Jovian electron jets in interplanetary space

The COSPIN/KET experiment onboard Ulysses has been monitoring the flux of 3–20 MeV electrons in interplanetary space since the launch of Ulysses in October 1990. The origin of these electrons has been known for a long time to be the Jovian magnetosphere. Propagation models assuming interplanetary diffusion of these electrons in the ideal Parker magnetic field were successfully developed in the past. The average electron flux measured by our experiment agrees with these models for most of the times before and after the Jovian flyby of February 1992, i.e. in and out of the ecliptic down to 28° S of heliographic latitude for the last data presented here (end of March 1993).However, in addition to this average flux level well accounted for by diffusion in an ideal Parker field, we have found very short duration electron events which we call “jets”, characterized by: (i) a sharp increase and decrease of flux; (ii) a spectrum identical to the electron spectrum in the Jovian magnetosphere; and (iii) a strong first-order anisotropy. These jets only occur when the magnetic field at Ulysses lies close to the direction of Jupiter, and most of the time (86% of the events) points outwards from Jupiter, i.e. has the same polarity after the flyby as the Jovian dipole (North to South). These events are interpreted as crossings by Ulysses of magnetic flux tubes or sheets directly connected to the location of the Jovian magnetosphere from which electrons escape into interplanetary space. The average thickness of these sheets is 10¹¹cm or 14 Jovian radii. These jets are clearly identified up to 0.4 a.u. before the Jupiter flyby in the ecliptic plane, and up to 0.9 a.u. out of the ecliptic.Moreover, the characteristic rocking of the electron spectrum in the Jovian magnetosphere with a 10 h periodicity is found to be present during the jets, and predominantly during them. In the past, this modulation has been reported to be present in interplanetary space as far as 1 a.u. upwind of Jupiter, a fact which cannot be accounted for by diffusion in the average Parker magnetic field. Our finding gives a simple explanation to this phenomenon, the 10 h modulation being carried by the “jet” electrons which travel with no appreciable diffusion along magnetic field lines with a direction far from the ideal Parker spiral.     

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.     

Boundary layer ion composition at Jupiter during the inbound pass of the Ulysses flyby

During the inbound segment of the Ulysses flyby of Jupiter, there were multiple incursions into the dawnside low-latitude boundary layer, as identified by Bame et al. (Science257, 1539–1542, 1992) using plasma electron data. In the present study, ion composition and spectral measurements provide independent collaborative evidence for the existence of distinct boundary layer regions. Measurements are taken in the energy-per-charge range of 0.6–60 keV/e and involve mass as well as mass-per-charge identification by the Ulysses/SWICS experiment. Ion species of Jovian magnetospheric origin (including O⁺, O²⁺, S²⁺, S³⁺) and sheath origin (including He²⁺ and high charge state CNO) have been directly identified for the first time in the Jovian magnetospheric boundary layer. Protons of probably mixed origin and He⁺ of possibly sheath (ultimately interstellar pickup) origin were also observed in the boundary layer. Sheath-like ions are observed throughout the boundary layer; however, the Jovian ions are depleted or absent for portions of two boundary layer cases studied. Ions of solar wind origin are observed within the outer magnetosphere. and ions of magnetospheric origin are found within the sheath, indicating that transport across the magnetopause boundary can work both ways, at least under some conditions. Although their source cannot be uniquely identified, the proton energy spectrum in the boundary layer suggests a sheath origin for the lower energy protons.     

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.     

Hydrogen from Jupiter's atmosphere in the Jovian magnetosphere

The passage of Ulysses through Jupiter's magnetosphere presents a new opportunity to investigate the contribution to the Jovian magnetosphere of ions of atmospheric origin. A determination of the magnetospheric H⁺/He²⁺ flux ratio allows an estimate of the relative abundance of ionospheric material in the Jovian magnetosphere. We find that the H⁺/He²⁺ flux ratio, measured in the energy/charge range between 0.65 and 60 keV/e, steadily increases from a solar wind level of 25 at the magnetopause to a value of 700 at the point of closest approach, and then steadily decreases whilst approaching the magnetopause on the outbound path. We conclude from this that: (1) there is a significant solar wind component throughout the outer and middle magnetosphere; and (2) a significant fraction of the protons in the middle magnetosphere are of nonsolar origin.     

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.     

Impulsive bursts of relativistic electrons discovered during Ulysses' traversal of Jupiter's dusk-side magnetosphere

During its flyby of Jupiter in February 1992, the Ulysses spacecraft passed through the Southern Hemisphere dusk-side Jovian magnetosphere, a region not previously explored by spacecraft. Among the new findings in this region were numerous, sometimes periodic, bursts of high energy electrons with energies extending from less than 1.5 MeV to beyond 16 MeV. These bursts were discovered by the High Energy Telescope (HET) and the Kiel Electron Telescope (KET) of the COSPIN Consortium. In this paper we provide a detailed analysis of observations related to the bursts using HET measurements. At the onset of bursts, the intensity of > 16 MeV electrons often rose by a factor of > 100 within 1 min, and multiple, pulsed injections were sometimes observed. The electron energy spectrum also hardened significantly at the onset of a burst. In most bursts anisotropy measurements indicated initial strong outward streaming of electrons along magnetic field lines that connect to the southern polar regions of Jupiter, suggesting that the acceleration and/or injection region for the electrons lies at low altitudes near the South Pole. The initial strong outward anisotropies relaxed to strong field-aligned bidirectional anisotropies later in the events. The bursts sometimes appeared as isolated events, but at other times appeared in quasi-periodic series with a period of 40 min. For smaller events shorter periods of the order 2–3 min were also observed in a few cases. For large events, multiple injections were sometimes observed in the first few minutes of the event. Radio bursts identified by the Ulysses URAP experiment in the frequency range 1–50 kHz were correlated with many of the electron bursts, and comparison of the time-intensity profiles for radio and electrons shows that the radio emission typically started several minutes before the electron intensity increase was observed. For the strongest electron bursts, small increases in the low energy (> 0.3 MeV) proton counting rates were also observed. Using a computerized identification algorithm to pick out bursts from the data record using a consistent set of criteria, 121 events were identified as electron bursts during the outbound pass, compared to only three events that satisfied the same criteria during the inbound pass through the day-side magnetosphere. No similar electron burst events have been found outside the magnetopause. Estimates of the electron content of a typical large burst (> 10²⁷ electrons) suggest that these bursts may make significant contributions to the fluxes of electrons observed in Jupiter's outer magnetosphere, and in interplanetary space.     

Energetic neutral atom imaging of Mercury's magnetosphere 3. Simulated images and instrument requirements

The paper presents simulations of the energetic neutral atom (ENA) production in the Mercury magnetosphere and the obtained ENA images for the equatorial and polar vantage points. The ENA fluxes are found to be 10²–10³ (cm² srskeV)⁻¹ and up to 10⁴–10⁵ (cm² srskeV)⁻¹ in the energy range 10–50 keV. Due to the small size of the magnetosphere, the particles injected in the tail can fill up the entire dayside magnetosphere making possible ENA imaging of the magnetospheric shape. The high variability of the Hermean magnetosphere gives rise to pulsating ENA emissions (ENA “flashes”) which can be used to study the global dynamics. The ENA instrument requirements, 10°×10° angular resolution and 20 s accumulation time, can be easily met by modern ENA instrumentation. Therefore, ENA imaging of the Mercury magnetosphere is feasible.     

Relations between turbulent regions of interplanetary magnetic field and Jovian decametric radio wave emissions from the main source

Jovian decametric radio wave emissions that were observed at Goddard Space Flight Center, U.S.A. for a period from 1 October to 31 December, 1974 and data obtained at Mt Zao observatory, Tohoku University, Japan, for a period from 14 July to 6 December, 1975 have been used to investigate the relationship of the occurrence of the Jovian decametric radio waves (JDW), from the main source, to the geomagnetic disturbance index, ΣK_p. The dynamic cross-correlation between JDW and ΣK_p indicates an enhanced correlation for certain values of delay time. The delay time is consistent with predicted values based on a model of rotating turbulent regions in interplanetary space associated with two sector boundaries of the interplanetary magnetic field, i.e. the rotating sector boundaries of the interplanetary magnetic field first encounter the Earth's magnetosphere producing the geomagnetic field disturbances, and after a certain period, they encounter the Jovian magnetosphere. There are also cases where the order of the encounter is opposite, i.e. the sector boundaries encounter first Jovian magnetosphere and encounter the Earth's magnetosphere after a certain period.     

Electron transport and precipitation at Mercury during the MESSENGER flybys: Implications for electron-stimulated desorption

To examine electron transport, energization, and precipitation in Mercury's magnetosphere, a hybrid simulation study has been carried out that follows electron trajectories within the global magnetospheric electric and magnetic field configuration of Mercury. We report analysis for two solar-wind parameter conditions corresponding to the first two MESSENGER Mercury flybys on January 14, 2008, and October 6, 2008, which occurred for similar solar wind speed and density but contrasting interplanetary magnetic field (IMF) directions. During the first flyby the IMF had a northward component, while during the second flyby the IMF was southward. Electron trajectories are traced in the fields of global hybrid simulations for the two flybys. Some solar wind electrons follow complex trajectories at or near where dayside reconnection occurs and enter the magnetosphere at these locations. The entry locations depend on the IMF orientation (north or south). As the electrons move through the entry regions they can be energized as they execute non-adiabatic (demagnetized) motion. Some electrons become magnetically trapped and drift around the planet with energies on the order of 1–10 keV. The highest energy of electrons anywhere in the magnetosphere is about 25 keV, consistent with the absence of high-energy (>35 keV) electrons observed during either MESSENGER flyby. Once within the magnetosphere, a fraction of the electrons precipitates at the planetary surface with fluxes on the order of 10⁹ cm⁻² s⁻¹ and with energies of hundreds of eV. This finding has important implications for the viability of electron-stimulated desorption (ESD) as a mechanism for contributing to the formation of the exosphere and heavy ion cloud around Mercury. From laboratory estimates of ESD ion yields, a calculated ion production rate due to ESD at Mercury is found to be on par with ion sputtering yields.     

A mathematical magnetospheric field model with independent physical parameters

A quantitative magnetospheric magnetic field model has been calculated in three dimensions. The model is based on an analytical solution of the Chapman-Ferraro problem. For this solution, the magnetopause was assumed to be an infinitesimally thin discontinuity with given geometry. The shape of the dayside magnetopause is in agreement with measurements derived from spacecraft boundary crossings.The magnetic field of the magnetopause currents can be derived from scalar potentials. The scalar potentials result from solutions of Laplace's equation with Neumann's boundary conditions. The boundary values and the magnetic flux through the magnetopause are determined by all magnetic sources which are located inside and outside the magnetospheric cavity. They include the Earth's dipole field, the fields of the equatorial ring current and tail current systems, and the homogeneous interplanetary magnetic field. In addition, the flux through the magnetopause depends on two constants of interconnection which provide the possibility of calculating static interconnection between magnetospheric and interplanetary field lines. Realistic numerical values for both constants have been derived empirically from observed displacements of the polar cusps which are due to changes in the orientation of the interplanetary field. The transition from a closed to an open magnetosphere and vice versa can be computed in terms of a change of the magnetic boundary conditions on the magnetopause. The magnetic field configuration of the closed magnetosphere is independent of the amount and orientation of the interplanetary field. In contrast, the configuration of the open magnetosphere confirms the observational finding that field line interconnection occurs primarily in the polar cusp and high latitude tail regions.The tail current system reflects explicitly the effect of dayside magnetospheric compression which is caused by the solar wind. In addition, the position of the plasma sheet relative to the ecliptic plane depends explicitly on the tilt angle of the Earth's dipole. Near the tail axis, the tail field is approximately in a self-consistent equilibrium with the tail currents and the isotropic thermal plasma.The models for the equatorial ring current depend on the D_st-parameter. They are self-consistent with respect to measured energy distributions of ring current protons and the axially symmetric part of the magnetospheric field.     

Evidence for accelerated electrons upstream from Jupiter's bow shock: Ulysses results

Several transient increases of electrons with energies in the range 40–100 keV have been detected upstream and immediately downstream from the Jovian bow shock (and only in these regions), by instruments on the Ulysses spacecraft during February 1992. The energy spectra of these electrons differ markedly from the energy spectrum of the trapped magnetospheric electrons measured by the same instrument. Two populations of the upstream electrons were identified. Type I electrons appear at times when the direction of the interplanetary magnetic field at the spacecraft could have been tangent to the Jovian bow shock surface thus paralleling, for the first time at another planetary bow shock, the rather well understood situation at Earth's bow shock. Type II electrons have the same energy spectrum as Type I electrons, but are not so clearly associated with the tangent field-line condition. They occur at high southerly latitudes only while the Type I electrons are detected both on the inbound and outbound passages. Type II electrons have never been reported at the Earth's bow shock or any other planetary bow shock. Under the assumption that the field line that goes through Ulysses connects to the bow shock in a straight line, two possible explanations for the Type II electrons may be: (1) very large distortions of the bow shock surface, perhaps caused by deformations of the magnetopause, may permit the tangent condition; and (2) upstream electrons are preferentially, but not necessarily, accelerated when the IMF is tangent to the bow-shock surface.     

MESSENGER observations of the plasma environment near Mercury

The MESSENGER Fast Imaging Plasma Spectrometer (FIPS) measured the bulk plasma characteristics of Mercury's magnetosphere and solar wind environment during the spacecraft's first two flybys of the planet on 14 January 2008 (M1) and 6 October 2008 (M2), producing the first measurements of thermal ions in Mercury's magnetosphere. In this work, we identify major features of the Mercury magnetosphere in the FIPS proton data and describe the data analysis process used for recovery of proton density (n_p) and temperature (T_p) with a forward modeling technique, required because of limitations in measurement geometry. We focus on three regions where the magnetospheric flow speed is likely to be low and meets our criteria for the recovery process: the M1 plasma sheet and the M1 and M2 dayside and nightside boundary-layer regions. Interplanetary magnetic field (IMF) conditions were substantially different between the two flybys, with intense reconnection signatures observed by the Magnetometer during M2 versus a relatively quiet magnetosphere during M1. The recovered ion density and temperature values for the M1 quiet-time plasma sheet yielded n_p∼1–10 cm⁻³, T_p∼2×10⁶ K, and plasma β∼2. The nightside boundary-layer proton densities during M1 and M2 were similar, at n_p∼4–5 cm⁻³, but the temperature during M1 (T_p∼4–8×10⁶ K) was 50% less than during M2 (T_p∼8×10⁶ K), presumably due to reconnection in the tail. The dayside boundary layer observed during M1 had a density of ∼16 cm⁻³ and temperature of 2×10⁶ K, whereas during M2 this region was less dense and hotter (n_p∼8 cm⁻³ and T_p∼10×10⁶ K), again, most likely due to magnetopause reconnection. Overall, the southward interplanetary magnetic field during M2 clearly produced higher T_p in the dayside and nightside magnetosphere, as well as higher plasma β in the nightside boundary, ∼20 during M2 compared with ∼2 during M1. The proton plasma pressure accounts for only a fraction (24% for M1 and 64% for M2) of the drop in magnetic pressure upon entry into the dayside boundary layer. This result suggests that heavy ions of planetary origin, not considered in this analysis, may provide the “missing” pressure. If these planetary ions were hot due to “pickup” in the magnetosheath, the required density for pressure balance would be an ion density of ∼1 cm⁻³ for an ion temperature of ∼10⁸ K.     

Correlations between magnetic field and electron density observations during the inbound Ulysses Jupiter flyby

The spacecraft Ulysses flew through the Jovian magnetosphere during February 1992. This paper compares the magnetic field observations recorded during the inbound pass of the flyby with the electron density as derived from the URAP instrument. In general, it is expected that the density variations will anti-correlate with the magnetic field strength in order to maintain pressure balance, although there may be instances when a temperature or energy rise alone could balance the static stress. Furthermore, there is the possibility that a dynamic process could occur which would cause both the density and field magnitude to rise in unison. In the middle magnetosphere, anti-correlation is found to exist between the two data sets; however, in the outer magnetosphere (which was characterized by very disturbed fields) and in the transition region between the outer and middle magnetospheres, there is no simple relationship between the density and field. Examples of anti-correlation, temperature or energy increases and dynamic processes are found.     

Stream Interactions and Interplanetary Coronal Mass Ejections at 5.3 AU near the Solar Ecliptic Plane

We have performed a survey of the characteristics of two types of large spatial-scale solar-wind structures, stream interaction regions (SIRs), and interplanetary coronal mass ejections (ICMEs), near 5.3 AU, using solar-wind observations from Ulysses. Our study is confined to the three aphelion passes of Ulysses, and also within ± 10° of the solar ecliptic plane, covering a part of 1992, 1997 – 1998, and 2003 – 2005, representing three slices of different phases of the solar activity cycle. Overall, there are 54 SIRs and 60 ICMEs in the survey. Many are merged in hybrid events, suggesting that they have undergone multiple interactions prior to reaching Jovian orbit. About 91% of SIRs occur with shocks, with 47% of such shocks being forward – reverse shock pairs. The solar-wind velocity sometimes stays constant or even decreases within the interaction region near 5.3 AU, in contrast with the gradual velocity increase during SIRs at 1 AU. Shocks are driven by 58% of ICMEs, with 94% of them being forward shocks. Some ICMEs seem to have multiple small flux ropes with different scales and properties. We quantitatively compare various properties of SIRs and ICMEs at 5.3 AU, and study their statistical distributions and variations with solar activity. The width, maximum dynamic pressure, and peak perpendicular pressure of SIRs all become larger than ICMEs. Dynamic pressure (P _dyn) is expected to be important for Jovian magnetospheric activity. We have examined the distributions of P _dyn of SIRs, ICMEs, and general solar wind, but these cannot explain the observed bimodal distribution of the location of the Jovian magnetopause. By comparing the properties of SIRs and ICMEs at 0.72, 1, and 5.3 AU, we find that the ICME expansion slows down significantly between 1 and 5.3 AU. Some transient and small streams in the inner heliosphere have merged into a single interaction region. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Electric fields within the martian magnetosphere and ion extraction: ASPERA-3 observations

Observations made by the ASPERA-3 experiment onboard the Mars Express spacecraft found within the martian magnetosphere beams of planetary ions. In the energy (E/q)-time spectrograms these beams are often displayed as dispersive-like, ascending or descending (whether the spacecraft moves away or approach the planet) structures. A linear dependence between energy gained by the beam ions and the altitude from the planet suggests their acceleration in the electric field. The values of the electric field evaluated from ion energization occur close to the typical values of the interplanetary motional electric field. This suggests an effective penetration of the solar wind electric field deep into the martian magnetosphere or generation of large fields within the magnetosphere. Two different classes of events are found. At the nominal solar wind conditions, a ‘penetration’ occurs near the terminator. At the extreme solar wind conditions, the boundary of the induced magnetosphere moves to a more dense upper atmosphere that leads to a strong scavenging of planetary ions from the dayside regions.     

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%.     

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.     

Stochastic models of hot planetary and satellite coronas: Atomic oxygen in Europa’s corona

This paper analyzes the formation, kinetics, and transport of hot oxygen atoms in the atmosphere of the Jovian satellite Europa. Atmospheric sources of suprathermal oxygen atoms are assumed to be represented by the processes of dissociation of molecular oxygen, which is the main component of the atmosphere, by solar UV radiation and electron fluxes from the inner magnetosphere of Jupiter, as well as by the reaction of dissociative recombination of the main ionospheric ion O ₂ ⁺ which thermal electrons. It is shown that dissociation in Europa’s near-surface atmosphere is balanced by the processes of the loss of atomic oxygen due to the effective escape of suprathermal oxygen atoms into the inner magnetosphere of Jupiter along the orbit of Europa and due to ionization by magnetospheric electrons and catalytic recombination of oxygen atoms on the icy surface of the satellite. It thus follows that atomic oxygen is only a small admixture to the main atmospheric component—molecular oxygen—in the near-surface part of the atmosphere. However, the outer exospheric layers of Europa’s atmosphere are populated mostly by suprathermal oxygen atoms. The near-surface molecular envelope of Europa is therefore surrounded by a tenuous extended corona of hot atomic oxygen.