Modulation of Young Injection Events at Saturn at the Rotation Period of Perturbations in the Winter Hemisphere: A Proposed Mechanism

Abstract:

Non-dispersive or “young” plasma injection events observed near midnight at Saturn are modulated at the period associated with the winter hemisphere [Kennelly et al., 2013]. Most other periodic dynamics of the magnetosphere are dominated by responses at the period of the summer hemisphere. The anomalous periodicity of plasma injection has not been explained. We present a theoretical explanation of ionospheric control, noting (as do Kennelly et al.) that the growth rate of the interchange instability is controlled by ionospheric conductance although the instability condition does not involve the ionosphere [Southwood and Kivelson, 1989]. The motion of the foot of a flux tube through the ionosphere is impeded by high conductance (line tying). Low conductance allows slippage and rapid growth of the instability. When the ionospheres have very different conductances, flux tube motion may be asymmetrical, with rapid displacement occurring only in the low conductance, winter hemisphere. A rotationally modulated low conductivity in that hemisphere would impose periodicity on injections. Pre-equinox (2009) at Saturn, the northern hemisphere conductance was low but probably varied with rotation phase because of the pattern of field-aligned currents (FACs) thought to rotate about the pole at the northern period, T_N[Jia and Kivelson, 2012]. The upward current region in the ionosphere was probably more highly ionized than the downward current region because of electron precipitation. Two predictions follow. (1) The probability of an injection event in the midnight sector should maximize when the downward FAC in the winter hemisphere (conductivity minimum) has rotated into the midnight sector and (2) in northern winter, the tilt of the interchanging flux tube should produce positive radial field perturbations at and above the equator for inward-moving flux tubes and negative radial field perturbations for outward-moving flux tubes. Tests of these predictions will be reported.

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Jia, X., and M. G. Kivelson (2012), J. Geophys. Res., doi:10.1029/2012JA018183

Kennelly, T. J., J. S. Leisner, G. B. Hospodarsky, and D. A. Gurnett (2013),J. Geophys. Res. Space Phys, doi:10.1002/jgra.50152

Southwood, D. J., and M. G. Kivelson (1989), J. Geophys. Res., 94(A1), doi:10.1029/JA094iA01p00299.