Characterization of Foreshock Plasma Populations at Mercury

Abstract

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

Short Large-Amplitude Magnetic Structures (SLAMS) at Mercury observed by MESSENGER

“…The favourable conditions mentioned above should include the SLAMS field line to be connected to the quasi-parallel bow shock, and it is puzzling that no such trend can be see in Figure 10. However, it is known that properties characteristic of the quasi-parallel foreshock can extend into regions mapping to a θ Bn of up to at least 60 • (Bonifazi and Moreno, 1981;Karlsson et al, 2021b;Glass et al, 2023), especially during conditions of more radial IMF (Blanco-Cano et al, 2009). This is consistent with the drop-off of SLAMS observations for θ Bn > 60 • , and the fact that SLAMS occur during times with a lower cone angle than the general solar wind distribution.…”

“…The SLAMS observations are seen to be relatively uniformly distributed along the bow shock. This is consistent with the findings of Glass et al (2023), who 145 noted that foreshock ion populations were observed rather uniformly with respect to local time. This is probably due to the more radially directed IMF at Mercury, as well as the small and dynamic magnetospheric system.…”

“…This region is also associated with a hotter plasma than the surrounding solar wind. This might be the signature of a mildly energized diffuse population (Glass et al, 2023), although a closer investigation would be necessary to verify this. Fluctuations in this region have a ∆B B0 up to about 1, and have a less clear sinusoidal character, indicating that they are evolving towards a more non-linear state.…”

“…For the focus point x 0 , and the eccentricity , we use the best-fit values of Winslow et al (2013): x 0 = 0.5 ; R M , = 1.04, while we let the focal parameter p vary. For each SLAMS event, we determine p from the closest bow shock crossing, in the same way as was done by Glass et al (2023). The values of p so determined vary between 1.87 and 3.44 R M , which can be compared with the best fit value of Winslow et al (2013) of p = 2.75 R M .…”