Every planet in our solar system, including the Earth, is 'blown' by the solar wind at supersonic speed.
The particles that make up this wind create an invisible magnetic field that protects us from the rest of interstellar space. For decades, astronomers have been analyzing this system of radiation and magnetism known as the heliosphere, mapping its boundaries to figure out what it looks like.
A new model from experts at several universities now suggests that this is a strange amalgamation of almost all of our theories. For many years, scientists believed the heliosphere was more like a comet with a round nose at one end and a tail at the other.
This is how it is usually portrayed in textbooks and articles, but in recent years, two other forms have emerged that seem more likely.
NASA
In 2015, data from the Voyager 1 spacecraft indicated the presence of two tails, which made the heliosphere more like a croissant. Two years later, data from the Cassini mission showed that we must completely get rid of the whole tail, turning it into a giant beach ball.
“You don't easily accept this kind of change,” says Tom Crimigis, who conducted experiments on Cassini and Voyager.
“The entire scientific community working in this area has assumed for over 55 years that the heliosphere has a cometary tail.”
Now we may have to rethink our assumptions once again, because if the new model is correct, the heliosphere could very well be in the shape of a deflated beach ball or a bulging croissant, it just depends on where and how you define the boundary.
The heliosphere is believed to extend twice as far as Pluto, when the solar wind constantly strikes interstellar matter, protecting us from charged particles that could otherwise destroy our solar system.
But figuring out where that border exists is like trying to figure out what shade of gray should distinguish black from white.
Using data from the New Horizons spacecraft, which is now outside Pluto, astronomers have found a way to separate the two sides.
Instead of assuming charged particles are all the same, the new model divides them into two groups: charged particles from the solar wind and neutral particles drifting in the solar system.
Unlike charged particles in interstellar space, these neutral 'absorbing ions' can easily slip through the heliosphere before their electrons are charged.
By comparing the temperature, density, and speed of these absorbing ions to solar waves, the team has found a way to determine the shape of the heliosphere.
'The depletion of [absorbing ions] due to charge exchange with neutral hydrogen atoms of the interstellar medium cools the heliosphere,' lowering 'it and leading to a narrower and more rounded shape, confirming the shape proposed by Cassini.
In other words, depending on which 'shade of gray' you choose to define the boundary, the heliosphere may look like a deflated sphere or a crescent moon.
“If we want to understand the environment, we have a better understanding of this entire heliosphere,” says astronomer Avi Loeb of Harvard.
But we still need a lot more data. While we are gradually beginning to harmonize our models, they are still limited by how little we know about the heliosphere itself.
Apart from the two Voyager spacecraft launched over four decades ago, not a single machine has flown beyond its limits. And even the two space probes that have passed this line do not have instruments to measure ions at the periphery.
The study was published in the journal Nature Astronomy.
Sources: Photo: Nature Astronomy, 2020
