This week the Earth has seen some increased magnetic activity in the upper atmosphere, and that means we got to see aurore! Across Northern Europe and the Northern USA people looked to the skies to see the northern lights. An aurora is one of the most beautiful sights in the natural world, and a phenomenon that actually tells us a lot about the Earth and how it interacts with its environment.
Those who followed me on Twitter (@aidanatcern) may have already seen some of the wonderful images of aurorae. There are dedicated webcams that capture the night sky, and you can see some sample images at the Aurora Webcam archive.
When charged particles accelerate or decelerate, or recombine in pairs, they emit electromagnetic radiation, and it is this radiation that we see in the aurora. The color of the light depends on the wavelength of the radiation, and the intensity of the light depends on how much radiation is emitted. That means that there is always an aurora above us, but if the energy of the radiation is too low, or the intensity is too weak, we won’t see anything. Once we know how to interpret the light we can learn something about the radiation that is emitted. Usually we see a variety of colors in an aurora and each color corresponds to a different wavelength, so if we can see a region of the sky that is all one color, we know that the wavelength (and hence the energy, ignoring the effects of aberration) must be the same. That means we can “map” the sky and find contours of wavelength.
Since the particles are accelerating, there must be something that causes the acceleration. The Earth’s core is made of (among other materials) molten iron. The rotation of the Earth means that this core is also rotating, and a rotating fluid magnetic medium creates a magnetic dipole, giving the Earth magnetic North and South poles. These poles are aligned near the geographic North and South poles of the Earth, but not exactly. (In fact, magnetic North and South keep moving and from time to time they even swap places. The exact mechanism behind this is not yet fully understood, but geological records show it happens every few hundred thousand years. Simulations suggest that the rotating magnetic fluid is a chaotic system, so the reversals occur at stochastic, or random, intervals of time.)
The sun produces a stream of particles, known as the solar wind, and they create their own electromagnetic field. The two fields, from the Earth and the sun, interact and they force charged particles in the upper atmosphere along curved paths. As the particles move along these paths they accelerate, decelerate and recombine, and that is what produces the aurorae. The most recent increase in magnetic activity can be traced back to a huge coronal mass ejection that arrived from the sun. This video shows the arrival of the flare:
The effect looks impressive, but don’t be scared, solar winds like this are perfectly harmless. Far bigger winds have hit the Earth in the past few billions years and life has continued to flourish in spite of them. Life has adapted to the Earth’s magnetic field and this field protects us from the high energy particles.
It turns out that while looking up at the night sky is a beautiful and moving experience in itself, it is also important to particle physicists. Some of the most important discoveries in the last century came from a different phenomena, cosmic rays. Cosmic rays are very high energy particles (usually protons) that travel huge interstellar distances and rain down on the Earth in much the same way that the solar wind does. They interact with the upper atmosphere to create cascades of particles, and usually the muons are the only detectable particles that reach sea level. Interactions of these cosmic rays gave rise to the discovery of the muon (“Who ordered that?!”), the pion and the kaon, the lightest forms of mesonic matter. It was around this time that large scale accelerators were developed, and we found hundreds of new mesons and baryons. Cosmic rays gave us a very small glimpse into a rich “zoo” of particles that has occupied physicists ever since.
Eventually, when we have exhausted our ability to accelerate particles to higher energies we might need to rely on cosmic rays again. There are proposals to develop ground based detectors to study the interactions of extremely high energy particles from outer space. Those particles have the potential to reach energy regimes we can only dream of at the moment. (Incidentally, this is one of the ways that we know for sure that the LHC cannot destroy the world. The universe creates much more energetic particles than we could ever hope to create in our accelerators, and since the universe seems to be in one piece we can conclude that the LHC is safe on Earth!)
If you’re fortunate enough to see an aurora then take a few moments to think about the huge forces at work, the vast distances involved, and how the colors tell us so much about how the Earth and solar wind behave. It really is one of the most beautiful phenomena in the universe.