Robert J. Strangeway

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.

Current disruptions in the near-Earth neutral sheet region

Observations from the Charge Composition Explorerin 1985 and 1986 revealed fifteen current disruption events in which the magnetic field fluctuations were large and their onsets coincided well with ground onsets of substorm expansion or intensification. These events are of short durations locally (∼1–5 min). They are mostly confined to within ∼0.5 RE of the neutral sheet and 1 hour local time from the magnetic midnight. Over the disruption interval, the local magnetic field can change by as much as a factor of ∼7. In general, the stronger the current buildup and the closer to the neutral sheet, the larger the resultant field change. There is also a tendency for a larger subsequent enhancement in the AE index with a stronger current buildup prior to current disruption. For events with good pitch angle coverage and extended observation in the neutral sheet region we find that the particle pressure increases toward the disruption onset and decreases afterward. Just prior to disruption, either the total particle pressure is isotropic, or the perpendicular component (P⊥) dominates the parallel comment (P∥), the plasma beta is seen to be as high as ∼70, and the observed plasma pressure gradient at the neutral sheet is large along the tail axis. The deduced local current density associated with pressure gradient is ∼27–80 nA/m² and is ∼85–105 mA/m when integrated over the sheet thickness. We infer from these results that just prior to the onset of current disruption, (1) an extremely thin current sheet requiring P∥ > P⊥ for stress balance does not develop at these distances, (2) the thermal ion orbits are in the chaotic or Speiser regime while the thermal electrons are in the adiabatic regime and, in one case, exhibit peaked fluxes perpendicular to the magnetic field, thus implying no electron orbit chaotization to possibly initiate ion tearing instability, and (3) the neutral sheet is in the unstable regime specified by the cross-field current instability. Subsequent to the disruption onset, enhancement of magnetic noise over a broad frequency range, magnetic field aligned counterstreaming electron beams, ion energization perpendicular to the magnetic field, and current reduction in the amount similar to that of current buildup during the growth phase are observed. These features seem to be compatible with the predicted development of the cross-field current instability.

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.

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.

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.

parallel electric fields in discrete arcs

We present a large-scale model of the parallel electric fields in the upward current region of the aurora. The model, called the “Transition Layer Model”, applies to intense, discrete arcs. It is based on 1-D spatial, 2-D velocity static Vlasov simulations and observations from the Fast Auroral Snapshot (FAST) satellite. The model depicts three regions along the magnetic field that are separated by two transition layers which contain parallel electric fields. The current-voltage properties closely follow the Knight relation.

FAST observations of electron distributions within AKR source regions

Observations of high-time resolution 3-D electron distributions within the source regions of Auroral Kilometric Radiation (AKR) are reported. In general, the electron data display a broad plateau over a wide range of pitch angles indicating that these distributions have been rapidly stabilized by AKR wave growth. The source of the electron instability appears to come from several features in the distribution, including an isotropic beam feature and its mirroring components, occasional electrons in the “trapped” region, as well as steep gradients present in the atmospheric loss cone. Taken together these features may provide a nearly continuous region of ∂f/∂v⟂ which could contribute to the relativistic cyclotron maser instability. Computer simulations of the evolution of the electron distribution which assume plasma conditions similar to the parameters measured by FAST show similar results to the observed electron distributions. The FAST observations also show that relativistic corrections to the AKR dispersion relation may enable a small k|| mode with a resonance condition that is able to take maximum advantage of the initial instability in the mono-energetic electron distributions within the auroral acceleration regions.

Magnetosphere Sawtooth Oscillations Induced by Ionospheric Outflow

Numerical simulations show that a magnetospheric disturbance is caused by an influx of O+ ions from the ionosphere. The sawtooth mode of convection of Earth’s magnetosphere is a 2- to 4-hour planetary-scale oscillation powered by the solar wind–magnetosphere–ionosphere (SW-M-I) interaction. Using global simulations of geospace, we have shown that ionospheric O+ outflows can generate sawtooth oscillations. As the outflowing ions fill the inner magnetosphere, their pressure distends the nightside magnetic field. When the outflow fluence exceeds a threshold, magnetic field tension cannot confine the accumulating fluid; an O+-rich plasmoid is ejected, and the field dipolarizes. Below the threshold, the magnetosphere undergoes quasi-steady convection. Repetition and the sawtooth period are controlled by the strength of the SW-M-I interaction, which regulates the outflow fluence.

Ion Cyclotron Waves in the Solar Wind Observed by STEREO Near 1 AU

Using high-resolution magnetic field data from the STEREO mission, we have observed strong narrow-band ion cyclotron waves (ICWs) in the solar wind near 1 AU. The waves propagate nearly parallel to the magnetic field and are below the local proton gyrofrequency in the solar wind frame. Because the twin STEREO spacecraft were far away from any planet, including the Earth, the waves do not have a planetary source. The ICWs often appear when the interplanetary magnetic field is more radial than the nominal Parker spiral. Since the wave frequency in the spacecraft frame is higher than the local proton gyrofrequency, the waves are not locally generated by ion pickup. Observations are consistent with wave generation closer to the Sun and outward transport by the super-Alfvenic solar wind. The waves are intrinsically left-hand polarized in the solar wind frame, but appear to be both left- and right-handed in the spacecraft frame, associated with outward and inward propagation, respectively. The relative amplitudes of the left-handed and right-handed waves support the closer-to-Sun generation scenario.