K. L. Ackerson

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.

Quadrispherical LEPEDEAS for ISEE's-1 and -2 Plasma Measurements

A new generation of spaceflight instrumentation for comprehensive measurements of plasmas within the earths magnetosphere and its environs is described. These quadrispherical low energy proton and electron differential energy analyzers (LEPEDEAS) for the ISEEs-l and -2 spacecraft are capable of determining the directional intensities of positive ions and electrons over all but 2 percent of the 4¿ sr solid angle for charged-particle velocity vectors at the spacecraft positions. The energy range of this instrumentation is 1 eV < E/Q < 45 keV with good energy and angular resolutions. An example of inflight observations within the earths magnetosheath is presented.

The plasma instrumentation for the Galileo Mission

The plasma instrumentation (PLS) for the Galileo Mission comprises a nested set of four spherical-plate electrostatic analyzers and three miniature, magnetic mass spectrometers. The three-dimensional velocity distributions of positive ions and electrons, separately, are determined for the energyper-unit charge (E/Q) range of 0.9 V to 52 kV. A large fraction of the 4 π -steradian solid angle for charged particle velocity vectors is sampled by means of the fan-shaped field-of-view of 160°, multiple sensors, and the rotation of the spacecraft spinning section. The fields-of-view of the three mass spectrometers are respectively directed perpendicular and nearly parallel and anti-parallel to the spin axis of the spacecraft. These mass spectrometers are used to identify the composition of the positive ion plasmas, e.g., H+, O+, Na+, and S+, in the Jovian magnetosphere. The energy range of these three mass spectrometers is dependent upon the species. The maximum temporal resolutions of the instrument for determining the energy (E/Q) spectra of charged particles and mass (M/Q) composition of positive ion plasmas are 0.5 s. Three-dimensional velocity distributions of electrons and positive ions require a minimum sampling time of 20s, which is slightly longer than the spacecraft rotation period. The two instrument microprocessors provide the capability of inflight implementation of operational modes by ground-command that are tailored for specific plasma regimes, e.g., magnetosheath, plasma sheet, cold and hot tori, and satellite wakes, and that can be improved upon as acquired knowledge increases during the tour of the Jovian magnetosphere. Because the instrument is specifically designed for measurements in the environs of Jupiter with the advantages of previous surveys with the Voyager spacecraft, first determinations of many plasma phenomena can be expected. These observational objectives include field-aligned currents, three-dimensional ion bulk flows, pickup ions from the Galilean satellites, the spatial distribution of plasmas throughout most of the magnetosphere and including the magnetotail, and ion and electron flows to and from the Jovian ionosphere.

Magnetopause expansions for quasi‐radial interplanetary magnetic field: THEMIS and Geotail observations

We report THEMIS and Geotail observations of prolonged magnetopause (MP) expansions during long-lasting intervals of quasi-radial interplanetary magnetic field (IMF) and nearly constant solar wind dynamic pressure. The expansions were global: the magnetopause was located more than 3 RE and ~7 RE outside its nominal dayside and magnetotail locations, respectively. The expanded states persisted several hours, just as long as the quasi-radial IMF conditions, indicating steady-state situations. For an observed solar wind pressure of ~1.1-1.3 nPa, the new equilibrium subsolar MP position lay at ~14.5 RE, far beyond its expected location. The equilibrium position was affected by geomagnetic activity. The magnetopause expansions result from significant decreases in the total pressure of the high-beta magnetosheath, which we term the low-pressure magnetosheath (LPM) mode. A prominent LPM mode was observed for upstream conditions characterized by IMF cone angles less than 20 ~ 25 grad, high Mach numbers and proton plasma beta<1.3. The minimum value for the total pressure observed by THEMIS in the magnetosheath adjacent to the magnetopause was 0.16 nPa and the fraction of the solar wind pressure applied to the magnetopause was therefore 0.2, extremely small. The equilibrium location of the magnetopause was modulated by a nearly continuous wavy motion over a wide range of time and space scales.The pressure balance at the magnetopause is formed by magnetic field and plasma in the magnetosheath, on one side, and inside the magnetosphere, on the other side. In the approach of dipole earths magnetic field configuration and gas-dynamics solar wind flowing around the magnetosphere, the pressure balance predicts that the magnetopause distance R depends on solar wind dynamic pressure Pd as a power low R ~ Pd^alpha, where the exponent alpha=-1/6. In the real magnetosphere the magnetic filed is contributed by additional sources: Chapman-Ferraro current system, field-aligned currents, tail current, and storm-time ring current. Net contribution of those sources depends on particular magnetospheric region and varies with solar wind conditions and geomagnetic activity. As a result, the parameters of pressure balance, including power index alpha, depend on both the local position at the magnetopause and geomagnetic activity. In addition, the pressure balance can be affected by a non-linear transfer of the solar wind energy to the magnetosheath, especially for quasi-radial regime of the subsolar bow shock formation proper for the interplanetary magnetic field vector aligned with the solar wind plasma flow.

Observations of plasmas and magnetic fields in Earth's distant magnetotail: Comparison with a global MHD model

We are reporting the first direct comparison of in situ observations of plasmas and magnetic fields in Earths distant magnetotail with the results of a time-dependent, global magnetohydrodynamic (MHD) simulation of the interaction of the solar wind with the magnetosphere. The magnetotail observations were taken with the Geotail spacecraft during the period 0300–0630 UT on October 27, 1992 at a position near the dawnside magnetopause at a downstream distance of about 81 RE. During this period a dense, cold ion stream similar in density and speed to that expected for the magnetosheath plasmas was intermittently observed. When the cold ion stream was not present, the spacecraft was located in the northern magnetotail lobe. The dense, cold ion stream differed from that expected for the magnetosheath in the Y and Z components of ion bulk flow and in the Y component of the magnetic field. These cold ion streams are associated with a magnetopause accommodation region positioned just outside the classical magnetopause, as identified by a well-defined transition from magnetic fields typical of those found in the lobe to the lesser and more fluctuating fields in the magnetosheath. This accommodation region exhibits perturbations in plasma flows and magnetic fields that appear to be related to the complex topology of the magnetopause at these large downstream positions. Simultaneous observations of the solar wind ions and the interplanetary magnetic field (IMF) with the IMP 8 spacecraft upstream from Earth provided the driving input for a global MHD model. The solar wind ion flow was steady during this period, and the IMF exhibited a series of rotations from northward to duskward. The dynamics of the magnetotail were controlled by the Y and Z components of the IMF. When this By was strongly positive, the magnetotail lobe appeared at the downstream Geotail position. Examination of the modeled plasma parameters in the Y-Z plane through the spacecraft position shows that this By provides a torque on the magnetotail about its central axis. The MHD model also accurately positions the spacecraft alternately in the magnetopause accommodation region and the magnetotail lobe as the IMF clock angle varied from northward to duskward, respectively. The temporal variations of modeled parameters, i.e., ion densities, temperatures, and bulk flow velocities and the magnetic field components, are directly compared with the Geotail measurements. This first comparison of the Geotail observations with the modeled plasma parameters and magnetic fields provides substantial encouragement that a global MHD model can provide a valid description of important aspects of the large-scale topology and dynamics of the magnetotail.

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.

Galileo plasma observations at Europa: Ion energy spectra and moments

Observations from the Galileo plasma analyzer (PLS) recorded during two near encounters with Europa are reported. The measured ion energy spectra show that the ions near Europa are a mix of thermalized torus plasmas with approximately Maxwellian ion velocity distributions and partially thermalized pickup ions with ring distributions. The measurements are used to determine plasma moments including ion number densities, bulk flow velocities, and ion temperatures. These parameters provide information concerning the interaction that occurs as corotating torus plasmas sweep past this moon. The first encounter on December 19, 1996, took the spacecraft through the wake of the moon with the altitude at closest approach approximately 700 km. The trajectory for the second encounter on February 20, 1997, was on the upstream side of the moon with closest approach at an altitude approximately 600 km. Features of the interaction are found to include (1) deflection of plasma flow away from the moon on the upstream side and into the wake on the downstream side, (2) evidence of boundaries in the near wake that indicate a structured wake, and (3) maximum heavy-ion densities near closest approach ∼40/cm3 that place a limit on the density of a high-altitude ionosphere at Europa.

Three-Dimensional Plasma Measurements within the Earth’s Magnetosphere

First magnetospheric measurements of the three-dimensional velocity distributions for positive ions and electrons within the energy range 1 eV ≤E/Q≤45 keV are reported. These velocity distributions are gained with quadrispherical Lepedeas on board the spacecraft ISEE-1 and -2. Three-dimensional bulk flows of protons in the vicinity of the magnetopause and within the dayside magnetosphere and dawn sector of the magnetotail are presented. Proton drift velocities within the magnetosphere and magnetotail can be directly determined and employed to calculate the corresponding quasi-static perpendicular electric fields and to provide quantitative analyses of kinematical models for plasma motions. Nonmonotonic features in the electron velocity distributions are found simultaneously with the presence of electron cyclotron harmonic electrostatic waves in the dayside magnetosphere. The relationship of the observed electron velocity distributions to expectations for resonant pitch-angle and energy diffusion is discussed, as well as the possibility of the existence of proton cyclotron harmonic instabilities. Examples of the signature of field-aligned acceleration of protons into the magnetosphere and the presence of low-energy ionospheric ions in the near-earth magnetotail are also presented. Perpendicular electrostatic fields can be calculated from the observed three- dimensional velocity distributions and are found to have typical magnitudes of ~ 1 mVm-1.

Properties of the mantle‐like magnetotail boundary layer: GEOTAIL data compared with a mantle model

Near the tail boundary beyond about 100 Re, GEOTAIL often encounters a plasma mantle-like boundary layer in which the plasma flowing tailward transitions smoothly from magnetosheath values of speed and density to much smaller values, more characteristic of the tail lobe. This boundary layer had earlier been recognized on the basis of ISEE 3 measurements. GEOTAIL confirms the existence of this layer and extends the documentation on its behavior. Boundary oscillations sweep the boundary layer over the spacecraft enabling GEOTAIL to “sound” the layers profile of plasma parameters. The density-versus-speed correlogram of the mantle-like part—a useful diagnostic for comparisons—is reasonably well simulated by a 1-D, MHD slow-mode expansion fan model of the plasma mantle. Flow directions are consistent with an open plasma mantle on the northern, duskside flank of the tail, as expected for the standard magnetic merging model of mantle formation.

SEVERAL RECENT FINDINGS CONCERNING THE DYNAMICS OF THE EARTH'S MAGNETOTAIL*

Several recent results concerning the nature of the Earths magnetotail are briefly reviewed. These observational findings include: (1) the three-dimensional character of the plasma sheet via a comprehensive survey of proton bulk flows, (2) a region of earthward flowing plasmas at the interfaces of the plasma sheet and magnetotail lobes during magnetic substorm recovery, and (3) the signature of electrostatic acceleration for protons within the jetting plasmas from magnetotail fireballs.