A Cassini/CAPS Survey of Ionospheric Outflow at Saturn and Its Internal and External Drivers
10 affiliations
- 1Boston University
- 2Lancaster University
- 3Johns Hopkins University Applied Physics Laboratory
- 4University of Colorado Boulder
- 5University of Saskatchewan
- 6University College London
- 7The Centre for Planetary Sciences at UCL/Birkbeck
- 8California Institute of Technology
- 9Southwest Research Institute
- 10Institute for Space Astrophysics and Planetology (IAPS)
Abstract
We present the first systematic survey of Saturnian ionospheric outflow using calibrated Cassini Plasma Spectrometer ion moments and Magnetometer data from 2004 to 2012. Applying criteria based on visible ion energy dispersion, field-aligned tailward flow, light-ion composition, and magnetic-lobe geometry, we identify 200 ion outflow detections on 24 unique days between 2006 and 2010 and trace each event to an ionospheric footprint at 10,000 km altitude. The inferred number fluxes span 4.38 × 10 ¹⁰ –3.68 × 10 ¹³ m ⁻² s ⁻¹ , extending the “polar wind”-like outflow range predicted by A. Glocer et al. Bulk kinetic energies, m / q = 2 abundances, and adiabatic mirror-altitude estimates rule out mirrored solar wind as the dominant source and support escape from Saturn’s ionosphere. Most detections map poleward of the statistical auroral oval of A. Bader et al., but also some fall on the oval and equatorwards from it. Applying the mass-flux range estimated from number flux and plasma composition, summing both polar caps yields a total polar cap mass rate of ∼0.08–112 kg s ⁻¹ ; summing both auroral ovals yields ∼0.007–66 kg s ⁻¹ , and 3–2700 kg s ⁻¹ for the broad equatorial belt. Clear outflow signatures occur on only ∼5.3% of geometrically eligible observing days, suggesting that CAPS might primarily sample enhanced, instrument-accessible events rather than the full low-energy background ion outflow. The largest outflow number fluxes are associated with enhanced solar wind dynamic pressure and reconnection, while the remaining events show PPO-phase-dependent modulation that is strongest for southern hemisphere outflows and weaker for northern hemisphere outflows. By combining in situ CAPS and MAG detections with magnetic field-line tracing, ENLIL-modeled upstream solar wind conditions, contemporaneous UV auroral imaging, and the PPO magnetic phase systems, this survey provides the first observational characterization of both the internal and the external drivers of ionospheric outflow at Saturn.
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BibTeX
@article{felici2026cassinicaps,
title = {{A Cassini/CAPS Survey of Ionospheric Outflow at Saturn and Its Internal and External Drivers}},
author = {M. Felici and L. C. Ray and S. V. Badman and O. Agiwal and L. Moore and H. T. Smith and R. J. Wilson and C. J. Martin and L. Gilbert and A. J. Coates and J. M. Jasinski and N. Achilleos and P. Valek and A. Mura},
journal = {The Planetary Science Journal},
year = {2026},
month = nov,
doi = {10.3847/psj/aeb1d4},
url = {https://iopscience.iop.org/article/10.3847/PSJ/aeb1d4},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - A Cassini/CAPS Survey of Ionospheric Outflow at Saturn and Its Internal and External Drivers
AU - M. Felici
AU - L. C. Ray
AU - S. V. Badman
AU - O. Agiwal
AU - L. Moore
AU - H. T. Smith
AU - R. J. Wilson
AU - C. J. Martin
AU - L. Gilbert
AU - A. J. Coates
AU - J. M. Jasinski
AU - N. Achilleos
AU - P. Valek
AU - A. Mura
PY - 2026
DA - 2026/11/04
JO - The Planetary Science Journal
DO - 10.3847/psj/aeb1d4
UR - https://iopscience.iop.org/article/10.3847/PSJ/aeb1d4
AB - We present the first systematic survey of Saturnian ionospheric outflow using calibrated Cassini Plasma Spectrometer ion moments and Magnetometer data from 2004 to 2012. Applying criteria based on visible ion energy dispersion, field-aligned tailward flow, light-ion composition, and magnetic-lobe geometry, we identify 200 ion outflow detections on 24 unique days between 2006 and 2010 and trace each event to an ionospheric footprint at 10,000 km altitude. The inferred number fluxes span 4.38 × 10 ¹⁰ –3.68 × 10 ¹³ m ⁻² s ⁻¹ , extending the “polar wind”-like outflow range predicted by A. Glocer et al. Bulk kinetic energies, m / q = 2 abundances, and adiabatic mirror-altitude estimates rule out mirrored solar wind as the dominant source and support escape from Saturn’s ionosphere. Most detections map poleward of the statistical auroral oval of A. Bader et al., but also some fall on the oval and equatorwards from it. Applying the mass-flux range estimated from number flux and plasma composition, summing both polar caps yields a total polar cap mass rate of ∼0.08–112 kg s ⁻¹ ; summing both auroral ovals yields ∼0.007–66 kg s ⁻¹ , and 3–2700 kg s ⁻¹ for the broad equatorial belt. Clear outflow signatures occur on only ∼5.3% of geometrically eligible observing days, suggesting that CAPS might primarily sample enhanced, instrument-accessible events rather than the full low-energy background ion outflow. The largest outflow number fluxes are associated with enhanced solar wind dynamic pressure and reconnection, while the remaining events show PPO-phase-dependent modulation that is strongest for southern hemisphere outflows and weaker for northern hemisphere outflows. By combining in situ CAPS and MAG detections with magnetic field-line tracing, ENLIL-modeled upstream solar wind conditions, contemporaneous UV auroral imaging, and the PPO magnetic phase systems, this survey provides the first observational characterization of both the internal and the external drivers of ionospheric outflow at Saturn.
SN - 2632-3338
ER -