MESSENGER Disappearing Dayside Magnetosphere Events: Evidence for Severe Dayside Erosion and/or Compression?

Tuesday, 16 December 2014
Helen R Middleton1, James A Slavin2, Jim M Raines2, Xianzhe Jia3, Brian J Anderson4, M. Leila Mays5 and Thomas Zurbuchen6, (1)ESA/ESAC, Villanueva De La Can, Spain, (2)University of Michigan Ann Arbor, Ann Arbor, MI, United States, (3)University of Michigan, Ann Arbor, MI, United States, (4)Johns Hopkins University, Baltimore, MD, United States, (5)NASA/GSFC, Greenbelt, MD, United States, (6)Univ Michigan, Ann Arbor, MI, United States
During northward passes over Mercury's dayside hemisphere, the MESSENGER spacecraft normally enters the dayside magnetosphere before it descends to below ~ 500 km altitude. However, for some of these dayside passes the dayside magnetosphere is completely absent in the magnetometer (MAG) and Fast Imaging Plasma Spectrometer (FIPS) observations. During these “disappearing" dayside magnetosphere passes the MESSENGER measurements indicate that the spacecraft passed directly from the magnetosheath into the northern magnetospheric cusp and, finally, the high latitude nightside magnetosphere. Likely causes of these unusual events are severe reconnection-driven erosion and/or solar wind compression of the dayside magnetosphere, to the point where the closed field line dayside magnetosphere lies equatorward and/or below the orbit of MESSENGER. A survey of such cases in the MESSENGER data is presented. The hypothesis that these events are produced by extreme dayside magnetosphere reconnection and/or solar wind compression, is tested using ENLIL predictions of solar wind conditions at Mercury orbit and analysis of the MESSENGER measurements in the cusp and high-latitude nightside magnetosphere.