Modeling Coronal States, Eruptions, and Acceleration

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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
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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.