MESSENGER Observations of Cusp Plasma Filaments at Mercury

Friday, 19 December 2014
Gang Kai Poh1, James A Slavin1, Gina A DiBraccio2, Xianzhe Jia3, Jim M Raines1, Suzanne M Imber4, Brian J Anderson5, Haje Korth6, Daniel J Gershman7, Thomas Zurbuchen8, Ralph L McNutt Jr6 and Sean C Solomon9, (1)University of Michigan Ann Arbor, Ann Arbor, MI, United States, (2)NASA Goddard Space Flight Center, Solar System Exploration Division, Greenbelt, MD, United States, (3)University of Michigan, Ann Arbor, MI, United States, (4)Radio and Space Plasma Physics, Leicester, United Kingdom, (5)Johns Hopkins Univ, Laurel, MD, United States, (6)The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States, (7)NASA Goddard Space Flight Center, Heliophysics Sci. Div., Greenbelt, MD, United States, (8)Univ Michigan, Ann Arbor, MI, United States, (9)Lamont-Doherty Earth Observatory, Palisades, NY, United States
At Mercury, MESSENGER has documented ~1-2-s-long reductions in the dayside magnetospheric magnetic field with amplitudes up to 90% of the ambient intensity. These field reductions which we have termed cusp filaments are observed from just poleward of the magnetospheric cusp to mid-latitudes. During these events, MESSENGER’s Fast Imaging Plasma Spectrometer (FIPS) measured H+ ions with magnetosheath-like energies. Minimum variance analysis of the Magnetometer (MAG) data indicates that the filaments are simple two dimensional flux tubes filled with magnetosheath plasma that has a diamagnetic effect on the local background field. Here we analyze 139 filaments identified in 3 years of MESSENGER magnetic field and plasma data to determine the physical properties of these structures. Our results indicate that cusp filaments are common phenomena for all solar wind conditions. They occur over a range of magnetic latitudes from ~50 to 80oN, with durations of ~0.1─2.5s and magnetic field decreases of ~50─300 nT. If the filaments are associated with flux transfer events (FTEs) and move over the spacecraft at speeds comparable to the flank magnetosheath flow speed of 300 km/s, then these filaments have dimensions of ~30─750 km, which is larger than the gyro-radius of a 1 keV H+ ion, i.e., ~ 23 km. Correlation analyses show no obvious dependence of the duration or magnitude of the diamagnetic decrease on magnetic latitude. Overall, the MAG and FIPS observations analyzed here appear consistent with an origin for cusp plasma filaments by the inflow of magnetosheath plasma associated with the localized magnetopause reconnection process that produces FTEs. Further analysis will be required to confirm this hypothesis.