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

摘要

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 H~-_3 emissions. The Doppler effect also modifies auroral resonance emission line shapes. We present a new radiative transfer equation for the hydrogen Lyman-a 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-a emission line. We apply this code to long slit echelle spectra of Jupiter's auroral Lyman-a 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 mbar pressure altitude (1500 km above the 1 bar level). The velocity is much higher than velocity derived from emision H~-_3 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,ubar.