D region ionization by lightning‐induced electromagnetic pulses

Abstract

[1] 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 N 2 + ion. This is clear evidence that ionization takes place in elves. The ratio of cross sections for N 2 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 N 2 Lyman-Birge-Hopfield (LBH) and the N 2 + 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 N 2 + first positive and Lyman-Birge-Hopfield (LBH) band emission in the elves. On the basis of the ratio between the N 2 + first negative emission and the time-integrated ionization production, we estimate that the elves produced an average electron density of 210 electrons cm À3 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.

The 762 nm emissions of sprites

“…The ISUAL AP contains blue (370-450 nm) and red (530-650 nm) bands of multiple-anode photometers. Each band of AP has 16 vertically stacked photomultiplier tubes (PMTs) with a combined field of view (FOV) of 22°(H) × 3.6°(V) [Chern et al, 2003;Mende et al, 2005]. The ISUAL imager, SP and AP are coaligned at the center of their views.…”

“…The ISUAL payload globally surveys transient luminous events (TLEs) and other luminous emissions in the upper atmosphere with a side-looking view from the 891 km altitude orbit [Chern et al, 2003;Mende et al, 2005]. Using the satellite information recorded with the TLE events, the altitude and location of the events can be determined by assuming that the bright center of the observed accompanying lightning-illuminated cloud has a height of 10 km [Kuo et al, 2008].…”

Early VLF perturbations observed in association with elves

“…An overview of this event is given in the next section. The only other evidence on ionisation production during an elve event has recently been provided by Mende et al (2005), based on multiwavelength photometric observations on board the FORMOSAT-2 satellite.…”

“…There exist a few conference presentations (Reising et al, 1995;Takahashi et al, 1995;Dowden et al, 1995), as cited by Strangeways (1996) and Fukunishi et al (1996), but no refereed publications dedicated to the topic. Recently, Mende et al (2005) presented indirect evidence of electron density enhancements in relation to an elve event observed by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) instrument on board the Taiwanese FORMOSAT-2 (former ROCSAT-2) satellite. The average electron density was estimated to be 210 electrons cm −3 over a circular region with a diameter of ∼165 km at ∼90 km altitude having an assumed height extent of 10 km.…”

ISUAL Imager and far-ultraviolet spectrophotometer degradation

“…This discrepancy may come from the temperature changes between calibration and on orbit operations. This was mentioned by Mende et al (2005). Using the error propagation equation, the error bar of each data points is calculated using the following formula: (5) where t is the MET in units of days, and a is the fitting coefficient.…”