Spotlight on Jupiter’s aurorae
7/7/15

Observations conducted with the help of the Hubble and Hisaki satellites has made it possible to understand the process at work behind the appearance of some aurorae on Jupiter. These aurorae are caused by a redistribution of plasma in a region of the planet's magnetosphere. Researchers have also been able to completely rule out the solar wind as being the cause of these particular aurorae. The dynamics of the aurorae on Jupiter depend on a number of different processes which are often very different to those we can observe on Earth. The mechanisms involved have not yet revealed all their secrets. The singular nature of these mechanisms within the solar system could hide the fact that they are quite common in the wider universe given that the gas giant Jupiter is very similar to the majority of exoplanets that have been discovered to date. Understanding these physical phenomena and differentiating them from what we already know should enable us to develop tools in order to better understand different mechanisms spread throughout the cosmos.

“The isn’t such a thing as “The” aurora at Jupiter, but there are many aurorae.", affirms Bertrand Bonfond, a post-doctoral researcher at the FNRS at the Laboratory for Planetary and Atmospheric Physics of the University of Liege (LPAP), to illustrate the multiplicity of physical mechanisms behind the Jovian aurorae. By way of a reminder, an aurora is a light phenomenon linked to the excitation of molecules or atoms in the upper atmosphere following impacts by charged particles (usually electrons) from the magnetosphere. On Jupiter, this neutral atmospheric gas is Hydrogen (nitrogen and oxygen on Earth). On Earth, the aurora is dominated by a phenomenon of reconnection between the magnetic field of the planet and that of the solar wind. (See articles “A transpolar arc discovered on Saturn” and “Surprise around Jupiter’s aurorae”). The aurorae of the gas giant are mostly linked to an internal process. “The Jovian system also has another very important ingredient, the moon Io which is the most volcanic body in the solar system. This satellite which is constantly experiencing eruptions, releases a large quantity of gas which essentially remains along its orbital path at first and rotates at the same speed as Io around Jupiter. It is then transformed into plasma. Captured by Jupiter’s magnetic field, it is then accelerated until it reaches the rotation speed of the planet.  But this plasma cannot accumulate indefinitely. It will migrate towards the exterior and will finally be released by the magnetosphere." It is the circulation of this plasma in the magnetosphere which causes the electric currents necessary for the formation of a large amount of the Jovian aurorae. But this is not the full story.  

Jupiter aurorae 2001 (c)ULg
Difficult to know exactly what is happening

The influence of the solar wind remains a great unknown. We know the role the solar wind plays on Earth but it is difficult to assess its role on Jupiter. The gas giant is much further away from the sun so the impression of the solar wind is much weaker there. In addition, Jupiter’s magnetic field is incomparably more intense than Earth’s and the internal phenomena linked to the circulation of plasma dominate. “Having said that, explains the planetologist, “it is not correct to say that the solar wind has no influence even if it is only of secondary importance because when the wind is stronger, it can considerably compress the Jovian magnetosphere and influence how the plasma circulates in its interior”. Based on this observation, the challenge is to differentiate between aurorae linked to the solar wind and those that are not. This is the real puzzle. It is certainly true that there are models which make it possible to estimate the variations in the solar wind on Jupiter and their probable evolutions based on measurements taken on Earth. But the precision of these models is limited and does not provide an understanding of the time delay observed between the estimated time of arrival of an enhancement of the solar wind and the level of intensity of the aurora linked to this enhancement.

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