From Dave McComas, IBEX Principal Investigator
October 19, 2012 marks exactly four years since the launch of the IBEX spacecraft. On that same date in 2008, the many in the team and I watched from Dulles, Virginia as IBEX was carried into space by a Pegasus rocket launched from Kwajalein, an atoll in the middle of the Pacific Ocean. It was a tense day, but in the end, everything worked out just fine and IBEX was safely in its initial orbit around Earth. In the last weeks of December, we were elated when IBEX began detecting energetic neutral atoms (ENAs) coming from the boundary of our Solar System. Little did we know that day the amazing discoveries that were in store for us. In October 2009, IBEX's first science results were released, highlighting the surprising and completely unexpected "IBEX Ribbon," an arc–shaped region in the sky that is producing many more ENAs than was expected. During the past three years, we also have detected neutral atoms from outside our heliosphere as they make their way to Earth’s region of our Solar System, and we have discovered ENAs coming from the surface of our Moon and from regions just outside and inside Earth’s magnetosphere. Most recently, we announced that there is no bow shock for our heliosphere, a complete paradigm shift from what was thought before. To date, scientists have already published 100 papers related to this small but remarkable mission — what an incredible scientific harvest!
And the science hits just keep on coming for IBEX. This month, a major update has been published in the Astrophysical Journal Supplement Series assembling, correcting, and validating the first three years of IBEX data, including new boundary region maps, identification of a strong candidate for the source of the IBEX Ribbon, and the first complete set of ENA sky maps using data from the IBEX–Lo sensor. All of these data have been posted on the IBEX website and provided to NASA’s data repository, so the entire science community can and should be using these new data for future studies. This is such an amazing time for the study of our Solar System’s last frontier, and we are beginning to unravel its mysteries. Please read on for more of the revolution in the understanding of our heliosphere. Go IBEX!
What IBEX studies
IBEX studies energetic neutral atoms. The boundary of our Solar System emits no light, so information about it cannot be collected by conventional telescopes. Charged particles, such as protons and electrons, stream off of our Sun at high speed; this wind of particles is called the "solar wind." The solar wind heads outward, far past the planets. Billions of miles from the Sun, the solar wind particles interact with material between the stars, called the "interstellar medium." This interaction process 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. Some of the ENAs happen to be moving inward and travel back through the Solar System toward Earth where IBEX can detect them. IBEX provides the only way we currently have of studying the entire boundary of our Solar System all at once.
3 Years of Observations from IBEX
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.
The IBEX Ribbon
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.
What we originally saw is that the pattern of ENAs coming from other parts of the sky is essentially dominated by this ENA ribbon. There are so many more ribbon ENAs that the ribbon is the main feature seen in all of the IBEX maps, even though ENAs from other parts of the sky are in that data, too.
3 Years of IBEX–Hi Maps
The first two sets of maps released by the IBEX science team were from IBEX–Hi, and the team has now released three full years of data from late 2008 through 2011 in new sets of IBEX–Hi maps:
Each column of images above shows a yearly set of IBEX–Hi maps for 2009 (left), 2010 (middle), and 2011 (right). Each row of images shows a particular energy level of ENAs detected by the IBEX–Hi sensor, five energy levels in total. The maps corresponding to the lowest energy level are in the top row and the maps corresponding to the highest energy level are in the fifth row. The red and orange colors in all of the maps represent a higher number of ENAs detected by IBEX, yellow and green represent a lower number, and the blue and purple colors represent the lowest numbers of ENAs detected by IBEX. In the past three years, the ribbon has been a fairly stable feature, although it has been dimming slightly as have emissions from the rest of the sky; it is visible in all of the map sets to varying degrees, depending on the energy level. At the highest energy levels, the ribbon is very hard to distinguish.
For the first time, the IBEX team has released complete ENA maps from the IBEX–Lo sensor:
Over the course of one day, IBEX–Hi detects, on average, about 500 particles; during that same amount of time, IBEX–Lo detects, on average, fewer than 100 particles, so it takes a much longer period of time for IBEX–Lo to detect enough particles to make a single set of maps. The maps above are laid out as the IBEX–Hi maps are, with the lowest energy level of IBEX–Lo at the top and the highest energy level at the bottom.
There are two columns of maps corresponding to the entire three year observation period. Here, the left side column corresponds to the data gathered by IBEX when the IBEX–Lo sensor was facing in the same direction that the IBEX spacecraft was traveling, known as the "ram direction." "Anti-ram" corresponds to the data gathered by IBEX–Lo when the sensor was facing in the direction of travel opposite to that of the IBEX spacecraft. The IBEX ribbon can be seen easily in the "ram" maps on the left, and it is most easily seen at an energy level that is similar to where IBEX–Hi sees it.
A Strong Ribbon Source Candidate
Ever since the ribbon was detected, the IBEX team has been trying to figure out what is causing it. It appears to be produced by the alignment of magnetic fields outside our heliosphere, suggesting that the interstellar environment has a great influence on the structure of our heliosphere. What the team has seen, though, is a declining number of ENAs coming from the ribbon over the course of about two and a half years, corresponding to a lower amount of solar wind pressure during that time. Additionally, the team has seen that at different energy levels, the ribbon appears substantially different, corresponding to different speeds of solar wind charged particles coming from different latitudes on the Sun.
So, what does this mean? It appears as though there is a direct connection between the ribbon and some solar wind source.
We’re starting to get a more complete picture of our heliosphere! Provided the IBEX spacecraft remains healthy, it will be very interesting to see if there are changes in the ribbon and in other parts of our Solar System’s boundary over the next five to ten years as we approach solar maximum in 2013 or 2014 and head toward another solar minimum. Is the solar wind truly a direct source of the IBEX Ribbon? What, exactly, is the process that creates the ribbon? Where exactly is the ribbon located? We hope IBEX can discover the answers to these questions — and many more!
To access the latest set of IBEX data, Data Release 4, please visit the IBEX Data Releases page in the Researchers section of this site.
To access the article titled "The First Three Years of IBEX Observations and Our Evolving Heliosphere," please visit the Astrophysical Journal Supplement Series website.