Collecting high-speed atoms has enabled researchers to trace the shape of our Sun’s protective bubble.
The extent of the Sun’s influence extends far beyond our home star, carving out a region in the interstellar medium (ISM) called the heliosphere. Imagine the heliosphere as the Sun’s “personal space” bubble, filled with solar wind particles that push out against the ISM to protect the solar system from harmful incoming radiation.
Astronomers have a vague sense of the heliosphere’s dimensions based on dependable but limited data, like that gathered by the Voyager spacecraft as they crossed the heliosphere’s boundary, known as the heliopause. However, a new study by Daniel Reisenfeld (Los Alamos National Laboratory) and his colleagues, published in the June Astrophysical Journal Supplement Series, uses data covering the entire sky to map the heliosphere in three dimensions. Their findings confirm what solar scientists have long theorized: The heliosphere is not a sphere at all but instead is compressed on one side, with a tail of debatable size on the other.
Using the Interstellar Boundary Explorer (IBEX) satellite, the team recorded the distance to the heliopause with a method they call sounding, for its resemblance to sonar techniques. The researchers detected energetic neutral atoms (ENAs) that originate in the outer layer of the heliosphere, or heliosheath, and form when fast-moving solar wind particles steal an electron from atoms in the less-turbulent ISM to become neutral atoms themselves. Following the electron robbery, some of the new ENAs fly straight back towards IBEX at very high speeds.
Astronomers found that changes in the amount of ENAs that IBEX catches are related to disturbances seen earlier in the pressure of the outgoing solar wind. If researchers can connect a unique wind disturbance to a variation in the amount of detected ENAs, then they can use the time that passed between these two events to calculate a trace-back time — essentially, how long it took solar wind particles to whizz out towards the heliopause, knock into the ISM, and come flying back as ENAs. With this time and estimations of how fast the relevant particles should be traveling, the researchers can then determine the distance to the heliopause.
However, the trace-back time depends on the thickness of the heliosheath, and the researchers must assume that ENAs are formed somewhere in the middle. Going outward from the Sun, the beginning of the heliosheath is marked by the termination shock, where the solar wind collides with the ISM and slows to less than the speed of sound. To turn IBEX’s observations into a 3D map of the heliosphere, the team had to use several simulations of the termination shock to put their trace-back times into context.
Using those assumptions and data over a full, 11-year solar cycle, Reisenfeld’s team calculated the distance to the heliopause in all directions around the Sun. The results show that the shape of the heliosphere does not depend much on the shape of the termination shock assumed. The largest difference between the models and the direct measurements made by either Voyager spacecraft is only 13 astronomical units (a.u.) — a relatively small distance compared with the shortest distance from the Sun to the heliopause at the “nose” (120 a.u.), which is compressed as it faces the interstellar wind. The rest of the heliosphere flares out in the opposite direction as the Sun hurls through space.
The Tail End
So far, there are no big surprises from the sounding project. But that’s because the current debate among astronomers isn’t about the heliosphere’s nose, but about its tail: Just how far does it stretch into space? It might extend for only hundreds of a.u. or trail for thousands. Reisenfeld’s team cannot distinguish between the two options with their data. All they can say is that the tail is at least 350 a.u. long, as this is the distance limit of their method.
“The issue is that at the IBEX energies, the particles that produce these images are only present until ~200 a.u. in each direction, so you cannot see much farther than that down the heliospheric tail,” says Merav Opher (Boston University), who recently showed that heliospheric plasma is shaped by the solar magnetic field, which may cause the heliosphere to look like a deflated croissant. Opher states that while these new maps are great confirmation of existing models, particularly those of the nose region, they do not bring much new information to the tail debate.
IBEX should be online long enough to observe part of the new activity cycle our Sun began in 2019, allowing Reisenfeld’s team to improve their maps over time. The upcoming Interstellar Mapping and Acceleration Probe (IMAP), set to launch in 2024, will specifically investigate the interactions between the solar wind and the ISM. Since this new mission will be able to observe at higher energies than IBEX and probe much farther than a few hundred astronomical units, Opher hopes that IMAP will settle the tail debate once and for all.