Exploring the Validity of ideal Magnetohydrodynamics in the Inner Magnetosphere
Abstract
Ideal magnetohydrodynamics (MHD) relies on various approximations. These include the frozen-in approximation, isotropic pressure, and the assumption that the divergence of the heat flux is zero. Here we will investigate two of these assumptions: the frozen-in condition, and the divergence of the heat flux. For the frozen-in condition it can be shown that provided the pressure is isotropic and the flows are barotropic (i.e., density and pressure gradients are parallel) then the frozen-in condition can be rewritten using a generalized magnetic field that includes plasma vorticity. Simulations run using the Community Coordinated Modeling Center show, however, that the flows are not barotropic in the inner magnetosphere. This may be a consequence of processes such as localized reconnection introducing structure in the plasma pressure and density. The next approximation is to assume that the frozen-in condition is determined by the electron fluid, that is, including the Hall term in the frozen-in condition. This implicitly assumes that the electrons are cold. The simulations do indicate that the frozen-in electron condition can yield different electric fields than found for the frozen-in ion condition. This could affect the evolution of the system through the change in the induction, and the change in the mapping of field-aligned currents to the ionosphere. Another possible consequence of the assumption of cold electrons is the affect on the heat flux. For equal temperature ions and electrons, the transport equations include a heat flux correction directly proportional to the current density, corresponding to the difference in the ion and electron fluid velocities. For cold electrons the additional heat flux is related to the ion temperature gradient. We will use the simulations to explore the expected heat flux correction, and whether or not this is significant in comparison to the heat flux associated with the fluid transport of internal energy and pressure.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMSM32A..08S
- Keywords:
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- 2730 MAGNETOSPHERIC PHYSICS / Magnetosphere: inner;
- 2740 MAGNETOSPHERIC PHYSICS / Magnetospheric configuration and dynamics