MESSENGER observations of the dayside low‐latitude boundary layer in Mercury's magnetosphere

Abstract

Observations from MErcury Surface Space ENvironment GEochemistry, and Ranging (MESSENGER)'s Magnetometer and Fast Imaging Plasma Spectrometer instruments during the first orbital year have resulted in the identification of 25 magnetopause crossings in Mercury's magnetosphere with significant low‐latitude boundary layers (LLBLs). Of these crossings 72% are observed dawnside and 65% for northward interplanetary magnetic field. The estimated LLBL thickness is 450 ± 56 km and increases with distance to noon. The Na+ group ion is sporadically present in 14 of the boundary layers, with an observed average number density of 22 ± 11% of the proton density. Furthermore, the average Na+ group gyroradii in the layers is 220 ± 34 km, the same order of magnitude as the LLBL thickness. Magnetic shear, plasma β and reconnection rates have been estimated for the LLBL crossings and compared to those of a control group (non‐LLBL) of 61 distinct magnetopause crossings which show signs of nearly no plasma inside the magnetopause. The results indicate that reconnection is significantly slower, or even suppressed, for the LLBL crossings compared to the non‐LLBL cases. Possible processes that form or impact the LLBL are discussed. Protons injected through the cusp or flank may be important for the formation of the LLBL. Furthermore, the opposite asymmetry in the Kelvin‐Helmholtz instability (KHI) as compared to the LLBL rules out the KHI as a dominant formation mechanism. However, the KHI and LLBL could be related to each other, either by the impact of sodium ions gyrating across the magnetopause or by the LLBL preventing the growth of KH waves on the dawnside.

MESSENGER Observations of Disappearing Dayside Magnetosphere Events at Mercury

“…This value is comparable to the ratios determined for the dayside magnetopause at Mercury in previous studies (Slavin et al, 2008DiBraccio et al, 2013;Liljeblad et al, 2015;Jia et al, 2019;Dong et al, 2019). This value is comparable to the ratios determined for the dayside magnetopause at Mercury in previous studies (Slavin et al, 2008DiBraccio et al, 2013;Liljeblad et al, 2015;Jia et al, 2019;Dong et al, 2019).…”

Investigation of ∼ 20–40 mHz ULF waves and their driving mechanisms in Mercury's dayside magnetosphere

“…As shown by Liljeblad et al (2015), MESSENGER covers the Hermean magnetosphere almost symmetrically during the year 2011. Therefore, observing a large part of the general ULF waves at the dawnside magnetosphere is surprising, assuming they are (as the duskside general ULF waves) likely driven by the KHI.…”

“…This either means that the dawnside general ULF waves are not generated by the KHI (although our results indicate that they are) or that dawnside KHWs are more common on Mercury than previously thought. Perhaps these dawnside KHWs do indeed frequently develop at the magnetopause but are constantly repressed by certain conditions of the surrounding environment (such as a broader velocity shear layer or a low-latitude boundary layer present most often at the dawnside as reported in Liljeblad et al (2015)) and will therefore not be as clearly visible as the duskside KHWs. Thus, they are more difficult to identify with the criteria used in Liljeblad et al (2014).…”

Observations of magnetospheric ULF waves in connection with the Kelvin‐Helmholtz instability at Mercury

“…At Mercury the occurrence of the KHI is clearly asymmetric: the large majority of the KH waves have been observed at the duskside magnetopause [ Liljeblad et al , ]. This distinct asymmetry, which has not been observed on Earth, may have different explanations: many have suggested that it most likely arises as a result of the finite Larmor radius effect via protons and/or sodium ions [e.g., Glassmeier and Espley , ; Nakamura et al , ; Sundberg et al , ; Gershman et al , ], while others have proposed that it could be an effect from an asymmetric low‐latitude boundary layer [ Sundberg and Slavin , ; Liljeblad et al , ]. The highly dynamic and small Hermean magnetosphere results in a KH wave periodicity of 30 s on average [ Liljeblad et al , ], i.e., a periodicity 4–10 times shorter as compared to what has been observed on Earth [ Kivelson and Chen , ; Fairfield et al , ; Hasegawa et al , ].…”