Previous studies suggested that Mercury’s terrestrial-like magnetosphere could possess Earth-like field-aligned currents (FACs) despite having no ionosphere. However, due to the limited coverage of spacecraft observations, our knowledge about Mercury’s FACs is scarce. Here, to survey the establishment and global pattern of Mercury’s FACs, we used Amitis, a hybrid-kinetic plasma model, to simulate the response of Mercury’s FACs to different interior conductivity profiles and various orientations of the upstream interplanetary magnetic field (IMF). We find that the planet with a conductive interior favors the establishment of FACs, and that IMF orientation controls the pattern of FACs in a similar manner as it does on Earth. But the response of R2-like FACs to IMF orientation differs, thus we cannot simply regard Mercury’s FACs as a scaled-down version of Earth’s. Comparison between our simulations and the previous data analysis suggests that the effective interior conductance to close Mercury’s FACs is ~2.4-3.4 S.

Mercury has a terrestrial-like magnetosphere which is usually taken as a scaled-down-version of Earth’s magnetosphere with a similar current system. We examine Mercury’s magnetospheric current system based on a survey of Mercury’s magnetic field measured by the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft as well as computer simulations. We show that there is no significant Earth-like ring current flowing westward around Mercury, instead, we find, for the first time, an eastward current encircling the planet near the night side magnetic equator with an altitude of ~500–1000 km. The eastward current is closed with the dayside magnetopause current and could be driven by the gradient of plasma pressure as a diamagnetic current. Thus, Mercury’s magnetosphere is not a scaled-down Earth magnetosphere, but a unique natural space plasma laboratory. Our findings offer fresh insights to analyze data from the BepiColombo mission, which is expected to orbit Mercury in 2025.

This paper presents a new highly accurate Martian crustal magnetic field model (G90). Mars Global Surveyor (MGS) and Mars Atmosphere and Volatile evolution (MAVEN) satellite vector magnetic data are combined to represent the Martian crustal magnetic fields by spherical harmonic (SH) functions up to degree 90. We use the conventional least squares measure to determine the Gauss coefficients, while no regularization was applied. The internal field and external field are separated by using a new magnetic field activity proxy from MAVEN data. Dayside and nightside data selection criteria are also employed to minimize the influence of the ionospheric field. The resulting model delineates the details of the crustal field with a spatial resolution of ∼245 km at 120 km altitude. At satellite altitudes, this model shows a lower misfit than any other presented model, which indicates that this model is more reliable for studying the crustal field topology. Due to its high accuracy, this model will help to address some of the open questions about the Martian crustal field.

We investigate sunward planetary ions in the Martian magnetotail that potentially reduce the amount of escaping ions. The global properties of sunward flows in the Martian magnetotail are characterized, based on over 13-years of ion data (May 2007–December 2020) collected by the ASPERA-3 instrument on Mars Express. We find that sunward flows mainly occur 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 higher during solar maximum rather than solar minimum. However, we identify a relatively low occurrence rate of sunward flows and low sunward flux, suggesting that sunward flows have negligible influence on total ion escape at Mars. This is different from those at Venus, where sunward flows can significantly decrease the total escape rates of ions.

Using over 6 years of magnetic field data (2014.10-2020.12) collected by the Mars Atmosphere and Volatile EvolutioN (MAVEN), 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 towards the nightside magnetic equatorial plane, which looks like 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 towards) are more stretched and “pinched” towards 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 force at +E-flank, but much thinner and with a weaker J×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; however, the rotation of the planet, against expectations, modulate the configuration of the tail current sheet insignificantly.

The ion composition of the tail current sheet is essential to understanding the Martian ion escape, however, our current knowledge is poor. Using measurements by Mars Atmosphere and Volatile EvolutioN (MAVEN), we investigate the density of different ion species in the Martian magnetotail current sheet. We find that events we survey of the current sheet, the average occurrence rate of current sheets with dominant heavy ions (O+ and O2+) is about 50%, while the rate of those with dominant H+ is about 20%. A current sheet closer to the terminator is mostly dominated by heavy ions, regardless of upstream solar wind, but it could be dominated sometimes by H+ when it is beyond the downstream distance of 0.75 RM (RM is Mars’ radius), where the occurrence rate of current sheets with dominant H+ (heavy ions) weakly increases (decreases) with solar wind density and dynamic pressure.

Quantitatively estimating magnetotail flapping motion is critical to understanding and characterizing the dynamics of flapping behaviors. Such an estimation could be achieved in principle by the multipoint analysis of spacecraft tetrahedron, e.g. 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 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. A comparison with the application of multi-point analysis of Cluster demonstrates that our method can be reasonably be applied to infer the average parameters over a flapping period. Thus, this method could be applied widely to the “big dataset” accumulated by single-point spacecraft missions in order to study magnetotail flapping dynamics.

Topological configurations of the magnetic field play key roles in the evolution of space plasmas. This paper presents a novel algorithm that can estimate the quadratic magnetic gradient as well as the complete geometrical features of magnetic field lines, based on magnetic field and current density measurements by a multiple spacecraft constellation at 4 or more points. The explicit estimators for the linear and quadratic gradients, the apparent velocity of the magnetic structure and the curvature and torsion of the magnetic field lines can be obtained with well predicted accuracies. The feasibility and accuracy of the method have been verified with thorough tests. The algorithm has been successfully applied to exhibit the geometrical structure of a flux rope. This algorithm has wide applications for uncovering a variety of magnetic configurations in space plasmas.

With Magnetospheric Multiscale Mission (MMS) observation of a magnetic flux rope of ion scale in magnetopause, we apply the single-point method presented by Rong et al., [2013] to study the magnetic field structure of flux rope. The calculated geometric parameters, e.g. axis orientation, helical handedness, current density, curvature radius, and boundaries of flux rope show well consistency with those derived from the multi-point methods. Thus, the single-point method of Rong et al., [2013] is reliable for studying the interior field structure of magnetic flux rope and could be applied widely to single-point spacecraft missions that examine the dynamics of flux rope.

A quick and effective technique is developed to diagnose the geomagnetic dipole field based on an unstrained single circular current loop model. In comparsion with previous studies, this technique is able to separate and solve the loop parameters successively. With this technique, one can search the optimum full loop parameters quickly, including the location of loop center, the loop orientation, the loop radius, and the electric current carried by the loop, which can roughly indicate the locations, sizes, orientations of the interior current sources. The technique tests and applications demonstrate that this technique is effective and applicable. This technique could be applied widely in the fields of geomagnetism, planetary magnetism and palaeomagnetism. The further applications and constrains are discussed and cautioned.