Long‐term modulations of Saturn's auroral radio emissions by the solar wind and seasonal variations controlled by the solar ultraviolet flux

Abstract

[1] Saturn's auroral activities have been suggested to be controlled by the seasonal variations of the polar ionospheric conductivities and atmospheric conditions associated with the solar extreme ultraviolet (EUV) flux. However, they have not yet been explained self-consistently by only the seasonal solar EUV effects. This study investigates the long-term variations of Saturnian Kilometric Radiation (SKR) as a proxy of the auroral activities, which were observed by Cassini's Radio and Plasma Wave Science experiment mostly during the southern summer (DOY (day of year) 001 2004001 to DOY 193 2010. We deduced the height distribution of the SKR source region in the Northern (winter) and Southern (summer) Hemispheres from the remote sensing of SKR spectra. The peak spectral density of the southern (summer) SKR was found to be up to 100 times greater than that of the northern (winter) SKR, and the altitude of the peak flux was similar ( 0.8 R s ) in the Northern and Southern Hemispheres. The spectral densities in both hemispheres became comparable with each other around equinox in August 2009. These results suggest a stronger SKR source region during the summer than the winter related to the seasonal EUV effect, which is opposite to the trend observed in the Earth's kilometric radiation. A long-term correlation analysis was performed for the SKR, solar EUV flux, and solar wind parameters extrapolated from Earth's orbit by an magnetohydrodynamical simulation focusing on variations on timescales longer than several weeks. We confirmed clear positive correlations between the solar wind dynamic pressure and peak flux density in both the Southern and Northern Hemispheres during the declining phase of the solar cycle. We conclude that the solar wind variations on the timescale of the solar cycle control the SKR source region. In addition, it was also confirmed that the south-to-north ratios of SKR power flux and source altitudes are positively correlated with the solar EUV flux. This result strongly supports a seasonal EUV effect on the SKR source region. The variations of SKR activity over both seasonal and solar cycles are discussed in comparison to the terrestrial case.Citation: Kimura, T., et al. (2013), Long-term modulations of Saturn's auroral radio emissions by the solar wind and seasonal variations controlled by the solar ultraviolet flux,

Planetary period oscillations in Saturn's magnetosphere: Examining the relationship between abrupt changes in behavior and solar wind‐induced magnetospheric compressions and expansions

“…It is undoubtedly true that Saturn's magnetospheric dynamics is influenced by several different drivers. Kimura et al [2013] suggested that at Saturn solar wind variations on solar cycle timescale affect the long-term magnetosphere-ionosphere coupling process on top of the baseline of the seasonal variations. This would seem to agree with the results presented here where we have observed a seasonal cause of the north-south asymmetry in the PPO periods seemingly associated with solar illumination and the abrupt changes in PPO behavior associated solar wind compressions and expansions.…”

“…They also reported that the pulsing of the SKR during these disturbed intervals was not significantly altered relative to that during nondisturbed intervals. Kimura et al [2013] presented a long-term correlation analysis between SKR, solar EUV flux, and propagated solar wind parameters, where they focused on variations with timescales in excess of several weeks. The study was performed during southern hemisphere summer from 2004 to 2010 during the declining phase of the solar cycle.…”

A flux comparison of northern and southern Saturn kilometric radio bursts during southern summer

“…The SKR energy flux and spectral range depend on the spacecraft location, because of the non-uniform spatial distribution of SKR sources and and the hollow cone emission pattern [Lamy et al, 2008;Kimura et al, 2013]. SKR emissions are usually not observed in the equatorial shadow zone (ESZ), which extends in radial distance from Saturn out to 4-7 R S , depending on frequency [Lamy et al, 2008].…”

“…SKR emissions are usually not observed in the equatorial shadow zone (ESZ), which extends in radial distance from Saturn out to 4-7 R S , depending on frequency [Lamy et al, 2008]. The N-and S-SKR are mainly observed in the latitude of −20 -+70 deg and −60 -+10 deg, respectively [Kimura et al, 2013]. In observer's local time (LT), SKR is stronger in the dawn side sector, ∼2-10 h LT.…”

“…Unfortunately, this meant that when one hemispheric component was stronger than the other for the 3 min integration time, the flux of the weaker hemisphere was artificially determined as 0 [Cecconi, private communication]. Under the southern summer conditions studied in this paper, the S-SKR is generally more intense than the N-SKR [Lamy et al, 2008;Kimura et al, 2013]. Therefore, the N-SKR flux was frequently seen as 0, masked by the simultaneous stronger S-SKR.…”