Astrophysicists at BUs NASA-funded SHIELD team reaches another milestone in their quest to understand the heliosphere.
A multi-institutional team of astrophysicists headquartered at Boston University, led by BU astrophysicist Merav Opher, has made a groundbreaking discovery in our understanding of the cosmic forces that shape the protective bubble that surrounds our solar system – a bubble that protects life on Earth and is known by space scientists as the heliosphere.
Astrophysicists believe that the heliosphere protects the planets in our solar system from powerful radiation coming from supernovae, the last explosions of dying stars in the entire universe. They believe that the heliosphere extends far beyond our solar system, but despite the massive buffer against cosmic radiation that the heliosphere provides to Earth’s life forms, no one really knows the shape of the heliosphere – or for that matter the size of it.
“How is it relevant to society? The bubble that surrounds us, produced by the sun, offers protection against galactic cosmic rays, and the shape of it can affect how these rays enter the heliosphere,” says James Drake, an astrophysicist at the University of Maryland, who collaborates with Opher. “There are many theories, but the way galactic cosmic rays can penetrate can, of course, be affected by the structure of the heliosphere – does it have wrinkles and folds and the like?”
Ofer’s team has constructed some of the most compelling computer simulations of the heliosphere, based on models based on observable data and theoretical astrophysics. At BU, at the Center for Space Physics, Opher, a College of Arts & Sciences professor of astronomy, heads a NASA DRIVE (Diversity, Realize, Integrate, Venture, Educate) Science Center supported by the $ 1.3 million NASA financing. This team, made up of experts Opher recruited from 11 other universities and research institutes, develops predictive models of the heliosphere in an effort that the team calls SHIELD (Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics).
Since BU’s NASA DRIVE Science Center first received funding in 2019, Opher’s SHIELD team has been searching for answers to several enigmatic questions: What is the overall structure of the heliosphere? How do its ionized particles evolve and affect heliosphere processes? How does the heliosphere interact and affect the interstellar medium, matter, and radiation that exist between stars? And how are cosmic rays filtered off or transported through the heliosphere?
“SHIELD combines theory, modeling and observations to build comprehensive models,” says Opher. “All of these different components work together to help understand the mysteries of the heliosphere.”
And now a paper published by Opher and partners in Astrophysical journal reveals that neutral hydrogen particles flowing outside our solar system are most likely to play a crucial role in the way our heliosphere takes shape.
In their latest study, Ofer’s team wanted to understand why heliosphere jets – flowering pillars of 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 – emit unstable jets?” Opher says. “We see these jets projecting like irregular bars, and [astrophysicists] has for years wondered why these forms cause instability. “
Similarly, SHIELD models predict that the heliosphere that travels with our sun and encompasses our solar system does not appear to be stable. Other models of the heliosphere developed by other astrophysicists tend to depict the heliosphere as having a comet-like shape, with a ray – or a “tail” – flowing behind it in its wake. In contrast, Ofer’s model suggests that the heliosphere is shaped more like a croissant or even a donut.
The reason for that? Neutral hydrogen particles, so-called, because they have equal amounts of positive and negative charge, which gives no charge at all.
“They come flowing through the solar system,” Opher says. Using a computational model as a recipe to test the effect of ‘neutrals’ on the shape of the heliosphere, “she took an ingredient out of the cake – the neutrals – noting that the rays coming from the sun, which shape the heliosphere, “When I put them back in, things start to bend, the center axis starts to wiggle, and that means something inside the heliosphere jets becomes very unstable.”
Instability as it would theoretically cause disturbances in the solar winds and rays coming from our sun, causing the heliosphere to split its shape – in a croissant-like shape. Although astrophysicists have not yet developed ways to observe the actual shape of the heliosphere, Ofer’s model suggests that the presence of neutrals hitting our solar system would make it impossible for the heliosphere to float uniformly like a shooting comet. And one thing is for sure – neutrals are definitely throwing themselves through space.
Drake, a co-author of the new study, says Ofer’s model “provides the first clear explanation for why the shape of the heliosphere breaks in the northern and southern regions, which may affect our understanding of how galactic cosmic rays enter Earth. and the near – the Earth’s environment. ” It can affect the threat that radiation poses to life on Earth and also to astronauts in space or future pioneers trying to travel to March or other planets.
“The universe is not quiet,” Opher says. “Our BU model is not trying to remove the chaos, which has allowed me to find the cause [of the heliosphere’s instability]…. The neutral hydrogen particles. “
Specifically, the presence of neutral substances colliding with the heliosphere triggers a phenomenon well known to physicists, called the Rayleigh-Taylor instability, which occurs when two materials of different densities collide, with the lighter material pushing against the heavier material. . This is what happens when oil is suspended over water and when heavier liquids or materials are suspended over lighter liquids. Gravity plays a role and gives rise to some wildly irregular shapes. In the case of the cosmic jets, the resistance between the neutral hydrogen particles and charged ions creates a similar effect as gravity. The “fingers” seen in the famous Horsehead Mist, for example, are caused by Rayleigh-Taylor’s instability.
“This discovery is a really big breakthrough, it has really set us in a direction to discover why our model gets its distinct croissant-shaped heliosphere, and why other models do not,” Opher says.
Reference: “A Turbulent Heliosheath Driven by the Rayleigh – Taylor Instability” by M. Opher, JF Drake, G. Zank, E. Powell, W. Shelley, M. Kornbleuth, V. Florinski, V. Izmodenov, J. Giacalone, S. Fuselier, K. Dialynas, A. Loeb and J. Richardson, December 1, 2021, Astrophysical journal.
DOI: 10.3847 / 1538-4357 / ac2d2e