The rarity of terrestrial gamma‐ray flashes: 2. RHESSI stacking analysis

Abstract

We searched for gamma‐ray emission from lightning using the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite by identifying times when RHESSI was near over 2 million lightning discharges localized by the Worldwide Lightning Location Network (WWLLN). We then stacked together the gamma‐ray arrival times relative to the sferic times, correcting for light propagation time to the satellite. The resulting stacked gamma‐ray time profile is sensitive to an average level of gamma‐ray emission per lightning discharge far lower than what can be recognized above background for a single terrestrial gamma‐ray flash (TGF). The summed signal from presumed small, previously unknown TGFs simultaneous with WWLLN discharges is remarkably weak: for the region from 0 to 300 km beneath RHESSI's footprint, (6.2 ± 3.8) × 10−3 detector counts/discharge are measured, as opposed to a typical range of 12–50 detector counts for TGFs identified solely from the gamma‐ray signal. Under the assumption of a broken power law differential distribution of TGF intensities, we find that the index must harden dramatically or cut off just below the sensitivity limit of current satellites and that for most scenarios less than 1% of lightning can produce a TGF that belongs anywhere in the same distribution as those that are observable. For the minority of scenarios where more than a few percent of flashes produce a TGF, most of these “TGFs” are less than 10−4 of the luminosity of the faintest RHESSI TGFs and therefore closer to the luminosity of lightning stepped leaders. The rarity of TGFs holds not only for TGFs simultaneous with the sferic observed by WWLLN but also for any time within 10 ms of the sferic, allowing (for example) for the possibility that different events within the upward propagation of a negative leader in positive intracloud lightning triggered the TGF and WWLLN's detection.

Observationally Weak TGFs in the RHESSI Data

“…This corresponds to 0.22 of the effective detection threshold 13.625 (Smith et al, , Appendix A). One should also note that we do not see any signs of a cutoff (nor a steep change) in the TGF distribution, as was presented as possible scenarios by Smith et al (). Summing up all net events from 3 to 20 counts we find ∼2,171 events.…”

“…The figure shows an excess of 13,554 counts on top of a mean background of 2.084 million counts from 6.38 million lightning flashes (note that the x ‐axis does not start at 0). Comparing this to Smith et al (), we see that we get a lower counts/flash ratio, ∼2.1·10 −3 compared to their 7.43·10 −3 . This can likely be attributed to the differences in search radius, as we use 900 km versus their 700 km.…”

“…In Figure we see a clear population of weak TGFs going all the way down to three counts in the detector (less than this is associated with some very high uncertainties). This corresponds to 0.22 of the effective detection threshold 13.625 (Smith et al, , Appendix A). One should also note that we do not see any signs of a cutoff (nor a steep change) in the TGF distribution, as was presented as possible scenarios by Smith et al ().…”

Characteristics of Radio Emissions Associated With Terrestrial Gamma‐Ray Flashes

“…On the other hand, RF pulses occurring within a longer time‐window of 20 ms of the TGF peak‐times were thought to be produced by “related IC discharges” (Connaughton et al, ) occurring in the same thunderstorm system as the TGF. Such pulses could probably also be directly associated with the TGFs (e.g., Smith et al, ), but the exact nature of the relationship remains unknown.…”

A Terrestrial Gamma‐Ray Flash inside the Eyewall of Hurricane Patricia

“…It is estimated that worldwide ~1,000 TGFs capable of being observed by satellite occur per day (Briggs et al, ). Although TGFs seem to occur in a small fraction of lightning (Smith et al, ), the radiation dose received from flying through a TGF beam aboard a passenger jet can present a significant hazard to both individuals (Dwyer et al, ) and avionics (Tavani et al, ) and is a motivation for the continued study of these high‐energy atmospheric phenomena.…”