MESSENGER Observations of Magnetotail Loading and Unloading: Implications for Substorms at Mercury

We present the first statistical study of loading and unloading of magnetic flux in Mercury's magnetotail. These events describe the global circulation of magnetic flux through the magnetosphere and provide strong evidence that terrestrial-type substorms take place at Mercury. 438 events were identified over the 4 years of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission by a gradual, short-lived increase in the magnetotail lobe magnetic field strength, coincident with an outward flaring of the magnetotail. Substorm duration ranged from tens of seconds to several minutes, with a median of 195 s and a mean of 212 s. The median amplitude of lobe magnetic field increase was ~11.5 nT, which represents an increase of 23.4% on the background lobe field strength, compared with ~10% for terrestrial substorms. The magnetotail lobes were found to contain ~2–3 MWb of magnetic flux based on 1031 tail passes, with a mean of 2.52 MWb and a standard deviation of 0.48 MWb. An estimate of the change in open flux content during the loading phase of each substorm ranged from 0.08 to 3.7 MWb with a mean value of 0.69 MWb and a standard deviation of 0.38 MWb. These changes in open flux content are an underestimate as the change in magnetotail radius during the events was not accounted for. The maximum lobe flux content during each substorm (~3 MWb) represented ~40% of the total available magnetic flux in the system (~7.5 MWb). During terrestrial substorms, the maximum lobe magnetic flux content is ~10–12% of the total flux from the dipole. A typical substorm at Mercury therefore cycles through a significantly larger fraction of the available magnetic flux than all but the largest substorms at the Earth.