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Session 56 - The Sun.
Oral session, Tuesday, January 14
Harbour C,
Coronal mass ejections (CMEs) involve the expulsion of some 10^16 g of solar material into interplanetary space, at hundreds of kilometers per second. Where does the energy for these ejections come from? Many CME theories invoke magnetic energy, often associated with field-aligned (force-free) currents. However, a body of analytic work and numerical experiments suggests that the energy of force-free magnetic fields is sufficient to open the coronal magnetic field--a necessary condition for a CME--but not to propel the ejected material outward nor to lift it against solar gravity. In this work we build on earlier models exploring the role that magnetic energy associated with cross-field currents might play in CMEs. These models require that gradients in gas pressure balance magnetic forces in the pre-CME equilibrium corona. We find that, in the simple models used to date, the excess negative gravitational energy associated with pressure gradients always exceeds the excess positive magnetic energy, thus making CME-like eruptions energetically unlikely. Our results suggest that more complex coronal structures-- including the helmet streamers with high-density regions overlying underdense cavities--may be energetically more conducive to CME development. This may help explain why the majority of CMEs occur in pre-existing coronal helmet streamers.
