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The Io Trigger: Radio Waves at Jupiter

Our recent discussion about Europa (Europa: Below the Impact Zone) has me thinking about those tempting Galilean moons and the problems they present for exploration. With a magnetic field 20,000 times stronger than Earth’s, Jupiter is a radiation generator. Worlds like Europa may well have a sanctuary for life beneath the ice, but exploring the surface will demand powerful radiation shielding for sensitive equipment, not to mention the problem of trying to protect a fragile human in that environment.

Radiation at Europa’s surface is about 5.4 Sv (540 rem), although to be sure it seems to vary, with the highest radiation areas being found near the equator, lessening toward the poles. In human terms, that’s 1800 times the average annual sea-level dose. Europa is clearly a place for robotic exploration rather than astronaut boots on the ground.

Jupiter offers up an environment where the solar wind, hurling electrically charged particles at ever-shifting velocities, interacts with the powerful magnetosphere, stretching it out almost 1000 kilometers away from the Sun. It’s essential that we learn more about the behavior of the magnetic fields generated by gas giants, and on that score new work out of Goddard Space Flight Center offers some insight. GSFC’s Yasmina Martos and team have been using the inner Galilean moon, Io, as a probe, studying what sets off one type of radio emissions known to emanate from Jupiter.

Io’s volcanoes have been observed since the first Voyager flyby, driven by internal heat as the moon experiences the gravitational pull not only of Jupiter but neighboring large moons. Gas and particles released by this activity are ionized and swiftly captured by Jupiter’s magnetic field, being accelerated along the field toward the Jovian poles.

Out of this we get decametric radio emissions (DAM) as electrons spiral in the magnetic field, waves that the Juno spacecraft’s Juno Waves Instrument has been detecting. Jupiter also produces radio waves at centimeter and decimeter wavelengths, caused by atmospheric phenomenon as well as activity in the magnetosphere apart from the Io interactions. The planet is, in fact, the noisiest radio emitter in the Solar System apart from the Sun. Homing in on the Io emissions, the GSFC work deploys a new magnetic field model with higher accuracy near the moon and targets the particular geometric configurations of planet and moon needed for Juno to detect the emissions.

Studying the radio emissions mediated by Io doesn’t help us cope with the radiation problem, but it does offer clues about this particular magnetosphere, a phenomenon we’ll come to know much better as future missions arrive. The researchers, reporting in the Journal of Geophysical Research: Planets, found that the decameter radio waves are controlled by not just the strength but the shape of Jupiter’s magnetic field. They emerge from a cone-like space thus formed, so that the spacecraft can only receive the radio signal when Jupiter’s rotation moves that cone across the instrument. The effect is similar to a lighthouse beacon sweeping out to sea.

Image: The multicolored lines in this conceptual image represent the magnetic field lines that link Io’s orbit with Jupiter’s atmosphere. Radio waves emerge from the source and propagate along the walls of a hollow cone (gray area). Juno, its orbit represented by the white line crossing the cone, receives the signal when Jupiter’s rotation sweeps that cone over the spacecraft. Credit: NASA/GSFC/Jay Friedlander.

I can remember trying to pick up radio emissions from Jupiter with my first shortwave receiving set — they’ve been a known phenomenon since 1955, detectable from Earth at between 10 and 40 MHz. What we get thanks to the Juno observations is a clarification about why decametric radio waves originating in the northern hemisphere seem more abundant than those from the southern. The paper explains for the first time the particular geometric configurations producing these effects. The authors offer up a plain language summary to go along with the paper’s abstract, from which this:

Thanks to Juno, the geometry of the magnetic field has been better constrained as waves and magnetic field data have been continuously collected within the Jovian environment since July 2016. In this study, we estimate where the radio waves generate and the energy of the electrons that generate these waves, which is up to 23 times higher than previously proposed. We ultimately demonstrate that the geometry of Jupiter’s magnetic field is a primary controller for the higher observation likelihood of radio wave groups originating in the northern hemisphere relative to those originating in the southern hemisphere.

Video: The decametric radio emissions triggered by the interaction of Io with Jupiter’s magnetic field. The Waves instrument on Juno detects radio signals whenever Juno’s trajectory crosses into the beam which is a cone-shaped pattern. This beam pattern is similar to a flashlight that is only emitting a ring of light rather than a full beam. Juno scientists then translate the radio emission detected to a frequency within the audible range of the human ear. Credit: University of Iowa/SwRI/NASA.

What a fascinating place the Jovian system is. Learning the precise locations within the magnetosphere where the decametric emissions originate helps to pin down the needed magnetic field strength and electron density to fit the Juno data. “The radio emission is likely constant,” says Martos, “but Juno has to be in the right spot to listen.”

The paper is Martos et al, “Juno Reveals New Insights Into Io‐Related Decameter Radio Emissions,” Journal of Geophysical Research: Planets (18 June 2020). Abstract.

{ 11 comments… add one }
  • Pat G July 20, 2021, 14:19

    The Jovian people are known for their beautiful poetry in Ionic Decameter.

    • Paul Gilster July 20, 2021, 19:58

      Pat! Remind me to buy you a drink next time I’m in London…

  • John walker July 20, 2021, 14:35

    Paul, you might want to note that the average radiation level of 5.4Sv is a value per day. This equates to 2kSv/ Earth year. Roughly 10,000 times the level on Mars surface or 800,000 times the level on Earth.

    • Paul Gilster July 20, 2021, 19:58

      Excellent point, John.

  • Stan Clark July 20, 2021, 14:41

    Reading this article about Jupiter, causes me to wonder if anyone has considered the Jupiter system as a giant generator? If a Dyson Sphere is consider very high tech. there has to be an intermediate point. Such as using planets and their moons, rotating in a magnetic field to generate electricity. Yes there are many problems with this but if we can overcome them…..

    • Michael July 21, 2021, 2:51

      Its seen as a 100 Terrawatt system.

      Io appears to be the main antagonist of radiation, perhaps covering Io to prevent material loss and acceleration would be helpful or a complete deorbit of it.

      • Andrei July 21, 2021, 21:10

        100 terrawatt would certainly power Earth and leave some small change. 15 TW, where US use 3 TW all on their own.
        Now that many have mentioned SciFi here, I will do so also. In the film Outland one criminal in a spacesuit is kicked into what look like a solar panel. My friend ask why there’s all that electric arcs, and that it couldn’t have been supposed to be a solar panel that far from the Sun.
        No, I say, if the director of the film done his homework, such panels would rather tap into the radiation belts which got a lot more local energy.
        The location for the film is indeed Io. And to my knowledge the very first major Scifi film showing that locality.

  • Michael July 21, 2021, 2:58
  • George King July 21, 2021, 19:57

    Not saying that they’ll do this anytime soon, but . . .

    . . . they perhaps could put a lightsail craft in a more-or-less polesitter orbit above Jupiter’s north pole and intersecting a slice of that cone. See generally G. Vulpetti, L. Johnson, and G. Matloff, Solar Sails: A Novel Approach to Interplanetary Travel, at 88 (2d ed. 2015).

    That possibly could allow them to monitor the signal from the cone more continuously while staying out of the teeth of Jupiter’s radiation, at least if a an otherwise viable polesitter orbit intersected that cone at some point.

    As I understand it, Juno is slicing through that particular area in the first place because it exposes the spacecraft to less radiation. See generally https://en.wikipedia.org/wiki/Juno_(spacecraft) (“Flight Trajectory,” then “Orbit & Environment”);
    see also https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter (“Interaction with Rings & Moons,” image labeled “Jupiter’s variable radiation belts”).

    “It’s a thought anyway,” one possible way to retrieve a more continuous signal from that cone, assuming Paul’s old shortwave receiver couldn’t be tweaked to pick it up, lol.

    Sails of course do open up a wide range of possible missions, at least in theory, albeit our sail technology has a ways to go yet.

  • Eniac July 25, 2021, 23:48

    It should be noted that the Juno Radiation Vault, with 1 cm of titanium, shields the radiation at Jupiter by 800 fold. So, 2 cm should be sufficient to protect humans, and glass should work nearly as well, in case someone really wants to go!

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