Fig. 1: Spacecraft location.

Locations of the exosphere and magnetospheric boundaries, and the trajectory of MESSENGER during the heavy-ion observations. The coordinate system is in the aberrated Mercury solar orbital frame (MSO). a View from dusk (X’-Z’ plane) in the noon–midnight meridian, with the Sun to the left. The solar wind is to the left of the bow shock, and magnetospheric model field lines⁵¹, help visualize Mercury’s magnetosphere. The location of the spacecraft is seen to be inside the bow shock in (a), however the spacecraft has a large component of its position directed out of the plane, and is actually in the solar wind. b The view from the Sun (Y’-Z’ plane). Also shown is a simple sodium exospheric model with a scale height of 100 km which was the measured scale height at the subsolar point for a true anomaly angle (TAA) of ~180° (i.e., at aphelion) in Mercury’s elliptical orbit^3,11. This model exosphere is extremely simple and does not capture many of the details of the actual exosphere. The model is only intended to show how high neutral sodium densities (>10² cm⁻³) are typically expected to be close to the dayside surface in comparison to MESSENGER’s altitude of 5300 km. The bow shock and magnetopause were drawn using a model derived from MESSENGER observations, for the observed conditions for this event²⁸. Source data are provided as a Source Data file.