September 5, 2013

From Dave McComas, IBEX Principal Investigator

Dave McComas

In January 2012, the IBEX science team released new observations of neutral atoms drifting in from outside our heliosphere toward Earth’s region of the Solar system. Thanks to IBEX’s amazing sensitivity and resolution, we can use these observations to probe the local region outside our heliosphere. Previous IBEX neutral atom results included observations of neutral hydrogen, oxygen, neon, and helium. Prior to IBEX, the Ulysses spacecraft also detected neutral helium atoms, and others had used the spectral signature to infer the neutral helium. For the first time, we have now combined these results to look for change over time. A new paper published in the journal Science by IBEX Co–Investigator Dr. Priscilla Frisch and others on the IBEX Science team (and other missions), shows the amazing result that the direction the local interstellar wind flowing into the solar system has varied over only a few decades. We look forward to continuing to use IBEX to unlock the secrets within, at the edge of, and outside our heliosphere. Congratulations to the talented IBEX science team for these exciting results! Go IBEX!


How does IBEX detect interstellar neutral atoms?

To detect interstellar neutral atoms, we get a lot of help from our Sun. Our Sun emits a "wind" of material outward, at an average of one million miles (1.6 million kilometers) per hour. As the solar wind streams away from the Sun, it races out toward the space between the stars. We think of this space as "empty" but it contains traces of gas, dust, and charged, or "ionized," gas — together called the "interstellar medium." The solar wind blows against this material and clears out a cavity–like region in the ionized gas. This cavity is called our "heliosphere."

An artist’s rendition of a portion of our heliosphere, with the solar wind streaming out past the planets and forming a boundary as it interacts with the material between the stars.
An artist’s rendition of a portion of our heliosphere, with the solar wind streaming out past the planets and forming a boundary as it interacts with the material between the stars. The termination shock is the boundary layer where the bubble of solar wind particles slows down when the particles begin to press into the interstellar medium. The heliopause is the boundary between the Sun’s solar wind and the interstellar medium. The bow wave is the region where the interstellar medium material piles up in front of our heliosphere, similar to how water piles up in front of a moving boat. Image Credit: IBEX Team/Adler Planetarium

Our entire heliosphere, which contains our Sun, the planets, and everything else in our Solar System, is moving through the interstellar medium. Because of this motion, a "breeze" of interstellar material moves toward our heliosphere’s boundary. The interstellar neutral atoms are just that — "neutral" — meaning they do not interact with magnetic fields. Interstellar neutral atoms, or ISNs, move through the boundary of our heliosphere without the boundary itself affecting them.

As the atoms approach the region containing the Sun and the planets, the breeze of ISNs is deflected by the Sun’s gravity into a curved path. Different atoms are deflected in different amounts based on their masses, and these deflections can be calculated. Based on these calculations, the IBEX team knows when to look for them as the IBEX spacecraft passes through these deflection regions in Earth’s orbit.

Curved ISM paths
An artist’s rendition of our heliosphere, showing the Sun, the orbits of the outer planets and Pluto, the termination shock, the heliopause, and bow wave. The heliosphere bubble is vaguely comet–shaped, with a rounded area to the left in this rendition and a region that sweeps out farther to the right, like a tail. Image Credit: NASA/GSFC/UNH

What were the prior observations that the IBEX team used to make their new conclusions?

The IBEX science team used a variety of data to develop their conclusions. Measurements of ultraviolet radiation emitted from interstellar helium were made by such spacecraft as STP 72–1 (1972–1973), Mariner 10 (1973–1975), SOLRAD 11B (1976), Prognoz 6 (U.S.S.R., 1977–1978), Extreme Ultraviolet Explorer (1992–2001), Nozomi (Japan, 1998–2003) and others. The team also used interstellar helium measurements from the long–lived joint NASA and European Space Agency Ulysses satellite (1990–2002) and, of course, IBEX (2009 to present). In all, the data came from eleven different spacecraft that operated at various times from 1972 to 2012.

Why did the team use helium observations?

The team used helium observations because helium is abundant, and it is not affected much by processes occurring at the edge of our Solar System. Also, there are sets of observation data involving helium covering a total span of forty years from which the IBEX team could draw their conclusions. While measurements of interstellar hydrogen atoms inside of our heliosphere have been conducted for several years longer than those of helium, these measurements are less useful because hydrogen is highly reactive with components of our heliosphere, and a hydrogen atom can have its electron stripped away much more easily than electrons of helium.


What has the IBEX science team concluded?

IBEX data and that of other spacecraft showed previously that our Sun appears to be close to the boundary of an interstellar cloud of gas and dust and there appears to be a network of gas and dust clouds in our local galactic vicinity. Even though they are very dilute and thin, the general positions of these clouds can still be measured. As our heliosphere (and everything in it) orbits the center of our galaxy, we pass into and out of these clouds at various times.

IBEX data revealed that interstellar neutrals enter our heliosphere at a speed of roughly 52,000 miles per hour (83,000 kilometers per hour), about 7,000 mph (11,000 kph) slower than what was inferred from Ulysses observations.

Interstellar neutral atoms entering from outside our Solar System
Interstellar neutral atoms entering from outside our Solar System Credit: IBEX team, M. Paternostro (The Adler Planetarium), Dr. P. Frisch (University of Chicago), Dr. S. Redfield (Wesleyan University)

What is the significance of this new look at the data?

The most significant result is that the direction in the sky from which interstellar neutral helium enters our Solar System has changed by about 7 degrees over the past forty years. This would equate to a shift of about 2.2 billion miles (3.5 billion kilometers). This variation may be telling us about changing conditions in our immediate region of the Milky Way Galaxy. More work will need to be done to characterize these conditions.

"One thing that also should not be overlooked when considering the science is the multinational nature of science and these observations," says Dr. Priscilla Frisch, lead author on the Science article. "Many nations, including the United States, member countries of the European Space Agency, the former Soviet Union, and Japan, developed the spacecraft that allowed us to work with such a rich set of data, and the science teams themselves — including that of IBEX today — are from many parts of the world. Science is about working together to answer questions, and this effort is a great example of that."

To access the article in Science, please visit the journal website.

For more information about past IBEX science discoveries, visit our Archived Updates.