From Dave McComas, IBEX Principal Investigator
Happy 5th anniversary, IBEX! IBEX’s mission is to gather particles coming in all directions toward Earth from the edge of our Solar System so we can learn about the processes occurring there. Little did we know when IBEX launched on October 19, 2008 how amazing those interactions would be and the fantastic science that IBEX would give us! As a celebration of the past five years, we are featuring many of IBEX’s most significant science results compiled together. The table below lists a number of IBEX’s major "firsts" and discoveries:
1) Discovery of an enhanced ENA Ribbon and its connection to the interstellar magnetic field.
2) First observations of globally distributed ENAs from the inner heliosheath.
3) Discovery of rapid (~6 mo) time variations in the heliosphere's interstellar interaction.
4) First observations of the heliotail and the influence of the interstellar magnetic field on it.
5) Discovery of our Sun's slower motion with respect to the interstellar medium.
6) Discovery that the heliosphere is very likely to have a bow wave and no bow shock.
7) First direct observations of interstellar Hydrogen, Oxygen, and Neon.
8) Discovery that the very local interstellar medium is rotating ahead of the heliosphere.
9) First images of the magnetospheric cusps and sub-solar magnetosheath from outside.
10) First ENA images of the Earth's plasma sheet and a possible disconnection event.
11) Discovery of neutralized and backscattered solar wind from the Moon.
Rocketry and Orbital Dynamics
12) First flight of a solid rocket on top of, and attaining high altitude orbit from, a Pegasus.
13) Discovery and first spacecraft to fly in a lunar resonance orbit.
Recently, I participated in an hour–long live interview with NPR’s David Schlom for the "Blue Dot Report" radio program. The interview, in its entirety, is here, which you can listen to in a series of three MP3 files [audio file 1, audio file 2, audio file 3]. I also invite you to read more about IBEX’s fantastic science results in our Archived Updates.
The IBEX team is composed of scientists, engineers, educators, and others from the United States and many countries around the world. I want to sincerely thank all of the hard–working men and women who have made IBEX the success that it has been. We all look forward to many more outstanding discoveries in the future! Go IBEX!
IBEX: A significant mission from the start
IBEX is a breakthrough mission that was achieved with a small, lightweight spacecraft launched from Kwajalein Atoll, Marshall Islands in the middle of the Pacific Ocean. IBEX was launched using a Pegasus rocket dropped from an L–1011 aircraft. Prior to the IBEX mission, all other NASA spacecraft that had used the Pegasus launch system only reached low–Earth orbit — just hundreds of miles/kilometers in altitude. But for IBEX, we supplied our own additional solid rocket motor and used that and our spacecraft’s onboard propulsion system to boost up into a high altitude orbit that, at its farthest point, reaches about 200,000 miles (320,000 kilometers) from Earth — about 5/6 of the distance to the Moon’s orbit. IBEX was the first (and thus far only) mission to go far beyond low–Earth orbit starting with a Pegasus rocket.
Why do we need to study the boundary of our Solar System?
The boundary of our Solar System emits no light, so conventional telescopes cannot collect information about it. Our Solar System is composed of several parts. Charged particles, such as protons and electrons, stream radially off of our Sun in all directions at around one million miles (1.6 million kilometers per hour); these moving particles are called the "solar wind." As the solar wind streams away from the Sun, it races out far past the planets 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." Billions of miles from the Sun, 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.
This interaction process between the solar wind and the interstellar medium creates "energetic neutral atoms," or ENAs. ENAs are atoms that travel very quickly and have no charge (i.e., equal numbers of protons and electrons). Because ENAs 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.
Is IBEX used only to study our Solar System’s boundary?
No — and this shows IBEX’s versatility. IBEX’s two sensors collect ENAs all year long, but measurements at various times of year allow the scientists to not only study neutral atoms created at our Solar System’s outer boundaries, but to also directly detect interstellar neutral atoms drifting in from outside our Solar System, neutral atoms created by the interaction of the solar wind with the surface of our Moon, and neutral atoms created by processes occurring in Earth’s magnetosphere.
Some of the significant IBEX Discoveries: 2009 to 2013
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 a globally distributed ENA flux that is somewhat similar to what the scientists thought they would see. However, what they also saw was an unexpected arc–shaped region in the sky that is creating several times more ENAs, showing up as a bright, narrow "ribbon" on the maps, which has subsequently been called the IBEX Ribbon.
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 five years, the ribbon has been a fairly stable feature, although it has been dimming slightly as have ENA 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 appears broader and less intense.
A Strong Ribbon Source Candidate
Ever since the IBEX Ribbon was detected, the science team has worked to determine what causes it. It appears to be related to the alignment of magnetic fields outside our heliosphere, suggesting that this interstellar magnetic field 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 past few years, corresponding to a lower amount of solar wind streaming out during that time. Additionally, the team has seen that the ribbon appears substantially different at different energy levels, 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 a solar wind source for it.
As for identifying the precise interactions that produce the ribbon, this has been a difficult part of the science work for the IBEX mission because no single model for the ribbon has been able to explain all of the observed features. The science team is still developing and refining models to explain the observations — 13 different models for the ribbon, at last count! One of these models may be the correct one, or the ribbon may be due to a combination of several processes, or a model that the team has not considered yet may be the correct one. There is still much work to be done!
No Bow Shock for our Heliosphere
The boundary of our Solar System is formed at the location where the solar wind streaming outward meets the interstellar medium. 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."
Results from IBEX have shown that our Solar System is traveling slower with respect to the interstellar medium. That difference in speed, about 7,000 miles per hour (11,000 kilometers per hour) 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; rather, a broader bow wave is present.
Observations of Interstellar Neutral Atoms
IBEX has directly 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 directly 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.
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 do not interact with magnetic fields and move through the boundary of our heliosphere without the boundary affecting them.
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. Significant IBEX findings include the following:
- IBEX data and that of other spacecraft showed previously that our Sun appears to be close to the boundary of an interstellar cloud of gas and dust and there appears to be a network of gas and dust clouds in our local galactic vicinity.
- Our local interstellar region does not currently match the characteristics where the Sun originally formed 5 billion years ago.
- The most significant result is that the direction in the sky from which interstellar neutral helium enters our Solar System has changed by about 7 degrees over the past forty years. This would equate to a shift of about 2.2 billion miles (3.5 billion kilometers). This variation may be telling us about changing conditions in our immediate region of the Milky Way Galaxy. More work will need to be done to characterize these conditions.
Our Solar System’s Heliotail
New data from IBEX is filling in the unknowns regarding our heliotail. IBEX data show the heliotail is the region where the solar wind flows downwind and ultimately escapes the heliosphere.
First, the ENAs detected show that the heliotail appears large, spanning nearly 180 degrees from one side to the other. As somewhat of a surprise, the lower energy, slower solar wind in the tail appears thinner in the middle and bulbous to either side; IBEX scientists have described these tail structures as "lobes." Our heliotail appears to have two of these lobes of low energy solar wind in addition to higher energy regions at higher southern and northern latitudes. Overall, the two lobes are slightly twisted, or tilted, at an angle. IBEX scientists hypothesize that this tilting is due to the force of the magnetic field outside our heliosphere acting upon the heliotail, rotating it slightly and squeezing it into an oval cross–sectional shape:
What is next for IBEX?
Thanks to IBEX data, we are getting not just the first but an increasingly more complete picture of our heliosphere. It will be very interesting to see if there are changes in parts of our heliosphere over the next five to ten years as we are currently passing solar maximum and heading toward another solar minimum over the next half decade. Scientists will also need to develop mathematical models that reproduce the IBEX results to be able to provide solid theories as to the production of the ribbon, the heliotail, the bow wave, and other regions.
"We think we know what we’re going to study in science, but the work often takes us in unexpected directions," says Dave McComas, IBEX Principal Investigator. "That has certainly been the case with IBEX, and we are lucky to have such a versatile tool to help us continue to learn so much about our home in the Milky Way Galaxy."
For details about all of the science discoveries mentioned above, please visit the IBEX website Archived Updates.