Analysis of Sodium Ions in the Solar Wind at Mercurys high-altitude Exosphere: MESSENGER Spacecraft Measurements

Due to its closer proximity to the Sun, Mercury experiences different solar wind conditions compared to Earth, and it is distinctive for having a tenuous, collisionless sodium exosphere rather than a dense atmosphere or ionosphere. This sodium exosphere is formed by a variety of source and loss processes, where sodium atoms are released from the surface of the planet. Particularly, the sputtering and meteoroid impact vaporization processes can release sodium atoms from Mercurys surface into its exosphere with high enough velocities that they can reach high altitudes. Neutral sodium in the high-altitude exosphere can then be photoionized and picked up by the solar wind. However, sodium that has just been ionized close to Mercurys surface cannot escape the planets magnetosphere due to the strong magnetic field of Mercury, making these sodium ion events detected in the solar wind unusual. Sputtering is a source process that is not well understood; therefore, studying it provides further understanding of Mercurys magnetospheric coupling with the solar wind and exosphere. Because sputtering is regulated by magnetic reconnection at the planets dayside magnetopause, an analysis of the Interplanetary Magnetic Field (IMF) orientation is necessary to understand if the identified Na+ events are caused by sputtering or by another source process such as meteoroid impact vaporization. In addition, studying the compression (or expansion) of Mercurys magnetopause can help us determine if sodium events are more likely to occur due to the magnetospheres interaction with the solar wind. We suggest that the identified events with count rates of ~3.0-4.5 counts s-1 occurred most likely due to a meteoroid impact and that a planetward IMF might play an important role in the sputtering process.