What's new here is the direct observation of a spectral feature--the well-known 511 keV line--which corresponds to annihilation of the positrons produced by the

What's new here is the direct observation of a spectral feature--the well-known 511 keV line--which corresponds to annihilation of the positrons produced by the TGF with electrons. Gamma-rays and electrons had been observed before, but it wasn't known that positrons were produced as well. Compton didn't observe it because its energy resolution was too coarse, and RHESSI (another satellite which has detected many TGFs) doesn't have enough collecting area to do spectroscopy. There are current ground and airborne experiments looking for high energy emission from lightning. (I'm a gamma-ray astrophysicist who works with these satellites on other sources.)