Global Forces in Eruptive Solar Flares: The Lorentz force Acting on the Solar Atmosphere and the Solar Interior

TL;DR

This paper analyzes the Lorentz force changes during large eruptive solar flares, deriving a more accurate model of force perturbations in the solar interior and atmosphere, and explores their observational and physical implications.

Contribution

It generalizes previous Lorentz force change expressions to include horizontal components and provides a more precise vertical component, linking magnetic eruptions to force changes and plasma ejection limits.

Findings

Magnetic eruptions make the photospheric magnetic field more horizontal.

Downward force changes act on the photosphere and interior during flares.

An upper limit on erupting plasma mass is derived from Lorentz-force considerations.

Abstract

We compute the change in the Lorentz force integrated over the outer solar atmosphere implied by observed changes in vector magnetograms that occur during large, eruptive solar flares. This force perturbation should be balanced by an equal and opposite force perturbation acting on the solar photosphere and solar interior. The resulting expression for the estimated force change in the solar interior generalizes the earlier expression presented by Hudson, Fisher and Welsch (CS-383, ASP, 221, 2008), providing horizontal as well as vertical force components, and provides a more accurate result for the vertical component of the perturbed force. We show that magnetic eruptions should result in the magnetic field at the photosphere becoming more horizontal, and hence should result in a downward (towards the solar interior) force change acting on the photosphere and solar interior, as recently argued from an analysis of magnetogram data by Wang and Liu (Astrophys. J. Lett. 716, L195, 2010). We suggest the existence of an observational relationship between the force change computed from changes in the vector magnetograms, the outward momentum carried by the ejecta from the flare, and the properties of the helioseismic disturbance driven by the downward force change. We use the impulse driven by the Lorentz-force change in the outer solar atmosphere to derive an upper limit to the mass of erupting plasma that can escape from the Sun. Finally, we compare the expected Lorentz-force change at the photosphere with simple estimates from flare-driven gasdynamic disturbances and from an estimate of the perturbed pressure from radiative backwarming of the photosphere in flaring conditions.