Electron magnetohydrodynamic simulations of magnetic island coalescence

Abstract

The role of electrons in driving magnetic island coalescence in the electron magnetohydrodynamic limit is investigated. In particular, the dependence of the coalescence time tc(f) (defined as the time required for a fixed fraction f of the initial island magnetic flux to reconnect) on the resistivity η for constant Lundquist number Sλ (defined in terms of the island wavelength λ) is studied. Two distinct island coalescence regimes are observed: (1) a “resistive diffusion regime” where tc(f)∝tw/η (tw≡(λ2/Ωcidi2), where Ωci is the ion cyclotron frequency, and di is the ion inertial length), and (2) an “electron driven regime” where tc(f)∝tw. Defining the “ion Lundquist number,” Si≡Sλdi/λ, the resistive diffusion regime is observed when Si≲50, while the electron driven regime is observed when Si≳50. In the electron driven regime, the coalescence time is insensitive to Si over the range 50≲Si≲250.