Oh, Northern Lights … on Jupiter?

Date posted: December 2, 2010
Posted in: Science and Research | Space science
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Jupiter's aurorae, courtesy of NASA

Jupiter's aurorae, courtesy of NASA

The first time Peter Delamere saw an aurora, he sort of wished it would get out of the way.

Delamere was at the time an undergraduate student at Carleton College in Northfield, Minnesota, and he was taking an observational astronomy course. Those pesky Northern Lights really obscured his view of the stars, he complained. One of his professors admonished him to appreciate every chance to see such a wonder.

Delamere has done better than that.  At a public lecture on Tuesday, he described a trajectory of investigation that has led him not only to chase aurorae on Earth — but to reach as far as Jupiter for clues about the showy polar phenomena.

Long gone are the days when he failed to appreciate the aurorae.

“I can’t impress upon those of you who haven’t seen the aurora: If the opportunity arises, take hold of it,” he told a packed house, in the auditorium at the Laboratory for Atmospheric and Space Physics.

The lab  happens to be quite literally across the street from my home here in Boulder, and so I walked over to learn about aurorae on Jupiter. Astronomers and probably plenty of amateur space buffs have known about Jupiter’s aurorae (and Saturn’s for that matter) for years, but they were news to me — which is why I was motivated to bundle up and leave my house on a dark, chilly evening.

Delamere began by showing incredible videos of other-worldly aurorae here on Earth, ribbons of green light flowing over northern landscapes. Aurorae begin when solar plasma is belched (not Delamere’s word) from the surface of the Sun and sent hurtling toward Earth. At that point, it’s called solar wind, and it comprises highly charged particles that would ordinarily bombard Earth’s surface and make life impossible, if it weren’t for Earth’s protective magnetic field. Charged particles can’t cross magnetic field lines, so they get diverted down them, which stretches and elongates those field lines on the side of Earth opposite the Sun. If the ejection of solar material was powerful enough, the particles will snap a magnetic field line like a rubber band, Delamere said, although he added that the physics of the rupture is one of the most compelling mysteries in his field. However it happens, the two ends of this figurative rubber band release showers of the charged solar particles that go hurtling toward Earth, agitating elements in Earth’s atmosphere and causing them to glow as Northern and Southern lights simultaneously — the aurorae.

Northern lights

Northern Lights

Following Delamere’s undergraduate education (and his introduction to the aurorae), he moved on to graduate school in Fairbanks, Alaska, where he got to participate in experiments to test the behavior of Earth’s magnetic field lines by shooting barium along their paths. The experiments confirmed the basics of the interactions between charged particles and the field lines.

But these experiments fell short, because the charged particles in question were heading in a direction opposite that of solar wind. Delamere really wanted to artificially create an aurorae for study by bombaring Earth’s magnetic field lines with plasma that could mimic the solar wind. Turns out, he didn’t have to — there was an experiment already set up elsewhere in the solar system.

“Jupiter’s got its own plasma-injecting experiment. It’s called Io,” he said, referring to Jupiter’s innermost, volcanically active moon (pronounced E-o). “I thought, why bother with these artificial experiments when we’ve got one occurring naturally in the solar system? Let’s go and study it.”

Io, as it turns out, is a plasma-generating machine, orbiting in a torus of its own volcanic gas, causing charged particles (electrons) to stream along the giant planet’s magnetic field lines and bombard Jupiter’s atmosphere along the giant planet’s magnetic field lines, as shown in the top of the next image. That’s one key difference between the aurorae on Earth and Jupiter — Earth’s is caused by solar wind, and most of Jupiter’s is not. There, Io (along with fellow moons Ganymede and Europa) appears to be the root cause of an internally-driven polar light show.

Credit: J. Clarke and G Ballester (University of Michigan), J. Trauger and R. Evans (Jet Propulsion Laboratory) and NASA

Credit: J. Clarke and G Ballester (University of Michigan), J. Trauger and R. Evans (Jet Propulsion Laboratory) and NASA

There are other, more mysterious parts of Jupiter’s aurorae, likely caused by the planet’s hugely powerful magnetosphere and possibly, to a much lesser extent, by a bit of solar wind. Delamere said the aurorae related to the moons create sporadic rings of light around the poles, while the other sources fill in that ring. So, whereas Earth’s aurorae form giant rings of light around the poles, Jupiter’s poles get caps of lighted weirdness, as shown in the Hubble images at the bottom. There’s no real seeing Jupiter’s aurorae, even for amateur astronomers with powerful telescopes. Even though they’re 100 times as energetic as the Northern and Southern lights, the Jovian aurorae happen primarily in the UV — out of the range of human sight.

We’ll just have to be satisfied with our own aurorae, spectacular enough to have driven ancient cultures to war and religion. The best places to see them, on about 100 nights a year, are Alaska and Canada. Probably best to wait a few years, as the Sun is just now emerging from a low point in its 11-year activity cycle. Alternatively, hopeful viewers could stay home and hope for the best. The aurorae are not nearly as spectacular in Boulder, Colorado or Mexico City, but they do appear — about once a year in Boulder and once every 10 years over Mexico.

While I wait, I’ll be attending some other lectures over at LASP. That was fun!

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