Cassini and IBEX Set to Rewrite the Textbooks on the Solar System

The sun's environment in interstellar space — the heliosphere — is essentially a bubble that encompasses the entire solar system and has a diameter about 100 times the distance from the Earth to the sun.

As the solar wind flows from the sun, it carves out a bubble in the interstellar medium. Models of the boundary region between the heliosphere and interstellar medium have been based on the assumption that the relative flow of the interstellar medium and its collision with the solar wind dominate the interaction. This would create a foreshortened “nose” in the direction of the solar system’s motion, and an elongated “tail” in the opposite direction.

Images from one of the Magnetospheric Imaging Instrument’s sensors, the Ion and Neutral Camera (MIMI/INCA), on NASA’s Cassini spacecraft suggest that the heliosphere may not have the comet-like shape predicted by existing models.

“These images have revolutionized what we thought we knew for the past fifty years; the sun travels through the galaxy not like a comet but more like a big, round bubble” said Stamatios Krimigis, principal investigator for MIMI, which is orbiting Saturn. “It’s amazing how a single new observation can change an entire concept that most scientists had taken as true for nearly fifty years.”

Launched one year ago into very high altitude Earth orbit, IBEX has been busy measuring how ions in the solar wind interact with the plasma from interstellar space. IBEX will tell us more about the shape of the solar system’s protective “bubble” –- called the heliosphere –- which is created by the solar wind and shields us from harmful galactic cosmic rays.

"Nature is full of surprises, and IBEX has been lucky to discover one of those surprises," said Priscilla Frisch, a senior scientist in astronomy & astrophysics at the University of Chicago.

What the [IBEX] mission has not found is what they were expecting, that is, evidence of large-scale dynamic processes that might be analogous to storms and tornados from the collision of a cold front and a warm front. A striking result is that "our maps show structure and energy spectra that are completely different from what any model has predicted," [Herbert Funsten] noted.

"We were expecting tie-dye and instead found noodle soup," Funsten said.

The ribbon also has fine structure -- small filaments of ENA [energetic neutral atom] emission no more than a few degrees wide ... The fine structure is as much of a mystery as the ribbon itself, researchers say.

Are the ENAs spread evenly across the sky?
No! This is an amazing discovery! There is an arc-shaped region in the sky that is creating a large amount of ENAs, showing up as a bright, narrow ribbon on the maps. Before the IBEX spacecraft was launched, scientists used models to predict the ENA pattern across the sky. These models predicted variations of only tens of percents across the sky—in contrast, IBEX observes variations that are literally hundreds of percents over small angles in the sky. In other words, we are not only seeing ENAs in an unexpected pattern, we are seeing many more ENAs from small regions than we thought we would.

In the paper, author Herbert Funsten notes "We have discovered an arc-shaped ribbon of high-pressure material that looks to be piled-up material from the Sun. The IBEX maps and the discovery of the ribbon are completely different from what we thought it should look like."

The ribbon is the source of intense emissions of energetic neutral atoms (ENAs), which IBEX specializes in detecting. Its presence appears to be at odds with the current model of the heliosphere, which scientists believed is shaped like a comet by the collision of the outgoing solar wind and the galactic wind, which blows outside the heliosphere.

Previously, the only direct evidence about this giant bubble came from the two Voyager spacecraft, which reached the inner edge of this boundary zone about two years ago and provided in-situ glimpses of two particular points on that shell.

The narrow ribbon snakes along this bubble's inner wall between Voyager 1 and 2, twin spacecraft that have been exploring the solar system's boundary since 2004 and 2007, respectively.

Voyager data, taken from specific regions within the boundary zone, had offered no hint that the ribbon existed. But from its orbit around Earth, NASA's Interstellar Boundary Explorer, or IBEX, spacecraft was able to give researchers a wider view.

"They were both out there making these local observation and had no idea that the main 'storm' so to speak was running right down between them," added McComas, a senior executive director with the San Antonio, Texas-based Southwest Research Institute.

"Can you imagine trying to determine the weather of the entire Earth from two weather stations? You can't do it," Christian told reporters at a press conference this afternoon.

"IBEX is like our first weather satellite, and it gives us the full picture [of the heliosphere]."

IBEX provides "the first all-sky view of this region, and no one was expecting what we've seen," [Geoffrey] Crew says. Oddly enough, the energetic ribbon that circles the whole sky doesn't seem to follow the ecliptic (the plane in which all of the planets orbit). Nor does it follow the plane of the galaxy we inhabit, the Milky Way. "Who ordered this?" Crew exclaims. "It doesn't line up with anything!"

The results from Cassini complement and extend findings from NASA's Interstellar Boundary Explorer, or IBEX, spacecraft. Data from IBEX and Cassini have made it possible for scientists to construct the first comprehensive sky map of our solar system and its location in the Milky Way galaxy.

In their first big-picture view of the heliosphere, scientists discovered a well-defined ribbon of neutrally charged particles, precisely tailored -- process unknown -- by magnetic fields in the interstellar sea.

One important clue: The ribbon runs perpendicular to the direction of the galactic magnetic field just outside the heliosphere...

"We're still trying to understand this unexpected structure, and we believe that the interstellar magnetic forces are associated with the enhanced ENA production at the ribbon," [Priscilla] Frisch said.

The bright ribbon appears to be shaped by the direction of the interstellar magnetic field outside the heliosphere. Scientists think this suggests that the galactic environment just outside the solar system has far more influence on the structure of the heliosphere than previously believed.

"This ribbon is organized around this magnetic field," said Rosine Lallement, senior scientist at the French National Center for Scientific Research in Paris. "It is truly new physics."

"[The ribbon is] aligned by and dominated by the external magnetic field," McComas said in a briefing Thursday. "That's a huge clue as to what's going on. But still we're missing some really fundamental aspect of the interaction -- some fundamental physics is missing from our understanding."

"That cannot be a coincidence," says [David] McComas. But what does it mean? No one knows. "We're missing some fundamental aspect of the interaction between the heliosphere and the rest of the galaxy. Theorists are working like crazy to figure this out."

"Theories are flying," [Geoffrey] Crew says. One suggestion is that the ribbon of energetic neutral atoms may have to do with the effect of the local galactic magnetic field pushing against the heliosphere. These observations suggest that the ribbon is perpendicular to that [interstellar] magnetic field, he says.

One idea is that, wherever the Milky Way's magnetic field presses on the heliosphere, more ENAs are created.

"Exactly where the [galaxy's] magnetic field is most wrapped around the outer boundary of the heliosphere, that's where the ribbon runs," McComas said.

"That could be an unbelievably remarkable coincidence, or it could be a fabulous clue that somehow this external magnetic field is actually imprinting onto our heliosphere through some process that we don't yet understand."

The INCA images [from Cassini] suggest that the solar wind’s interaction with the interstellar medium is instead more significantly controlled by particle pressure and magnetic field energy density.

“The map we’ve created from INCA’s images suggests that pressure from a hot population of charged particles and interaction with the interstellar medium’s magnetic field strongly influence the shape of the heliosphere,” says Don Mitchell, MIMI/INCA co-investigator at APL.

"The ribbon [discovered in the IBEX data] follows a circular arc of high pressure that we believe is centered on the direction of the magnetic field of the interstellar cloud through which we are moving," Funsten said. This magnetic field seems to fundamentally organize the interaction region.

The results of IBEX not only reveal fundamental properties of the heliosheath but also provide key information about the properties of the interstellar cloud through which our galaxy is moving. We will be moving out of the cloud in about 10,000 years; the IBEX results will help us understand how the Earth's space environment might be different when this happens.

IBEX data suggest the alignment of the ribbon is related to the local interstellar magnetic field, which could mean that its origins lie outside of the solar system. The ribbon also appears to have a fine structure, suggesting that the ion concentrations vary along its length.

The first all-sky maps developed by NASA's Interstellar Boundary Explorer (IBEX) spacecraft, the initial mission to examine the global interactions occurring at the edge of the solar system, suggest that the galactic magnetic fields had a far greater impact on Earth's history than previously conceived, and the future of our planet and others may depend, in part, on how the galactic magnetic fields change with time.

The interaction between the solar wind and the medium of our galaxy creates a complex host of interactions, which has long fascinated scientists, and is thought to shield the majority of harmful galactic radiation that reaches Earth and fills the solar system.

"The magnetic fields of our galaxy may change the protective layers of our solar system that regulate the entry of galactic radiation, which affects Earth and poses hazards to astronauts," says Nathan Schwadron of Boston University's Center for Space Physics and the lead for the IBEX Science Operations Center at BU.

"The most striking feature is the ribbon [discovered in the IBEX data] that appears to be controlled by the magnetic field of our galaxy," says Schwadron.

Although scientists knew that their models would be tested by the IBEX measurements, the existence of the ribbon is "remarkable" says Geoffrey Crew, a Research Scientist at MIT and the Software Design Lead for IBEX. "It suggests that the galactic magnetic fields are much stronger and exert far greater stresses on the heliosphere than we previously believed."

Adds Schwadron,"Any changes to our understanding of the heliosphere will also affect how we understand the astrospheres that surround other stars. The harmful radiation that leaks into the solar system from the heliosphere is present throughout the galaxy and the existence of astrospheres may be important for understanding the habitability of planets surrounding other stars."

"This is a shocking new result," says IBEX principal investigator Dave McComas of the Southwest Research Institute. "We had no idea this ribbon existed -- or what has created it. Our previous ideas about the outer heliosphere are going to have to be revised."

"It shows that what we thought we understood about this interaction is definitely not right," McComas said. "We kind of have to go back and start over."

The textbook descriptions of the heliosphere, according to Science's accompanying news story, will have to be entirely rewritten.