One‐dimensional models of quasi‐neutral parallel electric fields

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Abstract
Parallel electric fields can exist in the magnetic mirror geometry of auroral field lines if they conform to the quasi‐neutral equilibrium solutions first suggested by Alfvén and by Persson. Such solutions may contain double‐layer discontinuities, reflecting the disparity between the two sources that contribute to their plasma population—the hot and rarefield magnetosphere, magnetically confined, and the cool dense ionosphere, held down by gravity. This study reviews previous results on quasi‐neutral equilibria and on double layers and then examines the effects on such equilibria due to nonunique solutions, potential barriers, and field‐aligned current flows, using as inputs monoenergetic isotropic distribution functions. Among the conclusions reached are the following: (1) Double layer solutions not involving any net current flow are readily constructed; (2) Such layers may occur naturally and may involve a significant fraction of the total field aligned voltage drop; (3) Nonuniqueness of quasi‐neutral solutions must be utilized to determine the position of such layers; (4) The gravitational potential barrier that confronts escaping ions plays an important role and must be taken into account; (5) Outbound field‐aligned currents are carried primarily by precipitating electrons and only a very small fraction of them is due to ionospheric positive ions; (6) Inbound field‐aligned currents require an appreciable voltage to drive them, and their density is no greater than that of outbound currents; (7) Precipitation with no net j∥ sets up a ‘thermoelectric potential’ similar to the one predicted by Hultqvist. Overall, the model suggests that quasi‐neutral equilibria can explain many of the observed features of field‐aligned currents and of their associated electric fields, though solutions at this stage do not extend to voltages exceeding 2.2 kV.

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