Thunderstorms can the thought of as being driven by one of nature's particle accelerators. The electric fields in thunderstorms can run up to at least 100 kV/m and maybe more, so getting potential drops well into the mega-volt range over relatively short distances is not too difficult: http://science.ksc.nasa.gov/amu/journals/jamc-2008.pdf
Electron-positron pairs require an energy of just over 1 MeV to be created (the energy of a single electron charge dropping through a potential of 1 MV). So there is more than enough energy kicking around to generate pairs fairly copiously (positrons are always produced with a companion electron in this kind of process, although they may be produced alone in certain types of radioactive decay.)
The interesting part of this work is it seems there was a large volume where there were a lot of positrons, and the specific mechanism capable of producing that situation is not known. Really interesting science in an extreme and transient environment!
Electron-positron pairs require an energy of just over 1 MeV to be created (the energy of a single electron charge dropping through a potential of 1 MV). So there is more than enough energy kicking around to generate pairs fairly copiously (positrons are always produced with a companion electron in this kind of process, although they may be produced alone in certain types of radioactive decay.)
The interesting part of this work is it seems there was a large volume where there were a lot of positrons, and the specific mechanism capable of producing that situation is not known. Really interesting science in an extreme and transient environment!