Spectra of the hydrogen Lyman alpha. (Ly-alpha) emission line profiles of the jovian dayglow, obtained by the Goddard High Resolution Spectrograph on the Hubble Space Telescope, appear complex and variable on time scales of a few minutes. Dramatic changes occur in the Ly-alpha bulge region at low latitudes, where the line profiles exhibit structures that correspond to supersonic velocities of the order of several to tens of kilometers per second, This behavior, unexpected in a planetary atmosphere, is evidence for the particularly stormy jovian upper atmosphere, not unlike a star’s atmosphere.