Imager Of Sprites And Upper Atmospheric Lightning Research Articles

Radiative emission and energy deposition in transient luminous events

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment on the FORMOSAT-2 satellite has recently reported that an elve is the most dominant type of transient luminous events (TLEs) and deduced the global occurrence rates of sprites, halos and elves to be ∼1, ∼1 and 35 events/min, respectively (Chen et al 2008 J. Geophys. Res. 113 A08306). In this paper, we report the computed radiative emission and energy precipitation of the TLEs in the upper atmosphere. By analysing 1415 ISUAL TLEs, we found that for sprites, halos and elves the spatially averaged brightness are 1.5, 0.3 and 0.17 MR, and the energy deposition is 22, 14 and 19 MJ per event. After factoring in the global occurrence rates, the global energy deposition rates in the upper atmosphere are 22, 14 and 665 MJ min−1 from sprites, halos and elves.

Global distributions and occurrence rates of transient luminous events

We report the global transient luminous event (TLE) distributions and rates based on the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment onboard the FORMOSAT‐2 satellite. ISUAL observations cover 45°S to 25°N latitude during the northern summer and 25°S to 45°N latitude during the northern winter. From July 2004 to June 2007, ISUAL recorded 5,434 elves, 633 sprites, 657 halos, and 13 gigantic jets. Surprisingly, elve is the dominant type of TLEs, while sprites/halos are a distant second. Elve occurrence rate jumps as the sea surface temperature exceeds 26 degrees Celsius, manifesting an ocean‐atmosphere‐ionosphere coupling. In the ISUAL survey, elves concentrate over the Caribbean Sea, South China Sea, east Indian Ocean, central Pacific Ocean, west Atlantic Ocean, and southwest Pacific Ocean; while sprites congregate over central Africa, Japan Sea, and west Atlantic Ocean. The ISUAL experiment observed global rates of 3.23, 0.50, 0.39, and 0.01 events per minute for elves, sprites, halos, and gigantic jets, respectively. Taking the instrumental detection sensitivity and the restricted survey area into account, the corrected global occurrence rates for sprites and elves likely are a factor of two and an order of magnitude higher, respectively. ISUAL observations also indicate that the relative frequency of high peak current lightning (>80 kA) is 10 times higher over the oceans than over the land. On the basis of the corrected ISUAL elve global occurrence rate, the total electron content at the lower ionosphere above elve hot zones was computed to be elevated by more than 5%.

Broadband very low frequency measurement of D region ionospheric perturbations caused by lightning electromagnetic pulses

Prompt early/fast perturbations on narrowband sub‐ionospherically propagating very low frequency (VLF) signals are the primary evidence for the direct coupling of energy released by lightning discharge to the lower ionosphere. Different mechanisms have been advanced to explain the fast ionospheric perturbations, such as heating and ionization from the lightning electromagnetic pulse (EMP) associated with elves or from quasielectrostatic fields associated with sprites and halos. By comparing the broadband VLF spectra (3–25 kHz) of lightning discharges that shortly followed high peak current lightning discharges with the spectra of lightning discharges that did not, we detect D region perturbations caused by these intense lightning strokes over the U.S. East Coast and the U.S. High Plains. The electron density changes are measured by analyzing the broadband VLF propagation changes, and the perturbed electron density profiles from both regions are found to be consistent with those theoretically predicted for strong lightning EMP. In one case, a D region perturbation was detected following a lightning stroke that produced an isolated elve recorded by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) instrument on the FORMOSAT‐2 satellite, confirming the EMP origins of these ionospheric perturbations. For this case, we measure electron density enhancements of 460 cm−3 averaged over a 220‐km radius and 10‐km‐high perturbation region, in good agreement with the 210 cm−3 measured optically by Mende et al. (2005) for a different elve event. The characteristics of the lightning responsible for these ionospheric perturbations are investigated by comparing high peak current lighting strokes that do and do not generate detectable ionospheric perturbations.

Early VLF perturbations observed in association with elves

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.

D region ionization by lightning‐induced electromagnetic pulses

The electromagnetic pulses (EMP) from tropospheric lightning produce transient luminous events (TLEs), known as elves, in the 80–90 km region above the lightning. The luminosity is evidence that the EMP carries sufficient electric field to excite optical emissions at these altitudes; however, it is still unknown whether the field is sufficient to ionize the atmosphere. The first multiwavelength, quantitative observatory on a free‐flying satellite dedicated to observing TLEs, the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) instrument on FORMOSAT‐2, formerly called ROCSAT‐2, confirmed that a significant number of lightning events are accompanied by elves. The instrument consists of a low light level imager and a set of multichannel photometers. In a few cases where the lightning occurred beyond the solid Earth limb, pure spectral measurements of the elves were obtained. Here we analyze such an event and show that the elves contained significant 391.4 nm emission of the N2+ ion. This is clear evidence that ionization takes place in elves. The ratio of cross sections for N2 ionization and the production of the upper state of 391.4 nm emission is not a constant for low‐energy electrons found in TLEs. We attempted to find this ratio by comparing our photometric measurements of the TLE produced emissions to theoretically derived emission intensities. The electron energy distribution and the ratios of the modeled N2 Lyman‐Birge‐Hopfield (LBH) and the N2+ first negative to the second positive were computed as a function of the reduced electric field. From these ratios it was possible to obtain the reduced electric field from the ratios. We estimated that the reduced electric field, which characterizes the local electron energy distribution, was >200 Td. We also made comparisons of the theoretically derived intensities to our measurements of the N2+ first positive and Lyman‐Birge‐Hopfield (LBH) band emission in the elves. On the basis of the ratio between the N2+ first negative emission and the time‐integrated ionization production, we estimate that the elves produced an average electron density of 210 electrons cm−3 over a large (165 km diameter) circular region having an assumed 10 km altitude extent. These observations indicate that thunderstorms are a significant source of ionization in the low‐ to midlatitude nighttime D region.

ANALYZING ISUAL SPECTROPHOTOMETER DATA USING A TWO-COLOR DIAGRAM METHOD

Transient luminous events (TLEs; sprites, elves, jets and etc.) are lightning-related optical flashes occurring above thunderstorms. Since the first discovery of sprites in 1989, scientists have learned a great deal about the morphological, spectroscopic and electromagnetic characteristics of TLEs through ground and spacecraft campaigns. However, most of the TLE studies were based on events recorded over US High Plains. To elucidate the possible biasing effects, space-borne observations are needed and have their merits. Imager of sprites and Upper Atmospheric Lightning (ISUAL) on the FORMOSAT-2 satellite is the first instrument to carry out a true global measurement of TLEs from a low- earth orbit. In this short paper, we apply a common astronomical data analysis technique, two-color diagram, on the ISUAL spectrophotometer (SP) data. By choosing appropriated bandpasses and converting the measured flux of TLEs into the unit of magnitude, two-color diagrams of TLEs can be constructed. We demonstrate that two-color diagrams, which were constructed from the narrow-band spectrophotometer data, can be used to classify different types of TLEs and trace their temporal evolution. The amount of reddening due to Earth's atmosphere can also be estimated from two-color diagrams assembled from the broad-band spectrophotometer data.