This study presents a new spherical harmonic (SH) model of the crustal magnetic field of Mars, based on the magnetic field data set measured by the Mars Global Surveyor (MGS) and the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. To minimize the influence of external fields due to solar wind interaction with Mars, we rejected data that were observed dayside and above an altitude of 500 km. The data points of MAVEN were reduced by using a proxy of solar wind activity that identified and rejected any data measured during magnetically disturbed intervals. We used a conventional least squares technique to estimate the Gauss coefficients fitted to the reduced data set and made a compromise between model misfit and model roughness by truncating the SH model at degree 110. This model is capable of representing crustal fields with a spatial resolution approaching ∼200 km at 120 km altitude and ∼260 km at the Martian surface. Since our model fits MAVEN's observational data better than previous models, especially the data obtained during MAVEN's low altitude periapsis passes, we conclude that it may more accurately approximate the low‐altitude crustal field. We calculate the crustal field power spectrum of various models and find that small‐scale fields at low altitudes were underestimated by most previous models. This new model could benefit future studies associated with the Martian crustal field and its interaction with the solar wind.

Using the data sets of Mars Atmosphere and Volatile EvolutioN and OMNI for the period 2014 October 10–2020 February 14 and the heliocentric distance of 1–1.66 au, we investigate the statistical properties of solar wind upstream of Mars for the first time. The key parameters, including interplanetary magnetic field (IMF), proton density (N), bulk velocity (∣ V ∣), and dynamic pressure (P _dyn), are surveyed with regard to variations of solar activity level and heliocentric distance. We find that the parameters ∣IMF∣, N, and P _dyn monotonously decrease with heliocentric distance. Both ∣IMF∣ and P _dyn are generally stronger at a higher solar activity level (F _10.7 ≥ 70 sfu), while such activity has little relevance to N. In contrast, ∣ V ∣ basically keeps a median of about 370 km s⁻¹ and is insensitive to the solar activity level and heliocentric distance. We also find that the IMF upstream of Mars at the higher solar activity level has a much smaller spiral angle in the inward sector; thus, IMF seems “straighter” than that in the outward sector, although that is not so for the inward sector of the upstream of Earth. Our statistical survey can be used as a reference for upstream solar wind of Mars at 1.4 ∼ 1.7 au, and could benefit the studies on solar wind as well as the Martian space environment.

Flapping motion of the current sheets of planetary magnetotails is a common dynamic phenomenon. Previous studies of the Earth's magnetotail suggest that its flapping motion has two forms, that is, kink‐like flapping that can propagate as waves toward both flanks and steady flapping that moves up and down but does not propagate. Although some models have been proposed to explain the kink‐like flapping, its mechanism remains unclear. This paper surveys 87 flapping events statistically with respect to their flapping types, using the multipoint measurements of Cluster. The statistical results show that the up‐down steady flapping events tend to occur around the midnight region, and the kink‐like flapping events tend to occur near both flanks of the magnetotail. Thus, we propose that kink‐like flapping motion is causally related to steady flapping motion; that is, the up and down motion of steady flapping around the midnight region induces kink‐like flapping waves, which propagate toward both flanks of the magnetotail.

Today's geomagnetic field can prevent energetic particles, including solar energetic particles and galactic cosmic rays, from directly hitting the Earth's atmosphere. However, when the geomagnetic field strength is significantly decreased during geomagnetic field excursions or reversals, the geomagnetic field shielding effect becomes less prominent. Geomagnetic cutoff rigidity, as a quantitative estimation of the shielding effect, can be calculated using trajectory tracing or theoretical equations. We use a recent high‐resolution continuous geomagnetic field model (LSMOD.2) to study the geomagnetic cutoff rigidity during the Laschamps excursion. Global grids of the geomagnetic cutoff rigidities are presented, in particular for the excursion midpoint when the geomagnetic field is weak and not dipole‐dominated anymore at Earth's surface. We compare the cutoff rigidity calculation results between a trajectory tracing program and theoretical equations and we find that the influence of the non‐dipole component of the geomagnetic field cannot be ignored during the excursion. Our results indicate that the exposure of Earth's atmosphere to energetic particles of cosmic and solar origin is high and nearly independent of latitude in the middle of the Laschamps excursion. Our results will be useful for future studies associated with cosmic radiation dose rate and cosmogenic isotope production rate during the Laschamps excursion.