Comment on “Polar cap voltage saturation” by J. W. MacDougall and P. T. Jayachandran

[1] In their interesting paper MacDougall and Jayachandran [2006] attribute the phenomenon of transpolar potential saturation to the shunting off of the region 1 current by a nonlinearly growing region 2 current. Their idea is this: The region 1 current grows under a strengthening of a southward interplanetary magnetic field (IMF), which causes the transpolar potential to increase via the ionospheric Ohm's law. There is nothing controversial in this statement unless it is the assumption that the current is antecedent to the voltage. However, since current and voltage are algebraically related by the ionospheric Ohm's law, which does not say which is the cause of the other, at this point it is a matter of choice whether to emphasize current or voltage. The new part of the argument is that the region 2 current, which, in the MacDougall and Jayachandran's view, is fed by the region 1 current, grows faster than the region 1 current. In support of this statement they offer empirical evidence. Thus, as the southward IMF continues to strengthen, the nonlinearly increasing region 2 current progressively reduces the fraction of the linearly increasing region 1 current that crosses the polar cap to produce the transpolar potential. Eventually, they assert, so much of the region 1 current is consumed in feeding the region 2 current that the fraction left over to produce the transpolar potential is in effect choked off, leading to saturation. They support their idea by constructing an algebraic relation between the transpolar potential and the region 1 and region 2 currents parameterized by IMF Bz. The formula exhibits saturation, but as it is constructed with the above scenario in mind, this is not unexpected. [2] Our purpose here is to point out that there are several simple explanations for transpolar potential saturation that do not invoke region 2 currents [Siscoe et al., 2004] and that are supported by global MHD simulation [e.g., Siscoe et al., 2002a; Raeder and Lu, 2005; Ridley, 2005]. Nearly all invoke the region 1 current as the causative agent. To cite one nonregion 2 explanation, transpolar potential saturation happens when the region 1 current generates a field at the stagnation point comparable in strength to the dipole field there (the Hill criterion [Hill et al., 1976]) or, equivalently, when the magnetic pressure produced by the region 1 current becomes comparable to the solar wind dynamic pressure (the Vasyliunas formulation of the Hill criterion [Vasyliunas, 2004]). This criterion has been parameterized in terms of solar wind and ionospheric variables [Siscoe et al., 2002a, 2002b], and the predictions of the parameterized model have been found to agree with observations [e.g., Hairston et al., 2003; Ober et al., 2003]. [3] Several global MHD codes have demonstrated the phenomenon of transpolar potential saturation without the benefit of region 2 currents. Indeed, one of the known failings of global MHD simulations is their inability to reproduce region 2 currents of observed strength. This failing is being remedied by the inclusion of non-MHD modules that operate on particle drift physics in the inner magnetosphere [e.g., De Zeeuw et al., 2004], but the pertinent point is that these codes discovered transpolar saturation in their outputs before incorporating region 2 generating subsystems. Thus, as far as global MHD simulations are concerned, the cause of the phenomenon of transpolar potential saturation cannot be primarily region 2 currents. [4] This is not to say that region 2 currents do not affect the transpolar potential. As Crooker and Siscoe [1981] noted, region 2 currents effectively increase the ionospheric conductance as seen by the magnetosphere or solar wind. In the domain of transpolar potential saturation an increase in conductance reduces the saturation potential [Siscoe et al., 2002a]. It is the hope of MHD modelers that once the models reproduce realistic region 2 currents the potential-reducing effect just mentioned will bring the saturation potentials that the models presently find, which are too high by about a factor of 2, down to observed values. [5] In summary, by reminding us that region 2 currents affect the transpolar potential, MacDougall and Jayachandran [2006] have enlarged the conceptual space within which to look for explanations for the transpolar-potential-saturation phenomenon. Whereas others have dismissed region 2 currents in this connection on the basis of their making too small a contribution to the magnetic field at the stagnation point, where the Hill and Vasyliunas criteria apply [Siscoe et al., 2002a], and have regarded them as potentially being able to reduce saturation potential levels predicted by global MHD codes, MacDougall and Jayachandran have put them at the center of an explanation. However, the success that global MHD codes have had in simulating the phenomenon with virtually no region 2 currents suggests that some region 1 current mechanism is operating (possibly one of those proffered in the cited references) as well as or instead of a region 2 current mechanism. Eventually, global MHD simulations augmented with a subsystem capable of self-consistently simulating region 2 currents should be able definitively to sort out the relative roles of the region 1 and region 2 current systems. At present, the most powerful tools that the field has to investigate the issue (global MHD simulations sansregion 2 currents) suggest that region 1 currents play a major role. [6] Wolfgang Baumjohann thanks the reviewers for their assistance in evaluating this paper.