The true fluence distribution of terrestrial gamma flashes at satellite altitude

Abstract

[1] In this paper we use the fluence distributions observed by two different instruments, RHESSI and Fermi GBM, corrected for the effects of their different orbits, combined with their different daily TGF detection rates and their relative sensitivities to make an estimate of the true fluence distribution of TGFs as measured at satellite altitudes. The estimate is then used to calculate the dead-time loss for an average TGF measured by RHESSI. An independent estimate of RHESSI dead-time loss and true fluence distribution is obtained from a Monte Carlo (MC) simulation in order to evaluate the consistency of our results. The two methods give RHESSI dead-time losses of 24-26% for average fluence of 33-35 counts. Assuming a sharp cut-off the true TGF fluence distribution is found to follow a power law with l = 2.3 AE 0.2 down to $5/600 of the detection threshold of RHESSI. This corresponds to a lowest number of electrons produced in a TGF of $10 14 and a global production rate within AE38 latitude of 50000 TGFs/day or about 35 TGFs every minute, which is 2% of all IC lightning. If a more realistic distribution with a roll-off below 1/3 (or higher) of the RHESSI lower detection threshold with a true distribution with l ≤ 1.7 that corresponds to a source distribution with l ≤ 1.3 is considered, we can not rule out that all discharges produce TGFs. In that case the lowest number of total electrons produced in a TGF is $10 12 .

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

“…Although the distribution in Figure 9 is quite similar to what was found by Østgaard et al [2012] and Tierney et al [2013], a direct comparison cannot be made for the following reasons: The TGFs used in Østgaard et al [2012] are only the TGFs observed by RHESSI before the degradation of the instrument in 2005–2006, while we use the TGFs measurements made up until to 2011. As the strong TGFs from after 2005 are mixed with the weaker TGFs from before 2005, this causes a softer distribution which becomes apparent when using all our TGFs.…”

“…The figure shows that a relatively larger number of the second catalog TGFs originates from distances between 400 and 500 km from satellite foot point than the TGFs in the first catalog. The catalog 2 TGFs that are seen to come from close distances might either be in the lower end of a power law distribution as indicated by Østgaard et al [2012] or be equally strong TGFs originating from further below the tropopause.…”

Étranges phénomènes lumineux et bouffées de rayons gamma observés au-dessus des orages

“…En effet, on estime que les TGF produits sur l'ensemble de la planète et potentiellement détectables par l'instrument Gamma ray Burst Monitor (GBM) à bord de Fermi, sont au nombre de 400 000 par an [14]. Il s'agit là d'un minimum car les instruments opération-nels actuellement ne sont pas optimisés pour la détection des TGF, et il n'est pas impossible qu'en fait, tous les éclairs intranuages produisent des TGF [15]. Dans ce contexte, il est intéressant de noter qu'on estime que 30 éclairs intranuages se déclenchent par seconde à la surface du globe, soit environ 3 millions par jour.…”

TARANIS XGRE and IDEE detection capability of terrestrial gamma-ray flashes and associated electron beams

“…2a. As indicated in Ostgaard et al (2012), RHESSI has an effective area for detecting TGF σ TGF R of about 256 cm 2 . The Astro-Rivelatore Gamma a Immagini Leggero (AG-ILE) is a satellite from the Italian Space Agency dedicated to the study of high-energy gamma rays (typically above 50 MeV) in the universe.…”

“…Gjesteland et al (2012) indicate that n min R = 11 (for the RHESSI second catalogue) and Ostgaard et al (2012) radial distance between the TGF source (projection at satellite altitude) and the satellite for photons (various altitudes) and electrons (seen at the hemisphere of production and in the opposed hemisphere). The two electron profiles correspond to a detection altitude of 560 km, but are very close for 490 or 700 km altitude.…”