Neutral hydrogen distorts the heliosphere – a possible explanation for the croissant shape of the protective plasma bubble around the solar system

Deformed protective shell: Researchers have found a possible reason for the unusual shape of the heliosphere – the protective plasma bubble that surrounds our solar system. According to this, the flow of neutral hydrogen from interstellar space causes a turbulence that disrupts the flow of plasma. Instead of a neat tail or round ball shape, the heliosphere is given a croissant-like shape, simulations suggest.

The solar wind and the magnetic field of our home star create a protective bubble around our solar system. This heliosphere protects us from streams of high-energy interstellar particles. For a long time, astronomers assumed that this protective bubble was attracted to the tail of plasma thousands of astronomical units long. But measurement data from space probes and newer models cast doubt on this picture.

Deformed like a croissant: This is what the heliosphere might look like. © Merav Ofir et al.

Round, distorted or with a tail?

“There is currently an active debate about the shape and structure of the heliosphere and its tail,” explain Merav Over of Boston University and colleagues. While their model suggests a severely distorted croissant-like structure of the heliosphere based on the data, others assume a more rounded shape. The determining factor for the actual shape is the question of the extent to which streams of interstellar particles can penetrate the heliosphere and affect its tail.

In search of an answer, the victims and her team looked at an important fundamental component of the heliosphere – the planes of the heliosphere. These currents of particles and magnetic field lines emanating from the sun’s poles are essential to the shape of the protective plasma bubble. So the scientists used model simulations to analyze the factors that cause instability in these jets.

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Neutral hydrogen is critical

The simulations revealed something surprising: heliosphere jets remained stable in almost all conditions. Then the heliosphere formed a long, straight tail similar to a comet. But once the flow of neutral hydrogen atoms was programmed from interstellar space, the picture changed: “The central axis started to wobble and that meant something in the plane had become very unstable,” Ophir explains.

Under the influence of this neutral particle flow, the previously straight jets and their curve are distorted. As a result, charged interstellar particles can penetrate the resulting gap and cause more turbulence. “We show that the driving force behind this perturbation is the Rayleigh-Taylor instability,” the researchers explain. This instability occurs when media of different densities collide – in this case neutral hydrogen atoms and charged particles of the solar wind.

Explanation of the distorted structure

“This result is a huge advance because it explains why our model gives a clear croissant shape to the heliosphere, while other models don’t,” Over says. This model arises only when the flow of neutral hydrogen is taken into account. This fits with the new measurement data, according to which the density of neutral hydrogen at the edge of the heliosphere is 40 percent higher than it has long been assumed.

In accordance with this, more of these neutral atoms penetrate the heliosphere than some models show – and this could confirm the deformation of the protective shield identified by Ophir and her team. At the same time, new modeling shows that the turbulence is also causing ionized interstellar particles to penetrate the heliosphere and mix with the solar wind. Conversely, the instability also transfers solar material to the local interstellar environment. (Astrophysical Journal, 2021; doi: 10.3847/1538-4357/ac2d2e)

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These are: Boston University

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