Peeling Back the Sun

Different atoms emit light at specific wavelengths when they reach certain temperatures. Iron ions at 630,000°C emit light at exactly 171 Ångströms. At 6 million degrees, the same iron emits at 94 Å. By filtering for these specific wavelengths, SDO's cameras can isolate plasma at precise temperatures.

The false colours you see aren't the actual colours of light — these wavelengths are all in the extreme ultraviolet, invisible to human eyes. Scientists assign colours to make different channels distinguishable and to roughly correspond to temperature (cooler = warmer colours, hotter = cooler colours, counterintuitively).

Here's a puzzle: the Sun's surface is 5,700°C. But the corona — the Sun's outer atmosphere — is over a million degrees. That's like walking away from a campfire and finding it gets hotter the further you go. This shouldn't happen.

After 80 years, we still don't fully understand coronal heating. The leading theories involve magnetic field lines that twist, tangle, and violently reconnect, releasing energy in bursts too small to see individually but together maintaining the corona's extreme temperature. The 171, 193, and 211 Å channels are the workhorses of this research.

Sunspots appear dark at 4500 Å because they're cooler than surroundings. But in UV and EUV, they blaze with magnetic activity.

Coronal holes — dark patches in the 193 Å channel — are where magnetic field lines open into space. These are the source of the fast solar wind.

Solar flares light up dramatically in the hottest channels (94, 131 Å) but are barely visible in cooler ones. Watching a flare sweep through the channels shows how the plasma heats and cools.