References of "Stallard, Tom"
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See detailSaturn's aurorae
Stallard, Tom; Badman, Sarah; Dyudina, Ulyana et al

Scientific conference (2014, August 05)

The aurora at Saturn represents a direct manifestation of the interaction between the planet’s surrounding space environment and its upper atmosphere. Our understanding of this interaction has greatly ... [more ▼]

The aurora at Saturn represents a direct manifestation of the interaction between the planet’s surrounding space environment and its upper atmosphere. Our understanding of this interaction has greatly improved over the past decade, as a result of both in-situ and remote sensing of the aurora by Cassini, as well as through Earth-based observations. On Earth, the interaction is dominated by the connection between the magnetosphere and the Solar Wind, with opening and closing of magnetic field lines leading to sporadic aurora that are strongly controlled by changes in the solar wind. On Jupiter, internal plasma sources combine with a rotationally-dominated magnetosphere to produce intense currents associated with the breakdown in co-rotation in the magnetosphere, producing powerful and continuous aurora. Investigations into Saturn’s aurora have shown that the overall morphology changes dramatically with the arrival of compressions in the solar wind, suggesting a strong interaction with the solar wind at Earth. However, the varying rotation rate of Saturn’s magnetosphere, first identified by measurements of Saturn’s radio emission, can also be measured in many aspects of the auroral emission. This in turn suggests a degree of rotational control within the aurora. As such, the better we understanding the relative strength of these influences on the aurora of Saturn, the more we can understand how the magnetosphere interacts the planet and how, in turn, the planet drives changes in the magnetosphere. Here, we will present observations of the auroral emission directly produced by particles precipitating into Saturn’s atmosphere (radio emission), the resultant atmospheric auroral excitation this produces within of atomic hydrogen (UV and visible emission) and molecular hydrogen (UV emission), as well as thermal emission from both ions produced through auroral ionization and neutral species heated within the auroral region (IR emission). These observations show a wide variety of different auroral features ranging from the rotational pole, though the main auroral emission and down to latitudes where Saturn’s atmosphere interacts with Enceladus. Observations at these different wavelength, when compared and contrasted, reveal details about the particle precipitation process that drive them, as well as the affect these currents have on the surrounding neutral atmosphere. In-situ measurements by Cassini of the particles and magnetic field above the polar region allow us to measure and understand the field-aligned currents that produce the aurora. In comparing these currents with the auroral emission at the foot of these field lines, it is possible to understand the magnetospheric origin for Saturn’s auroral emission, as well as understanding the two-way interaction between the atmosphere and magnetosphere that is driven though the currents that produce this aurora. [less ▲]

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See detailAuroral evidence of Io's control over the magnetosphere of Jupiter
Bonfond, Bertrand ULg; Grodent, Denis ULg; Gérard, Jean-Claude ULg et al

in Geophysical Research Letters (2012), 39

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 ... [more ▼]

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. [less ▲]

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See detailEstimated energy balance in the jovian upper atmosphere during an auroral heating event
Melin, Henrik; Miller, Steve; Stallard, Tom et al

in Icarus: International Journal of Solar System Studies (2006), 181(1), 256-265

We present an analysis of a series of observations of the auroral/polar regions of Jupiter, carried out between September 8 and 11, 1998, making use of the high-resolution spectrometer, CSHELL, on the ... [more ▼]

We present an analysis of a series of observations of the auroral/polar regions of Jupiter, carried out between September 8 and 11, 1998, making use of the high-resolution spectrometer, CSHELL, on the NASA InfraRed Telescope Facility (IRTF), Mauna Kea, Hawaii; these observations spanned an "auroral heating event." This analysis combines the measured line intensities and ion velocities with a one-dimensional model vertical profile of the jovian thermosphere/ionosphere. We compute the model line intensities both assuming local thermodynamic equilibrium (LTE) and, relaxing this condition (non-LTE), through detailed balance calculations, in order to compare with the observations. Taking the model parameters derived, we calculate the changes in heating rate required to account for the modelled temperature profiles that are consistent with the measured line intensities. We compute the electron precipitation rates required to give the modelled ion densities that are consistent with the measured line intensities, and derive the corresponding Pedersen conductivities. We compute the changes in heating due to Joule heating and ion drag derived from the measured ion velocities, and modelled conductivities, making use of ion-neutral coupling coefficients derived from a 3-D global circulation model. Finally, we compute the cooling due to the downward conduction of heat and the radiation-to-space from the H-3(+) molecular ion and hydrocarbons. Comparison of the various heating and cooling terms enables us to investigate the balance of energy inputs into the auroral/polar atmosphere. Increases in Joule heating and ion drag are sufficient to explain the observed heating of the atmosphere; increased particle precipitation makes only a minor heating contribution. But local cooling effects-predominantly radiation-to-space-are shown to be too inefficient to allow the atmosphere to relax back to pre-event thermal conditions. Thus we conclude that this event provides observational, i.e. empirical, evidence that heat must be transported away from the auroral/polar regions by thermally or mechanically driven winds. (c) 2005 Elsevier Inc. All rights reserved. [less ▲]

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See detailNon-LTE effects on H-3(+) emission in the jovian upper atmosphere
Melin, Henrik; Miller, Steve; Stallard, Tom et al

in Icarus: International Journal of Solar System Studies (2005), 178(1), 97-103

Calculations of column intensities are performed for a number of infrared transitions of the H-3(+) molecular ion, using a model atmosphere recently produced by Grodent et al. [2001. A self-consistent ... [more ▼]

Calculations of column intensities are performed for a number of infrared transitions of the H-3(+) molecular ion, using a model atmosphere recently produced by Grodent et al. [2001. A self-consistent model of the jovian auroral thermal structure. J. Geophys. Res. 106, 12933-12952]. The line intensities integrated along the line of sight through the model atmosphere are first computed assuming that all of the emitting energy levels are in local thermodynamic equilibrium (LTE). These results are compared with those derived from a detailed balance calculation using a method recently proposed by Oka and Epp [2004. Non-thermal rotational distribution of H-3(+). Astrophys. J. In press]. It is shown that the population of excited vibrational levels starts to depart from that derived from LTE at attitudes higher than 500 km (above the jovian cloud tops). This effect has been noted previously by Kim et it. [1992. Densities and vibrational distribution of H-3(+) in the jovian auroral atmosphere. J. Geophys. Res. 97, 6093-6101]. By 2000 km, all of the excited vibrational levels are Populated at less than 10% of the expected LTE value. This has important implications for the jovian upper atmosphere. In particular, the H-3(+) cooling effect will be greatly reduced high in the atmosphere. Modelled LTE line emission is greater than that derived from non-LTE modelling. Comparison of the non-LTE modelling with recent spectral measurements of the jovian auroral/polar regions in the L- and K-infrared windows shows that the Grodent et al. [2001. A self-consistent model of the jovian auroral thermal structure. J. Geophys. Res. 106, 12933-12952] profile overestimates the measured line intensity by similar to 3. Allowing for this, the non-LTE modelling shows that the column densities derived from (quasi-)LTE treatment of the measured line intensities may underestimate the real H-3(+) abundance by a factor of between 6 and 200. This means that attempts to derive important ionospheric properties, such as conductivity and related energy inputs due to magnetosphere-ionosphere coupling, front observed spectra will need to take this into account, if they are not to be seriously in error. (c) 2005 Elsevier Inc. All rights reserved. [less ▲]

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