Its source could be the nuclear reactions that occur in the atmosphere and the incentive to spread the rays.
The explosions of gamma rays, which occur after the microseconds of the rays, were already known. The rays have millions of volts and create large electric fields in the sky. The fields accelerate electrons, which collide strongly against atoms and release gamma rays.
Now, however, they have found rays that come more backward. This delay can be a few minutes or a few hours, which seems to indicate that they come from the nuclear reactions of the atmosphere. However, they do not know how these reactions occur. Many believe that protons, accelerated as electrons and colliding with common elements in the atmosphere, can cause chain reactions.
In any case, it is not clear that the rays have enough energy to put protons in motion, nor what atoms produce gamma rays.