Energetic Electron Periodicities During the Cassini Grand Finale

Abstract

The Cassini F ring and Proximal orbits took the spacecraft closer to Saturn than any previous mission and allowed determination of energetic charged particles (E > 20 keV) in the inner magnetosphere of the planet. The periodicities of the energetic electrons show a remarkable consistency when analyzed using Lomb periodograms. From the beginning of the F ring orbits in October 2016 to the end of the mission in September 2017, the particles manifest a strong main period of 10.79 h ± 0.01 h, with small Doppler‐related signals slightly above and below this period. The signal‐to‐noise ratio of the main period indicates one of the strongest periods ever seen in Saturn's magnetosphere. As implied by latitude separation, the main period seems to be associated with the northern hemisphere. The 10.79 h period is exactly the same as the single period observed during the early part of the Cassini mission in 2005–6 when Saturn experienced southern summer. A simple rotating searchlight model can simulate the periodograms.

Planetary Period Oscillations in Saturn's Magnetosphere: Comparison of Magnetic and SKR Modulation Periods and Phases During Northern Summer to the End of the Cassini Mission

“…These values are very close to those attained by mid-2015 after the interval of period coalescence at 10.66 hr between mid-2013 and mid-2014 and subsequent period reversal Ye et al, 2016). Consistent with these results, Carbary et al (2017) have also reported energetic electron modulations during the F ring and proximal orbit intervals with a near-constant period of 10.79 ± 0.01 hr, as observed most clearly in northern hemisphere data. However, they did not detect the southern system period in either southern or northern hemisphere electron data.…”

Saturn's Planetary Period Oscillations During the Closest Approach of Cassini's Ring‐Grazing Orbits

“…The energetic electron periodicities during 2016/2017 exhibit a very similar dominant period associated with the northern hemisphere. This is approximately equal to the summer southern hemisphere period from earlier in the Cassini mission (Carbary et al, 2017). showed the amplitude of the equatorial azimuthal field component varied with radial distance and local time with the nightside amplitude being~2 times larger compared to the dayside at 10 R S .…”

“…The PPOs are present in many of the magnetospheric observations, namely, radio and magnetic field measurements, plasma data, and auroral emissions, and act to modulate the behavior of the magnetosphere (e.g., Carbary et al, 2017;Carbary & Mitchell, 2013;Jackman et al, 2016;Ramer et al, 2017;Thomsen et al, 2017;Ye et al, 2016). The PPOs are present in many of the magnetospheric observations, namely, radio and magnetic field measurements, plasma data, and auroral emissions, and act to modulate the behavior of the magnetosphere (e.g., Carbary et al, 2017;Carbary & Mitchell, 2013;Jackman et al, 2016;Ramer et al, 2017;Thomsen et al, 2017;Ye et al, 2016).…”

“…Saturn's magnetosphere is permeated with oscillations that have a period close to the expected planetary rotation of~10.6 hr; these phenomena have been termed as "planetary period oscillations" (PPOs). The PPOs are present in many of the magnetospheric observations, namely, radio and magnetic field measurements, plasma data, and auroral emissions, and act to modulate the behavior of the magnetosphere (e.g., Carbary et al, 2017;Carbary & Mitchell, 2013;Jackman et al, 2016;Ramer et al, 2017;Thomsen et al, 2017;Ye et al, 2016). From observations of both the Saturn kilometric radiation emission and magnetic field oscillations, it has been shown that there are two distinct periods, one associated with the northern hemisphere and the other with the southern hemisphere (Andrews, Coates, et al, 2010;Gurnett et al, 2009;Kurth et al, 2008;Southwood, 2011).…”

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

“…Despite the fact that Saturn's internal magnetic field is highly symmetric with respect to its rotational axis (Burton et al, ), rotational modulation has been observed in Saturn radio emissions, magnetic field perturbations, charged particles, energetic neutrals, and motions of the plasma sheet, auroral oval, and magnetopause (Desch & Kaiser, ; Espinosa & Dougherty, ; Espinosa et al, ; Gurnett et al, ; Carbary et al, ; Paranicas et al, ; Andrews et al, ; Andrews, Coates et al, ; Andrews et al, ; Clarke, Andrews, Arridge et al, ; Clarke, Andrews, Coates et al, ; Nichols et al, ; Khurana et al, ; Provan et al, , ; Carbary & Mitchell, , and references therein; Carbary et al, ). A period very close to the modulation period of Saturn kilometric radiation (SKR) was originally adopted by the IAU as the rotation period of Saturn (10.656 hr) due to the lack of a trackable solid surface on the gas giant.…”