May 10, 2012

From Dave McComas, IBEX Principal Investigator

Dave McComas

Back in 2003, the team that developed IBEX sat down to consider the questions that the mission would be designed to answer. What did we want to find out using IBEX? Four major questions came out of those discussions. One of those questions had to do with how the material between the stars, called the "interstellar medium," interacted with the outer reaches of the boundary of our Solar System. In January, we announced new science results from the IBEX mission that highlighted detections of neutral atoms of hydrogen, helium, oxygen, and neon drifting in from the interstellar medium, outside our heliosphere. Those observations showed that the neutral atoms were coming from a slightly different direction and a slightly lower speed than had been found previously by the Ulysses spacecraft. Using that new information and combining it with improved inferences of the local interstellar magnetic field strength, we are announcing that one of our major questions has been at least partially answered: the bow shock, a region where the interstellar density was thought to discontinuously jump up as it tries to avoid colliding with our heliosphere, does not, in fact, exist. These results are now available in a new paper published online in the May 10, 2012 issue of Science Express

It is too early to know all of the implications of this for our heliosphere. Decades of research have explored scenarios that included a bow shock. Those models now have to be redone using the latest IBEX results. Already, we know there are likely implications for how charged particles and galactic cosmic rays propagate around and sometimes enter the Solar System, which is relevant for human space travel, and I am excited to see what future repercussions this research will have. There is a lot to think about in these results, but I am so pleased with the contributions to the study of the structure of our Solar System and its boundary that IBEX has already made! I want to especially thank IBEX team members from the University of New Hampshire, University of Alabama in Huntsville, Moscow State University, Space Research Centre of the Polish Academy of Sciences in Warsaw, and the University of Bonn in Germany for making this paper possible. Go IBEX!

What is the structure of our Solar System?

Our Solar System is composed of several parts. Our Sun emits a "wind" of material outward in all directions, at typically about a 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." Our entire heliosphere, which contains our Sun, the planets, and everything else in our Solar System, is moving through the interstellar medium.  

What is the boundary of our Solar System?

The most simple explanation is that the boundary of our Solar System is formed at the location where the solar wind streaming outward meets the interstellar medium. The distance at which this occurs is about 10-15 billion miles, or 16-24 billion kilometers, from the Sun in the direction in which our Solar System is traveling. Scientists thought that if our Solar System was traveling fast enough, the pressure of our heliosphere plowing through the interstellar medium material would cause that material to greatly slow down and pile up in front of our heliosphere, forming a "bow shock," in a similar fashion to how air piles up in front of a supersonic jet forming a sound wave shock or "sonic boom." 

The termination shock is the boundary layer where the bubble of solar wind particles slow down when they begin to press into the interstellar medium.
The termination shock is the boundary layer where the bubble of solar wind particles slow down when they begin to press into the interstellar medium. The zone of slower–moving solar wind particles is the heliosheath. The heliopause is the boundary between the Sun’s solar wind and the interstellar medium. The bow shock is the region where the interstellar medium material would pile up in front of our heliosphere.   Image Credit: IBEX Team/Adler Planetarium

What is the new model of our boundary?

Recent scientific results from the IBEX spacecraft have shown that our Solar System is traveling slower with respect to the interstellar medium, and interstellar material is coming into our Solar System from a slightly different direction than previously thought. That difference in speed, about 7,000 miles per hour (11,000 kilometers per hour), does not sound like much but it means that there is about 25% less pressure in the region in front of our heliosphere. What this also means is that there is not the right combination of density, speed, pressure, and magnetism to cause a bow shock to form. 

An artist’s rendition of our heliosphere, showing the Sun, the orbits of the outer planets and Pluto, the termination shock, the heliosheath, and heliopause. The bow shock has been crossed–out with an "X," representing the new IBEX findings.
The latest IBEX results show that our heliosphere’s bow shock does not exist.   Image Credit: IBEX Team/Adler Planetarium

Instead, a "bow wave" is more likely present, which is an area where material is more dense than the surrounding interstellar medium, but no shock forms. Bow waves are what you see in front of the bow of a boat as it pushes through still water.

This is a schematic drawing of our heliosphere based on the most recent IBEX science results
This is a schematic drawing of our heliosphere based on the most recent IBEX science results. The heliosphere bubble is vaguely comet-shaped, with a more rounded area to the right in this rendition and a region that sweeps farther out to the left like a tail. The interstellar magnetic field is represented by the angled lines running from lower left to upper right. Perpendicular to those magnetic field lines is the IBEX Ribbon, shown in red. The previously inferred bow shock has been removed, and a "bow wave" is present in its place in front of our heliosphere. In this drawing, the direction our heliosphere would be moving is from left to right.   Image Credit: IBEX Team


So what is the significance of this new information?

Given these findings, scientists will need to go back and redo the work of the previous few decades and revise the models for how our Solar System interacts with the interstellar medium in our part of the galaxy. Our heliosphere is our home in the galaxy, and understanding how it protects us as it interacts with local interstellar material is important as we plan future space travel beyond Earth and think about the conditions surrounding our Solar System in the distant past and future. 

Our heliosphere is like a protective cocoon being inflated in the interstellar medium by the Sun's million mph solar wind. As our Sun orbits the center of the galaxy every couple hundred million years, it bobs in and out of the disk of the galaxy like a horse on a merry-go-round. As it does this, it passes through areas of the interstellar medium that are more and less dense, causing the heliosphere to change in shape and size. Denser areas with larger speeds relative to the Sun can compress the heliosphere more, while slower and less dense regions allow the bubble to expand. 

Understanding how all of these things affect the heliosphere is important so that we can better understand how the heliosphere protects us. It is a crucial layer of protection against dangerous cosmic rays that are harmful to living things. As cosmic rays approach and try to enter the heliosphere, they are deflected, and the majority of them are not able to pass into the inner Solar System. Fortunately, our Earth's magnetic field is usually able to shield life on Earth from the remaining cosmic rays. However, astronauts on deep space missions cannot bring the Earth's protection with them. We must also consider how the heliosphere will protect us in the distant future or how it did protect us in the past. Understanding the heliosphere and how it protects us is part of understanding our home in the galaxy.

A new paper detailing these investigations and results is available online in the May 10, 2012 issue of Science Express.