Energetics of Coronal Mass Ejections: Role of the Streamer Cavity

Abstract

Coronal mass ejections (CMEs) are large-scale outbursts from the solar corona that propel typically 2 × 1016 g of material into interplanetary space at speeds of typically 450 km s-1. Energy is required not only to accelerate the ejected mass, but also to lift it against solar gravity and, additionally, to open the coronal magnetic field. That energy is widely believed to be magnetic. However, a considerable body of research shows that energy stored in force-free magnetic arcades is sufficient only to open the field. Thus a magnetic explanation for coronal mass ejections involving simple magnetic arcades must include cross-field currents and therefore thermal pressure gradients. But in such models, the addition of cross-field currents can result in a large negative gravitational energy whose magnitude exceeds that of the available magnetic energy. The model corona then becomes more tightly bound to the Sun and is thus a poor candidate for the pre-CME coronal state. This paper introduces more complicated coronal pressure and density profiles that model the observed structure of the coronal helmet streamers in which most CMEs originate. In particular, our model includes a relatively low-density polar corona, a higher density arcade corresponding to the streamer helmet, and a low-density cavity in the equatorial region underlying the helmet. Solution of the resulting nonlinear equation for the pre-CME corona shows that it is indeed possible to build up net positive energy in excess of that needed to propel and lift the associated excess mass. Assuming that force-free currents (which are not included here) store sufficient energy to open the magnetic field, our results provide viable models for energy storage in the pre-CME corona.