H. C. Stenbaek-Nielsen

Imaging of elves, halos and sprite initiation at time resolution

Abstract Elves, halos and sprites were observed during August 1999 with a 1 ms high speed imager. The higher time resolution compared to conventional television cameras (17 or 20 ms ) allowed excellent images of the three phenomena temporally separate from each other to be obtained. Analysis of images of elves and halos indicates that the causal lightning-generated electromagnetic pulse and quasi-electro static fields are homogeneous and any small-scale (sub- 10 km ) structure, if visible, is most likely due to a structured atmosphere. Observations of sprites initiated to the side of a halo, without a halo, and from beads left over from a previous sprite, respectively, all suggest sub-pixel ( km ) background structures in atmospheric pressure or composition as being the dominant factors in determining the sprite “seed” location, or site of sprite initiation.

New evidence for the brightness and ionization of blue starters and blue jets

Blue jets and blue starters are partially ionized luminous cones of primarily blue light that propagate upward out of the top of thunderstorms at speeds of order 100 km s−1. Blue jets propagate up ∼40 km, but blue starters, which resemble blue jets, terminate abruptly after only a few kilometers of upward travel. Theories on the origin of blue jets have proposed that they are due to either positive or negative streamers or runaway electrons. Quantitative analysis of new multi-instrument observations of a blue starter from an aircraft during the Energetics of Upper Atmospheric Excitation by Lightning, 1998 (EXL98) campaign of July 1998, shows that the ionization accounts for ∼3% of the observed intensity. Quantitative analysis of a remarkable color photograph of a blue jet taken from Reunion Island in the Indian Ocean shows that the minimum optical energy deposition was ∼0.5 MJ. The same photograph shows details of streamers never before seen.

Sprites over Europe

Results are presented from the first European campaign for observation of sprites, conducted during the summer of 2000 from the French astronomical observatory, Observatoire Midi-Pyrenees. The primary objective was to establish if sprites are generated over Europe and to identify the characteristics of the associated thunderstorms. During the one-month campaign local weather conditions allowed observations approximately half of the nights. Sprites were observed two nights over the Alps and one night over southeastern France in connection with cold fronts moving in from the Atlantic. In all, 40 sprites were recorded, including dancing sprites, multiple carrot sprites and c-sprites. The weather conditions were almost identical during the 3 nights, with the active area forming on the front-side of the cold fronts. The storms are not of the same magnitude as active systems often observed over the North American plains. Even so, sprites seem to be a common occurrence also over Europe.

Conjugate auroras and the interplanetary magnetic field

Earlier results from analyses of auroral all-sky data obtained on several series of conjugate flights along the College, Alaska, magnetic meridian are reevaluated, and it is suggested that while the diffuse aurora is conjugate, the discrete aurora is nonconjugate but still on closed field lines. The suggestion that the discrete aurora is nonconjugate is contrary to an earlier analysis of the same data set. The old analysis ascribed observed shifts in conjugate points within the discrete auroras to auroral-activity-related perturbations in the geomagnetic field, but it has been impossible to find a physical mechanism for the perturbations. The nonconjugacy likely reflects that the processes creating the visual discrete auroral forms operate independently in each hemisphere. Assuming that the discrete aurora is a consequence of field-aligned currents, we suggest that the observed hemispherical differences in the discrete auroras may be caused by an interhemispherical field-aligned current component. Such a current component will result from the penetration of the interplanetary magnetic field into the magnetosphere and should primarily depend on the sign of By. The conjugate flight data previously analyzed and published appear to qualitatively support this conclusion.

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.

Detailed analysis of substorm observations using SuperDARN, UVI, ground‐based magnetometers, and all‐sky imagers

[1] A case study of a small-magnitude isolated substorm is presented. The substorm was observed by a variety of instruments including the Super Dual Auroral Radar Network (SuperDARN), the Polar Ultra Violet Imager (UVI), CANOPUS magnetometers, the Alaska chain magnetometers, the Poker Flat meridian-scanning photometer, and the Poker Flat all-sky imager. It was determined that the initial brightening was centered over the combined fields of view of the various instruments. Temporal and spatial relationships among plasma flows, auroral luminosity, and magnetometer perturbations are investigated. It is found that the initial substorm signature was observed in the plasma flows, followed by the auroral brightening, and finally followed by the magnetometer perturbation. Enhanced plasma flows were observed in a spatially confined region near the auroral oval for a period of ∼5 minutes prior to the brightening. After the brightness peaked, the plasma flow velocity decreased back to a preenhancement level.

Observation of electromagnetic oxygen cyclotron waves in a flickering aurora

Instruments on the Auroral Turbulence rocket detected several intervals of weak electromagnetic oscillations at frequencies of 6–13 Hz in a strongly flickering auroral arc. These oscillations have amplitudes of up to δB ∼ 3 nT and δE ∼ 4 mV/m and have downward field-aligned Poynting fluxes of up to ∼10−5 W/m². Fluctuations in the parallel electron flux at about 9 Hz were observed in association with the strongest of these oscillations. Simultaneous ground-based optical data show that the arc was flickering at frequencies of 8–15 Hz. The observed frequencies would match the oxygen cyclotron frequency at ∼4500 km altitude. In one wave/particle event the apparent lag of the waves behind the modulated electrons implies a modulation source altitude of 2500–5000 km. We interpret these waves as electromagnetic ion cyclotron waves originating in the auroral acceleration region.

A three-dimensional hybrid code simulation of the December 1984 solar wind AMPTE release

A three-dimensional hybrid code has been developed to study the interaction between small dense plasma clouds and an ambient plasma. The primary advantage of this code is a seamless interface between kinetic particles (plasma cloud) and an MHD fluid (ambient plasma). This interface provides momentum coupling between two distinct ion populations. As a preliminary test of our code, we have simulated the first 3 minutes of the December 1984 AMPTE artificial comet. The results show good agreement with observations of the magnetic field distribution as well as the lateral motion of the comet head. Examples of other applications for this code include comets, coronal mass ejections, Ios plasma torus, and plasma injection experiments.

Fast photometry of flickering in discrete auroral arcs

We present high speed photometric measurements of the auroral intensity of flickering aurora. These measurements reveal the existence of intensity modulations in the discrete aurora above the Nyquist frequency of standard 30 frame per second TV cameras. The intensity fluctuations observed are primarily below 80 Hz, although frequencies up to 180 Hz have been detected. Changes in the spectral characteristics from essentially band-limited to broadband intensity fluctuations and then to narrowband fluctuations are seen in individual discrete arcs within a few minutes. The center frequency of the observed fluctuations also change during this period. Data obtained from a standard narrow field TV camera observing the same arc show that only the low ( ≈ 10 Hertz) narrowband modulations would be considered standard flickering aurora.

Plasma irregularities in the D-region ionosphere in association with sprite streamer initiation

Sprites are spectacular optical emissions in the mesosphere induced by transient lightning electric fields above thunderstorms. Although the streamer nature of sprites has been generally accepted, how these filamentary plasmas are initiated remains a subject of active research. Here we present observational and modelling results showing solid evidence of pre-existing plasma irregularities in association with streamer initiation in the D-region ionosphere. The video observations show that before streamer initiation, kilometre-scale spatial structures descend rapidly with the overall diffuse emissions of the sprite halo, but slow down and stop to form the stationary glow in the vicinity of the streamer onset, from where streamers suddenly emerge. The modelling results reproduce the sub-millisecond halo dynamics and demonstrate that the descending halo structures are optical manifestations of the pre-existing plasma irregularities, which might have been produced by thunderstorm or meteor effects on the D-region ionosphere.