Physics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008espm...12.3.55z&link_type=abstract
"12th European Solar Physics Meeting, Freiburg, Germany, held September, 8-12, 2008. Online at http://espm.kis.uni-freiburg.de/,
Physics
Scientific paper
The possible role of magnetic flux emergence as triggering mechanism for the initiation of Coronal Mass Ejections (CMEs) is studied in the framework of the ideal magnetohydrodynamics (MHD) model. The full MHD equations are solved numerically on a spherical, axisymmetric (2.5D) domain.
All simulations are performed with a modified version of the Versatile Advection Code (VAC) (Toth 1996). The magnetic field of the solution is maintained divergence-free at machine precision by exploiting an approach similar to that of Balsara and Spicer (1999): instead of storing the magnetic field components on a staggered mesh, we use the vector potential components in the nodes.
In order to get satisfactorily solar wind properties, the Manchester et al. (2004) source term is implemented in the energy equation and gravity is taken into account as well in the model.
Finally, a magnetic vector potential is superimposed at the inlet boundary of the Parker wind solution so that, when the steady state is reached, the Antiochos et al. (1999) triple arcade 'break out' magnetic field configuration (symmetric with respect to the equator) of a helmet streamers is obtained.
When the steady state has been reached, we impose a magnetic flux emergence at the inlet boundary that is linearly growing in time during a time interval of ? t = 24 hours. After this time the vector potential at the solar base is again fixed. Due to the magnetic flux emergence at the solar base, extra radial magnetic field, is built up near the neutral line of the central arcade that expands outward. This generates an extra upward magnetic pressure force. As a consequence, the central flux system expands outward. Also the overlying field expands and, therefore, the downward magnetic tension increases. As a result, the X-point is flattened.
When the distance between the central expanding arcade field and the overlying streamer field is of the order of the grid resolution, the (numerical) reconnection between these fields sets in. A flux rope is formed and, later, accelerated.
Height-time and velocity-height plots of the ejected material are produced. The obtained eruption corresponds to a slow CME. The time evolution of the magnetic energy, kinetic energy and internal energy in the entire domain shows that magnetic energy is converted into kinetic energy, as expected.
The energy evolution plots show, however, that only a small amount of magnetic energy is released in the system, so that the system evolves to a higher energy state. We think that the explanation of this behavior lies in the role of the magnetic helicity, which we neglected by only emerging radial magnetic field.
In conclusion, we stress that by imposing a reasonable (Romano et al. (2007)) flux emergence rate, in a large but realistic active region (with, of course, model dimensionality limitations), quite realistic velocity profiles and energetics of slow CMEs are obtained.
Poedts Stefaan
Soenen Alexander
Zuccarello F. P.
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