Solar System’s Heliosphere

May Be Smaller and Rounder than Previously Thought

The heliosphere is a giant magnetic bubble that contains our Solar System, the solar wind and the solar magnetic field. Outside the heliosphere is the interstellar medium — the ionized gas and magnetic field that fills the space between stellar systems in our Milky Way Galaxy. The shape of the heliosphere has been explored in the past six decades. There was a consensus that its shape is comet-like. New research led by Boston University and Harvard University provides an alternative shape that lacks this long tail: the deflated croissant.

New model suggests the shape of the Sun’s bubble of influence, the heliosphere (seen in yellow), may be a deflated croissant shape, rather than the long-tailed comet shape suggested by other research. Image credit: Opher et al, doi: 10.1038/s41550-020-1036-0.

The shape of the heliosphere is difficult to measure from within. The closest edge of the heliosphere is more than 16 billion km (10 billion miles) from Earth.

Only NASA’s twin Voyager spacecraft directly measured this region, leaving us with just two points of ground-truth data on the shape of the heliosphere.

From near Earth, scientists study our boundary to interstellar space by capturing and observing particles flying toward Earth.

This includes charged particles that come from distant parts of the Galaxy, called galactic cosmic rays, along with those that were already in our Solar System, travel out towards the heliopause, and are bounced back towards Earth through a complex series of electromagnetic processes.

These are called energetic neutral atoms, and because they are created by interacting with the interstellar medium, they act as a useful proxy for mapping the edge of the heliosphere.

This is how NASA’s Interstellar Boundary Explorer (IBEX) studies the heliosphere, making use of these particles as a kind of radar, tracing out our Solar System’s boundary to interstellar space.

To make sense of these data, scientists use computer models to turn the data into a prediction of the heliosphere’s characteristics.

In the new research, Boston University’s Professor Merav Opher and colleagues used data from several NASA missions to characterize the behavior of material in space that fills the bubble of the heliosphere and get another perspective on its borders.

NASA’s Cassini mission carried an instrument — designed to study particles trapped in Saturn’s magnetic field — that also made observations of particles bouncing back towards the inner Solar System. These measurements are similar to IBEX’s, but provide a distinct perspective on the heliosphere’s boundary.

Additionally, NASA’s New Horizons mission has provided measurements of pick-up ions, particles that are ionized out in space and are picked up and move along with the solar wind.

Because of their distinct origins from the solar wind particles streaming out from the Sun, pick-up ions are much hotter than other solar wind particles — and it’s this fact that the new work hinges on.

“There are two fluids mixed together. You have one component that is very cold and one component that is much hotter, the pick-up ions,” Professor Opher said.

“If you have some cold fluid and hot fluid, and you put them in space, they won’t mix — they will evolve mostly separately.”

“What we did was separate these two components of the solar wind and model the resulting 3D shape of the heliosphere.”

Considering the solar wind’s components separately, combined with an earlier work by the team using the solar magnetic field as a dominant force in shaping the heliosphere, created a deflated croissant shape, with two jets curling away from the central bulbous part of the heliosphere, and notably lacking the long tail predicted by many scientists.

“Because the pick-up ions dominate the thermodynamics, everything is very spherical,” Professor Opher said.

“But because they leave the system very quickly beyond the termination shock, the whole heliosphere deflates.”

The research was published in the journal Nature Astronomy.

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M. Opher et al. 2020. A small and round heliosphere suggested by magnetohydrodynamic modelling of pick-up ions. Nat Astron 4, 675-683; doi: 10.1038/s41550-020-1036-0