image: The magnetic flux circulation of a Dungey cycle corresponds to a magnetospheric substorm, which lasts 2 to 3 minutes at Mercury. The location of the magnetopause, the occurrence of dayside magnetopause reconnection, and the shape of the plasma sheet at the start of the substorm (a) growth phase, (b) expansion phase, and (c) recovery phase. view more
Credit: ©Science China Press
This review paper is led by Dr. Weijie Sun, who is currently an assistant research scientist from the Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor. Mercury is the smallest and innermost planet in the solar system. “Mercury’s diameter is approximately one-third of Earth’s diameter, and is even slightly smaller than moons such as Ganymede and Titan,” says Dr. Weijie Sun. “Mercury has a global intrinsic magnetic field and experiences the strongest driving from the solar wind among the planets in the solar system,” Dr. Sun continues.
Due to its proximity to the Sun, spacecraft would require a relatively high speed to reach Mercury and would be difficult to insert into or stay at a stable orbit around Mercury. This paper summarizes the research of Mercury’s magnetosphere, which has been made mostly based on NASA-MESSENGER’s measurements. MESSENGER inserted into and stayed approximately four Earth years around Mercury, which is the only spacecraft that had done this by far.
The review paper starts by introducing the interplanetary environment, magnetosphere, exosphere, and conducting core of Mercury. The latest measurements from the NASA-Parker Solar Probe mission were used to reveal the interplanetary environment of Mercury. “The Parker Solar Probe has provided comprehensive particle and field measurements of Mercury’s orbital zone, which has revealed the large variations of the solar wind at Mercury’s perihelion and aphelion. The parameters that control the dynamics of Mercury’s magnetosphere, including solar wind dynamic pressure, convection electric field, plasma beta, and Alfvénic Mach number show large variations between Mercury’s perihelion and aphelion,” says Dr. Weijie Sun.
The frequent and intense magnetic reconnection on the dayside magnetosphere is revealed by the unique flux transfer event “shower”, which corresponds to a large number of magnetic flux ropes around the magnetosphere. The magnetopause reconnection relies on both magnetosheath plasma beta and the magnetic shear angle across the magnetopause. The flux transfer event showers can significantly contribute to the fundamental flux circulation, i.e., the Dungey cycle, and magnetosphere-surface-exosphere coupling at Mercury. In the next, the feedbacks from Mercury’s iron core due to the extreme solar events, for example, the coronal mass ejections (CMEs), are reviewed. The harder the push by the CMEs, the larger the induction currents of Mercury’s iron core. And then, the dawn-dusk asymmetries of Mercury’s magnetotail, which is opposite to the properties of Earth’s magnetotail, and the particle energizations in Mercury’s magnetosphere, in which ions are subject to strong kinetic effect but electrons are not, are summarized.
Following each topic, the authors discuss the open questions related to the topic, which can be considered by the computational simulations and the future spacecraft mission, especially the BepiColombo project, which is a joint mission between ESA and JAXA currently en route to Mercury. “BepiColombo made its first Mercury flyby on October 1st, 2021, and it plans to insert into Mercury’s orbit by the end of 2025. BepiColombo will also enable simultaneous two-point observations of Mercury’s magnetosphere for the first time. At times, one spacecraft will measure solar wind conditions upstream of the bow shock and serve as a solar wind monitor for the other spacecraft inside the magnetosphere,” says Dr. Weijie Sun.
However, the study of Mercury’s dynamic magnetosphere shall not only be limited to the Solar System. “Mercury is the only example of a hot terrestrial planet in our Solar System. Understanding of the solar wind-magnetosphere-surface coupling at Mercury is likely to contribute to the process of exoplanet research relating to small terrestrial-type planets,” says Prof. James A. Slavin also from the University of Michigan, who coauthors on this review paper.
See the article:
Sun W., Dewey R.M., Aizawa S., Huang J., Slavin J.A., Fu S., Wei Y., Bowers C..F. Review of Mercury’s dynamic magnetosphere: Post-MESSENGER era and comparative magnetospheres. SCIENCE CHINA Earth Sciences, 65 (1), 25-74 (2022), https://doi.org/10.1007/s11430-021-9828-0
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Science China Earth Sciences