It is also possible to detect “small” round spots which are the auroral footprint of Io on Jupiter. Although they do not seem to be very large, the Io footprint reaches the sizeable length of 1000 kilometers. It is also possible to observe bigger structures on the exterior of the main oval whose contours are less clear. This is the region that was studied during this campaign. These aurorae are linked to the redistributions of plasma, to the injection of hot plasma and the ejection of cold plasma (see below). They are therefore also linked to the circulation of plasma from Io but it is not the same mechanism as the one that gives rise to the main oval nor is the same part of the magnetosphere involved. “Moreover, in the center of the oval there are all these extremely dynamic regions. We are trying to establish whether they are linked to the solar wind but we have just begun to study them. Jupiter’s aurorae have not yet revealed all their secrets”.

Two satellites are better than one

At the beginning of 2014, Bertrand Bonfond joined a campaign led by Sarah Badman, of the Department of Physics of the University of Lancaster in England, and Tomoki Kimura, of the Japan Aerospace Exploration Agency (Jaxa). The analyses, recently published in Geophysical Research Letters (1), have made it possible to identify the phenomena of a precise zone of the Jovian aurorae. “The observations were jointly conducted by means of telescopes from two different satellites. We were planning the observations of Hubble and the Japanese team was planning the observations of the Hisaki satellite”, he explains.

Hubble (HST), which is a space telescope developed by NASA and the ESA, is one of the major tools in our quest to conquer the cosmos. Orbiting the Earth for 25 years, it has contributed to a better understanding of the expansion of the universe, the confirmation of the presence of supermassive black holes at the center of certain galaxies or even the existence of dark matter. It also makes it possible to observe the planets of the solar system in high definition. But it is in great demand and is typically used for the study of Jovian aurorae for a few periods of 90 minutes every year (or rather 45 minutes, because the rest of the time the Earth is between Hubble and Jupiter!). Hisaki is much smaller and can only observe in the extreme ultraviolet (where Hubble can observe at wavelengths from infrared to the far ultraviolet), with a more limited spatial resolution. “But it is dedicated to the observation of the interaction between the solar wind and the atmospheres and magnetospheres of the planets of the solar system. In accordance with its position and the alignment of the Earth and Jupiter, it can observe the Jovian aurorae continuously for long periods. For this campaign, Hisaki was able to gather data for the first two months of 2014. ” For two weeks during this period, the European researchers were able to observe Jupiter for 45 minutes every day by means of Hubble. This is a relatively long period for a telescope that is in such high demand.  

The Japanese telescope therefore observed Jupiter almost without interruption, which is unprecedented, but its resolution did not enable a very precise level of analysis. The aurora studied was integrated in a single pixel. It was possible to detect variations in the auroral brightness, but not to determine the most affected regions. This was an advantage of using Hubble but for a much shorter observation time. Combining the data gathered by the two tools, i.e. gathering continuity and high special resolution, led to an understanding of these different phenomena.

(1) Kimura, T., et al. (2015), Transient internally driven aurora at Jupiter discovered by Hisaki and the Hubble Space Telescope, Geophys. Res. Lett., 42, 1662–1668, doi:10.1002/2015GL063272.