How simulated fluence of photons from terrestrial gamma ray flashes at aircraft and balloon altitudes depends on initial parameters

Abstract

[1] Up to a few years ago, terrestrial gamma ray flashes (TGFs) were only observed by spaceborne instruments. The aircraft campaign ADELE was able to observe one TGF, and more attempts on aircraft observations are planned. There is also a planned campaign with stratospheric balloons, COBRAT. In this context an important question that arises is what count rates we can expect and how these estimates are affected by the initial properties of the TGFs. Based on simulations of photon propagation in air we find the photon fluence at different observation points at aircraft and balloon altitudes. The observed fluence is highly affected by the initial parameters of the simulated TGFs. One of the most important parameters is the number of initial photons in a TGF. In this paper, we give a semi-analytical approach to find the initial number of photons with an order of magnitude accuracy. The resulting number varies over several orders of magnitude, depending mostly on the production altitude of the TGF. The initial production altitude is also one of the main parameters in the simulations. Given the same number of initial photons, the fluence at aircraft and balloon altitude from a TGF produced at 10 km altitude is 2-3 orders of magnitude smaller then a TGF originating from 20 km altitude. Other important parameters are altitude distribution, angular distribution and amount of feedback. The differences in altitude, altitude distribution and amount of feedback are especially important for the fluence of photons observed at altitudes less than 20 km, and for instruments with a low-energy threshold larger than 100 keV. We find that the maximum radius of observation in 14 km for a TGF with the intensity of an average RHESSI TGF is smaller than the results reported by Smith et al. (2011), and our results support the conclusion in Gjesteland et al. (2012) and Østgaard et al. (2012) that TGFs probably are a more common phenomenon than previously reported.Citation: Hansen, R. S., N. Østgaard, T. Gjesteland, and B. Carlson (2013), How simulated fluence of photons from terrestrial gamma ray flashes at aircraft and balloon altitudes depends on initial parameters,

Characterizing the source properties of terrestrial gamma ray flashes

“…If instead, we used 10 keV, we would expect approximately 4 × 10 17 gamma rays, in approximate agreement with the Xu et al result. In summary, our work is in very good agreement with results from Geant3 and SWORD/Geant4 and in approximate agreement with the Xu et al [2012] results, although not enough information was provided in that paper to do a precise comparison. On the other hand, our results disagree with the Carlson et al [2007], the Hansen et al [2013], and the Gjesteland et al [2015] work, being an order of magnitude larger than the first and about an order of magnitude smaller than the latter two. Such differences will have a significant impact on the predicted currents and optical emissions and so need to be addressed.…”

An altitude and distance correction to the source fluence distribution of TGFs

“…With an angular distribution of around 40°, we have adjusted for the angular dependence by using C for α < 40 and 2* C for α > 40. To get the numbers of 10 17 and 10 18 as given in literature [ Hansen et al , 2013; Dwyer et al , 2012], the value is of the order of 10 4 .…”

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

“…Both Østgaard et al [] and Hansen et al [] suggest that all lightning could produce TGFs if they are faint enough not to be observed from space, and of course that is possible; and as far as space observations are concerned, it must also be possible that even bright TGFs could be associated with all lightning if most of them are hidden from observation from space by being either very low in the atmosphere or beamed downward instead of upward. As we discuss limits on the number of lightning flashes that can producing TGFs based on what is seen from orbit, we will have to consider two possibilities: that there is a very large population of very faint TGFs and that there is a population of brighter TGFs (whether or not they are as bright as those seen from orbit) buried deep in the atmosphere.…”

“…Østgaard et al [2012] combined ADELE's detection rate (1/1213 flashes within 10 km and 0/133 flashes within 4 km) with a cutoff power law distribution derived from spacecraft data (see below) to suggest that approximately 2% of lightning produces a TGF somewhere in the distribution, noting also that a distribution that flattens out at low luminosity could give a TGF yield of up to 100% of lightning with TGFs occurring down to about 10 12 relativistic electrons. Hansen et al [2013] performed a second set of simulations that gave a different estimate for ADELE's sensitivity, suggesting that those limits should be weaker by about an order of magnitude, but even under that assumption they represent significant limits on TGF production at low altitude that could not be obtained from space. The distribution of TGF intensities observed from space was suggested by Collier et al [2011] to be qualitatively consistent with a power law, based on both the observed intensity distribution and the decrease of maximum observed intensity with the distance along Earth's surface between the subsatellite point on the Earth and the TGF source position.…”