Dunn, William; (2018) The Auroral Dynamic Duo: spatial, spectral and temporal trends of Jupiter's Northern and Southern X-ray aurorae. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Jupiter's soft X-ray aurora is concentrated into a bright and dynamic hot spot that is dominated by charge exchange from precipitating high charge-state ions (e.g. Gladstone et al. 2002; Cravens et al. 2003; Elsner et al. 2005). These highly energetic planetary emissions exhibit pulsations over timescales of 10s of minutes. In one observation these pulsations were found to have a very regular periodic pulsation timescale of 45 minutes (Gladstone et al. 2002), but in all subsequent observations the timescale for pulsations was found to be irregular (e.g. Elsner et al. 2005; Branduardi-Raymont et al. 2004). Surrounding this pulsating soft X-ray (< 2 keV) spot, there is a transient auroral oval of hard X-rays (> 2 keV) produced by precipitating electrons that also produce the co-located UV main oval (e.g.Branduardi-Raymont et al. 2007; 2008). The hot spot's magnetic field lines have been suggested to map beyond 30 Jupiter Radii (RJ) to Jupiter's outer magnetosphere and possibly to the Jovian cusp (Pallier et al. 2001). This led Bunce et al. (2004) to propose that Jupiter's X-ray aurora was produced by the influence of dayside reconnection on the current systems at the magnetopause and therefore by the interaction between the solar wind and Jupiter's magnetosphere. In this thesis, we analyse the spatial, spectral and temporal characteristics of Jupiter's X-ray aurora from Chandra and XMM-Newton observation campaigns during 2007, 2011 and 2016. By applying the latest magnetosphere model mapping (Vogt et al. 2011; 2015), we identify that Jupiter's X-ray hot spot may originate from regions along the pre-noon to dusk magnetopause and that the hard X-rays mostly map to the middle magnetosphere, with a slight dawn preference. Our mapping for the hot spot supports previous suggestions that one may expect a solar wind relationship to the emissions. Indeed, we find that during solar wind density enhancements and magnetic field rotations from Interplanetary Coronal Mass Ejections and Corotating Interaction Regions, the Jovian soft X-ray aurora brightens, expands, changes spectral populations and exhibits pulsations on quicker timescales. The hard X-ray aurora also brightens during solar wind compressions but behaves independently of the soft X-ray emissions. During 4 (of 5) observed solar wind compressions, Jupiter's X-ray aurora pulses with a characteristic regular period of 9-13 minute. During solar wind rarefactions, the aurora dims and exhibits longer time-scale pulsations. It is unlikely that the X-ray aurora exists in isolation from other well-studied Jovian auroral wavebands, so we compare the X-ray aurora with radio, IR and UV emissions. We find that non-Io decametric radio emission bursts associated with solar wind compressions (e.g. Hess et al. 2014) occur during X-ray brightening. We note connections between the IR methane layer hot spot and the co-located X-ray hot spot, which may suggest that deeply penetrating ions producing the near-instantaneous X-ray emissions subsequently heat the stratosphere (e.g. through secondary electrons or subsequent photon emissions). We find (for limited available UV comparisons) that when UV dusk polar arcs form, the X-ray hot spot also brightens in a similar region. The normally transient hard X-ray emissions are observed to brighten and occur co-located with UV dusk polar arcs and UV dawn storms. Perhaps our most surprising result is that during some observations the Northern and Southern X-ray hot spots appear to behave independently of one another. Brightening in each hot spot is not correlated and a regular 9-11 minute pulsation period in the South (observed by both Chandra and XMM-Newton) is not also observed in the North, which exhibits more complex irregular pulsations. The two hot spots both map to the noon-magnetopause, but the Northern hot spot also maps along the dusk flank. We propose a few possible explanations: 1. The North and South X-ray hot spots are produced by cusp processes in-line or slightly adapted (non-sub-solar or high-latitude reconnection) from Bunce et al. (2004). 2. In addition to the cusp, the Northern aurora also maps to the dusk sector, which may be associated with tail reconnection. The viewing geometry may prevent this from being observed in the South. The combination of cusp and tail pulsations would produce a more complex lightcurve for the North, while for the South the observed lightcurve would be more regular because it only consists of pulsed transits to the noon magnetopause. This would also exhibit a solar wind relationship. Large-scale tail reconnection can be triggered by magnetospheric compressions. Associated X-ray emissions would therefore 'switch on' with solar wind compressions, causing the observed X-ray brightening and 'expanded hot spot' (actually constituting two sources in the North: tail and cusp). 3. Kelvin Helmholtz Instabilities (KHI) that form in the pre-noon sector (e.g. Ma et al. 2015), and grow along the dusk flank, generate field line resonances. The characteristic period associated with these resonances depends on the field line length. For regions further from noon, this would produce increasingly long timescales, which would explain the more complex Northern lightcurve. This can also generate intermittent reconnection and uni-directional currents (through field line twisting) which may explain differing hemispheric brightnesses. The scales of the KHI depend on the magnetic field strength, density and velocity of the plasmas on either side of the magnetopause and thus vary with solar wind conditions. 4. Alternative wave-particle interactions that propagate to different altitudes for each pole might also be responsible. While deciphering between these mechanisms is beyond the scope of this thesis, we close by proposing future investigations that may identify the mechanism/s responsible for producing the high-energy precipitation that generates Jupiter's enigmatic X-ray aurora.

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