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 (Pdyn). 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: Blobe(r) = (135 ± 8) * r(−2.1±0.3) + (31 ± 8). The lobe magnetic field exhibits both small‐scale (∼3 min) and orbit‐to‐orbit (∼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 Pdyn. Thus, our findings provide evidence for the pressure balance between the inward facing Pdyn on the nightside magnetopause and the outward facing magnetic pressure supplied by the lobes. Plain Language Summary: Of all the planets in the Solar System, Mercury orbits closest to the Sun. As a result, it is subjected to an extreme interplanetary environment driven by solar dynamics. One consequence of the electro‐magnetic interaction of the Sun and Mercury is the formation of the magnetotail, which is the result of magnetic field lines connected to the planet being dragged toward the nightside. A portion of the magnetotail, known as the lobe, is characterized by a relative lack of plasma density compared to the surrounding environment. The strength of the magnetic field in the lobe is directly correlated with the conditions in the interplanetary space surrounding Mercury, making it an important region to study the response of a planetary environment to the extreme conditions at Mercury's orbit. We identify and analyze times in which the Mercury Surface, Space Environment, Geochemistry and Ranging spacecraft observed this important region of Mercury's plasma environment. We find direct evidence for the magnetic field lobe's response to the plasma properties emanating from the Sun. Key Points: 1,242 intervals in which Mercury Surface, Space Environment, Geochemistry and Ranging observed the southern lobe of Mercury were identified in 4 years of orbital dataThe lobe field strength (in nT) falls off with distance downtail according to Blobe(r) = (135 ± 8) * r(−2.1±0.3) + (31 ± 8)Orbit‐to‐orbit variability in lobe field strength is directly attributable to changing upstream solar wind dynamic pressure [ABSTRACT FROM AUTHOR]