Main Auroral Oval Research Articles

On the nature and location of the proposed twin vortex systems in Saturn's polar upper atmosphere

AbstractTwin vortices in Saturn's northern and southern polar ionospheres have been proposed as the drivers of the ∼10.7 h periodicities in the magnetosphere. We critically analyze this model from an atmospheric perspective, identifying two broad problems: first, the difficulty in reconciling a ∼10.7 h periodicity with the strongly subcorotational flows that are observed in the polar ionosphere; second, the very large energy requirement for sustaining the vortices against Joule dissipation. We propose a model in which the twin vortex originates neither in the plasma nor in the thermosphere but deeper in the atmosphere, around 750 km altitude. At this altitude the gas is expected to be close to corotation, and Joule dissipation is low. A twin vortex system at this altitude blowing laterally across the Hall conductance gradient at the main auroral oval generates field‐aligned currents which are qualitatively similar to those driven by a plasma vortex or thermospheric vortex at higher altitudes.

Multispectral simultaneous diagnosis of Saturn's aurorae throughout a planetary rotation

From 27 to 28 January 2009, the Cassini spacecraft remotely acquired combined observations of Saturn's southern aurorae at radio, ultraviolet, and infrared wavelengths, while monitoring ion injections in the middle magnetosphere from energetic neutral atoms. Simultaneous measurements included the sampling of a full planetary rotation, a relevant timescale to investigate auroral emissions driven by processes internal to the magnetosphere. In addition, this interval coincidentally matched a powerful substorm‐like event in the magnetotail, which induced an overall dawnside intensification of the magnetospheric and auroral activity. We comparatively analyze this unique set of measurements to reach a comprehensive view of kronian auroral processes over the investigated timescale. We identify three source regions for the atmospheric aurorae, including a main oval associated with the bulk of Saturn Kilometric Radiation (SKR), together with polar and equatorward emissions. These observations reveal the coexistence of corotational and subcorototational dynamics of emissions associated with the main auroral oval. Precisely, we show that the atmospheric main oval hosts short‐lived subcorotating isolated features together with a bright, longitudinally extended, corotating region locked at the southern SKR phase. We assign the substorm‐like event to a regular, internally driven, nightside ion injection possibly triggered by a plasmoid ejection. We also investigate the total auroral energy budget, from the power input to the atmosphere, characterized by precipitating electrons up to 20 keV, to its dissipation through the various radiating processes. Finally, through simulations, we confirm the search‐light nature of the SKR rotational modulation and we show that SKR arcs relate to isolated auroral spots. We characterize which radio sources are visible from the spacecraft and we estimate the fraction of visible southern power to a few percent. The resulting findings are discussed in the frame of pending questions as the persistence of a corotating field‐aligned current system within a subcorotating magnetospheric cold plasma, the occurrence of plasmoid activity, and the comparison of auroral fluxes radiated at different wavelengths.

Modeling Jupiter's magnetosphere: Influence of the internal sources

We introduce a new model to study Jupiter's magnetosphere and how it interacts with the solar wind. We first derive a set of one‐fluid MHD equations to consistently include the ion‐neutral collisions in Jupiter's ionosphere and the mass loading in the Io torus. The mass loading and the subsequent radial mass transport in Jupiter's magnetosphere leads to a deviation from full corotation of the magnetospheric plasma. Ion‐neutral collisions in Jupiter's ionosphere and subsequent transport of angular momentum out into the magnetosphere acts to spin up the magnetosphere's plasma. Our model explicitly includes mass loading in the Io plasma torus and an inner boundary region, which represents the effects of Jupiter's ionosphere. We present the results of five model runs where different mass loading rates and ionospheric conductances are used. For these model runs, we consider an antiparallel interplanetary magnetic field and a strong solar wind dynamic pressure, resulting in a compressed magnetosphere. The results are compared with analytical models, in situ measurements, and remote‐sensing observations. Our azimuthal velocity profiles and the position of the corotation breakdown are in quantitative agreement with theoretical predictions by Hill (1979, 2001) and Saur et al., (2004a), and Voyager observations. The total current flowing into and out of the ionosphere is 48.7 MA, which is in agreement with estimates from measurements and analytical models. Using the field aligned electric current j∥ to determine the position of the aurorae, we find that our main auroral oval is associated, as expected, with the position of the corotation breakdown (between 20.6 RJ and 30.1 RJ for the different model runs). The discontinuity in the main oval observed by Radioti et al. (2008) is also present in our results, where it is caused by an asymmetry in the pressure distribution, due to the interaction between the rotating plasma and the magnetopause.

Io's volcanism influences Jupiter's magnetosphere

Volcanic emissions from Jupiter's moon Io supply plasma to the planet's magnetosphere and lead to its main auroral emissions. New observations show that the main auroral oval expanded and outer emissions brightened in spring 2007. Some studies have suggested that magnetospheric changes such as these could be caused by changes in the incoming solar wind. Bonfond et al. present several lines of evidence—including images from the Hubble Space Telescope and observations of a volcanic plume on Io from the New Horizons probe along with measurements of increased emissions from Jupiter's sodium cloud—that indicate that Io's volcanism controls changes in Jupiter's magnetosphere. (Geophysical Research Letters, doi:10.1029/2011GL050253, 2012)

Auroral evidence of Io's control over the magnetosphere of Jupiter

Contrary to the case of the Earth, the main auroral oval on Jupiter is related to the breakdown of plasma corotation in the middle magnetosphere. Even if the root causes for the main auroral emissions are Io's volcanism and Jupiter's fast rotation, changes in the aurora could be attributed either to these internal factors or to fluctuations of the solar wind. Here we show multiple lines of evidence from the aurora for a major internally‐controlled magnetospheric reconfiguration that took place in Spring 2007. Hubble Space Telescope far‐UV images show that the main oval continuously expanded over a few months, engulfing the Ganymede footprint on its way. Simultaneously, there was an increased occurrence rate of large equatorward isolated auroral features attributed to injection of depleted flux tubes. Furthermore, the unique disappearance of the Io footprint on 6 June appears to be related to the exceptional equatorward migration of such a feature. The contemporary observation of the spectacular Tvashtar volcanic plume by the New‐Horizons probe as well as direct measurement of increased Io plasma torus emissions suggest that these dramatic changes were triggered by Io's volcanic activity.

Ultraviolet and infrared observations of Saturn's auroral oval

New observations of Saturn's southern auroral oval made simultaneously in ultraviolet (UV) and infrared (IR) wavelengths show how complex and dynamic the auroral oval is. Melin et al. present high‐spatial‐resolution observations from the Cassini spacecraft's UV and IR instruments. The auroral oval varies over short time scales and has many small‐scale structures. The researchers observed three different arcs within the auroral oval, which they suggest are probably caused by precipitation of particles with different energies. In addition, the researchers found that the main auroral oval is morphologically similar in UV and IR, but in the auroral emissions equatorward and poleward of the main oval, there are noticeable differences in UV and IR, highlighting the need for observations at a range of wavelengths. (Geophysical Research Letters, doi:10.1029/2011GL048457, 2011)

Simultaneous Cassini VIMS and UVIS observations of Saturn's southern aurora: Comparing emissions from H, H2 and H3+ at a high spatial resolution

Here, for the first time, temporally coincident and spatially overlapping Cassini VIMS and UVIS observations of Saturn's southern aurora are presented. Ultraviolet auroral H and H2 emissions from UVIS are compared to infrared H3+ emission from VIMS. The auroral emission is structured into three arcs – H, H2 and H3+ are morphologically identical in the bright main auroral oval (∼73°S), but there is an equatorward arc that is seen predominantly in H (∼70°S), and a poleward arc (∼74°S) that is seen mainly in H2 and H3+. These observations indicate that, for the main auroral oval, UV emission is a good proxy for the infrared H3+ morphology (and vice versa), but for emission either poleward or equatorward this is no longer true. Hence, simultaneous UV/IR observations are crucial for completing the picture of how the atmosphere interacts with the magnetosphere.

Improved mapping of Jupiter's auroral features to magnetospheric sources

[1] The magnetospheric mapping of Jupiter's polar auroral emissions is highly uncertain because global Jovian field models are known to be inaccurate beyond ∼30 RJ. Furthermore, the boundary between open and closed flux in the ionosphere is not well defined because, unlike the Earth, the main auroral oval emissions at Jupiter are likely associated with the breakdown of plasma corotation and not the open/closed flux boundary in the polar cap. We have mapped contours of constant radial distance from the magnetic equator to the ionosphere in order to understand how auroral features relate to magnetospheric sources. Instead of following model field lines, we map equatorial regions to the ionosphere by requiring that the magnetic flux in some specified region at the equator equals the magnetic flux in the area to which it maps in the ionosphere. Equating the fluxes in this way allows us to link a given position in the magnetosphere to a position in the ionosphere. We find that the polar auroral active region maps to field lines beyond the dayside magnetopause that can be interpreted as Jupiter's polar cusp; the swirl region maps to lobe field lines on the night side and can be interpreted as Jupiter's polar cap; the dark region spans both open and closed field lines and must be explained by multiple processes. Additionally, we conclude that the flux through most of the area inside the main oval matches the magnetic flux contained in the magnetotail lobes and is probably open to the solar wind.

Quasi-periodic polar flares at Jupiter: A signature of pulsed dayside reconnections?

[1] The most dynamic part of the Jovian UV aurora is located inside the main auroral oval. This region is known to regularly show localized but dramatic enhancements on timescales of several tens of seconds, called polar flares. They have often been associated with the polar cusp, based on their location in the polar cap. The present study is based on the longest high-time resolution image sequences ever acquired by the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. We report the first observations of a regularity in the occurrence of these flares, with a timescale of 2–3 minutes. We use a magnetic flux mapping model to identify the region corresponding to these emissions in the equatorial plane: the radial distance ranges from 55 to 120 Jovian radii and the local times are between 10:00 and 18:00. The analogy with similar phenomena observed at Earth suggests that these quasi-periodic auroral flares could be related to pulsed reconnections at the dayside magnetopause. Indeed, the flares' projected location in the equatorial plane and their rate of re-occurrence show some similarities with the properties of the flux transfer events observed by the Pioneer and Voyager probes.

Latitudinal variations in Saturn's ionosphere: Cassini measurements and model comparisons

We present a study of latitudinal variations in Saturn's ionosphere using Cassini Radio Science Subsystem (RSS) measurements and Saturn‐Thermosphere‐Ionosphere‐Model (STIM) simulations. On the basis of Cassini RSS observations, the peak electron density (NMAX) and the total electron content (TEC) both exhibit a clear increase with latitude, with a minimum at Saturn's equator. When compared with these RSS trends, current model simulations overestimate NMAX and TEC at low latitudes and underestimate those parameters at middle and high latitudes. STIM is able to reproduce the RSS values for NMAX and TEC at low latitude when an additional low‐latitude loss process, such as a water influx, is introduced near Saturn's equator. The lack of auroral precipitation processes in the model likely explains some model/data discrepancies at high latitude; however, most of the high‐latitude RSS data are from latitudes outside of Saturn's typical main auroral oval. Using Cassini RSS electron density altitude profiles combined with ion density fractions and neutral background parameters calculated in STIM, we also present estimates of the latitudinal variations of Saturn's Pedersen conductance, ΣP. We find ΣP to be driven by ion densities in Saturn's lower ionosphere and to exhibit a latitudinal trend with a peak at mid‐latitude. Model calculations are able to reproduce low‐latitude conductances when an additional loss process is introduced, as before, but consistently underestimate most of the mid‐ and high‐latitude conductances derived from Cassini observations, perhaps indicating a missing ionization source within the model.

LOCATION AND MAGNETOSPHERIC MAPPING OF SATURN'S MID-LATITUDE INFRARED AURORAL OVAL

Previous observations of Saturn's infrared aurorae have shown that a mid-latitude aurora exists significantly equatorward of the main auroral oval. Here, we present new results using data from four separate observing runs in 1998, 2003, 2008, and 2010. When combined, these provide a view of the mid-latitude aurora under a considerable range of viewing conditions, allowing the first calculation of the latitudinal position of this aurora to be made. This has shown that the mid-latitude aurora is located at the magnetic footprint of the region within the magnetosphere where the initial breakdown in corotation occurs, between 3 R S and the orbit of Enceladus (~3.95 R S). We also confirm that this aurora is a continuous stable feature over a period of more than a decade and that an oval morphology is likely. When combined, these results indicate that the mid-latitude auroral oval is formed by currents driven by the breakdown process within the magnetosphere, in turn caused by mass loading from the torus of Enceladus, analogous with the volcanic moon Io's dominant role in the formation of Jupiter's main auroral oval.

Magnetosphere‐ionosphere coupling at Jupiter: Effect of field‐aligned potentials on angular momentum transport

We present a time‐independent model of Jupiter's rotation‐driven aurora based on angular momentum conservation, including the effects of a field‐aligned potential () and an ionospheric conductivity that is modified by precipitating electrons. We argue that arises from a limit to field‐aligned current at high latitudes, and hence, we apply a current‐voltage relation, which takes into account the low plasma densities at high latitudes. The resulting set of nonlinear equations that govern the behavior of angular momentum transfer is underconstrained and leads to a set of solutions, including those derived in earlier work. We show that solutions with high angular momentum transfer, large radial currents, and small mass transport rates ( ≤ 1000 kg/s) exist. Our set of solutions can reproduce many of the observed characteristics of Jupiter's main auroral oval, including the energy of the precipitating electrons, the energy flux into the ionosphere, the width of the aurora at the ionosphere, and net radial current across the field for a radial mass transport value of ∼500 kg/s.

Reimei observation of highly structured auroras caused by nonaccelerated electrons

We present observations of highly structured, thin auroras cased by precipitation of nonaccelerated electrons on the basis of optical and particle measurements performed by the Reimei satellite near the equatorward edge of the main auroral oval. The aurora has the following characteristics: (1) A full width at half maximum (FWHM) value is as low as only ∼1.8 km from optical measurements, and ∼0.6 km from particle measurements at the ionospheric altitude, which is much smaller than previously determined. (2) The FWHM value of 0.6 km corresponds to 9 km in the equatorial plane, which is ∼10 times smaller than the gyroradius of typical protons trapped in the near‐Earth plasma sheet. (3) At high energies greater than ∼1 keV, the velocity distribution function of precipitating electrons is comparable to that of the trapped ones and does not demonstrate any plateau or positive gradient in the distribution. (4) The aurora was observed in geomagnetically quiet condition. (5) A geosynchronous satellite observed a significant increase in the plasma pressure of hot electrons in comparison with that of hot ions. The structuring of the aurora may be attributed to scattering processes of hot electrons. If this were the case, hot electrons would be scattered by whistler mode chorus, or electrostatic electron cyclotron harmonic waves, and the structured aurora would be a visual manifestation of the highly structured, cold plasma that determines the growth of the waves scattering the hot electrons.

A NEW MINIMALLY INVASIVE TREATMENT FOR MALE STRESS URINARY INCONTINENCE: THE ADVANCE® MALE SLING, ONE YEAR DATA FROM A MULTICENTER PROSPECTIVE STUDY

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H2 band systems and H Lyman-α from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2–4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5–16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassini Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325–326 (November 20–21). The global brightening events on November 20–21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for ∼2 min is associated with auroral flares inside the main auroral ovals. On January 8, 2001, from 18:45–19:35 UT UVIS H2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to auroral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6–5 kHz Galileo PWS data.

Coupled rotational dynamics of Jupiter's thermosphere and magnetosphere

Abstract. We describe an axisymmetric model of the coupled rotational dynamics of the thermosphere and magnetosphere of Jupiter that incorporates self-consistent physical descriptions of angular momentum transfer in both systems. The thermospheric component of the model is a numerical general circulation model. The middle magnetosphere is described by a simple physical model of angular momentum transfer that incorporates self-consistently the effects of variations in the ionospheric conductivity. The outer magnetosphere is described by a model that assumes the existence of a Dungey cycle type interaction with the solar wind, producing at the planet a largely stagnant plasma flow poleward of the main auroral oval. We neglect any decoupling between the plasma flows in the magnetosphere and ionosphere due to the formation of parallel electric fields in the magnetosphere. The model shows that the principle mechanism by which angular momentum is supplied to the polar thermosphere is meridional advection and that mean-field Joule heating and ion drag at high latitudes are not responsible for the high thermospheric temperatures at low latitudes on Jupiter. The rotational dynamics of the magnetosphere at radial distances beyond ~30 RJ in the equatorial plane are qualitatively unaffected by including the detailed dynamics of the thermosphere, but within this radial distance the rotation of the magnetosphere is very sensitive to the rotation velocity of the thermosphere and the value of the Pedersen conductivity. In particular, the thermosphere connected to the inner magnetosphere is found to super-corotate, such that true Pedersen conductivities smaller than previously predicted are required to enforce the observed rotation of the magnetosphere within ~30 RJ. We find that increasing the Joule heating at high latitudes by adding a component due to rapidly fluctuating electric fields is unable to explain the high equatorial temperatures. Adding a component of Joule heating due to fluctuations at low latitudes is able to explain the high equatorial temperatures, but the thermospheric wind systems generated by this heating cause super-corotation of the inner magnetosphere in contradiction to the observations. We conclude that the coupled model is a particularly useful tool for study of the thermosphere as it allows us to constrain the plausibility of predicted thermospheric structures using existing observations of the magnetosphere.

Complex structure within Saturn’s infrared aurora

The majority of planetary aurorae are produced by electrical currents flowing between the ionosphere and the magnetosphere which accelerate energetic charged particles that hit the upper atmosphere. At Saturn, these processes collisionally excite hydrogen, causing ultraviolet emission, and ionize the hydrogen, leading to H(3)(+) infrared emission. Although the morphology of these aurorae is affected by changes in the solar wind, the source of the currents which produce them is a matter of debate. Recent models predict only weak emission away from the main auroral oval. Here we report images that show emission both poleward and equatorward of the main oval (separated by a region of low emission). The extensive polar emission is highly variable with time, and disappears when the main oval has a spiral morphology; this suggests that although the polar emission may be associated with minor increases in the dynamic pressure from the solar wind, it is not directly linked to strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn's magnetosphere. The equatorward arc of emission exists only on the nightside of the planet, and arises from internal magnetospheric processes that are currently unknown.

Evidence for subauroral proton flashes on the dayside as the result of the ion cyclotron interaction

A series of proton aurora flashes produced by the precipitation of energetic protons equatorward of the main auroral oval was observed between 0700 and 1300 UT on 31 May 2005 on the dayside with the Imager for Magnetopause‐to‐Auroral Global Exploration (IMAGE) Far Ultraviolet (FUV) imager. At the same time multiple bursts of geomagnetic pulsations within the Pc1 range known as “hydromagnetic emission bursts” were observed on several ground stations located in the conjugate MLT sector. The pulsation bursts are well correlated with the appearance of the proton aurora conjugated with the ground stations. The frequency width of the pulsation burst correlates and the upper frequency anticorrelates, respectively, with the latitudinal width and lowest latitude of the proton aurora at the meridian of the ground station. Observations at the meridional network of pulsation magnetometers show that the maximum pulsation spectral density is detected at stations conjugated with proton flashes. The close relationship between pulsations and proton precipitation (aurora) indicates their common source. We conclude that the source is the cyclotron instability of the ring current ions that is stimulated by impulsive magnetospheric compressions. The magnetosphere compressions are, indeed, confirmed by plasma data from the Geotail spacecraft in the duskside magnetosheath. They show a series of plasma pressure pulses during the time interval of interest. Also standard midlatitude magnetograms indicate large‐scale magnetospheric compressions.

Jovian-like aurorae on Saturn

Planetary aurorae are formed by energetic charged particles streaming along the planet's magnetic field lines into the upper atmosphere from the surrounding space environment. Earth's main auroral oval is formed through interactions with the solar wind, whereas that at Jupiter is formed through interactions with plasma from the moon Io inside its magnetic field (although other processes form aurorae at both planets). At Saturn, only the main auroral oval has previously been observed and there remains much debate over its origin. Here we report the discovery of a secondary oval at Saturn that is approximately 25 per cent as bright as the main oval, and we show this to be caused by interaction with the middle magnetosphere around the planet. This is a weak equivalent of Jupiter's main oval, its relative dimness being due to the lack of as large a source of ions as Jupiter's volcanic moon Io. This result suggests that differences seen in the auroral emissions from Saturn and Jupiter are due to scaling differences in the conditions at each of these two planets, whereas the underlying formation processes are the same.

Simultaneous appearance of isolated auroral arcs and Pc 1 geomagnetic pulsations at subauroral latitudes

We have been conducting observations of aurora and geomagnetic pulsations at Athabasca, Canada, located at a subauroral latitude (magnetic latitude: 62°, L ∼ 4.6), using an all‐sky imager and an induction magnetometer. Isolated auroral arcs at wavelengths of 557.7 nm, 630.0 nm, and 486.1 nm (Hβ) were often observed at latitudes separated equatorward from the main auroral oval. From a 1‐year observation (4 September 2005 to 3 September 2006), we found 13 isolated arc events. All these isolated arcs occurred coincidentally with Pc 1 geomagnetic pulsations, although there were nine other Pc 1 events without isolated arcs in the field of view of the imager. The arcs were observed in both pre‐ and post‐midnight sectors and tended to appear during the late recovery phase of geomagnetic storms. The isolated arcs had limited latitudinal and longitudinal widths of less than 230 km and 250–800 km, respectively. We found that as isolated arcs moved equatorward (poleward), the frequencies of the simultaneous Pc 1 pulsations increased (decreased). Using the Tsyganenko‐02 magnetic field model, the observed Pc 1 frequencies were almost the same as the frequencies of He+ electromagnetic ion cyclotron (EMIC) waves at the equatorial plane connected to observed isolated arcs. These results indicate that interactions of spatially localized EMIC waves with ring current ions cause high‐energy ion precipitation and associated isolated auroras at subauroral latitudes. These results also imply that the dynamics and instabilities in the inner magnetosphere can be monitored as low‐latitude auroral emissions away from the ordinary auroral oval.

Emission-Line Imaging of Saturn's H[FORMULA][F][SUP]+[/SUP][INF]3[/INF][/F][/FORMULA] Aurora

We present the first-ever infrared image of Saturn's auroral emission taken from Earth, using the CSHELL instrument on NASA's InfraRed Telescope Facility. We adapt the emission-line imaging technique to use the spectrometer as an ultra-narrowband imager, measuring the H3+ line at 3.953 μm with a spectral resolution of ~5000. Calibrating this technique with Jupiter shows a significant increase in sensitivity over the narrowest H3+ filters available. The resulting image of Saturn's auroral region confirms that the infrared main auroral oval is broadly similar to that seen in ultraviolet observations with a partially filled-in polar cap. It also shows that significant variations in brightness occur within the oval itself.