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. The solar wind is to the left of the bow shock, and magnetospheric model field lines 51, help visualize Mercury’s magnetosphere. a View from dusk ( X’- Z’ plane) in the noon–midnight meridian, with the Sun to the left. The coordinate system is in the aberrated Mercury solar orbital frame (MSO). Locations of the exosphere and magnetospheric boundaries, and the trajectory of MESSENGER during the heavy-ion observations. 1b, (where the bow shock boundary and the spacecraft are aligned in the same plane). Therefore, the appearance of the spacecraft inwards of the bow shock is a projection effect, and MESSENGER lies outside the bow shock in the solar wind as can more accurately be visualized in Fig. The spacecraft however, is positioned dawnward of Mercury (i.e., Y' ~ −2 R M, where R M = 2440 km) where the bow shock is northward of MESSENGER. 1a is in the noon–midnight meridian (i.e., at Y = 0). The spacecraft was traveling northwards and was located in the SW outside of the bow shock (red line). Figure 1 shows the location and trajectory of MESSENGER during these observations. On December 21, 2013, MESSENGER measured unexpectedly large heavy-ion counts (of planetary origin) in the solar wind where only protons or alpha particles are usually measured. The MESSENGER spacecraft frequently spent time in the solar wind outside of Mercury’s magnetosphere and the bow shock 10. These particles were subsequently photoionized in the solar wind and observed by the MESSENGER spacecraft.
We conclude that the cause of the event is the impact of a meteroid at Mercury, which vaporized Na and Si from the surface. We analyze the pickup ion’s velocity and infer the neutral densities from the observed ion fluxes. Here we show an extreme event where the MESSENGER spacecraft observed newly ionized particles from a neutral cloud of exospheric particles at high altitudes. Similarly, the magnetosphere is compressed on the dayside by the solar wind (SW), and the nightside magnetic field is stretched out to form a magnetotail, and so the exosphere mostly lies within the magnetosphere of Mercury.
Due to its proximity to the Sun, Mercury’s sodium exosphere experiences radiation pressure which compresses the exosphere at the subsolar region and accelerates sodium atoms to escape velocities on the nightside to form a cometary-like tail 4, 5, 6, 7, 8, 9. This sodium exosphere has small dayside scale heights of up to ~100 km at perihelion, with subsolar densities of 10 3 cm −3 measured at altitudes of ~450 km 3. Sodium is the most abundant observed species in the exosphere and has been the most well studied from ground observations as well as by MESSENGER. The most continuously observed species by the MESSENGER spacecraft were Na, Mg, and Ca 2.
The composition of this exosphere is now known to contain H, He, Na, K, Ca, Mg, Al, Fe, and Mn 1.
Mercury has a tenuous exosphere, which is supplied by particles released from its surface.