W. R. Paterson

Plasma Observations at Io with the Galileo Spacecraft

Plasma measurements made during the flyby of Io on 7 December 1995 with the Galileo spacecraft plasma analyzers reveal that the spacecraft unexpectedly passed directly through the ionosphere of Io. The ionosphere is identified by a dense plasma that is at rest with respect to Io. This plasma is cool relative to those encountered outside the ionosphere. The composition of the ionospheric plasmas includes O++, O+ and S++, S+, and SO2+ ions. The plasma conditions at Io appear to account for the decrease in the magnetic field, without the need to assume that Io has a magnetized interior.

Observations of thermal plasmas in Jupiter's magnetotail

[i] A survey of thermal plasmas and magnetic fields is presented for the orbit of the Galileo spacecraft around Jupiter that occurred during the period May 4 through June 22, 1997. This orbit traversed the magnetotail out to Jovian radial distances of 100.2 R J in the magnetotail. Perijove was positioned at 9.3 Rj. Three primary ion populations were detected with the plasma analyzer: cool hydrogen ions with temperatures of 10 eV, hot hydrogen ions with temperatures of ∼10 keV, and a third population of heavy ions such as O ++ , O + , S ++ , and S +++ with temperatures in the range of 500 eV. Plasma flows near perijove were in the corotational direction but with speeds ∼60% of those for rigid corotation with the planet out to radial distances of ∼18 R J . In the radial range of 18-26 R J there were significant radial components for the bulk flows, and the flow components in the corotational direction reached values expected for rigid corotation when the current sheet was crossed. The transient character of the plasma parameters suggests that strong ion plasma acceleration is occurring in this region. The temperatures of the heavy ions increased from 5 x 10 6 K at 9.3 Rj to ∼10 8 K at 26 R J . At distances 50 Rj it varied as R -1.19 . The thermal plasma pressure in the current sheet is a factor of ∼10 less than the magnetic pressure at 9.3 R J at positions above or below the sheet but becomes equal to this magnetic pressure at radial distances >30 R J . The corresponding values of the ratio of the plasma to magnetic pressure, β, are in the range of 10-100 in the current sheet. The number densities and temperatures of these plasmas are 0.05-0.1 /cm 3 and 0.5-1 x 10 8 K, respectively. In the magnetotail the bulk flows of the thermal plasmas exhibit substantial components in the corotational and radially outward directions, but the bulk speeds of 50-200 km s -1 are significantly less than those for rigid corotation. For this single orbit the average bulk flows were ∼50 km s -1 in the premidnight sector and 200 km s -1 in the early morning sector at radial distances >∼50 R J . At apojove of 100 R J an antisunward flow of ∼200 km s -1 is found that is supportive of the magnetospheric wind reported for Voyager measurements of energetic charged particles. The 10-hour periodicity of the pairs of current sheet crossings at the position of the Galileo spacecraft includes a variety of dynamical signatures, which are suggested to be due to the changes in direction and pressures in the solar wind and due to the transient acceleration of plasmas in the current sheet.

Observations of nonadiabatic acceleration of ions in Earth's magnetotail

We present observations of the three-dimensional velocity distributions of protons in the energy range 20 eV to 52 keV at locations within and near the current sheet of Earths magnetotail at geocentric radial distances 35 to 87 RE. These measurements were acquired on December 8, 1990, with a set of electrostatic analyzers on board the Galileo spacecraft during its approach to Earth in order to obtain one of its gravitational assists to Jupiter. It is found that the velocity distributions are inadequately described as quasi-Maxwellian distributions such as those found in the central plasma sheet at positions nearer to Earth. Instead the proton velocity distributions can be categorized into two major types. The first type is the “lima bean” shaped distribution with high-speed bulk flows and high temperatures that are similar to those found nearer to Earth in the plasma sheet boundary layer. The second type consists of colder protons with considerably lesser bulk flow speeds. Examples of velocity distributions are given for the plasma mantle, a region near the magnetic neutral line, positions earthward and tailward of the neutral line, and the plasma sheet boundary layer. At positions near the neutral line, only complex velocity distributions consisting of the colder protons are found, whereas both of the above types of distributions are found in and near the current sheet at earthward and tailward locations. Bulk flows are directed generally earthward and tailward at positions earthward and tailward of the neutral line, respectively. Only the high-speed, hot distribution is present in the plasma sheet boundary layer. The observations are interpreted in terms of the nonadiabatic acceleration of protons that flow into the current sheet from the plasma mantle. For this interpretation the hot, “lima bean” shaped distributions are associated with meandering, or Speiser, orbits in the current sheet. It is suggested that the colder, lower-speed proton velocity distributions are the result of fractional or few gyromotions before ejection out of the current sheet, but this speculation must be further investigated with appropriate kinetic simulation of trajectories.

Relation between electrostatic solitary waves and hot plasma flow in the plasma sheet boundary layer: GEOTAIL observations

The authors present studies which show correlations between plasma ion flow properties in the plasma sheet boundary layer, and the spectral properties of the broadband radiation observed there by GEOTAIL, referred to as electrostatic solitary waves. The width and spacing between the pulses is observed to change on time scales of milliseconds.

Outflow of hydrogen ions from Ganymede

On 6 September 1996 plasma measurements were obtained in the vicinity of Ganymede as the Galileo spacecraft passed by this moon with a closest approach distance of 261 km. Near Ganymede a dense, cold plasma region was found to be embedded in Jupiters hot plasma sheet. The cold plasmas are hydrogen ions flowing outwards from Ganymede at supersonic speeds. Temperatures and maximum number densities of these ions were about 4 × 104 K and 100 /cm³, respectively. Over Ganymedes polar caps there is strong plasma convection with speeds in the range of 50 km/s, i.e., the flow is not primarily directed parallel to the local magnetic fields. The corresponding potentials across a Ganymede diameter are in excess of 100 kV. The primary source of the hydrogen ions is believed to be the water ices on the surface of Ganymede. However, the anticipated oxygen ion outflow is not present, which implies that the oxygen is left on the moons surface. The loss of hydrogen from Ganymedes surface is about 3 × 109 grams/year.

Survey of plasma parameters in Earth`s distant magnetotail with the Geotail spacecraft

The three-dimensional velocity distributions for both electrons and positive ions as determined with plasma instrumentation on board the Geotail spacecraft are used to compute number densities, ion bulk velocities, and electron and ion temperatures. The authors present a statistical survey of these plasma parameters for the magnetotail at distances from Earth in the range of 10 to 210 R{sub E} for the period 19 October 1992 through 10 July 1993. The ion temperature is used to discriminate {open_quotes}hot{close_quotes} plasmas from {open_quotes}cold{close_quotes} plasmas. The hot ion plasmas are frequently found to stream either towards or away from Earth and the authors identify these with the plasma sheet, plasma sheet boundary layer, and regions of acceleration. The cold ion plasmas are typically streaming away from Earth. These cold plasmas appear to be associated with the plasma mantle, the magnetosheath, and boundary layers interior to the magnetosheath, although the contribution from the ionosphere has not yet been evaluated. 11 refs., 3 figs., 2 tabs.

Cross-tail magnetic flux ropes as observed by the GEOTAIL spacecraft

Ten transient magnetic structures in Earths magnetotail, as observed in GEOTAIL measurements, selected for early 1993 [at (−) XGSM = 90 - 130 RE], are shown to have helical magnetic field configurations similar to those of interplanetary magnetic clouds at 1 AU but smaller in size by a factor of ≈ 700. Such structures are shown to be well approximated by a comprehensive magnetic force-free flux-rope model. For this limited set of 10 events the rope axes are seen to be typically aligned with the YGSM axis and the average diameter of these structures is ≈ 15 RE.

Ion sources and acceleration mechanisms inferred from local distribution functions

This study investigates the sources of the ions making up the complex and nonisotropic H + velocity distribution functions observed by the Geotail spacecraft on May 23, 1995, in the near-Earth magnetotail region and recently reported by Frank et al. [1996]. A distribution function observed by Geotail at ∼10 R E downtail is used as input for the large scale kinetic (LSK) technique to follow the trajectories of approximately 90,000 H + ions backward in time. Time-dependent magnetic and electric fields are taken from a global magnetohydrodynamic (MHD) simulation of the magnetosphere and its interactions with appropriate solar wind and IMF conditions. The ion population described by the Geotail distribution function was found to consist of a mixture of particles originating from three distinct sources: the ionosphere, the low latitude boundary layer (LLBL), and the high latitude plasma mantle. Ionospheric particles had direct access along field lines to Geotail, and LLBL ions convected adiabatically to the Geotail location. Plasma mantle ions, on the other hand, exhibited two distinct types of behavior. Most near-Earth mantle ions reached Geotail on adiabatic orbits, while distant mantle ions interacted with the current sheet tailward of Geotail and had mostly nonadiabatic orbits. Ions from the ionosphere, the LLBL, and the near-Earth mantle were directly responsible for the well-separated, low energy structures easily discernible in the observed and modeled distribution functions. Distant mantle ions formed the higher energy portion of the Geotail distribution. Thus, we have been successful in extracting useful information about particle sources, their relative contribution to the measured distribution and the acceleration processes that affected particle transport during this time.

Galileo observations of electron beams and thermal ions in Jupiter's magnetosphere and their relationship to the auroras

The thermal ion parameters within the energy range 7 V to 53 kV and the electron velocity distributions for the energy range 1 kV to 53 kV are presented for three Galileo spacecraft orbits around Jupiter. The orbital segments extended from perijove at radial distances about 9 R J out to positions in the plasma sheet at 50 Rj. Inbound segments and outbound segments were positioned in the local morning and premidnight sectors, respectively, in the Jovian equatorial magnetosphere. Because the magnetic equatorial plane is tilted with respect to the spacecraft orbital plane, the plasma sheet is traversed about every 5 hours at System III longitudes about 110° and 290° alternatively. At radial distances extending to about 30 Rj, there is a strong dependence of ion temperatures on System III longitude. The ion temperatures are higher at 290° relative to those at 110°. At perijove of 9 Rj the ion temperatures were about 5 x 106 K and similar to those in the hot lo torus at 7 Rj. Ion temperatures increased over the radial distance range of 10 to 25 R J to values of about 5 x 10 7 to 10 8 K. At 9 to 20 R J the ion bulk flows were about 60% of rigid corotational values and were well ordered and directed in the corotational direction. At greater distances in the plasma sheet the bulk motions became highly variable with large reductions in the corotational direction but with large radial outflows. This behavior exhibits a significant dependence on the local-time position of the spacecraft with greater radial flows in the premidnight sector. Intense electron beams which were aligned parallel and antiparallel to the magnetic field were detected at radial distances of about 20 to 30 R J . For Jupiters plasma sheet at these radial distances it appears likely that a region of intense plasma heating at low altitudes is responsible for the electron beams. These electron beams are identified with the main auroral ring. The electron energies in the beams span the range of several keV to tens of keV. Their projection into the auroral ionosphere provides for arc widths in the range of 1000 km or less. The energy fluxes at the Jovian auroral ionosphere, after correction for the angular width of the field of view of the electrostatic analyzer, extend up to 100 ergs/cm2-s and are sufficient to excite maximum far-ultraviolet emissions in the megarayleigh range as remotely observed with cameras.

Pressure, volume, density relationships in the plasma sheet

The entropy parameter Pn � 5/3 was found to be relatively uniform throughout the region studied. The energy parameter TV 2/3 decreased by 40% for ions and 10% for electrons near midnight between � 29.5 and � 11.5 RE. These energy parameter changes suggest that the most energetic ions and electrons are either being deenergized or preferentially lost, processes that may be associated with gradient and curvature drifts through the sides of the convecting flux tubes or by wave instabilities and a parallel heat flux. INDEX TERMS: 2764 Magnetospheric Physics: Plasma sheet; 2744 Magnetospheric Physics: Magnetotail; 2760 Magnetospheric Physics: Plasma convection; 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; KEYWORDS: plasma sheet, magnetotail, pressure balance inconsistency, 3-D models