Evolution of the Toroidal Flux of CME Flux Ropes during Eruption

TL;DR

This study investigates how the toroidal magnetic flux of CME flux ropes evolves during eruptions, revealing a two-phase pattern of rapid increase and subsequent decrease, and correlating it with flare activity.

Contribution

It provides the first quantitative analysis of the toroidal flux evolution during CME eruptions using a novel identification method.

Findings

Toroidal flux shows a rapid increase followed by decrease during eruptions.

The flux evolution correlates with soft X-ray flux, with a slight delay.

Reconnection processes are inferred to build up and reduce the flux.

Abstract

Coronal mass ejections (CMEs) are large-scale explosions of the coronal magnetic field. It is believed that magnetic reconnection significantly builds up the core structure of CMEs, a magnetic flux rope, during the eruption. However, the quantitative evolution of the flux rope, particularly its toroidal flux, is still unclear. In this paper, we study the evolution of the toroidal flux of the CME flux rope for four events. The toroidal flux is estimated as the magnetic flux in the footpoint region of the flux rope, which is identified by a method that simultaneously takes the coronal dimming and the hook of the flare ribbon into account. We find that the toroidal flux of the CME flux rope for all four events shows a two-phase evolution: a rapid increasing phase followed by a decreasing phase. We further compare the evolution of the toroidal flux with that of the Geostationary Operational Environmental Satellites soft X-ray flux and find that they are basically synchronous in time, except that the peak of the former is somewhat delayed. The results suggest that the toroidal flux of the CME flux rope may be first quickly built up by the reconnection mainly taking place in the sheared overlying field and then reduced by the reconnection among the twisted field lines within the flux rope, as enlightened by a recent 3D magnetohydrodynamic simulation of CMEs.