Researchers Measure Pressure in Solar System’s Heliosheath

Using data from NASA’s Voyager spacecraft, a team of scientists has calculated the total pressure from particles in the outer region of the Solar System, known as the heliosheath — and it was found to be greater than expected.

NASA’s Voyager spacecraft, one in the heliosheath and the other just beyond in interstellar space, took measurements as a solar event known as a global merged interaction region passed by each spacecraft four months apart. Image credit: NASA’s Goddard Space Flight Center / Mary Pat Hrybyk-Keith.

NASA’s Voyager spacecraft, one in the heliosheath and the other just beyond in interstellar space, took measurements as a solar event known as a global merged interaction region passed by each spacecraft four months apart. Image credit: NASA’s Goddard Space Flight Center / Mary Pat Hrybyk-Keith.

On Earth, we have air pressure created by air molecules drawn down by gravity. In space, there’s also a pressure created by particles like ions and electrons. These particles, heated and accelerated by the Sun create a giant balloon known as the heliosphere extending millions of miles out past Pluto.

The edge of this region, where the Sun’s influence is overcome by the pressures of particles from other stars and interstellar space, is where the Sun’s magnetic influence ends.

In order to measure the pressure in the heliosheath, Princeton University researcher Jamie Rankin and colleagues used the Voyager spacecraft, which have been traveling steadily out of the Solar System since 1977.

At the time of the observations, Voyager 1 was already outside of the heliosphere in interstellar space, while Voyager 2 still remained in the heliosheath.

“There was really unique timing for this event because we saw it right after Voyager 1 crossed into the local interstellar space,” Dr. Rankin said.

“And while this is the first event that Voyager saw, there are more in the data that we can continue to look at to see how things in the heliosheath and interstellar space are changing over time.”

Dr. Rankin and co-authors from Princeton University, MIT and the University of New Hampshire used an event known as a global merged interaction region, which is caused by activity on the Sun.

The Sun periodically flares up and releases enormous bursts of particles, like in coronal mass ejections. As a series of these events travel out into space, they can merge into a giant front, creating a wave of plasma pushed by magnetic fields.

When one such wave reached the heliosheath in 2012, it was spotted by Voyager 2.

The wave caused the number of galactic cosmic rays to temporarily decrease. Four months later, the researchers saw a similar decrease in observations from Voyager 1, just across the Solar System’s boundary in interstellar space.

Knowing the distance between the spacecraft allowed them to calculate the pressure in the heliosheath as well as the speed of sound.

In the heliosheath sound travels at around 300 km/sec — thousand times faster than it moves through air.

The scientists noted that the change in galactic cosmic rays wasn’t exactly identical at both spacecraft.

At Voyager 2 inside the heliosheath, the number of cosmic rays decreased in all directions around the spacecraft. But at Voyager 1, outside the Solar System, only the galactic cosmic rays that were traveling perpendicular to the magnetic field in the region decreased.

This asymmetry suggests that something happens as the wave transmits across the Solar System’s boundary.

“Trying to understand why the change in the cosmic rays is different inside and outside of the heliosheath remains an open question,” Dr. Rankin said.

The results were published in the September 25, 2019 issue of the Astrophysical Journal.

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J.S. Rankin et al. 2019. Heliosheath Properties Measured from a Voyager 2 to Voyager 1 Transient. ApJ 883, 101; doi: 10.3847/1538-4357/ab3d9d

This article is based on text provided by the National Aeronautics and Space Administration.

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