Maybe. If we collectively, and you personally, are lucky, then maybe you might see auroras — quantum physics in the sky — tonight.
Before I tell you about the science, I’m going to tell you where to get accurate information, and where not to get it; and then I’m going to give you a rough idea of what auroras are. It will be rough because it’s complicated and it would take more time than I have today, and it also will be rough because auroras are still only partly understood.
First though — as usual, do NOT get your information from the mainstream media, or even the media that ought to be scientifically literate but isn’t. I’ve seen a ton of misinformation already about timing, location, and where to look. For instance, here’s a map from AccuWeather, telling you who is likely to be able to see the auroras.
See that line below which it says “not visible”? This implies that there’s a nice sharp geographical line between those who can’t possibly see it and those who will definitely see it if the sky is clear. Nothing could be further than the truth. No one knows where that line will lie tonight, and besides, it won’t be a nice smooth curve. There could be auroras visible in New Mexico, and none in Maine… not because it’s cloudy, but because the start time of the aurora can’t be predicted, and because its strength and location will change over time. If you’re north of that line, you may see nothing, and if you’re south of it you still might see something. (Accuweather also says that you’ll see it first in the northeast and then in the midwest. Not necessarily. It may become visible across the U.S. all at the same time. Or it may be seen out west but not in the east, or vice versa.)
Auroras aren’t like solar or lunar eclipses, absolutely predictable as to when they’ll happen and who can see them. They aren’t even like comets, which behave unpredictably but at least have predictable orbits. (Remember Comet ISON? It arrived exactly when expected, but evaporated and disintegrated under the Sun’s intense stare.) Auroras are more like weather — and predictions of auroras are more like predictions of rain, only in some ways worse. An aurora is a dynamic, ever-changing phenomenon, and to predict where and when it can be seen is not much more than educated guesswork. No prediction of an aurora sighting is EVER a guarantee. Nor is the absence of an aurora prediction a guarantee one can’t be seen; occasionally they appear unexpectedly. That said, the best chance of seeing one further away from the poles than usual is a couple of days after a major solar flare — and we had one a couple of days ago.
Good Information and How to Use it
If you want accurate information about auroras, you want to get it from the Space Weather Prediction Center, click here for their main webpage. Look at the colorful graph on the lower left of that webpage, the “Satellite Environment Plot”. Here’s an example of that plot taken from earlier today:
There’s a LOT of data on that plot, but for lack of time let me cut to the chase. The most important information is on the bottom two charts.
The bottom row, the “Estimated Kp index”, tells you, roughly, how much “geomagnetic activity” there is (i.e., how disturbed is the earth’s magnetic field). If the most recent bars are red, then the activity index is 5 or above, and there’s a decent chance of auroras. The higher the index, the more likely are auroras and the further away from the earth’s poles they will be seen. That is, if you live in the northern hemisphere, the larger is the Kp index, the further south the auroras are likely to be visible. [If it’s more than 5, you’ve got a good shot well down into the bulk of the United States.]
The only problem with the Kp index is that it is a 3-hour average, so it may not go red until the auroras have already been going for a couple of hours! So that’s why the row above it, “GOES Hp”, is important and helpful. This plot gives you much more up-to-date information about what the magnetic field of the earth is up to. Notice, in the plot above, that the magnetic field goes crazy (i.e. the lines get all wiggly) just around the time that the Kp index starts to be yellow or starts to be red.
Therefore, keep an eye on the GOES Hp chart. If you see it start to go crazy sometime in the next 48 hours, that’s a strong indication that the blast of electrically-charged particles from the Sun, thrown out in that recent solar flare, has arrived at the Earth, and auroras are potentially imminent. It won’t tell you how strong they are though. Still, this is your signal, if skies near you are dark and sufficiently clear, to go out and look for auroras. If you don’t see them, try again later; they’re changeable. If you don’t see them over the coming hour or so, keep an eye on the Kp index chart. If you’re in the mid-to-northern part of the U.S. and you see that index jump higher than 5, there’s a significant geomagnetic storm going on, so keep trying. And if you see it reach 8 or so, definitely try even if you’re living quite far south.
Of course, don’t forget Twitter and other real-time feeds. These can tell you whether and where people are seeing auroras. Keeping an eye on Twitter and hashtags like #aurora, #auroras, #northernlights is probably a good idea.
One more thing before getting into the science. We call these things the “northern lights” in the northern hemisphere, but clearly, since they can be seen in different places, they’re not always or necessarily north of any particular place. Looking north is a good idea — most of us who can see these things tonight or tomorrow night will probably be south of them — but the auroras can be overhead or even south of you. So don’t immediately give up if your northern sky is blocked by clouds or trees. Look around the sky.
Auroras: Quantum Physics in the Sky
Now, what are you seeing if you are lucky enough to see an aurora? Most likely what you’ll see is green, though red, blue and purple are common (and sometimes combinations which give other colors, but these are the basic ones.) Why?
The typical sequence of events preceding a bright aurora is this:
- A sunspot — an area of intense magnetic activity on the Sun, where the sun’s apparent surface looks dark — becomes unstable and suffers an explosion of sorts, a solar flare.
- Associated with the solar flare may be a “coronal mass ejection” — the expulsion of huge numbers of charged (and neutral) particles out into space. These charged particles include both electrons and ions (i.e. atoms which have lost one or more electrons). (Coronal mass ejections, which are not well understood, can occur in other ways, but the strongest are from big flares.)
- These charged particles travel at high speeds (much faster than any current human spaceship, but much slower than the speed of light) across space. If the sunspot that flared happens to be facing Earth, then some of those particles will arrive at Earth after as little as a day and as much as three days. Powerful flares typically make faster particles which therefore arrive sooner.
- When these charged particles arrive near Earth, it may happen (depending on what the Sun’s magnetic field and the Earth’s magnetic field and the magnetic fields near the particles are all doing) that many of the particles may spiral down the Earth’s magnetic field, which draws them to the Earth’s north and south magnetic poles (which lie close to the Earth’s north and south geographic poles.)
- When these high-energy particles (electrons and ions) rain down onto the Earth, they typically will hit atoms in the Earth’s upper atmosphere, 40 to 200 miles up. The ensuing collisions kick electrons in the struck atoms into “orbits” that they don’t normally occupy, as though they were suddenly moved from an inner superhighway ring road around a city to an outer ring road highway. We call these outer orbits “excited orbits”, and an atom of this type an “excited atom”.
- Eventually the electrons fall from these “excited orbits” back down to their usual orbits. This is often referred to as a “quantum transition” or, colloquially, a “quantum jump”, as the electron is never really found between the starting outer orbit and the final inner one; it almost instantaneously transfers from one to the other.
- In doing so, the jumping electron will emit a particle of electromagnetic radiation, called a “photon”. The energy of that photon, thanks to the wonderful properties of quantum mechanics, is always the same for any particular quantum transition.
- Visible light is a form of electromagnetic radiation, and photons of visible light are, by definition, ones that our eyes can see. The reason we can see auroras is that for particular quantum transitions of oxygen and nitrogen, the photons emitted are indeed those of visible light. Moreover, because the energy for each photon from a given transition is always the same, the color of the light that our eyes see, for that particular transition, is always the same. There is a transition in oxygen that always gives green light; that’s why auroras are often green. There is a more fragile transition that always gives red light; powerful auroras, which can excite oxygen atoms even higher in the atmosphere, where they are more diffuse and less likely to hit something before they emit light, can give red auroras. Similarly, nitrogen molecules have a transition that can give blue light. (Other transitions give light that our eyes can’t see.) Combinations of these can give yellows, pinks, purples, whites, etc. But the basic colors are typically green and red, occasionally blue, etc.
So if you are lucky enough to see an aurora tonight or tomorrow night, consider what you are seeing. Huge energies involving magnetic fields on the Sun have blown particles — the same particles that are of particular significance to this website — into space. Particle physics and atomic physics at the top of the atmosphere lead to the emission of light many miles above the Earth. And the remarkable surprises of quantum mechanics make that light not a bland grey, with all possible colors blended crudely together, but instead a magical display of specific and gorgeous hues, reminding us that the world is far more subtle than our daily lives would lead us to believe.