An international team of researchers from Japan Aerospace Exploration Agency (JAXA), National Aeronautics and Space Administration (NASA), Boston University, University of Leicester, Space Research Corporation, and the National Institute of Information and Communications Technology (NICT), led by Dr. James O’Donoghue, found that Jupiter’s intense aurora, the most powerful in the solar system, is responsible for heating the entire planet’s upper atmosphere to surprisingly high temperatures.

Overview / Background

Sitting more than five times the distance from the Sun as the Earth, Jupiter is not expected to be particularly warm. Based on the amount of sunlight received, the average temperature in the giant planet’s upper atmosphere should be about 200 K or a chilly -73 Celsius. Instead, the measured value sits around 700 K or 420 Celsius. The source of this global heat has remained elusive for 50 years, causing scientists to refer to the discrepancy as an “energy crisis” for the planet. The idea that the powerful Jovian aurora at the poles could be the source of Jupiter’s mysterious energy had been proposed previously but observations have been unable to confirm or deny this until now. In addition, a global model of Jupiter’s upper atmosphere suggested that winds heated by the aurora and headed to the equator are prevented from energy redistribution by the fast planetary rotation of ~10 hours.

Approach / Results / Outputs

The team observed Jupiter with the 10-metre Keck II telescope on Mauna Kea in Hawai’i for five hours on two separate nights in April 2016 and January 2017. Using the Near-Infrared Spectrometer (NIRSPEC) on the Keck II, emission from H3+ ions in Jupiter’s atmosphere was detected from the planet’s poles down to the equator. H3+ ions are a major constituent of the ionized part of Jupiter’s upper atmosphere and the intensity of the emission can be used to derive the temperature of that region. The team created five maps of the atmospheric temperature at different spatial resolutions. Referring to the higher spatial temperature information as much as possible while maintaining sufficient accuracy, they succeeded to derive a detailed global temperature map (Fig. 1).
The temperature maps of Jupiter's upper atmosphere show clear gradients, with temperatures decreasing from the polar auroral regions to the equator. This demonstrated that Jupiter’s aurora was circulating auroral energy planet-wide, with winds carrying the heated atmosphere to lower latitudes and adjacent longitudes. Comparing observation results of the two nights, the temperature at the pole and mid-to-low latitude observed in 2017 was clearly high (Fig. 1). Compared with the estimation results of solar wind variation at Jupiter provided by NICT (Fig. 2), January 2017 was the time when a large increase in solar wind pressure arrived at Jupiter.
This research report was published in Nature on August 5, 2021.

Future work / Prospects

This research is an example that space weather information such as solar wind variation, which NICT is working on, contributes to the understanding of the planetary environment (= scientific research). This study revealed that changes caused by the Sun affect the global heating of the upper atmosphere through the aurora driven by the giant magnetosphere of Jupiter located far from the Sun. Atmospheric circulation (wind field) is thought to play an important role in global heating, but how atmospheric wind is driven under the constraints of the fast rotation effect is an issue for the future work. Even in the closest terrestrial environment, energy distribution from the polar auroral regions is an important issue from the perspective of space weather, which can be related to communication and positioning failures in the mid-to-low latitudes.

Journal： Nature
Paper title： Global upper-atmospheric heating at Jupiter by the polar aurorae
Authors： J. O’Donoghue1,2, L. Moore3, T. Bhakyapaibul3, H. Melin4, T. Stallard4, J. E. P.Connerney5,2, C. Tao6
Affiliations ：1. ISAS, JAXA, Japan, 2. NASA Goddard Space Flight Center, USA, 3. Boston University, USA, 4. University of Leicester, UK, 5. Space Research Corporation, USA, 6. NICT, Japan
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Fig.1
Fig.1
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Fig.1 [from paper] (a) Jupiter's column-averaged H3+ temperatures on 14 April 2016 and 25 January 2017. A visible computer-generated globe of Jupiter based on Hubble space telescope imagery is shown underneath the H3+ temperature projection, imagery credit: NASA's Goddard Space Flight Center and the Space Telescope Science Institute. Note that Jupiter is tilted differently on each date in order to reveal different features. (b) Median Jovian H3+ temperatures found for each latitude across all longitudes.
Fig.2
Fig.2
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Fig.2 [from paper] Dynamic pressure of the solar wind (SW) at Jupiter from a 1-D model of solar wind propagation closely surrounding the dates of the observations reported here. The blue shaded regions mark the periods of ground-based observations. The 1σ uncertainty in arrival time of the solar wind at Jupiter is denoted by the horizontal, arrowed lines.

Contact

Space Environment Laboratory, Radio Propagation Research Center, Radio Research Institute

TAO Chihiro

E-mail: chihiro.tao_attmark_nict.go.jp