The MEFISTO and WPT Electric Field Sensors of the Plasma Wave Investigation on the BepiColombo Mio Spacecraft: Measurements of Low and High Frequency Electric Fields at Mercury

This paper describes the design of MEFISTO (Mercury Electric Field In-Situ Tool) and WPT (Wire Probe Antenna) electric field sensors for Plasma Wave Investigation (PWI) on the BepiColombo Mio spacecraft (Mercury Magnetospheric Orbiter, MMO). The two sensors will enable the first observations of electric fields, plasma waves and radio waves in and around the Hermean magnetosphere and exosphere. MEFISTO and WPT are dipole antennas with 31.6 m tip-to-tip length. Each antenna element has a spherical probe at each end of the wire (15 m length). They are extended orthogonally in the spin plane of the spacecraft and enable measurements of the electric field in the frequency range of DC to 10 MHz by the connection to two sets of receivers, EWO for a lower frequency range and SORBET for higher frequencies. In the initial operations after the launch (20 Oct. 2018), we succeeded to confirm the health of both antennas and to release the launch lock of the WPT. After Mercury orbit insertion planned at the end of 2025, both sensors will be fully deployed and activate full operations of the PWI electric field measurements.