Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsm71b..06r&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #SM71B-06
Other
2407 Auroral Ionosphere (2704), 2712 Electric Fields (2411), 2740 Magnetospheric Configuration And Dynamics
Scientific paper
MHD does not allow parallel electric fields. However, electric fields parallel to the magnetic field have been experimentally well-established in the auroral region. What features of the auroral region cause MHD to be inapplicable ? It is argued that the answer lies in the superposition of two magnetic structures, one consisting of the rotating magnetic dipole, and the other consisting of the external magnetic field which is fixed in the inertial frame. The conductivity condition requires that the net force on a charged particle is zero, which means that either the condition E + VxB =0 (Case I) is satisfied or E and V are individually zero (Case II) in a given frame of reference. For example, for plasma co-rotating with the magnetic dipole, Case I is satisfied in the inertial frame while Case II is satisfied in the rotating frame by the appropriate transformation. On the other hand, if Case I is imposed in the rotating frame and Case II in the inertial frame there is no co-rotation. How does nature decide between these two possibilities? Laboratory experiments that were performed in the early 1900's show that a magnet rotating about its axis of symmetry produces a detectable motional electric field only in the rotating frame. In other words, the rotating dipole creates an electrostatic motional electric field in the rotating frame which is canceled by a polarization of the conducting plasma, consistent with Case II (Etot'=0). The effect of the resulting polarization electric field is seen in the inertial frame as EpxBd plasma drift, co-rotating with the dipole. In short, the rotating dipole naturally entrains an initially stationary plasma and causes it to rotate . On the other hand, the stationary external magnetic field does not contribute to the rotation of the plasma and must satisfy Case I in the rotating frame and Case II in the inertial frame, which is opposite to that for the rotating dipole. Since it is impossible for the combined magnetic field to simultaneously satisfy the conductivity condition for both rotating and stationary magnetic structures, MHD does not apply here. In fact, one can easily show by incorporating both magnetic fields in an MHD treatment that inconsistencies become clear in the limit as one of the magnetic fields is set to zero. Note that the magnetic force from the stationary magnetic field is partially perceived as an equivalent electric force in the rotating frame which is perpendicular to the stationary magnetic field, but not that of the rotating dipole. The polarization electric field from the rotating dipole exists in the inertial frame and is perpendicular to the dipole magnetic field, but not that of the stationary magnetic field. Therefore, in the auroral region where both magnetic field structures are significant, there is the same finite E-parallel in both reference frames. This E-parallel is sustained by the Earth's rotation and provides a continuous energy source for the aurora.
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