Comment on “Nonlinear response of the polar ionosphere to large values of the interplanetary electric field” by C. T. Russell et al.

[1] The topic of the proper functional form of the ionospheric-magnetospheric response to solar wind conditions is a subject of much discussion and debate. While numerous studies over the decades have shown linear relationships [e.g., Burton et al., 1975; Reiff et al., 1981; Boyle et al., 1997], others have found nonlinear relationships [e.g., Hill et al., 1976; Wygant et al., 1983; Weimer, 2001]. Recently, Russell et al. [2001] (hereinafter RLL) discuss a saturation effect in two high-latitude ionospheric parameters, the cross polar cap potential (CPCP) and the integrated Joule heating (IJH), for large values of the y-component of the interplanetary electric field (IEF), or the southward component of the interplanetary magnetic field (IMF Bz). They illustrate this effect by analyzing data and modeling results from five storms between 1995 and 1998, as well as evidence of saturation in other data sets. They conclude that the ionospheric response is saturated for IEF levels greater than 3 mV m . Finally, they discuss the inner magnetosphere exhibiting a linear relationship with the IEF. [2] With this comment, we hope to continue the discussion regarding the functional form of the magnetospheric response to the solar wind. We feel this is a critical issue of space physics, and it should be a top priority of our field to understand this relationship. That the CPCP exhibits nonlinearity and even saturates for large IEF values is not being disputed [see, e.g., Liemohn et al., 2002]. However, the IEF at which the CPCP begins to saturate may be interpreted differently than presented by RLL. The nonlinear response may only be just beginning at 3 mV m , where RLL state that it is in full effect. Additionally, there are several uses of data and model results discussed by RLL, which deserve further explanation. It is the hope of this comment to elucidate this analysis and to show that the data presented by RLL support the conclusion that nonlinear saturation effects become significant at substantially higher IEF values than 3 mV m . [3] Throughout this comment, the terms saturation and nonlinearity are taken to have distinct meanings. Saturation is when the slope of a curve asymptotically reaches zero. This is in contrast to the broader terms of nonlinearity and ‘‘the beginning of saturation,’’ which refer to the deviation of the functional form away from a straight line.