Conditions for fast magnetic reconnection in astrophysical plasmas

We investigate favourable circumstances for fast magnetic reconnection in astrophysical plasmas based on recent results by Rogers et al. (2001). Given that a critical magnetic field structure with antiparallel field lines exists, our analysis demonstrates that a sufficient condition for fast reconnection is that the ratio of the thermal pressure to the magnetic field pressure $\beta$ should be significantly larger than $2 m_e/m_p$ (twice the ratio of electron mass to proton mass). Using several examples (like the different components of the interstellar medium, the intergalactic medium, active galactic nuclei and jets) we show that in almost all astrophysical plasmas, magnetic reconnection proceeds fast i.e. independent of the resistivity, with a few percent of the Alfv{\'e}n speed. Only for special cases like neutron stars and white dwarfs is $\beta$ smaller than $2 m_e/ m_p$.