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.