Statistical Analysis of Mercury's Magnetotail Lobe Field Using MESSENGER Observations

The magnetotail lobe region at Mercury is characterized by low plasma density and low magnetic field variability compared to the nightside magnetosheath and central plasma sheet. At Mercury, as well as other planets, lobe magnetic fields play a crucial role in storing and releasing magnetic flux in response to changing upstream solar wind conditions such as interplanetary magnetic field (IMF) orientation and solar wind dynamic pressure (P-dyn). This makes the region significant for studying the magnetospheric interaction with the intense solar wind conditions at Mercury's orbit. Here, we identify and analyze magnetotail lobe observations made by the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft during its 4 years orbital phase. We empirically determined a set of criteria using magnetometer (MAG) and the Fast Imaging Plasma Spectrometer instruments onboard MESSENGER to identify lobe magnetic field intervals. From 3,332 MESSENGER orbits, we identify 1,242 lobe field intervals. We derive an expression for the average lobe magnetic field strength in nanotesla with respect to radial distance downtail: B-lobe(r) = (135 +/- 8) * r((-2.1 +/- 0.3)) + (31 +/- 8). The lobe magnetic field exhibits both small-scale (similar to 3 min) and orbit-to-orbit (similar to 8-12 hr) variability in magnetic field strength compared to this averaged field strength expression. The orbit-to-orbit variability in lobe field strength is not significantly correlated with estimated IMF orientation, but is directly correlated with P-dyn. Thus, our findings provide evidence for the pressure balance between the inward facing P-dyn on the nightside magnetopause and the outward facing magnetic pressure supplied by the lobes.