Makenzie B. Lystrup

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 ∼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.

First Vertical Ion Density Profile in Jupiter's Auroral Atmosphere: Direct Observations Using the Keck II Telescope

We present the first vertical ion density profiles of Jupiters upper atmosphere derived directly from ground-based observations. Observations of infrared H+3 emissions in Jupiters auroral/polar regions were collected by the high-resolution spectrometer NIRSPEC on the Keck II telescope. We have calculated vertical density profiles for a latitude in the southern auroral region using the measured column densities and a shell model of the Jovian ionospheric H+3 emission. We compare our resultant profiles to those generated by a recent one-dimensional model in both local thermodynamic equilibrium (LTE) and non-LTE conditions. We find good agreement with the model profiles up to 1800 km. Above that, however, our measurements show that more H+3 is produced than is predicted by the model. Our observational method is a new tool for probing Jupiters upper atmosphere from Earth and can possibly be extended to the study of other gas giant planets.

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 H3+ 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.

the driver of giant planet atmospheres

We present a review of recent developments in the use of molecular ion as a probe of physics and chemistry of the upper atmospheres of giant planets. This ion is shown to be a good tracer of energy inputs into Jupiter (J), Saturn (S) and Uranus (U). It also acts as a ‘thermostat’, offsetting increases in the energy inputs owing to particle precipitation via cooling to space (J and U). Computer models have established that is also the main contributor to ionospheric conductivity. The coupling of electric and magnetic fields in the auroral polar regions leads to ion winds, which, in turn, drive neutral circulation systems (J and S). These latter two effects, dependent on , also result in very large heating terms, approximately 5×1012 W for Saturn and greater than 1014 W for Jupiter, planet-wide; these terms compare with approximately 2.5×1011 W of solar extreme UV absorbed at Saturn and 1012 W at Jupiter. Thus, is shown to play a major role in explaining why the temperatures of the giant planets are much greater (by hundreds of kelvin) at the top of the atmosphere than solar inputs alone can account for.

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

Previous observations of Saturns 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 Ios dominant role in the formation of Jupiters main auroral oval.

DUSK-BRIGHTENING EVENT IN SATURN'S H+3 : AURORA

We report on a unique dusk-brightening event within Saturns aurorae. Measurements of the H infrared aurora using the CSHELL instrument on NASAs Infrared Telescope Facility (IRTF), made in 2005 December, show an auroral intensity structure unlike anything previously detected. The aurora has a significantly brighter dusk sector over three Earth nights, a period in excess of 5 Saturnian days, suggesting a consistent source for this emission, stable in position within the magnetosphere. However, unlike previously detected dawn-brightening events, the overall auroral brightness remains low and the ion wind structure appears unaffected. Using the location of magnetopause crossings as a proxy for the solar wind pressure, the solar wind appears to be exceptionally rarefied. This leads us to conclude that the dusk-brightening event is strongly linked with the unusual solar wind conditions at the time of the observations.

Emission-line imaging of Saturn's H + 3 aurora

We present the first-ever infrared image of Saturns auroral emission taken from Earth, using the CSHELL instrument on NASAs 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 Saturns 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.

Location and magnetospheric mapping of Saturn's mid-latitude infrared auroral oval

Previous observations of Saturns 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 Ios dominant role in the formation of Jupiters main auroral oval.