Modulation of Jovian middle magnetosphere currents and auroral precipitation by solar wind-induced compressions and expansions of the magnetosphere: initial response and steady state

We present results from a theoretical model which has been used to investigate the modulation of the magnetosphere-ionosphere coupling currents in the Jovian middle magnetosphere by solar wind-induced compressions and expansions of the magnetosphere. We consider an initial system in which the current sheet field lines extend to 50R_J in the equatorial plane, and where the iogenic plasma in the current sheet undergoes steady outward radial diffusion under the influence of the ionospheric torque which tends to maintain corotation with the planet. We show using typical Jovian parameters that the upward-directed field-aligned currents flowing throughout the middle magnetosphere region in this system peak at values requiring the existence of significant field-aligned voltages to drive them, resulting in large precipitating energy fluxes of accelerated electrons and bright ‘main oval’ UV auroras. We then consider the changes in these parameters which take place due to sudden expansions or compressions of the magnetosphere, resulting from changes in the solar wind dynamic pressure. Two cases are considered and compared, these being first the initial response of the system to the change, determined approximately from conservation of angular momentum of the radially displaced plasma and frozen-in field lines, and second the subsequent steady state of steady outward radial diffusion applied to the compressed or expanded system. We show that moderate inward compressions of the outer boundary of the current sheet field lines, e.g. from 50 to 40R_J, are effective in significantly reducing the coupling currents and precipitation in the initial state, the latter then recovering, but only partly so, during the evolution to the steady state. Strong inward compressions, e.g. to 30R_J cause significant super-corotation of the plasma and a reversal in sense of the current system in the initial state, such that bright auroras may then be formed poleward of the usual ‘main auroral oval’ due to the ‘return’ currents. The sense of the currents subsequently reverts back to the usual direction as steady-state conditions are restored, but they are weak, and so is the consequent electron precipitation. For outward expansions of the current sheet, however, the field-aligned currents and electron precipitation are strongly enhanced, particularly at the poleward border mapping to the outer weak field region of the current sheet. In this case there is little evolution of the parameters between the initial expansion and the subsequent steady state. Overall, the results suggest that the Jovian middle magnetosphere coupling currents and resulting ‘main oval’ auroral acceleration and precipitation will be strongly modulated by the solar wind dynamic pressure in the sense of anti-correlation, through the resulting compressions and expansions in the size of the magnetosphere.     

Centrifugal instability of the jovian magnetosphere and its interaction with the solar wind

The outer regions (r > 2.3 R_j; R_j = radius of Jupiter) of the magnetosphere of Jupiter will systematically accumulate plasma. If sufficient plasma accumulates, the field lines must open to allow the plasma to escape. Available energy sources appear able to supply plasma at a high enough rate to keep the field lines constantly open beyond about 60 R_J. We suggest that the solar wind interaction with Jupiter may be essentially different from that with the Earth, with the Jovian magnetosphere opening up to form a planetary wind.     

Slow-mode shock candidate in the Jovian magnetosheath

We discuss some interesting plasma observations in the Jovian magnetosheath by the onboard plasma instruments of the Cassini spacecraft during the 2000-2001 Jupiter flyby. We propose that the observations are consistent with a slow-mode shock transition. In the terrestrial magnetosheath, a number of observations have been made that are consistent with slow-mode waves or shocks. In addition, a number of observations have established that, at least occasionally, slow-mode structures form at the plasma sheet-lobe boundary in the terrestrial magnetotail, related to X lines associated with reconnection. There has been only one previously reported observation of a slow-mode shock-like transition in the Jovian plasma environment. This observation was made in the dayside magnetosheath. The observation we report here was made well downstream of the magnetosphere in Jupiter’s magnetosheath, at local time ∼19:10. For our analysis we have used the data from the Cassini Plasma Spectrometer (CAPS) the Magnetospheric Imaging Instrument (MIMI) and the Magnetometer (MAG). The bow shock crossings observed by Cassini ranged downstream to −600 R_J from the planet     

Ulysses observations of anti-sunward flow on Jovian polar cap field lines

We examine the energetic (MeV) ion data obtained by the Anisotropy Telescopes instrument of the Ulysses COSPIN package during two northern high-latitude excursions prior to closest approach to Jupiter, when the spacecraft left the region of trapped fluxes on closed magnetic field lines at lower latitudes and entered a region of open field lines which we term the polar cap. During these intervals the ion fluxes dropped by 4–5 orders of magnitude to low but very steady values, and the ion spectrum was consistent with the observation of an essentially unprocessed interplanetary population. Ion anisotropies observed at these distances (within 16R_J, of Jupiter) indicate that in the low-latitude, high-flux regions the flows are principally azimuthail and in the sense of corotation, with speeds which are within a factor of 2 (in either direction) of rigid corotation. In the higher latitude trapped flux regions the flows rotate to become northward as the polar cap is approached, while in the polar cap itself the flows rotate further to become anti-corotational (and anti-sunward in the morning sector) and northward. These results provide primary evidence of the existence of solar wind-driven flows in the outer Jovian magnetosphere mapping to the high-latitude ionosphere. Investigation of concurrent magnetic data for the signatures of related field-aligned currents reveals only weak signatures with an amplitude of order 1 nT. The implication is that the height-integrated Pedersen conductivity of the ionosphere to which the spacecraft was connected was low, of order 0.01 mho or less. We also examine the ion observations during the two northern high-latitude excursions previous to those discussed above. These data indicate that the spacecraft approached but did not penetrate the open flux region during these intervals.     

Reconnection sites in Jupiter’s magnetotail and relation to Jovian auroras

The Galileo spacecraft explored Jupiter’s magnetotail in a low-inclination orbit, where it detected the signatures of tail reconnection. In this paper, we examine and classify the tail reconnection signatures into four types: dipolarizations, strong northward B_θ excursions, tailward-moving plasmoids and planetward-moving plasmoids. The distribution of these four types of events is used to infer the most probable location of the Jovian tail reconnection site to be near 0200 LT at a planetocentric distance of 80 Jovian radii. Dipolarizations are mainly observed planetward of this point, and strong northward B_θ excursions and plasmoids are found mostly tailward. The observations also suggest that the Jovian tail reconnection starts at a point (neutral point), a localized region in the tail, instead of along an extended azimuthal line (X-line). Using the updated Khurana’s Jupiter’s magnetospheric model, which includes the external field and the effects of the swept-back configuration of tail field lines, we map the signatures of Jovian tail reconnection into the Jupiter’s ionosphere. We confirm that the dawn auroral storms or the polar dawn spots observed by the Hubble Space Telescope (HST) are located close to the extrapolated footpoints of tail dipolarizations and could be the auroral signatures of tail reconnection.     

Polar cap arcs and the open regions

After reviewing the basic characteristics of the polar cap arcs, it is suggested that their appearance can be explained if the open region splits into two, one located in the dawn sector and the other in the dusk sector. It is suggested that a distinct splitting occurs temporarily when an IMF tangential discontinuity passes by the magnetosphere and the sign of the IMF B_y component changes at the discontinuity, provided that the IMF B_z component is positive on both sides. As a result, the dawn or the dusk side of the polar region will be connected to either the front side or the hind side of the discontinuity, depending on the sign of the B_y component across the discontinuity. As the dynamo process is expected to operate in each of the two open regions (as is the case in the single open region), it is reasonable to infer that a sheet of plasma and of field-aligned currents forms in the region between the two open regions, resulting in the polar cap arcs across the polar region. The four-cell convection pattern may also appear. A model of the magnetosphere is constructed to demonstrate such a possibility.     

Plasma electron signatures of magnetic connection to the Jovian bow shock: Ulysses observations

Ulysses plasma electron observations of bidirectional and enhanced unidirectional electron heat fluxes within 4500 R_J (0.8 a.u. or 3 months on either side of closest approach) of Jupiter are presented as evidence for the magnetic connection of the spacecraft to the Jovian bow shock. These bursts of suprathermal electrons (> 30 eV) are observed when the interplanetary magnetic field points roughly parallel or antiparallel to the Jupiter-spacecraft line. Ninety-eight possible connection events were found over the 6 month period centered on the closest approach to Jupiter. The frequency of occurrence peaked with proximity to the bow shock, with most events occurring post-encounter. These are the first observations of backstreaming suprathermal electrons made in the vicinity of the Jovian bow shock.     

Observations of Electromagnetically Coupled Dust in the Jovian Magnetosphere

We report on dust measurements obtained during the seventh orbit of the Galileo spacecraft about Jupiter. The most prominent features observed are highly time variable dust streams recorded throughout the Jovian system. The impact rate varied by more than an order of magnitude with a 5 and 10 hour periodicity, which shows a correlation with Galileo's position relative to the Jovian magnetic field. This behavior can be qualitatively explained by strong coupling of nanometer-sized dust to the Jovian magnetic field. In addition to the 5 and 10 h periodicities, a longer period which is compatible with Io's orbital period is evident in the dust impact rate. This feature indicates that Io most likely is the source of the dust streams. During a close (3,095 km altitude) flyby at Ganymede on 5 April 1997 an enhanced rate of dust impacts has been observed, which suggests that Ganymede is a source of ejecta particles. Within a distance of about 25 R_J(Jupiter radius, R_J= 71,492 km) from Jupiter impacts of micrometer-sized particles have been recorded which could be particles on bound orbits about Jupiter. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ulysses observations of the planetary depletion layer at Jupiter

The repeated samplings of the Jovian magnetosheath during the Ulysses encounter with Jupiter provided an opportunity to probe the planetary depletion layer. Of the 10 complete crossings of the Jovian magnetopause, only three contained clear signatures of an overlying depletion layer. All of these occurred on the flanks of the magnetosphere near the dusk terminator; crossings on the dayside were ambiguous or clearly lacked a depletion layer signature. In this paper we present a detailed analysis of the observations by the Ulysses solar wind plasma and magnetometer experiments and discuss conditions favorable and unfavorable for depletion layer observation.     

Three-dimensional particle anisotropies in and near the plasma sheet of Jupiter observed by the EPAC experiment onboard the Ulysses spacecraft

During its inbound journey into Jupiter's magnetosphere, Ulysses had several encounters with the Jovian plasma sheet near the magnetic equator, which were related with intensity maxima in the energetic particles. We show for the first time anisotropies in three dimensions of three ion species (protons, helium and oxygen) in the energy range 0.24 < E < 0.77 [MeV/nucleon]. The data, obtained with the Energetic Particle Composition Experiment (EPAC) onboard Ulysses have been analysed by using spherical harmonics in three dimensions. We show that the first-order anisotropies of ions in or near the plasma sheet are strongest in a plane parallel to the ecliptic plane and more or less azimuthal with respect to the rotation of Jupiter. We show that the first-order anisotropy amplitude is larger for helium and oxygen ions than for protons in nearly the same energy per nucleon range. We find flow velocities for helium ions which are not consistent with corotation, but are larger by a factor of 2 in and near the Jovian plasma sheet on the dayside magnetosphere. In contrast for protons we observe nearly corotation. Far from the plasma sheet, at high magnetic latitudes, the flow velocities are less than corotation for protons, as well as for helium and oxygen. The azimuthal particle anisotropies are explained by intensity gradients perpendicular to the centre of the plasma sheet, by E × B particle drifts, and by nonadiabatic orbits of the particles near the Jovian plasma sheet. All of the three phenomena act in the same azimuthal direction, perpendicular to the mainly radial magnetic field direction. Each of them can be estimated, but their individual effects cannot be distinguished from each other. In addition, we find a radial component of the anisotropy which apparently is stronger for protons than for heavier ions. This radial anisotropy component is interpreted as a result of the radial outward displacement of ions in an azimuthally swept back magnetic field.     

A possible method of accelerating relativistic electrons in the neutral sheet in the Jovian outer magnetosphere

It is suggested that the periodic modulation of Mev electron fluxes observed by Pioneer 10 in its outbound orbit was due to crossing the magnetically neutral sheet in the Jovian outer magnetosphere. It is pointed out that these electrons are continually generated in this sheet formed in the outer magnetosphere beyond about 20 Jupiter radii from the planet.     

Measurements of hot plasmas in the magnetosphere of Jupiter

This paper presents an overview of a number of the principal findings regarding the hot plasmas (E 50 keV) in Jupiter's magnetosphere by the HISCALE instrument during the encounter of the Ulysses spacecraft with the planet in February 1992. The hot plasma ion fluxes measured by HI-SCALE in the dayside magnetosphere are similar to those measured in the same energy range in this region by the Voyager spacecraft in 1979. Within the dayside plasma sheet, the hot-ion energy densities are comparable with, or larger than, the magnetic field energy densities; these hot ions are found to corotate at about one-half the planetary corotational speed. For ions of energies 500 keV/nucleon, the protons contributed from 50–60% to as much as 80% of the energy content of these plasmas. Strong, magnetic-field-aligned streaming was found for both the ions and electrons in the high-latitude duskside magnetosphere. The ion and electron pitch-angle distributions could be characterized by cos²⁵ α throughout many of the high anisotropy intervals of the outbound pass. There is some evidence in the ion pitch-angle distributions for a corotational component in the hot plasmas at high Jovian latitudes. While there are limitations owing to the finite geometries of the detector telescope systems on the determination of the angular spreads of the ion and electron beams, the measurements show that there are intervals when the particle distributions are not bidirectional. At such times, locally the hot plasmas could be carrying currents of 10⁻⁴μAm⁻². The temporal variations in the streaming electron fluxes are substantially larger than the variations measured for the fluxes that are more locally mirroring. The temporal variations contain periodicities that may correspond to hydromagnetic wave frequencies in the magnetosphere as well as to larger scale motions of magnetospheric plasmas. On nearly half of the days for about a 130 day interval around the time of the Ulysses encounter with the planet, particles of Jovian origin were measured in the interplanetary medium. An event discussed herein shows evidence of an energy dependence of the particle release process from the planetary magnetosphere into the interplanetary medium.     

HEOS-2 observations of the boundary layer from the magnetopause to the ionosphere

HEOS-2 low energy electron data (10 eV–3.7 keV) from the LPS Frascati plasma experiment have been used to identify three different magnetospheric electron populations. Magnetosheathlike electron energy spectra (35–50 eV) are characteristic of the plasma mantle, entry layer and cusps from the magnetopause down to 2–3 R_E Plasma sheet electrons (energy > 1 keV) are found at all local times, with strong intensities in the early morning quadrant and weaker intensities in the afternoon quadrant. The plasma sheet shows a well defined inner edge at all local times and latitudes, the inner edge coinciding probably with the plasmapause. The plasma sheet does not reach the magnetopause, but it is separated from it by a boundary layer electron population that is very distinct from the other two electron populations, most electrons having energies 100–300 eV.We map these three electron populations from the magnetopause down to the high latitude near earth regions, by making use of the HEOS-2 low latitude inbound passes and the high latitude outbound passes (in Solar Magnetic (SM) coordinates). The boundary layer extends along the magnetopause up to 5–7 R_E above the equator; at higher latitudes it follows the magnetic lines of force and it is found closer and closer to the earth, so that it has the same invariant latitudes of the system 1 currents observed by Iijima and Potemra (1976) in their region 1. The plasma sheet can be mapped into their region 2 and the cusp-entry layer-plasma mantle can be mapped into their cusp currents region. The boundary layer is observed for any Interplanetary Magnetic Field (IMF) direction. We speculate that magnetosheath particles penetrate into the magnetosphere everywhere along the magnetopause. The electron energization, however, is observed only in the boundary layer, on both dawn and dusk side and could be due to the polarization electric field at magnetopause generated by the magnetosheath plasma bulk motion in the region where such motion is roughly perpendicular to the magnetospheric magnetic field. The electron energization is absent in the regions (entry layer and plasma mantle) where the sheath plasma motion is roughly parallel or antiparallel to the magnetospheric magnetic field.     

Quasiperiodic Jovian Radio bursts: observations from the Ulysses Radio and Plasma Wave Experiment

The Ulysses flyby of Jupiter has permitted the detection of a variety of quasiperiodic magnetospheric phenomena. In this paper, Unified Radio and Plasma Wave Experiment (URAP) observations of quasiperiodic radio bursts are presented. There appear to be two preferred periods of short-term variability in the Jovian magnetosphere, as indicated by two classes of bursts, one with 40 min periodicity, the other with 15 min periodicity. The URAP radio direction determination capability provides clear evidence that the 40 min bursts originate near the southern Jovian magnetic pole, whereas the source location of the 15 min bursts remains uncertain. These bursts may be the signatures of quasiperiodic electron acceleration in the Jovian magnetosphere; however, only the 40 min bursts occur in association with observed electron bursts of similar periodicity. Both classes of bursts show some evidence of solar wind control. In particular, the onset of enhanced 40 min burst activity is well correlated with the arrival of high-velocity solar wind streams at Jupiter, thereby providing a remote monitor of solar wind conditions at Jupiter.     

Characterization of Jovian plasma-embedded dust particles

As the data from space missions and laboratories improve, a research domain combining plasmas and charged dust is gaining in prominence. Our solar system provides many natural laboratories such as planetary rings, comet comae and tails, ejecta clouds around moons and asteroids, and Earth's noctilucent clouds for which to closely study plasma-embedded cosmic dust. One natural laboratory to study electromagnetically controlled cosmic dust has been provided by the Jovian dust streams and the data from the instruments which were on board the Galileo spacecraft. Given the prodigious quantity of dust poured into the Jovian magnetosphere by Io and its volcanoes resulting in the dust streams, the possibility of dusty plasma conditions exist. This paper characterizes the main parameters for those interested in studying dust embedded in a plasma with a focus on the Jupiter environment. I show how to distinguish between dust-in-plasma and dusty-plasma and how the Havnes parameter P can be used to support or negate the possibility of collective behavior of the dusty plasma. The result of applying these tools to the Jovian dust streams reveals mostly dust-in-plasma behavior. In the orbits displaying the highest dust stream fluxes, portions of orbits E4, G7, G8, C21 satisfy the minimum requirements for a dusty plasma. However, the P parameter demonstrates that these mild dusty plasma conditions do not lead to collective behavior of the dust stream particles.     

Mass-injection rate from Io into the Io plasma torus

Theoretical arguments have been presented to the effect that both plasma and energy are supplied to the Jovian magnetosphere primarily from internal sources. If we assume that Io is the source of plasma for the Jovian magnetosphere and that outward flow of plasma from the torus is the means of drawing from the kinetic energy of rotation of Jupiter to drive magnetospheric phenomena, we can obtain a new, independent estimate of the rate of mass injection from Io into the Io plasma torus. We explicitly assume the solar wind supplies neither plasma nor energy to the Jovian magnetosphere in significant amounts. The power expended by the Jovian magnetosphere is supplied by torus plasma falling outward through the corotational-centrifugal-potential field. A lower limit to the rate of mass injection into the torus, which on the average must equal the rate of mass loss from the torus, is therefore derivable if we adopt a value for the power expended to drive the various magnetospheric phenomena. This method yields an injection rate of at least 10³ kg/sec, a value in agreement with the results obtained by two other independent methods of estimating mass injection rate. If this injection rate from Io and extraction of energy from Jupiter's kinetic energy of rotation has been maintained over geologic time, then approximately 0.1% of Io's mass (principally in the form of sulfur and oxygen) has been lost to the Jovian magnetosphere, and Jupiter's spin rate has been reduced by less than 0.1%.     

Solar wind access to the plasma sheet along the flanks of the magnetotail

Low-energy particle trajectories in an idealized magnetotail magnetic field are investigated to determine the accessibility of magnetosheath protons and electrons to the plasma sheet along the flanks of the tail magnetopause. The drift motion of the positively (negatively) charged particles incident on the dawn (dusk) magnetotail flank causes such particles to penetrate deeper into the magnetotail. For certain combinations of particle energy, incident velocity vector and initial penetration point on the tail magnetopause, the incident particles can become trapped in the plasma sheet, after which their net drift motion then provides a current capable of supporting the entire observed magnetotail field. The results further indicate that the bulk of the solar wind plasma just outside the distant tail boundary, which streams preferentially in a direction along the magnetopause away from the Earth at velocities around 400 km s^?1, can be caught up in the tail if the initial penetration point is within about 2R_E, of the quasi-neutral sheet. It is suggested that a large fraction of the magnetotail plasma is composed of former solar wind particles which have penetrated the magnetospheric boundary at the tail flanks.     

Some possible effects of Jupiter's rings on the Jovian inner plasmasphere

The ionospheric plasma density on magnetic field lines threading the Jovian rings which are located inside ～1.8 R_J on the jovigraphic equatorial plane, is calculated by using a rotating ion exosphere model. It is found that the bulk of the ionospheric particles on these field lines are on ballistic trajectories. On field lines approximately symmetric with respect to the jovigraphic equator, the ring, which to a first approximation would absorb the population of trapped particles, consequently has little effect. On field lines which are made asymmetric by the higher-order multipoles of Jupiter's field and the tilt of the dipole axis, the rings may have a significant effect. It is suggested that better definition of the rings' atmospheric and ionospheric properties is required to model these localized effects. If the rings are found to be an important plasma source for the inner magnetosphere, the present exospheric model will have to be revised.     

Sodium emission from Comet Shoemaker-Levy 9 in the magnetosphere of Jupiter

On July 20, 1994, before the Q fragments of Comet Shoemaker-Levy 9 fell to Jupiter, more than 200 spectra of the Jovian features were obtained at the Crimean Astrophysical Observatory in the wavelength range 5700–7600 Å with a 26 s exposure time and a spectral resolution of 20 Å. We found a time-varying Na D line emission in the form of two components with Doppler shifts of about 30 Å. The brightest and most frequent sodium flares were detected when the Q fragments passed through the Jovian inner magnetosphere at a distance of about three the Jovian radii (3R_J) from its center, where they crossed the Io-Jupiter current tube. A frequency analysis of our data revealed a flare recurrence time scale of 1 min. We conclude that sodium was released from the cometary dust and from the surfaces of numerous cometary debris and that its amount was enough to produce the observed emission. The observed high-speed clouds of sodium atoms are assumed to have been formed through ionization, ion acceleration by the bidirectional electric fields of Alfvén waves in the Io-Jupiter current tube, and their neutralization.     

Effect of parallel electric fields on whistler mode waves in an anisotropic Jovian magnetospheric plasma

The effect of parallel electrostatic field on the amplification of whistler mode waves in an anisotropic bi-Maxwellian weakly ionized plasma for Jovian magnetospheric conditions has been carried out. The growth rate for different Jovian magnetospheric plasma parameters forL = 5.6R _j has been computed with the help of general dispersion relation for the whistler mode electromagnetic wave of a drifted bi-Maxwellian distribution function. It is observed that the growth or damping of whistler mode waves in Jovian magnetosphere is possible when the wave vector is parallel or antiparallel to the static magnetic field and the effect of this field is more pronounced at low frequency wave spectrum.