On the Mechanical Energy Available to Drive Solar Flares

TL;DR

This paper investigates the sources of mechanical energy in the solar convection zone that drive solar flares, comparing different models and concluding that emerging flux tubes can supply enough energy for even the largest flares.

Contribution

It provides a comparative analysis of three flare energy models, emphasizing the sufficiency of emerging flux in powering major solar flares.

Findings

Photospheric dynamo models can only power minor flares.

Coronal energy storage may power moderate flares but has coupling limitations.

Emerging flux models can supply enough energy for the largest flares.

Abstract

Where does solar flare energy come from? More specifically, assuming that the ultimate source of flare energy is mechanical energy in the convection zone, how is this translated into energy dissipated or stored in the corona? This question appears to have been given relatively little thought, as attention has been focussed predominantly on mechanisms for the rapid dissipation of coronal magnetic energy by way of MHD instabilities and plasma micro instabilities. We consider three types of flare theory: the steady state "photospheric dynamo" model in which flare power represents coronal dissipation of currents generated simultaneously by sub-photospheric flows; the "magnetic energy storage" model where sub-photospheric flows again induce coronal currents but which in this case are built up over a longer period before being released suddenly; and "emerging flux" models, in which new magnetic flux rising to the photosphere already contains free energy, and does not require subsequent stressing by photospheric motions. We conclude that photospheric dynamos can power only very minor flares; that coronal energy storage can in principle meet the requirements of a major flare, although perhaps not the very largest flares, but that difficulties in coupling efficiently to the energy source may limit this mechanism to moderate sized flares; and that emerging magnetic flux tubes, generated in the solar interior, can carry sufficient free energy to power even the largest flares ever observed.