Atmospheric Sciences Research Spotlight

Hubble Gazes at Europa's Aurora

Scientists get their best glimpse yet of the shimmering phenomena on one of Jupiter's most intriguing moons.

Source: Journal of Geophysical Research: Space Physics

By

The thin oxygen atmosphere of Jupiter’s icy moon Europa has been the subject of speculation for more than 2 decades, ever since scientists deduced its existence after spotting the telltale glow of ultraviolet auroras.

Now Roth et al. report the results of a 5-month observation campaign by NASA’s Hubble Space Telescope, from late 2014 through spring 2015. By combining that data set with previous Hubble observations as far back as 1999, this study is the most detailed yet of Europa’s auroras.

On Earth, auroras are mostly driven by the Sun, as plasma particles from the solar wind stream toward the poles and collide with the atmosphere. But Jupiter has its very own sources: the volcanoes of its moon Io and the fast rotating magnetic field. The plasma that the volcanoes spew gets trapped by Jupiter’s magnetic field, forming a disk of plasma. Because Jupiter’s magnetic field—like Earth’s—is tilted, this plasma disk is also tilted. This tilt creates a choppy environment for the moons of Jupiter like Europa, whose orbital path weaves in and out of the disk.

The team found that this weaving has a big influence on the strength of the auroras. When Europa is deep inside the disk, plowing through the plasma, the auroras light up the strongest. The brightest spots of aurora were at the poles, and they tend to be stronger at the pole that is facing the densest layer of the disk.

Other information the team gleaned from the data includes determining how much oxygen is in Europa’s atmosphere to better precision than ever before by measuring the relative strengths of different emission lines. Also, on several occasions, Hubble had a view of Europa while it was in Jupiter’s shadow—but nothing much changed, which suggests that the Sun doesn’t play much of a role in the auroras.

Many mysteries remain. One is that unlike on Io, Hubble didn’t see strong auroras near the equator of Europa. Another is that the aurora on the right half of Europa, as seen from Earth, is consistently brighter than that on the left side. One explanation for the difference could be that the local time on Europa affects the aurora. From Earth’s vantage point, the right-hand side of Europa corresponds to late afternoon and dusk, whereas the fainter left-hand side coincides with dawn. (Journal of Geophysical Research: Space Physics, doi:10.1002/2015JA022073, 2016)

—Mark Zastrow, Freelance Writer

Citation: Zastrow, M. (2016), Hubble gazes at Europa’s Aurora, Eos97, doi:10.1029/2016EO047887. Published on 17 March 2016.

© 2016. The authors. CC BY-NC 3.0
  • John W. Bonnell

    ah, but the best thing about Birkeland’s work was his hypothesis, later demonstrated to be true, that electric currents flow along the earth’s magnetic field in the aurora region, in addition to currents flowing perpendicular to the magnetic field in the auroral ionosphere (which was Chapman’s hypothesis at the time). Both sorts of currents are a significant part of the momentum transfer system from the solar wind flow to the magnetosphere and ionosphere – in essence, the way that the forces are transmitted from remote regions of the magnetosphere to the ionosphere and upper atmosphere.

    • Dennis Gallagher

      Agreed.

  • John W. Bonnell

    Most terrestrial auroras are not due to the direct access of solar wind particles to the upper atmosphere, but rather have their source in the acceleration of electrons and ions already trapped on closed terrestrial magnetic field lines.

    There are some interesting and important exceptions to this rule: proton aurora in the cusp region near magnetic noon, the dim auroral features that can be observed in the polar cap proper due to low-energy electrons from the magnetosheath (shocked solar wind plasma), and transient aurora during large solar energetic particle events.

    It is the case that the flow of the solar wind and the interplanetary magnetic field over the Earth’s magnetic field is one of the prime sources of momentum and energy for driving magnetospheric particle acceleration such as that found in the aurora, but again, most of the actual particles involved start out as populations trapped on terrestrial field lines.