M. G. Kivelson

Bursty bulk flows in the inner central plasma sheet

High speed flows in the Earths Inner Central Plasma Sheet (ICPS) occur during enhanced flow intervals that have been termed Bursty Bulk Flow (BBF) events. The importance of different flow magnitude samples for Earthward transport in the ICPS are statistically evaluated and several representative BBFs and their relevance to Earthward transport are discussed. The selection of BBFs is automated in a database and they are shown to be responsible for most of the Earthward transport that occurs within the ICPS. The BBF related transport is compared to the transport measured within the entire plasma sheet during the 1985 AMPTE/IRM crossings of the magnetotail. The results show that BBFs last only a small fraction of the time in the plasma sheet but can account for several tens of percent of the Earthward particle and energy transfer and possibly all of the Earthward magnetic flux transfer in the plasma sheet.

Introduction to Space Physics

Introduction 1. Brief history of solar terrestrial physics 2. Physics of space plasmas 3. The Sun 4. The solar wind 5. Collisionless shocks 6. Interactions with magnetized planets 7. Ionospheres 8. Interactions with unmagnetized bodies 9. Magnetopause, tail and reconnection 10. Magnetospheric configuration 11. Magnetic pulsations 12. Plasma waves 13. Magnetospheric dynamics 14. The aurora and the auroral ionosphere 15. Magnetospheres of outer planets Appendices Index.

The Cluster Magnetic Field Investigation

The Cluster mission provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment. Four-point measurements of the magnetic field will enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere. Difference measurements of the magnetic field data will be combined to derive a range of parameters, such as the current density vector, wave vectors, and discontinuity normals and curvatures, using classical time series analysis techniques iteratively with physical models and simulation of the phenomena encountered along the Cluster orbit. The control and understanding of error sources which affect the four-point measurements are integral parts of the analysis techniques to be used. The flight instrumentation consists of two, tri-axial fluxgate magnetometers and an on-board data-processing unit on each spacecraft, built using a highly fault-tolerant architecture. High vector sample rates (up to 67 vectors s-1) at high resolution (up to 8 pT) are combined with on-board event detection software and a burst memory to capture the signature of a range of dynamic phenomena. Data-processing plans are designed to ensure rapid dissemination of magnetic-field data to underpin the collaborative analysis of magnetospheric phenomena encountered by Cluster.

Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto

The Galileo spacecraft has been orbiting Jupiter since 7 December 1995, and encounters one of the four galilean satellites—Io, Europa, Ganymede and Callisto—on each orbit. Initial results from the spacecrafts magnetometer, have indicated that neither Europa nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede and possibly Io. Here we report perturbations of the external magnetic fields (associated with Jupiters inner magnetosphere) in the vicinity of both Europa and Callisto. We interpret these perturbations as arising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment. Electromagnetic induction requires eddy currents to flow within the moons, and our calculations show that the most probable explanation is that there are layers of significant electrical conductivity just beneath the surfaces of both moons. We argue that these conducting layers may best be explained by the presence of salty liquid-water oceans, for which there is already indirect geological evidence, in the case of Europa.

Mirror instability: 1. Physical mechanism of linear instability

The mirror instability is prevalent in planetary and cometary magnetosheaths and other high beta environments. We review the physics of the linear instability. Although the instability was originally derived from magnetohydrodynamic fluid theory, later work showed that there were significant differences between the fluid theory and a more rigorous kinetic approach. Here we point out that the instability mechanism hinges on the special behavior of particles with small velocity along the field. We call such particles resonant particles by analogy with other uses of the term, but there are significant differences between the behavior of the resonant particles in this instability and in other instabilities driven by resonant particles. We comment on the implications of these results for our understanding of the observations of mirror instability-generated signals in space.

Flow bursts, braking, and Pi2 pulsations

We examine six near-Earth dipolarization events during which rapid flows were observed by Geotail at distances between 8 and 15 RE in the magnetotail. Each flow event was associated with local dipolarization, auroral arc brightening, and Pi2 pulsations (periods of 40–150 s). Variations in earthward flow velocity delayed by 60–90 s match the Pi2 waveforms on the ground at low latitudes on the flank. We conclude that low-latitude Pi2 pulsations are directly driven by compressional pulses associated with braking of oscillatory earthward flows. In addition, we identify a new type of nightside Pi2, which is related to the oscillatory braking current that modulates the current in the substorm current wedge. For one event we estimate the magnitude of the braking current from the amplitude of ground perturbations and find I∼2–3×104 A. An independent calculation of the inertial current using the flow measurements yields 6×104 A. We separate Pi2 into three distinct physical types: low-latitude directly driven, nightside transient response, and nightside inertial current. We propose a phenomenological model linking flow bursts, the substorm current wedge, and the three types of Pi2 pulsations.

Generation of Pi2 pulsations by bursty bulk flows

We present the first observations directly relating bursty bulk flows (BBFs) to Pi2 pulsations. At ISEE 2, a large earthward flow was observed. The Institute of Geological Sciences (IGS) magnetometer chain located at the same local time as ISEE 2 recorded Pi2 pulsations, slightly delayed relative to flow onset, consisting of a long-period (7–8 mHz) component associated with oscillations of the substorm current wedge. We demonstrate that the ionospheric current began to increase 90 s before the arrival of these Pi2 pulsations. We argue that this precursor was generated by earthward convection of plasma sheet plasma. Phase skips at midlatitudes on the nightside are associated with new flow bursts at ISEE 2. The Air Force Geophysics Laboratory (AFGL) magnetometer chain, which was located on the dusk flank, measured Pi2 pulsations that started ∼90 s after the flow bursts. We demonstrate that there is a one-to-one correlation between impulsive flow onset in the tail and Pi2 pulsations. We also show that oscillations at AFGL are directly related to temporal variations of the flow. Thus we suggest that the characteristic frequencies of low-latitude Pi2 pulsations are established by the temporal structure of processes in the near-Earth magnetotail.

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.

Europa and Callisto: Induced or intrinsic fields in a periodically varying plasma environment

Magnetometer data from four Galileo passes by the Jovian moon Europa and three passes by Callisto are used to interpret the properties of the plasma surrounding these moons and to identify internal sources of magnetic perturbations. Near Europa the measurements are consistent with a plasma rich in pickup ions whose source is freshly ionized neutrals sputtered off of the moons surface or atmosphere. The plasma effects vary with Europas height above the center of Jupiters extended plasma disk. Europa is comet-like when near the center of the current sheet. It is therefore likely that the strength of the currents coupling Europa to Jupiters ionosphere and the brightness of a Europa footprint will depend on System III longitude. Magnetic perturbations on the scale of Europas radius can arise from a permanent dipole moment or from an induced dipole moment driven by the time-varying part of Jupiters magnetospheric field at Europas orbit. Both models provide satisfactory fits. An induced dipole moment is favored because it requires no adjustable parameters. The inductive response of a conductive sphere also fits perturbations on two passes near Callisto. The implied dipole moment flips direction as is predicted for greatly differing orientations of Jupiters magnetospheric field near Callisto in the two cases. For both moons the current carrying shells implied by induction must be located near the surface. An ionosphere cannot provide the current path, as its conductivity is too small, but a near surface ocean of ∼10 km or more in thickness would explain the observations.