The ESA-JAXA BepiColombo mission will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric The BepiColombo mission to Mercury Edited by Johannes Benkhoff, Go Murakami and Ayako Matsuoka B A. Milillo
Using over 6 years of magnetic field data (October 2014–December 2020) collected by the Mars Atmosphere and Volatile EvolutioN, we conduct a statistical study on the three‐dimensional average magnetic field structure around Mars. We find that this magnetic field structure conforms to the pattern typical of an induced magnetosphere, that is, the interplanetary magnetic field (IMF) which is carried by the solar wind and which drapes, piles up, slips around the planet, and eventually forms a tail in the wake. The draped field lines from both hemispheres along the direction of the solar wind electric field (E) are directed toward the nightside magnetic equatorial plane, indicating that they are “sinking” toward the wake. These “sinking” field lines from the +E‐hemisphere (E pointing away from the plane) are more flared and dominant in the tail, while the field lines from the –E‐hemisphere (E pointing toward) are more stretched and “pinched” toward the plasma sheet. Such highly “pinched” field lines even form a loop over the pole of the –E‐hemisphere. The tail current sheet also shows an E‐asymmetry: the sheet is thicker with a stronger tailward trueJ→×trueB→ $\overrightarrow{J}\times \overrightarrow{B}$ force at +E‐flank, but much thinner and with a weaker trueJ→×trueB→ $\overrightarrow{J}\times \overrightarrow{B}$ (even turns sunward) at –E‐flank. Additionally, we find that IMF Bx can induce a kink‐like field structure at the boundary layer; the field strength is globally enhanced and the field lines flare less during high dynamic pressure.
The Moon and Mercury are airless bodies, thus they are directly exposed to the ambient plasma (ions and electrons), to photons mostly from the Sun from infrared range all the way to X-rays, and to meteoroid fluxes. Direct exposure to these exogenic sources has important consequences for the formation and evolution of planetary surfaces, including altering their chemical makeup and optical properties, and generating neutral gas exosphere. The formation of a thin atmosphere, more specifically a surface bound exosphere, the relevant physical processes for the particle release, particle loss, and the drivers behind these processes are discussed in this review.
Early exploration of Mercury, based on data collected by Mariner 10 during its three flybys, revealed that it was the only terrestrial planet in our solar system, other than Earth, to possess a global dipolar magnetic field (Ness et al., 1974). A subsequent mission known as MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER), sent the first spacecraft to orbit around Mercury (Solomon et al., 2007). It confirmed the dipolar field and found it was similar to Earth's in that the magnetic field lines of Mercury are divergent near the south pole and convergent toward the north pole; Mercury's dipole moment, however, is only about 195 ± 10 nT R M 3 (R M = 2,440 km is the radius of Mercury)-much weaker than Earth's (4/10000 of Earth's dipole moment)-and Mercury's dipole center is shifted northward by about 484 ± 11 km (0.2 R M ) (Anderson, Johnson et al., 2011). Further, Mercury has no atmosphere but possesses a tenuous surface-bounded exosphere. As the closest planet to the Sun, Mercury encounters a much stronger impingement of solar wind, whose density and dynamic pressure are an order of magnitude higher than those at Earth. In comparison to Earth, the result is a much smaller, weaker and more dynamic magnetosphere (
Knowing the properties of the Martian magnetotail flapping waves is critical to understand the dynamics of the Martian induced magnetosphere. Based on the measurements by Mars Atmosphere and Volatile EvolutioN and a newly developed method, we provide the first quantitative study of Martian magnetotail flapping waves in comparison with Earth's magnetotail flapping waves. We found that the period of Martian magnetotail flapping waves ranges from ∼50 to ∼250 s. The estimated average wave amplitude is ∼380 km, and the wavelength is ∼1,100 km. The estimated propagation speed ranges from ∼3 to ∼30 km/s, which is about a third of that of Earth's magnetotail flapping waves, and the speed declines with the increase of the current sheet thickness. Intriguingly, we found that the flapping waves with shorter periods, or lower wavelengths, can propagate faster, showing similar dispersive signatures as that found in the Earth's magnetotail. Our results demonstrate that the fluctuated field energy carried by flapping waves within unit time along unit magnetotail length is one order higher than that of Earth's flapping waves, as normalized by the magnetic energy of planetary magnetotail. Thus, in comparison with Earth's magnetotail, the flapping waves of Martian induced magnetotail would play a more important role in affecting the magnetotail dynamics.
Unlike Earth, Mars does not possess an intrinsic global, dipolar magnetic field, which leads to the direct interaction of the Martian ionosphere/atmosphere with the solar wind and scavenging of the planetary ions (e.g., Luhmann & Brace, 1991, and references therein;Zhang et al., 2021). The "frozen-in" interplanetary magnetic field (IMF), carried by the solar wind plasma flow, is draped around Mars as solar wind approaches the planet. This draped IMF would then slip into a wake, resulting in an elongated induced magnetotail that is characterized by a pair of magnetic lobes having antiparallel field lines, together with a current sheet between the lobes, while this induced magnetotail configuration would be controlled by the IMF orientation (e.g.
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