There’s so much more to the Sun than the yellow orb that human eyes can perceive. High-energy light, colors that our eyes don’t detect, reveals a dramatically different landscape. Take a minute to enjoy the beauty of this perspective through the video above (mobile users click here), and then read on to learn all about it.
Upon seeing this event in October of last year, I immediately thought of the iconic Hokusai painting, The Great Wave off Kanagawa. The stormy waters over Mt. Fuji are replaced by super-heated gas arcing over a solar prominence. Where air currents and gravity conspired to produce Hokusai’s wave, the one in the video is driven by the complex tangle of the Sun’s magnetic field. And of course, the scale is dramatically different. The height of the Sun's wave is around 52,000 km, over 4x the diameter of Earth!
This vantage is afforded to us by the Solar Dynamics Observatory (SDO), a NASA satellite launched in 2010. Similar instruments have hinted at these wonders before, but none had the HD resolution and continuous observations of SDO. We are the first humans to see our star like this. That’s pretty special, I think.
The movie starts out in the Sun’s lower atmosphere, called the chromosphere. This layer rises about 2,000 km over the visible surface and is home to plasma upwards of 50,000° Celsius, 50x hotter than the surface. We’re seeing it here through extreme-ultraviolet (EUV) light. Since our eyes can’t see this far up the electromagnetic spectrum, any color scheme is “false color” by definition. When red overtakes blue about 25 seconds into the video, we’ve moved to the standard set of colors typically used for this wavelength. If you noticed the clock running in the upper-right, you saw that time runs backward through this segment.
We then zoom out and rotate, showing us the entire star with its true north-south orientation. The colors change again about 40 seconds in, but this time it’s more than a color change. It’s a switch to a different telescope on the satellite, which sees a different wavelength of light. For the aficionados, this change is from 304 to 171 Angstroms (Å), or 30.4 to 17.1 nanometers. Both telescopes are part of the Atmospheric Imaging Assembly (AIA), an array of four telescopes that each sees a different part of the atmosphere. The yellow (171 Å) shows us the upper part of the “transition region” and the lower part of the outer atmosphere, called the corona.
The transition region is aptly named for an abrupt transition to ridiculously high temperatures. Within just a few hundred kilometers, temperatures rocket from tens of thousands to millions of degrees. If this increase seems strange to you, you’re not alone. Scientists have known that the corona is hundreds of times hotter than the surface for about 75 years, but nobody has explained exactly why. In hopes of addressing this question, a new satellite called the Interface Region Imaging Spectrometer (IRIS) was launched just a few months ago to study the transition region in more detail.
While we don’t know exactly why the corona is so hot, we do know that the energy comes from the Sun’s magnetic field. But magnetic fields aren’t so easy to understand. You can’t see them unless there’s electrically charged material to trace them out. That’s just what the wave does, in fact, revealing the Sun’s magnetic field like iron filings reveal that of a bar magnet. The wave is forced to move along the field lines because it, like the rest of the Sun, is made of plasma.
Plasma is just gas that’s been heated enough to strip electrons from the atoms. The result is a soup of particles with electric charges, and anything with an electric charge is subject to magnetic forces. That’s why there’s just one term in physics to describe all this: electromagnetism. An enormous component to studying the Sun is using what we know about electromagnetism and the behavior of fluids to predict the structure of the magnetic field, which can then be checked with observations like these.
We’re 700 words in now and I still haven’t told you what actually caused the wave. What prompted this plasma to gracefully flow along the magnetic field? This, folks, is the hard part. Solar flares, eruptions, and events like this one are not very well understood. That said, the most likely driver for this event is a phenomenon called “magnetic reconnection”. Reconnection is a difficult concept to wrap one’s head around, but it can be summarized as the explosive joining of two separate magnetic field lines. Field lines resist being pushed together, similar to the repulsion between the positive (or negative) ends of two magnets. If magnetic field lines are forced to meet, the structure of the field abruptly changes and some of the stored magnetic energy gets dumped into heating and moving particles around. Check out this video for a nice example of reconnection using SDO images.
There’s evidence here for one of reconnection’s telltale signs, heating. Take another look at the GIF animation above. The dark structures on the left and in the middle are called solar prominences. They’re dark because they’re colder than the surrounding material. The wave takes shape when the dark prominence material on the left brightens up and is pushed along the field lines overlying the prominence in the middle. This brightening indicates that the wave plasma was heated enough to start emitting the wavelength of light detected by that telescope.
But it’s important to be cautious here. When the brightness of any structure increases, it’s impossible to know from just one set of images whether the increase was caused by an increase in temperature, as I'm claiming, or an increase in density. This is precisely why SDO/AIA has more than one telescope. Each telescope has at least two “channels”, each of which is sensitive to plasma at different temperatures. We can combine the observations from these different channels to get a more complete picture. When we look at the hotter channels, we see an increase in brightness in even the hottest temperature range, which is strongly indicative of heating.
The final piece of the puzzle is where the initial magnetic reconnection event took place. The GIF animation below shows another of AIA’s wavelength channels (131 Å). Right at the start, there’s a tiny jet followed by a small flare in the bright region to the left. Jets like this are caused by magnetic field loops poking up from beneath the surface and reconnecting with the overlying field system. Sometimes all it takes is a small nudge out of equilibrium for a large event to unfold. It’s difficult to say with certainty, but I suspect that the little jet was the first domino to fall, triggering the flare that ultimately drove our wave.
Remember that this was just another day on the Sun. Countless of amazing events have been observed by SDO and there are many more to come as the maximum phase of this solar cycle winds down over the next few years. You don’t have to wait for the next article like this one to see what’s happening. Images from SDO and a host of other satellites are available online through the Helioviewer project. You can make your own movies and upload them straight to YouTube. Check it out!