Modeling Coronal States, Eruptions, and Acceleration

1. Modeling Coronal States, Eruptions, and Acceleration PI: Joachim Birn, Los Alamos National Laboratory CoI: T. G. Forbes, UNH; M. Hesse, NASA/GSFC Initial state Partial eruption Late state Stable flux rope Unstable flux rope

2. Modeling Coronal States, Eruptions, and Acceleration PI: Joachim Birn, Los Alamos National Laboratory Co-Is: T. G. Forbes, UNH, M. Hesse, NASA/GSFC A crucial problem in the study of coronal mass ejections (CMEs) and solar flares is the identification of initial configurations and boundary conditions that can produce an eruption of the field configuration. We used (ideal) magnetohydrodynamic (MHD) simulations to investigate the stability and dynamic evolution of two (approximate) equilibrium configurations. The initial models were derived within a, previously developed, general framework for the construction of series of suitable coronal states. They consist of twisted flux ropes, connected to the photosphere and anchored in the corona by an overlying arcade, embedded in a helmet streamer type configuration. The two models studied differ by the magnitude of the toroidal field and, correspondingly, the degree of twist and the amount of plasma pressure. The model with the least twist remains stable and settles into an equilibrium that differs only slightly from the initial state. In contrast, the more strongly twisted flux rope becomes unstable. Some portion of it breaks out in a kinklike fashion and moves rapidly outward, while another portion remains below. The evolved stage is characterized by the formation of a thin current sheet below an outward moving rope. [Birn et al., 2005a]. Analytical theory and kinematical models were applied to magnetic reconnection in the solar corona. The emphasis of this investigation was on the relation between the reconnection electric field, relevant for particle acceleration, the reconnection rate, and the change in magnetic connectivity among photospheric magnetic footpoints. The results are not tied to the presence or absence of specific topological features of the magnetic field, such as separatrix layers or separators. It was shown that the critical element in determining the location of ribbon-like bright features on the solar surface may be the parallel electric field, integrated along magnetic field lines. A general relation between the change of the reconnected magnetic flux and the integrated parallel electric field was derived. The results of this analysis were applied to the solar coronal case by means of two kinematical models, one of which affords a fully analytical treatment. The results showed that reconnection-induced changes of magnetic connectivity on the corona-photosphere interface are directly related to the maximum value of the field line-integrated parallel electric field. [Hesse et al., 2005] Publications: Birn, J., T. G. Forbes, and M. Hesse, Stability and dynamical evolution of three-dimensional flux ropes, Astrophys. J., submitted, 2005a. Hesse, M., T. G. Forbes, and J. Birn, On the relation between reconnected magnetic flux and parallel electric fields in the solar corona, Astrophys. J., 631, 1227-1238, 2005.