A MAVEN Survey of Ionospheric Flux Ropes at Mars: Evidence for 3 Formation Mechanisms

Abstract:

Magnetic flux ropes have been analyzed in the ionosphere of Mars in order to understand their formation mechanism and contribution to atmospheric escape. Detected at objects throughout the solar system including unmagnetized bodies like Venus and Titan, flux ropes are a magnetic field phenomenon characterized by a filament of twisted magnetic field with an axial core and outer helical wraps. Flux ropes play a role in two classes of fundamental processes at Mars: (1) the rapid reconfiguration of magnetospheric magnetic fields and the acceleration of charged particles, and (2) atmospheric and magnetospheric mass loss through plasma channeling and entrainment in their helical wraps and escape down the magnetotail. Using magnetic field data taken by the magnetometer (MAG) instrument onboard the Mars Atmosphere and Volatlie EvolutioN (MAVEN) spacecraft, we identify flux ropes by their characteristic increase in total field magnetic field strength coinciding with the inflection point of a bipolar signature created by the surrounding helical wraps. These flux ropes display a wide range in amplitude (5 - 110 nT), latitude (-70° - +70°), and solar zenith angle (15°- 110°). We use both the fluxes of electrons and their energy distribution measured by the Solar Wind Electron Analyzer (SWEA) instrument onboard MAVEN to parameterize the likelihood of a loss cone distribution and whether the measured electrons are composed of primarily photoelectrons or solar wind electrons. We combine these parameters with MAG measurements to estimate the magnetic topology throughout the flux rope structure, and therefore their likely formation mechanism. We present the first survey of flux ropes at Mars with evidence for flux ropes formed by 3 distinct formation mechanisms: External Reconnection (ER) between the draped IMF and crustal anomalies, Internal Reconnection (IR) between the crustal anomalies themselves, and Boundary Wave Instabilities (BWI) similar to those observed at Venus. Of the 153 in our database, 50 (33%), 88 (58%) and 15 (10%) flux ropes are consistent with the ER, IR and BWI formation mechanisms, respectively. Our classification of the formation of flux ropes at Mars will further our understanding of the processes that comprise the complex coupling between the Martian magnetosphere and atmosphere.