Abstract
Abstract. The aurorae at Jupiter are made up of many different features associated with a variety of generation mechanisms. The main auroral emission, also known as the main oval, is the most prominent of them as it accounts for approximately half of the total power emitted by the aurorae in the ultraviolet range. The energy of the precipitating electrons is a crucial parameter to characterize the processes at play which give rise to these auroral emissions, and the altitude of the emissions directly depends on this energy. Here we make use of far-UV (FUV) images acquired with the Advanced Camera for Surveys on board the Hubble Space Telescope and spectra acquired with the Space Telescope Imaging Spectrograph to measure the vertical profile of the main emissions. The altitude of the brightness peak as seen above the limb is ~ 400 km, which is significantly higher than the 250 km measured in the post-dusk sector by Galileo in the visible domain. However, a detailed analysis of the effect of hydrocarbon absorption, including both simulations and FUV spectral observations, indicates that FUV apparent vertical profiles should be considered with caution, as these observations are not incompatible with an emission peak located at 250 km. The analysis also calls for spectral observations to be carried out with an optimized geometry in order to remove observational ambiguities.
Highlights
The far-UV (FUV) aurorae at Jupiter can be divided into three major regions: (1) the outer emissions, including the satellite footprints, relatively compact blobs associated with injection signatures as well as more diffuse emissions associated with the pitch angle diffusion boundary; (2) the main emission, an essentially closed auroral curtain associated with corotation enforcement electric currents; and (3) the polar emissions, which can themselves be divided into (a) an active region, the locus of very intense flares, (b) a chaotic and dynamic swirl region, and (c) a dark region, which may sometimes be flanked by polar dawn spots
Our model shows that the apparent vertical profile deduced from the FUV images alone cannot allow us to derive the vertical emission profile for the nightside main emission because the apparent peak altitude is too close to the methane homopause
Observational evidence indicates that the current density, the precipitated energy flux, and the electron energy are correlated (Gustin et al, 2004)
Summary
The far-UV (FUV) aurorae at Jupiter can be divided into three major regions: (1) the outer emissions, including the satellite footprints, relatively compact blobs associated with injection signatures as well as more diffuse emissions associated with the pitch angle diffusion boundary; (2) the main emission, an essentially closed auroral curtain associated with corotation enforcement electric currents; and (3) the polar emissions, which can themselves be divided into (a) an active region, the locus of very intense flares, (b) a chaotic and dynamic swirl region, and (c) a dark region, which may sometimes be flanked by polar dawn spots (see reviews by Grodent, 2015; Delamere et al, 2014, and references therein). Hubble Space Telescope (HST) FUV images regularly display portions of the main emission above the planetary limb, making it possible to directly measure its vertical emission profile on the nightside. Cohen and Clarke (2011) found some significant hemispheric differences in the scale height of the FUV emissions at a high altitude above the limb, which they attributed to temperature differences between the two polar regions They did not examine the peak altitude of the emissions, which reflects the energy of the precipitating electrons.
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