Small-scale variability in Saturn’s lower ionosphere

We perform and present a wavelet analysis on all 31 Cassini electron density profiles published to date (Nagy, A.F. et al. [2006]. J. Geophys. Res. 111 (A6), CiteID A06310; Kliore, A.J. et al. [2009]. J. Geophys. Res. 114 (A4), CiteID A04315). We detect several discrete scales of variability present in the observations. Small-scale variability (S < 700 km) is observed in almost all data sets at different latitudes, both at dawn and dusk conditions. The most typical scale of variability is 300 km with scales between 200 km and 450 km being commonly present in the vast majority of the profiles. A low latitude dawn/dusk asymmetry is noted in the prevalent scales with the spectrum peaking sharply at the 300 km scale at dusk conditions and being broader at dawn conditions. Compared to dawn conditions the dusk ionosphere also shows more significant variability at the 100 km scale. The 300 km vertical scale is also present in the few available profiles from the northern hemisphere. Early observations from 2005 show a dominant scale at 350 km whereas later in 2007–2008 the spectrum shifts to the shorter scales with the most prominent scale being 300 km. The performed wavelet analysis and the obtained results are independent of assumptions about the nature of the layers and do not require a definition for a “background” electron density profile. second part of the paper we present a gravity wave propagation/dissipation model for Saturn’s upper atmosphere and compare the wave properties to the characteristics of the observed electron density variability at different scales. The general features observed in the data are consistent with gravity waves being present in the lower ionosphere and causing layering of the ions and the electrons. The wave-driving mechanism provides a simultaneous explanation for several of the properties of the observed variability: (i) lack of variability in the electron density above the predicted region of wave dissipation; (ii) in most cases the peak amplitude of variability occurs within the altitude range for dissipation of gravity waves or below; (iii) shorter scales have smaller amplitudes than the longer scales; (iv) shorter scales are present at lower altitudes whereas longer scales persist to higher altitudes; and (v) several layers often form a system of equally spaced maxima and minima that can be traced over a large altitude range.