Asymmetry in the Jovian auroral Lyman-α line profile due to thermospheric high-speed flow

Abstract

The presence of a fast wind in the auroral region atmosphere of Jupiter has been detected from the Doppler shift in near-IR observations of H₃⁺ emissions. The Doppler effect also modifies auroral resonance emission line shapes. We present a new radiative transfer equation for the hydrogen Lyman-α line in the presence of a horizontal wind and solve it numerically using the Feautrier method. This model allows us to derive the theoretical profile of the Lyman-α emission line. We apply this code to long slit echelle spectra of Jupiter's auroral Lyman-α emission obtained by HST-STIS which exhibit a self-reversed and asymmetric profile. Using a simple expression for the vertical variation of the horizontal wind velocity, we find the HST-STIS observations are well reproduced by a velocity profile reaching ∼4-8 km/s near ∼10^-5 μbar pressure altitude (∼1500 km above the 1 bar level). The velocity is much higher than velocity derived from H₃⁺ emission peaking at ∼550 km and consistent with an increase of the velocity with regards to altitude but reaching larger values than predicted by circulation models at ∼10^-5 μbar.