Scientists discover inexplicable structures at the border of interstellar space

Image: Juan Bautista Ruiz via Getty Images

Scientists have discovered strange ripples and inexplicable structures at the boundary between our solar system and the vast expanses of interstellar space beyond, reports a new study.

The results show that the boundary of the heliosphere, a protective bubble created by the sun that envelops the solar system, is shifting in “intriguing and potentially controversial ways,” according to the study.

The heliosphere extends about 11 billion miles into space, more than twice the orbit of Pluto, and is shaped by a stream of charged particles emitted by the Sun called the solar wind. The edge of this structure is marked by a region called the heliopause, where the solar wind gives way to the forces of the interstellar medium. NASA’s Voyager 1 and 2 probes, launched in 1977, both broke that limit in the last decade, making them the first man-made objects to venture into interstellar space.

Voyager continues to send dispatches from beyond the heliosphere, but they can only report conditions at their respective locations. Scientists have learned to map some of the broader contours of the heliopause by looking for emissions from energetic neutral atoms (ENAs) produced by the interactions between the solar and interstellar winds, but much of this transitional region is still shrouded in mystery.

Now, scientists led by Eric Zirnstein, a space physics research scientist at Princeton University, have identified strange new details about this zone, illuminated by a month-long increase in solar wind dynamic pressure in 2014. A NASA satellite dubbed Interstellar Boundary Explorer (IBEX) captured a dramatic brightening of ENAs following that pressure front, revealing heliopause movements and “significant asymmetries” in the structure of the heliosphere that conflict with models released Monday, according to a study in natural astronomy, hence the potential for controversy.

“In early 2017, we noticed a brightening of energetic neutral atom (ENA) emissions in IBEX data originating from a small portion of the sky centered about 20 degrees below the ‘nose’ of the heliosphere,” Zirnstein said in an E- Mail “This brightening first appeared at the highest ENA energies we could observe.”

“Then the emission area got bigger and bigger over time across the sky and started to appear at lower ENA energies,” he continued. “We knew this had to be in response to a large change in solar wind pressure, which is a key factor in determining ENA emissions from the fringes of the heliosphere. The fact that the initial clearing point was not centered on the “nose” and spread asymmetrically across the sky motivated studies of this behavior to find out why.”

When the researchers examined the IBEX observations, they found that the surface of the heliosphere is distorted by giant waves appearing at an unexpected oblique (or tilted) angle. These spatial variations within these structures can reach up to ten astronomical units (AU), where one AU is the distance between the Earth and the Sun.

“We were quite surprised at how sloping the surfaces are, where the closest points are tilted ~30 degrees under the nose, in contrast to most known models of the heliosphere,” Zirnstein said. “The wave structures were also surprising because after doing our best to account for potential uncertainties in the analysis, we found that these waves are statistically significant. But in hindsight, considering how dynamic the solar wind actually is, that probably shouldn’t have been too surprising – but it was certainly interesting to watch.”

According to Zirnstein, the exact mechanisms that give rise to some of these intriguing observations are unknown.

“There seems to be a missing driving force for this asymmetry related to the solar wind and its interaction with the interstellar medium,” he noted. “Other star systems with astrospheres may be similar in their asymmetry, but it all depends on the properties of the star itself and the interstellar medium surrounding it.”

In addition to discovering these distortions at the gateways to interstellar space, the team was able to reconstruct the “substantial differences in the boundaries of the heliosphere” in recent years, according to the study. This particular finding sheds new light on Voyager’s journey through the heliospheric termination shock (HTS), the point at which the solar wind slows, and then through the heliopause.

“The Voyager spacecraft provides the only direct on-site measurement of the location of these boundaries. But only at one point in space and time,” said Zirnstein. “Comparing our results, which were observed at a different time in the solar cycle than when Voyager 1 or 2 crossed borders, made it complicated. The HTS surface locations compared well to Voyager 1 and 2, but what was most surprising was the [heliopause] Pop up.”

Voyager 1 crossed the 2012 heliopause at a distance of 121.6 AU from the Sun, while Voyager 2 crossed at 119 AU in 2018. Zirnstein and his colleagues discovered that the surface of the heliopause actually shifted tens of astronomical units over this period. For example, after Voyager 1 crossed that boundary, the heliopause stretched for years as if chasing after it, though the boundary never overtook the probe again. In contrast, Voyager 2 may have been right in the expanding heliopause, like a surfer on top of a wave, for several years before finally entering interstellar space

“Voyager 2 crossed the HP at 119 AU from the Sun in late 2018, and our analysis put it at 103 +/- 8 AU in mid-2016, which is much closer to the Sun,” Zirnstein explained. “However, we do know that in mid-2016 Voyager 2 was still within the heliopause and fairly close to the distance we derived (within the uncertainty of the analysis). This implies that Voyager 2 was near the heliopause in mid-2016, but only inside, and then the heliopause must have been moving outward for a number of years before the spacecraft finally crossed it in late 2018.”

“Both observations seem to imply significant movement of the heliopause surface over time and how close Voyagers may have been to the heliopause at different times,” he added.

In other words, the glow from ENAs at this dim entrance to the wider galaxy has revealed the shifting boundaries of the heliosphere and illuminated strange structures that require more observations and simulations to explain. Because we live in this heliosphere, which protects our solar system from harmful cosmic rays, it is important to map its contours and understand the complex forces that shape them. To that end, Zirnstein and his colleagues plan to further explore this region of space with IBEX and other missions.

“The next steps are twofold,” said Zirnstein. “First, we want to better understand the existence of this tilt and the waves, which will likely require comparisons with high-resolution, dynamic models of the heliosphere. It is likely that these new results will motivate the outer heliosphere community to improve their simulations in various aspects to try to understand the source of these observations.

“The other step is to wait for another big change in solar wind pressure, like the one that occurred in late 2014,” he concluded. “It appears that this type of event occurs at least once per solar cycle, so in a few years we might have another event to ‘map’ the HTS and HP surfaces again. This could also coincide with the launch of NASA’s Interstellar Mapping and Acceleration Probe (IMAP) in 2025, which will provide higher resolution observations of ENAs compared to IBEX and even better images of the heliosphere for us.”