Dynamics of the Io UV footprints: short timescale brightness variations

The electromagnetic interaction between Io and Jupiter's magnetic field leads to single or multiple UV spots in both jovian hemispheres. Variations of the multiplicity of the spots and the inter-spots distances have been observed and are linked to the position of Io in its plasma torus. These morphological changes have a timescale of a few hours. We have studied the footprint evolution with a time resolution of a few tens of seconds using the HST/STIS camera in the time-tag mode. This approach resulted in an evaluation of the impact of the rapid evolution of the jovian magnetic field lines when measuring the dimensions of the spots with a longer exposure time. More basically, evidence of strong brightness variations of the main spots (up to 40%) have been observed with a typical growth time of 1 minute. Finally, unexpected simultaneous fluctuations of both primary and secondary spots have also been found in the southern hemisphere. In conclusion, the characterisation of the variations mentioned herein provides clues for a complex and rapidly evolving energetic electrons precipitation mechanism.