The Solar System’s tail, mapped by IBEX
Astronomers have assumed for some time that our Sun has a tail, formed from material ejected in the solar wind, and shaped by the Solar System’s passage through the interstellar medium (ISM). Repeated announcements from NASA about the Voyager spacecraft’s passage from the Solar System through to interstellar space are really stories about how they’re passing through the surface of that tail. But new data from NASA’s Interstellar Boundary Explorer (IBEX) have given us our first clear view of what this “heliotail” looks like, and how it is shaped.
The intensely hot plasma making up the outer layers of the Sun ejects atomic nuclei in all directions with enough speed to overcome the Sun’s gravity. This outflowing material is steered and accelerated by the Sun’s magnetic field, and forms a wind of subatomic particles blowing at immense speed outwards through the Solar System (although it is a very insubstantial wind, being thinner than even the best vacuums we can create in laboratories on Earth). The intensity and structure of the solar wind varies constantly, as the turbulent solar surface boils and flares, so that sometimes it is barely detectable, while at others it smashes into our upper atmosphere, stripping electrons from air molecules and creating the ethereal light shows of the Aurora Borealis. The wind blows at hundreds of kilometers per second, more than fast enough to escape the Solar System forever, but as it gets further from the Sun, it runs into the interstellar medium, an even more diffuse gas filling the space between the stars (And make no mistake about just how insubstantial this stuff is. I’m talking about less than a single atom per cubic centimeter). It slows down, and the building pressure creates a shock-wave, which astronomers call the Heliosphere.
When the charged particles of the solar wind collide with the neutral atoms of the interstellar medium, the result is both particles shooting off in new directions. Some of these particles happen to travel back into the Solar System and strike the sensors on IBEX, which counts the number of collisions and the directions from which they came. This data, collected over years, was used to build a map of the heliotail, which was announced in the Astrophysics Journal of 10 July (doi:10.1088/0004-637X/771/2/83).
What IBEX has now confirmed is that, as expected, our Solar System’s passage through the interstellar medium sculpts the heliosphere, flattening it at the front and stretching it out into a long tail at the rear. Such stellar tails have been observed before in other stars, but it’s never been possible to observe our own tail directly. IBEX’s map reveals a wealth of detail that was not known before. Because the solar wind is strongest from the Sun’s poles, and weakest from the equator (thanks to the funneling effect of the Sun’s magnetic field), and because it is further bent by its passage through the weaker magnetic field of the galaxy itself, the heliosphere is not spherical to start with. The intensity of the solar wind is quite variable as well, from both daily variations and the 11 year solar cycle, causing the heliosphere to fluctuate over time. When the ram pressure of the ISM gets to work on this uneven structure, the end result is a heliotail with a four-lobed shape, described by mission scientists as a four-leafed clover. And interestingly, it doesn’t point away from the direction of the Sun’s movement, but is instead buffeted off to the side in response to the electromagnetic forces generated by the Milky Way’s magnetic field. It’s a lot like a comet, in other words – yet another example of how completely different phenomena keep creating the same familiar shapes throughout nature. Bath-time whirlpools echoing gigantic spiral galaxies, coastlines looking much the same whether viewed at a microscopic level or at the scale of continents, and now the swoosh of a comet compares to the trail of subatomic debris left behind by our Sun as it carves its orbit around the Galaxy.