Abstract
The water-group ion cyclotron waves (ICWs) in Saturn’s magnetosphere were studied using the magnetic field data provided by the MAG magnetometer on board the Cassini satellite. The period from January 2005 to December 2009, when the Cassini radial distance is smaller than 8 RS, was used. ICWs were identified by their left-hand circularly polarized magnetic perturbations and wave frequencies near the water-group ion gyrofrequencies. We obtained the spatial distribution of ICW amplitude and found that the source region of ICWs is mostly located in the low-latitude region, near the equator and inside the 6 RS radial distance. However, it can extend beyond 7 RS in the midnight region. In general, the wave amplitude is peaked slightly away from the equator, for all local time sectors in both the Northern and Southern Hemispheres. By assuming that the water-group ions are composed of pickup ions and background thermal ions, we obtained the local instability condition of the ICWs and estimated their growth rate along the field lines. If the wave amplitude is correlated with the growth rate, the observed latitudinal dependence of the wave amplitude can be well explained by the local stability analysis. Also, latitudinal location of the peak amplitude is found to depend on the local time. This implies a local time dependence for the water-group ion parallel temperature T|, as determined from the theoretical calculations. Graphical abstractFigure 7. The upper panel shows the projected orbit of Cassini in the cylindrical R–Z plane across the L-shell from the Northern Hemisphere to the Southern Hemisphere. The red line denotes Cassini’s orbit and the blue lines denote the L-shell field lines going through points A and B, respectively. The lower panel shows the power spectrum of delta B_{phi }, the blue dotted line shows the local W+ gyrofrequency (flocal), and the black line is the W+ gyrofrequency at the equator (feq) of the dipole L-shell of the Cassini orbit, which moves from A to B
Highlights
Ion cyclotron waves (ICWs) in Saturn’s magnetosphere were identified in early reports of magnetometer data analysis from the Pioneer 11 and Voyager 1 missions (e.g., Smith and Tsurutani 1983; Barbosa 1993)
Growth rate of ion cyclotron waves (ICWs) and its latitudinal dependence To understand why the maximum ICW wave amplitude is located off the equator, we investigate the stability of the left-hand circularly polarized ion cyclotron wave based on kinetic theory
We identify these waves as electromagnetic ion cyclotron waves (ICWs), destabilized by the pickup water-group ion velocity distribution
Summary
Ion cyclotron waves (ICWs) in Saturn’s magnetosphere were identified in early reports of magnetometer data analysis from the Pioneer 11 and Voyager 1 missions (e.g., Smith and Tsurutani 1983; Barbosa 1993). In the 2005 data (Fig. 1a), the W+ cyclotron waves are detected in the noon, dusk and midnight regions, with the magnetic latitude θ between −10° and 8° and the radial distance R between 3.5 RS and 7.8 RS.
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