Category Archives: solar flares

Dark Voids in Solar Flares

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.

Surviving the Radiation Belts

Understanding the Sun-Earth Connection

A Solar Dynamics Observatory (SDO) view of a dark prominence crossing beneath a coronal loop on the active surface of the Sun. This is part of a sequence of events that took place August 26-28th. Prominences are long strands of cooler gases that float above the solar surface. The loops are seen in extreme ultraviolet light by SDO. These are magnetic field lines being traced by spiraling particles above active regions of the Sun. Courtesy SDO.

Solar activity has been in the news a lot the past few months. So, unless you’ve been hiding under a rock with a tinfoil hat wrapped around your head, you’ve probably noticed numerous stories about solar flares and coronal mass ejections (CMEs) from the Sun and how they can cause everything from northern lights to power grid failures if they happen to send blasts of energized particles directly at Earth and our magnetic field.

Well, all that’s true.  And, the chances of solar activity affecting us and our technology are pretty high when the Sun is more active, as it is right now.  To be more precise, the Sun is going through a period called solar maximum, where it is more active.  Thus, we see more flares and mass ejections than during periods when the Sun is more quiescent.

This solar activity affects something called “space weather”, which refers to conditions and processes that occur in space that have the potential to affect Earth and its atmosphere.  Solar activity such as coronal mass ejections, solar flares, and the constant action of solar wind bring energy and particles from the Sun across space to Earth. Once they get here, they can disrupt Earth’s magnetic field, and they can cause radiation damage to spacecraft, and interrupt telecommunications, and affect global positioning satellite systems.  On the ground, disruptions to our magnetic field can interrupt power grids.

So, it makes sense that NASA and other space agencies are interested in studying the Sun’s influence on Earth’s radiation belts.  They’d like to be able to predict and understand solar outbursts, with an eye toward protecting us and our technology. And so, they are focusing some special attention on the near-Earth radiation environment.

You may have heard of the Van Allen belts. They were discovered and characterized in 1958 by James Van Allen, and surround our planet in a set of two torus-shaped nested belts that ranges from a thousand to 60,000 kilometers above Earth’s surface. Most of the particles that zip around in the Van Allen Belts come from the Sun, carried there by the solar wind.

Now, the interesting thing about the Van Allen Belts is that it is pretty dangerous to fly spacecraft through this region because of the intense radiation environment they contain. Anything that we want to send up to space either has to cross the belt quickly or stays pretty well away from it.  So, there haven’t been many spacecraft sent specifically to hang around IN the belt and study it for any length of time. One of the reasons is that a probe designed to spend time in the belt would need to have much of its electronics package shielded from the heavy radiation in the belt.

Two identical Radiation Storm Belt Probes will pass through the inner and outer radiation belts that surround our planet. Courtesy JHU/APL/NASA.

All that’s changing with the launch last week of the NASA Radiation Belt Storm Probes (RBSP).  These two heavily-shielded spacecraft will study the Van Allen Belts to figure out how particles get INTO the belts, what happens to them when they’re there, and where they go when they leave the belts.  The probes will also give solar physicists some insight into how such events as coronal mass ejections and solar flares affect the Van Allen Belts.

I think it’s pretty cool that we have a pair of spacecraft that are deliberately and carefully designed to survive in the Van Allen Radiation Belts for at least two years and possibly four years (when the mission is extended) in constant contact with high-energy particles.  They’lll give scientists the most in-depth look at just what’s happening in these dangerous radiation environments.

The Millstone Hill Radar installation at MIT's Haystack Observatory is part of the ground-based component of the RBSP mission. Courtesy MIT/Haystack Observatory.

What I also find interesting about this mission is that the  probes aren’t acting alone.  There is a very important ground-based portion of the RBSP mission that involves my friends over at MIT’s Haystack Observatory.

In about 60 days, when the spacecraft have gone through their commissioning period (that is, when they are tested and calibrated), then the prime science mission begins. At that time, the Millstone Hill installation at Haystack will make collaborative electric field measurements on the same magnetic field line that the RBSP is experiencing during its orbit through the inner and outer belts.  This will give scientists more than one point of view on activity in the radiation belts and help them understand the activities occurring in the belts in response to activity on the Sun.

The current solar maximum is, I think, one of THE most studied maximums in recent history.  Not only do we have spacecraft such as the RBSP probes, the Solar Dynamics Observatory, the Solar Heliospheric Observatory, STEREO and many other missions focused on the Sun and its activity, but places like Haystack Observatory are uniquely positioned to give the ground-based, almost “3-dimensional” view of what’s happening as the Sun sends its fury our way.

If you want to learn more about space weather, the RBSP mission, and others, here are a few links to help you out.  And, by all means, check it all out.  Living with a star like the Sun gives us a great chance to understand other stars and their environments, too.

Space Weather FX podcast series, MIT Haystack Observatory/Loch Ness Productions

MIT Haystack Observatory Atmospheric Physics page

Radiation Belt Storm Probes, NASA Mission to study solar effects on Earth’s radiation belts

Solar Dynamics Observatory, a NASA/JHU mission to study the Sun

STEREO, a twin-probe NASA mission to study the Sun in stereo

SOHO (Solar Heliospheric Observatory), a NASA mission to study the Sun