No abstract

[1] The ISUAL gigantic jets (GJs) are categorized into three types from their generating sequence and spectral properties. Generating sequence of the type I GJs resembles that reported previously; after the fully developed jet (FDJ) established the discharge channel, the ISUAL photometers registered a peak that was from a return-stroke-like process. The associated ULF (ultra-low-frequency) sferics of these type I GJs indicates that they are negative cloud-to-ionosphere discharges (−CIs). Type II GJs begin as blue jets and then developed into GJs in ∼100 ms. Blue jets also frequently occurred at the same region before and after the type II GJs. No identifiable ULF sferics of the type II GJs were found, though an extra event that has +CI ULF signature is probably a type II GJ. The FDJ streamer brightness of the type I GJs is ∼3.4 times of that of the type II GJs. These evidences suggest that the type II GJs are composed of positive streamers. Type III GJs were preceded by lightning, and a GJ subsequently occurred near this preceding lightning. The spectral data of the type III GJs are dominated by lightning signals and the ULF data have high background noise; thus both cannot be properly analyzed. However, the average brightness of the type III GJs falls between those of the other two types of GJs. We propose that the discharge polarity of the type III GJs can be either negative or positive, depending on the type of the charge imbalance left by the trigger lightning.

[1] The Imager of Sprites and Upper Atmospheric Lightnings (ISUAL) often recorded events that have significant far-ultraviolet (FUV) emissions in the spectrophotometer but have no discernible transient luminous events (TLEs) in the imager. These FUV events likely are dim TLEs. To confirm the conjecture, lightning emissions were simulated and proved to be completely absorbed by the atmosphere. The FUV emission of the FUV events follows the lightning OI emission within 1 ms, similar to the characteristics of elves. After analyzing the imager-N 2 1P brightness of the elves and their FUV intensity, a linear correlation was found, which is consistent with the work of Kuo et al. (2007). The intensity of the FUV events ranks among the dimmest elves and is less than 1 × 10 4 photons/cm 2 . Combining all the information, the FUV events are identified as dim elves that eluded the detection of the ISUAL imager. Also from the detection limits of the ISUAL spectrophotometer (SP) and the imager, for the before-the-limb elves the detection number of SP is found to be nearly 16 times higher than that of the imager. This result is consistent with a related factor of ∼13 that was inferred from the U.S. National Lightning Detection Network (NLDN) peak current distribution for the negative cloud-to-ground lightning. Hence the ISUAL spectrophotometer can be used to perform elve survey, to infer the peak current of the elve-producing lightning, and possibly to be used to deduce other lightning parameters. Evidence is also found for the existence of multielves, which are FUV events from the M-components or the multiple strokes in lightning flashes.

Abstract. Space-based measurements from an imager aboard the high-apogee NASA-IMAGE satellite allows for global-scale observations of nightside ionospheric densities and structure. Such a view cannot be provided by imagers in near-Earth orbit or based on the ground. The IMAGE Spectroscopic Imager (SI) isolates the Far-ultraviolet (FUV) O I 135.6 nm emission which is produced through radiative recombination of O + . These observations clearly show the distribution of FUV emissions of the equatorial airglow bands over the range of local times between the evening terminator to points well after midnight. Determination of plasma drift speeds in these local time sectors is performed by identification and subsequent tracking of localized depressions in the FUV emissions. This determination is made for nearly 200 plasma bubbles in the March-May period of 2002. Important findings of this study include (1) an unambiguous association between D st and zonal plasma drift speeds, and (2) a longitudinal dependence of the zonal plasma drift speeds, with a peak around the Indian sector. The first effect is attributed to penetrating ring current electric fields, while the second is apparently due to a longitudinal variability in the vertical polarization electric fields that directly affects the zonal plasma drift speeds.

Abstract. As a measure of the degree of coupling between the solar wind-magnetosphere-ionosphere systems, the rate at which the size of the polar cap (the region corresponding to ionospheric termini of open magnetic flux tubes) varies is of prime importance. However, a reliable technique by which the extent of the polar cap might be routinely monitored has yet to be developed. Current techniques provide particularly ambiguous indications of the polar cap boundary in the dawn sector. We present a case study of spaceand ground-based observations of the dawn-sector auroral zone and attempt to determine the location of the polar cap boundary using multi-wavelength observations of the ultraviolet aurora (made by the IMAGE FUV imager), precipitating particle measurements (recorded by the FAST, DMSP, and Cluster 1 and 3 satellites), and SuperDARN HF radar observations of the ionospheric Doppler spectral width boundary. We conclude that in the dawn sector, during the interval presented, neither the poleward edge of the wideband auroral UV emission (140-180 nm) nor the Doppler spectral width boundary were trustworthy indicators of the polar cap boundary location, while narrow band UV emissions in the range 130-140 nm appear to be much more reliable.