High pressure at the borders of interstellar space

Illustration of the heliosphere (Image NASA/IBEX/Adler Planetarium)
Illustration of the heliosphere (Image NASA/IBEX/Adler Planetarium)

A team of scientists led by Jamie Rankin of Princeton used measurements of the galactic cosmic rays detected by NASA’s Voyager space probes to obtain an estimate of the total pressure that plasma, magnetic fields and various particles exert on each other in the heliosheath, the outer region of the heliosphere, the big bubble around the Sun where solar wind exerts its influence. The result is higher than expected and will help to better understand the interactions between the Sun and its surroundings.

The Voyager space probes entered interstellar space over the past few years: the confirmation for Voyager 1 arrived in September 2013 while for Voyager 2 it arrived in December 2018. Thanks to the fact that some instruments of both probes are still working, it was possible to keep on conducting measurements of the surrounding environment. The heliosheath is about 15 billion kilometers from the Sun so its study is really complex and the trajectory of the two Voyagers allowed to carry out detections directly inside it.

The data used for this calculation refer to a period in which Voyager 1 had just entered interstellar space while Voyager 2 was still in the heliosheath. In that situation, the scientists exploited a wave of plasma generated by the Sun when it releases enormous amounts of particles, such as in a coronal mass ejection. When various fronts of particles merge as they travel in space, they form what’s called a global merged interaction region.

In 2012, a global merged interaction region reached the heliosheath and that event was recorded by Voyager 2 with a temporary drop in galactic cosmic rays. Four months later, scientists noticed a similar drop in Voyager 1 detections, which was reached later because it’s farther away and was in interstellar space. Knowing the distance between the two probes, it was possible to calculate the pressure in the heliosheath. In that area there are enough particles to transmit sounds and their speed was calculated at about 300 kps, about a thousand times faster than the speed of sound in the air.

A discovery that still needs to be explained is that of the change in cosmic rays, which was not identical for the two space probes. Voyager 2 detected a drop in cosmic rays in all directions around it. Voyager 1 detected in interstellar space only a drop in the galactic cosmic rays traveling perpendicular to the magnetic field in that region.

Each new study of data collected by the Voyager space probes offers new information on those areas of ​​the solar system that helps us better understand how the Sun influences space in the heliosphere and how the solar wind interacts with other interstellar phenomena. This can also offer an idea about the dynamics in other star systems that we can’t examine directly. It’s one of the frontiers of astronomy that gets expanded as the Voyagers move away from the Sun.

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