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From the UCL - Nature Astronomy paper 30 Oct 2017:
Analysis of the XMM-Newton EPIC spectra (Supplementary Information) shows that the dominant emissions from the north- ern and southern aurora are from precipitating ions of O7+,8+ and S6+,...,14+ and/or C5+,6+ and therefore relate to downward current regions9,10. To identify more precisely the sources for these precip- itating ions and the associated downward currents, we use a flux equivalence mapping model12,13 to connect magnetic field lines in the ionosphere with the equatorial magnetosphere (using the north- ern Grodent anomaly12 and southern VIP414 models).
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Kelvin Helmholtz instabilities (KHIs) are perhaps one of the most important large-scale instabilities that occur in coronal, mag- netospheric and astrophysical environments, transferring large quantities of energy, momentum and plasma between separate plasma regimes. They are also thought to occur at Jupiter’s mag- netopause21,24, and they offer an alternative mechanism capable of explaining the periodic X-ray signatures3,8.
For the Earth’s magne- tosphere, KHIs can trigger magnetopause fluctuations and excite compressional ultralow-frequency (ULF) magnetic field oscilla- tions and field line resonances, driving standing Alfvén waves in the ionosphere25,26.
At Jupiter, ULF waves have been observed with 10–20min periodicity27,28, the lower bound of which matches our 9–12min X-ray pulsations. The periodicity of ULF oscillations depends on the magnitude of the magnetospheric cavity, velocity shear and thickness of the interaction boundary. At Jupiter, the size of the magnetosphere varies bimodally between compressed and expanded states (respective standoff distances16 63-92RJ). This could explain the bimodal 9–12min and 40–45min X-ray aurora periodicity.
If the thickness of the magnetopause boundary, size of the magnetosphere and velocity shear were similar on 24 May and 1 June, then KHI-driven Alfvén waves could produce recurring periodicity.
Moreover, KHIs could generate different brightening in each pole by driving oppositely directed field-aligned currents in each hemisphere through Ampere’s law. Traditional KHI studies focus solely on the shear in the flow as the generation mechanism. However, magnetic field orientation, plasma characteristics and thickness of the magnetopause boundary all have critical roles to play in generating wave modes along the boundary. It is for these reasons that, contrary to expectations from planetary rotation, KHIs are often observed along the dusk flank of the magnetospheres of both Earth29 and Saturn30, as well as the dawn sector, where the velocity shear is largest. Indeed, at larger velocity shears, KHI may also be stabilized30.
The prevalence and locations of KHIs, alongside the possibility of KHI-generated acceleration of the order of the MeV amu-1 required for the observed X-ray signatures, remain to be fully explored at Jupiter. However, wave–particle inter- actions, KHI-driven reconnection and/or modulation of current systems and their associated potential drops are all possible accel- aeration mechanisms.
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These findings also highlight possible multi-wavelength connections for Jupiter’s aurora. Ultraviolet polar auroral flares31 sometimes coincide with X-ray brightenings5 and, like the X-ray pulsations, quasi-periodically enhance on roughly a 10min tim- escale32. Bright infrared auroral hot spots are also co-located with the X-ray hot spots, which may suggest that the pulses of precipitation of high-energy ions, and their associated drivers, provide an important heating mechanism for Jupiter’s stratosphere down to the 10 mbar pressure level33.
The independent behaviour of Jupiter’s pair of soft X-ray hot spots during these observations raises fundamental questions about what processes at rapidly rotating magnetospheres produce these auroras. For Jupiter, the spectral signatures of the precipitating ions suggest that the spots locate Jupiter’s downward currents10 and may identify the northern and southern cusps9. However, the observed distinctive behaviour could be indicative of non-equatorial recon- nection, magnetopause-driven ULF waves, tail reconnection or local magnetic conditions at each polar region.
Over the coming 2years, X-ray observing campaigns in conjunction with NASA’s Juno mission will offer the opportunity to determine whether the independent behaviour that we report here is commonplace or is unique to the observations presented here. Critically, they will help to identify the magnetospheric conditions and auroral processes that are able to generate Jupiter’s highest-energy emissions and the seemingly independent behaviour of the northern and southern soft X-ray hot spots.