Deciphering The Slow-rise Precursor of a Major Coronal Mass Ejection

TL;DR

This study investigates the early evolution of a coronal mass ejection by analyzing a long-duration precursor event, revealing the role of precursor reconnection and magnetic tension in the slow rise of the magnetic flux rope leading to eruption.

Contribution

It provides detailed observations and a 3D MHD simulation showing how precursor reconnection and magnetic tension drive the slow rise of the flux rope before eruption.

Findings

Precursor reconnection occurs above cusp-shaped loops, heating and slowly rising the flux rope.

The flux rope initially forms with an 'M' shape and separates from the loops.

Magnetic tension drives the slow rise, transitioning to magnetic pressure for rapid eruption.

Abstract

Coronal mass ejections (CMEs) are explosive plasma phenomena prevalently occurring on the Sun and probably on other magnetically active stars. However, how their pre-eruptive configuration evolves toward the main explosion remains elusive. Here, based on comprehensive observations of a long-duration precursor in an event on 2012 March 13, we determine that the heating and slow rise of the pre-eruptive hot magnetic flux rope (MFR) are achieved through a precursor reconnection located above cusp-shaped high-temperature precursor loops. It is observed that the hot MFR threads are built up continually with their middle initially showing an "M" shape and then being separated from the cusp of precursor loops, causing the slow rise of the entire MFR. The slow rise in combination with thermal-dominated hard X-ray source concentrated at the top of the precursor loops shows that the precursor reconnection is much weaker than the flare reconnection of the main eruption. We also perform a three-dimensional magnetohydrodynamics simulation that reproduces the early evolution of the MFR transiting from the slow to fast rise. It is also disclosed that it is the magnetic tension force pertinent to "M"-shaped threads that drives the slow rise, which, however, evolves into a magnetic pressure gradient dominated regime responsible for the rapid-acceleration eruption.