Dreicer order ambipolar electric fields at Parker's steady state solar wind sonic critical point

Abstract

The time‐independent ambipolar electric field E(r*) at Parker's solar wind sonic critical point r* is analytically shown to range between (0.6–2.0) ED(r*), where ED(r*) is the local Dreicer [1959, 1960] electric field. The ratio of ambipolar to Dreicer field strength scales as (〈T*〉/106° K)3/2 20/ln Λ, where 〈T*〉 is the average of electron and ion temperatures at the critical point. As a million degree corona is nearly certainly required for the wind as we observe it, E(r*) ≃ ED(r*). Since the steady state solar wind is characterized by no parallel currents, these large electric fields require generalizations of Dreicer's discussion of what are the results of such large electric fields. This is clear since even when E = 0.43ED in a homogeneous plasma, the electron fluid drifts at the electron thermal speed with respect to the ions in the absence of collective instabilities. Consequences of such a large current at the critical point are not found at 1 AU. The electric fields at the orbit of earth [Scudder, 1995b] and at the base of the fully ionized layer of the transition region [Scudder, 1995a] are nevertheless comparable to the local Dreicer electric field. From a theoretical point of view, the finding that such large electric fields are required in the plasma points to the need for basic modifications to the macroscopic description of a magnetized plasma that are outside of the Chapman‐Enskog‐Spitzer‐Braginskii closure schemes that assume perturbative corrections to homogeneous solutions as an expansion in the small parameter ε = E‖/ED. It is suggested that the possibility to form gradients, to allow heat to flow, and to allow the bulk plasma to move can short circuit the homogeneous expectations of bulk runaway implicit in the Dreicer discussion. However, the intrinsic bifurcation of the electron distribution reflects the physics of the Coulomb cross section in the presence of the very strong dc field and is suggested as the underlying reason for the omnipresent nonthermal distributions inside and out of the critical point. The ambipolar electric field at the solar wind critical point from two‐fluid theory and the electric field assumed in Spitzer‐Braginskii heat transport are shown to disagree at almost all the possible two‐fluid critical points. This same closure flaw also afflicts the fluid portion of any turbulence description that may be thought relevant for the wind.