Abstract
We analyzed two observations obtained in Jan. 2013, consisting of spatial scans of the jovian north ultraviolet aurora with the HST Space Telescope Imaging Spectrograph (STIS) in the spectroscopic mode. The color ratio (CR) method, which relates the wavelength-dependent absorption of the FUV spectra to the mean energy of the precipitating electrons, allowed us to determine important characteristics of the entire auroral region. The results show that the spatial distribution of the precipitating electron energy is far from uniform. The morning main emission arc is associated with mean energies of around 265keV, the afternoon main emission (kink region) has energies near 105keV, while the ‘flare’ emissions poleward of the main oval are characterized by electrons in the 50–85keV range. A small scale structure observed in the discontinuity region is related to electrons of 232keV and the Ganymede footprint shows energies of 157keV. Interestingly, each specific region shows very similar behavior for the two separate observations.The Io footprint shows a weak but undeniable hydrocarbon absorption, which is not consistent with altitudes of the Io emission profiles (∼900km relative to the 1bar level) determined from HST-ACS observations. An upward shift of the hydrocarbon homopause of at least 100km is required to reconcile the high altitude of the emission and hydrocarbon absorption.The relationship between the energy fluxes and the electron energies has been compared to curves obtained from Knight’s theory of field-aligned currents. Assuming a fixed electron temperature of 2.5keV, an electron source population density of ∼800m−3 and ∼2400m−3 is obtained for the morning main emission and kink regions, respectively. Magnetospheric electron densities are lowered for the morning main emission (∼600m−3) if the relativistic version of Knight’s theory is applied.Lyman and Werner H2 emission profiles, resulting from secondary electrons produced by precipitation of heavy ions in the 1–2MeV/u range, have been applied to our model. The low CR obtained from this emission suggests that heavy ions, presumably the main source of the X-ray aurora, do not significantly contribute to typical UV high latitude emission.
Highlights
This discrepancy, still not elucidated, is usually interpreted as being the result of our poor knowledge of the hydrocarbon density profiles in polar regions, which is thought to be very different from observed and modeled profiles determined at low latitudes because of the energy deposition induced by the aurora
The ratio between the images integrated in the 1550–1620 Å and the 1230–1300 Å ranges directly provides a map of the color ratios (CRs), which in turn can be converted to mean energy hEi of the precipitating electrons
Since the vertical thickness of the emission decreases as the emission moves to lower altitude, the shape of the vertical profile (VER) becomes similar to the ‘thin-Maxwellian type VER’, as used in the CR method
Summary
The ultraviolet jovian aurora is mainly produced by the interaction between the H2 atmosphere and precipitating magnetospheric electrons. Very dynamical, intense and sometimes quasi-periodic brightenings of several MR have been observed, possibly related to abrupt solar wind variations (Waite et al, 2001; Bonfond et al, 2011) The latter bright polar flares have never been directly observed with UV spectrographs, which makes it problematic to determine their characteristics, but STIS spectra obtained from ‘‘typical” polar emissions are generally associated with mean electron energies in the 30–100 keV range. By using Monte Carlo simulations, these authors showed that an observed emission peaking at 900 km is compatible with precipitation of electrons distributed as a kappa function with a mean energy of $1.1 keV, much lower than the values obtained by Gérard et al (2002) This discrepancy, still not elucidated, is usually interpreted as being the result of our poor knowledge of the hydrocarbon density profiles in polar regions, which is thought to be very different from observed and modeled profiles determined at low latitudes because of the energy deposition induced by the aurora. A fourth component consist of emissions equatorward of the main emission: transient emissions possibly due to magnetospheric injections (Dumont et al, 2014) and diffuse emissions, presumably associated with electron scattering by whistler mode waves (Radioti et al, 2009)
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