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The levitation of charged dust, which may cause serious hazards to astronauts and lunar rovers, has been one of the most significant challenges in lunar exploration. Here we simulate lunar sheath potentials in different solar wind conditions and solar zenith angles (SZAs) on the lunar surface by the particle-in-cell method. The simulated potentials exhibit two types of distributions as a function of height, depending on the SZAs. For SZA ∼ 0°–70°, the nonmonotonic distribution with positive surface potential dominates in the photoelectron sheath. For SZA >∼81°, the monotonic distribution with negative surface potential is observed in the plasma sheath. With the calculated potentials and the assumption that the dust radius distribution exponentially decreases, we further investigate spatial distributions of the dust levitated above the surface. It is found that number density of the levitating lunar dust is enhanced at the terminator (SZA ∼ 81°) in the plasma sheath. In the photoelectron sheath it gradually decreases as the SZA increases from 0° to 70°. Further calculations of the potential and the derived electrostatic field suggest that the dust spatial distributions can be influenced by the bulk velocity, number density, and temperature of the solar wind. Those findings deepen our understanding of lunar surface charging and the mechanism of lunar dust levitation, which can provide technical support for lunar explorations.

We have used the AMR-CESE-MHD model to investigate the influences of the IMF Bz and the upstream solar wind dynamic pressure (Dp) on Earth’s magnetopause and bow shock. Our results present that the earthward displacement of the magnetopause increases with the intensity of the IMF Bz. The increase of the northward IMF Bz also brings the magnetopause closer to the Earth even though with a small distance. Our simulation results show that the subsolar bow shock during the southward IMF is much closer to the Earth than during the northward IMF. As the intensity of IMF Bz increases (also the total field strength), the subsolar bow shock moves sunward as the solar wind magnetosonic Mach number decreases. The sunward movement of the subsolar bow shock during southward IMF are much smaller than that during northward IMF, which indicates that the decrease of solar wind magnetosonic Mach number hardly changes the subsolar bow shock location during southward IMF. Our simulations also show that the effects of upstream solar wind dynamic pressure (Dp) changes on both the subsolar magnetopause and bow shock locations are much more significant than those due to the IMF changes, which is consistent with previous studies. However, in our simulations the earthward displacement of the subsolar magnetopause during high solar wind Dp is greater than that predicted by the empirical models.

Magnetic reconnection between neighboring magnetic field loops, the so-called interloop reconnection, is a common process to drive flares in the solar atmosphere. However, there is no direct evidence that a similar but less explosive process can take place on planets. The strong crustal fields on Mars generate plenty of magnetic loops in the near-Mars regions, providing a unique environment to research the interloop reconnection on a planet. Here, we report magnetic reconnection events between crustal field loops in the Martian ionosphere observed by Mars Atmosphere and Volatile EvolutioN (MAVEN) for the first time. During the current layer crossing, MAVEN recorded the characteristic signals of collisionless magnetic reconnection, including the Hall magnetic field, Alfvénic outflow, and electron energization. This finding implies that the interloop reconnection in the Martian ionosphere could contribute to the localized energy deposition and particle energization, which provides the seed source for aurora in the Martian atmosphere. [ABSTRACT FROM AUTHOR]

We develop a general 3-D relativistic test particle (TP) simulation code to examine the trajectories and pitch angle variations of test electrons. We investigate the conditions when the electrons transit broadband, parallel propagating H+ band electromagnetic ion cyclotron (EMIC) waves to find whether the classical quasilinear theory (QLT) can be applied to interactions between H+ band EMIC waves and radiation belt relativistic (and ultra-relativistic) electrons. We find that to reach good consistency between TP scattering coefficients and QLT calculations, the wave-particle resonant interaction time (which indicates the time of particles traveling in the wave fields and being resonant with the wave fields) must be carefully calculated to ensure the validity of the linear diffusion process. This wave-particle resonant interaction time depends largely on the wave amplitude, the bandwidth, and the initial parameters of electrons for TP simulations; once the time of electrons traveling in the wave fields becomes large enough when the pitch angles of electrons scattered by wave fields reach the maximal value limited by the resonance condition and cannot be diffused any longer, the linear diffusion process of particles defined by QLT will not be valid any longer. When the resonant electrons start to travel through the wave field, they undergo the linear diffusion process resulting from the pitch angle scattering by the H+ band EMIC wave field. As the time of traveling in the wave fields increases, the test electrons will suffer effective pitch angle scattering and the pitch angles of these electrons will reach the maximum value limited by the resonance condition, which is determined by the wave and electron parameters. These electrons are then reflected by the wave fields and the linear wave-particle interactions defined by QLT will not be valid anymore. This kind of wave-particle interaction subsequently introduces considerable deviation of TP diffusion coefficients from those predicted by QLT calculations. Our results demonstrate that the general validity of QLT in describing how broadband EMIC waves affect radiation belt relativistic electrons is highly related to the time of electron traveling in the wave fields, which tends to increase for smaller wave power and broader wave frequency spectra. [ABSTRACT FROM AUTHOR]