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F—–‘ magnetospheres, how do they work?

22 April 2011

Artist's rendition of Earth's magnetosphere. How the fuck does it work? (Image courtesy of NASA)

It’s mid-November, 2010, and University of Colorado astrophysicist Peter Delamere has a public relations problem.

In early December, he will deliver a public lecture at the Laboratory for Atmospheric and Space Physics. The presentation should be informative about the details of his research, but must also be accessible for a lay audience.

“What do I talk about and how do I make it interesting to the general public?” Delamere muses during a conversation in his cluttered LASP office. He pauses, brow furrowed in concentration. He consults a calendar. “Wow, it’s only two weeks away.”

The trouble is that Delamere studies planetary magnetic fields, which are odd and invisible phenomena that defy easy explanation. Like gravity, magnetism is one of the fundamental forces of the universe. But gravity makes a certain intuitive sense — little things stick to big things — perhaps because we can feel its direct effects from the moment we are born. Magnets, on the other hand, are rare, unassuming and powerful beyond their size.

Those mysterious qualities can make magnetism seem a bit like magic, a perception made infamous by rap group Insane Clown Posse with the release of the music video for their song “Miracles”. The lyrics point out various mundane objects and phenomena the artists consider to be worthy of awe, and include the lines:

"Miracles" (Image copyright ICP, presumably)

Fuckin’ magnets, how do they work?/And I don’t wanna talk to a scientist/Ya’ll motherfuckers lyin’ and gettin’ me pissed.

Certain geekier portions of the Internet lit up with a mixture of hilarity and revulsion when the video hit YouTube, both for its ham-fisted message and for the explicit rejection of scientific explanations. But it must be admitted that “Fuckin’ magnets, how do they work?” is a profound question, and mind-bogglingly tough to answer. In another, more obscure YouTube video, the late Nobel-winning physicist Richard Feynman squirms defensively under the query before launching into a six-minute series of analogies that boils down to, “They just do.”

Delamere squirms, too, at times during a series of conversations in the weeks before his lecture. And for good reason: although anyone with a refrigerator is familiar with magnets, the planetary magnets Delamere and his colleagues study are no mere kitchen decorations. They are huge, invisible and unlike anything a general audience would be familiar with.

Most of us see the planets as a handful of lonely wanderers in an empty vacuum. But to Delamere and his colleagues, the planets — or, more specifically, their magnetic fields — are rocks in a surging river of invisible plasma.

That plasma is the solar wind, a maelstrom of ionized particles cast off the sun in all directions. Because ions carry an electromagnetic charge, they interact with planetary magnetic fields. And because they move at such great speeds, those interactions produce energetic and powerful results. The most familiar is the aurora, a fantastic light show appearing in polar latitudes as a planet’s magnetic field funnels electrons into the upper atmosphere.

The aurora must be seen to be believed. (Video copyright National Geographic)

Delamere compares magnetosphere research to studying a “black box” — he sees inputs and outputs, but not the internal machinery. Planetary magnetic fields are enormous but ephemeral objects, largely undetectable by ground-based observation. Auroras are only indirect signs of their existence, and what little direct data we do have often comes from a single point of measurement like a satellite or deep space probe. Untangling the chaotic relationship between magnetic fields and solar plasma based on such limited resources is an impressive feat of inductive reasoning.

“The beauty of the aurora is it is telling you a lot about what’s happening in the magnetosphere itself,” Delamere says. “The auroral morphology, the way that it varies in intensity, the boundaries that it maps out…that essentially paints a magnetoshperic boundary for you. So the observable aurora is actually a very important diagnostic of this complex [three-dimensional] structure.”

Delamere is part of the Magnetospheres of the Outer Planets research group within LASP. Much of MOP’s research focuses on Jupiter. As the largest planet, Jupiter boasts a magnetic field of incredible size: as much as five to ten times larger than the Sun. But what makes Jupiter’s magnetosphere really interesting is how different it is from Earth’s.

On Earth, the magnetosphere is essential to life. It keeps us from being bombarded with high-energy particles that could burn away the atmosphere and destroy every living organism. We owe this protective sheath to the liquid iron that flows through the outer core of the planet, deep within the Earth. The motion of the iron is what generates the planet’s magnetic field.

If we could see Earth’s magnetosphere, it would look a lot like the diagrams of dipole magnets we might remember from high school physics. Field lines would emerge from the south pole, curve around the sides of the planet, and re-enter the planet at the north pole. The field lines radiate outward in concentric loops, with the field getting exponentially weaker the farther the lines extend from the planet.

The point where the strength of the magnetic field is too low to push the solar wind back any further is called the magnetopause, and represents the Sunward boundary of the magnetosphere. Here, the ions of the solar wind break around the magnetosphere like water in a stream flowing past a large rock.

This video shows in 30 seconds what it takes me five paragraphs to describe.

Earth’s magnetosphere, by the way, isn’t actually spherical. As the solar wind flows past it interacts with the outermost field lines, dragging them into a tapering tail on the night side of the planet. That gives the magnetosphere an aerodynamic teardrop shape. It’s also the driving force behind Earth’s aurora.

As the magnetic field lines stretch farther and farther away from the planet, they store energy like the elastic bands on a slingshot being drawn back. As the field is distorted, each field line becomes parallel to itself; it points out from the south pole and in toward the north. The greater the distortion, the closer these opposing flows get until — SNAP! — the field line suddenly reconnects with itself, turning back into a smooth curve.

That moment of magnetic reconnection is the slingshot being released. As the field snaps inward toward the planet, massive waves of electrical energy are generated. They propagate back along the field lines toward the poles, accelerating any electrons in their path.

Moving at 1/10th the speed of light, the tsunami of electrons crashes down upon the polar regions. Each electron plays a high-speed game of pachinko with the gases in the upper atmosphere. Every collision stores energy in a molecule of oxygen or nitrogen, which is later released as visible light.

Oxygen glows green or dull red, nitrogen bright red or blue. The light is released in a phantasmal display of curtains, ribbons and spirals that cascades through the polar night sky.

But for all the sublime and startling intricacy of Earth’s aurora, it is nothing compared to the epic beauty of Jupiter’s.

Image courtesy of NASA

“If we could paint [Jupiter’s magnetosphere],” Delamere says, “it would cover a significant portion of our sky.”

Jupiter’s magnetosphere is the largest object in the solar system. Seen from Earth, it would appear five times larger than the full moon. A thousands suns could fit inside its boundaries. Jupiter’s magnetosphere isn’t that big just because the planet is big, however. It’s what’s on the inside that counts.

The driving force of Earth’s magnetosphere dynamics is the solar wind, which comes from the outside. Jupiter’s magnetosphere system, on the other hand, is internally driven. The process starts at Io (pronounced EE-oh), one of Jupiter’s innermost moons.

Io is slightly smaller than our moon, but boasts a thin sulfurous atmosphere. Over time, Jupiter’s magnetic field ionizes some of those particles and accelerates them along the path of Io’s orbit. This forms a donut-shaped ring of plasma around Jupiter that orbits about four times faster than Io itself does. The flow of plasma past Io constantly scrapes new particles out of the atmosphere. About 1 ton of material is ejected every second, ionized, and accelerated into the donut.

An early rendition of Io's plasma torus. (Image courtesy of NASA)

“It’s like a snake eating its own tail,” says Vincent Dols, another MOP researcher who focuses on Io’s contribution to Jupiter’s magnetosphere.

The snake sheds too, as plasma eventually spins outward from the edges of the ring. It forms a disc that spreads throughout the equatorial plane surrounding Jupiter.

Jupiter’s magnetic field is boosted by all that ionized plasma. It’s also distorted outwards along the plasma disc, similar to the way Earth’s magnetic field is distorted by the solar wind. Also like Earth, the distortion plays a role in creating Jupiter’s aurora. But the mechanism is entirely different.

The plasma disc rotates along with Jupiter, like a record around the spindle of a turntable. As the plasma spins, it moves outwards and decelerates. That creates a tug-of-war between plasma and planet, with the magnetic field lines acting as the rope.

This diagram shows how the plasma disc interacts with Jupiter's magnetic field to create an aurora. (Image courtesy of the LAPLACE mission proposal team)

Because of the huge size of both the plasma disc and Jupiter itself, the constant tension between the two generates huge electrical currents that flow back toward the planet. The system is more or less stable, producing a bright oval of aurora that forever crowns the planet’s night side.

To make things even more interesting, Jupiter boasts not one, but two separate auroral phenomena in addition to the main auroral oval.

The first is a small, bright spot of auroral emissions that tracks directly to Io’s orbit around Jupiter. It’s caused by Io’s plasmic self-flagellation, which transfers energy to the planet along a narrow portion of the magnetic field, known as a flux tube.

The second occurs in the polar region, within the boundaries of the main auroral oval. It is thought to be the only part of Jupiter’s aurora that is tied directly to the solar wind, but the mechanism remains unclear. Earth’s solar wind-driven aurora waxes and wanes in time with varying solar activity, but the polar emissions at Jupiter are chaotic, unpredictable and — most weirdly — constant.

Auroral phenomena at Jupiter's poles: the main auroral oval, Io's bright signature dot (left) and the stormy polar aurora. (Image courtesy of NASA)

Delamere and his colleagues hope that the Juno mission, launching next summer, will help explain the polar emissions. The spacecraft will take up an eccentric, highly elliptical orbit that will let it fly quite low over Jupiter’s poles — so close, in fact, that it will fly through the aurora itself.

But Juno won’t arrive until 2016. In the meantime, Delamere has a lecture to prepare for.

To bring magnetospheric research to life, he says, “you really have to appeal to the aurora,” because it’s observable and recognizable. The visuals get people interested, even if they don’t follow all the details of the science behind the aurora.

“I think everybody would love to understand what the hell it is that we’re doing,” Delamere says. “But, you know, there’s complexity in it.”

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