On the Relationship Between Coronal Magnetic Decay Index and CME Speed

TL;DR

This study investigates the correlation between the coronal magnetic decay index and CME speed, finding that the decay index increases with CME speed below 1000 km/s and stabilizes at higher speeds.

Contribution

It advances understanding by quantifying the relationship between decay index and CME speed using observational data and potential field extrapolation.

Findings

Decay index increases with CME speed below 1000 km/s

Decay index remains around 2.2 for CMEs faster than 1000 km/s

Case study shows different decay index values above erupted and non-erupted parts

Abstract

Numerical simulations suggest that kink and torus instabilities are two potential contributors to the initiation and prorogation of eruptive events. A magnetic parameter named decay index (i.e., the coronal magnetic gradient of the overlying fields above the eruptive flux ropes) could play an important role in controlling kinematics of eruptions. Previous studies have identified a threshold range of the decay index that distinguishes between eruptive and confined configurations. Here we advance the study by investigating if there is a clear correlation between the decay index and CME speed. 38 CMEs associated with filament eruptions and/or two-ribbon flares are selected using the Halpha data from the Global Halpha Network. The filaments and flare ribbons observed in Halpha associated with the CMEs help to locate the magnetic polarity inversion line, along which the decay index is calculated based on the potential field extrapolation using MDI magnetograms as boundary conditions. The speeds of CMEs are obtained from the LASCO C2 CME catalog available online. We find that the mean decay index increases with CME speed for those CMEs with a speed below 1000 km/s, and stays flat around 2.2 for the CMEs with higher speeds. In addition, we present a case study of a partial filament eruption, in which the decay indexes show different values above the erupted/non-erupted part.