Abstract
Physical background of the evolution of a coronal magnetic flux rope embedded in the magnetic arcade during the gradual-rise pre-eruptive stage is studied. It is assumed that this stage represents an externyly-driven evolution of the preeruptive structure through a series of quasi-equilibrium states, until a point when system losses equilibrium and erupts. In particular, three driving processes are considered: twisting motions of the flux-rope footpoints, emergence of new magnetic flux beneath the flux rope, and the mass leakage down the flux-rope legs. For that purpose, an analytical flux-rope model is employed, to inspect how fast the equilibrium height of the structure rises due to the increase of the poloidal-toaxial field ratio, the increase of axial electric current, and the decrease of mass. It is shown that the flux-rope twisting itself is not sufficient to reproduce the rising speeds observed during the pre-eruptive stage. Yet, it is essential for the loss-ofequilibrium process. On the other hand, the considered emerging flux and the mass loss processes reproduce well the rate at which the pre-eruptive structure rises before the main acceleration stage of the eruption sets in.
Highlights
There is a general consensus that solar eruptions, which lead to coronal mass ejections (CMEs) and are frequently associated with solar flares, are a consequence of instability of coronal structures, most often considered to be coronal magnetic arcades embedding a helically twisted magnetic flux rope
I and M represent the axial electric current and total mass within the flux rope, respectively, Bc is the background arcade field and μ0 stands for the permeability
Note that from the point of view of Equation (1), the mass loss related to the material draining down the prominence legs is equivalent to the increase of axial electric current I related to emerging flux and/or twisting processes, since the mass loss increases the value of CL [see Equation (2)]
Summary
There is a general consensus that solar eruptions, which lead to coronal mass ejections (CMEs) and are frequently associated with solar flares, are a consequence of instability of coronal structures, most often considered to be coronal magnetic arcades embedding a helically twisted magnetic flux rope (for a review see, e.g., Schmieder et al, 2015; Green et al, 2018). There are three different scenarios that can explain the presence of the flux rope within the eruptive structure: (i) an already-formed flux rope emerged from the subphotospheric layers; (ii) rope is formed gradually by a series of reconnections within a sheared arcade; (iii) it forms during the eruption itself (e.g., Green et al, 2018, and references therein). After that the system finds itself in an instability regime, dynamically erupting in trying to find a new equilibrium state Such type of evolution is usually denoted as loss-of-equilibrium scenario. In this paper we analyze the influence of these processes on the evolution of the pre-eruptive system, focusing on the effects of the flux rope twisting, electric current increase, and the mass loss. This paper is focused on a quantitative study of characteristics of the pre-acceleration stage
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