LTP 129: Aurorae

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• Bart Busschots (host) – @bbusschots – Flickr

In this solo show Bart explains what the northern light are, how they work, how to see then, and how to photograph them. But most importantly, since we’re entering the peak of the 11 year cycle they follow, Bart gives some advice on preparing to make the most of any aurorae visible in your neck of the woods!

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We recently had a very dramatic display of the northern lights above much of Europe and North America, and that’s no accident — we’re entering prime aurora time! So, now seems the perfect time to dedicate and instalment to seeing and photographing these natural wonders.

One Marvel of Nature, Many Names

Before we dive in to the meat of this instalment, just a note that what we could colloquially call the Northern Lights are technically Aurorae, or, if you prefer the modern pluralisation, Auroras.

Western culture obviously encountered aurorae in northern countries first, and Galileo of all people named them the Aurora Borealis, after the Roman goddess of the dawn and the Greek god of the north wind. When western culture discovered the same cool lights in the southern hemisphere, they were named Aurora Australis, after the Greek god of the south wind.

Having studied science in Europe a few decades ago I learned the irregular plurals for scientific words, so I say ‘one Aurora’ and ‘two Aurorae’. These days those archaic plurals are being replaced with regular plurals, so it’s just as correct to say ‘one Aurora’ and ‘two Auroras’, but my brain is still wired the old way, so I’m going to use Aurorae here.

Where do the Aurorae Come From?

We now know that the key to understanding Aurorae is the sun, but it took us a while to figure that out.

We now know that the sun has an approximately 22 year cycle during which it’s north and south magnetic poles flip around, and then flip back. At the points in the cycle when the poles are relatively stable the surface of the sun tends to be quite a tranquil place, but as the poles are flipping, the surface really comes to life.

We discovered the solar cycle all the way back in the 1800s by counting so-called sunspots on the surface of the sun, and noticing that their prevalence ebbs and flows over 11 year cycles, and, that we see more aurorae when we see more sunspots. So, we discovered the correlation the centuries ago, but it took us quite a bit longer to figure out the mechanism.

Sunspots are ‘cooler’ parts of the sun’s surface — just 3 to 4½ thousand degrees Celsius instead of the usual six thousand (5½ to 8 thousand degrees Fahrenheit and instead of 11 thousand)! They’re caused by small-scale changes in the sun’s magnetic field. The sunspots themselves don’t really do anything, but those magnetic fields do! They can cause hot electrically charged gas from the sun’s surface to get flung into space, and when things are lined up just right, those charged particles are thrown our way.

Because the earth has a magnetic field, and because those particles are charged, they get intercepted by the earth’s magnetic field, and because of its shape, the particles slam into the atmosphere in a big oval around the north and south poles. Those ovals are usually quite small, covering the countries near the north and south poles, but if the sun throws something at us with enough energy the ovals can expand, sometimes reaching down to the Mediterranean and the southern United States.

When charged particles hit the atmosphere they excite the molecules in the atmosphere, bumping them into higher energy states. When that happens the molecules return to their ground state by emitting light, and the energy the molecule had to lose determines the colour of the light. This is actually how fluorescent lightbulbs work!

Aurorae are literally the sky acting as a light bulb!

What do they Look Like?

At different levels of the atmosphere there are different molecules, so depending on how deep the charged particles from the sun penetrate into the atmosphere you’ll get different colours.

Up near the top of the atmosphere you have very diffuse atomic oxygen, so there’s not a lot of it, and it’s also not getting knocked around a lot, so what little there is gets excited enough to emit relatively dim red light.

A little lower in the atmosphere you still have atomic oxygen, but here it’s being jostled about a lot, so on average each atom tends to get less excited so most of them emit green rather than red light. This green is generally the easiest colour to see because even a relatively weak aurora will reach these altitudes, and since there are more molecules there they out-shine the much dimmer red glow from the very top of the atmosphere.

Staying with atomic oxygen, if the aurora is strong enough you might get a good mix red and green emissions even lower down in the atmosphere which can give you some pink or yellow hues, but this is much rarer.

When you move down lower into the atmosphere there’s very little atomic oxygen, but there’s lots nitrogen, and it comes in two forms — molecular nitrogen, and ionised molecular nitrogen. These molecules can emit light at quite a few different frequencies, and hence colours, but they’re constrained to the red and blue parts of the spectrum. The blue tends to dominate and sometimes it combines with the reds to give nice purples.

So, while we have four main colours — red, green, blue & purple, they’re all being made by just two elements — Nitrogen & Oxygen!

What’s more important to note is that different colours show up at different altitudes, so when you have tall features in an auroral display they show colour bands!

So we know what colours we can expect, but what about shapes? Aurorae can come in any shape really, but some shapes are more common, and, a single display is usually made up of a few different shapes combined.

1. A glow near the horizon — these are by far the most common, and often this is all you’ll see. An auroral glow can be easily confused with low cloud, but, the difference is that stars shine clearly through an auroral glow, but get blocked by clouds. Because they’re dim, our eyes won’t see their colour, but our cameras will!
2. When an aurora heats up a bit you’ll start to get patches of brighter aurora that look like glowing clouds. Again, you’ll see stars through them.
3. When things really get going, you’ll get tall arcs of aurora — these reach from low to the horizon right up into the sky, and because they cut through many layers of the atmosphere, they’ll change colour as they rise. If these arcs have folds in them they’re often described as ‘curtains’. These are the pretty ones that can slowly sway over and back as you watch.
4. Finally, when an aurora has been building for a while, you can get a corona that covers much of the sky and seems to radiate out from a point. You tend to see these later in the night. These can be easy to miss because the corona is usually high in the sky above your head!

Seeing and Photographing Aurorae

Note: to save myself being duplicative, I’m going to word this segment from a northern hemisphere POV, but if you’re in the southern hemisphere the same applies, you just need to replace ‘north’ with ‘south’.

The key to maximising your chances of seeing and photographing an aurora is a clear northern horizon with as few obstructions and as little light pollution as possible.

During solar maximum it’s always worth checking your northern horizon when the skies happen to be clear. If you see what might be a low haze, check if you can see any stars through it, if you can, it’s an auroral glow! If you see an auroral glow it’s worth checking back every hour or so because aurorae often build as the night goes on.

After the glow, look for more defined ‘clouds’ a little higher above the northern horizon, if you see stars through those then you have some auroral patches. If you see any of those, definitely keep checking the sky every hour, because there’s a good chance you’ll see some arcs from time to time during the night. If you’re lucky enough to have a good display, remember to look up, especially after midnight, because there could be an auroral corona right over your head!

These days, we live in a world where the cameras on our phones can see aurorae better than our human eyes!

For a start, our eyes have two main light sensors — rods & cones. The rods are way more sensitive to light than the cones, but they don’t detect colour, so, in low light we tend to see the world as if someone turned the saturation slider right down!

Our digital cameras on the other hand have no such limitation, they see dim colours just fine! So, rather than trying to look for stars peeping through suspicious low clouds, just take out your phone and take a quick snapshot, if you see greens and reds, then that’s not a cloud!

Once you know there is an auroral display you can try do better than a snapshot. Regardless of whether you shoot DSLR or phone camera, the following advice applies:

1. Try get some sort of silhouette of the horizon into the shot — it will help give context and a sense of scale. It can be the tiniest strip right at the base of the shot.
2. Aurorae are big, grand phenomena, shoot as wide as you can to capture their grandeur!

If you’re shooting with a phone camera I’ll offer the following additional advice:

1. If your phone has a night mode, use it!
2. Expose as long as your camera will allow — on iPhones you can use the chevron icon to show the advanced settings, then you can click on the night mode icon and slide the slider all the way to the right
3. Stabilise yourself as much as possible.
   1. If you have a tripod adaptor for your phone, use it!
   2. If not, find a wall or fence at a comfortable height, rest your elbow on it, and spread them wide apart to create a kind of human tripod.
   3. To shoot high in the sky without a tripod, find a low wall and lean your shoulder against it. It needs to be low enough not to obscure your shot, but high enough to reach your shoulder.

I’ll offer the following additoinal advice for shooting with a DSLR:

1. Use a tripod
2. Use the widest lens you have, and set it wide open at its lowest focal ratio
3. Focus on infinity
   1. Ideally, buy lenses with an infinity marker on the barrel, set the focus mode to manual, dial in infinity, take a test shot to verify, and you’re good to go!
   2. If your lens doesn’t have an infinity marker on the barrel, enable single-point autofocus mode, make sure the centre point is selected, then find something bright as far away from you as you can, put it right on that centre focus point, and and hope the camera can lock on. If it can, enable your focus lock and you’re good to go.
   3. If all else fails, you’ll need to set the lens to manual focus, turn it all the way to the direction to focus further from you, then start taking test exposures and slowly inch the focus back. Infinity is usually quite close to maximum twist on the lens.
4. Expose for as long as you can get away with — check what works by exposing longer and longer, zooming in, and checking if the stars have become too much like potatoes for your tastes, if they have, back off a little and you’re good to go.
5. Manually set the white balance to daytime

When it comes to processing the shots, I tend to do the following on my iPhone:

1. Dial back the black point and the shadows to get the silhouetted horizon black and the bare sky dark
2. Increase the vibrancy to bring out the colours in the aurora
3. Increase the definition to get the stars to stand out better

Preparation

As we come into solar maximum it’s well worth finding a few good places as close to your home as possible where you have good northern horizons, and, where it’s safe to be after dark.

It’s also worth keeping an eye on the news for predicted aurorae, or for related terms like:

• Coronal Mass Ejection (CME)
• Solar Storm
• Geomagnetic Storm

You might also want to keep an eye on some more scientific resources, or to install an app.

The least human-unfriendly website might be SpaceWeatherLive’s aurora forecast, and the most technically detailed is probably the the homepage of the US National Oceanic & Atmospheric Administration (NOAA) Space Weather Prediction Center’s homepage.

In terms of apps, none of the ones I’ve found are great — they all seem to suffer from the by nerds for nerds poor UI effect I notice in many geekier tools. But, FWIW, the one I’m least cranky with is the iOS app My Aurora Forecast & Alerts — it does successfully. send me push notifuications when I need to keep an eye on the sky, even if I find the UI very clunky.

Understanding the Kp-index & G-scale

A lot of websites and apps will give you their predictions in terms of something called the Kp-index. This is a global, i.e. planetary, average of the K-index which is a measure of disturbances in the earth’s magnetic field that goes from zero to nine, with zero being no disturbances, and 9 meaning things are as disturbed as we’ve ever seen! Because the earth’s magnetic field is different from place to place, the definition doesn’t map specific magnetic field strengths to specific indexes, but instead maps a 3-hour maximum reading against what ever is normal for that observatory when there’s nothing going on. The math to go from local K-index readings to the Kp-index is even more horrific!

Finally, just to add a little more confusion to the mix, NOAA decided they needed a simple 5-point scale for issuing warnings to power companies and spacecraft operators. They named their scale the G-scale, for geomagnetic storm, and it maps directly to the Kp-index.

As best as I can figure out — if you live in latitudes where aurae are normal then you want to keep an eye on the sky when the Kp-reaches 4, and if you live further south, you’re not going to see anything until you get into the G-scale, and how high it has to get for you to see anything depends on how far you are from your pole.

Final Thoughts

I hope I’ve inspired you to make the effort to try capture any aurorae you can this solar maximum, and to give you one final nudge, I snapped these shots from a fence a few hundred yards from my front door using my iPhone and the human-tripod technique — www.flickr.com/….