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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. [ABSTRACT FROM AUTHOR]

We investigate Mars‐ward planetary ions (O+ and O2+) in the Martian magnetotail that potentially reduce the amount of escaping ions. The global properties of Mars‐ward flows in the Martian magnetotail are characterized, based on over 13‐years of ion data (May 2007–December 2020) collected by the Analyzer of Space Plasma and Energetic Atoms instrument on Mars Express. We find that Mars‐ward flows are frequently observed in the vicinity of the crustal fields, implying that crustal fields may play a key role in producing such flows. The occurrence rate and sunward flux are found higher during solar maximum than solar minimum. However, the occurrence rate and flux of Mars‐ward flows are relatively low. This is different from the case at Venus, where the planetward flows can significantly decrease the total escape rates of ions. Plain Language Summary: Without the protection of an intrinsic magnetic field, solar wind can interact directly with the Martian atmosphere and drive ion escape, which plays a vital role in the evolution of the planetary atmosphere. It was shown that, on average, all Martian ions accelerated by the solar wind flow downstream and escape to interplanetary space. However, some planetary ions can flow sunward or Mars‐ward in the tail, possibly reducing total ion escape. The mechanism and role of these Mars‐ward ions remain unclear. Here, we present a statistical analysis that reveals the global properties of Mars‐ward flows at Mars based on over 13 years of Mars Express data. Our results suggest that crustal fields may play a role in driving Mars‐ward flows. We also show that Mars‐ward flows might have negligible influence on total ion escape at Mars regardless of solar activity level. This finding can help us understand ion escape and tail dynamics at Mars. Key Points: Mars‐ward ion flows in the magnetotail occur more frequently and with a higher flow flux during solar maximumThe occurrence rate and flux of planetward flows at Martian magnetotail are relatively lower than the case at Venusian magnetotailA strong correlation between crustal fields and Mars‐ward flows is identified [ABSTRACT FROM AUTHOR]

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 J→×B→ $\overrightarrow{J}\times \overrightarrow{B}$ force at +E‐flank, but much thinner and with a weaker J→×B→ $\overrightarrow{J}\times \overrightarrow{B}$ (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. Plain Language Summary: The dynamics and distribution of Martian space plasma are strongly coupled with the magnetic field structure. The knowledge of the magnetic field morphology of the Martian induced magnetosphere is essential to understanding the plasma dynamics of the Martian magnetosphere, for example, the ion escape. However, the real magnetic field morphology of the Martian magnetosphere remains unclear. To clarify the global magnetic field structure around Mars, we comprehensively study the three‐dimensional (3‐D) magnetic field structure of the Martian magnetosphere based on measurements from the Mars Atmosphere and Volatile EvolutioN spacecraft. We found that the reconstructed 3‐D magnetic field lines around Mars, governed by the orientation of upstream interplanetary magnetic fields (IMFs), are draped around the planet, and the morphology has an evident asymmetry along the direction of the solar wind electric field. We found the field structure is affected by the sunward component of the IMF, and the variations of solar wind dynamic pressure. We also quantitatively estimated the average structure of the tail current sheet and calculated the electromagnetic force exerted on the plasma in the current sheet, which is critical to interpreting the plasma escape through the tail current sheet. Key Points: The magnetic field structure has an evident hemispheric asymmetry along the direction of the solar wind electric fieldThe tail current sheet is thicker and has a stronger tailward J→×B→ $\overrightarrow{J}\times \overrightarrow{B}$ force near the flank of the +E‐hemisphereThe effects of interplanetary magnetic field Bx, the dynamic pressure of solar wind on the field structure are surveyed respectively [ABSTRACT FROM AUTHOR]

The geomagnetic field originates from interior dynamo currents and can be approximated well by a simple dipolar field in the vicinity of Earth's surface. Given that the dipolar field is induced by a current loop, it is possible to invert the loop parameters by tracing the sampled magnetic field geometry. Drawing on the analysis of field geometric structure with sampled field dataset, we develop, in this study, a new technique to invert the interior current source, which is based on a single circular current loop model. Unlike previous studies, this technique has the ability to separate and solve the optimal loop parameters successively, including the location of the loop center, the loop axis, the radius, and the carried electric current. Applications to the International Geomagnetic Reference Field and the sampled magnetic field dataset by the spacecraft of the Swarm mission demonstrate that our technique‐derived loop center, loop axis, and magnetic moment are consistent with previous estimations by the eccentric dipole model, which shows the reasonability and effectiveness of this technique. Moreover, our technique can be reduced to fit an eccentric dipole model and is particularly useful for inversions of the geometry of interior current sources. Thus, it could be applied widely in the fields of planetary magnetism and palaeomagnetism. Further applications and constraints are discussed, with some cautions given. Plain Language Summary: Most planets in our solar system possess a global dipolar magnetic field. The dipolar field can be approximatively generated by a current loop. The loop's parameters, including the location of the loop center, the loop axis, the radius, and the carried electric current, are key to characterizing the geometry and strength of the dipolar field. However, traditional methods that use least‐square fitting of all loop parameters simultaneously result in multiple local solutions, meaning there is no general way to solve it. By analyzing the sampled magnetic field dataset, we present a novel technique that separates and solves the loop parameters successively. Several tests show that our technique is reasonable and able to be applied in the fields of geomagnetism, planetary magnetism, and palaeomagnetism. Key Points: New technique is developed to diagnose the geomagnetic dipolar field based on a current loop modelTechnique is able to separate and solve the loop parameters according to the sampled data set of the magnetic fieldTests and applications show that technology is effective and applicable and could be applied widely in the fields of planetary magnetism [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: In contrast to Earth, Mars does not have a global dipole magnetic field. An earlier Mars Global Surveyor (MGS) mission discovered that Mars possesses localized magnetic field anomalies, which most likely originate in the Martian crust. The localized crustal magnetic fields on Mars are much stronger than the crustal fields usually observed on Earth. The strong Martian crustal field can significantly affect the interaction between solar wind and Mars. An accurate crustal field model is thus necessary and important for studying, among others, the Martian space environment and the geometry and origin of the crustal field. Here, we present a new crustal magnetic field model that combines the spacecraft data of Mars Atmosphere and Volatile EvolutioN and MGS. For low altitudes, the new model fits the data much better than previous models, giving a more accurate representation of the Martian crustal field. This new model could thus greatly improve our knowledge of the Martian crustal field, including its field morphology, its field map at low altitudes, and its field strength at different wavelengths. Key Points: A spherical harmonic Martian crustal field model is presented that combines both MAVEN and MGS dataThe global power spectrum of the Martian crustal field, up to spherical‐harmonic degree 90, is robustly determinedOur new model demonstrates that small‐scale crustal fields on Mars may have been largely underestimated by most previous models [ABSTRACT FROM AUTHOR]

Quantitatively estimating magnetotail flapping motion is critical for understanding and characterizing its dynamical behaviors. Such estimation can be achieved in principle by the multipoint analysis of spacecraft tetrahedron, for example, Cluster or MMS mission, but, owing to the inability of single-point measurement to separate the spatial-temporal variation of magnetic field, would be inadequate for a single spacecraft. Since single-point missions dominate explorations of planetary magnetotail, we have developed a single-point method based on the magnetic field measurement that quantitatively estimates the parameters of flapping motion, including spatial amplitude, wavelength, and propagation velocity. By comparing several applied cases with the multipoint analysis of Cluster, we demonstrate that our method can be reasonably applied to infer the average parameters over the whole flapping period when magnetotail is during quiet phase (magnetic field in magnetotail does not experience significant temporal variation). Thus, this method could be applied widely to the "big data set" accumulated by single-point spacecraft missions in order to study magnetotail flapping dynamics. [ABSTRACT FROM AUTHOR]