MESSENGER Observations of Standing Whistler Waves Upstream of Mercury's Bow Shock

This paper reports on the standing whistler waves upstream of Mercury's quasi‐perpendicular bow shock. Using MESSENGER's magnetometer data, 36 wave events were identified during interplanetary coronal mass ejections (ICMEs). These elliptic or circular polarized waves were characterized by: (a) a constant phase with respect to the shock, (b) propagation along the normal direction to the shock surface, and (c) rapid damping over a few wave periods. We inferred the speed of Mercury's bow shock as ∼26 km/s and a shock width of 1.87 ion inertial length. These events were observed in 20% of the MESSENGER orbits during ICMEs. We conclude that standing whistler wave generations at Mercury are generic to ICME impacts and the low Alfvén Mach number (MA) collisionless shock, and are not affected by the absolute dimensions of the bow shock. Our results further support the theory that these waves are generated by the current in the shock. Plain Language Summary The strength of planetary bow shocks varies with the planet's heliocentric distance from the Sun. Studying the bow shocks of other planets is important for extending our understanding of collisionless‐shock physics. In the solar system, the bow shocks of Mercury are unique as they are produced by low Mach numbers and low plasma beta solar wind blowing over a small magnetized body that is 1–2 orders smaller than Earth. The standing whistler waves upstream of the bow shock of Mercury were determined through statistical analyses. Similar to the observations at Earth, these waves were rapidly damping with a proportion of the wave periods; however, the damping distance at the spacecraft frame was considerably shorter at only a few kilometers upstream in the small‐scale bow shock of Mercury. The high occurrence rate of standing whistler waves suggests that Mercury's bow shock is a natural plasma laboratory, which can be used to further investigate low MA planetary shocks during the upcoming BepiColombo mission. Key Points First survey of standing whistler waves upstream of Mercury's bow shock Standing whistler waves are common at Mercury during interplanetary coronal mass ejections Our results support the theory that current in shock generates standing whistler waves