Studying the protective bubble of our solar system

Is this what the heliosphere looks like? Research conducted by BU suggests so. The size and shape of the magnetic “force field” that shields our solar system from deadly cosmic rays has long been debated by astrophysicists. Credit: Merav Opher, et. Al

A multi-institutional team of Boston University-based astrophysicists, led by BU astrophysicist Merav Opher, has made a revolutionary discovery in our understanding of the cosmic forces that shape the protective bubble surrounding our solar system, a bubble which harbors life on Earth and is known to space researchers as the heliosphere.

Astrophysicists believe that the heliosphere shields the planets in our solar system from the powerful radiation emanating from supernovae, the final explosions of dying stars throughout the universe. They believe that the heliosphere extends far beyond our solar system, but despite the enormous buffer against cosmic radiation that the heliosphere provides to life forms on Earth, no one really knows the shape of it. heliosphere – or, for that matter, its size.

“How is this relevant to society? The bubble around us, produced by the sun, provides protection against galactic cosmic rays, and its shape can affect how these rays enter the heliosphere,” explains James Drake, university astrophysicist. from Maryland who collaborates with Opher. “There are a lot of theories but, of course, the way galactic cosmic rays can enter can be affected by the structure of the heliosphere – does it have wrinkles and folds and that sort of thing?”

Opher’s team has built some of the most compelling computer simulations of the heliosphere, based on models built on observable data and theoretical astrophysics. At BU, in the Center for Space Physics, Opher, professor of astronomy at the College of Arts & Sciences, runs a NASA DRIVE (Diversity, Realize, Integrate, Venture, Educate) science center backed by $ 1.3 million in funding from NASA. This team, made up of Opher experts recruited from 11 other universities and research institutes, is developing predictive models of the heliosphere in an effort the team calls SHIELD (Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics).

Since the BU’s NASA DRIVE Science Center first received funding in 2019, Opher’s SHIELD team has sought answers to several puzzling questions: What is the overall structure of the heliosphere? How do its ionized particles evolve and affect heliospheric processes? How does the heliosphere interact and influence the interstellar medium, matter and radiation that exist between stars? And how are cosmic rays filtered or transported through the heliosphere?

“SHIELD combines theory, modeling and observations to create complete models,” Opher explains. “All of these different components work together to help understand the puzzles of the heliosphere.”

And now an article published by Opher and his collaborators in Astrophysics Journal reveals that neutral hydrogen particles coming from outside our solar system most likely play a crucial role in how our heliosphere takes shape.

In their latest study, Opher’s team set out to understand why heliospheric jets – columns of flourishing energy and matter similar to other types of cosmic jets found throughout the universe – become unstable. “Why do stars and black holes – and our own sun – eject unstable jets?” Said Opher. “We see these jets projecting in the form of irregular columns, and [astrophysicists] I have wondered for years why these shapes show instability. “

Another breakthrough for the team studying the protective bubble of our solar system |  the edge

New research by BU astrophysicist Merav Opher could explain why the heliosphere, a protective magnetic “force field” emanating from our sun and encompassing our solar system, is likely unstable and irregularly shaped. “The universe is not calm,” Opher says. “Our BU model does not try to eliminate chaos. Credit: Merav Opher, et. Al

Likewise, the SHIELD models predict that the heliosphere, traveling in tandem with our sun and encompassing our solar system, does not appear stable. Other models of the heliosphere developed by other astrophysicists tend to describe the heliosphere as having a comet-like shape, with a jet – or “tail” – dripping behind in its wake. In contrast, Opher’s model suggests that the heliosphere is more shaped like a crescent or even a donut.

The reason for that? Neutral hydrogen particles, so called because they have equal amounts of positive and negative charges which have no charge at all.

“They are streaming through the solar system,” Opher says. Using a computer model as a recipe to test the effect of “neutrals” on the shape of the heliosphere, she “took out an ingredient from the cake – the neutrals – and noticed the jets coming from the sun, shaping the heliosphere, became super stable. When I put them back in place things start to bend, the center axis starts to wiggle, and that means something inside the heliospherical jets is getting very unstable. “

Such instability would theoretically cause disturbances in solar winds and jets emanating from our sun, causing the heliosphere to split into a crescent shape. Although astrophysicists have yet to develop a way to observe the actual shape of the heliosphere, Opher’s model suggests that the presence of neutrals crashing into our solar system would prevent the heliosphere from flowing evenly. like a burning comet. And one thing’s for sure: Neutrals definitely make their way through space.

Drake, co-author of the new study, says that Opher’s model “offers the first clear explanation for the disruption of the shape of the heliosphere in the northern and southern areas, which could impact our understanding of. how galactic cosmic rays enter Earth and the near-Earth environment. ” This could affect the threat that radiation poses to life on Earth and also to astronauts in space or future pioneers trying to get to Mars or other planets.

“The universe is not calm,” Opher says. “Our BU model does not try to cut the chaos, which allowed me to identify the cause [of the heliosphere’s instability]…. Neutral particles of hydrogen. “

Specifically, the presence of neutrals colliding with the heliosphere triggers a phenomenon well known to physicists, called Rayleigh-Taylor instability, which occurs when two materials of different densities collide with the lighter material pushing against the heaviest material. This is what happens when oil is suspended above water and when heavier fluids or materials are suspended above lighter fluids. Gravity plays a role and gives rise to wildly irregular shapes. In the case of cosmic jets, the drag between neutral hydrogen particles and charged ions creates an effect similar to gravity. The “fingers” seen in the famous Horse’s Head nebula, for example, are caused by Rayleigh-Taylor instability.

“This discovery is a really major breakthrough, it really set us on the path to finding out why our model gets its distinct crescent shaped heliosphere and why other models don’t,” Opher said.

Discover the shape of our solar system

More information:
M. Opher et al, A Turbulent Heliosheath Driven by the Rayleigh-Taylor Instability, The Journal of Astrophysics (2021). DOI: 10.3847 / 1538-4357 / ac2d2e

Provided by Boston University

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