What are Voyager Activities

Voyager 2: First data from interstellar space

News from the frontier of the solar system: Exactly one year ago the NASA space probe Voyager 2 crossed the border to interstellar space. Now their data provide new, sometimes surprising insights into the nature of this border region - including the existence of a strong magnetic barrier. The comparison with Voyager 1 data also shows similarities, but also some crucial differences, as researchers report in five "Nature" articles.

The two Voyager probes are cosmic pioneers. Because they were the first man-made objects to leave the protective heliosphere of the solar system and reach interstellar space. Voyager 1 passed the border region of the heliopause in the summer of 2012, Voyager 2 followed on November 5, 2018. The latter in particular was eagerly awaited by researchers around the world. Because only the data from Voyager 2 can show whether the measurement data from Voyager 1 are representative of the nature of this border region of our solar system.

Now the long-awaited comparison data have arrived and five research teams have evaluated the results on plasma flows, cosmic rays and magnetic fields on both sides of the heliopause.

Structure of the heliosphere: The heliopause forms the outer boundary, followed by the helio envelope and inside the termination shock, a shock wave that marks the innermost contact area of ​​the penetrating integer stellar medium with the solar wind. © NASA / IBEX / Adler Planetarium

And yet it is round!

One of the most important results: the heliosphere - the protective sphere of influence of the sun - is apparently symmetrical. Because despite their widely spaced trajectories, both Voyager probes reached the boundary to the interstellar medium at almost the same distance from the sun - Voyager 1 at just under 122 astronomical units (AU), Voyager 2 at 119 AU. "This indicates that the heliosphere is symmetrical, at least in relation to the two transition points of the Voyager probes," says Bill Kurth of the University of Iowa.

The exciting thing about it: In the past, astronomers assumed an asymmetrical shape of the heliosphere that extended far backwards. But as early as 2017, data from the Cassini probe had provided the first indications that the heliosphere was instead surprisingly spherical and symmetrical. The new Voyager data now confirm this.

Striking differences

But there are also striking differences between the measurement data from Voyager 1 and 2, as the research teams report. For example, almost two years before the heliopause, Voyager 1 had already registered a decline in the plasma currents, but after the transition into the interstellar medium, several strong turbulences occurred in the surrounding plasma. This spoke in favor of an unstable but thick border region.

Not so with Voyager 2: "Voyager 2 crossed the heliopause in just half a day," report Leonard Burlaga from NASA's Goddard Space Flight Center and his team. In addition, the probe registered neither a previous long lull nor the strong “tsunamis” after the transition into the interstellar medium. "Accordingly, the heliopause observed by Voyager 2 was stable and thin, in contrast to the unstable and thicker heliopause that Voyager 1 crossed," says Burlaga.

But why? The researchers suspect the reason for these differences in solar activity: Voyager 1 passed the heliopause during a minimum in the solar activity cycle, which allowed interstellar magnetic fields and cosmic rays to penetrate further into the edge of the heliosphere. Voyager 2, on the other hand, crossed the line during a period of high activity and strong solar winds that may have stabilized the heliopause.

Position of the Voyager probes - both have already passed the heliopause. © NASA / JPL-Caltech

Magnetic barrier

And Voyager 2 discovered something else: On this side of the heliopause there is apparently an extensive magnetic barrier that acts as an additional shield against cosmic rays. "The existence of such a barrier was predicted, but only now have we proven it," report Burlaga and his team. "The magnetic field in this barrier was stronger than any magnetic field previously measured in the Helio hull."

From the data, the scientists conclude that the magnetic barrier of the heliosphere follows a dynamic development: It is created by magnetic currents that flow in the direction of the heliopause, thin out there and flow sideways. If the direction of the sun's magnetic field is reversed in the next solar cycle, this process starts all over again with reversed polarity, according to the researchers.

Many questions still open

Overall, the new data from Voyager 2 and its sister probe suggest that the border region of our solar system with the interstellar medium forms a complex and highly dynamic system, as the researchers explain. And still not all questions about this border region of the solar sphere of influence have been clarified.

Fortunately, the two Voyager probes will continue to provide valuable data from the unexplored vastness of interstellar space for at least ten years. A special savings program ensures that the power reserves for measuring instruments and data transmission last a long time. But even after that, the space probes will fly through space as mute messengers for humanity for thousands of years. (Nature, doi: 10.1038 / s41550-019-0928-3; doi: 10.1038 / s41550-019-0920-y; doi: 10.1038 / s41550-019-0929-2; doi: 10.1038 / s41550-019-0918-5 ; doi: 10.1038 / s41550-019-0927-4)

Source: Nature

5th November 2019

- Nadja Podbregar