Comment on ‘‘Role of dispersive Alfvén waves in generating parallel electric fields along the Io-Jupiter fluxtube’’ by S. T. Jones

[1] The authors of a recent paper [Jones and Su, 2008] have used two-fluid plasma theory [e.g., Streltsov et al., 1998] to predict the parallel electric field strength of a shear Alfvén wave traveling through the Jovian magnetosphere from the vicinity of Io to the Jovian ionosphere. [2] Two-fluid analyses give adequate predictions for the parallel electric field strength Ek in plasma regimes where the electron thermal speed vTe = (2kBTe/me) 1/2 is much larger, or much smaller, than the Alfvén speed vA = B0(m0r) . When vTe vA, shear Alfvén waves can have finite Ek when their perpendicular scale lengths are comparable to the electron skin depth le = c/wpe, whereupon the waves are called inertial Alfvén waves. Conversely, when vTe vA, shear Alfvén waves can have Ek when their perpendicular scales are comparable to le (vTe/vA), and are often known as kinetic Alfvén waves. Analytic studies using full kinetic theory (either with linear approximations [Lysak, 1998] or using a fully nonlinear simulation [Watt and Rankin, 2008]) have also shown that parallel electric fields are also supported where vTe vA, if the perpendicular scale length is comparable to the kinetic scale length. [3] If we consider a magnetic fluxtube connecting Io (at 5.9RJ radial distance) with the high-latitude ionosphere at roughly 65 latitude, then the lower portion of this fluxtube has vTe < vA and is in the inertial regime, whereas plasma in the vicinity of Io is more likely to have vTe vA due to the decreasing magnetic field strength and increasing ion number density in the Io torus. [4] Two-fluid analyses are inadequate in this plasma environment. By portraying the kinetic and inertial corrections to the Alfvén wave dispersion as ‘‘competing’’ effects, the underlying physics of Alfvén waves with finite perpendicular scale lengths becomes obfuscated. In this paper, we show the correct equation for the dispersive factor given in equation (3) of Jones and Su [2008], and we present an alternate explanation for the small ratio of parallel to perpendicular electric fields predicted in the vicinity of Io.