A multi-institutional team of astrophysicists led by Boston University astrophysicist Merav Opher has made a breakthrough in our understanding of the cosmic forces that shape the protective bubble surrounding our solar system, known to space researchers as the heliosphere, which protects life on Earth.
Astrophysicists believe the heliosphere shields our solar system’s planets from intense radiation emitted by supernovas, the universe’s ultimate explosions of dying stars.
They believe the heliosphere extends well beyond our solar system, but no one understands the shape or extent of the heliosphere, despite the fact that it supports Earth’s life-forms with a vast shield against cosmic radiation.
“How is this relevant for society? The bubble that surrounds us, produced by the sun, offers protection from galactic cosmic rays, and the shape of it can affect how those rays get into the heliosphere,” says James Drake, an astrophysicist at the University of Maryland who collaborates with Opher.
“There are lots of theories but, of course, the way that galactic cosmic rays can get in can be impacted by the structure of the heliosphere does it have wrinkles and folds and that sort of thing?”
Opher’s team has created some of the most intriguing computer simulations of the heliosphere, using models based on observable evidence and theoretical astrophysics. At BU, in the Center for Space Physics, Opher, a College of Arts & Sciences professor of astronomy, leads a NASA DRIVE (Diversity, Realize, Integrate, Venture, Educate) Science Center that’s supported by $1.3 million in NASA funding.
How is this relevant for society? The bubble that surrounds us, produced by the sun, offers protection from galactic cosmic rays, and the shape of it can affect how those rays get into the heliosphere.
James Drake
In an effort dubbed SHIELD (Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics), that team, made up of experts recruited from 11 different universities and research organizations, produces predictive models of the heliosphere.
Since BU’S NASA DRIVE Science Center first received funding in 2019, Opher’s SHIELD team has hunted for answers to several puzzling questions: What is the overall structure of the heliosphere? How do its ionized particles evolve and affect heliospheric processes? What role does the heliosphere play in the interstellar medium, the matter and radiation that exists between stars, and how does it interact with it? What is the mechanism by which cosmic rays are filtered or transferred via the heliosphere?
“SHIELD combines theory, modeling, and observations to build comprehensive models,” Opher says. “All these different components work together to help understand the puzzles of the heliosphere.”
Now, a report published in the Astrophysical Journal by Opher and associates demonstrates that neutral hydrogen particles coming in from beyond our solar system are most likely vital in the formation of our heliosphere.
Opher’s team wanted to know why heliospheric jets, which are blooming columns of energy and matter similar to other types of cosmic jets found across the cosmos, become unstable in their current study.
“Why do stars and black holes and our own sun eject unstable jets?” Opher says. “We see these jets projecting as irregular columns, and astrophysicists have been wondering for years why these shapes present instabilities.”
The heliosphere, which travels in parallel with our sun and encompasses our solar system, does not appear to be stable, according to SHIELD simulations. Other astrophysicists’ models of the heliosphere tend to represent the heliosphere as having a comet-like shape, with a jet or “tail” trailing behind in its wake. Opher’s concept, on the other hand, argues that the heliosphere is shaped like a croissant or perhaps a donut.
The reason for that? Because they have equal amounts of a positive and negative charge, neutral hydrogen particles have no charge at all.
“They come streaming through the solar system,” Opher says. Using a computational model like a recipe to test the effect of ‘neutrals’ on the shape of the heliosphere, she “took one ingredient out of the cake the neutrals, and noticed that the jets coming from the sun, shaping the heliosphere, become super stable. When I put them back in, things start bending, the center axis starts wiggling, and that means that something inside the heliospheric jets is becoming very unstable.”
Such instability may conceivably disrupt the solar winds and jets that emanate from our sun, causing the heliosphere to split into a croissant-like shape.
Opher’s model argues that the presence of neutrals slamming into our solar system would make it difficult for the heliosphere to flow uniformly like a shooting comet, despite the fact that astrophysicists have yet to find techniques to observe the real shape of the heliosphere. And one thing is certain: neutrals are pelting their way through the universe.
Opher’s model, according to Drake, “provide the first clear explanation for why the form of the heliosphere breaks up in the northern and southern areas, which could impact our knowledge of how galactic cosmic rays enter Earth and the near-Earth environment.”
This could have an impact on the hazard of radiation to life on Earth, as well as humans in space and future explorers seeking to reach Mars or other worlds.
“The universe is not quiet,” Opher says. “Our BU model doesn’t try to cut out the chaos, which has allowed me to pinpoint the cause of the heliosphere’s instability…. The neutral hydrogen particles.”
The presence of neutrals colliding with the heliosphere causes a physicist-recognized phenomenon known as the Rayleigh-Taylor instability, which occurs when two materials of different densities contact, with the lighter material pushing against the heavier material.
When oil is suspended over water, or when heavier fluids or solids are suspended above lighter fluids, this is what happens. Gravity also plays a part, resulting in some crazily uneven shapes.
The pull between the neutral hydrogen particles and charged ions generates a similar effect to gravity in cosmic jets. The Rayleigh-Taylor instability, for example, causes the “fingers” observed in the renowned Horsehead Nebula.
“This finding is a really major breakthrough, it’s really set us in a direction of discovering why our model gets its distinct croissant-shaped heliosphere and why other models don’t,” Opher says.
