The magnetically controlled ionopause boundary observed by LPW onboard MAVEN within magnetic pile-up region of Mars

Abstract

We have analysed plasma and magnetic field data obtained from 627 orbits of Mars Atmosphere and Volatile Evolution (MAVEN) that occurred during October 2014, April 2015, September 2017 and May 2018, when the spacecraft was crossing the magnetic pile-up boundary of Mars. Two broad peaks and a drop are observed in this magnetic field region. The magnetic cavity boundary lies between the ionosphere and the magnetic pile-up boundary. A steep ionopause-like boundary is observed in 24 profiles of 627 orbits, when MAVEN was passing from the magnetic pile-up region during the daytime ionosphere in presence of horizontal magnetic field of high strength, while their night side ionosphere did not show such boundary in presence of horizontal magnetic field of low strength. These profiles are unique, which have not been reported earlier. We propose that the horizontal magnetic field can form such ionopause within the magnetic pile-up boundary during the daytime if time and location of the magnetic anomaly coincide with the ion and electron density measurements. The nighttime ionosphere is thought to be produced within the magnetic pile-up boundary due to transportation of plasma from dayside to nightside across the terminator by a horizontal plasma flow velocity as has been predicted by the previous studies.