Properties and sources of the dayside martian magnetosphere

Abstract

This study is based on measurements of the energy-mass analyzer Suprathermal and Thermal Ion Composition instrument (STATIC, McFadden et al., 2015) and magnetometer (MAG, Connerney et al., 2015) on the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft (Jakosky et al., 2015). 48 passes of MAVEN at SZA < 70° in the northern hemisphere on the day side of Mars (in order to avoid the influence of remnant magnetic field) were selected with condition to have continuous observations of STATIC from ionosphere to magnetosheath (“complete” observation crossings). It was found that in the vast majority of such “complete” crossings there is a layer between the ionosphere and magnetosheath, where two oxygen ion populations are observed: (1) heated ionospheric ions and (2) higher energy oxygen ions. The heated ionospheric ions are located just above the cold ionospheric ions and is identified by the increase of energy and temperature and decrease of number density. The second ion population has a wide energy spread from a few eV to hundreds of eV and has a velocity distribution typical to pick-up ions. The quite similar characteristics of the pick-ions just upstream of the magnetopause and characteristics of the second ions component in the layer between magnetosheath and ionosphere suggests their common origin. Pick-up ions in this layer have energies from a few to hundred electron volts have gyroradii comparable or significantly larger (from ~80 to ~570 km) than the thickness of this layer (~90 km).Pick-up ions from magnetosheath penetrate into upper ionosphere and interact with ionospheric ions and upper atmosphere. The population of pick-up ions provides additional energy source to the ionosphere and the upper atmosphere of Mars.The layer between dayside magnetosheath and ionosphere is not sufficiently studied and has many different names (see Espley, 2018). The physical state and properties of this layer (formed by combination of hydrodynamic and kinetic processes) are very much different from magnetosheath and ionosphere. This is defined as the dayside magnetosphere of Mars (Vaisberg et al., 2018).