Although never actually observed, it has long been assumed that as our Solar System careers through the Universe, the heliosphere, or solar bubble, has a tail trailing behind it like a comet's. For the first time, NASA's Interstellar Boundary Explorer (IBEX), which was launched back in 2008, has mapped the boundaries of this heliotail, revealing it is shaped like a four-leaf clover.
While telescopes have managed to spot heliotails around other stars, it has been difficult to verify if our star has one. In 2003, Pioneer 10, which was launched in 1972, lost power before it was able to move into the tail, so there was no data with which to make a solid determination. Since the particles in a tail of this sort don't shine, they can't be seen conventionally.
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By combining three years of imagery from IBEX, the research team mapped out the tail, which contains a combination of fast and slow moving particles. The map shows that there are two lobes of slower particles with faster particles above and below. The tail has a twisted structure since it experiences the tugging of magnetic fields from outside our solar system. This forms the four-leaf clover shape which is caused by the fast solar wind that the Sun sends out near its poles, and a slower wind that is pushed out near its equator. The Sun has an 11-year activity cycle, and this is a common pattern in the most recent phase of this cycle.
The clover shape does not align with the Solar System precisely. The shape has a subtle rotation indicating that as it moves further from the Sun's magnetic influence, the charged particles are being nudged into a new orientation, aligning with magnetic fields from the local galaxy. The solar wind spreads in all directions from the Sun, eventually slowing down and bending back along the tail after moving far past the outermost planets. The particles join a migration of particles moving backwards inside the boundary of the heliosphere due to the incoming interstellar material. This boundary, where the solar wind is stopped by the interstellar medium, is known as the heliopause.
When slower neutral atoms originating from elsewhere in our galaxy collide with faster charged particles, they can exchange an electron, resulting in a slow charged particle and a fast neutral atom. The neutral atom is no longer bound to the magnetic fields so speeds straight off in whatever direction it was pointed at at that moment.
Using a technique called energetic neutral atom imaging, IBEX is able is able to measure neutral particles created by collisions at the heliosphere’s boundaries to observe far away structures and chart the shape of our solar bubble's tail. The mapping relies on the fact that the paths of neutral particles aren't affected by the heliosphere’s magnetic fields.
The researchers don't currently know how long the tail is, but they believe it gradually fades away and becomes indistinguishable from the rest of interstellar space.
Now that scientists have these observations, they can test their computer simulations of the heliosphere and improve their models. As they continue to improve simulations with data instruments in space and lab analysis on the ground, the researchers hope to improve our understanding of the comet-like tail streaming out behind us. Maybe even giving us more insight into where everything started or where we are headed (literally).
The team's research is described in a paper published in The Astrophysical Journal.
The video below gives a brief description of IBEX's work.