[UPDATE, midnight New York time: the cloud of particles from the solar flare arrived a few hours ago, but it didn’t impact the earth’s magnetic field quite as hard as the best-guess forecast. (Remember the probability of a geomagnetic storm was only 60%; i.e. the probability of no storm was 40%.) Right now, the auroras are likely visible in Canada but probably not in the US. This could change, but don’t get your hopes up too high; we may have to wait for the next solar flare.]
I’ve been sidelined with computer troubles and non-science activities, so first, a belated thanks to everyone who left a thoughtful comment after Monday’s post and question about communicating science to the public. I appreciate hearing your views, especially from readers with a diversity of backgrounds!
Now, many of you may have heard that there is a forecast of northern (and southern) lights, also known as auroras, tonight. What you’ve heard is correct: today’s NOAA space-weather forecast, from http://www.swpc.noaa.gov/forecast.html, says
VI. Geomagnetic Activity Probabilities 13 Apr-15 Apr A. Middle Latitudes Active 30/30/15 Minor Storm 35/45/05 Major-severe storm 10/15/01 B. High Latitudes Active 10/15/15 Minor Storm 20/25/20 Major-severe storm 60/35/20
i.e. it shows that even in mid-latitudes (meaning northern US and southern Canada, and northern Europe, along perhaps with parts of Australia and New Zealand [?]) they are estimating a 35%-45% probability of a minor `geomagnetic storm’ tonight, with a 10-15% probability of a major storm… and a geomagnetic storm, which literally means a lot of activity in the earth’s magnetic field, generally leads to auroras further away from the north and south pole than usual.
Auroras are caused when swarms of electrically charged particles (electrons and ions) from the sun arrive in the earth’s vicinity, are captured by the earth’s magnetic field, spiral down toward the north and south pole of the earth, and hit molecules in the earth’s atmosphere, causing them to glow. [Here’s a NASA flyer with a drawing and a little more information.] A strong solar flare (a giant explosion on the surface of the sun) can generate a huge cloud of charged particles (a CME, or coronal mass ejection), and if this cloud arrives at the earth it can not only generate auroras but also move the earth’s magnetic field around, with the effect that some of this glow is found well away from its usual location near the poles, over places where more humans live.
And because there was a big CME on Thursday that seemed to be aimed at the earth, and because it takes about two days for such a cloud to travel out to where the earth is,
the space weather forecasters are expecting the CME’s particles to hit the earth any time now It has apparently arrived! When that happens, if enough of those particles get caught in the earth’s magnetic field, we’ll start seeing auroras away from the earth’s poles.
Auroras are generally quite dim, though easy to see in a naturally dark sky — so you need to be looking after dark and (unless the auroras are exceptionally bright) ideally some distance from any city lights, with eyes adjusted to the darkness, and with few clouds and no haze. The lights are most likely to be found to the north of your location, but the further north you are, the more likely they are to be overhead. (Switch north and south if you’re in the Southern Hemisphere.)
Now, something else you may have seen is this map, due to accuweather.com. The map shows where there likely will be cloud cover (red and yellow) and clear skies (green), but in one respect it is badly misleading: it has a line on it that separates the region where the auroras might be visible from the region where they are “NOT VISIBLE”. Well, do not entirely lose heart if you are a bit south of that line, and also, don’t be very sure of things if you are north of it. No one can predict very precisely where that line will be, and it will move around (quite likely northward) as time wears on following the CME’s arrival.
By the way, let’s ignore the breathless headline of the accompanying AccuWeather article: “Dazzling Northern Lights Anticipated Tonight” — that’s journalistic license at work. (Not to mention the article’s scientific misstatement about protons.) The auroras generated by the solar flare may be anything from a dim, quietly beautiful green wash to a bright red and/or multi-color light show, and no one, certainly not AccuWeather, can tell you which it will be, especially not in a specific time or location. So be prepared for something that’s beautiful and subtle, not dazzling. If in fact you find yourself dazzled, count yourself very lucky!
Finally: one good way to know whether you should go outside to look at the auroras is to keep an eye on this graph. The bottom of the four plots on the graph is marked “Estimated Kp”, and you can see that it’s showing green bars most of the time. But if the most recent bar is red, that means the earth’s magnetic field is significantly disturbed, and there’s a good chance of auroras further south than usual, in which case you should definitely consider going outside and taking a look. The higher the red bar, the more southerly the auroras are likely to go. (If someone can remind me where one can find more timely updates of the Kp estimate, I’d appreciate that.)
Meanwhile, the later the CME arrives (and it is already a bit late) the less likely it is that the CME hits the earth head on, and the less likely it becomes that we’ll see auroras far from the poles tonight. UPDATE: The CME has arrived, a little while before sunset in New York. If in fact we do get auroras at all, it’s quite possible you’ll be able to see them an hour or two after sunset in the northern United States and Canada.
Wishing you good luck with one of the first aurora opportunities of 2013! And if you see one, think to yourself: physics is beautiful!