Helium in Mercury's Extended Exosphere Determined by Pick‐Up Generated Ion Cyclotron Waves

Helium (He) was first detected by remote spectroscopic observations of the Ultraviolet Visible Spectrometers (UVVS) instrument in Mercury's exosphere during the three Mariner 10 flybys in 1974 and 1975. Here, we derive the first in situ radial density profile of He in Mercury's extended exosphere by analyzing magnetic field and plasma measurements from 2011 to 2015 obtained by the MErcury Surface, Space ENvironment, Geophysics, and Ranging (MESSENGER) spacecraft. Our results indicate that most of the exospheric He follows a thermal trend. Interestingly, some events show density enhancements compared to the thermal profile which might be caused by sporadic (micro-)meteorite impact events. These energetic He population events yield average He surface densities of 150 cm−3. The thermal He population on Mercury's day side can be reproduced with a surface density of 1,100 cm−3. The main exospheric He source can be identified as thermal release and thermal recycling of solar wind implanted He particles that agree well with the observations made by the Probing Of Hermean Exosphere By Ultraviolet Spectrometer (PHEBUS) instrument on board BepiColombo during its first Mercury fly by and exospheric simulations.