We present a review of observations and theories of the dynamics of Jupiter's magnetosphere from Pioneer to New Horizons. We suggest that Jupiter's solar wind–driven magnetospheric flows are due ...primarily to viscous processes at the magnetopause boundary. Jupiter's magnetopause boundary is determined by a pressure balance between the solar wind dynamic pressure and the magnetospheric high‐β plasma. We discuss how this plasma‐on‐plasma interaction generates solar wind–imposed magnetic stresses that (1) generate the dawn‐dusk asymmetry in plasma flows and magnetic fields, (2) dictate the location of the magnetic x line in the tail, (3) enhance escape of Jovian plasma down the magnetotail, and (4) drive global plasma flows that are consistent with Jupiter's complex polar aurora without the requirement for a persistent region of open flux.
Electron acceleration by dispersive scale Alfvén waves at Jupiter is investigated using a Gyrofluid‐Kinetic‐Electron model. Specifically, the simulations consider the propagation of an Alfvén wave ...perturbation from the center of the Io plasma torus to high‐latitude regions that are consistent with recent Juno satellite observations (e.g., Allegrini et al., 2017, https://doi.org/10.1002/2017GL073180; Mauk, et al., 2017a, https://doi.org/10.1038/nature23648; Mauk, et al., 2017b, https://doi.org/10.1002/2016GL072286; Szalay et al., 2018, https://doi.org/10.1029/2018JE005752). As in those observations, the energized electron spectra is broadband in nature and the majority of the energization is under the interaction of inertial Alfvén waves at high latitudes. The extent of the energization associated with these waves is proportional to both the magnitude of the wave perturbation and the ratio of the torus to high‐latitude density.
Plain Language Summary
Recent observations of the Juno satellite at Jupiter illustrate that the electron energy spectrum at high latitudes is observed to be broadband—that is, ranging in energies from tens of electron volts to tens and hundreds of kiloelectron volts. At Earth, such electron spectra are associated with electron energization by Alfvén waves—which are transverse waves that travel along magnetic field lines in close analogy to waves on a string. In particular, at small scales (e.g., perpendicular scale lengths on the order of the ion orbit around the field line), kinetic effects allow for significant electric field generation that can efficiently accelerate electrons parallel to the field line. At these scales, the waves are known as dispersive Alfvén waves. In this work, we, for the first time, present global‐scale (entire dipolar field line) kinetic simulations of electron energization at Jupiter. We illustrate that these dispersive Alfvén waves, sourced in the Io plasma torus, lead to broadband electron energization close to the Jupiter ionosphere that is qualitatively consistent with the Juno observations. We additionally illustrate how the presence of the Io plasma torus (which is a feature unique to the Jupiter ionosphere) affects the characteristics of this broadband energization.
Key Points
The first full flux tube kinetic simulations of electron energization in dispersive Alfven waves at Jupiter are presented
The resulting electron energization is broadband in nature—consistent with recent Juno observations
The ratio of the torus to high‐latitude density plays an important role in regulating the magnitude of the energization
The internal sources of plasma in the giant magnetospheres of Jupiter and Saturn affect magnetospheric dynamics in terms of magnetosphere‐ionosphere coupling and auroral current systems, as well as ...the interaction with the solar wind. The radial transport of plasma at Jupiter is well constrained to vary between 300 kg/s and 1200 kg/s over timescales of 20–60 days. Saturn's neutral‐dominated inner magnetosphere has presented a challenge for determining the radial mass transport rates with values ranging between 20 kg/s and 280 kg/s over timescales of 20–80 days. We present an estimate of the radial mass transport rates associated with magnetospheric plasma production for Jupiter and Saturn, assuming that magnetospheric plasma loss (equal to the plasma source) can be treated as mass pickup by the solar wind. That is, the magnetospheric plasma is assumed to be at rest with respect to the solar wind, analogous to cometary mass loading of the solar wind flow. This Alfvénic coupling between the solar wind and the magnetosphere suggests mass loading rates of 500–1500 kg/s for Jupiter and 20–50 kg/s for Saturn, consistent with observations and physical chemistry models. We discuss implications for magnetotail structure by considering the contribution of the viscous interaction of the solar wind with the giant magnetospheres. We suggest that a significant portion of the magnetotail structure can be attributed to this viscous interaction.
Key Points
The magnetotail structure is tied to the viscous interaction
Internal mass loading rates determine the scale of the magnetosphere
Viscous drivers are key to the solar wind interaction
The Juno spacecraft had previously observed intense high frequency wave emission, broadband electron and energetic proton energy distributions within magnetic flux tubes connected to Io, Europa, ...Ganymede, and their wakes. In this work, we report consistent enhancements in <46 keV energy proton fluxes during these satellite flux tube transit intervals. We find enhanced fluxes at discrete energies linearly separated in velocity for proton distributions within Io wake flux tubes, and both proton and electron distributions within Europa and Ganymede wake flux tubes. We propose these discrete enhancements to be a result of resonances between particles' bounce motion with standing Alfvén waves generated by the satellite‐magnetosphere interaction. We corroborate this hypothesis by comparing the bounce and field‐line resonance periods expected at the satellites' orbits. Hence, we find bounce‐resonant acceleration is a fundamental process that can accelerate particles in Jupiter's inner magnetosphere and other astrophysical plasmas.
Plain Language Summary
The passage of the Galilean moons‐ Io, Europa, Ganymede, and Callisto, perturbs the plasma flow in Jupiter's magnetosphere, creating waves that travel from the moon and reflect off Jupiter's ionosphere. These waves have been proposed to accelerate charged particles, and such accelerated particles had been observed by the Juno spacecraft during its passage through magnetic field lines connected to the satellite wakes. In this work, we find instances when this acceleration occurs selectively at specific energies that have constant separation in speed. We propose that this selective acceleration is due to resonance between particle bounce motion and the waves arising from the satellite wake perturbation. Bounce‐resonant acceleration is a promising fundamental process which can accelerate particles in Jupiter's inner magnetosphere and other plasma systems with similar geometries.
Key Points
Proton and electron flux enhancements in satellite and wake flux tubes often occur at discrete energies linearly separated in speed
Broadband proton flux enhancements at <46 keV energies were also observed within satellite flux tube crossings
Particles can be accelerated via resonance between bounce motion and standing Alfvén waves generated by moon‐magnetosphere interactions
The giant‐planet magnetodiscs are shaped by the radial transport of plasma originating in the inner magnetosphere. Magnetic flux transport is a key aspect of the stretched magnetic field ...configuration of the magnetodisc. While net mass transport is outward (ultimately lost to the solar wind), magnetic flux conservation requires a balanced two‐way transport process. Magnetic reconnection is a critical aspect of the balanced flux transport. We present a comprehensive analysis of current sheet crossings in Saturn's magnetosphere using Cassini magnetometer data from 2004 to 2012 in an attempt to quantify the circulation of magnetic flux, emphasizing local time dependence. A key property of flux transport is the azimuthal bend forward or bend back of the magnetic field. The bend back configuration is an expected property of the magnetodisc with net mass outflow, but the bend forward configuration can be achieved with the rapid inward motion of mostly empty flux tubes following reconnection. We find a strong local time dependence for the bend forward cases, localized mostly in the postnoon sector, indicating that much of the flux‐conserving reconnection occurs in the subsolar and dusk sector. We suggest that the reconnection occur in a complex and patchy network of reconnection sites, supporting the idea that plasma can be lost on small scales through a “drizzle”‐like process. Auroral implications for the observed flux circulation will also be presented.
Key Points
Giant‐planet magnetodiscs are shaped by radial plasma transport
Magnetic flux is transported radially outward with the plasma
Magnetic reconnection circulates flux to the inner magnetosphere on the dayside
Using idealized models of the magnetosheath and magnetospheric magnetic fields, plasma densities, and plasma flow, we test for the steady state viability of processes mediating the interaction ...between the solar wind and the magnetosphere of Saturn. The magnetopause is modeled as an asymmetric paraboloid with a standoff distance of ∼25 RS. We test where on the magnetopause surface large‒scale reconnection may be affected by either a shear flow or diamagnetic drift due to a pressure gradient across the magnetopause boundary. We also test for the onset of the Kelvin‒Helmholtz instability. We find that, for the solar wind and magnetosphere states considered, reconnection is inhibited on the dawn flank due to the large shear flows in this region. Additionally, most of the dawn and dusk equatorial region of the magnetopause is Kelvin‒Helmholtz unstable, due to the presence of the dense magnetospheric plasma sheet and weak magnetic fields on either side of the magnetopause. This study is a follow‒up to a previously published study of the solar wind interaction with Jupiter's magnetosphere.
Key Points
Velocity shears limit reconnection on Saturn's dawn flank.
Diamagnetic drifts may limit reconnection but depend on energetic population.
The dawn flank of Saturn's magnetopause is Kelvin-Helmholtz unstable.
Saturn's rapid rotation combined with relatively weak magnetic fields in the outer magnetosphere and sheath lead to conditions that are favorable for the Kelvin‐Helmholtz (KH) instability. A ...Kelvin‐Helmholtz unstable magnetopause boundary has important consequences for Saturn's interaction with the solar wind due to mass, momentum, and energy transport that can occur at the magnetopause boundary. Previous attempts to identify vortices have been hampered by limited plasma data to unambiguously reveal vortical flow. The magnetic field data, on the other hand, may be able to identify the KH instability due to intense magnetic fluctuations that are associated with KH vortices. We have conducted two‐dimensional hybrid code simulations of Saturn's magnetopause boundary to illustrate the expected magnetic field signatures of KH. Specifically, our simulations show strong field‐aligned current sheet filaments or strong bipolar fluctuations of the in‐plane magnetic field components, bounding the KH vortices. A global search for these characteristic magnetic field signatures near the magnetopause boundary was made of the Cassini mission data from 2004 to 2009. We find that most of the potential KH activity is found on the dusk flank, contrary to expectations. We suggest that KH growth is supported in the prenoon and subsolar regions and that these vortices are transported through coupling to the rotating planet, past noon and tailward on the dusk flank. In addition, we find many instances in the subsolar magnetosphere of possible plasmoid formation (Bz northward) in conjunction with these intense magnetic field fluctuations.
Key Points
Identify Kelvin‐Helmholtz (KH) waves at Saturn
Determine how Saturn interacts with the solar wind
Assess the dawn‐dusk asymmetry of KH at Saturn
The temperatures of ions in the magnetospheres of Jupiter and Saturn were observed to increase substantially from about 10 to 30 planet radii. Different heating mechanisms have been proposed to ...explain such observations, including a heating model for Jupiter based on magnetohydrodynamic (MHD) turbulence with flux‐tube diffusion. More recently, an MHD turbulent heating model based on advection was shown to also explain the temperature increase at Jupiter and Saturn. We further develop this turbulent heating model by combining effects from both diffusion and advection. The combined model resolves the physical consistency requirement that diffusion should dominate over advection when the radial flow velocity is small and vice versa when it is large. Comparisons with observations show that previous agreements, using the advection only model, are still valid for larger radial distance. Moreover, the additional heating by diffusion results in a better agreement with the temperature observations for smaller radial distance.
Plain Language Summary
The temperatures of ions in the magnetospheres of Jupiter and Saturn were observed to increase substantially near the planet. This suggests that there should be some heating sources to counter the cooling effect due to expansion. There have been several models trying to explain such observation using different heating mechanisms, including a heating model for Jupiter based on turbulence and diffusion effects, as well as a model based on advection effects for Jupiter and Saturn. We further develop a heating model by combining effects from both diffusion and advection. The combined model resolves the physical consistency requirement that diffusion should be stronger than advection nearer to the planet, but shifting to the opposite farther away. Comparisons with observations show that previous agreements using the advection only model are still valid, and are improved by including diffusion nearer to the planet.
Key Points
A new model for the heating of the magnetospheres of Jupiter and Saturn by magnetohydrodynamic turbulence is developed
The model combines effects from diffusion and advection such that each is dominant when the radial velocity is small or large, respectively
Predictions of the temperature and radial velocity profiles agree better with Jupiter and Saturn observations than previous models
Jupiter's auroral emission is a spectacular phenomenon that provides insight into energy release processes related to the coupling of its magnetosphere and ionosphere. This energy release is ...influenced by solar wind conditions. Using joint observations from Juno and the Hubble Space Telescope (HST), we statistically investigate the relationship between auroral power and current sheet variations under different solar wind conditions. In this study, we reveal that during global main auroral brightening events that are closely connected to solar wind compressions, the dawn side current sheet is substantially thinner than during times when a quiet auroral morphology is present. Furthermore, the total current intensity in the current sheet is found to increase under solar wind compression conditions compared to the quiet period. These findings provide important observational evidence for how magnetospheric dynamics driven by solar wind behavior affect auroral activity, deepening our understanding of the coupling between Jupiter's magnetosphere and ionosphere.
Plain Language Summary
Jupiter, the largest planet in our solar system, has a fascinating and powerful auroral emission that can help us understand the interactions between its magnetic field and the charged particles in its atmosphere. These auroral emissions are influenced by solar wind conditions, which are streams of charged particles coming from the Sun. In this study, we used observations from the Juno spacecraft and the Hubble Space Telescope to investigate the relationship between Jupiter's auroral emissions and changes in the planet's magnetic field. We found that during periods of increased solar wind pressure, the magnetic field layer, known as the current sheet, becomes thinner compared to times when the aurora is quiet. These findings offer valuable evidence of how Jupiter's magnetic field is affected by solar wind behavior and improve our understanding of the relationship between the planet's magnetic field and its aurora. This research helps us better comprehend the complex processes occurring in Jupiter's magnetosphere and can potentially enhance our knowledge of similar phenomena occurring on other planets.
Key Points
The features of the current sheet in Jupiter's dawnside magnetosphere are highly relevant to auroral contexts
During a solar wind compression event, the dawnside current sheet becomes substantially thinner than during quiet times
The total current intensity of the current sheet is much higher during solar wind compression events than the quiet times
The Jovian magnetosphere is highly dynamic, influenced by both solar wind and internal processes associated with the rapid planetary rotation and Io's volcanic activities. Accompanying the mass and ...energy circulations driven by the magnetospheric dynamics, the magnetic configuration also changes dramatically. One of the crucial parameters to characterize the magnetic configuration is magnetic field line curvature (FLC), which generally describes how stretched the field line is. The curvature is pivotal to influence particle behaviors, for example, pitch angle scattering which may lead to auroral particle precipitation. In this work, a method is proposed to investigate the real‐time magnetic FLC in Jovian current sheet using the magnetic field data from the Juno spacecraft. The results indicate that the FLC scattering of ions and relativistic electrons are common in Jovian magnetosphere, providing a crucial insight to understand the particle behaviors.
Plain Language Summary
Both the Earth and the Jupiter have intrinsic magnetic field. When the planetary magnetic field interacts with the solar wind, a region called magnetosphere is formed. Particle behaviors in different planetary systems are different, due to the different magnetospheric dynamics. The curvature of magnetic field, describing the stretch level of a magnetic field line, is a basic parameter to describe a planetary space system, and it can significantly influence particle behaviors, for example, to scatter the magnetospheric particles to planetary atmosphere, causing auroral emissions. In this work, we proposed a method to calculate the magnetic field line curvature (FLC) near the equatorial plane inside the Jupiter's magnetosphere using Juno data set, for the first time to provide a global picture on the magnetic FLC. By comparing with the radius of particles' gyration motions, we suggest that ions and electrons can be strongly scattered by the magnetic FLC. We believe that the results in this study provide useful information on the different particle behaviors between the terrestrial system and the Jovian system.
Key Points
We proposed a method to investigate the magnetic field line curvature (FLC) in Jupiter's current sheet using data from Juno data set
50 events are selected by specific criteria. The magnetic FLC and different particles' Larmor radius are investigated
The FLC will scatter ions and relativistic electrons as a potential cause of auroral precipitation
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