On flux rope stability and atmospheric stratification in models of coronal mass ejections triggered by flux emergence

US Naval Research Laboratory, Washington DC 20375, USA

Received: 28 February 2014
Accepted: 14 August 2014

Abstract

Context. Flux emergence is widely recognized as playing an important role in the initiation of coronal mass ejections. The Chen & Shibata model (Chen & Shibata 2000, ApJ, 545, 524), which addresses the connection between emerging flux and flux rope eruptions, can be implemented numerically to study how emerging flux through the photosphere can affect the eruption of a pre-existing coronal flux rope.

Aims. The model’s sensitivity to the initial conditions and reconnection microphysics is investigated with a parameter study. In particular, we aim to understand the stability of the coronal flux rope in the context of X-point collapse, as well as the effects of boundary driving in both unstratified and stratified atmospheres.

Methods. A modified version of the Chen & Shibata model is implemented in a code with high numerical accuracy with different combinations of initial parameters governing the magnetic equilibrium and gravitational stratification of the atmosphere. In the absence of driving, we assess the behavior of waves in the vicinity of the X-point. With boundary driving applied, we study the effects of reconnection microphysics and atmospheric stratification on the eruption.

Results. We find that the Chen & Shibata equilibrium can be unstable to an X-point collapse even in the absence of driving due to wave accumulation at the X-point. However, the equilibrium can be stabilized by reducing the compressibility of the plasma, which allows small-amplitude waves to pass through the X-point without accumulation. Simulations with the photospheric boundary driving evaluate the impact of reconnection microphysics and atmospheric stratification on the resulting dynamics: we show the evolution of the system to be determined primarily by the structure of the global magnetic fields with little sensitivity to the microphysics of magnetic reconnection; and in a stratified atmosphere, we identify a novel mechanism for producing quasi-periodic behavior at the reconnection site behind a rising flux rope as a possible explanation of similar phenomena observed in solar and stellar flares.