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.Plain Language SummaryThe solar wind consists of a continual stream of plasma and magnetic field emitted from the Sun in all directions, permeating the heliosphere. The density and magnetic field strength in the solar wind decrease with distance from the Sun, so Mercury, located in the inner heliosphere (0.3–0.45 AU), experiences a very strong interaction with the solar wind. The surface of Mercury is protected from this powerful solar wind by the planetary magnetic field, which carves out a cavity in the solar wind called the magnetosphere. The primary means by which the solar wind drives global dynamics in Mercury’s system is via a mechanism known as magnetic reconnection, in which energy and momentum are transferred to the planetary magnetosphere. Energy builds up in the magnetosphere until it cannot be sustained and is explosively released, accelerating particles toward the planetary surface. At Earth, this energy release may be observed in the brightening of the aurora. This loading and unloading of energy into the terrestrial system is known as a substorm. This research paper characterizes the properties of analogous loading and unloading of energy in Mercury’s magnetosphere, using data from the MESSENGER spacecraft.Key PointsFirst statistical study of loading‐unloading events at Mercury using data from the entire MESSENGER missionCharacterizes the properties of loading‐unloading events to understand the magnetic flux content and transport in Mercury’s magnetosphereCompares the magnetic flux transport during substorms at Mercury with observations at the EarthPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141310/1/jgra53810.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141310/2/jgra53810_am.pd