Energy‐flux relationship in the FUV Jovian aurora deduced from HST‐STIS spectral observations

Abstract

Far ultraviolet spectral observations of the Jovian aurora have been made since 1997 with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope at low spectral resolution. The combination of the spectral resolution with the intensity variation along the STIS slit provides information on the latitudinal variation of the precipitating auroral electron energy flux and the mean electron energy, from which the electron current density at the top of the atmosphere can also be deduced. It is found that the mean electron energies associated with the main oval lie in the range 30–200 keV and show a tendency to increase with the precipitating energy flux. The current densities lie in the range ∼0.04–0.4 μA m−2, consistent with previous estimates, and are also positively correlated with the energy flux. The observed relationship between the auroral time‐integrated energy fluxes and the electron energies in the main oval is compatible with that expected from Knight's theory of field‐aligned currents. The best agreement between the observed data and the Knight curves is obtained for an electron temperature of Te = 2.5 keV and a source density N = 0.003 cm−3, that is within the range of values observed in the equatorial plane during the Voyager flybys. No systematic dependence of the electron energy with magnetic local time is found, but the morning sector around 0800 MLT shows greater variability than other regions of the oval. Analysis of time‐tagged data shows that the main oval energy flux usually varies steadily over the several minute intervals of observation and that the mean electron energy usually undergoes correlated variations such that the current density remains relatively constant. It is shown that these overall properties are also consistent with Knight's theory of auroral electron acceleration associated with field‐aligned current flow, from which it is inferred that the temporal variations observed are often due to slow changes in the magnetospheric “source” electron parameters in the presence of near‐steady magnetosphere‐ionosphere coupling currents. By contrast, time‐integrated emissions in the polar region are found to be associated with similar mean electron energies to the main oval but with typically smaller energy fluxes and current densities. Pressure balance arguments are advanced, which indicate that the brighter of these emissions must be associated with an auroral acceleration mechanism perhaps similar to that operative in the main oval, while it remains possible that the weaker emissions could result from precipitation from a quasi‐isotropic hot magnetospheric electron source.