[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 .

This letter presents a new search algorithm for identifying Terrestrial Gamma ray Flashes (TGFs) in the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data. The algorithm has been applied to data from the period 2004–2006 and we have found more than twice as many TGFs as previously reported. The new TGFs follow the same geographical and seasonal variations as the previously reported TGFs. The match percentage between the new TGFs and World Wide Lightning Location Network (WWLLN) data is comparable to the RHESSI catalog TGFs. Our results shows that previous searches only identified the most intense events, and that there might be a large population of faint TGFs.

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During the Northern Hemisphere summer of 2003 a sprite observation campaign was conducted from Southern Europe and the magnetically conjugate region in South Africa. The campaign brought together a wide variety of instrumentation to investigate the effects of sprites on the mesosphere, and to search for signatures of the relativistic electron acceleration process in the magnetically conjugate hemisphere. Measurements in Europe included optical video imaging from a remote-controlled, semi-automatic camera system located at the Observatoire du Pic du Midi in the Pyre´ne´es mountains in Southern France, infrasound observations from France, and ULF-HF electromagnetic observations from a number of locations. The measurements in South Africa included observations of optical emissions and VLF electromagnetic waves. The campaign was successful, with more than 100 sprites observed during 10 storms. The paper will give an overview of the campaign and some results. They include: (1) the first clear identification of infrasound from sprites, allowing independent (of optical observations) estimates of the energy input to the mesosphere, (2) the first detection of sprites triggered by intra-cloud lightning, a result that underscores the need for considering the complete thunderstorm source field resulting from both cloud-to-ground discharges and the intra-cloud discharges feeding them, and (3) a clear one-to-one relationship with sprites and early VLF events, suggesting that VLF ground transmitter signals can be an important tool for estimating ionisation and relaxation of ionised structures in sprites. No signatures of relativistic electrons were identified during the campaign. r