More spectacular displays on horizon


Did you see the display of the aurora borealis a few weeks ago? It was visible well into the southern states and with some reports as far south as Mexico.

The intensity and frequency of auroral displays correlate to sunspot activity, which varies according to an 11-year cycle. We have reached the peak of that cycle, a period called solar maximum.

As we experience that period of intense solar activity over the next year, we can expect, or at least hope for, other spectacular auroral displays.

First, let’s discuss why and how aurorae happen. Then, I’ll give some practical advice on seeing them.

The glorious display began 100,000 years ago in the sun’s tiny core. The pressures and temperatures there are so high that the sun fuses its component hydrogen into helium in a hydrogen-bomb reaction on an unimaginably epic scale.

The resulting energy boils and heaves its way to the surface in a chaotic journey that lasts 100,000 years or more. As it finally reaches the sun’s surface, it strips hydrogen atoms of their orbiting electrons.

What is left are naked protons and electrons, charged subatomic particles. The sun’s power ejects some charged particles from its surface into its corona, its outer atmosphere.

Particles build up in the corona until the pressure becomes so great that a part of the corona explodes outward into space as a coronal mass ejection (CME for short).

Every so often, those CMEs head in Earth’s direction. About 18 hours after a CME, a deadly wave of charged solar particles inundates Earth.

Luckily for us, Earth keeps on spinning. Deep at its center is a rotating core of mostly iron. Our spinning iron core acts like a dynamo by generating a field of electromagnetic energy.

That magnetic field repels 98% of the charged particles into space, where they can do us no harm. The lines of magnetic force carry the rest to the north and south magnetic poles.

A few of those particles travel the ever-changing, ever-shifting lines of magnetism that pass through Earth’s atmosphere.

Our atmosphere is mostly nitrogen (78%) and oxygen (21%. As the charged particles slam into the atmospheric molecules, those molecules light up and produce the varying colors of the display.

The green in the aurorae results from collisions with oxygen molecules in the lower atmosphere. Higher up in the ionosphere, collisions with oxygen produce red, and collisions with nitrogen create a violet hue.

Why do most aurorae look green if nitrogen is the predominant gas in our atmosphere? The fault, dear readers, lies not in the auroral light but in ourselves. The human eye is much more sensitive to green than red or violet.

Here’s some advice on seeing them. First, you’ll need some warning of when they might happen. The media cannot always provide sufficient advanced warning.

It’s not their fault. Even during intense solar activity, the CME might not be aimed at Earth. Or the most intense display might happen during daylight in Ohio.

So it helps to have advanced warning. Apps for iPhones and Android provide alerts of potential aurorae visible at your latitude.

I use “My Aurora Forecast” on my iPhone. It includes alert notifications for one’s specific location, live aurora cameras, and maps of projected auroral activity.

Second, it helps to have an eye more sensitive to auroral colors than the ones you were born with. Luckily, most people own one in the form of a smartphone. Turn on your camera and point it north. Your phone may see aurorae that your eyes missed.

Third, if possible, get to a dark rural sky, especially one with no large cities to your north. True aurora aficionados travel to dark skies when an aurora seems likely.

However, auroral displays are notoriously fickle. If you spot an aurora, and it looks short-term, stay wherever you are.

Finally, be patient. Check every couple hours if you don’t see a display after an auroral alert. Truly excellent displays will ebb and flow throughout the night.

Next week: We have known the causes of aurorae only since the beginning of the 20th century. Our current understanding arises from the work of Norwegian astronomer Kristian Birkeland and his sometimes frustrating struggle to get his research appreciated by the broader astronomical community. More on that next week.

Tom Burns is the former director of the Perkins Observatory in Delaware.

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