The Sun is a constant source of interest, particularly to solar scientists who monitor solar flares and other outbursts. A great many amateurs also observe it, looking for sunspots and solar filaments. And, of course, most of us love it when the Sun’s “out” and clouds aren’t obscuring it.
On the solar physics side, people are still working very hard to understand why the Sun does what it does—particularly when it erupts with a solar flare. Flares are massive outbursts that release a lot of energy and radiation. Solar physicists think they know quite a bit about these events, but even today, some puzzling things happen during flares that they don’t understand completely.
One of those things is a “supra-arcade downflow” (or SAD, for short), that looks almost like dark fingers reaching down toward the solar surface within a flare. Scientists want to know how they happened and what they are, so they turned to space to find out.
(Credit: NASA SDO)
Understanding What Causes a Solar Flare
Until the advent of specialized telescopes and space-borne instruments, it was kind of tough to monitor everything that happens during a solar flare. For one thing, these events are incredibly bright, particularly in x-rays. And, they happen very quickly. They also can’t be predicted accurately.
So, when one occurs, scientists have to move quickly to get data about them. Over the years, they’ve learned a lot about these flares and what causes them. The standard story is that flares occur during a process called “magnetic reconnection.” That’s when magnetic field lines on the Sun get twisted and pushed together. Eventually, they are stretched and compressed. At some point, they break, almost like a rubber band that snaps if you twist and pull it too much. When this action happens in solar magnetic fields, there’s a huge release of energy (the flare) and then the lines reconnect very quickly.
Solar Flares and SADS
This brings us back to the SADs. Most scientists assumed that they were dark downflows that occurred as a result of the broken magnetic fields and that they were part of the field line “snap back” to the Sun once the flare was ending. And, that would make sense, except for one thing: they move too slowly. Downflows of material along magnetic field lines move a lot faster than these SADs. So, what could they be?
To explain this, researchers studied images from a specialized instrument on the NASA Solar Dynamics Observatory. It’s been studying the Sun from space since its launch in 2010 and focuses on the solar atmosphere, its magnetic fields, and seismic waves that ripple through the Sun. It takes images of the Sun every twelve seconds, and all that data is useful in 3D simulations of the Sun’s atmosphere and flares. And, that data has shown scientists something they didn’t expect.
SADs and Mixing
It turns out that, after studying this data and the 3D models, scientists found that SADs aren’t part of the reconnection event itself. They are a result of it—they are voids in the solar plasma. Tis plasma is a superheated “fluid” made mostly of ions and electrons. It can heat up and cool, depending on activity in the Sun. The Sun’s surface, in particular, is a very turbulent place. As a result, regions of plasma right next to each other can have different temperatures and densities.
So, what does this have to do with SADs? It turns out that they form when solar plasmas of different densities try and fail to combine in the turbulent environment during the magnetic disconnection and reconnection that causes the solar flare. It’s like mixing oil and water. Those two fluids can’t combine. Instead, they separate, and that creates voids between them. The same thing is happening with the SADs—they are actually the voids between the plasmas of different densities. The plasmas can’t combine, so they separate and the SADs are created.
Solar Flare Predictions and SADs
The absence of plasma in these voids is all part of the study of flares and other space weather events. What scientists want to do, ultimately, is be able to predict these outbursts. Flares, for example, can affect and damage some of our technology here on Earth. That includes communications and electrical grids that are sensitive to space weather activity. Further studies, including more extensive 3D modeling of data from SDO and other spacecraft, will certainly contribute to our understanding of the Sun and its activities.
