Bow Shock Crossing Observations by MESSENGER From a Magnetic Point of View

Abstract

Mercury possesses a weak internal magnetic field, which creates a magnetosphere and consequently a bow shock that interacts with the solar wind. Understanding the dynamics of the bow shock is essential for deepening our knowledge of Mercury's interaction with the solar wind. In this study, we address the challenge of identifying and classifying bow shock crossings using magnetic field data from the MESSENGER mission, collected from 2011 to 2015. We develop an automated detection method that categorizes crossings into three confidence levels. Our analysis reveals a total of 13,502 crossings, with 1,765 classified as very clear crossings (category 1), 5,027 indicated as clear crossings (category 2), and 6,710 as crossing detections of poor quality (category 3). We apply the method proposed by (Schmid & Narita, 2023, https://doi.org/10.3847/1538-4357/aced07) to calculate the Alfvén Mach number based on the magnetic field jump characteristics. The shape and location of the bow shock are found to depend on upstream solar wind conditions, leading to a differentiation of crossings into quasi-parallel and quasi-perpendicular cases. We find that in the quasi-perpendicular case, the subsolar standoff distance decreases with increasing Alfvén Mach number, aligning with predictions from magnetohydrodynamic theory (Spreiter et al. (1966), https://doi.org/10.1016/0032-0633(66)90124-3; Chapman and Cairns (2003), https://doi.org/10.1029/2002ja009569). Conversely, no clear relationship is observed in the quasi-parallel case, likely due to complexities introduced by the foreshock region, which can alter the bow shock's structure. These findings illustrate the complex behavior of Mercury's bow shock and underscore the importance of further measurements, including plasma parameters, to improve our understanding of its dynamic interaction with the solar wind.