Category Archives: solar physics

solar flares, solar physics

Dark Voids in Solar Flares

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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.

Still image of several supra-arcade downflows, also described as “dark, finger-like features,” occurring in a solar flare. The downflows appear directly above the bright flare arcade. This solar flare occurred on June 18, 2015. CREDIT NASA SDO
Still image of several supra-arcade downflows, also described as “dark, finger-like features,” occurring in a solar flare. The downflows appear directly above the bright flare arcade. This solar flare occurred on June 18, 2015.
(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.

astronomy, solar physics, solar studies, sun

The Boiling Sun

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Living with a star, our Sun, is something we do every day. Of course, most of us just bask in the sunlight when it’s available. But, for solar physicists, the astronomers who study the Sun, our star presents a great opportunity to delve deeper into what makes it tick. Or, in the case of some recently released images and vids, what makes it look like a boiling Sun.

The Daniel K. Inouye Solar Telescope on the Hawaiian island of Maui is the latest giant telescope to turn its high-resolution instruments to look at our Sun. The video below shows a view of the Sun that we’d never see if we were to travel to it. But, the telescope is sensitive to light beyond the spectrum that we can see (which is a range between 400 and 700 nanometers). This view is of light that radiates at 705 nanometers, so just beyond what we can see.

Why a Boiling Sun?

What’s happening here? Well, we know the Sun is hot at its core, where it’s more than 15 million degrees! That heat travels out from the center and is released at the surface, called the chromosphere. There, we see a turbulent motion in the gases that make up the Sun’s atmosphere. That action is what makes it look like a boiling Sun in the video.

Those cells of superhot gases are a cooling mechanism. They bring the heat up from beneath the solar surface and then release it. It’s the way our star sends heat out to the rest of the solar system. Watch the video for a short bit and it might remind you of boiling fudge, or syrup on the stove. Those also operate under the same principle of heat “release”.

On the Sun, the heat rises up from this region into space through the outer layers of the Sun’s atmosphere. Temperatures there are about 6,000 C (10,800 F). A funny thing happens above the surface—the heat steadily rises until it’s more than a million degrees in the uppermost region called the corona. Solar scientists are busily figuring out why this superheating happens, and they know that solar magnetic fields are somehow involved.

The interesting thing about this video (and the images from DIST) is that these are the sharpest images of the boiling surface of the Sun ever taken. And, they were taken from Earth! Want to know more about this telescope and see additional images? Check out their website here.