[1] This study presents the O I 135.6 nm airglow observation of the middle-latitude electron density enhancement during local summer nighttime by Global Ultraviolet Imager (GUVI) on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft. The nighttime density enhancement at magnetic middle latitudes in the Southern Hemisphere, known as the Weddell Sea Anomaly (WSA), had been studied by using multipoint observations, and a similar anomalous nighttime enhancement was also found in the Northern Hemisphere recently. The resemblance of both anomalies at magnetic middle latitudes in the Northern and Southern Hemispheres suggests that they should be categorized as the midlatitude summer nighttime anomaly (MSNA). To further explore the three-dimensional structure of the MSNA and its day-to-day variation, the two-dimensional global radiance maps and the vertical electron density profiles derived from disk and limb scans of the TIMED/GUVI 135.6 nm airglow observations are utilized in this study. These global observations show that the northern MSNA mainly occurs in Asia, Europe, and the North Atlantic Ocean regions, while the southern MSNA occurs in the South America-Antarctica region, near the WSA region. The GUVI day-to-day observations in 2006 further illustrate that the southern MSNA appears nightly in January-February and November-December, while the northern MSNA appears in 36 out of 41 total observation nights in May-June.
The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) on the Taiwanese FORMOSAT‐2 (formerly ROCSAT‐2) spacecraft is the first global observatory of transient luminous events (TLEs). During the first 4 months of operation a large number of elves were observed. Elves are obvious with their far ultraviolet (FUV) signature that is less attenuated by atmospheric O 2 at their emission altitude compared to emission from sprites or lightning at lower altitude. About half of all elves were produced by lightning that shows a three‐step signature in the photometer signal: (1) An initial brightening in all except the FUV channels, (2) a slow brightness decrease for the next 2–5 milliseconds, and (3) an impulsive increase of signal in all channels. We interpret this specific signature as the initial breakdown with a beta‐type stepped leader followed by the bright return stroke of a negative cloud to ground (−CG) lightning. In contrast, sprites follow positive cloud to ground lightning (+CG) without a signature of initial breakdown and stepped leader. Many sprites are associated with the continuing current and can be delayed up to 100 ms after the lightning.
The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) on the FORMOSAT‐2 spacecraft observes Transient Luminous Events (TLE) like sprites, elves, and halos from space. We analyzed halos that were observed in Central America close enough to ELF/VLF receivers that allowed for the determination of the polarity of the parent lightning. All halos were created by negative cloud to ground lightning (−CG) strokes that occurred almost exclusively over the open water. Only three out of the 31 events happened over land. We conclude that the Central American region seems to be special with respect to the large proportion of −CG created halos. Such a behavior is very different from the occurrence of sprites that are mostly created by positive cloud to ground lightning.
This is the first report of the most comprehensive 630.0 nm airglow limb images taken using Imager of Sprites and Upper Atmospheric Lightnings (ISUAL) onboard FORMOSAT‐2. The limb scans reveal two distinct airglow layers: the upper one corresponds to the thermospheric O( 1 D) emission and the lower one corresponds to the OH (9–3) emissions. Sequences of such observations are combined to generate altitude‐latitude maps of the emissions, which reveal intensity enhancements of both the layers at certain locations where they often appear to be linked/joined vertically. A detailed analysis of the location and occurrence of the enhancements in the entire observations, together with simulations of the emissions suiting the ISUAL limb geometry are carried out to understand the causes and related processes.
This article reports the first high time resolution measurements of gigantic jets from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment. The velocity of the upward propagating fully developed jet stage of the gigantic jets was ∼10 7 m s −1 , which is similar to that observed for downward sprite streamers. Analysis of spectral ratios for the fully developed jet emissions gives a reduced E field of 400–655 Td and average electron energy of 8.5–12.3 eV. These values are higher than those in the sprites but are similar to those predicted by streamer models, which implies the existence of streamer tips in fully developed jets. The gigantic jets studied here all contained two distinct photometric peaks. The first peak is from the fully developed jet, which steadily propagates from the cloud top (∼20 km) to the lower ionosphere at ∼90 km. We suggest that the second photometric peak, which occurs ∼1 ms after the first peak, is from a current wave or potential wave–enhanced emissions that originate at an altitude of ∼50 km and extend toward the cloud top. We propose that the fully developed jet serves as an extension of the local ionosphere and produces a lowered ionosphere boundary. As the attachment processes remove the charges, the boundary of the local ionosphere moves up. The current in the channel persists and its contact point with the ionosphere moves upward, which produces the upward surging trailing jets. Imager and photometer data indicate that the lightning activity associated with the gigantic jets likely is in‐cloud, and thus the initiation of the gigantic jets is not directly associated with cloud‐to‐ground discharges.
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.
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