From Dave McComas, IBEX Principal Investigator
The science just keeps coming from IBEX! We are heavily involved in analyzing not only the particles coming from our heliosphere but also particles coming from regions that are much closer to home. In our August 2010 IBEX Update, we showed the first images of the dayside magnetosphere from IBEX. In our latest paper, presented today at the American Geophysical Union meeting in San Francisco, and awaiting publication in the Journal for Geophysical Research we show how IBEX can study the Earth's night side magnetosphere with unparalleled detail, including publication of the very first image of a region of our magnetosphere called the "plasma sheet." IBEX gives scientists yet another way to study this normally invisible part of our region of space. Read on for more information about this exciting news, including an unexpected observation. Yet another great step forward for space science!
Science Update – More Magnetosphere Observations
What is IBEX seeing?
Charged particles from the solar wind interact with the magnetosphere — a region of space that surrounds the Earth; the magnetosphere is produced by the Earth's magnetic field, which traps ionized gas or plasma within. Because of the pressure of the solar wind, the shape of the night side of our magnetosphere might look vaguely like comet's tail or the stretched out pointed end of a teardrop, streaming back far behind the Earth and forming what scientists call the "magnetotail". This region is dynamic, ebbing and flowing with the changes in the solar wind.
Surrounding Earth at the edge of the atmosphere, there is also is an extremely low-density region of neutral hydrogen atoms called the "geocorona". The trapped magnetospheric charged particles exchange electrons with the neutral hydrogen in the geocorona, and the magnetospheric particles become neutral during this process. At this point, these energetic neutral atoms (ENAs) are no longer affected by Earth's magnetic field and fly off in whatever direction they were going when they became neutral. Because some of these particles happen to be traveling in the direction of the IBEX spacecraft and its sensors, IBEX can detect them. Just like our heliosphere boundary and the part of our magnetosphere facing the Sun, this part of the magnetosphere does not give off light that we can detect, so we must use ENA particle sensors like those on IBEX to study regions like this.
What came before IBEX?
While this is IBEX's first detection of the energetic neutral atoms from this region, ENAs from the Earth's magnetosphere have been observed previously by several other spacecraft. These ENAs were spotted in the late 1970s by the International Sun-Earth Explorer 1 spacecraft (ISEE-1). Since then, other missions such as Polar, IMAGE, and TWINS have all detected magnetosphere ENAs. The difference is that those missions all viewed the Earth's magnetosphere from the inside looking outward. For IBEX's observations, it was positioned outside the Earth's magnetosphere looking inward.
When did IBEX make these observations?
During each fall and spring, IBEX and the Earth are arranged so that IBEX's view scrolls over the Earth's magnetotail and plasma sheet from the side. These particular observations were made in October 2009 during IBEX's fifty-first orbit of the Earth.
This diagram shows IBEX orbit #51 looking down on the north pole of the Earth (black/white circle). The direction of the Sun is upward in the diagram. As IBEX spins, its sensors sweep across the Earth's magnetosphere, as indicated by the gray FOV (Field of View) area. RE shows distances in terms of Earth radii (1 Earth radius equals approximately 3,900 miles, or about 6,200 kilometers). (The IBEX spacecraft in the diagram is not to scale.)
Why are these observations so remarkable?
The part of the magnetosphere that IBEX has imaged for the first time is the "plasma sheet". The plasma sheet is a component of the magnetotail made up of magnetic field lines that attach to the Earth at both ends, bottling up a denser region of positively charged atoms and negatively charged electrons. Missions like TWINS and IMAGE detected ENAs within about 30,000 miles of Earth and looking down the magnetotail. IBEX can observe them coming from much farther away from Earth in the plasma sheet. Dave McComas further explains, "IBEX is the first mission to have the sensitivity and the orbit necessary to view these ENA emissions from more distant parts of the magnetotail. The image of the plasma sheet and magnetotail ENAs alone is remarkable and it would have been the subject of a great science article all by itself because it's the first time we've imaged these important regions of the magnetosphere. However, there is something more that we have seen!"
What do the images show?
The image below is the first ever composite image of the plasma sheet in the magnetotail.
Earth is shown by the black/white circle in the middle. The curved lines represent model magnetic field lines in the Earth's magnetosphere. The Sun is toward the left. The red and yellow colors show the regions of the magnetosphere emitting the most ENAs, and the green, blue, and purple colors show the regions emitting fewer ENAs. This image represents the ENAs collected by IBEX in Orbit 52, from November 5 through 7, 2009. The line next to FOV in the lower right corner shows the width of IBEX's field of view at any one time. To build up this image, IBEX had to scan the magnetosphere for about two days.
However, in the data collected during the previous orbit (#51) shown below, the observations are quite different.
Notice there are more ENAs that can be seen in the most distant part of the plasma sheet to the far right, and fewer ENAs are seen in nearer plasma sheet region compared to the previous ENA image. Dave says, "Such a brightening would be consistent with dynamic changes in the magnetotail. We know these changes occur frequently." These observations were made from October 27 through October 29, 2009, and this map was created from observations built up from many scans of the magnetosphere during that time. It is important to note that neither of the two ENA images above is a snapshot but, instead, each is an average of the ENAs detected over the course of about two days.
Are there explanations for these changes?
Yes, there are several possibilities to explain the variation in ENAs in the distant plasma sheet. A closer look at the various images produced by multiple IBEX orbits has revealed what appears to have been a piece of the plasma sheet being bitten off and ejected down the tail. This is called a "magnetic disconnection", a dynamic event where the magnetic fields "reconnect" across the plasma sheet. Dave adds, "Imagine the magnetosphere as a long balloon used for "tying" animal shapes. If you squeeze the balloon with your hands, the squeezing pressure forces the air into another segment of the balloon. Similarly, the solar wind sometimes increases the pressure around the magnetosphere, resulting in a portion of the plasma sheet being pinched away from the rest and forced down the magnetotail - just like squeezing forces air to move away from the pinched part of the balloon. This ejected material is called a "plasmoid". We have never been able to see a magnetic disconnection event in any actual images, so it is extremely exciting that IBEX may very well have spotted one for the first time!"
Another possibility that the team is considering is an increase in solar wind pressure that would squeeze the magnetotail and cause it to heat up. This process would also create more ENAs in the plasma sheet in a similar pattern to what IBEX has seen.
A third hypothesis is that plasma may have been forced toward Earth, coming inward from farther away in the magnetotail. The evidence for this, though, is not as strong based on the conditions in and around the magnetosphere at the time of these observations.
So, what is in store for the future?
More observations are needed to show various events like these. The observations happened during a period where Earth's magnetosphere was relatively quiet. If there is more solar activity like we expect in the next few years and, therefore, more activity in the magnetosphere, IBEX should be able to see some pretty amazing things, including plenty of disconnections and plasmoids. Additionally, there are other current and new spacecraft teams to work with, including missions that will give direct measurements of the magnetosphere plasma as well as ENA imaging from the inside while IBEX can simultaneously view it from the outside. The next few years should be really exciting as we see our magnetosphere from the outside and learn even more about it!
An article detailing these observations has been accepted for publication in the American Geophysical Union's Journal of Geophysical Research. Check it out!
Go to the Public Data Page for access to research-level data from the IBEX mission.