Theoretical Approaches to the Description of Magnetic Merging: The Need for Finite βe, Anisotropic, Ambipolar Hall MHD

J Scudder

doi:10.1023/A:1004978021644

Theoretical models of magnetic reconnection are reviewed with a critical view of their suitability for astrophysical plasmas, with a focus on those sampled plasmas near the magnetopause. Frequently the approximations are more those of convenience than physically justified. It is concluded that magnetic reconnection cannot be qualitatively or quantitatively addressed with any one fluid MHD picture unless the Hall, ambipolar and inertial emfs are included in the Generalized Ohm's Law. The observed size of electron pressure anisotropies ensures that the thawing of magnetic flux is almost always determined by the often neglected ambipolar term of the Generalized Ohm's Law. Thus resistive MHD or even resistive Hall MHD cannot possibly give a correct structural picture of the reconnection current carrying layer at the magnetopause. In the magnetotail the ion inertial "resistivity" is much larger than coulomb resistivity with a similar structural form as the coulomb emf. However, until recently the ambipolar contributions there have not been considered. This change in viewpoint of the controlling factors for thawing of magnetic flux parallels the recent evolution of understanding of collisionless shocks, where initially stochastic wave-resistivities were thought to substitute for the coulomb dissipation of high density shock waves. Now these shocks are known to be controlled by coherent agents that can modify emf's such as the ambipolar electric field, the Hall contributions of the gyrating ions, and the electric electron drift in the shock layer to support the current without thawing flux and without any requirement of ohmic dissipation per se. The observational tests that reconnection is a viable process for plasma entry in the magnetosphere are briefly reviewed. Sites where these conservation laws are said to be approximately fulfilled are discussed with an eye toward systematic experimental issues of these tests. That magnetic shear poorly indexes "good" Walén testing layers may be an indication that the resistive dissipation is either not uniformly important across the data set or resistive emf's are not the appropriate agent for the thawing of flux. The ambipolar scale length clearly exceeds the resistive or electron skin depth regime with layers that pass the "good" Walén test layers which have β < 5; this may indicate the importance of the ambipolar violations to the frozen field description.

Theoretical Approaches to the Description of Magnetic Merging: The Need for Finite βe, Anisotropic, Ambipolar Hall MHD

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