Some of what the Sun spits out violently returns to its surface And might provide a glimpse of how stars get built.

Material getting blasted off the surface of the Sun. Some of it will make a violent return trip.

The Sun may look like a calm, steady presence from the safe distance of our vantage point on Earth, but just a slight bit of magnification shows that its surface is seething with violent activity. And every now and then some of that violence gets sent towards Earth in the form of a coronal mass ejection, causing auroras and general worries about the safety of the people and hardware we have in orbit.

In focusing on the material that gets sent toward Earth, however, it's easy to overlook the fact that not everything that gets shot out of the Sun is energetic enough to escape its gravity. A lot of material obeys the dictum "what goes up must come down" and ends up crashing back to the surface of the Sun. Now, scientists have imaged these events as the ejected material returns and strikes the surface of the Sun. Using that, they built a model that shows what happens below the Sun's surface during these impacts, a model that may have applications to the processes that build stars in the first place.

Some video of the solar eruptions, showing the material that returns to the Sun's surface at high speed.

The basic process at issue is fairly simple: the eruptions of the Sun that power coronal mass ejection send a lot of mass away from the Sun, but not all of it has sufficient momentum to escape the Sun's gravity. So at a certain point, it comes to a halt and then reverses, heading back toward the surface of the Sun. A lot of it reaches free-fall speeds before it impacts—which, given the environment, is somewhere around 300-450 kilometers a second.

If the sheer speed isn't enough to impress you, the size of the blobs and strings of material that return to the Sun just might. The authors estimate that, along their short axis, many of these are above 2,000 km across, and some up to 4,000 km. On their long axis, they can be much larger than 10,000 km. The upper figures would place the area they impact as roughly the size of the US (including Alaska). The observations show that the impact can be anything from a single droplet to a train of droplets and longer strings of material.

Different views of the material crashing back into the Sun, taken at different wavelengths. Arrows highlight some of the points of impact.

The data suggests that the Sun's magnetic fields are weak where the impacts take place, so the authors built a model of the events using gravity and fluid dynamics. The model shows that the material will plow straight through the Sun's chromosphere and procede through until it strikes a layer with equal density. At that point, the material creates a thin disk of very hot material, after which the center region sees a bit of a bounce-back effect, creating an upward surge of material heated to 10⁵K. For larger streams of material, the bounce-back structures could be as large as 10,000 km.

The authors' model, showing some of the bounce-back effect. Courtesy of Fabio Reale.

Although these events produce radiation across the visible spectrum and into the UV, a lot of the UV light seems to get absorbed by the surrounding layers of material and never escapes the Sun. And that, the authors say, can help explain a bit of a conundrum regarding the formation of stars.

At the later stages of their growth, young stars have already ignited fusion even as more material from the surrounding disk is being funneled into the surface of the star by its magnetic fields. Observations of young stars have allowed us to estimate the amount of material reaching the growing star but have given different estimates depending on the wavelengths being used for the imaging. The new work suggests that the UV observations may lead to a systematic underestimate of how much material is impacting the star, helping bring different estimates in line.