The significantly weaker magnetic field, smaller magnetosphere, and much faster timescale of processes around Mercury, when compared with Earth, enable charged particles to escape its magnetosphere more efficiently through magnetopause shadowing and direct bombard of the surface. A new analysis of data from NASA’s MESSENGER (MErcury Surface, Space Environment, GEochemistry, and Ranging) spacecraft proves that, despite these substantial differences, a bifurcated ring current — doughnut-shaped field of charged particles flowing laterally around the planet and excluding the poles — can form in the magnetosphere of Mercury and initiate magnetic storms under strong solar wind driving.
This color image of Mercury was generated from three MESSENGER images taken through filters sensitive to light in different wavelengths. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington.
Mercury’s magnetosphere was discovered by NASA’s Mariner 10 spacecraft in the 1970s. It has an intrinsic dipole field with northwards offset and a small magnetic moment.
Recent MESSENGER observations confirmed that Mercury’s magnetosphere resembles Earth’s in many aspects, such as magnetospheric structures (e.g., magnetotail, plasma mantle, and polar cusp), magnetospheric dynamic processes, and magnetic structures (e.g., dipolarization fronts and flux ropes).
However, in situ measurements to directly demonstrate the existence of Mercury’s ring current — a magnetospheric electric current mainly carried by ions trapped in a planetary magnetosphere — were lacking.
“Confirmation about geomagnetic storms on Mercury results from research made possible by a fortuitous coincidence – a series of coronal mass ejections from the Sun on April 8-18, 2015, and the end of NASA’s MESSENGER space probe, which launched in 2004 and crashed into the planet’s surface on April 30, 2015, at the expected end of its mission,” said Peking University planetary researcher Jiutong Zhao and colleagues.
The coronal mass ejection of April 14, 2015 compressed Mercury’s ring current on the Sun-facing side and increased the current’s energy.
The analysis of the MESSENGER data revealed the presence of a ring current intensification that is essential for triggering magnetic storms.
“The sudden intensification of a ring current causes the main phase of a magnetic storm,” said Professor Hui Zhang, a space physicist with the Geophysical Institute at the University of Alaska Fairbanks.
But this doesn’t mean Mercury has auroral displays like those on Earth.
On Earth, the storms produce aurora displays when solar wind particles interact with the particles of the atmosphere.
On Mercury, however, solar wind particles don’t encounter an atmosphere.
Instead, they reach the surface unimpeded and may therefore be visible only through X-ray and gamma ray examination.
The team’s results show that magnetic storms are potentially a common feature of magnetized planets.
“Our results obtained from MESSENGER provide a further fascinating insight into Mercury’s place in the evolution of the Solar System following the discovery of its intrinsic planetary magnetic field,” the authors said.
The findings appear in two papers in the journal Science China Technological Sciences and the journal Nature Communications.
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Q. Zong et al. Magnetic storms in Mercury’s magnetosphere. Sci. China Technol. Sci, published online February 18, 2022; doi: 10.1007/s11431-022-2009-8
JT. Zhao et al. 2022. Observational evidence of ring current in the magnetosphere of Mercury. Nat Commun 13, 924; doi: 10.1038/s41467-022-28521-3
