T. J. Hallinan

Small-scale auroral arc distortions

Small-scale spatially periodic distortions of auroral forms have been studied utilizing low-light level television observations made at various locations in the Northern and Southern Hemispheres. The most commonly observed features were folds and vortex-like curl formations. The curls, identified here with the Kelvin-Helmholtz instability due to fluid shear, invariably had a counterclockwise rotational shape and motion when viewed in a direction anti-parallel to the Earths magnetic field. The typical measured wavelength (∼5 km) and measured growth rate (4.2 sec−1) were used to evaluate the Kelvin-Helmholtz dispersion relation for the apparent shear ωs = ∂ νx∂y (28 sec−1). The apparent horizontal velocities of both folds (0–5 km/sec) and curls (0–22 km/sec) were invariably observed to be counterclockwise with respect to the multiple arc centre when viewed antiparallel to B. Consistent agreement between rotational shape and rotational motion suggests that the apparent growth rate and the apparent horizontal velocities closely approximate the actual values. If the shear results from E×B drifts in a space charge field, the calculated value for ωs, implies an unneutralized electron density ∼0–1 cm−3 and a ΔE across the arc element ∼500mV/m. The velocity measurements indicate that the ΔE values for individual elements can combine to produce transient electric fields at the edges of multiple arcs as high as 1000 mV/m.

Aircraft observations conjugate to FAST: Auroral are thicknesses

Optical observations conjugate to the FAST satellite show good agreement between the widths of auroral structures observed optically and those inferred from the measured electron energy flux. The implication is that these structures are imposed by processes at or above the -4000 km altitude of FAST. A variety of widths down to about 2 km were observed, but there were no examples of finer scale structures. A pre-breakup weak discrete arc at the poleward edge of the diffuse aurora showed electron produced optical structures located on either side of upward going ion beams. The optical emission in the equatorward part of the diffuse aurora was caused almost exclusively by precipitating ions. The optical observations were made over northern Alaska between Jan 31 and Feb 16, 1997, from a jet aircraft carrying an all-sky and three narrow-field TV cameras.

FLICKERING: A 10-cps FLUCTUATION WITHIN BRIGHT AURORAS.

Abstract Visually and with image orthicon television systems, a 10 ± 3 cps intensity variation has been observed in bright homogeneous auroras just prior to and during the breakup. It has been observed simultaneously at conjugate locations in the northern and southern hemispheres and at several other locations in the northern hemisphere. This phenomenon is called flickering ; it has characteristics which clearly distinguish it from other types of pulsing auroras.

A morphological study of black vortex streets

Black aurora has been defined as the lack of auroral emissions in well-defined regions within diffuse aurora [Royrvik, 1976; Davis, 1978]. A particularly intriguing class of black aurora is the occurrence of black vortex streets (black curls) exhibiting clockwise vorticity [Davis, 1978; Trondsen and Cogger, 1997.] An archive of low-light level all-sky and narrow field television camera data was searched, yielding 20 examples of black curls. Black curls are concluded to result from the Kelvin-Helmholtz instability occurring in regions of positive space charge. Calculated peak electric fields ranged from 80 to 300 mV/m. White curls seen in aurora that was intermediate between diffuse and discrete were sometimes found in association with black curls and even paired with black curls to form newly discovered Karman vortex streets. A spectroscopic measurement indicated mixed proton and electron precipitation. Electric field configurations are proposed for the black and white curl systems and the Karman vortex streets.

Observations of black auroral patches and of their relationship to other types of aurora

A morphological study of black patches and newly reported black rings (black patches with islands of embedded diffuse aurora) was conducted using an archive of low-light level all-sky and narrow-field television camera data. The overwhelming majority of black patches and rings drifted eastward, poleward of westward drifting pulsating aurora in the early recovery phase of the auroral substorm. Occasionally, black aurora would overlap with pulsating aurora. Sometimes the pulsations would add to the intensity in the black aurora more than in the surrounding diffuse aurora (called veiling). One example was documented where the pulsation was purely additive, adding to the intensity of the black aurora and the surrounding diffuse aurora equally. Cases of total veiling suggest an intensity limit, possibly associated with the strong pitch angle diffusion limit.

Lidar observations of a large high‐altitude sporadic Na layer during active aurora

A large sporadic Na layer was observed near 109 km during active aurora at Poker Flat, Alaska, on the night of 12–13 January 1996. This sporadic layer is significantly denser and broader than previously reported thermospheric sporadic layers; having a peak concentration equal to the peak concentration of the background Na layer, a FWHM of 4.7 km, and a column abundance of 40% of the background Na layer. The appearence of the Nas layer coincided with a significant OH temperature gradient across the sky but did not coincide with the presence of a discrete arc overhead nor a distinct signature in the local magnetometer data. The presence of such a large amount of neutral Na at these altitudes contradicts the current models of Nas layer formation. The observations are discussed in the light of other observations, current models, layered auroral phenomena, and meteoric ablation.

Relation between optical emissions, particles, electric fields, and Alfvén waves in a multiple rayed arc

Velocities of rays in auroral arcs were used to infer the perpendicular electric fields above the acceleration region. Using rocket measurements of electron energy as a proxy for the high-altitude potential, the high-altitude perpendicular electric fields were calculated and found to be in good agreement with those derived from the ray motions. Additionally, a 0.6 Hz oscillating electric field at high altitude was postulated on the basis of the passing rays. Such a field was also calculated from the electron energy measurements and was found to be closely related to an Alfven wave measured on the payload following a delay of 0.8 s. The measured electron energy flux agreed well with the auroral luminosity down to scale sizes of about 10 km. The combination of ground-based imaging and the measured energy flux also allowed a determination of the lower border altitude of the arcs. They were found to be somewhat higher (130 km) than expected on the basis of the electron energy. A tall rayed arc with a lower border height of 170 km was associated with a burst of suprathermal electrons on the poleward edge of the aurora.

Spectroscopic evidence for suprathermal electrons in enhanced auroras

Auroral arcs frequently exhibit bands of enhanced intensity along the bottom edge. These bands have sharp upper borders and are referred to as enhanced lower borders or auroral hems. They are thought to be related to thin layers of enhanced luminosity observed both in auroral arcs and in pulsating auroras. These anomalous distributions of intensity as a function of altitude are referred to collectively as enhanced aurora (EA) and are of interest because they suggest a more complex process for producing auroral ionization and luminosity than the collisional degradation of precipitating electrons. Spectra of eight auroral hems were obtained with an imaging spectrograph and showed consistently that within the hem, there is an enhanced ratio of N2 1st positive emissions (red) to N2+ 1st negative emissions (blue). The enhancement in the red/blue ratio ranged from 10% to 100%. There was a generally smaller and less consistent enhancement in the ratio of the O2+ 1st negative band at 5275 A to the N2+ 1st negative. Observations of the far red spectra of several hems showed no enhancement in the ratio of N2 1st positive to N2+ Meinel emissions. There were no simultaneous measurements of the Meinel and the N2+ 1st negatives. It is argued that the enhanced spectral ratios are indicative of a population of suprathermal electrons with a sharp energy cutoff near the ionization potential of N2 and that the electrons are energized in situ through wave-particle interactions or possibly by dc electric fields. It is also noted that some but not all of the hems are type b red lower borders.

VLF Wave Experiments in Space Using a Modulated Electron Beam

A sounding-rocket payload to study the generation and emission of electromagnetic waves from a modulated electron beam was developed and successfully flown from the Poker Flat Research Range, Alaska, in March 1992 on a Black Brant 11 sounding rocket. We describe the mission objectives, the instrumentation, flight operations, and preliminary results from the flight. The payload contained a modulated electron gun with a triode arrangement of electrodes allowing modulation of the beam current up to vlf frequencies. Vehicle charging was inhibited by a gas-release system, synchronized with the beam emissions. A network of ground stations was set up to try to receive signals from the modulated beam, and low-light systems were set up to look for evidence of the beam below the payload trajectory. No evidence of beam-induced wave radiation to the ground was detected. However, strong vlf frequencies following the preset program were clearly detected by the diagnostic free flyer and the tethered daughter payload segment. The effect of the gas releases was very marked. The vehicle potential dropped from over 1 kV to about 30 V when the gas was turned on. Some evidence of the light from beam-atmosphere interactions was seen near the end of the flight, but in general, the optical results were not very informative. The launch was successful, mission operations were as planned, and good-quality onboard data were collected throughout the operational part of the flight.

Observed Barium Emission Rates

The barium releases from the CRRES satellite have provided an opportunity for verifying theoretically calculated barium ion and neutral emission rates. Spectra of the five Caribbean releases in the summer of 1991 were taken with a spectrograph on board a U.S. Air Force jet aircraft. Because the line of sight release densities are not known, only relative rates could be obtained. The observed relative rates agree well with the theoretically calculated rates and, together with other observations, confirm the earlier detailed theoretical emission rates. The calculated emission rates can thus with good accuracy be used with photometric observations. It has been postulated that charge exchange between neutral barium and oxygen ions represents a significant source for ionization. If so, it should be associated with emissions at 4957.15 A and 5013.00 A, but these emissions were not detected.