From 1997 to 2006, the Mars Global Surveyor (MGS) spacecraft provided magnetic field measurements while orbiting Mars, extensively sampling the magnetic field at an altitude of about 400 km (Acuña et al., 1998) after periapsis was raised upon completion of the aerobraking phase. The MGS mission discovered that Mars possesses many localized remanent magnetic fields, which most likely originate in the Martian lithosphere (Acuña et al., 1999). Remanent magnetic fields, otherwise known as crustal fields or lithospheric magnetic fields, are widely believed to be induced by an ancient core dynamo. Mars currently does not have a global dipole magnetic field as in the case of Earth and Mercury (Langlais et al., 2010). The most intense crustal fields of Mars are located in the Southern Hemisphere. These fields are 1 to 2 orders of magnitude stronger than the crustal fields on Earth (Kother et al., 2015;Voorhies et al., 2002), 3 to 4 orders of magnitude stronger than the crustal fields on Moon (Purucker & Nicholas, 2010) and Mercury (Johnson et al., 2015).

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.

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.

Ion escape to space through the interaction of solar wind and Mars is an important factor influencing the evolution of the Martian atmosphere. The plasma clouds (explosive bulk plasma escape), considered an important ion escaping channel, have been recently identified by spacecraft observations. However, our knowledge about Martian plasma clouds is lacking. Based on the observations of the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we study a sequence of periodic plasma clouds that occurred at low altitudes (∼600 km) on Mars. We find that the heavy ions in these clouds are energy-dispersed and have the same velocity, regardless of species. By tracing such energy-dispersed ions, we find the source of these clouds is located in a low-altitude ionosphere (∼120 km). The average tailward moving flux of ionospheric plasma carried by clouds is on the order of 107 cm−2 s−1, which is one order higher than the average escaping flux for the magnetotail, suggesting explosive ion escape via clouds. Based on the characteristics of clouds, we suggest, similar to the outflow of Earth’s cusp, these clouds might be the product of heating due to solar wind precipitation along the open field lines, which were generated by magnetic reconnection between the interplanetary magnetic field and crustal fields that occurred above the source.

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 (