Raymond J. Walker

Io's Interaction with the Plasma Torus: Galileo Magnetometer Report

Galileo magnetometer data at 0.22-second resolution reveal a complex interaction between Io and the flowing plasma of the Io torus. The highly structured magnetic field depression across the downstream wake, although consistent with a magnetized Io, is modified by sources of currents within the plasma that introduce ambiguity into the interpretation of the signature. Highly monochromatic ion cyclotron waves appear to be correlated with the local neutral particle density. The power peaks in the range of molecular ion gyrofrequencies, suggesting that molecules from Io can remain undissociated over a region of more than 15 Io radii around Io.

Anomalous aspects of magnetosheath flow and of the shape and oscillations of the magnetopause during an interval of strongly northward interplanetary magnetic field

On February 15, 1978, the orientation of the interplanetary magnetic field (IMF) remained steadily northward for more than 12 hours. The ISEE 1 and 2 spacecraft were located near apogee on the dawnside flank of the magnetotail. IMP 8 was almost symmetrically located in the magnetosheath on the dusk flank and IMP 7 was upstream in the solar wind. Using plasma and magnetic field data, we show that (1) the magnetosheath flow speed on the flanks of the magnetotail steadily exceeded the solar wind speed by 20%, (2) surface waves of ∼5-min period and very nonsinusoidal waveform were persistently present on the dawn magnetopause and waves of similar period were present in the dusk magnetosheath, and (3) the magnetotail ceased to flare at an antisunward distance of 15 RE. We propose that the acceleration of the magnetosheath flow is achieved by magnetic tension in the draped field configuration for northward IMF and that the reduction of tail flaring is consistent with a decreased amount of open magnetic flux and a larger standoff distance of the subsolar magnetopause. Results of a three-dimensional magnetohydrodynamic simulation support this phenomenological model.

Intermittent short‐duration magnetic field anomalies in the Io torus: Evidence for plasma interchange?

On Galileos inbound pass through the Io plasma torus in December 1995, we detected intermittent, short-duration increases in the magnetic field magnitude, notable by their abrupt onset and subsequent abrupt recovery to the previous level. The events were small and brief, with an average field change of 15.7 nT (1–2% of the background field) and an average duration of ∼26 s. They were observed first shortly after data at high time resolution were first acquired at a radial distance of 7.7 RJ from Jupiters center. The last and most clearly-defined signature, with exceptionally sharp boundaries, was observed at 6.03 RJ, about 12 minutes before closest approach to Io. The magnetic pressure change across the boundaries of these structures was ∼10 to ∼33 nPa, the latter value in a region with background plasma pressure of 60 nPa. Thus, although of small amplitude in the magnetic field, they represent large changes in the pressure. Abrupt changes in the measurements of the other fields and particles instruments correlate with some of the magnetic signatures. In this paper, we present the magnetometer data for these unusual signatures, discuss the limits on particle density changes that they imply, the significance of their spatial distribution, and their relationship to transport of plasma in the torus.

A global magnetohydrodynamic simulation of the response of the magnetosphere to a northward turning of the interplanetary magnetic field

We have used a global magnetohydrodynamic simulation model to investigate the time series of events which occur in the magnetosphere when the interplanetary magnetic field (IMF) changes from southward to northward. Within 15 min of the northward turning the magnetopause reconnection site moves from the subsolar point to the high-latitude tail. The high-latitude reconnection converts tail lobe field lines into closed dayside field lines and new field lines in the IMF. The removal of lobe flux plus continuing reconnection at the near-Earth neutral line cause the field lines which thread the plasma sheet to relax to a more dipolar state and a very slow tailward retreat of the near earth neutral line. The newly closed dayside field lines are convected around the flanks of the magnetosphere and into the tail. The closed flux convected from the dayside to the tail is the main source of closed flux into the tail. As a result, the plasma sheet recovers first along the flanks of the magnetosphere and then nearer midnight. The near-Earth neutral line shrinks toward midnight and then moves rapidly tailward.

Magnetic field signatures near Galileo's closest approach to Gaspra

Two large magnetic field rotations were recorded by the spacecraft Galileo 1 minute before and 2 minutes after its closest approach to the asteroid Gaspra. The timing and the geometry of the field changes suggest a connection with Gaspra, and the events can be interpreted as the result of the draping of the solar wind field around a magnetospheric obstacle. Gaspras surface field is inferred to be within an order of magnitude of Earths surface field, and its magnetic moment per unit mass is in the range observed for iron meteorites and highly magnetized chondrites. The location of the magnetic signatures suggests that perturbations are carried by waves in the magnetosonic-whistler mode with wavelengths between electron and ion gyro radii.

A global magnetohydrodynamic simulation of the magnetosphere when the interplanetary magnetic field is southward: The onset of magnetotail reconnection

We have used a new high-resolution global magnetohydrodynamic simulation model to investigate the onset of reconnection in the magnetotail during intervals with southward interplanetary magnetic field (IMF). After the southward IMF reaches the dayside magnetopause reconnection begins and magnetic flux is convected into the tail lobes. After about 35 min, reconnection begins within the plasma sheet near midnight at x = −14RE. Later the x line moves toward dawn and dusk. The reconnection occurs just tailward of the region where the tail attaches onto the dipole-dominated inner magnetosphere. The simulation shows that prior to the onset of reconnection, the Poynting flux is concentrated in this region. The time required for the start of reconnection depends on the component of the magnetic field normal to the equator (BZ). Reconnection occurs only after the BZ component has been reduced sufficiently for the tearing mode to grow. Later, when all the plasma sheet field lines have reconnected, a plasmoid moves down the tail.

The magnetic field and magnetosphere of Ganymede

Within Jupiters magnetosphere, Ganymedes magnetic field creates a mini-magnetosphere. We show that the magnetic field measured during Galileo‧s second pass by Ganymede, with closest approach at low altitude almost directly over the moons polar cap, can be understood to a large measure in terms of the structure of a vacuum superposition model of a uniform field and a Ganymede-centered dipole field. Departures from the simple model can be attributed principally to magnetopause currents. We show that the orientation of the observed magnetopause normal is qualitatively consistent with expectations from the vacuum superposition model. The magnetopause currents inferred from the inbound boundary crossing are closely related to expected values, and the magnetic structure of the boundary is similar to that observed at the magnetopause of Earth. We use the vacuum magnetic field model to infer the magnetic field near Ganymedes surface, and thereby predict the particle loss cones that should be present along the spacecraft trajectory. By mapping a fraction of the corotation electric field into the polar cap, we determine expected flow velocities near closest approach to Ganymede as a function of reconnection efficiency. We conclude by discussing prospects for measurements on Galileos remaining passes by Ganymede.

A Magnetic Signature at Io: Initial Report from the Galileo Magnetometer

During the inbound pass of the Galileo spacecraft, the magnetometer acquired 1 minute averaged measurements of the magnetic field along the trajectory as the spacecraft flew by Io. A field decrease, of nearly 40 percent of the background jovian field at closest approach to Io, was recorded. Plasma sources alone appear incapable of generating perturbations as large as those observed and an induced source for the observed moment implies an amount of free iron in the mantle much greater than expected. On the other hand, an intrinsic magnetic field of amplitude consistent with dynamo action at Io would explain the observations. It seems plausible that Io, like Earth and Mercury, is a magnetized solid planet.

MHD simulations of Io's interaction with the plasma torus

We compare results from magnetohydrodynamic computations of Ios interaction with the plasma torus with observations from the Galileo flyby. Both conducting and intrinsically magnetized models of In are considered. Both models can reproduce many observed features of the interaction, including a high plasma density, a low plasma temperature, and a significant depression in magnetic field magnitude in Ios wake. The high plasma density in the wake is produced by ion pickup and concomitant slowing of the flow and is not of ionospheric origin. The models also show important quantitative differences with the data. In particular, neither model reproduces large-scale structure on the flanks of the geometric wake. Neither model can be ruled out at the present time, but intrinsically magnetized models most easily match the magnitude of the maximum observed magnetic field depression in the wake. Comparison of results with future Galileo flybys past Io (planned for 1999) may help to clarify whether Io is magnetized.

Mirror mode structures in the Jovian magnetosheath

[1] Mirror mode waves are commonly observed in planetary magnetosheaths. Their magnetic signatures are often periodic but occasionally appear as intermittent increases of field magnitude (peaks) or as intermittent decreases (dips). We define quantitative mirror structure identification criteria and statistically analyze the distributions of the various forms. A survey of all the relevant magnetometer data in the Jovian magnetosheath reveals that mirror mode structures are present 61.5% of the time. Two-thirds of the events include waves that are either quasi-periodic or aperiodic, while 19% contain dips and 14% contain peaks. The amplitude and period of quasi-periodic and periodic structures appear to increase as the residence time of the flowing plasma within the sheath increases. Peaks are primarily observed on the dayside in the high β plasmas of the middle magnetosheath. Dips are observed mostly in low β plasma near the magnetopause and on the flanks. A phenomenological model for the evolution of mirror structures that accounts for these observations has been developed. We propose that the mirror structures form near the bow shock and undergo an initial growth phase during which their amplitude increases linearly. Structures that dwell in anisotropic, high β plasma may saturate nonlinearly as described by Kivelson and Southwood [1996]. We interpret field magnitude peaks as the signatures of such nonlinear saturation. Finally, we ascribe the dip signatures to the process of stochastic decay of mirror structures as flow away from the subsolar point carries the structures into lower β plasma.