– From Dave McComas, IBEX Principal Investigator
In those first 2009 science results, we also announced the first–ever direct observations of neutral hydrogen and oxygen atoms drifting in from the interstellar medium, outside our heliosphere. Our talented science team has continued to make observations of these – and other – interstellar neutrals, and we are proud to showcase their work, published as a set of six papers in the February issue of Astrophysical Journal Supplements as a special section titled “IBEX Direct Observations of Interstellar Neutrals.” I hope you will enjoy learning more about this fascinating aspect of IBEX’s exquisite capabilities, which are even allowing us to probe the local region outside our heliosphere. Congratulations to the members of the science team who worked hard on these amazing results – and GO IBEX!
What has IBEX detected?
IBEX has measured hydrogen, oxygen, neon, and helium atoms drifting in from outside our heliosphere toward Earth’s region of our Solar System. Atoms such as these are called “interstellar neutral atoms” or “ISNs.” Prior to IBEX the only other spacecraft to detect ISNs was Ulysses, over a decade ago, and it could only measure neutral interstellar helium. IBEX’s measurements of interstellar hydrogen, oxygen, and neon are the first–ever detections of these atoms by any spacecraft. The IBEX–Lo instrument on IBEX detects all of these ISNs.
How do we know these ISNs are from outside our heliosphere?
Our Sun emits a “wind” of material outward, at an average of 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. Because of this motion, a sort–of “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. ISNs move through the boundary of our heliosphere without the boundary 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.
What have these observations told us?
Studying interstellar atoms can tell us a lot about the region outside our heliosphere and shows us how our Sun is interacting with material around it.
Our local interstellar region does not currently match the characteristics where the Sun originally formed 5 billion years ago.
Our Sun is currently located in a region of space that is very, very thin, in terms of the amounts of dust and gas found there. We do not think it was always this way, and studying the abundances of certain elements can tell us a bit about how it has changed compared to when the Sun formed. Many different elements are created during the lifetimes of massive stars – stars that are larger than our Sun. These stars “live fast and die young.” Their lifetimes are shorter than stars like our Sun, they contain more material, and their temperatures are hotter. These stars fuse hydrogen to form helium, as our Sun does, but later fusion reactions also form elements such as carbon, neon, oxygen, silicon, and iron. When these stars explode as supernovae, the region around that former star is seeded with those elements, and later stars that form in this region have these elements incorporated into them. We can look at the Neon–to–Oxygen ratio in our Sun to get a sense of what the abundances of those elements were like in the area of and at the time of the Sun’s formation. Measurements by IBEX of the ratio of the abundances of neon in relation to oxygen (Ne/O) in the interstellar region shows less oxygen than the science team would have expected to see. The IBEX data for the interstellar ratio does not match the solar ratio, which could mean that our local interstellar region is not like it was when the Sun formed. So, where is the oxygen? One of the ideas that the science team has is that the oxygen may be locked up in the dusty or icy grains that are in the local interstellar material.
Our Sun appears to be close to the boundary of an interstellar cloud of gas and dust.
There appears to be a network of gas and dust clouds in our local galactic vicinity. While 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 reveal 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; they also enter from a somewhat different direction. Magnetic forces play a major role in the interactions of the charged particles at the heliosphere’s boundaries. As the overall particle speeds drop, however, the magnetic forces play an even more dominant role. “With this lower speed, the external magnetic forces cause the heliosphere to become more squished and misshapen,” says Dave McComas. “Rather than being shaped like a bullet moving through the air, the heliosphere becomes flattened, more like a beach ball being squeezed when someone sits on it.”
Based on Ulysses results, previous science teams had concluded that our heliosphere was located in between two of the nearby clouds, the “Local Cloud” and the “G-Cloud” and transitioning into a new region of space. However, while the boundary of the Local Cloud is very close, IBEX results show the heliosphere remains fully in the Local Cloud, at least for the moment. “Sometime in the next hundred to few thousand years, the blink of an eye on the timescales of the galaxy, our heliosphere should leave the local interstellar cloud and encounter a much different galactic environment,” McComas says. Researchers will be able to add measurements about the charged particles outside the heliosphere to the neutral particle measurements provided by IBEX as the two Voyager spacecraft leave our Solar System and cross the heliosphere boundary, possibly within the next few years. “That will give us an even more complete picture of what’s happening in the regions surrounding our home in the Solar System,” says McComas.
Why are these observations important?
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 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 the bubble to expand. In addition, the strength of the solar wind varies over the Sun’s cycle, “breathing” periodically, also contributing to this.
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 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.
To access the Astrophysical Journal Supplement papers, please visit the journal website. The IBEX papers and an accompanying editorial will be published online on January 31.