When it comes to charting the outer regions of the Solar System, astronomers may as well use the colloquial phrase "here there be dragons." As much as we still have to learn about our direct planetary neighbors, like Mars or Venus, we are even more in the dark about the mysterious expanse beyond Pluto, where hulking undiscovered planets may roam and the Sun's influence wanes before the boundary to interstellar space.
For instance, one of the strangest phenomena observed in the far reaches of the Solar System is the IBEX ribbon, named after NASA's Interstellar Boundary Explorer (IBEX) satellite, which discovered it in 2008.
This so-called ribbon is an intense stream of neutral particles that has been called a "roadmap in the sky," with regards to studying the murky interstellar magnetic field beyond the Sun's influence.
Think of it like this: solar wind and magnetism create a kind of cosmic harbor, called the heliosphere, that extends about 90 astronomical units (AU) around the Sun (one AU is defined as the distance between the Earth and the Sun). All known planets are moored well inside these protected waters.
However, outside the harbor, in the interstellar medium, the ocean is much wilder and harder to predict. Moreover, just as eddies and riptides occur on the frontiers between harbors and open ocean, the IBEX ribbon is somehow generated by the transitional zone, or heliosheath, between the Solar System and the wider galactic void.
Now, in a new study published in the Astrophysical Journal Letters, a team led by Eric Zirnstein, a space scientist at the Southwest Research Institute in San Antonio, Texas, demonstrate that the underlying dynamic underneath the IBEX ribbon is likely to be solar blowback.
"The theory says that some solar wind protons are sent flying back towards the sun as neutral atoms after a complex series of charge exchanges, creating the IBEX ribbon," said Zirnstein in a statement. "Simulations and IBEX observations pinpoint this process—which takes anywhere from three to six years on average—as the most likely origin of the IBEX ribbon."
In other words, the Sun ejects a bunch of protons in the solar wind, many of which pick up hitchhiker electrons that help them sneak past the heliopause, the gateway to the heliosheath.
But if some of these now-neutral atoms happen to lose their electron friends in these brave new waters, then gain new ones at exactly the right time and place, they can be shot back through the heliopause, creating aurora-like phenomena such as the IBEX ribbon. It's as if these charge-hopping atoms are prodigal children, destined to return back to the heliosphere that bore them, while their brethren rush forth to larger galactic vistas.
Many recent studies support this model, as do the observations of Voyager 1 and 2, both of which recently passed into the heliosheath. On top of that, Zirnstein's team was able to simulate the strength and direction of the interstellar magnetic field with great accuracy using this model.
But there are still several competing theories about the ribbon out there, so it's not yet an open-and-shut case at this stage.
"The new findings can be used to better understand how our space environment interacts with the interstellar environment beyond the heliopause," said IBEX program scientist Eric Christian in a statement.
"In turn, understanding that interaction could help explain the mystery of what causes the IBEX ribbon once and for all."