with earlier Cluster results. In the 2019-2021 data, the pressure gradient force magnitudes are generally smaller and no clear distance for the acceleration reversal can be determined. The curvature forces for both 2017-2018 and 2019-2021 BBFs indicate earthward acceleration independent of distance, consistent with the Cluster measurements. The sum, the j × B-force, suggests for the 2017-2018 BBFs earthward acceleration tailward of XGSM ∼15 R_E and deceleration within that distance, also consistent with Cluster. In contrast, the 2019-2021 BBFs show general earthward acceleration by j × B independent of distance. In the GSM XY plane, the average (j × B)_xy vectors are earthward, and in the premidnight and postmidnight dawnward for the 2017-2018 BBFs. For 2019-2021, the average (j × B)_xy vectors have components toward the tail center. In the GSM YZ plane, the average (j × B)_yz vectors are toward the neutral sheet.