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February 2014: Using Cosmic Rays as a Magnetic Field Marker
February 13, 2014
Happy 2014, everyone! We are starting out the year with some great new results from the IBEX science team that are crucial to helping us understand how the heliosphere is linked out into the more distant local interstellar medium around us and narrow down the mechanism for the formation of the IBEX Ribbon. Since the discovery of the IBEX Ribbon, a region of enhanced energetic neutral atom emission that stretches across a significant portion of the sky, IBEX scientists have been hard at work to figure out what causes it. The Ribbon appears to run perpendicular to the direction of the interstellar magnetic field just outside the heliosphere. Because of this arrangement, determining and confirming the direction of the interstellar magnetic field has been a point of high interest for the IBEX scientists. Previously, others used polarized starlight to determine the magnetic field direction between us and nearby stars, but we needed another independent method so that we could be confident about mathematical models of the IBEX Ribbon that use the interstellar magnetic field. This is a critical part of our work because there are more than 10 individual IBEX Ribbon ideas or models that the team (and some outside the team) are working on now. In the new study, we used data from cosmic ray detections that turned out to align very well with the prior stellar polarization data and Ribbon direction. This is described in a new paper led by Nathan Schwadron and published in the February 13, 2014 issue of Science Express.
This process of determining the direction of the interstellar magnetic field is an excellent example of how results from vastly different types of science programs can be combined to help us learn more about our home in the galaxy. My thanks to the folks from the Super–K, Milagro, and IceCube science teams for producing great data that the IBEX scientists could utilize and work collaboratively with them on to combine with our data and get these great new results. I look forward to sharing more exciting results in the future — Go IBEX!
What is the background information I need to know?
First, we need to explain the formation of 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. 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 and dust, 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." The outer border of this cavity 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. Our entire heliosphere, which contains our Sun, the planets, and everything else in our Solar System, is moving through the interstellar medium.
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
The interaction between the solar wind and the interstellar medium creates energetic neutral atoms (ENAs), which are particles with no charge that move very fast. Because these particles have no charge, they are not affected by magnetic fields and travel in a straight line in the direction they were moving when they became neutral.
How does IBEX work?
Some of the ENAs happen to be moving inward and travel back through the Solar System toward Earth where IBEX can detect them. IBEX contains two ENA sensors, IBEX–Hi and IBEX–Lo. Each sensor detects different ENA energies, and the energy levels of the two sensors overlap so they can be checked against one another, which is important to make sure that the data are valid. IBEX provides the only way we currently have of studying the entire boundary of our Solar System all at once.
IBEX is a NASA–funded Small Explorer satellite mission that orbits Earth and maps the boundary of the Solar System. The IBEX spacecraft itself is about the size of a large bus tire.
Image Credit: Walt Feimer, NASA GSFC
What has IBEX shown us so far?
The first all–sky maps of ENAs coming from our Solar System’s boundary showed something expected and something else completely surprising. There are ENAs coming from various parts of the sky, in somewhat similar patterns as the scientists thought they would see. However, what was unexpected is an arc–shaped region in the sky that is creating a huge amount of ENAs, showing up as a bright, narrow "ribbon" on the maps, which has subsequently been called the IBEX Ribbon.
In our local neighborhood of the Milky Way Galaxy, there are magnetic fields between the stars, as well as the interstellar medium material. Some of these magnetic fields drape across our heliosphere and squeeze it in some areas. It appears as though the IBEX Ribbon is aligned perpendicular to the interstellar magnetic field direction.
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 are shown running from upper left to lower right around our heliosphere, and the area where the field lines "squeeze" our heliosphere corresponds to the Ribbon location. The red arrow at the front shows the direction of travel of our Solar System.
Image Credit: Adler Planetarium/IBEX Team
What does the Ribbon tell us?
One important aspect to all of the IBEX Ribbon studies is to confirm the direction of the interstellar magnetic field, because the formation of the Ribbon appears to be related to the interstellar magnetic field. There are no measurements, though, that scientists can point to that definitively show the direction of the magnetic field, so its direction must be inferred through several different types of observations. One way that the IBEX team and others have used to trace the direction of interstellar magnetic fields is to view starlight from stars within about 120 light years from Earth as that light passes through interstellar dust. Interstellar dust grains can become electrically charged and are then aligned in a way that follows those magnetic fields. Starlight passing through those magnetically aligned dust grains is then polarized, so the direction the light is polarized will trace the interstellar magnetic fields. By studying the starlight, we can "see" the magnetic fields.
What did the IBEX science team look at this time?
Confirming the direction of the interstellar magnetic fields has many implications for helping to determine the mechanisms that create the IBEX Ribbon and for understanding how the interstellar magnetic field interacts with and influences our heliosphere. Thus, the IBEX science team is very interested in using more than one way to confirm the interstellar magnetic field direction. In a new study published in the journal Science, the IBEX team looked at how cosmic rays are guided by interstellar magnetic fields and, thus, detection of cosmic rays can be another tracer for the interstellar magnetic fields.
What are cosmic rays?
The name "cosmic ray" is slightly misleading, as they are actually particles with mass that are traveling quickly. Cosmic rays, despite the word "ray" in the name, are not forms of light, such as infrared light, ultraviolet light, or gamma rays. The vast majority of cosmic rays are nuclei of hydrogen, simply one proton with no electron. Some are alpha particles, or the nuclei of helium atoms composed of two protons and two neutrons with no electrons, and a few cosmic rays are solitary electrons. Cosmic rays originate in sources such as supernovae, active galaxies, and others.
Cosmic rays, being charged particles, are affected by magnetic fields, including interstellar magnetic fields, our Sun’s magnetic field, and Earth’s magnetic field. As cosmic rays approach our heliosphere, they can be deflected, and the majority of them are not able to pass into the inner Solar System. Higher energy cosmic rays are more likely to pass through our heliosphere and some can even pass through Earth’s magnetic field to be detected through various ways due to their interactions with Earth’s atmosphere.
What do we use to detect cosmic rays?
There are a number of detectors on Earth that are used to detect cosmic rays or other particles that are associated with the high energy events that produce cosmic rays. The IBEX science team used data from several detectors, including Super–kamiokande, Milagro, and Ice Cube. Super–kamiokande
, or Super–K, is a detector situated deep in a mine in Japan. Operating until 2008, Milagro
was a detector located in the mountains of New Mexico. IceCube
is a detector that operates at the South Pole.
What did the IBEX team discover?
IBEX scientists found that the interstellar magnetic field direction found from looking at polarized starlight lined up very well with the direction determined using the cosmic ray data.
Ultimately, why is this information important?
Understanding how cosmic rays affect the heliosphere is important so that we can better understand how the heliosphere protects us. Our heliosphere is like a cocoon being inflated in the interstellar medium by the solar wind. As our Sun orbits the center of the galaxy every 225 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 can compress the heliosphere, while less dense regions allow this bubble to expand. In addition, the strength of the solar wind varies over the Sun’s cycle, "breathing" periodically, also contributing to these changes.
The heliosphere is a crucial layer of protection against dangerous cosmic rays that are harmful to living things. Fortunately, our Earth’s magnetic field is usually able to shield life on Earth from the cosmic rays that are not shielded by our heliosphere. However, astronauts on deep space missions cannot bring the Earth’s protection with them, so planning for future deep space travel beyond Earth will need to take this information into account. 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.
I want to learn more!
To access the article titled "Global Anisotropies in TeV Cosmic Rays Related to the Sun’s Local Galactic Environment from IBEX," please visit the Science Express