From Dave McComas, IBEX Principal Investigator
In May of 2012, the IBEX team announced amazing news: our heliosphere does not have a bow shock in front of it. A bow shock was long hypothesized to be present, though there was no way to test this hypothesis until IBEX data became available. We have released a revised artist’s rendition of the forward part of our heliosphere utilizing these results.
Have you noticed, though, that when we work with our artist to visualize the heliosphere, we cleverly cut off the images on the "back end" or "heliotail" (right side in the image below)? There is a very good reason for this. We did not have any data prior to IBEX to tell us if a tail existed, what it looked like, or how various particles behaved within that region.
When we designed the IBEX mission, one of the fundamental questions we hoped to answer involved determining the overall properties of the heliotail. Now, with three years of observations under our belt, we finally have enough data to start to summarize some of the observational features of this previously unexplored region. These new results are published in the July 10, 2013 issue of the Astrophysical Journal. In short, there does appear to be a heliotail with interesting shape and orientation properties, which we have highlighted below. More work will need to be done to identify a mathematical model that reproduces the observational features that we have seen so far, and more data will be needed to identify how changes in the Sun and solar wind structure over the solar cycle affect the heliotail. We are excited, though, to start to fill in the prior BIG blank in our knowledge of this important part of our heliosphere! Go IBEX!
Where is the heliotail in relation to our Solar System?
Our Solar System is composed of several parts. Our Sun emits a "wind" of material outward in all directions, at typically about 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 or bubble 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.
Have we seen other heliospheres and heliotails?
We have imaged other bubbles, called "astrospheres," around other stars, as well as the tails from these astrospheres.
This is an image of a bow shock and astrosphere surrounding a star called BZ Camelopardalis, released by the National Optical Astronomy Observatory:
An image from the Hubble Space Telescope shows the bow shock in front of the young star LL Orionis:
Possibly the most dramatic example, imaged by NASA’s GALEX spacecraft, shows the astrosphere around the speedy Mira, a star moving so quickly, its astrosphere is stretched into an enormous 13 light–year–long tail:
How does IBEX study the heliotail?
IBEX studies energetic neutral atoms. Our Solar System’s heliotail emits no light, so information about it cannot be collected by conventional telescopes. The solar wind emitted by our Sun is composed primarily of charged particles, such as protons and electrons, streaming off of our Sun at high speed. The solar wind heads outward, far past the planets. Billions of miles from the Sun, solar wind particles interact with individual neutral atoms from 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 to study our heliotail.
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.
So far, what does IBEX data tell us about our heliotail?
New data from IBEX is filling in the unknowns regarding our heliotail. IBEX data show the heliotail is the region where the Sun’s million mile per hour (1.6 million kilometers per hour) solar wind flows down 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 (the green, yellow and red regions in the figure labeled "c" below) 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 at low and mid latitudes (shown in green in the schematic below labeled "a") in addition to higher energy regions (shown in blue in the schematic below labeled "a") along the top and bottom of the tail.
"We chose the term ‘lobes’ very carefully," says Dr. Dave McComas, IBEX principal investigator and assistant vice president of the Space Science and Engineering Division at Southwest Research Institute. "It may be that these are separate structures bent back toward the down wind direction. However, we can’t say that for certain with the data we have today."
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:
As for the extent of the heliotail, that is difficult to determine. The ENAs detected by IBEX do not directly give an estimate of how far they have traveled to reach the IBEX sensors, but using IBEX data, scientists can estimate of the length of the heliotail to be about one thousand times the distance between the Earth and the Sun. This Earth–Sun distance, called an "Astronomical Unit," is about 93 million miles, so one thousand times this distance is approximately 93 billion miles.
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 the heliotail over time and in other parts of our heliosphere over the next five to ten years as we are currently passing solar maximum and head 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 heliotail.
"We think we know what we’re going to study in science, but the work often takes us in unexpected directions," says McComas. "That was certainly the case with this study, which started by simply trying to better quantify a small structure we had incorrectly identified as an "offset heliotail." The heliotail we found was much bigger and very different from what we expected. We hope and expect that continuing new IBEX data will take us in even more interesting and unexpected directions!
To access the article titled "The Heliotail Revealed by the Interstellar Boundary Explorer," please visit the Astrophysical Journal website.