C. A. Cattell

FAST satellite observations of large‐amplitude solitary structures

We report observations of “fast solitary waves” that are ubiquitous in downward current regions of the mid-altitude auroral zone. The single-period structures have large amplitudes (up to 2.5 V/m), travel much faster than the ion acoustic speed, carry substantial potentials (up to ∼100 Volts), and are associated with strong modulations of energetic electron fluxes. The amplitude and speed of the structures distinguishes them from ion-acoustic solitary waves or weak double layers. The electromagnetic signature appears to be that of an positive charge (electron hole) traveling anti-earthward. We present evidence that the structures are in or near regions of magnetic-field-aligned electric fields and propose that these nonlinear structures play a key role in supporting parallel electric fields in the downward current region of the auroral zone.

Detection of localized, plasma‐depleted flux tubes or bubbles in the midtail plasma sheet

Recent studies have shown that most Earthward transport hi the midtail, high-beta plasma sheet takes place in the form of short-lived, high-speed plasma flow bursts. Bursty bulk flows are observed both when the plasma sheet is thin, such as during substorm expansion, and when it is thick, such as during substorm recovery. We present multi-instrument observations from the ISEE1 and ISEE 2 spacecraft to argue that when the plasma sheet becomes thick and close to its equilibrium state, the plasma and magnetic field signatures of high-speed flow events are consistent with the theoretically predicted signatures of plasma-depleted flux tubes or “bubbles” [Pontius and Wolf, 1990; Chen and Wolf, 1993]. These signatures consist of a decrease in the plasma pressure and an increase in the Bz-component of the magnetic field accompanying the high speed flow. We show that the Earthward moving bubbles are separated from the plasma ahead of them by a sharp tangential discontinuity. The layer ahead of the bubbles exhibits flow and magnetic field shear consistent with flow around an Earthward moving obstacle. The bubble is in approximate total pressure balance with the surrounding medium. We show that there is a systematic difference in the orientation of the discontinuity measured at ISEE 1 and 2, implying a small (about 1–3 RE) cross-tail size of the bubbles.

FAST observations in the downward auroral current region: Energetic upgoing electron beams, parallel potential drops, and ion heating

Observations of plasma particles and fields by the FAST satellite find evidence of acceleration of intense upgoing electron beams by quasi-static parallel electric fields. The beam characteristics include a broad energy spectrum with peak energies between 100 eV and 5 keV, perpendicular temperatures less than 1 eV, and fluxes greater than 109/cm²sec. Diverging electrostatic shocks associated with the beams have integrated potentials that match the beam energy. These beams are found in regions of downward Birkeland current and account for the total field-aligned current when they are present. The most energetic ion conics in the auroral zone are found coincident with these beams, in agreement with the model for “trapped” conics. The measured particle densities of the electron beams and associated ion conics are approximately equal and typically range from 1 to 10 cm−3, with no evidence for additional cold density. The beams are seen frequently at altitudes between 2000 and 4000 km in the winter auroral zone. Their probability of occurrence has a strong dependence on season and altitude and is similar to that for upgoing ion beams in the adjacent upward current regions. This similarity suggests that the density and scale height of ionospheric ions play an important role in the formation of both types of beams.

FAST satellite observations of electric field structures in the auroral zone

Electric field and energetic particle observations by the Fast Auroral Snapshot (FAST) satellite provide convincing evidence of particle acceleration by quasi-static, magnetic-field-aligned (parallel) electric fields in both the upward and downward current regions of the auroral zone. We demonstrate this by comparing the inferred parallel potentials of electrostatic shocks with particle energies. We also report nonlinear electric field structures which may play a role in supporting parallel electric fields. These structures include large-amplitude ion cyclotron waves in the upward current region, and intense, spiky electric fields in the downward current region. The observed structures had substantial parallel components and correlative electron flux modulations. Observations of parallel electric fields in two distinct plasmas suggest that parallel electric fields may be a fundamental particle acceleration mechanism in astrophysical plasmas.

Polar Spacecraft Based Comparisons of Intense Electric Fields and Poynting Flux Near and Within the Plasma Sheet-Tail Lobe Boundary to UVI Images: An Energy Source for the Aurora

In this paper, we present measurements from two passes of the Polar spacecraft of intense electric and magnetic field structures associated with Alfven waves at and within the outer boundary of the plasma sheet at geocentric distances of 4-6 R(sub E), near local midnight. The electric field variations have maximum values exceeding 100 mV/m and are typically polarized approximately normal to the plasma sheet boundary. The electric field structures investigated vary over timescales (in the spacecraft frame.) ranging front 1 to 30 s. They are associated with strong magnetic field fluctuations with amplitudes of 10-40 nT which lie predominantly ill the plane of the plasma sheet and are perpendicular to the local magnetic field. The Poynting flux associated with the perturbation fields measured at these altitudes is about 1-2 ergs per square centimeters per second and is directed along the average magnetic field direction toward the ionosphere. If the measured Poynting flux is mapped to ionospheric altitudes along converging magnetic field lines. the resulting energy flux ranges up to 100 ergs per centimeter squared per second. These strongly enhanced Poynting fluxes appear to occur in layers which are observed when the spacecraft is magnetically conjugate (to within a 1 degree mapping accuracy) to intense auroral structures as detected by the Polar UV Imager (UVI). The electron energy flux (averaged over a spatial resolution of 0.5 degrees) deposited in the ionosphere due to auroral electron beams as estimated from the intensity in the UVI Lyman-Birge-Hopfield-long filters is 15-30 ergs per centimeter squared per second. Thus there is evidence that these electric field structures provide sufficient Poynting flux to power the acceleration of auroral electrons (as well as the energization of upflowing ions and Joule heating of the ionosphere). During some events the phasing and ratio of the transverse electric and magnetic field variations are consistent with earthward propagation of Alfven surface waves with phase velocities of 4000-10000 kilometers per second. During other events the phase shifts between electric and magnetic fields suggest interference between upward and downward propagating Alfven waves. The E/B ratios are about an order of magnitude larger than typical values of C/SIGMA(sub p), where SIGMA(sub p), is the height integrated Pedersen conductivity. The contribution to the total energy flux at these altitudes from Poynting flux associated with Alfven waves is comparable to or larger than the contribution from the particle energy flux and 1-2 orders of magnitude larger than that estimated from the large-scale steady state convection electric field and field-aligned current system.

Cluster observations of electron holes in association with magnetotail reconnection and comparison to simulations

Cluster observations of electron holes in association with magnetotail reconnection and comparison to simulations

FAST satellite wave observations in the AKR source region

The Fast Auroral SnapshoT (FAST) satellite has made observations in the Auroral Kilometric Radiation (AKR) source region with unprecedented frequency and time resolution. We confirm the AKR source is in a density depleted cavity and present examples in which cold electrons appeared to have been nearly evacuated (nhot> ncold). Electron distributions were depleted at low-energies and up-going ion beams were always present. Source region amplitudes were far greater than previously reported, reaching 2×10−4 (V/m)²/Hz (300 mV/m) in short bursts with bandwidths generally <1 kHz. Intense emissions were often at the edge of the density cavity. Emissions were near or below the cold plasma electron cyclotron frequency in the source region, and were almost entirely electromagnetic. The |E|/|B| ratio was constant as a function of frequency and rarely displayed any features that would identify a cold plasma cutoff or resonance.

Comparisons of Polar satellite observations of solitary wave velocities in the plasma sheet boundary and the high altitude cusp to those in the auroral zone

Characteristics of solitary waves observed by Polar in the high altitude cusp, polar cap and plasma sheet boundary are reported and compared to observations in the auroral zone. The study presented herein shows that, at high altitudes, the solitary waves are positive potential structures (electron holes), with scale sizes of the order of 10s of Debye lengths, which usually propagate with velocities of a few thousand km/s. At the plasma sheet boundary, the direction of propagation can be either upward or downward; whereas at the leading edge of high altitude cusp energetic particle injections, it is downward. For these high altitude events, explanations based on ion modes and on electron modes are both examined, and the electron mode interpretation is shown to be more consistent with observations.

FAST observations of VLF waves in the auroral zone: Evidence of very low plasma densities

The Fast Auroral SnapshoT (FAST) explorer frequently observes the auroral density cavity, which is the source region for Auroral Kilometric Radiation (AKR). An important factor in the generation of AKR is the relative abundance of hot and cold electrons within the cavity, since hot electrons introduce relativistic modifications to the wave dispersion. VLF wave-form data acquired by FAST within the auroral density cavity show clear signatures of whistler-mode waves propagating on the resonance cone. This allows us to obtain the electron plasma frequency, and the cavity often has densities <1 cm−3. Moreover, the hot electrons can be the dominant electron species, enabling AKR to be generated below the cold electron gyro-frequency.

Geotail observations of substorm onset in the inner magnetotail

On April 26, 1995, while Geotail was in the near-equatorial magnetotail at 13 RE and 2300 LT, a substorm onset occurred that was documented by ground magnetograms, auroral kilometric radiation, and magnetic field and particle data from four spacecraft at and near geosynchronous orbit. Although Geotail was initially outside a greatly thinned current sheet, plasma sheet thickening associated with the substorm dipolarization quickly caused Geotail to move into the plasma sheet where it observed field-aligned earthward moving ions with velocities of 400 km/s. During the subsequent few minutes as the magnetic field became more northward, the velocities increased with particles moving increasingly into the energy range of the energetic particle experiment. These flows culminated with 1-min worth of earthward flow of 2000 km/s that was perpendicular to the northward B field. Such flow, probably the largest ever detected at 13 RE, was confirmed by the observation of an intense dc electric field of 50 mV/m (0.3 megavolts/RE). This large field is probably inductive, caused by reconnection that occurred tailward of the spacecraft, and related to the acceleration processes associated with particle injection at geosynchronous orbit. Energy and magnetic flux conservation arguments suggest that this rapid flow has a small cross-tail dimension of the order of 1 RE. The data appear to support a simulation of Birn and Hesse [1996] which showed rapid earthward flows from a reconnection line at 23 RE that caused a tailward expansion of a region of dipolarized flux. Subsequent to the onset, Geotail observed plasma vortices with typical velocities of 50–100 km/s that occurred in a high-beta plasma sheet with a 15-nT northward magnetic field. The vortices were punctuated by occasional flow bursts with velocities up to 400 km/s, one of which was accompanied by a violently varying magnetic field where north/south field components were as large as 30 nT and as small as −8 nT.