We perform the first statistical analysis of the main properties of waves observed in the 0.05-0.41 Hz frequency range in the Hermean foreshock by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Magnetometer. Although we find similar polarization properties to the "30 s" waves observed at the Earth's foreshock, the normalized wave amplitude ( B∕|B 0 | ∼ 0.2) and occurrence rate (∼0.5%) are much smaller. This could be associated with relatively lower backstreaming proton fluxes, the smaller foreshock size and/or less stable solar wind (SW) conditions around Mercury. Furthermore, we estimate that the speed of resonant backstreaming protons in the SW reference frame (likely source for these waves) ranges between 0.95 and 2.6 times the SW speed. The closeness between this range and what is observed at other planetary foreshocks suggests that similar acceleration processes are responsible for this energetic population and might be present in the shocks of exoplanets.

<p>In this work we perform the first statistical analysis of the main properties of waves observed in the 0.05–0.41 Hz frequency range in the Hermean foreshock by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Magnetometer. Although we find similar polarization properties to the '30 s' waves observed at the Earth's foreshock, the normalized wave amplitude (∼0.2) and occurrence rate (∼0.5%) are much smaller. This suggests significant lower backstreaming proton fluxes, due to the relatively low solar wind Alfvenic Mach number around Mercury. These differences could also be related to the relatively smaller foreshock size and/or more variable solar wind conditions. Furthermore, we estimate that the speed of resonant backstreaming protons in the solar wind reference frame (likely source for these waves) ranges between 0.95 and 2.6 times the solar wind speed. The closeness between this range and what is observed at other planetary foreshocks suggests that similar acceleration processes are responsible for this energetic population and might be present in the shocks of exoplanets.</p>

Mercury has been a target of interest and study since the flybys by Mariner 10 in 1974 and 1975 and has been subject to intense subsequent study by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft (Solomon et al., 2007), which orbited Mercury from 2011 until early 2015. As many previous works have noted, Mercury occupies a unique place among the solar system's planets. It is situated closest to the Sun, in a plasma and magnetic environment that is an order of magnitude more intense than at Earth and more than 2 orders of magnitude more intense than Jupiter on average (Slavin & Holzer, 1981). As a result, although Mercury has an Earth-like intrinsic dipolar magnetic field and a magnetosphere that obeys the Dungey cycle, its response to external solar forcing is far more dynamic and rapid than is observed at Earth-the reconfiguration time of Mercury's magnetosphere is approximately 2 min, compared to approximately 1 h at Earth (Dungey, 1961;Siscoe et al., 1975;Slavin et al., 2009).At both Earth and Mercury, the planetary cusps are a key region of the dayside magnetosphere. The cusps of both planets are defined as the region at and above the planetary surface (at Mercury, with its lack of ionosphere) or atmosphere (at Earth) colocated with the foot points of field lines that have just undergone reconnection on the dayside (Cowley & Owen, 1989). These field lines convect through the cusp toward the magnetotail, carrying magnetic and plasma flux as they move (Dungey, 1961). Crucially, these field lines

The Mercury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft was the first spacecraft to orbit the planet Mercury. Previous analysis of MESSENGER data has established that of all the planets in the solar system, Mercury's magnetosphere is the most like Earth's, dominated by the Dungey cycle in its dynamic response of the magnetosphere to solar wind forcing. In this work, we identify and describe for the first time Mercury's northern plasma sheet horn—a Dungey cycle feature key to plasma precipitation. We find three possible geometries for potential horn observation by MESSENGER and describe a case study of each. Two additional case studies are presented with geometries particularly favorable to estimating plasma precipitation within the horns. Estimates of proton precipitation flux are performed, which show precipitation levels on the order of 107 per cm2 per second, on the same order of magnitude as the estimated proton precipitation flux in the dayside cusp despite the higher average energy of the protons in the horn. Potential paths for future study of the horns are discussed.

The terrestrial bow shock has been the primary focus for studies of the acceleration of ions at collisionless shock for more than half a century (Burgess et al., 2012;Parks et al., 2017). The region directly upstream of the bow shock hosts a variety of different plasma populations whose creation stems, directly and indirectly, from the interaction of solar wind ions with the shock. This "foreshock" region, which is magnetically connected to the bow shock, exhibits a large variety of waves and energized particles. The nature of the magnetic connections between the interplanetary magnetic field (IMF) and bow shock controls the spatial distribution of these populations.In regions where the angle between the IMF and the bow shock normal ( 𝐴𝐴 𝐴𝐴𝑏𝑏𝑏𝑏 ) is greater than 45° (referred to as quasi-perpendicular), collimated ion beams with energies of a few keV are seen to propagate in the sunward direction along the IMF direction (Paschmann et al., 1980). These ion populations are typically referred to as Field Aligned Beams (FABs), for this reason. Apart from their narrow pitch angle extent, FABs are distinguished by their temperature anisotropies (Paschmann et al., 1981), depletion of He 2+ relative to the solar wind (Ipavich et al., 1988), and large variation in velocity and density with varying 𝐴𝐴 𝐴𝐴𝑏𝑏𝑏𝑏 (Oka et al., 2005). The most widely accepted acceleration mechanism for FABs is the Shock Drift Acceleration (SDA) mechanism (Burgess, 1987). In this mechanism, FAB ions have multiple encounters with the shock as they drift along the convective electric field (in the frame of the shock). This mechanism is broadly consistent with earlier work in which conservation of the magnetic moment, μ, during reflection at the shock in the deHoffman-Teller frame was used to explain FAB acceleration (Sonnerup, 1969). The low relative abundance of alpha particles in FABs was also confirmed in hybrid simulation (Burgess, 1989). A model for FAB acceleration involving the leakage of heated downstream plasma was proposed by Edmiston et al. (1982) andTanaka et al. (1983), but unlike SDA has not been sufficient to explain observations of FABs at Earth (Kucharek et al., 2004;Möbius et al., 2001;Oka et al., 2005).The second foreshock population documented by the earliest studies of Earth's foreshock is the diffuse population, which at Earth is generally limited to the quasi-parallel foreshock (where 𝐴𝐴 𝐴𝐴𝑏𝑏𝑏𝑏 < 45 • ). Diffuse ions are