Steady State Aspects of Magnetic Field Line Merging
- Edward W. Hones Jr.
Published Online: 19 MAR 2013
Copyright 1984 by the American Geophysical Union.
Magnetic Reconnection in Space and Laboratory Plasmas
How to Cite
Vasyliunas, V. M. (1984) Steady State Aspects of Magnetic Field Line Merging, in Magnetic Reconnection in Space and Laboratory Plasmas (ed E. W. Hones), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM030p0025
- Published Online: 19 MAR 2013
- Published Print: 1 JAN 1984
Print ISBN: 9780875900582
Online ISBN: 9781118664223
- Electric field;
- Magnetic field line merging;
- Magnetic merging;
Although a true steady state does not seem ever to occur in the magnetosphere, there are topics profitably discussed as steady-state aspects, including (1) description of phenomena whose essential features are the same with or without time variations, and (2) description of the time-averaged configuration, important for understanding and classifying the basic structures which may then serve as a framework for the more complex time-dependent effects. Magnetic field line merging (I consider this term preferable to its synonym “magnetic reconnection,” for linguistic reasons) exhibits such steady-state aspects on both a local and a global scale. The local aspects are primarily those concerned with the region centered about the magnetic X line (also called the separator) formed by the intersection of two branches of the separatrix surface (between volumes occupied by topologically different magnetic field lines). Bulk flow of the plasma across the separatrix surface implies an electric field along the X line, a conclusion that can be derived independently of the presence or absence of time variations by applying Faraday's law to a loop lying within the separatrix surface, whether steady or not. The pivotal point of the argument is that the global length scales of the system (e.g. the radius of curvature of the X line) are very large compared to the microscopic length scales (e.g. gyroradii, resistive lengths) at which the MHD approximation breaks down, so that intermediate length scales, macroscopic but local, exist. On a global scale, the complex three-dimensional geometrical configuration may be visualized with the help of several diagrams, including in particular representations of (1) magnetic field lines and flow streamlines on electric equipotential surfaces and (2) potential contours on a surface passing through the magnetic X lines. The large-scale magnetic topology associated with the open magnetosphere is characterized by a single X line that closes on itself to form a complete ring (nonplanar in general); it can be divided, on the basis of the relative direction of the electric field, into two distinct segments generally referred to as the “dayside” and “nightside” merging lines, plus possibly some additional “inactive” segments where the electric field along the X line vanishes. Despite the names, the actual locations of these segments even in a time-averaged configuration are very uncertain, and a number of distinct models can be proposed without conflicting with the (as yet rather limited) observations.