Voyager Makes an Interstellar Discovery

The solar system is passing through an interstellar cloud that physics says should not exist. In the Dec. 24th issue of Nature, a team of scientists reveal how NASA's Voyager spacecraft have solved the mystery.

"Using data from Voyager, we have discovered a strong magnetic field just outside the solar system," explains lead author Merav Opher, a NASA Heliophysics Guest Investigator from George Mason University. "This magnetic field holds the interstellar cloud together and solves the long-standing puzzle of how it can exist at all."

Voyager flies through the outer bounds of the heliosphere en route to interstellar space. A strong magnetic field reported by Opher et al in the Dec. 24, 2009, issue of Nature is delineated in yellow.

The discovery has implications for the future when the solar system will eventually bump into other, similar clouds in our arm of the Milky Way galaxy.

Astronomers call the cloud we're running into now the Local Interstellar Cloud or "Local Fluff" for short. It's about 30 light years wide and contains a wispy mixture of hydrogen and helium atoms at a temperature of 6000 C. The existential mystery of the Fluff has to do with its surroundings. About 10 million years ago, a cluster of supernovas exploded nearby, creating a giant bubble of million-degree gas. The Fluff is completely surrounded by this high-pressure supernova exhaust and should be crushed or dispersed by it.

"The observed temperature and density of the local cloud do not provide enough pressure to resist the 'crushing action' of the hot gas around it," says Opher.

So how does the Fluff survive? The Voyagers have found an answer.
"Voyager data show that the Fluff is much more strongly magnetized than anyone had previously suspected—between 4 and 5 microgauss*," says Opher. "This magnetic field can provide the extra pressure required to resist destruction."

An artist's concept of the Local Interstellar Cloud, also known as the "Local Fluff."

NASA's two Voyager probes have been racing out of the solar system for more than 30 years. They are now beyond the orbit of Pluto and on the verge of entering interstellar space—but they are not there yet.

"The Voyagers are not actually inside the Local Fluff," says Opher. "But they are getting close and can sense what the cloud is like as they approach it."

The Fluff is held at bay just beyond the edge of the solar system by the sun's magnetic field, which is inflated by solar wind into a magnetic bubble more than 10 billion km wide. Called the "heliosphere," this bubble acts as a shield that helps protect the inner solar system from galactic cosmic rays and interstellar clouds. The two Voyagers are located in the outermost layer of the heliosphere, or "heliosheath," where the solar wind is slowed by the pressure of interstellar gas.

Voyager 1 entered the heliosheath in Dec. 2004; Voyager 2 followed almost 3 years later in Aug. 2007. These crossings were key to Opher et al's discovery.

The anatomy of the heliosphere. Since this illustration was made, Voyager 2 has joined Voyager 1 inside the heliosheath, a thick outer layer where the solar wind is slowed by the pressure of interstellar gas.

The size of the heliosphere is determined by a balance of forces: Solar wind inflates the bubble from the inside while the Local Fluff compresses it from the outside. Voyager's crossings into the heliosheath revealed the approximate size of the heliosphere and, thus, how much pressure the Local Fluff exerts. A portion of that pressure is magnetic and corresponds to the ~5 microgauss Opher's team has reported in Nature.

The fact that the Fluff is strongly magnetized means that other clouds in the galactic neighborhood could be, too. Eventually, the solar system will run into some of them, and their strong magnetic fields could compress the heliosphere even more than it is compressed now. Additional compression could allow more cosmic rays to reach the inner solar system, possibly affecting terrestrial climate and the ability of astronauts to travel safely through space. On the other hand, astronauts wouldn't have to travel so far because interstellar space would be closer than ever. These events would play out on time scales of tens to hundreds of thousands of years, which is how long it takes for the solar system to move from one cloud to the next.

"There could be interesting times ahead!" says Opher.

To read the original research, look in the Dec. 24, 2009, issue of Nature for Opher et al's article, "A strong, highly-tilted interstellar magnetic field near the Solar System."

Colliding Auroras Produce Explosions of Light

A network of cameras deployed around the Arctic in support of NASA's THEMIS mission has made a startling discovery about the Northern Lights. Sometimes, vast curtains of aurora borealis collide, producing spectacular outbursts of light. Movies of the phenomenon were unveiled at the Fall meeting of the American Geophysical Union today in San Francisco.

"Our jaws dropped when we saw the movies for the first time," says space scientist Larry Lyons of UCLA, a leading member of the team that made the discovery. "These outbursts are telling us something very fundamental about the nature of auroras."

Colliding auroras photographed by THEMIS all-sky imagers (ASIs) on Feb. 29, 2008. [short movie with labels] [full-length movie]

The collisions occur on such a vast scale, isolated observers on Earth with limited fields of view had never noticed them before. It took a network of sensitive cameras spread across thousands of miles to get the big picture.

NASA and the Canadian Space Agency created such a network for THEMIS, short for "Time History of Events and Macroscale Interactions during Substorms." THEMIS consists of five spacecraft launched in 2006 to solve a long-standing mystery: Why do auroras occasionally erupt in an explosion of light called a substorm? Twenty all-sky imagers (ASIs) were deployed across the Alaskan and Canadian Arctic to photograph auroras from below while the spacecraft sampled charged particles and electromagnetic fields from above. Together, the cameras and spacecraft would see the action from both sides and be able to piece together cause and effect—or so researchers hoped.
It seems to have worked.

The breakthrough came earlier this year when UCLA researcher Toshi Nishimura completed the Herculean task of assembling continent-wide movies from the individual ASI cameras.

Twenty all-sky imagers (ASIs) were deployed by researchers from the University of California Berkeley, the University of Calgary, and the University of Alaska in support of the THEMIS mission. This map shows their locations and field of view. Image

"It can be a little tricky," Nishimura says. "Each camera has its own local weather and lighting conditions, and the auroras are different distances from each camera. I've got to account for these factors for six or more cameras simultaneously to make a coherent, large-scale movie."

The first movie he showed Lyons was a pair of auroras crashing together in Dec. 2007.

"It was like nothing I had seen before," Lyons recalls. "Over the next several days, we surveyed even more events. Our excitement mounted as we became convinced that the collisions were happening over and over."

The explosions of light, they believe, are a sign of something dramatic happening in the space around Earth—specifically, in Earth's "plasma tail." Millions of kilometers long and pointed away from the sun, the plasma tail is made of charged particles captured mainly from the solar wind. Sometimes called the "plasma sheet," the tail is held together by Earth's magnetic field.

The same magnetic field that holds the tail together also connects it to Earth's polar regions. Because of this connection, watching the dance of Northern Lights can reveal much about what's happening in the plasma tail.

A schematic diagram of Earth's magnetosphere. Earth is the circle near the middle and the plasma tail is denoted in yellow.

By examining many collisions, Lyons and Nishimura have identified a common sequence of events. It begins with two elements: (1) a broad curtain of slow-moving auroras and (2) a smaller knot of fast-moving auroras, initially far apart. The slow curtain is quietly glowing over the Arctic when the speedy knot rushes in from the north. The two auroras collide and an eruption of light ensues.

How does this sequence connect to events in the plasma tail?

"It took some creative thinking to come up with an answer, but I believe this team has done it," says THEMIS project scientist Dave Sibeck of the Goddard Space Flight Center.

Lyons believes that the fast-moving knot is associated with a stream of relatively lightweight plasma jetting through the plasma tail. The stream gets started in the outer regions of the plasma tail and moves rapidly inward toward Earth. The fast knot of auroras moves in synch with this stream.

Meanwhile, the broad curtain of auroras is quietly hanging over the Arctic, gently glowing, more or less minding its own business. This curtain is connected to the stationary inner boundary of the plasma tail and is fueled by plasma instabilities there.

Auroras poised to collide. Click on the image to view a labeled sequence of events.

When the lightweight stream reaches the inner boundary of the plasma tail—bang!--there is an eruption of plasma waves and instabilities. This collision of plasma is mirrored by a collision of auroras over the poles.

National Science Foundation radars located in Alaska and Greenland confirm this basic picture. They have detected echoes from streams of material rushing through Earth's upper atmosphere just before the auroras collide and erupt.
The five THEMIS spacecraft also agree. They have been able to fly through the plasma tail and confirm the existence of lightweight material rushing toward Earth.

"By putting together data from ground-based cameras, ground-based radar, and the THEMIS spacecraft themselves, we now have a nearly complete picture of what causes explosive auroral substorms," says Sibeck.And what a picture it is. Click here for movies.