Statistical Study on Spatial Distribution and Polarization of Saturn Narrowband Emissions

Abstract

The spatial distribution and polarization of Saturn narrowband (NB) emissions have been studied by using Cassini Radio and Plasma Wave Sciences data and goniopolarimetric data obtained through an inversion algorithm with a preset source located at the center of Saturn. From 2004 January 1 to 2017 September 12, NB emissions were selected automatically by a computer program and rechecked manually. The spatial distribution shows a preference for high latitude and intensity peaks in the region within 6 Saturn radii (R s ) for both 5 and 20 kHz NB emissions. 5 kHz NB emissions also show a local time preference roughly in the 18:00−22:00 sector. The Enceladus plasma torus makes it difficult for NB emissions to propagate to the low latitude regions outside the plasma torus. The extent of the low latitude regions where 5 and 20 kHz NB emissions were never observed is consistent with the corresponding plasma torus density contour in the meridional plane. 20 kHz NB emissions show a high circular polarization while 5 kHz NB emissions are less circularly polarized with | | < V 0.6 for majority of the cases. And cases of 5 kHz NB emissions with high circular polarization are more frequently observed at high latitude especially at the northern and southern edges of the Enceladus plasma torus.

Reflection and Refraction of the L‐O Mode 5 kHz Saturn Narrowband Emission by the Magnetosheath

“…The pink region marks the location where the 5 kHz Z‐mode NB was observed in the statistical study of S. Wu et al. ( 2021 ). The dashed black circular line represents a rough picture of the cut‐off frequency of the 5 kHz NB.…”

“…The second is the catalog of 5 kHz NB from S. Wu et al. ( 2021 ). The global distribution of the 5 kHz L‐O mode NB (the 5 kHz Z‐mode NB is not plotted) in the meridional plane is shown in Figure 1 panel (a).…”

“…These “trapped” Z‐mode NB can mode convert to the “free space” L‐O mode NB at density gradients (Menietti et al., 2019 ; Ye, Menietti, et al., 2010 ). Due to the blockage of the high density plasma torus (centered at the equatorial plane) generated by the cryovolcanic activity of the moon Enceladus, the L‐O mode NB originally generated interior to the plasma torus would propagate to high latitudes unhindered, creating a low latitude shadow zone outside the plasma torus (S. Wu et al., 2021 ; Ye, Menietti, et al., 2010 ). However, 5 kHz NB is still observed at low latitudes outside the plasma torus.…”

“…S. Wu et al. ( 2021 ) proposed that Saturn's magnetosheath may reflect the lowest frequencies of L‐O mode NB, as electron density measurements in Saturn's magnetosheath (Sergis et al., 2013 ) sometimes went to above 0.3 cm −3 , which is high enough to reflect the 5 kHz NB. However, that work only covered the statistical characteristics of 5 and 20 kHz NB and did not provide evidence on the reflection mechanism.…”

Statistics of Water-group Band Ion Cyclotron Waves in Saturn's Inner Magnetosphere Based on 13 yr of Cassini Measurements

“…Saturnʼs magnetosphere hosts a variety of plasma waves, e.g., whistler-mode waves (Menietti et al 2013(Menietti et al , 2014Woodfield et al 2022), electron cyclotron harmonic (ECH) waves (Tao et al 2010;Menietti et al 2017;Long et al 2021;Teng et al 2021), and Z-mode waves (Ye et al 2009(Ye et al , 2010Menietti et al 2016;Wu et al 2021). Via resonant wave-particle interactions, these waves are recognized to contribute significantly to the local acceleration and loss of Saturnʼs radiation belt electrons (e.g., Menietti et al 2012;Gu et al 2013;Kollmann et al 2018;Woodfield et al 2018Woodfield et al , 2019Yu et al 2019;Yao et al 2021;Woodfield et al 2022).…”