On the supply of heavy planetary material to the magnetotail of Mercury

Abstract

Abstract.We examine the transport of low-energy heavy ions of planetary origin (O + , Na + , Ca + ) in the magnetosphere of Mercury. We show that, in contrast to Earth, these ions are abruptly energized after ejection into the magnetosphere due to enhanced curvature-related parallel acceleration. Regardless of their mass-to-charge ratio, the parallel speed of these ions is rapidly raised up to ∼ 2 V E×B (denoting by V E×B the magnitude of the local E × B drift speed), in a like manner to Fermi-type acceleration by a moving magnetic mirror. This parallel energization is such that ions with very low initial energies (a few tenths of eVs) can overcome gravity and, regardless of species or convection rate, are transported over comparable distances into the nightside magnetosphere. The region of space where these ions reach the magnetotail is found to extend over altitudes similar to those where enhanced densities are noticeable in the MESSENGER data, viz., from ∼ 1000 km up to ∼ 6000 km in the pre-midnight sector. The observed density enhancements may thus follow from E × B related focusing of planetary material of dayside origin into the magnetotail. Due to the planetary magnetic field offset, an asymmetry is found between drift paths anchored in the Northern and Southern hemispheres, which puts forward a predominant role of heavy material originating in the Northern Hemisphere in populating the innermost region of Mercury's magnetotail.

Plasma Sources in Planetary Magnetospheres: Mercury

“…While the typical yield for sputtered ions by proton 1320 impingement is in the range of 10 -4 to 10 -1 per impacting ion [Benninghoven, 1975], 1321 the yields measured for ESD could be ten times higher, especially the more energetic 1322 the incident electrons [Wang et al, 1984]. ejection into the magnetosphere [Delcourt, 2013]. In this latter study, it was found 1415 that the numerous populations that are released at very low energies such as those due 1416 to thermal desorption are rapidly accelerated up to ∼2V E×B (V E×B being the E×B drift 1417 speed) in a like manner to the acceleration due to a moving magnetic mirror [Cowley, 1418[Cowley, 1984.…”

Plasma Sources in Planetary Magnetospheres: Mercury

“…The answer varies from model to 1341 model, of course, but despite their major differences, all sodium models estimate the 1342 content to be on the order of 10 28 sodium atoms [the exosphere content ranges 0. ejection into the magnetosphere [Delcourt, 2013]. In this latter study, it was found 1415 that the numerous populations that are released at very low energies such as those due 1416 to thermal desorption are rapidly accelerated up to ∼2V E×B (V E×B being the E×B drift 1417 speed) in a like manner to the acceleration due to a moving magnetic mirror [Cowley, 1418[Cowley, 1984.…”

MESSENGER observations of multiscale Kelvin‐Helmholtz vortices at Mercury

“…Numerical simulations of planetary ion trajectories [ Delcourt et al ., ; Seki et al ., ; Delcourt , ] have treated Na + ions as independent test particles, although planetary ions have been reported in sufficient abundance (e.g., 0.1–1 cm −3 in the premidnight plasma sheet) [ Gershman et al ., ] to contribute to bulk plasma parameters such as thermal pressure (Σ i P i ), mass density (Σ i m i n i ), and instability growth, where n i , P i , and m i are the number density, partial pressure, and mass of each species i in a plasma. Furthermore, it is in regions containing keV Na + ions that we expect kinetic scale physics to be most prominent.…”