Planetary Magnetospheres Research Articles

MESSENGER Observations of Multiple Magnetic Energy Releases during Mercury’s Substorm

Substorms are energy storage and dissipation processes that occur in planetary magnetospheres accompanied by numerous phenomena, such as magnetic dipolarizations, plasma sheet thinning and thickening, magnetic flux ropes, and plasmoid ejections. Recent MESSENGER observations have revealed substorm activity in Mercury’s near magnetotail, with average durations of 2–3 minutes for both the growth and expansion phases. Herein, we report MESSENGER spacecraft observations of sporadic and short-term energy dissipation processes during the substorm expansion phase in Mercury’s magnetotail. Three magnetic dipolarizations lasting for ∼4 minutes were detected during the expansion phase and manifested as step-like enhancements in the northward component of the magnetic field. The magnetic field strength decreased during each dipolarization, indicating an intermittent release of magnetic energy. They arise owing to multiple magnetic pileup effects caused by planetward bursty bulk flows ejected from distant magnetotail reconnections. Additionally, in close proximity to each dipolarization, we observed the occurrence of pulse-like fluctuations in the B y component. These new MESSENGER observations suggest that during the expansion phase of Mercury’s substorm, the energy dissipation can be caused by multiple local transient processes, although the duration is much shorter than that of Earth.

Heliocentric Distance and Solar Activity Dependence of Sustained Quasi-radial Interplanetary Magnetic Field Occurrence

Planets close to their stars experience an interplanetary magnetic field (IMF) that is dominantly quasi-radial. Our solar system serves as a laboratory to study how the occurrence of quasi-radial IMF varies away from the star and under different stellar activities. Furthermore, on time and spatial scales relevant to magnetospheric physics, solar wind variability prevails in the form of structures generated both at the Sun and locally in the interplanetary space. The stationary Parker spiral model only approximates the large-scale structure of the IMF. Deviations from the Parker spiral often result in strongly radial magnetic fields that give rise to kinetic foreshock turbulence, which in turn can impact planetary magnetospheres. The relative significance of this type of interaction can be estimated statistically based on the occurrence rate of cases where the IMF is directed along the radial direction, leading to the entire day-side magnetosphere being downstream of the ion foreshock. We use observations covering radial distances from 0.1 to 10 au and more than 2 solar cycles to quantify the prevalence of radial IMFs throughout the heliosphere. Near Earth’s orbit, it is found that the occurrence rates of quasi-radial and southward IMF orientations are similar, and that the Pearson correlation coefficient is calculated between quasi-radial IMF occurrence rate and solar activity. A negative correlation is demonstrated for radial distances extending to at least Mars but not to Saturn.

The case for studying other planetary magnetospheres and atmospheres in Heliophysics

The case for studying other planetary magnetospheres and atmospheres in Heliophysics

Direct evidence of substorm-related impulsive injections of electrons at Mercury

Mercury’s magnetosphere is known to involve fundamental processes releasing particles and energy like at Earth due to the solar wind interaction. The resulting cycle is however much faster and involves acceleration, transport, loss, and recycling of plasma. Direct experimental evidence for the roles of electrons during this cycle is however missing. Here we show that in-situ plasma observations obtained during BepiColombo’s first Mercury flyby reveal a compressed magnetosphere hosts of quasi-periodic fluctuations, including the original observation of dynamic phenomena in the post-midnight, southern magnetosphere. The energy-time dispersed electron enhancements support the occurrence of substorm-related, multiple, impulsive injections of electrons that ultimately precipitate onto its surface and induce X-ray fluorescence. These observations reveal that electron injections and subsequent energy-dependent drift now observed throughout Solar System is a universal mechanism that generates aurorae despite the differences in structure and dynamics of the planetary magnetospheres.

Discovery of double Hall pattern associated with collisionless magnetic reconnection in dusty plasmas

ABSTRACT Magnetic reconnection is prevalent in magnetized plasmas in space and laboratories. Despite significant investigations on reconnection in electron–ion plasmas, studies of reconnection in magnetized plasmas with negatively charged dust grains are quite sparse. Here, we report the first fully kinetic simulations of collisionless reconnection in a three-species (i.e. electron, proton, and negatively charged dust grain) dusty plasma, through which the discovery of double Hall pattern is made. The double Hall pattern consists of a traditional Hall quadruple current in between the ion and electron diffusion region and a reversed Hall current in between the boundary of the ion and dust diffusion region. The analysis of the reconnection rate is also given. This study may be applicable to explain observations of planetary magnetospheres and the astrophysical objects, and may be realized in the laboratory studies of dusty plasmas.

Resolved imaging confirms a radiation belt around an ultracool dwarf

Radiation belts are present in all large-scale Solar System planetary magnetospheres: Earth, Jupiter, Saturn, Uranus and Neptune1. These persistent equatorial zones of relativistic particles up to tens of megaelectron volts in energy can extend further than ten times the planet’s radius, emit gradually varying radio emissions2–4 and affect the surface chemistry of close-in moons5. Recent observations demonstrate that very low-mass stars and brown dwarfs, collectively known as ultracool dwarfs, can produce planet-like radio emissions such as periodically bursting aurorae6–8 from large-scale magnetospheric currents9–11. They also exhibit slowly varying quiescent radio emissions7,12,13 hypothesized to trace low-level coronal flaring14,15 despite departing from empirical multiwavelength flare relationships8,15. Here we present high-resolution imaging of the ultracool dwarf LSR J1835 + 3259 at 8.4 GHz, demonstrating that its quiescent radio emission is spatially resolved and traces a double-lobed and axisymmetrical structure that is similar in morphology to the Jovian radiation belts. Up to 18 ultracool dwarf radii separate the two lobes, which are stably present in three observations spanning more than one year. For plasma confined by the magnetic dipole of LSR J1835 + 3259, we estimate 15 MeV electron energies, consistent with Jupiter’s radiation belts4. Our results confirm recent predictions of radiation belts at both ends of the stellar mass sequence8,16–19 and support broader re-examination of rotating magnetic dipoles in producing non-thermal quiescent radio emissions from brown dwarfs7, fully convective M dwarfs20 and massive stars18,21.

Anisotropic Energy Transfer and Conversion in Magnetized Compressible Turbulence

We present a spatial filtering (or coarse-graining) analysis on 3D magnetized magnetohydrodynamic (MHD) turbulence simulations. The filtered compressible MHD formulae show transfer of kinetic and magnetic energies from large to small scales, as well as energy conversion between kinetic, magnetic, and thermal energies. The anisotropic filtering enables separate analyses of the energy flows perpendicular and parallel to the global mean magnetic field. Anisotropy in energy cascade is demonstrated by the larger perpendicular energy cascade rate and also the larger perpendicular wavenumbers associated with the peak energy transfer rate. We also find that the “inertial range” along the parallel (perpendicular) direction in the anisotropic energy cascade formulation is no longer strictly dissipation-free, because it includes the dissipation in the perpendicular (parallel) direction. A change in the driving force (kinetic only versus kinetic and magnetic) affects the energy conversion between kinetic and magnetic energies. While the compressibility of the driving force changes the partition of different channels of energy transfer and conversion, and also increases the total energy transfer rate, the global energy flow remains unaffected by compressibility qualitatively. Our analysis can be applied to multispacecraft observations of turbulence in the solar wind or a planetary magnetosphere.

Terrestrial High‐Power LF Signals as Contributing Factors to the Heliospheric Environment

AbstractLow‐frequency radio signals of natural origins have been registered in the heliosphere for more than 50 years, mainly being a product of interaction between the solar wind and the planetary magnetospheres. Withing these signals, series of man‐made transmissions have been detected, coming from the LORAN‐C navigation system and the terrestrial longwave broadcasting stations. The paper compares these two types of signals in the terms of their power distribution, which affects their readability and achieved effective range in space, as well as their possible usability in future research basing on the type of information they deliver. It is indicated that the less‐investigated longwave signals are more efficient, spectrum‐powerful and information‐rich that the LORAN‐C system, offering more possibilities for heliospheric and near‐Earth space research.

Particle-sounding of the spatial structure of kinetic Alfvén waves

Kinetic Alfvén waves (KAWs) are ubiquitous throughout the plasma universe. Although they are broadly believed to provide a potential approach for energy exchange between electromagnetic fields and plasma particles, neither the detail nor the efficiency of the interactions has been well-determined yet. The primary difficulty has been the paucity of knowledge of KAWs’ spatial structure in observation. Here, we apply a particle-sounding technique to Magnetospheric Multiscale mission data to quantitatively determine the perpendicular wavelength of KAWs from ion gyrophase-distribution observations. Our results show that KAWs’ perpendicular wavelength is statistically 2.4pm 0.7 times proton thermal gyro-radius. This observation yields an upper bound of the energy the majority proton population can reach in coherent interactions with KAWs, that is, roughly 5.76 times proton perpendicular thermal energy. Therefore, the method and results shown here provide a basis for unraveling the effects of KAWs in dissipating energy and accelerating particles in a number of astrophysical systems, e.g., planetary magnetosphere, astrophysical shocks, stellar corona and wind, and the interstellar medium.

A Brief Introduction of Current and Future Magnetospheric Missions

In this paper, I briefly introduce recently terminated, current, and future scientific spacecraft missions for in situ and remote-sensing observations of Earth’s and other planetary magnetospheres as of February 2023. The spacecraft introduced here are Geotail, Cluster, Time History of Events and Macroscale Interactions during Substorms / Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (THEMIS / ARTEMIS), Magnetospheric Multiscale (MMS), Exploration of energization and Radiation in Geospace (ERG), Cusp Plasma Imaging Detector (CuPID), and EQUilibriUm Lunar-Earth point 6U Spacecraft (EQUULEUS) for recently terminated or currently operated missions for Earth’s magnetosphere; Lunar Environment Heliospheric X-ray Imager (LEXI), Gateway, Solar wind Magneto-sphere Ionosphere Link Explorer (SMILE), HelioSwarm, Solar-Terrestrial Observer for the Response of the Magnetosphere (STORM), Geostationary Transfer Orbit Satellite (GTOSat), GEOspace X-ray imager (GEO-X), Plasma Observatory, Magnetospheric Constellation (MagCon), self-Adaptive Magnetic reconnection Explorer (AME), and COnstellation of Radiation BElt Survey (CORBES) approved for launch or proposed for future missions for Earth’s magnetosphere; BepiColombo for Mercury and Juno for Jupiter for current missions for planetary magnetospheres; Jupiter Icy Moons Explorer (JUICE) and Europa Clipper for Jupiter, Uranus Orbiter and Probe (UOP) for Uranus, and Neptune Odyssey for Neptune approved for launch or proposed for future missions for planetary magnetospheres. I discuss the recent trend and future direction of spacecraft missions as well as remaining challenges in magnetospheric research. I hope this paper will be a handy guide to the current status and trend of magnetospheric missions.

Interaction between the turbulent solar wind and a planetary magnetosphere: A 2D comet example

Context. Using the newly developed code Menura, we present the first global picture of the interaction between a turbulent solar wind and a planetary obstacle in our solar system, namely a comet. Aims. This first publication sheds light on the macroscopic effect of the upstream solar wind turbulence on the induced magnetosphere of a comet. Methods. Using a hybrid particle-in-cell simulation code, we modelled a medium activity comet using turbulent and laminar solar wind input, for a direct comparison between the two regimes. Results. We show how the turbulent characteristics of the solar wind lead to a smaller obstacle size. We then present how the upstream turbulent structures, traced by the perpendicular magnetic field fluctuations absent in the laminar case, self-consistently drape and pile up around the denser inner coma, forming intense plasmoids downstream of the nucleus, pulling away dense cometary ion bubbles. This pseudo-periodic erosion phenomenon re-channels the global cometary ion escape; as a result, the innermost coma is found to be on average 45% less dense in the turbulent case than predicted by simulating a laminar upstream flow.

Unique Banded Structures of Plasmaspheric Hiss Waves in the Earth's Magnetosphere

AbstractPlasmaspheric hiss is an electromagnetic wave mode that occurs ubiquitously in the high‐density plasmasphere and contributes crucially to the dynamic behavior of the Earth's Van Allen radiation belts. While plasmaspheric hiss is commonly considered to be a broadband emission with frequencies from ∼100 Hz to several kHz, here we report Van Allen Probes measurements of unambiguous banded signatures of plasmaspheric hiss, uniquely characterized by an upper band above ∼200 Hz, a lower band below ∼100 Hz and a power gap in between. Banded plasmaspheric hiss occurs with the probability ∼8% in the postnoon sector within 2.5–5.0 Earth radii, showing strong dependence on geomagnetic and solar wind conditions. Observations also suggest that banded hiss waves result possibly from two combined sources, the upper band originating from the transformation of chorus waves propagating from outside the plasmasphere, and the lower band from localized excitation inside the plasmasphere, which however requires future investigation. The banded hiss waves shed new light on the evolution of the Earth's radiation belts and have implications for understanding whistler‐mode waves in planetary magnetospheres.

Force‐Free Current Sheets in the Jovian Magnetodisk: The Key Role of Electron Field‐Aligned Anisotropy

AbstractCurrent sheets are an essential element of the planetary magnetotails, where strong plasma currents self‐consistently support magnetic field gradients. The current sheet configuration is determined by plasma populations that contribute to the current density. The most commonly investigated configuration is supported by diamagnetic cross‐field currents of hot ions, typical for the magnetospheres of magnetized planets. In this study, we examine a new type of the current sheet configuration supported by field‐aligned currents from electron streams in the Jovian magnetodisk. Such bi‐directional streams increase the electron thermal anisotropy close to the fire‐hose instability threshold and lead to strong magnetic field shear. The current sheet configuration supported by electron streams is nearly force‐free, with |B| ≈ const across the sheet. Using Juno plasma and magnetic field measurements, we investigate the internal structure of such current sheets and discuss possible mechanisms for their formation.

The dependence of radial diffusion coefficients on solar/interplanetary drivers

Radial diffusion driven by Ultra Low Frequency (ULF) waves is very important for magnetospheric dynamics, as it contributes to both acceleration and loss of relativistic electrons in the outer Van Allen radiation belt, as well as to other planetary magnetospheres. The dependence of ULF wave power spectral density and radial diffusion coefficients (DLL) on solar wind parameters has already been investigated but their dependence on the various solar and interplanetary drivers is poorly studied. In this study, we conduct a statistical analysis of radial diffusion coefficients DLL to look into their dependence on different interplanetary drivers, i.e. Interplanetary Coronal Mass Ejections (ICMEs) and Stream Interaction Regions (SIRs), which originate from different solar drivers: CMEs and coronal holes. We use the ”SafeSpace” database (https://synergasia.uoa.gr/modules/document/?course=PHYS120), which includes radial diffusion coefficients DLL and ULF wave power spectral density, created using magnetic and electric field measurements by the THEMIS satellites in the 2011–2019 time period. We study how the properties of these solar wind drivers influence the behavior of radial diffusion coefficients. The results indicate significant differences between drivers of different solar origin at the ratio of the electric over the magnetic DLL component (DLLE over DLLB), especially when the solar wind dynamic pressure is maximized due to shock existence. Furthermore, differences are observed between SIRs with shock and SIRs without shock. This feature introduces a significant energy dependence to the radial diffusion coefficients, which is further depending on the radial distance and the different first adiabatic invariant (μ) values. These results emphasize the importance of the energy dependence of the magnetic component of the DLL, especially during the occurrence of interplanetary shocks, which is neglected by most semi-empirical models used in radiation belt simulations.

Diffuse auroral precipitation driven by lower-band chorus second harmonics

Diffuse aurora at the Earth’s high latitude regions is mainly caused by the low-energy (0.1–30 keV) electron precipitation which carries the major energy flux into the nightside upper atmosphere. Previous studies have demonstrated that combined scattering by the upper- and lower- band chorus waves acts as the dominant cause of diffuse auroral precipitation, but that is not necessarily the case as these two types of waves do not always occur simultaneously, with the lower-band more often. Here we report that the lower-band chorus satisfying the preferred condition can generate their second harmonics so as to trigger the diffuse auroral electron precipitation. We find that the lower-band chorus alone can only cause the precipitation of electrons greater than 4 keV, while the self-consistently generated second harmonic is weak but still able to result in the electron precipitation below 4 keV. The combined effect of those modes results in the observed pancake electron distributions and the diffuse aurora. Our results clearly demonstrate an alternative but universal mechanism of chorus-driven diffuse aurora in the Earth, which may also apply to the auroral formation in other planetary magnetospheres.

Acceleration in Exoplanet Magnetospheres

We briefly review the various acceleration mechanisms working in exoplanetary magnetospheres. The proposed scenarios are based on the investigation of planetary magnetospheres of the Solar System. Primary attention is paid to Earth and Jupiter, as the most characteristic examples of the various types of magnetospheric features determining different acceleration processes.

Using machine learning to diagnose relativistic electron distributions in the Van Allen radiation belts

Abstract The behaviour of relativistic electrons in the radiation belt is difficult to diagnose as their dynamics are controlled by simultaneous physical processes, some of which may be still unknown. Signatures of these physical processes are difficult to identify in large amounts of data; therefore, a machine learning approach is developed to classify energetic electron distributions which have been driven by different mechanisms. A series of unsupervised machine learning tools have been applied to 7 yrs of Van Allen Probe Relativistic Electron-Proton Telescope data to identify six different typical types of plasma conditions, each with a distinctly shaped energy-dependent pitch angle distribution (PAD). The PADs at lower energies have shapes as expected from previous studies – either butterfly, pancake, or flattop, providing evidence that machine learning has been able to reliably classify the relativistic electrons in the radiation belts. Further applications of this technique could be applied to other space plasma regions, and data sets from inner heliospheric missions such as Parker Solar Probe and Solar Orbiter, to planetary magnetospheres and the JUICE mission. Understanding PADs across the heliosphere enables researchers to determine the physical mechanisms that drive pitch angle evolution and investigate their spatial and temporal dependence and physical properties.

Closed field line vortices in planetary magnetospheres

ABSTRACTIn a rotation-dominated magnetosphere, there is a region where closed field lines rotate around the planet, and also a region where the open field lines stretch away from the planet, forming the lobes of the magnetotail. This paper shows that there could be a third, significantly different region, where the closed field lines form twisted vortex structures anchored in the magnetotail. Such patterns form when there are significant plasma sources inside the magnetosphere and the time-scale of the plasmoid formation process is substantially larger than the planetary rotation period. In the presence of vortices, the Dungey and Vasyliunas cycles act differently. The Dungey flow does not penetrate the central region of the polar cap. Tail reconnection events are rare, thus leaving the plasma time enough to participate in the essentially 3D vortex-forming plasma motion. The above conditions are fulfilled for Saturn. We discovered vortex-like patterns in the plasma and magnetic field data measured by the Cassini spacecraft in the nightside magnetosphere of Saturn. The plasma whirling around in these vortices never reaches the dayside, instead, it performs a retrograde motion in the high latitude regions of the magnetotail. Low-energy plasma data suggest that the observed patterns correspond to the closed field line vortices.

The effect of wave frequency drift on the electron nonlinear resonant interaction with whistler-mode waves

Electron resonant interaction with electromagnetic whistler-mode waves plays a crucial role for electron flux dynamics in planetary magnetospheres. One of the most intense types of whistler-mode waves consists of chorus waves generated via nonlinear resonant interaction with hot anisotropic electrons and propagating with time-varying (drifting) wave frequency. Electron nonlinear resonant interactions with such waves in a dipole magnetic field are well described analytically within the Hamiltonian approach under the approximation of monochromatic waves (of constant frequency). This paper aims to generalize this description to waves with drifting frequency. We show how frequency drift modifies two main nonlinear resonant effects: phase trapping and phase bunching. The obtained results contribute to the development of the Hamiltonian approach for wave–particle resonant interactions.

The three-fluid generalized Ohm's law: A theoretical study

The two-fluid generalized Ohm's law (GOL) is based on the assumption that plasma is composed of only protons and electrons. The three-fluid GOL is obtained theoretically for the three-fluid plasma consisting of heavy ions, light ions, and electrons, which prevails in planetary ionospheres and magnetospheres. Three inertial lengths corresponding to the three-scale diffusion region in the three-fluid magnetic reconnection are derived. The ion inertial lengths and reconnection rate as well as the Hall magnetic and electric fields are modified due to the two-step decoupling process of ions. Our results provide a framework to extend the reconnection theory for even more ion species.