[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.

[1] We report the ionization emission associated with N 2 + 1N band in a halo event without visible sprite streamers. To avoid the lightning contamination to the ionization emission, we find halos whose parent lightning light is blocked by the Earth's limb. , and 1.6 × 10 21 photons, respectively. In the halo region, the electron density increased as 1-2 orders of magnitude higher than ambient electron density. From the emission ratio of N 2 + 1N to N 2 2P, the reduced electric field is estimated to be 275-325 Td that is higher than the conventional breakdown electric field. The recorded electric field related to this halo event is produced by a lightning discharge that has a total charge moment change of À1450 C km. Based on the estimated electric field from optical emissions, it is found that the lightning-induced electric field in the bright halo region is significantly relaxed with a rate faster than that estimated using ambient electron density, in agreement with previous modeling results showing that the electron density enhancement due to the ionization processes leads to a short dielectric relaxation time inside the halo region.

[1] We report the 762 nm emissions in sprites recorded by the ISUAL experiment onboard the FORMOSAT-2 satellite. The 762 nm imager filter is centered at 763.3 nm with a 7 nm bandwidth at 50% transmittance. Sprite emissions in this passband include the N 2 first positive (1PN 2 ) bands, (2, 0) and (3, 1), the O 2 atmospheric (atm) band (0, 0), and the hydroxyl (4, 0) emissions. Because these mixed emissions cannot be resolved in the 762 nm narrowband filter, a zero-dimensional plasma chemistry model is used to estimate the expected relative intensities of these emission bands in sprites. The computed 1PN 2 brightness in a single streamer is 1.4 MR and 2.6 kR for the O 2 atm band emissions at frame integration times of 30 ms. In the 762 nm passband, the 1PN 2 emissions are the dominant emissions in sprites, and the ratio of 1PN 2 to O 2 atmospheric emissions is ∼500, while the hydroxyl emissions can be neglected. In this ISUAL 762 nm campaign, the brightest sprite out of the four recorded events has possible O 2 atm band emissions that lasted more than 90 ms, and its observed brightness is consistent with the model prediction. Even though the lightning 762 nm emissions are strongly absorbed by O 2 below 60 km, the ISUAL observed parent lightning emissions in this passband are still more than a factor of two brighter than those from ISUAL observed sprites. Hence for spacecraft nadir TLE detection missions, 762 nm bands may not be used as the sole signature to identify sprites, and auxiliary emission bands are needed.

[1] We analyzed the ISUAL-FORMOSAT2 elves, the LIS-TRMM lightning, the sea surface temperature (SST), and the El Niño southern oscillation (ENSO) indices (the Niño 3.4 Index and the Southern Oscillation Index -SOI) in the period between June 2005 and May 2010 to explore the impacts of ENSO on the occurrences of the mesospheric elves and the troposphere lightning. The standardized anomalies of the elve and the lightning occurrence densities are used to quantify the deviation of the elve and lightning occurrences during an ENSO event. The areas in the ENSO-sensitive western Pacific, central Pacific and Tahiti regions with a significant event anomaly are taken to be the impact indicators of ENSO. Also the SOI is used to examine the correlation of the temporal intensity variation between ENSO, elve and lightning. The results indicate that elve shows clear responses to ENSO with a correlation over 0.6 in the coastal and the oceanic regions. The lightning occurrence is responsive to ENSO in the oceanic regions, but shows a low correlation in the coastal regions, due the overwhelming influence of the landmass. Therefore, between elve and lightning as proximity indices of ENSO, elve has a broader applicable geographic range. However, elve is known to be the mesospheric luminous manifestations of the high-peak-current lightning, and the response of the intense lightning to ENSO would be similar to that of the elve. Hence, alternatively, the intense lightning can also be used as an indicator of ENSO.

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