The Morphology of Saturn's Aurorae Observed During the Cassini Grand Finale

Bader, Alexander; Cowley, S. W. H.; Ray, L. C.; Kinrade, Joe; Palmaerts, Benjamin; Pryor, W. R.

doi:10.1029/2019gl085800

Cited by 5 publications

(5 citation statements)

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“…In addition to the main oval, Saturn's aurora also has a dimmer emission equatorward of the main oval called the outer emission. The outer emission boundaries map to a distance of~6-7 R S in the magnetic equator plane (Bader et al, 2020) corresponding to a striking plasma boundary at 7 R S as identified in Schippers et al (2008). Inward of that boundary, the electron pitch angle distributions are trapped and have pancake or butterfly distributions, whereas outward of that boundary, the pitch angle distributions are bidirectional.…”

Section: 1029/2019ja027693mentioning

confidence: 97%

Energy Flux and Characteristic Energy of Electrons Over Jupiter's Main Auroral Emission

et al. 2020

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Jupiter's ultraviolet (UV) aurorae, the most powerful and intense in the solar system, are caused by energetic electrons precipitating from the magnetosphere into the atmosphere where they excite the molecular hydrogen. Previous studies focused on case analyses and/or greater than 30-keV energy electrons. Here for the first time we provide a comprehensive evaluation of Jovian auroral electron characteristics over the entire relevant range of energies (~100 eV to~1 MeV). The focus is on the first eight perijoves providing a coarse but complete System III view of the northern and southern auroral regions with corresponding UV observations. The latest magnetic field model JRM09 with a current sheet model is used to map Juno's magnetic foot point onto the UV images and relate the electron measurements to the UV features. We find a recurring pattern where the 3-to 30-keV electron energy flux peaks in a region just equatorward of the main emission. The region corresponds to a minimum of the electron characteristic energy (<10 keV). Its polarward edge corresponds to the equatorward edge of the main oval, which is mapped at M shells of~51. A refined current sheet model will likely bring this boundary closer to the expected 20-30 R J . Outside that region, the >100-keV electrons contribute to most (>~70-80%) of the total downward energy flux and the characteristic energy is usually around 100 keV or higher. We examine the UV brightness per incident energy flux as a function of characteristic energy and compare it to expectations from a model.

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Section: 1029/2019ja027693mentioning

confidence: 97%

Energy Flux and Characteristic Energy of Electrons Over Jupiter's Main Auroral Emission

et al. 2020

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“…Pulsations in the ultraviolet (UV) auroral emissions have further been linked with magnetodisc reconnection and are observed to preferentially occur at dusk with patchy, diffuse signatures (Bader et al, 2019(Bader et al, , 2020. These phenomena highlight the quieter dawn magnetodisc that maps to the aurora along field-aligned currents compared to the active and more variable dusk magnetodisc and cushion that generate these structures.…”

Section: Discussionmentioning

confidence: 99%

The Cushion Region and Dayside Magnetodisc Structure at Saturn

Staniland

Dougherty

Masters

et al. 2021

Geophysical Research Letters

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At the gas giants, the presence of an internal plasma source coupled with their rapid rotation (∼10 h) significantly perturbs their magnetic field configuration. Neutrals ejected from the moons Enceladus and Io in the inner magnetosphere of Saturn and Jupiter, respectively, become ionized, locking onto magnetic field lines and are accelerated toward corotation. The newly formed plasma is centrifugally confined to the equator, radially stretching the magnetic field into a magnetodisc. This structure has been observed at all local times under expanded conditions at Saturn (Arridge, Russell, et al., 2008). At Jupiter, a region adjacent to the magnetopause where the magnetodisc structure breaks down and the field is quasi-dipolar, referred to as the "cushion region," has been identified (Went, Kivelson, et al., 2011) and is argued to be populated by mass-depleted flux tubes following tail reconnection (Kivelson & Southwood, 2005). However, this region has yet to be identified at Saturn (Went, Kivelson, et al., 2011), despite the similarities between these two systems. At Saturn, mass that is loaded into the magnetosphere by Enceladus must be lost from the system. These water group ions (W +) are eventually driven radially outwards in the low plasma beta (β < 1) inner magnetosphere via an interchange instability with the more tenuous hot plasma population in the outer magnetosphere (e.g., Azari et al., 2018; Gold, 1959). At larger radial distances, plasma pressure dominates (β > 1) and the magnetic field balloons until closed field lines reconnect and mass is lost in the magnetotail (Vasyliunas, 1983). Hence, mass-depleted flux tubes following nightside reconnection via this cycle, or the solar-wind driven Dungey cycle (Dungey, 1961), convect along the dawn flank toward noon and are subsequently refilled, thus restarting the mass transport cycle. It has been suggested that a turbulent channel

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“…Figures 3b and 3c also suggests a tendency for the currents in the southern hemisphere to be a little larger than those in the northern hemisphere irrespective of season. This could be a consequence of the weaker ionospheric magnetic fields in the southern hemisphere compared with the northern hemisphere, which may act both to somewhat enhance weak diffusion particle precipitation into the southern hemisphere compared with the northern hemisphere as reported by Bader et al (2020), as well as to increase the conductivity (the mobility) of the ionization produced.…”

Section: Comparison Ofmentioning

confidence: 90%

Seasonal Dependence of the Magnetospheric Drag Torque on Saturn's Northern and Southern Polar Thermospheres and its Relation to the Periods of Planetary Period Oscillations

et al. 2020

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We calculate the magnetospheric drag torques on Saturn's northern and southern polar thermospheres during late southern summer in 2008 and northern spring in 2012-2013 using previously derived profiles of ionospheric meridional coupling currents determined from high-latitude Cassini magnetic field data. We show that the drag torques in the "winter" and "summer" auroral regions are near equal at~2.3 × 10 16 N m, contrary to the recent discussion of Brooks et al. (2019, https://doi.org/10.1002/ 2019JA026870) who suggest that significant seasonal differences should occur in these regions. Instead, seasonally dependent torques occur in the adjacent polar open field regions, where the "winter" and "summer" torques are~0.3 × 10 16 and~1.8 × 10 16 N m, respectively. We derive a simple rotating disc model of the polar thermosphere and estimate the speed of the poleward flow from midlatitudes required to balance these torques in steady state, finding values of tens of m s −1 consistent with previous numerical modeling. Comparison of the calculated torques with concurrent periods of the northern and southern planetary period oscillations (PPOs) does not suggest a direct connection between these quantities as proposed by Brooks et al., 2019, showing at the least that significant additional factors must be involved. We further note some issues with their scenario for dual modulation of radio emissions, previous observations having shown that the principal oscillatory PPO field-aligned currents that modulate the emissions rotate in the auroral region with periods~10.7 ± 0.1 hr, propagating through the more slowly rotating~15-20 hr period outer magnetospheric plasma, with implications for the proposed "atmospheric flywheel" picture.

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The Morphology of Saturn's Aurorae Observed During the Cassini Grand Finale

Cited by 5 publications

References 63 publications

Energy Flux and Characteristic Energy of Electrons Over Jupiter's Main Auroral Emission

Energy Flux and Characteristic Energy of Electrons Over Jupiter's Main Auroral Emission

The Cushion Region and Dayside Magnetodisc Structure at Saturn

Seasonal Dependence of the Magnetospheric Drag Torque on Saturn's Northern and Southern Polar Thermospheres and its Relation to the Periods of Planetary Period Oscillations

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