Middle Magnetosphere Research Articles

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.

On the Dynamics of the Jovian Ionosphere and Thermosphere: I. The Measurement of Ion Winds

We present profiles of the line-of-sight ionospheric wind velocities in the northern auroral/polar region of Jupiter. Our velocities are derived from the measurement of Doppler shifting of the H 3 +ν 2 Q(1,0 −) line at 3.953 μm. The data for this study were obtained using the facility high-resolution spectrometer CSHELL on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, during the nights of September 7–11, 1998 (UT). A detailed analysis, allowing for several effects on the measured wavelength, is presented. The velocity profiles finally derived are consistent with an H 3 + electrojet of ∼1.2 to 1.5 km s −1 flowing clockwise (as viewed from above the north rotational pole)—i.e., counter to the rotation of the planet—around the main auroral oval, for the night of September 11, and with somewhat lower velocities for September 8 and 10. In addition, we note two main regions inside (poleward of) the main auroral oval, which we call the Dark Polar Region (DPR) and the Bright Polar Region (DPR). These features correspond to the yin–yang structure noted by T. Satoh and J. E. P. Connerney (1999, Icarus 141, 236–252). The DPR appears to be strongly redshifted, with poleward winds in the line of sight up to −2.5 km s −1. The BPR may have weakly blueshifted emission. The detection of the auroral electrojet continuously over our observing run confirms the initial electrojet detection of D. Rego, N. Achilleos, T. Stallard, S. Miller, R. Prangé, M. Dougherty, and R. D. Joseph (1999, Nature 399, 121–124), as well the mechanism proposed by T. W. Hill (1979, J. Geophys. Res. 84, 6554–6558) to explain how Jupiter's equatorial magnetospheric plasmasheet is kept in corotation with the planet. We discuss the results in light of recent proposals and analyses of the jovian middle magnetosphere, and suggest that our DPR winds may be under solar wind control.

Influence of solar wind on Jupiter's magnetosphere deduced from currents in the equatorial plane

Galileo is the sixth spacecraft to have visited Jupiter's magnetosphere. Among them the six spacecraft have sampled a fairly large expanse of magnetospheric local times and latitudes under varying solar wind conditions. An adequate (though not optimum) database of field and particles now exists that makes it possible to map the current circuits at a global scale. We have used the magnetic field observations from all six of the spacecraft to compute the electric current density in the equatorial plane of Jupiter's magnetosphere by making certain appropriate assumptions. We show that in the middle magnetosphere the azimuthal currents are much stronger on the nightside (∼ 144 MA between the radial distances of 10 and 50 RJ) than they are on the dayside (∼ 88 MA in the same distance range). From current continuity considerations we conclude that the nightside partial ring current is fed and emptied by field‐aligned currents in the dusk and the dawn sectors, respectively. These currents are similar (but opposite in polarity) to Region 2 field‐aligned currents observed in the Earth's magnetosphere. Because the presence of Region 1 or 2 sense field‐aligned currents in a magnetosphere indicates the presence of solar wind driven convection in a magnetosphere, a surprising conclusion of the present analysis is that the solar wind influence reaches deep into the heart of Jupiter's magnetosphere. Other findings of this study are that (1) the equatorial field strength is remarkably constant over all local times, (2) the equatorial source of the outward field‐aligned currents required for the generation of aurorae is located between the radial distances of 10 and 30 RJ with a peak near 20 RJ, and (3) the Jovian magnetosphere displays a magnetic field configuration intermediate to a Parker spiral and a magnetosphere driven by solar wind.

Origin of the main auroral oval in Jupiter's coupled magnetosphere–ionosphere system

We show that the principal features of the main auroral oval in the jovian system are consistent with an origin in the magnetosphere–ionosphere coupling currents associated with the departure of the plasma from rigid corotation in the middle magnetosphere, specifically with the inner region of field-aligned current directed upwards from the ionosphere to the magnetosphere. The features we refer to include its location, its continuity in local time, its width, and the precipitating particle energy flux and auroral luminosity. A simple empirical model of the field and flow in the middle magnetosphere is used to estimate the field-aligned currents flowing into and out of the equatorial current sheet associated with the breakdown of corotation. The models indicate that the current flows outwards from the ionosphere into the current sheet through most of the middle magnetosphere. Mapped to the ionosphere, the upward field-aligned current density is of order ∼1 μA m −2 , confined to circumpolar annular rings around each pole of latitudinal width ∼1° (∼1000 km ), centred near ∼16° dipole latitude. The upward current is carried principally by downward-precipitating magnetospheric electrons from the tenuous hot plasma which extends outside the cooler denser equatorial plasma sheet to high latitudes. For reasonable observed values of the magnetospheric electron parameters it is found that such currents require the existence of field-aligned voltages of order ∼100 kV . The auroral primaries are thus ∼100 keV electrons, consistent with deep penetration of the jovian atmosphere and low-altitude auroras, as observed. The peak ionospheric energy flux associated with the accelerated precipitating electrons is of order ∼0.1– 1 W m −2 , sufficient to drive a UV aurora of 1–10 MR at ∼20% conversion efficiency. In addition, to produce the current, the acceleration region must extend in altitude typically above ∼3–4 R J. The spatially extended energetic auroral electron beams so formed are suggested to form a principal source of free energy for non-Io-related radio emissions. An important implication of the model is that the main oval auroras and radio emissions will respond principally to the dynamic pressure of the solar wind, in the sense of anticorrelation.

Divergence of the equatorial current in the dawn sector of Jupiter's magnetosphere: analysis of Pioneer and Voyager magnetic field data

Averaged values of the azimuthal component of the magnetic field observed outside the jovian middle magnetosphere equatorial current sheet have been used to derive radial profiles of the radial current intensity over the jovicentric distance range 20–50 R J. Data from four spacecraft flybys have been used, spanning the dawn sector from ∼0100 to ∼0900 MLT (i.e. inbound Pioneer-11, and outbound Pioneer-10, and Voyagers-1 and -2). These profiles have been combined with a recent empirical model of the azimuthal current intensity to estimate the total divergence of the current in the current sheet along the trajectory, and hence the density of the field-aligned current that couples the current sheet and the ionosphere. For the Voyager passes the inferred field-aligned current flows from the ionosphere into the current sheet, with ( j ||/ B)≈2– 10×10 −13 A m −2 nT −1 . The inferred field-aligned currents at the ionosphere require significant field-aligned acceleration of thermal magnetospheric electrons (of 2–3 keV thermal energy), through voltages of ∼40–150 kV. The latitude of these currents, together with the estimated electron energy flux deposited in the ionosphere (0.01–0.1 W m −2), suggests a direct connection with the main jovian auroral oval. Related currents are present on the Pioneer passes as well, but lie equatorially inside of ∼20 R J. Between 20 and 50 R J, the inferred field-aligned currents on these passes are either small (Pioneer-11), or reversed in sense with similar densities (Pioneer-10). The radial profiles of the radial current associated with magnetosphere–ionosphere coupling have also been used to derive radial profiles of the angular velocity of the magnetospheric plasma, for given values of the effective Pedersen conductivity of the jovian ionosphere. Reasonable profiles are obtained for effective conductivities of several tenths of a mho. The Voyager data then indicate slowly falling values from near-rigid corotation at ∼20 R J, to ∼50% of rigid corotation at ∼50 R J. For the same values of the conductivity the Pioneer data indicate smaller angular velocities in the inner region (∼70% of rigid corotation), remaining either constant or even increasing with distance. This behaviour may either indicate the presence of some magnetospheric dynamic process operating in the outer magnetosphere on these passes, or could alternatively be due to falling conductivities with distance in the conjugate ionosphere.

More about the structure of the high latitude Jovian aurorae

This study is based on the determination of a ‘reference’ main oval for Jupiter's aurora from a series of high-resolution images taken with the Faint Object Camera on board the Hubble Space Telescope in the H 2 Lyman bands centered near 1550 A ̊ . We have taken advantage of the visibility of the northern auroral oval over a large range of longitudes on June 24, 1994, especially for longitudes smaller than 170° where it is generally very faint and thus undetectable. In the south, where there is no such problem, we combined images taken from various points of view between June 1994 and September 1996. We find that the northern main oval is consistent in size and in general aspect with the footprint locus, in the Connerney (1998)'s VIP4 model, of magnetic field lines crossing the equator near 20 R J. However, the precise shape of this oval differs from the model (and from previous ‘reference’ main ovals) in that it exhibits a ‘bean-like’ aspect with excursions toward lower latitudes in the S III longitude range 190–240° (an already reported feature), and toward higher latitudes in the poorly documented 120–150° range. In the south, our reference oval covers an area about that of the 30 R J VIP4 model. As in the north, it is shifted from a VIP4 model oval, toward lower latitudes from 110 to 200° and toward higher latitudes from 310 to 100°. The accurate definition of these (magnetically conjugate) oval loci puts additional strong constraints on magnetic field models at high latitude. Based on our reference main oval, we have then extrapolated still higher latitude ovals. Very interestingly, we find that we can fit (i) the highest latitude arc of oval detected well inside the main oval at longitudes greater than 170°, and (ii) the high latitude edge of what we had previously named the ‘transpolar emission’ at longitudes less than 170° (both also detected on images taken at other dates), by a single empirical oval. We suggest that this oval indicates the location of the northern polar cap boundary, within the uncertainty related to the temporal variability of the actual emission features. We can also fit another secondary arc of oval and a second branch of our ‘transpolar emission’ by a slightly larger empirical oval, presumably connected to the outer magnetosphere. This implies that the region of permanent high latitude diffuse emission seen inside the main oval in the dusk sector must be at the footprint of closed field lines connected all the way out from the middle magnetosphere to the magnetopause. Finally a bright spot is sometimes observed just equatorward of the northern polar cap boundary with an average brightness of 0.5-1 MR (comparing with a bright main oval). We establish that this spot does not rotate with the planet, but rather remains close to magnetic noon. We thus tentitatively identify it, by reference to the Earth aurora, with the footprint of the northern Jovian polar cusp, or with a transient dayside aurora. We also highlight the differences observed in the southern high latitude structure.

On the energy source for diffuse Jovian auroral emissivity

Data from the Energetic Particle Detector on Galileo have been used to investigate whether the radiation belt population can act as a direct energy reservoir for Jovian auroral emissions. Energetic (E > 42 keV) ion distributions in the middle magnetosphere (9 < RJ < 26) yield peak precipitation energy fluxes comparable to a few ergs‐cm−2‐s−1, which are too weak to produce significant auroral emissivity. Detailed analysis of energetic electrons (15‐884 keV) indicates the presence of an enhanced population near the loss cone that may provide precipitation energy flux above 10 ergs‐cm−2‐s−1 over an extended region (10 ≤ RJ ≤ 25) of the middle magnetosphere. The available precipitation electron energy flux decreases at higher radial distance, falling to values below 1 erg‐cm−2‐s−1 outside 30 RJ. Additional acceleration mechanisms such as field‐aligned currents are required to account for observed auroral intensities at higher L. The precipitation flux values presented here have been calculated under the assumption of strong pitch‐angle scattering due to wave‐particle interactions in the essentially collisionless middle magnetosphere. Plasma waves with relatively modest amplitude (Bw≥ 25 pT or Ew≥ 5 mV/m) are sufficient to induce strong diffusion scattering outside 10 RJ. Such scattering can account for energy deposition over a broad latitude range (Δ λ ≥5 °) associated with the diffuse Jovian aurora.

Spectroscopic Evidence for High-Altitude Aurora at Jupiter from Galileo Extreme Ultraviolet Spectrometer and Hopkins Ultraviolet Telescope Observations

The Galileo Extreme Ultraviolet Spectrometer (EUVS) and the Hopkins Ultraviolet Telescope (HUT) acquired UV spectra of Jupiter Aurora in the period from 1995 through 1997. The EUVS spectra spanned the wavelength range 540–1280 Å and the HUT spectra measured the extreme ultraviolet and far ultraviolet (EUV+FUV) wavelength range 830–1850 Å. Both sets of spectra present evidence of high-altitude, optically thin H 2 band emissions from the exobase region. The analysis of the UV spectra with a two-stream electron transport model and a jovian model auroral atmosphere indicates that the primary electron flux is composed of both soft and hard electrons with characteristic energies in the soft electron energy range of 20–200 eV and the hard electron range of 5–100 keV. The soft electron flux causes enhanced EUV emission intensities below 1100 Å. The soft electron flux may explain the high temperature of the upper atmosphere above the homopause as measured from H 3 + rovibrational temperatures in the IR. For the deep aurora, a high primary characteristic energy above 5 keV is known to be present. The Galileo Energetic Particle Detector (EPD) has measured the electron distribution functions for energies above 15 keV in the middle magnetosphere. The high-energy distribution functions can be modeled by a combination of Maxwellian and kappa distributions. However, the EUV (800–1200 Å) portion of the HUT spectrum cannot be modeled with a single distribution of hard electrons as was possible in the past for the FUV (1200–1650 Å) spectrum measured by itself. The combination of EUV and FUV spectral observations by HUT serves to identify the amount of soft electron flux relative to the hard primary flux required to produce the high-altitude aurora in the neighborhood of the exobase.

The Jovian auroral oval

Jupiter's aurora shows, among other features, a persistent, continuous oval of luminosity that encircles each magnetic pole and maps along magnetic field lines to the middle magnetosphere (r ≈ 30 Rj). This auroral oval is interpreted here as the ionospheric footprint of the upward Birkeland (magnetic field aligned) current that enforces partial corotation of magnetospheric plasma moving outward from its source (the Io plasma torus) to its sink (the outer magnetosphere and ultimately the solar wind). A simplified model of this current system, based on the assumption of a constant, uniform plasma outflow in a spin‐aligned dipole magnetic field, has a maximum upward Birkeland current density at an ionospheric latitude that maps to the middle magnetosphere. Strong upward Birkeland currents are known, from terrestrial studies, to produce bright aurora.

A new perspective concerning the influence of the solar wind on the Jovian magnetosphere

The solar wind exerts a strong influence on the Jovian magnetosphere in changing its volume, in energizing plasma, and in stimulating the aurora and a host of other associated effects. However, whereas at Earth the dominant solar terrestrial coupling process is magnetic reconnection, the dominant energy reservoir in Jupiter's magnetospheric plasma, continually present, is the kinetic energy of its rotating plasma disk. This “flywheel”’ produces effects with no terrestrial analogy, some of which we describe here. The most surprising prediction from the analysis of this paper is that remotely sensed symptoms of Jovian magnetospheric activity are likely to occur in conjunction with solar wind pressure decreases. Compressions of the magnetosphere produced by forward shocks and other solar wind pressure increases will heat the magnetospheric plasma but substantially reduce the ionosphere‐magnetosphere current systems. The intensity of dayside aurora and of radio wave emissions associated with increased ionospheric‐magnetospheric current systems will tend to anticorrelate with magnetospheric compressions and correlate with expansions. The link to the aurora is based on an argument that the auroral zone maps to the plasma disk of the middle magnetosphere and is thus linked to plasma sheet dynamics. The effect of expansion on the plasma sheet is to increase the parallel pressure, setting up conditions that can produce detached plasma “blobs” and enhance mass loss. The analysis is particularly apposite in light of the opportunities for observing solar wind‐Jovian interactions using data from both the Galileo and the Cassini spacecraft during the Cassini flyby of Jupiter in late 2000, ideally supplemented by auroral imaging with ground‐based and Hubble telescopes.

Magnetohydrodynamic simulations of the effects of the solar wind on the Jovian magnetosphere

We have used a three-dimensional magnetohydrodynamic simulation of the interaction between the solar wind and a rapidly rotating magnetosphere to study the effects of the solar wind dynamic pressure and the interplanetary magnetic field (IMF) on the configuration of the Jovian magnetosphere. Both the solar wind dynamic pressure and the IMF can cause substantial changes in the magnetosphere. On the dayside when the pressure increases the bow shock and magnetopause move toward Jupiter and the equatorial magnetic field in the middle magnetosphere becomes more dipole-like. When the pressure decreases the boundaries move farther from Jupiter and the dayside magnetic field becomes stretched out into a more tail-like configuration. For northward IMF the boundaries move toward Jupiter but the field becomes more tail-like. Finally, for southward IMF the boundaries move away and the field becomes more dipole-like. These changes are qualitatively consistent with those observed on spacecraft passing through the dayside magnetosphere. However, we were not always able to get quantitative agreement. In particular the model does not reproduce the extremely tail-like magnetic field observed during the Pioneer 10 and Ulysses inbound passes. The solar wind and IMF also influence the configuration of the middle magnetosphere in the magnetotail. Tailward flows were found in the nightside equatorial plasma sheet for most IMF orientations. Both inertial effects and the IMF influence reconnection in the tail. The only time the tailward flow in the magnetotail stopped was during prolonged intervals with southward IMF. Then reconnection in the polar cusp caused the flow to move out of the equatorial plane.

Waves close to the crossover frequency in the Jovian middle magnetosphere

The presence of heavy ions in a plasma introduces additional characteristic frequencies one of which is a crossover frequency between each adjacent pair of gyrofrequencies of the ions present. The crossover frequency is controlled by the fractional ion charge densities. The phase speed of the L and R wave modes have the same phase velocity at this frequency. For oblique propagation waves become linearly polarized at the crossover frequency. They reverse their natural polarization from left to right (or vice versa) as the wave frequency transverses the crossover frequency. Here we present polarization analysis of a portion of the Ulysses magnetometer observations in the middle Jovian magnetosphere during the inbound pass and close to a plasma sheet approach. The analysis show transverse waves with frequency between the gyrofrequencies of SO+ and S+. We observe polarization reversal at this frequency, which could be an indication of wave observations at the crossover frequency. On this assumption these observations can then be used to derive heavy ion composition of the resident plasma.

Energetic ion (>∼50keV) and electron (>∼40keV) bursts observed by Ulysses near Jupiter

A careful analysis of data collected by the HISCALE experiment on board Ulysses suggests that the quasi-periodic (QP) modulation of several or tens of minutes in flux and/or anisotropy/spectral observations is an almost permanent characteristic of the energetic (>∼50 keV) ion population in the outer and the high latitude middle magnetosphere of Jupiter. In most cases a periodicity of ∼5–20 min is evident, although periodicities of ∼40 min and ∼80 min were also observed. The ∼40 min and ∼80 min periodicities were often accompanied by other harmonics, for instance ∼20 min and ∼40 min periodicities respectively, and were preferentially observed in the dusk magnetosphere. Observations of QP series of energetic ion and electron bursts in the dusk high latitude magnetosphere, as for instance, strong anisotropy, forward velocity dispersion and time delay between the ion and the electron onsets, are consistent with a periodic acceleration / injection of particles, in the form of field-aligned beams, originated from the low altitude environment of the planet (∼8 R J from the planet surface); the QP intensity gradients perpendicular to the field observed in the dusk middle magnetosphere are consistent with spatial structures of energetic particle sheets crossed by the spacecraft. The QP bursts of energetic particles are often accompanied by bursts of radio and aurora emissions and are possible sources of these Jovian phenomena.

ULF wave observations in Jupiter's magnetosphere

In previous work we have examined magnetometer data from the Ulysses Jupiter flyby, and in particular middle magnetosphere observations in the vicinity of the magnetodisk. Waves transverse to the background magnetic field were found in the heavy ion gyrofrequency regime, a signature of Ion Cyclotron (IC) waves. We examined the dayside and high latitude duskside high resolution 1 sec data. Wave signatures were observed on several occasions both close to the magnetic equator and at some distance from it. Lower mass ion signatures were observed further away from the planet and the heaviest mass ion signatures appear closer to the Io torus although still at some distance from it. IC waves and their propagation properties can be used as a diagnostic of the particle species in a multicomponent plasma. Here we present polarization analysis of a portion of the Ulysses magnetometer wave observations in the middle Jovian magnetosphere during the inbound pass and close to plasma sheet approaches. The analysis show transverse waves in the heavy ion gyrofrequency regime with left-hand polarization in some cases and right-hand polarisation in others. We discuss these results in terms of polarization reversal properties of ULF waves propagating in a multicomponent plasma.

Wave signatures in the Jovian magnetosphere

Magnetometer data from the Ulysses Jupiter flyby are examined, in particular middle magnetosphere observations in the vicinity of the magnetodisk. Ion cyclotron waves are searched for in the heavy ion gyrofrequency regime (SO + 2, SO +, K +, S +, Na +, O + and S ++). Power spectral peaks in the ion cyclotron frequency range from some of the dayside Ulysses magnetometer data have previously been described. Here we examine the dayside and high latitude duskside high resolution 1-s data. Ion cyclotron waves signatures are observed on several occasions both close to the magnetic equator and at some distance from it. Rippling and warping of the magnetodisk, as observed by Ulysses, could be the cause of such ion cyclotron signatures arising some distance away from the magnetic equator. Lower mass ions are observed further away from the planet and the heaviest mass ion signatures appear closer to the Io torus. Preliminary polarisation analysis of the observations is also presented and some theoretical implications of the wave signatures are discussed.

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.

Circulation and dynamics in the jovian magnetosphere

Io continually adds mass to the Io torus and this density builds up until centrifugal force is sufficient to overcome the line tying of the jovian ionosphere. The magnetic flux is carried outward with this slow radial convection. Under the assumption that 1 ton per second is added to the torus at Io and that the Voyager observed densities and average ion mass are pertinent for the Pioneer and Galileo epochs, a consistent flow pattern arises. The outward flow near Io is a few meters per second. Near Europa the velocity, based on the observations of a Europa plume, rises to about 400 m/s. Beyond 24 R J in the magnetodisk it is possible to again estimate the velocity. It is about 20 km/s at 25 R J and about 50 km/sec at 40 R J consistent with the Voyager LECP measurements. This outflow of magnetic flux is replenished by reconnection of magnetic flux in the near tail and the subsequent inward convection of empty flux tubes into the inner magnetosphere. These flux tubes are seen in the middle magnetosphere and the Io torus. Their scarcity as judged by the fact that they constitute only about 0.4% of the observing time indicates that they are moving inward at a much higher rate than the mass-loaded magnetospheric plasma moves outward. In the neighborhood of the magnetodisk the outflow appears not to be steady as the magnetic flux crossing the current sheet varies greatly from orbit to orbit. This unsteadiness may lead the occasional auroral storms seen in the ionosphere at the feet of these field lines.

Outstanding issues in understanding the dynamics of the inner plasma sheet and ring current during storms and substorms

The paper deals with five selected issues of the dynamical coupling of the near-Earth plasma sheet and magnetosphere, (1) substorm initiation, (2) dipolarization, (3) pressure release of the outer magnetosphere via the auroral energy conversion process, (4) magnetization of the very high beta plasma assembling at the inner edge of the tail, and (5) penetration of energetic particles into the ring current below L ≅ 4. One outstanding and strongly debated subject is not discussed here, the origin of the substorm current wedge. The main conclusions (or personal preferences) are: (1) the substorm is initiated by formation of a near-Earth neutral line; (2) dipolarization occurs through magnetic flux transport by the earthward reconnection flow and not by current diffusion; (3) the auroral energy conversion process, the “auroral pressure valve”, contributes substantially to the pressure release during the substorms; (4) high beta (≫ 10) plasma breaks up into smaller scale blobs under slow magnetization; and (5) deep and prolonged penetration of hot plasma sheet plasma into the middle magnetosphere produces currents and electric fields which lead to the growth of the storm-time ring current.

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.

Storm‐like dynamics of Jupiter's inner and middle magnetosphere

The discovery of energy‐time dispersed, charged particle signatures of dynamic, longitudinally confined charged particle injections within Jupiter's inner magnetosphere has been reported previously as measured in >20‐keV particle intensities by the Galileo energetic particles detector (EPD). While these events have similarities to so‐called substorm injections observed within the Earth's magnetosphere, it is unknown whether the driving mechanisms are similar. Over 100 Jovian injection events have now been documented between radial distances of ∼9 RJ (the inner boundary of most of the observations) and 27 RJ Injections occur at all System III longitudes and local time positions. Similar to Earth magnetospheric injections, the Jovian injections occur throughout the broad radial region of transition between the quasi‐dipolar and neutral sheet magnetic field configurations, and where the charged particle energy densities are competitive with the magnetic energy densities. The Jovian injections can be clustered in time, analogous to what often happens during well‐known magnetic storms that occur in the Earth's magnetosphere. During one particular periapsis of Galileo's orbital trajectory, the magnetosphere was observed by EPD to become suddenly very disturbed with multiple injections following a prolonged period (>24 Earth hours) of relative quiescence. Because of the prestorm coincidence of a signature of an apparent Earth‐like global magnetospheric disturbance, we hypothesize that this Jovian storm occurred when the inner and middle magnetosphere were triggered out of marginal stability by the passage of a magnetohydrodynamic fast mode wave launched at the magnetopause by a pressure variation in the interplanetary (solar wind) environment.