Confining the angular distribution of TGF emission

We present the first constraints on the TGF emission cone, based on accurately geolocated TGFs. The distribution of the observation angles for 106 TGFs are compared to Monte Carlo simulations. We find that TGF emissions within an isotropic half angle >;30° can generate the observations. In addition, 36 events are used for spectral analysis. The result is a significant softening of the TGF energy spectrum for large observation angles, which is consistent with a TGF emission half angle <;40°. The constraints 30°-40° indicates that the electrons which produce TGFs are accelerated in nearly vertical electric fields.