ISUAL‐Observed Blue Luminous Events: The Associated Sferics

Abstract

The blue luminous events (BLEs) recorded by ISUAL (Imager of Sprites and Upper Atmospheric Lightning) radiate unambiguous middle ultraviolet to blue emissions (230–450 nm) but contain dim red emissions (623–754 nm). The BLE appears to be dot‐like on one ISUAL image with an integration time of 29 ms. A few BLEs develop upward into blue jets/starters or type II gigantic jets (GJs). The associated sferics of the BLEs in the extremely low frequency to very low frequency band and in the low‐frequency band exhibit similar patterns to the narrow bipolar events (NBEs) identified in the very low frequency and low‐frequency band. The ISUAL BLEs are conjectured to be the accompanied light emissions of the NBEs. Both upward and downward propagating current obtained from the associated sferics of the BLEs have been found. The source heights of the six BLEs related to negative NBEs are estimated in the range of 16.2–17.8 km. These six events are suggested to occur between the upper positive charge layer and the negative screen charge layer on the top of the normally electrified thunderstorm. The six blue starters, one blue jet, and one type II GJ are inferred to be positive upward discharges from their associated sferics in the extremely low frequency to very low frequency band. Based on the simultaneous radio and optical observations, a NBE is conjectured to be the initiation discharge with rapidly flowing current within the thunderstorm, while a blue jet/starter or a type II GJ is suggested to be the ensuing discharge with slowly varying current propagating upward from the thunderstorm.

On negative Sprites and the Polarity Paradox

“…The analysis showed that the timing uncertainty of the ISUAL lightning and other transient events can be greatly reduced to a deviation of 2.3 ms (Huang, ), comparable to the selection criteria used in Williams et al (). The existing studies of the ISUAL sprites and jets (Chang et al, ; Chou et al, ; Lee et al, ) have proved that a timing accuracy of 2.3 ms for ISUAL event indeed were achieved and the polarity of the parent lightning can be identified with minimal unambiguity.…”

Blue Optical Observations of Narrow Bipolar Events by ASIM Suggest Corona Streamer Activity in Thunderstorms

“…Having said the above, the fact that no 777.4 photometric signals are simultaneously detected with the recorded blue (337 nm) signal—or that the 337 and 777 nm cameras of MMIA show spatially separated structures—together with the derived altitudes of the discharge sources in the thunderclouds leads us to think that the recorded single‐pulse blue events are the optical manifestation of corona discharges like positive NBEs occurring at different depths inside thunderclouds in the 8.5–14 km range between the midlevel main negative and upper positive charge regions of thunderclouds (Bandara et al., 2019; Rison et al., 2016; Smith et al., 2004; Wu et al., 2011). Negative NBEs can occur at higher altitudes (16–20 km), between the upper positive charge cloud layer and the negative screening charge layer above the cloud where normal lightnings rarely occur (Smith et al., 2004; Wu et al., 2011), and might be connected to emerged blue starters (Chou et al., 2018; Liu et al., 2018) in regions with active deep convection.…”

A study of the context in which compact intracloud discharges occur

“…More recently, Chou et al . 48 observed 6 “blue luminous events” that were accompanied by upper-level CIDs, occurring between the main positive and screening negative charge regions, at heights ranging from 16 to 18 km above ground level. Similarly, Liu et al .…”