Reply to comment by C. E. J. Watt and R. Rankin on “Role of dispersive Alfvén waves in generating parallel electric fields along the Io-Jupiter fluxtube”

[1] In our recent paper [Jones and Su, 2008] (hereinafter referred to as JS) we discuss the effect of dispersive Alfven waves (DAW) in relation to observed particle acceleration at the Io-Jupiter auroral footprint using a two-fluid model. Watt and Rankin [2009] (hereinafter WR) assert that a kinetic wave model should be used when discussing the region separating the inertial and kinetic Alfven regimes. We agree with this analysis, but point out that the same fundamental conclusions are reached with both approaches. The transition between the two regimes should occur within the Io torus, and parallel electric fields within the torus are smaller than the fields in the lower-density region outside the torus. [2] We wish to thank WR for calling attention to the important work done in kinetic Alfven wave theory by Lysak and Lotko [1996] and Lysak [1998] and how it relates to our recent paper (JS). We agree with point 1 of WR, that using the kinetic DAW theory does provide a more complete description of dispersive Alfven wave (DAW) behavior at the transition between inertial and kinetic regions (b ’ me/mi). In JS we chose to use an analytic two-fluid model of DAW [Streltsov et al., 1998; Stasiewicz et al., 2000] to study relative magnitudes of Alfvenic Ek/E? along the entire Io-Jupiter fluxtube. The kinetic approach relies on numeric solutions of the plasma dispersion function Z(z) as done by Lysak and Lotko [1996]. While this does give a more accurate calculation of Ek in the transition region, it is cumbersome in regions where b is not close to me/mi. [3] The fundamental conclusions of JS remain valid whether considering the two-fluid treatment of the aforementioned kinetic treatments. For the assumed initial wave parameters of f and k? the ratio of Ek/E? at higher latitudes of the Io-Jupiter fluxtube will be much larger than within the Io torus (see JS Figure 1f and WR Figure 1a). [4] We also agree with point 2 of WR, that the uncertainty about the values of k? in the torus region make it difficult to predict what the amplitudes of DAW Ek will be. The model values of k? considered in JS were motivated by previous studies of Su et al. [2006] and Hess et al. [2007] and the common assumption of k? / B. It was not our intent to state what k? or Ek should be in the torus but rather what the relative amplitudes of Ek may be for a wide range of parameters both inside and out of the torus. As stated by WR and discussed in JS, increasing k? within the torus will generate larger theoretical values of Ek (see JS Figure 5). However, unless k? is modified as it propagates out of the torus, model values of Ek at higher latitudes will still be two orders of magnitude larger than within the torus. [5] In closing, we appreciate the interest WR have shown in our research and thank them for providing the community with this additional analysis. The model discussed in JS shows that for reasonable models of the Io-Jupiter plasma parameters, the transition between the inertial and kinetic DAW regimes should occur within the torus. Both kinetic and fluid approaches show that theoretical parallel Alfvenic electric fields along the Io-Jupiter fluxtube should be larger at high latitudes than within the torus.