Atmospheric positron flux modulation during thunderstorms

We have discovered a new phenomenon in high-energy atmospheric physics involving an increased fraction of ground-level positrons during a thunderstorm. This increase is linked to developing a lower positively charged region (LPCR) at the thundercloud bottom. When the electric field strength in the lower atmosphere exceeds a critical value, seed electrons from cosmic rays accelerate and multiply, creating relativistic runaway avalanches of millions of electrons and gamma rays with energies up to 50 MeV. The emerging LPCR changes the polarity of the electric field nearly to the Earth's surface, causing electrons to decelerate and positrons to accelerate, increasing the flux of 511 keV ``annihilation'' gamma rays. By measuring these gamma rays, we can reveal the formation of the LPCR and follow the dynamics of the charge structure in the lower part of the thundercloud.