Jupiter’s intense auroras superheat its upper atmosphere

Jupiter’s upper atmosphere is hundreds of degrees warmer than expected. After a decades-long search, scientists may have pinned down a likely source of that anomalous heat. The culprit, a new study suggests, is the planet’s intense auroras, Jupiter’s version of Earth’s northern and southern lights (SN: 6/8/21).

The temperature of the upper atmosphere of Jupiter, which orbits an average distance of 778 million kilometers from the sun, should be about –73° Celsius, says James O’Donoghue, a planetary scientist at the JAXA Institute of Space and Astronautical Science in Sagamihara, Japan. That’s largely due to the feeble illumination of the sun there, which amounts to less than 4 percent of the energy per square meter that hits Earth’s atmosphere. Instead, the region several hundred kilometers above the planet’s cloud tops has an average temperature of about 426° C.

Scientists first noticed this mismatch more than 40 years ago. Since then, researchers have come up with several ideas about where the upper atmosphere’s thermal boost might originate, including pressure waves or gravity waves created by turbulence lower in the atmosphere. But observations by O’Donoghue and his colleagues now provide convincing evidence that the auroras pump heat throughout the planet’s upper atmosphere.

The researchers used the 10-meter Keck II telescope atop Hawaii’s dormant Mauna Kea volcano to observe Jupiter on one night each in 2016 and 2017. Specifically, the team looked for infrared emissions that betray the presence of positively charged hydrogen molecules (H₃⁺). Those molecules are created when charged particles in the solar wind, among other sources, slam into the planet’s atmosphere at hundreds or thousands of kilometers per second, painting polar auroras.

Measuring the intensities of these molecules’ infrared emissions let the team pin down how hot it gets high above the cloud tops. In those polar regions, temperatures in the upper atmosphere likely top out at about 725° C, the team reports in the Aug. 5 Nature. But at equatorial latitudes, the team’s heat map showed that the temperature falls to about 325° C. That pattern of a gradual drop-off in temperature toward lower latitudes bolsters the notion that Jupiter’s auroras are the source of anomalous heat in the upper atmosphere and that winds disperse that warmth from the polar regions.

One of the nights the team observed Jupiter — January 25, 2017 — was particularly well-timed because Jupiter was experiencing a strong solar flare at the time. Besides an intense aurora, data revealed a broad swath of warmer-than-normal gases at mid-latitudes, which the researchers interpret as a wave of warmth rolling southward. “It was pure luck that we captured this potential heat-shedding event,” says O’Donoghue.

The team’s observations “are close to a ‘smoking gun’ for the redistribution of auroral energy,” says Tommi Koskinen, a planetary scientist at the University of Arizona in Tucson. The next challenge, he notes, is to understand the underlying mechanisms of heat production and heat transfer and to then incorporate them into researchers’ simulations of Jupiter’s atmospheric circulation.