An illustration of MESSENGER flying through Mercury’s magnetosphere.
An illustration of Mercury’s magnetosphere experiencing magnetic reconnection (‘x’). Curly arrows show solar wind plasma either impacting the planet (near the North pole) or bouncing off the magnetic field there and traveling into the magnetotail (on the night side of the planet).
Mercury’s exosphere and surface are (mostly) shielded from the solar wind by a planetary magnetic field, just like at the Earth. The protected region around the planet carved out by this magnetic field is called the magnetosphere (see left images, above). MESSENGER found that Mercury’s magnetosphere is very dynamic due to the planet’s small magnetic field and proximity to the Sun. One process, called magnetic reconnection, is nearly 100 times faster than at Earth. Magnetic reconnection joins magnetic field lines carried in the solar wind with those of the planet, which then transfers large amounts of energy into the magnetosphere (see right image). Data from MESSENGER’s magnetometer showed that magnetic reconnection formed twisted helical tubes of magnetic field lines, at a rate about 60 times higher than at Earth. This process can actually cause high-energy plasma, travelling around 1 million miles per hour, to impact Mercury’s surface. The neutral atoms and ions that are liberated (blasted!) from the surface as a result of such impacts act as a source of neutral atoms to the exosphere and ions to the magnetosphere. MESSENGER’s Fast Ion Plasma Spectrometer (FIPS) instrument was able to measure both of these types of ions, as well as ions formed by photoionization of exospheric neutral atoms by the Sun’s light. This data showed plasma in Mercury’s magnetosphere that originates from the solar wind and the planet itself behaves in many interesting ways.