When the solar wind warms Jupiter's atmosphere
Following observations made by an international group of researchers including Denis Grodent and Bertrand Bonfond of LPAP (STAR Research Unit / Faculty of Science), it appears that when the solar wind is stronger, the heating of the base of the polar atmosphere of the gas giant Jupiter is more significant. These observations are published in the journal Nature Astronomy (1).
W
hile it is known that the phenomenon of polar aurorae is occurring at the poles of Jupiter, researchers were far from expecting that they could have such a significant impact on the largest planet in our solar system. On Earth, polar aurorae (called aurora borealis at the North Pole and aurora australis at the South Pole) result from the interaction between particles ejected by the sun - the solar wind - and Earth's magnetic field. These particles react directly to the bursts of the solar wind, and create these beautiful lights that we know. Aurorae that occur at Earth have a warming impact that is limited to the uppermost layers of the atmosphere, above 100 km, and it is therefore not perceived near the surface.
An international team of researchers, led by astronomer James Sinclair of NASA's Jet Propulsion Laboratory - including Denis Grodent and Bertrand Bonfond, planetologists at ULiège's Laboratory for Planetary and Atmospheric Physics (LPAP) - has just examined a rather unexpected mechanism linked to the phenomenon of polar aurorae occurring on Jupiter. Using infrared images taken by the COMICS (Cooled-Mid-Infrared Camera and Spectrograph) instrument of the Subaru telescope of the Japan National Observatory located atop Mauna Kea in Hawaii, researchers were able to observe that polar aurorae warm Jupiter's deep atmosphere. "We already knew that the solar wind was strongly impacting the ultraviolet and X-ray auroras on Jupiter," explains Professor Denis Grodent, Director of LPAP, "and these new observations indicate that it is not only the upper atmosphere that reacts to the tumults of the interplanetary environment, but its influence extends even further down to the stratosphere, which also heats up when the solar wind comes to life. »
Infrared images captured during the January, February and May 2017 observation campaign clearly show hot spots near the poles, where Jupiter's aurora heats the atmosphere. "Paradoxically, it is the infrared cooling effect that is observed. But this is all the more important as the temperature of the atmosphere increases," explains Denis Grodent. Warming occurs when the magnetosphere and the solar wind interact. "Not only does Jupiter's polar atmosphere react to the vagaries of the solar wind, but it also reacts very quickly," says Bertrand Bonfond, a qualified F.R.S.-FNRS researcher at LPAP, "it's a surprising discovery! " Based on the pictures obtained by the team, it is clear that barely a day after the solar wind hits Jupiter, the chemistry and structure of its atmosphere changes and its temperature increases significantly.
Infrared images of Jupiter recorded by the COMICS (Cooled Mid-Infrared Camera and Spectrograph) instrument at the Subaru telescope on top of Mauna Kea, Hawaii. These images were captured a few hours apart, from January 11 to 12, 2017, and illustrate how quickly the atmosphere showed the effects of the solar wind. Credit: NAOJ and NASA / JPL-Caltech
This phenomenon discovered by the researchers could explain why on Jupiter the atmospheric temperature at the poles is higher than at the equator, which is exactly the opposite of Earth. These observations also suggest the presence of a self-regulatory mechanism that prevents the temperature of the polar thermosphere from increasing too much, which could lead to its escape. The heating would cause hydrocarbons (such as methane CH4) to rise, which have the ability to cool down the atmosphere by transforming heat into infrared radiation, a radiation that was observed by the team. This type of mechanism also exists in Earth's upper atmosphere, where constituents such as carbon dioxide (CO2) or nitrogen monoxide (NO) can cool the atmosphere during periods of high auroral warming. It is therefore possible to observe (and therefore validate) similar physical and chemical processes in atmospheres as different as those of Earth and Jupiter.
Scientific reference
(1) J. A. Sinclair, G. S. Orton, J. Fernandes, Y. Kasaba, T. M. Sato, T. Fujiyoshi, C. Tao, M. F. Vogt, D. Grodent, B. Bonfond, J. I. Moses, T. K. Greathouse, W. Dunn, R. S. Giles, F. Tabataba-Vakili, L. N. Fletcher & P. G. J. Irwin, A brightening of Jupiter’s auroral 7.8-μm CH4 emission during a solar-wind compression, Nature Astronomy, 8 April 2019
Cover illustration
Scientists used red, blue and yellow to represent Jupiter's infrared emissions and highlight the warming that occurs at Jupiter's poles due to solar wind. Infrared images of Jupiter, sensitive to its stratospheric temperatures, recorded by the COMICS (Cooled Mid-Infrared Camera and Spectrograph) instrument at the Subaru telescope on top of Mauna Kea, Hawaii. This image was captured on January 12, 2017. Credit: NAOJ and NASA / JPL-Caltech
