Solar Wavelength

Look at a photograph of the Sun and you see a bright disk - the photosphere. But the photosphere is just the surface. The Sun has an atmosphere too, extending millions of kilometres into space, and it's far more turbulent than the serene surface suggests.

The problem is you can't see it. Not in visible light. The corona is millions of degrees hotter than the surface, but it's also millions of times fainter. To see it, you need to look in ultraviolet - wavelengths invisible to your eyes but precisely suited to the temperatures of the solar atmosphere.

This is exactly what NASA's Solar Dynamics Observatory does. Launched in 2010, SDO photographs the Sun every 12 seconds in ten different wavelengths simultaneously. Each wavelength reveals a different temperature layer, a different set of structures, a different story about the same star.

The Sun's temperature is not what you'd expect. The surface (photosphere) is about 5,700°C. Move upward through the chromosphere and you'd expect it to get cooler - like climbing a mountain on Earth. It does, briefly. Then something extraordinary happens.

In a thin layer called the transition region, the temperature rises from tens of thousands to over a million degrees in just a few hundred kilometres. The corona - the Sun's outermost atmosphere - blazes at 1 to 3 million degrees. During flares, localised regions can reach 10 million degrees or more.

Nobody fully understands why. This is the "coronal heating problem" - one of the biggest unsolved questions in solar physics. The energy must come from the magnetic field, but the exact mechanism remains debated.

Each wavelength channel in the explorer above corresponds to a specific temperature. When you slide from 4500Å to 94Å, you're climbing from the 5,700°C surface to the 6-million-degree flare plasma. The structures you see change completely because different temperatures reveal different physical processes.

SDO doesn't just take pretty pictures. It's a diagnostic tool, and its ten wavelength channels function like a set of thermal filters, each tuned to a specific temperature.

At 171Å (630,000°C), you see the quiet corona - delicate magnetic loops arching between sunspots, tracing the magnetic field like iron filings around a magnet. At 193Å (1.2 million°C), coronal holes become dramatically visible - dark regions where the magnetic field opens into space, the source of the fast solar wind that rushes past Earth.

At 304Å (50,000°C), you're looking at the chromosphere. Prominences and filaments are vivid here - vast curtains of plasma suspended by magnetic fields. When they erupt, they become coronal mass ejections - billions of tonnes of magnetised plasma flung into the solar system.

The hottest channels (94Å and 131Å) only light up during flares - the most violent events in the solar system. When the magnetic field suddenly reconnects, it accelerates plasma to millions of degrees in minutes. These are the events that can disrupt satellites, power grids, and communications on Earth.