The Sun may seem steady and calm to the eye, but in reality it’s anything but. Magnetic fields pierce the surface, barely constraining the dozens of millions of tons of ionized plasma that flow along them. These loops of magnetic force store vast amounts of energy, and if released, can explode with the force of millions of nuclear bombs. And when they do, that matter is hurled into space at speeds hundreds of even thousands of times faster than a rifle bullet.
Screaming across the solar system, most of that material misses the Earth, such a small target we are. But sometimes the geometry is right, and that debris bears down on us. Slamming into the Earth’s own magnetic field, the interaction focuses the subatomic particles from the Sun to our poles, where they cascade down the geomagnetic field lines into our atmosphere. The atoms and molecules in our air become energized by this gust of solar wind, causing them to glow. And when they do—and you see them above, from space—it looks like this:
That lovely picture is from the Suomi NPP Earth-observing satellite. It has a camera on board that can see light from visible light out to the infrared, beyond what our eyes can see. It’s very sensitive to this light, able to pick up the faintest glimmer, and it also has very keen vision, able to resolve tiny details.
The picture shows the aurora australis, the glow of the air over Antarctica, in this case over Queen Maud Land, due south of Africa, on Jul. 15, 2012. This is a huge swath of the Earth; the width of the image is about 2000 km (1200 miles). In reality, the aurora was probably mostly green with hints of red and purple, but in this image the colors weren’t separated as they would be in a color picture; for this shot a green photon counts as much as a red one, and all were added together to map the brightness of the aurora.
The Moon was a crescent at the time this picture was taken, so really the only ambient light is from the aurora itself. And yet you can see the edge of the ice sheet going more-or-less horizontally across the screen, illuminated solely by the glow (the yellow line marks the coastline of Antarctica, with Antarctica below the line, and the Southern ocean above; the winter ice extends past the coast to the north). The jagged streaks in the aurora are not real; they are due to motion in the lights as the Earth’s magnetic field is buffeted by the solar winds, combined with the motion of the satellite itself as it took the image.
Suomi NPP is a powerful tool; in the hi-res version of this image you can easily see cracks in the ice and separate floes. The superb camera used here is designed to detect low-level light sources such as the aurorae, airglow, forest fires, and city lights. This can be used to carefully measure the human impact on light levels around the globe, in turn an indicator of our footprint on the planet. We know for rock solid fact that humans have a very large influence on many factors on our planet—climate, water flow, heat exchange, air pollution, and much more—so it is absolutely critical we have a better grasp on what we are doing. Suomi NPP is part of a fleet of satellites that have that as their purpose.