The low-frequency source of Saturn’s kilometric radiation

Lamy, Laurent; Zarka, Philippe; Cecconi, Baptiste; Prangé, Renée; Kurth, W. S.; Hospodarsky, G. B.; Persoon, A. M.; Morooka, M.; Wahlund, Jan‐Erik; Hunt, G. J.

doi:10.1126/science.aat2027

Cited by 27 publications

(41 citation statements)

References 52 publications

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“…In image a, this secondary oval appears at 69-72 ∘ from 06:00 LT (or 00:00 LT ≤ 70 ∘ ) to 18:00 LT, with peak brightnesses ∼ 10 kR. Interestingly, during this exposure, Cassini intercepted the secondary oval near 11:00 LT during which MAGnetometer (MAG) measurements (Dougherty et al, 2004) of the azimuthal magnetic component (supporting information Figure S2) reveal successive small-scale abrupt gradients consistent with field-aligned current signatures, preceded ∼1 hr earlier by a large positive gradient indicating a strong upward current layer associated with the poleward main emission (e.g., Bunce et al, 2008;Hunt et al, 2014;Lamy et al, 2018;Talboys et al, 2009Talboys et al, , 2011.…”

Section: A Variety Of Variable Componentsmentioning

confidence: 82%

Saturn's Northern Aurorae at Solstice From HST Observations Coordinated With Cassini's Grand Finale

Lamy

Prangé

Tao

et al. 2018

Geophysical Research Letters

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Throughout 2017, the Hubble Space Telescope (HST) observed the northern far-ultraviolet aurorae of Saturn at northern solstice, during the Cassini Grand Finale. These conditions provided a complete viewing of the northern auroral region from Earth and a maximal solar illumination, expected to maximize the ionosphere-magnetosphere coupling. We analyze 24 HST images concurrently with Cassini measurements of Saturn's kilometric radiation and solar wind parameters predicted by two magnetohydrodynamic models. The aurorae reveal highly variable components, down to timescales of minutes, radiating 7 to 124 GW. They include a nightside-shifted main oval, unexpectedly frequent and bright cusp emissions, and a dayside low-latitude component. On average, these emissions display a strong local time dependence with two maxima at dawn and premidnight, the latter being newly observed and attributed to nightside injections possibly associated with solstice conditions. These results provide a reference frame to analyze Cassini in situ measurements, whether simultaneous or not. Plain Language SummaryIn 2017, the Hubble Space Telescope regularly observed the northern ultraviolet aurorae of Saturn in coordination with Cassini in situ measurements obtained during the Grand Finale, when the spacecraft flew across magnetic field lines connected to the aurorae. Hubble imaged Saturn's aurorae at 24 occasions spread over 7 months during northern solstice, when the northern auroral region was both fully visible from Earth and permanently illuminated by the Sun. The observed aurorae display a variety of components observed poleward of 68 ∘ latitude with different properties, some of which were unreported before. These emissions strongly vary with time, down to a few minutes, and radiate from 7 to 124 GW. On average, the auroral intensity also strongly varies with local time (a Sun-referenced frame) and peaks at dawn, as previously observed, and also premidnight, pointing to a recurrent nightside activity of the magnetosphere. These results provide a reference basis to analyze Cassini in situ measurements.

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Section: A Variety Of Variable Componentsmentioning

confidence: 82%

Saturn's Northern Aurorae at Solstice From HST Observations Coordinated With Cassini's Grand Finale

Lamy

Prangé

Tao

et al. 2018

Geophysical Research Letters

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“…Mitchell et al (2009) also observed ion conics similar to those found in relation to the terrestrial and, later on, Jovian aurorae and concluded that the driving mechanisms may be closely related, although Cassini's position was far above the acceleration region (>5 R S ), and no auroral imagery was available for these observations. However, the height and structure of Saturn's auroral acceleration region remains a matter of ongoing research-so far based on the analyses of Saturn Kilometric Radiation (e.g., Lamy et al, 2011Lamy et al, , 2018 and modeling efforts (e.g., Ray et al, 2013). Their modeling suggests that this process can produce significant ion heating as previously investigated for Earth's auroral region (Singh et al, 1981).…”

Section: 1029/2019ja027403mentioning

confidence: 99%

Energetic Particle Signatures Above Saturn's Aurorae

et al. 2020

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Near the end of its mission, NASA's Cassini spacecraft performed several low-altitude passes across Saturn's auroral region. We present ultraviolet auroral imagery and various coincident particle and field measurements of two such passes, providing important information about the structure and dynamics of Saturn's auroral acceleration region. In upward field-aligned current regions, upward proton beams are observed to reach energies of several tens of keV; the associated precipitating electron populations are found to have mean energies of about 10 keV. With no significant wave activity being apparent, these findings indicate strong parallel potentials responsible for auroral acceleration, about 100 times stronger than at Earth. This is further supported by observations of proton conics in downward field-aligned current regions above the acceleration region, which feature a lower energy cutoff above ∼50 keV-indicating energetic proton populations trapped by strong parallel potentials while being transversely energized until they can overcome the trapping potential, likely through wave-particle interactions. A spacecraft pass through a downward current region at an altitude near the acceleration region reveals plasma wave features, which may be driving the transverse proton acceleration generating the conics. Overall, the signatures observed resemble those related to the terrestrial and Jovian aurorae, the particle energies and potentials at Saturn appearing to be significantly higher than at Earth and comparable to those at Jupiter.Plain Language Summary NASA's Cassini spacecraft orbited closer to Saturn than ever before during the last stage of its mission, the "Grand Finale". This allowed the onboard instruments to measure charged particles and plasma waves directly above the auroral region while simultaneously providing high-resolution imagery of the ultraviolet aurorae. Based on observations of highly energetic ions streaming away from the planet in regions of low plasma wave activity, we infer the existence of strong electric fields which act to accelerate electrons down into the atmosphere, driving the bright auroral emissions. Our estimates of the average energy of the precipitating electrons support this finding. Charged ions sometimes seem to be energized by plasma waves above the aurorae before they can escape, but the exact process in which this happens is not fully understood. Most signatures presented here resemble those observed in relation to Earth's aurorae, suggesting that the mechanisms acting at both planets are quite similar although Saturn's acceleration mechanism is significantly stronger.

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“…The top of the SKR emission region was sampled directly and repeatedly to determine its source (121) and elucidate how these planetary radio emissions are generated. SKR was strongly time-variable from orbit to orbit, with a dependence on local time around Saturn.…”

Section: Close To Saturnmentioning

confidence: 99%

Cassini-Huygens’ exploration of the Saturn system: 13 years of discovery

Spilker

2019

Science

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The Cassini-Huygens mission to Saturn provided a close-up study of the gas giant planet, as well as its rings, moons, and magnetosphere. The Cassini spacecraft arrived at Saturn in 2004, dropped the Huygens probe to study the atmosphere and surface of Saturn’s planet-sized moon Titan, and orbited Saturn for the next 13 years. In 2017, when it was running low on fuel, Cassini was intentionally vaporized in Saturn’s atmosphere to protect the ocean moons, Enceladus and Titan, where it had discovered habitats potentially suitable for life. Mission findings include Enceladus’ south polar geysers, the source of Saturn’s E ring; Titan’s methane cycle, including rain that creates hydrocarbon lakes; dynamic rings containing ice, silicates, and organics; and Saturn’s differential rotation. This Review discusses highlights of Cassini’s investigations, including the mission’s final year.

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The low-frequency source of Saturn’s kilometric radiation

Cited by 27 publications

References 52 publications

Saturn's Northern Aurorae at Solstice From HST Observations Coordinated With Cassini's Grand Finale

Saturn's Northern Aurorae at Solstice From HST Observations Coordinated With Cassini's Grand Finale

Energetic Particle Signatures Above Saturn's Aurorae

Cassini-Huygens’ exploration of the Saturn system: 13 years of discovery

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