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.

This paper reports on the standing whistler waves upstream of Mercury’s quasi-perpendicular bow shock. Using MESSENGER’s magnetometer data, 36 wave events were identified during interplanetary coronal mass ejections (ICMEs). These elliptic or circular polarized waves were characterized by: (1) a constant phase with respect to the shock, (2) propagation along the normal direction to the shock surface, and (3) rapid damping over a few wave periods. We inferred the speed of Mercury’s bow shock as ~31 km/s and a shock width of 1.76 ion inertial length. These events were observed in 20% of the MESSENGER orbits during ICMEs. We conclude that standing whistler wave generations at Mercury are generic to ICME impacts and the low Alfvén Mach number (MA) collisionless shock, and are not affected by the absolute dimensions of its bow shock. Our results further support the theory that these waves are generated by the current in the shock.

Aurora displays provides an essential diagnostic to spatial and temporal variations of terrestrial space environment and is also an important proxy of solar activity. Contemporary auroral observations have just continued for more than half a century. In the long history prior to modern era, visual auroral observations can dates back to 1450 AD in mid-latitudes and polar regions in Europe. In mid- and low-latitude regions in East Asia, official historical books in China, Korea, and Japan also recorded numerous visual auroral phenomena began from 1000 AD until modern times. In this study, we compiled a new auroral catalogue from ancient Korean historical books, including 2013 auroral records with day-level resolution from 1012 to 1811 AD, especially for the records searched from the . The occurrence of the aurora in the new catalogue is generally consistent with previous datasets. This extended dataset provides valuable support for various studies related to solar-terrestrial space weather and ancient climates.

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.

The Neutral Gas and Ion Mass Spectrometer of the Mars Atmosphere and Volatile Evolution provides a large data set to explore the ion composition and structure of the Martian ionosphere. Here the dayside measurements are used to investigate the minor ion density profiles with distinctive peaks above 150 km, revealing a systematic trend of decreasing peak altitude with increasing ion mass. We specifically focus on a subset of species including O$^+$, N$_2^+$/CO$^+$, C$^+$, N$^+$, He$^+$, and O$^{++}$, all of which are mainly produced via direct photoionization of parent neutrals. Our analysis reveals weak or no variation with solar zenith angle (SZA) in both peak density and altitude, which is an expected result because these ion peaks are located within the optically thin regions subject to the same level of solar irradiance independent of SZA. In contrast, the solar cycle variations of peak density and altitude increase considerably with increasing solar activity, as a result of enhanced photoionization frequency and atmospheric expansion at high solar activities. He$^+$ serves as an exception in that its peak density increases towards large SZA and meanwhile shows no systematic variation with solar activity. The thermospheric He distribution on Mars should play an important role in determining these observed variations. Finally, the peak altitudes for all species are elevated by at least several km within the weakly magnetized regions, possibly attributable to the suppression of vertical diffusion by preferentially horizontal magnetic fields in these regions.

Plasmoid is a key process for transferring magnetic flux and plasma in planetary magnetospheres. At Earth，plasmoid is a key media transferring energy and mass in the “Dungey Cycle” magnetospheric circulation. For giant planets, plasmoid is primarily generated by the dynamic processes associated with “Vasyliunas Cycle”. It is generally believed that planetary magnetotails are favourable for producing plasmoids. Nevertheless, recent study reveals that magnetic field lines could be sufficiently stretched to allow magnetic reconnection (Guo et al. 2018a) in Saturn’s dayside magnetodisc. And in the study, we report direct observations of plasmoid in Saturn’s dayside magnetodisc. Moreover, we perform a statistical investigation on the global plasmoid electron density distribution. The results show an inverse correlation between the nightside plasmoid electron density and local time, and the maximum plasmoid electron density around prenoon local time on the dayside. These results are consistent with the magnetospheric circulation picture associated with“Vasyliunas Cycle”.

Energetic electron depletions are a notable feature of the nightside Martian upper atmosphere. In this study, we investigate systematically the variations of the occurrence of depletions with both internal and external conditions, using the extensive Solar Wind Electron Analyzer measurements made on board the Mars Atmosphere and Volatile Evolution. In addition to the known trends of increasing occurrence with decreasing altitude and increasing magnetic field intensity, our analysis reveals that depletions are more easily observed when the ambient magnetic fields are more horizontally inclined and under lower Solar Wind (SW) dynamic pressures. We also find that the occurrence increases with increasing atmospheric CO$_2$ density but this trend is restricted to low altitudes and within weakly magnetized regions only. These observations suggest that the formation of electron depletions is two folded: (1) Near strong crustal magnetic anomalies, closed magnetic loops preferentially form and shield the atmosphere from direct access of SW electrons, a process that is modulated by the upstream SW condition; (2) In weakly magnetized regions, SW electrons precipitate into the atmosphere unhindered but with an intensity substantially reduced at low altitudes due to inelastic collisions with ambient neutrals. In addition, our analysis reveals that both the ionospheric plasma content and thermal electron temperature are clearly reduced in regions with depletions than those without, supporting SW electron precipitation as an important source of external energy driving the variability in the deep nightside Martian upper atmosphere and ionosphere.