The BepiColombo–Mio Magnetometer en Route to Mercury

Space Sci Rev (2020) 216:125 https://doi.org/10.1007/s11214-020-00754-y The BepiColombo–Mio Magnetometer en Route to Mercury W. Baumjohann1 · A. Matsuoka2 · Y. Narita1 · W. Magnes1 · D. Heyner3 · K.-H. Glassmeier3,4 · R. Nakamura1 · D. Fischer1 · F. Plaschke1 · M. Volwerk1 · T.L. Zhang1 · H.-U. Auster3 · I. Richter3 · A. Balogh5 · C.M. Carr5 · M. Dougherty5 · T.S. Horbury5 · H. Tsunakawa6 · M. Matsushima7 · M. Shinohara8 · H. Shibuya9 · T. Nakagawa10 · M. Hoshino11 · Y. Tanaka12 · B.J. Anderson13 · C.T. Russell14 · U. Motschmann15,16 · F. Takahashi17 · A. Fujimoto18 Received: 1 December 2019 / Accepted: 3 October 2020 © The Author(s) 2020 Abstract The fluxgate magnetometer MGF on board the Mio spacecraft of the BepiColombo mission is introduced with its science targets, instrument design, calibration report, and scientific expectations. The MGF instrument consists of two tri-axial fluxgate magnetometers. Both sensors are mounted on a 4.8-m long mast to measure the magnetic field around Mercury at distances from near surface (initial peri-center altitude is 590 km) to 6 The BepiColombo mission to Mercury Edited by Johannes Benkhoff, Go Murakami and Ayako Matsuoka B Y. Narita 1 Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria 2 World Data Center for Geomagnetism, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan 3 Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, 38106 Braunschweig, Germany 4 Max-Planck-Institut für Sonnensystemforschung, 37077 Göttingen, Germany 5 Blackett Laboratory, Imperial College, London SW72AZ, UK 6 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, 229-8510, Japan 7 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan 8 National Institute of Technology, Kagoshima College, Kirishima, Kagoshima 899-5193, Japan 9 Department of Earth Sciences, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan 10 Department of Information and Communication Engineering, Tohoku Institute of Technology, Sendai, Miyagi 982-8577, Japan 11 Department of Earth and Planetary Science, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan 12 National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan 13 Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA 125 Page 2 of 33 W. Baumjohann et al. planetary radii (11640 km). The two sensors of MGF are operated in a fully redundant way, each with its own electronics, data processing and power supply units. The MGF instrument samples the magnetic field at a rate of up to 128 Hz to reveal rapidly-evolving magnetospheric dynamics, among them magnetic reconnection causing substorm-like disturbances, field-aligned currents, and ultra-low-frequency waves. The high time resolution of MGF is also helpful to study solar wind processes (through measurements of the interplanetary magnetic field) in the inner heliosphere. The MGF instrument firmly corroborates measurements of its companion, the MPO magnetometer, by performing multi-point observations to determine the planetary internal field at higher multi-pole orders and to separate temporal fluctuations from spatial variations. Keywords Magnetic field · Mercury · Magnetosphere · Inner heliosphere 1 Introduction Understanding the magnetic field environment around Mercury is one of the primary science targets in the BepiColombo mission (Benkhoff et al. 2010). From a space plasma point of view, Mercury is distinct among the solar system’s planets in that 1. the planet possesses an intrinsic magnetic field and magnetosphere even though the planet itself is rather small (a radius of about 2440 km) and rotating slowly (cf., other “terrestrial” bodies such as Venus, Mars, and Earth’s Moon do not have an intrinsic field), 2. the size of magnetosphere is very small and comparable to the gyro-radius of heavy ions (Na+ , for example, has a gyro-radius of about 1000 km), making the magnetosphere respond quickly to the changes in the solar wind condition such as flow speed, density variation, magnetic field direction with a characteristic time scale of the magnetosphere about 1–2 minutes (Balogh 1997; Baumjohann et al. 2006; Slavin et al. 2009)), 3. the lack of an ionosphere makes the magnetospheric dynamics (through the electric current configuration) different from Earth’s magnetosphere with its ionosphere (Glassmeier 1997; Slavin et al. 1997). Mercury’s planetary magnetic field was discovered by Mariner 10’s flybys in 1974 and 1975 (Ness et al. 1974, 1975). The discovery was the most surprising result of the mission because the thermal condition, the rotation rate, and the presumed core state (believed to be a solid iron core) seemingly excluded the possibility of a dynamo mechanism in operation. Already Mariner 10 observed a variety of magnetospheric structures and processes such as a dipolar-like intrinsic field, magnetopause, magnetotail, substorm-like disturbances (Siscoe 14 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA 15 Institut für Theoretische Physik, Technische Universität Braunschweig, 38106 Braunschweig, Germany 16 DLR Institute of Planetary Research, 12489 Berlin, Germany 17 Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Fukuoka 819-0395, Japan 18 School of Computer Science and Systems Engineering, Kyushu Institute of Technology, Kawazu, Fukuoka 820-8502, Japan The BepiColombo–Mio Magnetometer en Route to Mercury Page 3 of 33 125 et al. 1975), and ultra-low-frequency (ULF) waves (Russell 1989). Various currents flowing in Mercury’s magnetosphere system were identified such as magnetopause current, magnetotail current, field-aligned currents, reconfiguration currents, and induced currents within the planet (Glassmeier 2000). The MESSENGER mission (Solomon et al. 2007), launched in 2004 and orbiting Mercury from 2011 to 2015, improved our understanding of Mercury’s magnetosphere after Mariner 10 significantly. The surface equatorial field is estimated at about 250 to 290 nT with a dipole field contribution at surface level in the range from 180 to 220 nT. The magnetic equator in the tail is shifted northward, giving an offset dipole magnetosphere as the lowest-order picture (Anderson et al. 2011, 2012). From a dynamo theoretical point of view, the northward offset of the magnetic equator implies that higher-order terms, in particular the quadrupole field, play a more important role than in the other magnetospheres like at Earth, Jupiter, and Saturn (the quadrupole term also plays an important role at both Uranus and Neptune). For a more comprehensive review of Mercury’s magnetic field see, e.g., Anderson et al. (2010) and Wicht and Heyner (2014). The MESSENGER observations revealed the magnetospheric structure and processes in more (...truncated)