elves, which were discovered in the years immediately preceding the first TGF observations. For instance, that field may be due to the separation of charges in a thundercloud ("DC" field) often associated with sprites, or due to the electromagnetic pulse (EMP) produced by a lightning discharge, often associated with elves. There is also some evidence that certain TGFs occur in the absence of lightning strikes, though in the vicinity of general lightning activity, which has evoked comparisons to blue jets. The DC field model requires a very large thundercloud charge to create sufficient fields at high altitudes (e.g. 50–90 km, where sprites form). Unlike the case of sprites, these large charges do not seem to be associated with TGF-generating lightning. Thus the DC field model requires the TGF to occur lower down, at the top of the thundercloud (10–20 km) where a local field can be stronger. This hypothesis is supported by two independent observations. First, the spectrum of the gamma-rays seen by RHESSI matches very well to the prediction of relativistic runaway at 15–20 km. Second, TGFs are strongly concentrated around Earth's equator when compared to lightning. (They may also be concentrated over water compared to lightning in general.) Thundercloud tops are higher near the equator, and thus the gamma-rays from TGFs produced there have a better chance of escaping the atmosphere. The implication would then be that there are many lower-altitude TGFs not seen from space, particularly at higher latitudes. An alternative hypothesis, the EMP model, relaxes the requirement on thundercloud charge but instead requires a large current pulse moving at very high speed. The required current pulse speed is very restrictive, and there is not yet any direct observational support for this model. Another hypothetical mechanism is that TGFs are produced within the thundercloud itself, either in the strong electric fields near the lightning channel or in the static fields that exist over large volumes of the cloud. These mechanisms rely on extreme activity of the lightning channel to start the process (Carlson et al. 2010) or on strong feedback to allow even small-scale random events to trigger production. The Atmosphere-Space Interactions Monitor (ASIM), dedicated to measuring simultaneously optical signals of lightning and signals of terrestrial gamma-ray flashes, revealed that TGFs are usually associated with optical flashes, strongly suggesting that relativistic electrons as precursors of TGFs are produced in the strong electric fields in the proximity of