Jupiter's Middle Magnetosphere Research Articles

Radial and Vertical Structures of Plasma Disk in Jupiter's Middle Magnetosphere

Radial and Vertical Structures of Plasma Disk in Jupiter's Middle Magnetosphere

Galileo Observation of Electron Spectra Dawn‐Dusk Asymmetry in the Middle Jovian Magnetosphere: Evidence for Convection Electric Field

AbstractIn Jupiter's middle magnetosphere, debate remains on the controlling physics of electron acceleration to the ultrarelativistic regime. Some local‐time asymmetric processes regulating MeV electrons may have their hold across electron spectra, leaving imprints down to 10–100s keV energies. Spectra at these lower energies can be measured with finite energy resolution, which is hardly achieved for MeV electrons. We investigate 10–100s keV electron spectra using Galileo measurements. We show that at 15–20 RJ, the power‐law exponents exhibit a previously unexpected dawn‐dusk asymmetry. This asymmetry is more prominent for 100s‐keV spectra, persistent but enhanced from 1996 to 2001. These match the theory of transport driven by a dawn‐to‐dusk electric field. Contrary to past expectations of dawn‐to‐dusk electric field at Io plasma torus (IPT) extending outward from 6 RJ, the origin of the field we infer may be completely different, despite its orientation and impacts on electron energization similar with the IPT one.

Ganymede's Auroral Footprint Latitude: Comparison With Magnetodisc Model

AbstractVariations of Ganymede's auroral footprint locations are presented based on observations by the Hubble Space Telescope in 2007 and 2016. The poleward and equatorward shifts of Ganymede's footprint could be influenced by the mass outflow rate from Io and the solar wind compression, as the internal and external factors respectively. We compare our results with Ganymede's footprint mapping based on the magnetodisc model. The mapped footprint in Jupiter's ionosphere shifts equatorward with increased hot plasma parameter, Kh, which is associated with hot plasma pressure. We analyzed the effect of cold plasma number density (Nc), related to the mass outflow rate and connected to the material produced by Io. The results show that the magnetic footprint is shifted equatorward by 0.37° when the mass outflow rate is increased from 800–2,000 kg s−1. Iogenic plasma has a strong influence on the stretching of the magnetic field lines in Jupiter's middle magnetosphere, causing the equatorward shift of Ganymede's footprint. For external factors, Ganymede's footprint shifted poleward by 0.62° under the influence of solar wind compression while the mass outflow is kept constant at 1,000 kg s−1. We present similar locations of Ganymede's footprint based on the field lines mapped as a result of the compensation between an increase of Kh and the solar wind compression. Overall, the location of Ganymede's auroral footprint corresponds with the mass loading rate from Io and the solar wind dynamic pressure.

Energetic Proton Distributions in the Inner and Middle Magnetosphere of Jupiter Using Juno Observations

AbstractJupiter is known to have a complex magnetosphere containing energetic (above 10s of keV) electrons, protons, and heavy ions. However, a global distribution of these energetic particles is not fully understood before the era of the polar‐orbiting Juno mission. In this study, we focus on the energetic proton distribution at M < 50 by taking advantage of Juno's measurement covering various magnetic latitudes and M‐shells, and find that energetic proton fluxes are higher off‐equator than that near the equator at M > ∼20, and become comparable or lower at high latitudes than those near the equator at low M‐shells. Pitch angle distributions of energetic protons are field‐aligned, isotropic, and weak pancake‐like from high to low M‐shells. Proton phase space density shows a weak M‐shell dependence at M > 30 and a large positive slope at M < 30, suggesting their potential source and loss processes.

Survey of Juno Observations in Jupiter's Plasma Disk: Density

AbstractWe explore the variation in plasma conditions through the middle magnetosphere of Jupiter with latitude and radial distance using Juno‐JADE measurements of plasma density (electrons, protons, sulfur, and oxygen ions) surveyed on Orbits 5–26 between March 2017 and April 2020. On most orbits, the densities exhibit regular behavior, mapping out a disk between 10 and 50 RJ (Jovian radii). In the disk, the heavy ions are confined close to the centrifugal equator which oscillates relative to the spacecraft due to the ∼10° tilt of Jupiter's magnetic dipole. Exploring each crossing of the plasma disk shows there are some occasions where the density profiles are smooth and well‐defined. At other times, small‐scale structures suggest temporal and/or spatial variabilities. There are some exceptional orbits where the outer regions (30–50 RJ) of the plasma disk show uniform depletion, perhaps due to enhanced ejection of plasmoids down the magnetotail, possibly triggered by solar wind compression events.

Envelope solitons and rogue waves in Jupiter's magnetosphere

Considering a system of positively charged different sizes of dust grains which interacts with streaming electrons and ions, modulational instability (MI) together with the formation of envelope solitons and rogue waves are studied in Jupiter's middle magnetosphere. A nonlinear Schrödinger equation (NLSE) is derived and three distinct wave modes are observed among which the slowest mode is significantly affected by power law dust size distribution (DSD). For this mode, DSD enhances the MI growth rate in the unstable domain. The effects of densities of streaming electron and ion beams on the instability and different characteristics of the envelope solitons are investigated. How the number densities of electron and ion beams influence the spatiotemporal evolution of the associated rogue waves are also discussed.

Magnetosphere-ionosphere coupling in Jupiter's middle magnetosphere: Computations including a self-consistent current sheet magnetic field model

In this paper we consider the effect of a self-consistently computed magnetosdisc field structure on the magnetosphere-ionosphere coupling current system at Jupiter. We find that the azimuthal current intensity, and thus the stretching of the magnetic field lines, is dependent on the magnetosphere-ionosphere coupling current system parameters, i.e. the ionospheric Pedersen conductivity and iogenic plasma mass outflow rate. Overall, however, the equatorial magnetic field profiles obtained are similar in the inner region to those used previously, such that the currents are of the same order as previous solutions obtained using a fixed empirical equatorial field strength model, although the outer fringing field of the current disc acts to reverse the field-aligned current in the outer region. We also find that, while the azimuthal current in the inner region is dominated by hot plasma pressure, as is generally held to be the case at Jupiter, the use of a realistic plasma angular velocity profile actually results in the centrifugal current becoming dominant in the outer magnetosphere. In addition, despite the dependence of the intensity of the azimuthal current on the magnetosphere-ionosphere coupling current system parameters, the location of the peak field-aligned current in the equatorial plane also varies, such that the ionospheric location remains roughly constant. It is thus found that significant changes to the mass density of the iogenic plasma disc are required to explain the variation in the main oval location observed using the Hubble Space Telescope.

Equatorial electron beams and auroral structuring at Jupiter

It has been reported that low‐altitude regions of downward electric current on auroral magnetic field lines are sites of dramatic upward magnetic field‐aligned electron acceleration that generates intense magnetic field‐aligned electron beams within Earth's equatorial middle magnetosphere. Field‐aligned equatorial electron beams also are observed within Jupiter's middle magnetosphere. The mystery about these Jovian beams is that they are observed in a region thought to map to Jupiter's brightest aurora and on field lines that generally carry electric current away from Jupiter's atmosphere rather than toward the atmosphere as anticipated at Earth. Here we develop procedures for quantifying the character of the Jupiter electron beams (for example, how confined they are with respect to the magnetic field‐aligned directions). We apply the procedures to the highest time resolution electron data available from the Galileo Energetic Particle Detector (EPD). We find that the Jupiter equatorial electron beams are spatially and/or temporally structured (down to <20 km at auroral altitudes, or less than several minutes), with regions of intense beams intermixed with regions absent of such beams. We suggest that, as with the situation at Earth, Jupiter's circuit of electric currents that supports its brightest aurora is structured, with regions of upward current intermixed with regions of downward current.

Interchange instability of the plasma disk in Jupiter's middle magnetosphere and its relation to the radial plasma density distribution

Abstract. We analyse the interchange or flute instability of the equatorial plasma disk in Jupiter's middle magnetosphere. Particular attention is paid to wave coupling between the dense plasma in the equatorial disk and the more rarefied plasma at higher latitudes, and between the latter plasma and the conducting ionosphere at the feet of the field lines. It is assumed that the flute perturbations are of small spatial scale in the azimuthal direction, such that a local Cartesian approximation may be employed, in which the effect of the centrifugal acceleration associated with plasma rotation is represented by an "external" force in the "radial" direction, perpendicular to the plasma flow. For such small-scale perturbations the ionosphere can also be treated as a perfect electrical conductor, and the condition is determined under which this approximation holds. We then examine the condition under which flute perturbations are at the threshold of instability, and use this to determine the corresponding limiting radial density gradient within the plasma disk. We find that when the density of the high-latitude plasma is sufficiently low compared with that of the disk, such that coupling to the ionosphere is not important, the limiting radial density profile within the disk follows that of the equatorial magnetic field strength as expected. However, as the density of the high-latitude plasma increases toward that of the equatorial disk, the limiting density profile in the disk falls increasingly steeply compared with that of the magnetic field, due to the increased stabilising effect of the ionospheric interaction. An initial examination of Galileo plasma density and magnetic field profiles, specifically for orbit G08, indicates that the latter effect is indeed operative inside radial distances of ~20 RJ. At larger distances, however, additional density smoothing effects appear to be important.

Synchronized oscillations in energetic electron fluxes and whistler wave intensity in Jupiter's middle magnetosphere

Synchronized oscillations in energetic electron fluxes and electromagnetic wave emissions at the planetary rotation period have been observed in and near Jupiter's magnetosphere. In this work we theoretically examine time‐dependent processes in Jupiter's middle magnetosphere energetic electron radiation belts which are relevant to these oscillations, where intervals of particle accumulation are followed by precipitation into the ionosphere during a whistler electromagnetic radiation pulse. These oscillations are described by a relativistic system of quasi‐linear equations, in which diffusion of particles in adiabatic invariant space and evolution of the electromagnetic radiation are taken into account. Linear analysis shows that the natural oscillation frequency of the Jovian radiation belt is almost independent of magnetic shell and is close in magnitude to the planet's rotation frequency, thus suggesting the possibility of a global resonance. We propose that such near‐resonant oscillations can be excited by periodic modulations of the whistler decay rate due to ionospheric absorption, which has components that depend both on local time in the magnetosphere and on asymmetries that rotate with the planet. These dependencies are shown to combine to produce a modulation at the planetary rotation period that is independent of local time and also independent of the angular velocity of rotation of the plasma. The response to this synchronized driving is investigated in both the linear and nonlinear regimes, and the conditions required for a synchronized response at the planetary period are examined.

Magnetosphere-ionosphere coupling currents in Jupiter's middle magnetosphere: effect of magnetosphere-ionosphere decoupling by field-aligned auroral voltages

Abstract. We consider the effect of field-aligned voltages on the magnetosphere-ionosphere coupling current system associated with the breakdown of rigid corotation of equatorial plasma in Jupiter's middle magnetosphere. Previous analyses have assumed perfect mapping of the electric field and flow along equipotential field lines between the equatorial plane and the ionosphere, whereas it has been shown that substantial field-aligned voltages must exist to drive the field-aligned currents associated with the main auroral oval. The effect of these field-aligned voltages is to decouple the flow of the equatorial and ionospheric plasma, such that their angular velocities are in general different from each other. In this paper we self-consistently include the field-aligned voltages in computing the plasma flows and currents in the system. A third order differential equation is derived for the ionospheric plasma angular velocity, and a power series solution obtained which reduces to previous solutions in the limit that the field-aligned voltage is small. Results are obtained to second order in the power series, and are compared to the original zeroth order results with no parallel voltage. We find that for system parameters appropriate to Jupiter the effect of the field-aligned voltages on the solutions is small, thus validating the results of previously-published analyses.

Magnetosphere-ionosphere coupling currents in Jupiter's middle magnetosphere: effect of precipitation-induced enhancement of the ionospheric Pedersen conductivity

Abstract. We consider the effect of precipitation-induced enhancement of the Jovian ionospheric Pedersen conductivity on the magnetosphere-ionosphere coupling current system which is associated with the breakdown of the corotation of iogenic plasma in Jupiter's middle magnetosphere. In previous studies the Pedersen conductivity has been taken to be simply a constant, while it is expected to be significantly enhanced in the regions of upward-directed auroral field-aligned current, implying downward precipitating electrons. We develop an empirical model of the modulation of the Pedersen conductivity with field-aligned current density based on the modelling results of Millward et al. and compute the currents flowing in the system with the conductivity self-consistently dependent on the auroral precipitation. In addition, we consider two simplified models of the conductivity which provide an insight into the behaviour of the solutions. We compare the results to those obtained when the conductivity is taken to be constant, and find that the empirical conductivity model helps resolve some outstanding discrepancies between theory and observation of the plasma angular velocity and current system. Specifically, we find that the field-aligned current is concentrated in a peak of magnitude ~0.25µAm-2 in the inner region of the middle magnetosphere at ~20 RJ, rather than being more uniformly distributed as found with constant conductivity models. This peak maps to ~17° in the ionosphere, and is consistent with the position of the main oval auroras. The energy flux associated with the field-aligned current is ~10mWm-2 (corresponding to a UV luminosity of ~100kR), in a region ~0.6° in width, and the Pedersen conductivity is elevated from a background of ~0.05mho to ~0.7mho. Correspondingly, the total equatorial radial current increases greatly in the region of peak field-aligned current, and plateaus with increasing distance thereafter. This form is consistent with the observed profile of the current derived from Galileo magnetic field data. In addition, we find that the solutions using the empirical conductivity model produce an angular velocity profile which maintains the plasma near to rigid corotation out to much further distances than the constant conductivity model would suggest. Again, this is consistent with observations. Our results therefore suggest that, while the constant conductivity solutions provide an important indication that the main oval is indeed a result of the breakdown of the corotation of iogenic plasma, they do not explain the details of the observations. In order to resolve some of these discrepancies, one must take into account the elevation of the Pedersen conductivity as a result of auroral electron precipitation.Key words. Magnetospheric physics (current systems, magnetosphere-ionosphere interactions, planetary magnetospheres)70d

Turbulent Heating of Jupiter's Middle Magnetosphere

We give a possible explanation for the high ion temperatures of ~108 K measured in the middle magnetosphere of Jupiter. In the absence of a significant heat source, much lower temperatures are expected due to adiabatic cooling of the expanding cloud of plasma from the Io torus region of the magnetosphere. We show that the weak magnetohydrodynamic turbulent fluctuations observed in the middle magnetosphere dissipate energy at a rate that can account for the high ion temperatures. A simple estimate gives a turbulent heating rate of 2.5 × 107 K day-1 at a radial distance of around 20 Jupiter radii, where the turbulence maximizes. We present a one-dimensional radial temperature model and find a temperature profile in good agreement with the values observed by the Galileo spacecraft.

A simulation study of currents in the Jovian magnetosphere

We have used a global magnetohydrodynamic simulation of the interaction of Jupiter's magnetosphere with the solar wind to investigate the effects of the solar wind on the structure of currents in the jovian magnetosphere. A thin equatorial current sheet with currents flowing around Jupiter dominates Jupiter's middle magnetosphere. However, in our simulations this current is not uniform in azimuth. It is weaker on the day side than the night side with local regions where the current density decreases by more than 50%. In addition to this ring current the current sheet contains strong radial “corotation enforcement” currents. Outward radial currents are found at most local times but there are regions with currents directed toward Jupiter. The current pattern is especially complex in the local afternoon and evening regions. In the near equatorial magnetosphere the field-aligned current pattern also is complex. There are regions with currents both toward and away from Jupiter's ionosphere. However, when we mapped the currents from the inner boundary of the simulation to the ionosphere we found a pattern more like that expected for the ionosphere to drive corotation with currents away from Jupiter at lower latitudes and currents toward Jupiter at higher latitudes. Since upward field-aligned currents are associated with aurora at the Earth they may be associated with aurora at Jupiter. The upward field-aligned currents map to larger distances on the night side (40 R J to 60 R J) than on the day side (20 R J to 30 R J). In the simulations changing the solar wind dynamic pressure did not make major changes in the current sheet or field-aligned currents (both were slightly stronger for higher pressures). The interplanetary magnetic field had a stronger effect on the currents with the strongest currents for northward IMF. However, it took a very long time for the magnetosphere to respond to the changes in the IMF.

Distributions of current and auroral precipitation in Jupiter's middle magnetosphere computed from steady-state Hill–Pontius angular velocity profiles: solutions for current sheet and dipole magnetic field models

We present an analysis of the magnetosphere–ionosphere coupling current system which is associated with the maintenance of plasma corotation in Jupiter's middle magnetosphere. The formulation follows Cowley and Bunce (Planet. Space Sci. 49 (2001) 1067), but here the angular velocity of the equatorial plasma is computed using the steady-state theory of Hill (J. Geophys. Res. 84 (1979) 6554) and Pontius (J. Geophys. Res. 102 (1997) 7137) rather than being determined by a simple empirical model. Results are compared both for a realistic magnetospheric current sheet magnetic field model and for a simple dipole field. The results for the current sheet model confirm previous conclusions concerning current and auroral precipitation distributions based on the empirical angular velocity model. Specifically, we find that upward field-aligned currents flow from the ionosphere to the equatorial current sheet over the whole radial range from ∼20 R J to the outer boundary of the region considered at 100 R J, thus returning from the current sheet to the ionosphere at larger radial distances outside this region. The upward currents are of sufficient intensity to require the existence of field-aligned voltages which accelerate magnetospheric electrons down into the atmosphere, thus creating intense auroras. At ionospheric heights, the upward field-aligned current densities are several tenths of a μA m −2 , flowing in a narrow latitudinal band less than ∼1° wide, located at ∼16° dipole co-latitude. The associated field-aligned voltages are several tens of kV, resulting in precipitated electron energy fluxes capable of exciting UV auroras of hundreds of kR intensity. The model thus forms an appropriate basis on which to understand the origin of Jupiter's ‘main auroral oval’ emissions. For the dipole field, the steady-state equatorial angular velocity profile is found to be very similar to that for the current sheet model. Nevertheless, the different mapping of the field lines to the ionosphere results in a magnetosphere–ionosphere coupling current system which has significantly different properties. In particular, the height-integrated ionospheric Pedersen current intensity is found to be factors of ∼2–3 less than that for the current sheet model, and since the field-aligned current is also found to be spread over a much broader latitudinal band, ∼5° wide centred at ∼10° dipole co-latitude, the field-aligned current density is reduced by more than an order of magnitude. Only small (few kV) field-aligned voltages are thus required to carry the upward current in this case, resulting in much weaker electron precipitation, and much weaker, few kR, UV auroras. Our results thus emphasise that while the magnetic field model may not greatly effect the steady-state angular velocity profile, it nevertheless is of crucial significance in determining the consequent magnetosphere–ionosphere coupling current system, and the resulting auroral precipitation.

A simple empirical model of the equatorial radial field in Jupiter's middle magnetosphere, based on spacecraft fly-by and Galileo orbiter data

In this paper we consider empirical models of the radial field and azimuthal current in Jupiter's middle magnetosphere region, at distances in the range 20– 45 R J . We first of all compare the model derived previously by Bunce and Cowley (Planet. Space Sci. 49 (2001) 261) using Pioneer, Voyager and Ulysses fly-by data, with a combined data set that now also incorporates data from the first twenty orbits of the Galileo orbiter. The overall RMS fractional residual is found to be 12.7%, such that the model does provide a good description of the combined data set. In particular, it is shown that the Galileo data also exhibit the same local time asymmetry as found in the fly-by data, in which the radial field (and azimuthal current) are stronger at a given radial distance on the nightside compared with the dayside. However, it is also shown that if the combined data are separated into 2 h bins of local time and then fitted to individual power law curves, the overall RMS fractional residual is reduced to 7.7%, thus showing scope for improvement in the empirical model. Based on the combined data set, in our revised model the field is taken as asymmetric outside of 14.5 R J , and to fall with radial distance with an exponent which is taken to vary sinusoidally with local time, varying between −1.5 at noon and −1.0 at midnight, such that the field becomes increasingly asymmetric with increasing distance. The overall RMS residual for this four-parameter model is found to be 9.7%, only slightly higher than that of the free-fits to the 2 h MLT binned data, and representing a worthwhile improvement over the original Bunce and Cowley model. The implied divergence of the azimuthal current for the revised model peaks at ∼15 kA R J −2 near the dawn-dusk meridian at a radial distance of ∼23 R J . The implied difference in the total azimuthal current flowing in the current sheet between 20 and 50 R J at midnight compared with noon is 19 MA , in a total (at dawn and dusk) of 59 MA .

Evidence for weak MHD turbulence in the middle magnetosphere of Jupiter

In this paper we study the small scale-magnetic eld fluctuations of the middle Jovian magnetosphere that are embedded in the strong background eld 0 of Jupiter in the framework of turbulence theory. We perform a statistical analysis of these fluctuations using a ve year set of Galileo spacecraft magnetic eld data. Calculating power spectra of the fluctuations, we identify for the rst time a spectral index of minus two for wave vectors perpendicular to 0. These results strongly support a description of the fluctuations within the framework of weak magnetic turbulence and recent theoretical developments therein by Galtier et al. (2000). In addition, we show that there is little variation of the spectral index upon radial distance in the middle magnetosphere. We comment also on how the presence of such fluctuating elds might have interesting consequences concerning transport properties in the Jovian magnetosphere.

Energetic ion observations in the middle Jovian magnetosphere

The middle magnetosphere of Jupiter has not been well studied prior to the Galileo mission. This region (10–30 RJ) has been traversed by Galileo numerous times allowing spatial and temporal features to be differentiated. We present observations of energetic ions (E ∼4.5–18.4 MeV nucleon−1) in the middle Jovian magnetosphere obtained with the Heavy Ion Counter (HIC) on Galileo. The radial dependence of the intensity, spectrum, and composition of these particles indicates that solar material dominates the 15 to 30 RJ region and diffuses without loss toward Jupiter. Near 10 RJ the composition is more Iogenic and shows evidence of strong losses. These observations can be understood in terms of a common energy per charge spectrum for all heavy ions. Due to the substantially different charge to mass ratios of the Iogenic and solar ions, the O/C and S/C abundance ratios vary with magnetic moment and therefore radial distance when observed over the fixed energy interval of the HIC instrument. Near 25 RJ there is evidence of acceleration of ions to MeV nucleon−1 energies. The location of this acceleration has been suggested previously but not directly observed before the Galileo mission.

Evidence provided by Galileo of ultra low frequency waves within Jupiter's middle magnetosphere

The Voyager and Ulysses spacecraft measurements from the dayside of Jupiter's middle magnetosphere provided evidence for ultra low frequency waves in the magnetometer data, in particular waves of periods 10–20 minutes. Galileo is the first orbiter mission around Jupiter and the first to extensively study the nightside magnetosphere. Similar waves are discernible when Galileo passed through the equatorial current sheet with very few features being noticed outside of the current sheet regions. An overview of the data shows that only 12% of crossings do not have any wave signatures in any component of magnetic field in the 10–20 minute period regime. The majority of these waves are found in the perpendicular field component. These waves are thus prevalent over the nightside magnetosphere and when combined along with the earlier work on the dayside seems to suggest that these ultra low frequency waves are a global phenomenon.

Multispectral observations of Jupiter's aurora

Remote sensing of Jupiter's aurora from x-ray to radio wavelengths has revealed much about the nature of the jovian aurora and about the impact of ionosphere-magnetosphere coupling on the upper atmosphere of Jupiter. As indicated by the combination of x-ray and ultraviolet observations, both energetic heavy ions and electrons energized in the outer magnetosphere contribute to auroral excitation. Imaging with the Hubble Space Telescope in the ultraviolet and with the InfraRed Telescope Facility at infrared wavelengths shows several distinct regions of interaction: 1) a dusk sector where turbulent auroral patterns extend well into the polar cap; 2) a morning sector generally characterized by a single spatially confined auroral arc originating in the outer or middle magnetosphere of Jupiter; 3) diffuse emissions associated with the Io plasma - spectroscopy has provided important information about the thermal structure of Jupiter's auroral atmosphere and the altitude distribution of auroral particle energy deposition, while Lyman alpha line profiles yield clues to the nature of thermospheric dynamical effects. Galileo observations at visible wavelengths on the nightside offer a new view of the jovian aurora with unprecedented spatial information. Infrared observations have added much to the understanding of thermal structure at all latitudes, the dynamics of the thermospheric wind system, and auroral morphology, and may hold the key to understanding the role of Joule heating in Jupiter's thermosphere. ROSAT observations have revealed soft x-ray emissions from Jupiter's lower latitudes as well as from the auroral zones, implying that energetic particle precipitation also occurs at low latitudes in regions magnetically linked to the inner radiation belts. In this review, multispectral observations of jovian auroral emissions are presented within a theoretical/modeling framework that is intended to provide some insight into magnetosphere-ionosphere coupling and its effects on the upper atmosphere.