This plot shows the measured magnitude of the magnetic field of Mercury as MESSENGER executed its first flyby of that planet. MESSENGER's Magnetometer (MAG) provided definitive identification of all boundaries of the Mercury magnetosphere system, consistent with the observations made with the Fast Imaging Plasma Spectrometer (FIPS) on the Energetic Particle and Plasma Spectrometer (EPPS) instrument, and revealed a much more quiescent system than was seen during the first Mariner 10 flyby. This state of the system was also consistent with the absence of energetic particles as documented by the Energetic Particle Spectrometer (EPS) portion of MESSENGER's EPPS instrument. Mercury lacks radiations belts similar to the Van Allen belts at the Earth discovered by James Van Allen with a simple particle experiment on Explorer I launched 50 years ago.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington