Electron Inertia and Magnetic Reconnection

Abstract: The finite electron mass can cause magnetic reconnection even in the absence of any other non-ideal effect in a magnetic evolution. It will be shown that when electron inertia is the only non-ideal effect in the evolution of the magnetic field $\vec{B}$, there is a related field that evolves ideally. This field is $$\vec{\mathcal{B}} \equiv \vec{B} + \vec{\nabla}\times \left( (m_e/n e²⁾ \vec{j} \right)$$ with $m_e$ the electron mass, $n$ the electron number density, and $\vec{j}$ the current density. Although the magnetic field is modified from its ideal evolution form by the electron inertia, the effect on particle trajectories, even electron trajectories, is small unless the current lies in thin sheets, which make $\vec{j}$ extremely large. The field $\vec{\mathcal{B}}$ is closely related to Voigt normalized magnetic field, which is defined by a Laplacian smoothing of $\vec{B}$. The difference between $\vec{\mathcal{B}}$ and $\vec{B}$ involves the relativistically invariant four-space Laplacian acting on $\vec{B}$ with a $c/\omega_{pe}$ smoothing distance; $\omega_{pe}$ is the plasma frequency.