F. V. Coroniti

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.

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.

Three‐dimensional stability of thin quasi‐neutral current sheets

In a thin current sheet (ρi0/L ≲ 1, where ρi0 is the ion gyroradius in the lobe field and L is the current sheet half thickness) of the generalized Harris type, the relative ion-electron cross-field drift is comparable to the ion thermal velocity. The three-dimensional stability properties of such a thin current sheet are investigated by means of nonlocal two-fluid theory and two-dimensional and three-dimensional full particle simulations. As was suggested originally by Zhu et al. [1992], the drift kink mode is found to be of critical importance. For the simple case of no initial Bz field, the fluid theory demonstrates that the drift kink mode is a non-MHD mode with a polarization structure such that E1y is an antisymmetric function of z while E1z is a symmetric function with E1z(0) ≠ 0. Two-dimensional (y,z) particle simulations indicate that the nonlinear behavior of this mode is dominated by long-wavelength modes with kyL ∼ 1 and frequency ωr ∼ Ωi0, where Ωi0 is the ion gyrofrequency in the lobe field. Three-dimensional particle simulations performed on a massively parallel computer show that while the growth rates for the drift kink mode are reduced by the finite Bz, they can still be appreciable (γ/Ωi0 ≲ 0.05–0.10). The kyL ∼ 1 drift kink modes are always the first to grow in the simulations; subsequently, tearing-like modes with a dominant kx wave vector also become unstable. Implications of these results for the triggering of substorms are discussed.

Collisionless reconnection in two‐dimensional magnetotail equilibria

A two-dimensional (x, z) particle simulation model based on the Darwin approximation to Maxwells equations is developed for studying collisionless reconnection in the magnetotail. The particles and fields are initialized in accord with a general equilibrium configuration which includes a pressure gradient along the tail axis and tail flaring. The model is used to investigate a number of theoretical issues regarding the spontaneous ion tearing instability under the assumption that the electron dynamics are unimportant. It is demonstrated both numerically and analytically that in a thin current sheet with ρ i0 ∼ λ (ρ i0 is the ion Larmor radius based on the lobe field and λ is the characteristic thickness of the current sheet) the growth rates in the absence of a normal field component are much smaller than expected based upon the analytic theory for a thick sheet (ρi0 ≪ λ). For such a thin current sheet the presence of a normal field Bz on axis of even a few percent strongly inhibits the growth of the instability. This result is not altered by the addition of a constant By component smaller than the lobe field. It is demonstrated further that the transition to stability occurs when the cyclotron frequency based on Bz equals the growth rate of the Bz = 0 tearing mode. This requires typically a normal field of the order of 6% of the lobe field. If a sufficiently large external perturbation of the lobe magnetic field reduces the normal field on axis below the stabilization threshold over a significant fraction of a tearing mode wavelength, then one can recover the rapid instability of the one-dimensional neutral sheet.

Plasma wave observations at comet Giacobini-Zinner

The plasma wave instrument on the International Cometary Explorer (ICE) detected bursts of strong ion acoustic waves almost continuously when the spacecraft was within 2 million kilometers of the nucleus of comet Giacobini-Zinner. Electromagnetic whistlers and low-level electron plasma oscillations were also observed in this vast region that appears to be associated with heavy ion pickup. As ICE came closer to the anticipated location of the bow shock, the electromagnetic and electrostatic wave levels increased significantly, but even in the midst of this turbulence the wave instrument detected structures with familiar bow shock characteristics that were well correlated with observations of localized electron heating phenomena. Just beyond the visible coma, broadband waves with amplitudes as high as any ever detected by the ICE plasma wave instrument were recorded. These waves may account for the significant electron heating observed in this region by the ICE plasma probe, and these observations of strong wave-particle interactions may provide answers to longstanding questions concerning ionization processes in the vicinity of the coma. Near closest approach, the plasma wave instrument detected broadband electrostatic noise and a changing pattern of weak electron plasma oscillations that yielded a density profile for the outer layers of the cold plasma tail. Near the tail axis the plasma wave instrument also detected a nonuniform flux of dust impacts, and a preliminary profile of the Giacobini-Zinner dust distribution for micrometer-sized particles is presented.

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.

First plasma wave observations at Neptune

The Voyager 2 plasma wave instrument detected many familiar plasma waves during the encounter with Neptune, including electron plasma oscillations in the solar wind upstream of the bow shock, electrostatic turbulence at the bow shock, and chorus, hiss, electron cyclotron waves, and upper hybrid resonance waves in the inner magnetosphere. Low-frequency radio emissions, believed to be generated by mode conversion from the upper hybrid resonance emissions, were also observed propagating outward in a disklike beam along the magnetic equatorial plane. At the two ring plane crossings many small micrometer-sized dust particles were detected striking the spacecraft. The maximum impact rates were about 280 impacts per second at the inbound ring plane crossing, and about 110 impacts per second at the outbound ring plane crossing. Most of the particles are concentrated in a dense disk, about 1000 kilometers thick, centered on the equatorial plane. However, a broader, more tenuous distribution also extends many tens of thousands of kilometers from the equatorial plane, including over the northern polar region.

Convection and the formation of thin current sheets in the near‐Earth plasma sheet

Particle simulations are used to investigate plasma sheet convection in a realistic near-Earth magnetic field model including dipolelike and taillike regions. Convection leads to the formation of a thin current region characterized by a strong electrostatic potential and a depressed equatorial magnetic field component in which the cross tail current is carried predominantly by the electrons as a result of their E x B and diamagnetic drifts. Implications for the thin current sheets observed in the near-Earth magnetotail during substorm growth phase are discussed. 18 refs., 4 figs.

Ergosphere Driven Winds

This is by far the shortest chapter in the book, yet it is the most important as it synthesizes the fundamental physics of all ergospheric dynamos that create and sustain a toroidal magnetic flux (and the currents that support it). The content is not more than a typical section of the book, but is broken off into its own chapter for emphasis.

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.