Search

In Jupiter's magnetosphere, the electric current system within the "current sheet" generates a magnetic field, which is comparable to or dominating the Jovian intrinsic field in the magnetosphere. However, updates of an existing model of the magnetospheric field by Khurana (1997, https://doi.org/10.1029/97JA00563) using newly acquired data by Galileo and Juno has never been conducted since it was first formulated. Here we used the data by Voyager 1/2, Galileo, and Juno to revise the current sheet shape model parameters as well as those of the magnetospheric field based on each spacecraft data. We derived fits that reproduced each data well, and revealed long‐term variations of both current sheet and magnetospheric field parameters over several decades. The updated model parameters are useful for detecting dynamic events in the magnetosphere such as magnetopause deformation and plasmoid generation. They can also be used as external fields necessary for probing into the Galilean icy moons by electromagnetic induction methods. Plain Language Summary: Above Jupiter's atmosphere and ionosphere, there is a vast region coined "magnetosphere" where magnetic fields govern physical phenomena. The magnetic fields in the magnetosphere are divided mainly into two components: one arising from inside Jupiter, and the other generated by electric currents flowing in a current sheet in the magnetosphere. Although both had been modeled so far, the models are, as usual, not perfect and should be updated on arrival of new datasets. Unfortunately, the magnetospheric field model is especially outdated compared with the intrinsic field. In this study, we focused on the magnetospheric field and its associated current sheet shape models using three datasets by four spacecraft, pre‐Galileo (Voyager 1 and Voyager 2), Galileo, and Juno. By updating the model parameters for each dataset, we determined three sets of models for current sheet shape and magnetospheric field that showed long‐term variations of Jupiter's magnetosphere over nearly half a century. The three sets of models are useful to detect dynamic events in the Jovian magnetosphere such as temporal changes of its shape and coherent plasma releases, and for probing deep into the Galilean icy moons that are thought to be possible cradles of extraterrestrial life. Key Points: We updated existing models of both current sheet and magnetic field in the Jovian magnetosphere using pre‐Galileo, Galileo, and Juno dataDifferences among the updated model parameters may represent long‐term variations of the current system in the Jovian magnetosphereThe new parameters are useful for study on magnetospheric dynamics and prediction of external magnetic variations for Galilean moons [ABSTRACT FROM AUTHOR]

We present initial results of low‐energy ion observations from BepiColombo's first Mercury flyby. Unprecedentedly high time resolution measurements of low energy ions at Mercury by BepiColombo Mio reveal rapid (a few seconds) and large (1–2 orders of magnitude) fluctuations of ion flux around the magnetopause and within the magnetosphere. Around the magnetic equator in the pre‐midnight magnetotail, Mio observed plasma sheet ions consistent with previous observations. In the midnight magnetotail near the closest approach, Mio observed the co‐existence of high‐energy (∼keV/q) and low‐energy (<∼300 eV/q) ion components. The low‐energy component is inferred to be cold with a temperature well below 100 eV and have a major contribution to the total density as opposed to previously reported cold tenuous ions. Future observations by Mio will provide insights into the sources, transport, and acceleration of the newly identified ion components. Plain Language Summary: BepiColombo, launched on 20 October 2018, is now en route to Mercury. Along its 7‐year journey to the Mercury orbit insertion, BepiColombo conducted, and will conduct, nine planetary flybys in total. This paper presents brand‐new observations of ions (positively charged particles) in Mercury's magnetosphere by BepiColombo Mio during its first encounter with Mercury. Thanks to high‐time resolution measurements by Mio, it is revealed that ions around Mercury change dramatically and rapidly just over a few seconds. Mio also observed an unexpectedly dense ion population with low energies, for which we do not have a definitive explanation yet. Future flyby and in‐orbit observations by BepiColombo will provide key information for unraveling the dynamics and sources of ions in Mercury's magnetosphere. Key Points: We present initial reports on low energy ion observations during BepiColombo's first Mercury flybyMio observed large fluctuations of ion flux with time scales down to a few seconds around the magnetopause and within the magnetosphereIon energy spectra obtained in the midnight magnetotail suggest the presence of an unexpectedly dense cold component [ABSTRACT FROM AUTHOR]

This review paper summarizes the research of Mercury's magnetosphere in the Post-MESSENGER era and compares its dynamics to those in other planetary magnetospheres, especially to those in Earth's magnetosphere. This review starts by introducing the planet Mercury, including its interplanetary environment, magnetosphere, exosphere, and conducting core. The frequent and intense magnetic reconnection on the dayside magnetopause, which is represented by the flux transfer event "shower", is reviewed on how they depend on magnetosheath plasma β and magnetic shear angle across the magnetopause, following by how it contributes to the flux circulation and magnetosphere-surface-exosphere coupling. In the next, Mercury's magnetosphere under extreme solar events, including the core induction and the reconnection erosion on the dayside magnetosphere, the responses of the nightside magnetosphere, are reviewed. Then, the dawn-dusk properties of the plasma sheet, including the features of the ions, the structure of the current sheet, and the dynamics of magnetic reconnection, are summarized. The last topic is devoted to the particle energization in Mercury's magnetosphere, which includes the energization of the Kelvin-Helmholtz waves on the magnetopause boundaries, reconnection-generated magnetic structures, and the cross-tail electric field. In each chapter, the last section discusses the open questions related to each topic, which can be considered by the simulations and the future spacecraft mission. We end this paper by summarizing the future BepiColombo opportunities, which is a joint mission of ESA and JAXA and is en route to Mercury. [ABSTRACT FROM AUTHOR]

We analyzed Mars Atmosphere and Volatile Evolution (MAVEN) and Venus Express (VEX) data of magnetic flux ropes observed in both Mars and Venus nightside induced magnetosphere, in order to understand their statistical characteristics and possible formation processes. We statistically identified 382 (286) events at Mars (Venus) via the minimum variance analysis and investigated the flux rope properties including their geometrical configurations of axial core field direction and spatial distributions. Interestingly, there is no significant difference with respect to the variety of the flux rope axial orientation between Mars and Venus, indicating that about at least one fourth of events do not necessitate magnetotail reconnection for their formations. However, the spatial distribution shows that flux ropes at Venus tend to be observed near the central plasma sheet, whereas those at Mars are more spread out. Moreover, the events tend to be observed more frequently in the -E hemisphere, where the solar wind electric field (ESW) is pointing toward the planet, rather than in +E hemisphere, where the ESW direction is away from the planet. The geographical distribution shows that flux ropes at Mars tend to be observed more frequently in the southern hemisphere where the strong crustal magnetic fields are primarily distributed. Therefore, such a tendency indicates that the Martian crustal magnetic fields could play significant roles on generating flux ropes in the nightside magnetosphere of Mars. [ABSTRACT FROM AUTHOR]

Mercury is embedded in a tenuous and highly anisotropic sodium exosphere, generated mainly by plasma‐surface interactions. The absolute values of the sodium ion density are still under debate. Observations by MESSENGER's Fast Imaging Plasma Spectrometer (FIPS) instrument suggest the density of exospheric ions to be several orders of magnitude lower than the upstream solar wind density, indicating that the sodium exosphere has no substantial influence on the magnetospheric current systems. However, MESSENGER magnetic field observations of field line resonances revealed sodium ion densities comparable to the upstream solar wind density. To investigate how a dense exosphere would affect the current systems within Mercury's magnetosphere, we apply an established hybrid (kinetic ions, fluid electrons) model and conduct multiple model runs with gradually increasing exospheric density, ranging from no sodium ions at all to comet‐like configurations. We demonstrate how a sufficiently dense exosphere leads to self‐shielding of the sodium ion population from the ambient electric field and a significant inflation and symmetrization of Mercury's magnetosphere, which is decreasingly affected by the dipole offset. Once the sodium ion density is sufficiently high, Region 2 field‐aligned currents emerge close to the planet. The modeled Region 2 currents are located below the orbit of MESSENGER, thereby providing a possible explanation for the absence of these currents in observations. The sodium exosphere also closes a significant fraction of the Region 1 currents through Pedersen and Hall currents before the "guiding" magnetic field lines even reach the planetary surface. The modeled sodium ion and solar wind densities agree well with observations. Key Points: A hybrid model is applied to study the influence of Mercury's sodium exosphere on the currents in the magnetosphereA dense sodium exosphere based on Gamborino et al. (2019) partially closes Region 1 currents before they reach the dayside surfaceWith a dense sodium exosphere included, Region 2 currents can form below the altitude of MESSENGER's orbit [ABSTRACT FROM AUTHOR]

The Galilean moons play major roles in the giant magnetosphere of Jupiter. At the same time, the magnetospheric particles and fields affect the moons. The impact of magnetospheric ions on the moons' atmospheres supplies clouds of escaping neutral atoms that populate a substantial fraction of their orbits. At the same time, ionization of atoms in the neutral cloud is the primary source of magnetospheric plasma. The stability of this feedback loop depends on the plasma/moon‐atmosphere interaction. The purpose of this review is to describe the physical processes that shape the space environment around the two innermost Galilean moons—Io and Europa—and to show their impact from the planet Jupiter out into interplanetary space. Key Points: Io and Europa substantially impact the Jovian magnetosphere while interactions of plasma with moons affects their surfaces and atmospheresTen components are described, including neutrals, thermal plasma, and energetic particles, spanning from Jupiter to interplanetary mediumCurrent understanding of the systems and processes are reviewed with a summary of outstanding questions [ABSTRACT FROM AUTHOR]

Mercury's nightside magnetosphere is investigated under the impact of a coronal mass ejection (CME) and a high‐speed stream (HSS) with MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) observations. The CME was shown to produce a low plasma β (ratio of thermal pressure to magnetic pressure) magnetosheath, while the HSS creates a higher β magnetosheath. Reconnection at the dayside magnetopause was found to be stronger during the CME than the HSS, but both were stronger than the average condition (Slavin et al., 2014, https://doi.org/10.1002/2014JA020319). Here we show that the CME and HSS events produced large numbers of flux ropes and dipolarization fronts in the plasma sheet. The occurrence rates for the structures were approximately 2 orders of magnitude higher than under average conditions with the rates during CME's being twice that of HSS's. The flux ropes appeared as quasiperiodic flux rope groups. Each group lasted approximately 1 min and had a few large flux ropes followed by several smaller flux ropes. The lobe magnetic flux accounted for around half of the Mercury's available magnetic flux with the flux during CME's being larger than that of HSS's. The CME produced a more dynamic nightside magnetosphere than the HSS. Further, for the CME event, the tail magnetic reconnection produced a distorted Hall magnetic field pattern and the X‐line had a dawn‐dusk extent of 20% of the tail width. No magnetic flux loading and unloading events were observed suggesting that, during these intense driving conditions, Mercury's magnetosphere responded with a type of quasi‐steady convection as opposed to the tail flux loading‐unloading events seen at Earth. Key Points: Coronal mass ejections drive more intense nightside reconnection than high speed streamsUnder extreme conditions, magnetic reconnection produces a distorted Hall magnetic field pattern in the plasma sheetContinued intense solar wind forcing does not produce substorm magnetic flux loading and unloading of tail lobe instead steady convection [ABSTRACT FROM AUTHOR]

Abstract: The variations of plasma sheet proton properties during magnetospheric substorms at Earth and Mercury are comparatively studied. This study utilizes kappa distributions to interpret proton properties at both planets. Proton number densities are found to be around an order of magnitude higher, temperatures several times smaller, and κ values broader at Mercury than at Earth. Protons become denser and cooler during the growth phase, and are depleted and heated after the dipolarizations in both magnetospheres. The changes of κ at Earth are generally small (<20%), indicating that spectrum‐preserving processes, like adiabatic betatron acceleration, play an important role there, while variations of κ at Mercury are large (>60%), indicating the importance of spectrum‐altering processes there, such as acceleration due to nonadiabatic cross‐tail particle motions and wave‐particle interactions. This comparative study reveals important intrinsic properties on the energization of protons in both magnetospheres. [ABSTRACT FROM AUTHOR]

Abstract: We analyze 94 traversals of Mercury's southern magnetospheric plasma mantle using data from the MESSENGER spacecraft. The mean and median proton number densities in the mantle are 1.5 and 1.3 cm−3, respectively. For sodium number density these values are 0.004 and 0.002 cm−3. Moderately higher densities are observed on the magnetospheric dusk side. The mantle supplies up to 1.5 × 108 cm−2 s−1 and 0.8 × 108 cm−2 s−1 of proton and sodium flux to the plasma sheet, respectively. We estimate the cross‐electric magnetospheric potential from each observation and find a mean of ~19 kV (standard deviation of 16 kV) and a median of ~13 kV. This is an important result as it is lower than previous estimations and shows that Mercury's magnetosphere is at times not as highly driven by the solar wind as previously thought. Our values are comparable to the estimations for the ice giant planets, Uranus and Neptune, but lower than Earth. The estimated potentials do have a very large range of values (1–74 kV), showing that Mercury's magnetosphere is highly dynamic. A correlation of the potential is found to the interplanetary magnetic field (IMF) magnitude, supporting evidence that dayside magnetic reconnection can occur at all shear angles at Mercury. But we also see that Mercury has an Earth‐like magnetospheric response, favoring −BZ IMF orientation. We find evidence that −BX orientations in the IMF favor the southern cusp and southern mantle. This is in agreement with telescopic observations of exospheric emission, but in disagreement with modeling. [ABSTRACT FROM AUTHOR]

A simple Monte Carlo model is presented that considers the effects of spacecraft orbital sampling on the inferred distribution of magnetic flux ropes, generated through magnetic reconnection in the magnetotail current sheet. When generalized, the model allows the determination of the number of orbits required to constrain the underlying population of structures: It is able to quantify this as a function of the physical parameters of the structures (e.g., azimuthal extent and probability of generation). The model is shown adapted to the Hermean magnetotail, where the outputs are compared to the results of a recent survey. This comparison suggests that the center of Mercury's neutral line is located dawnward of midnight by 0.37−1.02+1.21RM and that the flux ropes are most likely to be wide azimuthally (∼50% of the width of the Hermean tail). The downtail location of the neutral line is not self‐consistent or in agreement with previous (independent) studies unless dissipation terms are included planetward of the reconnection site; potential physical explanations are discussed. In the future the model could be adapted to other environments, for example, the dayside magnetopause or other planetary magnetotails. Key Points: Monte Carlo model allows the estimation of X‐line location and reconnection frequency given sampling with a single spacecraftMercury's magnetotail reconnection site is consistent with a center offset (0.37‐1.02+1.21RM) dawnward of midnightMercury's downtail X‐line location is only self‐consistent if dissipation terms are included planetward of the X‐line [ABSTRACT FROM AUTHOR]

We present the first observation of fast plasma flows in Mercury's magnetotail. Mercury experiences substorm activity phenomenologically similar to Earth's; however, field‐of‐view limitations of the Fast Imaging Plasma Spectrometer (FIPS) prevent the instrument from detecting fast flows in the plasma sheet. Although FIPS measures incomplete plasma distributions, subsonic flows impart an asymmetry on the partial plasma distribution, even if the flow directions are outside the field of view. We combine FIPS observations from 387 intervals containing magnetic field dipolarizations to mitigate these instrument limitations. By taking advantage of variations in spacecraft pointing during these intervals, we construct composite plasma distributions from which mean flows are determined. We find that dipolarizations at Mercury are embedded within fast sunward flows with an averaged speed of ~300 km/s compared to a typical background flow of ~50 km/s. Plain Language Summary: Similar to Earth, Mercury has a global magnetic field that forms a protective cavity, known as the magnetosphere, within the solar wind. The solar wind compresses the dayside magnetosphere, while stretching the nightside magnetosphere behind the planet. Variations within the solar wind cause dynamic activity within Mercury's magnetosphere, with a process known as magnetic reconnection mediating the interaction. Magnetic reconnection changes the topology of magnetic field lines and transfers energy and momentum from the magnetic field to the plasma within it. At Earth, magnetic reconnection in the nightside magnetosphere drives fast flows of plasma toward the planet, which when nearing the planet are slowed and diverted. These flows cannot be identified directly at Mercury because of limitations of the MESSENGER spacecraft measurements collected there. This research paper develops a new statistical technique to identify and characterize these fast flows at Mercury. Key Points: Multiple FIPS plasma observations from the MESSENGER spacecraft have been combined statistically to determine average flowsObservations collected during dipolarizations produce an average plasma flow of ~300 km/s compared to ~50 km/s during background intervalsSeveral dipolarizations are required to unload Mercury's magnetotail during a substorm, and some flows may reach the planet's surface [ABSTRACT FROM AUTHOR]