[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.
Abstract. VLF remote sensing is used to detect lower-ionospheric electron density changes associated with a certain type of transient luminous events known as elves. Both ground- and satellite-based observations of elves are analysed in relation to VLF data acquired at various receiver sites in Europe, the United States and Antarctica. Ground-based observations were performed during the EuroSprite2003 campaign, when five elves were captured by low-light cameras located in the Pyrenees. Analysis of VLF recordings from Crete shows early VLF perturbations accompanying all of the elves. A large dataset consisting of elves captured by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) payload on Taiwan's FORMOSAT-2 satellite over Europe and North America has also been analysed. Early/fast VLF perturbations were found to accompany some of the elves observed over Europe. However, no VLF perturbations were detected in relation to the elves observed by ISUAL over North America. The present analysis – based on the largest database of optical elve observations used for VLF studies so far – constitutes evidence of processes initiated by the lightning EMP (electromagnetic pulse) causing electron density changes in the lower ionosphere in line with theoretical predictions. It also proves that sub-ionospheric electron density changes associated with elves can intrude to lower heights and thus perturb VLF transmissions. The possibility of VLF detection, however, depends on several factors, e.g., the distance of the elve from the receiver and the transmitter–receiver great circle path (GCP), the altitude of the ionised region and the characteristics of the VLF transmitter, as well as the EMP energy, which occasionally may be sufficient to cause optical emissions but not ionisation.
[1] On 22 July 2007, 37 blue jets/starters and 1 gigantic jet occurring over a thunderstorm in the Fujian province of China were observed from the Lulin observatory on the central mountain ridge of Taiwan. The majority of the jets were observed to occur in a 5 min window during the mature phase of the jet-producing thunderstorm. These jets have significant red band emissions. However, the blue emissions from these jets were not discernible due to severe atmospheric scattering. A model estimation of the emissions from a streamer reveals that the red emissions in blue starters and blue jets are mainly from the nitrogen first positive band (1PN2). The type II gigantic jet is the first of this type that was observed from the ground. The generation sequence of the gigantic jet begins with a blue starter, then a blue jet occurs at the same cloud top after ∼100 ms and finally develops into a gigantic jet ∼50 ms later. Using “optical strokes” as surrogates of the lightning strokes, the correlations between jets and the cloud lightning are explored. The results indicate that the occurrence of jets can be affected by the preceding local cloud-to-ground (CG) lightning or nearby lightning (intracloud (IC) or CG), while in turn the jets might also affect the ensuing lightning activity.
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.
'/%3e%3cuse xlink:href='%23a' transform='rotate(60)'/%3e%3ccircle r='17' fill='%23000095'/%3e%3ccircle r='15' fill='%23fff'/%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
