February 5, 2013

From Dave McComas, IBEX Principal Investigator

Dave McComas

So what process creates the IBEX Ribbon? Why do we see so many more energetic neutral atoms coming from a particular ring in the sky? Those are questions that have perplexed the IBEX science team and the rest of the science community since we first saw the Ribbon in IBEX’s Solar System boundary region maps released in 2009. The Ribbon was completely unexpected and not predicted in previous models of our heliosphere. The IBEX science team has been hard at work to come up with a mathematical and scientific model that explains the Ribbon, its various features, and from where in space the Ribbon originates. Our original papers in Science magazine, released in 2009, outline six different models that could explain the Ribbon, and there have been several more ideas proposed since then – thirteen, in all. The latest model, detailed in a new paper published in the Astrophysical Journal, is a bit different from the rest, however. For the first time, we may have a mechanism that may explain all of the key features of the Ribbon that we have observed: the Ribbon's broadening at increasing energy levels, its overall structure, its features at different energy levelshow it changes over time, and the appearance of the feature nicknamed "the knot."  

This is a perfect example of the fantastic process of science: we observe something completely new with IBEX, we develop various hypotheses to explain the observations, and we develop mathematical models to try to validate the hypotheses. Have we found a way to explain the Ribbon? We cannot say for sure right now, but we are looking forward to getting more data from IBEX to see how well this one (and all of the others) hold up!

What forms the boundary of our Solar System?

To understand the new IBEX Ribbon model, first we need to find out what forms the boundary of our Solar System. What do we mean when we say something has an edge or a boundary? Some things, like a table or a soccer field, have clear edges or painted boundaries that are easy to spot. Other objects, like cities and towns, have boundaries that aren’t as easy to see, especially from a distance. Our Solar System is more like a city than a table or soccer field. 

The parts of our Solar System’s boundary are defined by the interactions between the solar wind and the interstellar medium. Material streaming off of the Sun, called the "solar wind," races out well past all of the planets and toward the space between the stars. We think of this space as "empty" but it contains traces of gas and dust, called the "interstellar medium." The solar wind blows against the interstellar medium and clears out a bubble–like region in this gas. This bubble that surrounds the Sun and the Solar System is called the "heliosphere." This is not a bubble like a soap bubble, but more like a cloud of foggy breath that you breathe into chilly winter air. The outer border of this bubble is where the solar wind’s strength is no longer great enough to push back the interstellar medium. This border is known as the "heliopause," and is often considered to be the outer edge of our Solar System.

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

In our local neighborhood of the Milky Way Galaxy, there are magnetic fields between the stars, as well. Some of these magnetic fields drape across our heliosphere and squeeze it in some areas.

The heliopause and these magnetic fields will both play a part in the story of this IBEX Ribbon model. 

What are the processes involved in the model?

The particles of solar wind are mostly made of atoms of hydrogen. These atoms can be in two forms: ions and neutral atoms. A neutral atom of hydrogen contains one proton whose positive charge is balanced by a single negatively charged electron. If the neutral hydrogen atom has its electron stripped away, all that is left is the positively charged proton. This charged particle would be an ion. Neutral atoms are not affected by magnetic fields, while ions are affected by magnetic fields. Both ions and neutral particles make up the solar wind. 

For this model, we will concentrate on the neutral hydrogen atoms in the solar wind. The neutral hydrogen atoms flow straight out, well past all the planets, past the heliopause, and into the local interstellar medium (LISM). The LISM contains a thin "soup" of neutral and charged particles. The neutral hydrogen atoms encounter the region just outside the heliopause where the local galactic magnetic field squeezes our heliosphere. If a positively charged proton in the LISM encounters a neutral solar wind hydrogen atom, the proton can steal the electron from the neutral hydrogen atom. The LISM particle is now neutral, and the once–neutral solar wind hydrogen particle is now positively charged. This solar wind proton, and other ions, can now interact with the local galactic magnetic field. 

A diagram showing the processes involved in the Retention Region model

A diagram showing the processes involved in the Retention Region model. Neutral solar wind particles, identified by the red arrows, flow outward from the Sun, past the heliopause, and encounter charged interstellar medium particles. After losing an electron to the interstellar medium particles, the solar wind particles then interact with the local interstellar magnetic field. The Ribbon is thus a region where these particles become trapped or "retained" due to intense waves and vibrations in the interstellar magnetic field, and the Ribbon is an enhancement that we see at 90 degree angles relative to the magnetic field (identified in this diagram by the physics equation BR=0). 
Image Credit: IBEX Team

According to this model, the Ribbon exists in a special location where the neutral hydrogen atoms move across the local galactic magnetic field. When these particles lose their electrons and become charged, they begin to gyrate rapidly around magnetic field lines. That rapid rotation creates waves or vibrations in the magnetic field. Let’s use an analogy of boats in a harbor. Big waves outside the harbor make it difficult for the boats to move beyond into the vast ocean. The boats can be trapped in the harbor if the ocean waves are powerful enough. This is the nature of the new Ribbon model. It is a region where particles, originally from the solar wind, become trapped or "retained" due to intense waves and vibrations in the magnetic field.

If ions that are gyrating around the magnetic field in this trapped region encounter neutral LISM atoms, they can steal electrons from the neutral atoms. Because the solar wind particles are now neutral, again, and are not affected by magnetic fields, they travel in a straight line in the direction they were going when they became neutral. Many of these particles happen to travel inward to our region of the Solar System and can be detected by the IBEX spacecraft as it orbits Earth. 

Because so many ions are confined in the trapped region, so many more neutral atoms come from this region, and it looks like a brighter swath in the IBEX maps. This region corresponds to the IBEX Ribbon and runs perpendicular to the interstellar magnetic field. Imagine the Ribbon source, then, as a thick band of trapped ions, like a life preserver around a person in a pool. 



This is a 3–dimensional diagram of the "Retention Region," with the region shown as a "life preserver" around our heliosphere bubble along with the original IBEX Ribbon image. The interstellar magnetic field lines shown running upper left to lower right

Image Credit: Adler Planetarium/IBEX Team

The new model described above – call it the "Retention Model" – shares many aspects with one of the first six mechanisms first suggested in the IBEX issue of Science magazine in 2009. There, it was hypothesized that the ribbon might come from the solar wind, namely, that neutral atoms from the solar wind spill out into local interstellar space and then when they lose an electron and become ions, they begin gyrating about the magnetic field. All of this is similar to the idea in the Retention Model. One of the main differences though is that the Retention Model develops the idea that the Ribbon is truly a region in space where particles become trapped. However, in the former model from 2009, the formation of the Ribbon is really an optical effect. When particles gyrate about the magnetic field, they move in directions perpendicular (at 90 degrees) to the magnetic field. Therefore, due to this gyration, you would expect to see an enhancement in regions where we look at 90 degree angles relative to the magnetic field – thus the Ribbon. Figuring out whether the Ribbon is an optical effect, a Retention Region in space, or some other effect will depend on developing ways to distinguish between these models. Even with all of these ideas on the table, it is still very possible that we are missing something important. This underscores the importance of keeping an open mind, and allowing the observations to point the way to the real answer. 

What was it like to figure out this model?

Dr. Nathan Schwadron, head of the IBEX Science Operations Center, worked on this model with Dr. David McComas, Principal Investigator for the IBEX mission. How did Nathan feel when he first saw the results that showed that this model of the Ribbon actually produced features that looked very much like the images from IBEX? "Very relieved," he said. "I thought, ‘Wow, this thing might actually be right? Really?’"

Is this model the right one?

We cannot say for sure just yet. We need more data from IBEX to see how well the observed data fits with the features of the model. One major test for this model will be to see if the Ribbon changes in step with observed changes in the solar wind. If what we observe matches what the model predicts should happen to the Ribbon as the solar wind pressure changes, then that will go a long way toward validating the model. "This is a very distinctive event that shows almost in real time how mysteries of science are opened and then solved. I am not pretending that this is the ultimate solution. Nevertheless, I think we have taken a pretty large step in understanding the origins of the Ribbon," says Nathan.

What is next for this model?

Nathan explains, "We can now can use the Ribbon as a bright marker in the sky giving us the direction of the interstellar magnetic field and its strength. We still have a lot of work to do, though. Our next step is to work out the details of the wave generation process, and in turn, figure out the implications of the structure of the Ribbon for understanding the structure of the heliopause and the area just outside the heliopause. Then, we need to understand how the local interstellar magnetic field varies with time. Understanding how all of these things affect the heliosphere is important so that we can better understand how the heliosphere protects us as a crucial barrier against dangerous cosmic rays coming in from other parts of the galaxy and elsewhere."

How can I learn more?

The new model for the IBEX Ribbon is detailed in a paper – "Spatial Retention of Ions Producing the IBEX Ribbon" – published recently in the Astrophysical Journal.