The Voyager probes were launched in the 1970s with the goal of studying the planets of our Solar System. Now, their decades-long journey through the cosmos has allowed astronomers to learn about how the Sun shapes the space environment far beyond the orbit of Neptune.
Luckily, the probes were in the right place at the right time to detect an ejection of plasma from the Sun, allowing researchers to uncover new information about the distant edges of the Solar System. The findings are reported in The Astrophysical Journal.
The Sun continuously emits a stream of particles known as the solar wind. These have carved a cavity within the interstellar medium and we call this region the heliosphere. By analyzing the properties of the solar plasma, astronomers have been able to define certain regions within the heliosphere.
All the known planets are located within a boundary known as the termination shock. Beyond this boundary, the speed of the solar wind drops abruptly due to the pressure of interstellar material. This region is called the heliosheath. The edge of the heliosheath (and heliosphere), is called the heliopause. Crossing it officially takes you into interstellar space.
Voyager 1 got to the heliopause in August 2012 and Voyager 2 crossed it just last year. So for a while, the two spacecraft were on opposite sides of the boundary. And their positions were key to obtaining an important measurement.
In mid-2012, a global merged interaction region (GMIR) formed. A GMIR can appear when several releases of particles from the Sun combine in a denser region with a higher magnetic field. The Voyager probes had experienced and studied GMIRs before, but the one from 2012 gave researchers the opportunity to look at it from both sides of the heliopause. The GMIR moved through space and hit Voyager 2, with Voyager 1 recording a similar signal about 130 days later. That’s just over three months since Voyager 1 had entered interstellar space.
Although the data are limited, under the assumption that the two signals correspond to the same event, the team was able to estimate the speed of sound in the heliosheath. They also estimated its pressure, which is very close to the lowest possible pressure we can create in the lab.