•Study of 2000 UVIS FUV spectra with hydrocarbon and Lyα/H2 methods and 2 polar atmospheres.•Mean primary electron energy mostly between 1 and 17 keV.•Energy flux - mean energy correlated: plasma ...characteristics compatible with electron temperature near 0.1 keV and density near 3 ×103m−3.•Lyα/H2 method allows calculation of electron energy maps.•First statistical maps of the auroral electrons mean energy show clear local-time dependence.
About 2000 FUV spectra of different regions of Saturn's aurora, obtained with Cassini/UVIS from December 2007 to October 2014 have been examined. Two methods have been employed to determine the mean energy 〈E〉 of the precipitating electrons. The first is based on the absorption of the auroral emission by hydrocarbons and the second uses the ratio between the brightness of the Lyman-α line and the H2 total UV emission (Lyα/H2), which is directly related to 〈E〉 via a radiative transfer formalism. In addition, two atmospheric models obtained recently from UVIS polar occultations have been employed for the first time. It is found that the atmospheric model related to North observations near 70° latitude provides the results most consistent with constraints previously published.
On a global point of view, the two methods provide comparable results, with 〈E〉 mostly in the 7–17keV range with the hydrocarbon method and 〈E〉 in the 1–11keV range with the Lyα/H2 method. Since hydrocarbons have been detected on ∼20% of the auroral spectra, the Lyα/H2 technique is more effective to describe the primary auroral electrons, as it is applicable to all spectra and allows an access to the lowest range of energies (≤5keV), unreachable by the hydrocarbon method. The distribution of 〈E〉 is found fully compatible with independent HST/ACS constraints (emission peak in the 840–1450km range) and FUSE findings (emission peaking at pressure level ≤0.2µbar). In addition, 〈E〉 exhibits enhancements in the 3LT–10LT sector, consistent with SKR intensity measurements.
An energy flux–electron energy diagram built from all the data points strongly suggests that acceleration by field-aligned potentials as described by Knight's theory is a main mechanism responsible for electron precipitation creating the aurora. Assuming a fixed electron temperature of 0.1keV, a best-fit equatorial electron source population density of 3 ×103m−3 is derived, which matches very well to the plasma properties observed with Cassini MAG and CAPS/ELS instruments. However, several auroral regions are characterized by relatively high 〈E〉 and low energy flux, suggesting that additional processes such as plasma injections or magnetic reconnections must be accounted for to explain the emission in these regions.
The Lyα/H2 ratio technique can be used to build maps of 〈E〉 from single spectral images. As expected, preliminary results show that the spatial distribution of 〈E〉 is not uniform, as seen on Jupiter.
Our study reveals that a fraction of the aurora is due to very low energy electrons (<1keV). Even in this case, comparisons between observed and modeled spectra show that 100eV is a suitable value to represent the average energy of the secondary electrons.
Cassini's mission exploring the Saturn system ended with the Grand Finale, a series of orbits bringing the spacecraft closer to the planet than ever before and providing unique opportunities for ...observations of the ultraviolet aurorae. This study presents a selection of high‐resolution imagery showing the aurorae's small‐scale structure in unprecedented detail. We find the main arc to vary between a smooth and a rippled structure, likely indicating quiet and disturbed magnetospheric conditions, respectively. It is usually accompanied by a diffuse and dim outer emission on its equatorward side which appears to be driven by wave scattering of hot electrons from the inner ring current into the loss cone. The duskside is characterized by highly dynamic structures which may be signatures of radial plasma injections. This image set will be the only high‐resolution data for the foreseeable future and hence forms an important basis for future auroral research on Saturn.
Plain Language Summary
At the end of its mission, the Cassini spacecraft performed a set of orbits bringing it closer to Saturn than ever before. By passing over the planet's polar regions at such low altitude, its ultraviolet camera could observe Saturn's aurorae in unprecedented resolution. The observations show for the first time the detailed structure of the main auroral arc which varies between a smooth and a rippled shape, likely depending on how quiet or disturbed the plasma near Saturn is. We further find a host of small arcs and blobs near dusk whose origins are not readily explained with the current understanding of how Saturn's aurorae are driven. Diffuse features surrounding the brightest auroral emissions are attributed to hot electrons from the equatorial plane which are scattered such that they can reach Saturn's atmosphere. These observations are of unique quality and invaluable for future auroral studies.
Key Points
We present observations of Saturn's ultraviolet aurorae in unprecedented resolution, revealing previously unseen small‐scale features
The main aurorae can be smooth or rippled, likely depending on magnetospheric conditions, and multiple parallel arcs are observed near dusk
An outer emission is, although variable in brightness, always present and suggested to be driven by hot electrons from the ring current
•We document pulsation at about a 1h period in Saturn’s aurora.•The B-field, energetic electrons, and auroral hiss at the magnetopause are modulated together.•The pulsating forms in the high latitude ...auroral zone rotate at about 40% of corotation speed.•A large fraction of the polar cap is involved in the pulsation, although phase varies with location.
Over the course of about 6h on Day 129, 2008, the UV imaging spectrograph (UVIS) on the Cassini spacecraft observed a repeated intensification and broadening of the high latitude auroral oval into the polar cap. This feature repeated at least 5 times with about a 1h period, as it rotated in the direction of corotation, somewhat below the planetary rotation rate, such that it moved from noon to post-dusk, and from roughly 77° to 82° northern latitudes during the observing interval. The recurring UV observation was accompanied by pronounced ∼1h pulsations in auroral hiss power, magnetic perturbations consistent with small-scale field aligned currents, and energetic ion conics and electrons beaming upward parallel to the local magnetic field at the spacecraft location. The magnetic field and particle events are in phase with the auroral hiss pulsation. This event, taken in the context of the more thoroughly documented auroral hiss and particle signatures (seen on many high latitude Cassini orbits), sheds light on the possible driving mechanisms, the most likely of which are magnetopause reconnection and/or Kelvin Helmholtz waves.
While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, there is considerable evidence that auroral emissions on Jupiter ...and Saturn are driven primarily by internal processes, with the main energy source being the planets' rapid rotation. Prior observations have suggested there might be some influence of the solar wind on Jupiter's aurorae and indicated that auroral storms on Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of auroral processes on solar wind conditions, a large campaign of observations of these planets has been undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft and solar wind conditions both propagated from 1 AU and measured near each planet. The data indicate a brightening of both the auroral emissions and Saturn kilometric radiation at Saturn close in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical relationship between Saturnian auroral processes and solar wind conditions. At Jupiter the correlation is less strong, with increases in total auroral power seen near the arrival of solar wind forward shocks but little increase observed near reverse shocks. In addition, auroral dawn storms have been observed when there was little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of Jupiter and Saturn.
During the Grand Finale Phase of Cassini, the Ultraviolet Imaging Spectrograph on board the spacecraft detected repeated detached small-scale auroral structures. We describe these structures as ...auroral beads, a term introduced in the terrestrial aurora. Those on DOY 232 2017 are observed to extend over a large range of local times, i.e., from 20 LT to 11 LT through midnight. We suggest that the auroral beads are related to plasma instabilities in the magnetosphere, which are often known to generate wavy auroral precipitations. Energetic neutral atom enhancements are observed simultaneously with auroral observations, which are indicative of a heated high pressure plasma region. During the same interval we observe conjugate periodic enhancements of energetic electrons, which are consistent with the hypothesis that a drifting interchange structure passed the spacecraft. Our study indicates that auroral bead structures are common phenomena at Earth and giant planets, which probably demonstrates the existence of similar fundamental magnetospheric processes at these planets.
Auroral observations capture the ionospheric response to dynamics of the whole magnetosphere and may provide evidence of the significance of reconnection at Saturn. Bifurcations of the main dayside ...auroral emission have been related to reconnection at the magnetopause and their surface is suggested to represent the amount of newly opened flux. This work is the first presentation of multiple brightenings of these auroral features based on Cassini ultraviolet auroral observations. In analogy to the terrestrial case, we propose a process, in which a magnetic flux tube reconnects with other flux tubes at multiple sites. This scenario predicts the observed multiple brightenings, it is consistent with subcorotating auroral features which separate from the main emission, and it suggests north‐south auroral asymmetries. We demonstrate that the conditions for multiple magnetopause reconnection can be satisfied at Saturn, like at Earth.
Key Points
Multiple magnetopause reconnection like at Earth is possible at Saturn
North‐south asymmetries at Saturn's dayside aurora are predicted
Magnetopause reconnection at Saturn is of certain significance
The Alice spectrograph on New Horizons performed several far-ultraviolet (FUV) airglow observations during the July 2015 flyby of Pluto. One of these observations, named PColor2, was a short (226 s) ...scan across the dayside disk of Pluto from a range of ∼34,000 km, at about 40 minutes prior to closest approach. The brightest observed FUV airglow signal at Pluto is the Lyman alpha (Lyα) emission line of atomic hydrogen, which arises primarily through the resonant scattering of solar Lyα by H atoms in the upper atmosphere, with a brightness of about 30 Rayleigh. Pluto appears dark against the much brighter (∼100 Rayleigh) sky background; this sky background is likewise the result of resonantly scattered solar Lyα, in this case by H atoms in the interplanetary medium (IPM). Here we use an updated photochemical model and a resonance line radiative transfer model to perform detailed simulations of the Lyα emissions observed in the Alice PColor2 scan. The photochemical models show that H and CH4 abundances in Pluto’s upper atmosphere are a very strong function of the near-surface mixing ratio of CH4, and could provide a useful way to remotely monitor seasonal climate variations in Pluto’s lower atmosphere. The morphology of the PColor2 Lyα emissions provides constraints on the current abundance profiles of H atoms and CH4 molecules in Pluto’s atmosphere, and indicate that the globally averaged near-surface mixing ratio of CH4 is currently close to 0.4%. This new result thus provides independent confirmation of one of the primary results from the solar occultation, also observed with the New Horizons Alice ultraviolet spectrograph.
•Lyα emissions observed during the New Horizons flyby constrain the abundance profiles of H and CH4 in Pluto’s atmosphere.•These constraints indicate that the near-surface mixing ratio of methane is 0.4%, in agreement with solar occultation results.•The abundance levels of H and CH4 in Pluto’s upper atmosphere are very sensitive to small changes in Pluto’s lower atmosphere.
We present Cassini auroral observations obtained on 11 November 2016 with the Ultraviolet Imaging Spectrograph at the beginning of the F‐ring orbits and the Grand Finale phase of the mission. The ...spacecraft made a close approach to Saturn's southern pole and offered a remarkable view of the dayside and nightside aurora. With this sequence we identify, for the first time, the presence of dusk/midnight arcs, which are azimuthally spread from high to low latitudes, suggesting that their source region extends from the outer to middle/inner magnetosphere. The observed arcs could be auroral manifestations of plasma flows propagating toward the planet from the magnetotail, similar to terrestrial “auroral streamers.” During the sequence the dawn auroral region brightens and expands poleward. We suggest that the dawn auroral breakup results from a combination of plasma instability and global‐scale magnetic field reconfiguration, which is initiated by plasma flows propagating toward the planet. Alternatively, the dawn auroral enhancement could be triggered by tail magnetic reconnection.
Key Points
We identify a novel type of auroral arc at Saturn: “azimuthally extended polar to equator arc”
The arc could be ionospheric signature of moving plasma flow released from tail reconnection, similar to the terrestrial auroral streamer
Dawn auroral enhancements and poleward expansion could be initiated by the auroral arc
Stagnation of Saturn's auroral emission at noon Radioti, A.; Grodent, D.; Gérard, J.‐C. ...
Journal of geophysical research. Space physics,
June 2017, Letnik:
122, Številka:
6
Journal Article, Web Resource
Recenzirano
Odprti dostop
Auroral emissions serve as a powerful tool to investigate the magnetospheric processes at Saturn. Solar wind and internally driven processes largely control Saturn's auroral morphology. The main ...auroral emission at Saturn is suggested to be connected with the magnetosphere‐solar wind interaction, through the flow shear related to rotational dynamics. Dawn auroral enhancements are associated with intense field‐aligned currents generated by hot tenuous plasma carried toward the planet in fast moving flux tubes as they return from tail reconnection site to the dayside. In this work we demonstrate, based on Cassini auroral observations, that the main auroral emission at Saturn, as it rotates from midnight to dusk via noon, occasionally stagnates near noon over a couple of hours. In half of the sequences examined, the auroral emission is blocked close to noon, while in three out of four cases, the blockage of the auroral emission is accompanied with signatures of dayside reconnection. We discuss some possible interpretations of the auroral “blockage” near noon. According to the first one, it could be related to local time variations of the flow shear close to noon. Auroral local time variations are also suggested to be initiated by radial transport process. Alternatively, the auroral blockage at noon could be associated with a plasma circulation theory, according to which tenuously populated closed flux tubes as they return from the nightside to the morning sector experience a blockage in the equatorial plane and they cannot rotate beyond noon.
Key Points
The auroral emission at Saturn as it rotates from midnight to dusk via noon occasionally is blocked near noon
The blockage of the emission near noon is often accompanied by dayside magnetopause reconnection
The auroral blockage could be related to flow shear variations or/and to fast rotating flux tubes which experience a blockage at noon
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