Keogram analysis of ENA images at Saturn

Abstract

Keograms are constructed from azimuthal profiles of energetic hydrogen atoms (25-55 keV) from Saturn's magnetosphere. The keograms exhibit linear structures or "tracks" that reveal prograde rotational motion of features or "blobs" in the energetic neutral atom (ENA) images. From polynomial fits, the first derivatives of these tracks are used to estimate the rotational speeds of the blobs. The total blob speed consists of plasma convective drift plus gradient drift, so the convective speed can be approximated by subtracting the gradient drift of the protons from which the ENA derive. This subtraction gives plasma convection speeds that are ∼28°/h at ∼5 RS and decrease to a constant ∼21°/h between 10 RS and 20 RS, which are consistently below corotation (∼33.3°/h) and in substantial agreement with estimates of plasma convection made in situ. The speeds also show a local time dependence, decreasing as much as 4-6°/h as the blobs move from midnight through noon to midnight. Key Points First use of keograms to find plasma convection speeds at Saturn Speed peaks near midnight and then decreases 4-6°/h over the rest of local time Speed is ∼28°/h at ∼5 RS but is constant 21°/h outside ∼10 RS ©2014. American Geophysical Union. All Rights Reserved.