An SLS5 Longitude System Based on the Rotational Modulation of Saturn Radio Emissions

Abstract

Despite the axisymmetry of Saturn's internal field, Saturn radio emissions like Saturn kilometric radiation (SKR) are modulated due to planetary rotation. With the completion of Cassini mission in September 2017, we now have around 14 years of observation of Saturn radio emissions, roughly from southern solstice to northern solstice. In this study, we extend the SLS4 longitude system to the end of the Cassini mission using a phase tracing method. The new Saturn longitude system (SLS5) organizes the observed SKR maxima around 0° subsolar longitude in both northern and southern hemispheres and can be used to organize other phenomena observed in Saturn's magnetosphere, for example, hot plasma injection events. SKR is modulated like a clock when the main source on the morning side is visible. To convert the observed phase to the clock phase, the phase of the morning side source, we also define a second longitude system SLS5*, which takes the spacecraft position into account based on a simple visibility model.

Seasonal structures in Saturn's dusty Roche Division correspond to periodicities of the planet's magnetosphere

“…The seasonal appearance of resonant structures in Saturn's Roche Division fits in well with the observed seasonal variations in Saturn's PPO rotational modulations. This is demonstrated in Figure 7, which shows the rotation speeds of the northern (blue) and southern (red) PPO systems derived from the SKR (solid line) and MAG (dashed line) data over the Cassini mission (Ye et al 2016(Ye et al , 2018Provan et al 2016Provan et al , 2018). An equivalent scale of PPO periods is shown on the right hand vertical axis.…”

“…The fractional variation of the brightness profile at each of these radii is plotted below with the same color scheme for north/south and a different symbol for each ring observation. The SKR maximum emission in this MAG longitude system is marked by the vertical dashed lines with appropriate symbols on each end (Ye et al 2018). The minimum fractional variations of the ring pattern appears to be consistently at quadrature with the PPO system that is responsible for the resonance in the Roche Division at any given time.…”

Energetic Electron Patterns in the New SLS5 Longitude System

“…The third coordinate is SLS5 longitude λ SC of the spacecraft, which is given by (Kurth et al, , ; Ye et al, ) as where ϕ SC = (12/ π )tan −1 ( y , x )+12 is the local time of the spacecraft in hours, measured clockwise from midnight in hours, and λ SUN is the Sun's SLS5 longitude. Here, ( x , y ) are rectangular coordinates of the spacecraft in a right‐handed system in which z is aligned with the spin axis, where y points toward dusk and the xz plane contains the direction from Saturn to the Sun.…”

“…Here, ( x , y ) are rectangular coordinates of the spacecraft in a right‐handed system in which z is aligned with the spin axis, where y points toward dusk and the xz plane contains the direction from Saturn to the Sun. The north and south values of λ SUN are provided in the supporting information of Ye et al (). Note that all SLS systems are left‐handed and measure longitude westward from the prime meridian.…”

“…The third coordinate is SLS5 longitude λ SC of the spacecraft, which is given by (Kurth et al, , 2008Ye et al, 2018) as…”