We investigate localized magnetosheath and solar wind density enhancements, associated with clear magnetic field changes, and therefore referred to as magnetosheath/solar wind plasmoids, respectively. Using Cluster data, we show that there are two distinct populations of magnetosheath plasmoids, one associated with a decrease of magnetic field strength (diamagnetic plasmoids), and one with an increased magnetic field strength (paramagnetic plasmoids). The diamagnetic magnetosheath plasmoids have scale sizes of the order of 1–10 RE, while the paramagnetic ones are an order of magnitude smaller. The diamagnetic plasmoids are not associated with any change in the magnetosheath plasma flow velocity, and they are classified as embedded plasmoids in the terminology of Karlsson et al. (2012). The paramagnetic plasmoids may either be embedded or associated with increases in flow velocity (fast plasmoids). A search for plasmoids in the pristine solar wind resulted in identification of 62 diamagnetic plasmoids with very similar properties to the magnetosheath diamagnetic plasmoids, making it probable that the solar wind is the source of these structures. No paramagnetic plasmoids are found in the pristine solar wind, indicating that these are instead created at the bow shock or in the magnetosheath. We discuss the relation of the plasmoids to the phenomenon of magnetosheath jets, with which they have many properties in common, and suggest that the paramagnetic plasmoids can be regarded as a subset of these or a closely related phenomenon. We also discuss how the results from this study relate to theories addressing the formation of magnetosheath jets.

A large study of Kelvin-Helmholtz (KH) waves at the magnetopause of Mercury covering 907 days of data from the MErcury Surface Space ENvironment GEochemistry Ranging spacecraft have resulted in 146 encounters of not only nonlinear KH waves but also linear surface waves, including the first observations of KH waves at the dawnside magnetopause. Most of the waves are in the nonlinear phase (90%) occur at the duskside magnetopause (93%), under northward magnetosheath magnetic field conditions (89%) and during greater magnetosheath B z (23 nT) values than in general. The average period and amplitude is 30 ± 14 s and 14 ± 10 nT, respectively. Unlike duskside events, dawnside waves do not appear at the magnetopause flank (< 6 magnetic local time). This is in agreement with previous observations and modeling results and possibly explained by finite Larmor radius effects and/or a lack of a large-scale laminar flow at the dawnside magnetopause boundary.

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The magnetic field data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is investigated to establish the presence of magnetospheric ultralow frequency (ULF) waves in connection with 131 previously observed nonlinear Kelvin‐Helmholtz (KH) waves at Mercury. Distinct ULF wave signatures are detected in 44 out of the 131 magnetospheric traversals prior to or after observing the KH waves. Of these ULF events, 39 out of 44 are highly coherent at the frequency of maximum power spectral density and occur more often on the dayside magnetosphere than away from it. The waves observed at the dayside magnetosphere, which appear mainly at the duskside and naturally following the KH wave occurrence asymmetry, are significantly different from the eveningside or morningside events and have the following distinct wave characteristics: a polarization mainly in the perpendicular (azimuthal) direction to the mean magnetic field, a wave normal angle closer to the parallel than the perpendicular direction, an absolute ellipticity increasing away from noon, almost exclusively a right‐hand polarization, and frequencies in the narrow range of 0.02–0.04 Hz (well below the local Na+ gyrofrequency and in the same range as the KH waves). The results strongly suggest that the large majority of the ULF waves at the dayside observed in this study are driven by KH waves at the magnetopause and that they occur in the vicinity of a field line resonance, which in turn manifests the importance of the Kelvin‐Helmholtz instability in terms of energy and momentum transport throughout Mercury's magnetosphere.

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.