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Using the Mars Atmosphere and Volatile EvolutioN measurements, we observed a wave train of vortices in different developing phases of the Kelvin‐Helmholtz (K‐H) instability at the ionopause above the Martian northern hemisphere during 09:35:00–09:55:00 Universal Time on 24 March 2017. Based on the sawtooth‐like oscillations in the magnetic fields associated with the quasi‐periodic fluctuations in the ion densities, velocities, and total pressure, we identified three fully and two partially developed vortices. The wavelength of the K‐H waves is ∼2 Martian radii and the period is ∼2 min. The magnetic topologies associated with the vortices are observed to change from draped in the magnetosheath to open in the transition region and finally to closed approaching the ionosphere. Significant ion loss caused by the K‐H instability is observed. The total oxygen loss rate is estimated to be ∼1025 s−1, which is comparable to the global ion loss as previously estimated. Plain Language Summary: Shear‐driven Kelvin‐Helmholtz (K‐H) instability can form along the boundary of the solar wind interaction with non‐magnetic planets (such as Venus and Mars). Previous studies reported partially and fully developed K‐H vortices at the Martian induced magnetosheath. In this study, we directly observed both types of the K‐H vortices during the developing process of the K‐H instability at the Martian ionopause. The magnetic topologies associated with the K‐H vortices change from draped in the magnetosheath to open in the transition region and to closed approaching the ionosphere. The K‐H instability can be an effective mechanism that leads to ion escape at Mars. A large number of ions within the K‐H vortices are observed to escape from Mars. The loss rate of oxygen caused by the K‐H instability is estimated to be significantly high. Our results confirm that the K‐H instability is important for the Martian ion loss. Key Points: We report a wave train of Kelvin‐Helmholtz (K‐H) vortices during different developing phases of the K‐H instability at the ionopause of MarsThe wavelength of the K‐H waves is ∼2 Martian radii and the period is ∼2 min; the magnetic topologies within the K‐H vortices changedThe estimated loss rate of oxygen caused by the K‐H instability is on the order of 1025 s−1 [ABSTRACT FROM AUTHOR]

The Martian‐induced magnetopause (IMP), characterized by the sudden changes of magnetic fields and ion compositions, is the transition boundary separating the magnetosheath from Martian induced magnetosphere. Based on the magnetic field and plasma measurements by Mars Atmosphere and Volatile EvolutioN, we for the first time report a magnetic reconnection at the dayside IMP. The clear plasma jet and bipolar Hall magnetic field provide solid evidence for the identification of the magnetic reconnection. Our observations demonstrate that the newly arrived interplanetary magnetic field and the anchored magnetic field in the induced magnetosphere can reconnect with each other at the IMP. Such reconnection allows the planetary ions to escape from Mars through the reconnected magnetic fields across the IMP, resulting in a significant increase in the local loss rate of the ionospheric ions. Plain Language Summary: Magnetic reconnection is one of the most important topics in space physics since it can not only alter the topology of the magnetic field but also quickly convert magnetic energy to kinetic energy. Around Mars, magnetic reconnections are identified in various regions, such as the induced magnetosphere, ionosphere, magnetotail. However, the magnetic reconnection on the Martian induced magnetopause, which separates the induced magnetosphere from the shocked solar wind, has not been reported and analyzed yet. In this letter, we for the first time report a magnetic reconnection at the Martian magnetopause with Hall magnetic field and ion jet flow. The magnetic reconnections on the magnetopause are crucial because they can accelerate planetary heavy ions in the induced magnetosphere or ionosphere to facilitate their escape from Mars. Key Points: Magnetic reconnection at the Martian induced magnetopause is reportedHall magnetic field associated with the reconnection are detected by Mars Atmosphere and Volatile EvolutioNThe thickness of the reconnection region and local oxygen loss rate are estimated to be 575 km and ∼1.0⋅1024s−1, respectively [ABSTRACT FROM AUTHOR]

The lunar wake is usually rapidly refilled when the background plasma is substantially subsonic. Here we present observational case studies, by the two ARTEMIS spacecraft, of the lunar wakes in the magnetosheath near the magnetopause under transonic plasma flows with relatively large plasma beta and different magnetic field orientations. The associated background plasma betas are near 1, suggesting that the plasmas are not strongly controlled by the magnetic fields. As a result, the part of the lunar wake under subsonic plasma was rapidly refilled. Pitch angle distributions reveal that a significant portion of the refilling ions are from the downstream wake. This is consistent with the fact that higher sound speed than bulk speed of the background plasma enables downstream particles to get access to upstream wake due to the pressure gradient. The other part under supersonic plasma remains a typical wake structure with high plasma density or flux depletions. Our results show that the Mach number and plasma beta primarily determine the rapid refilling of the lunar wake, which is allowed only in subsonic plasma with plasma beta greater than or close to 1 and hardly influenced at all by the magnetic field orientation. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is a fundamental and important physical process that can release magnetic energy and change magnetic topology. A statistic study of reconnected magnetic field lines has been carried out by Cluster spacecraft 1 during 58 current sheet crossing events. We calculated the values of at different distances from reconnection site to describe the loosing process of magnetic field lines. Our results suggest that current sheets in ion diffusion regions have small values of. As the distance from reconnection site increases, the reconnected magnetic field lines relax, and subsequently have relatively large values of. However, such large values of are found to be closely related to open magnetic field lines that connect the Earth's magnetosphere and the solar wind. [ABSTRACT FROM AUTHOR]

We report a sudden expansion of the lunar crustal field observed by ARTEMIS P1 at analtitude of 1035 km near the optical shadow boundary of the Moon. The lunar crustal field islocated at 81˚ E and 8˚ N on the lunar surface and is characterized by strong depletions ofion and electron fluxes indicative of closed magnetic field lines. Such expansion of crustalfield is found to be the combination of the crustal field, the interplanetary magnetic field andthe induced magnetic field of a solar wind current sheet. Our analysis show that the currentsheet is a rotational discontinuity with nonzero normal magnetic fields. The calculated J⋅E<0indicates that the current sheet is a small-scale dynamo. The poynting flux of this dynamo,which transfers other energies into electromagnetic energy, can provide electromagneticenergy that is required to support the expansion of the lunar crustal field in this study. [ABSTRACT FROM AUTHOR]