Extreme Southerly Aurora

In early May this year Arizonans were treated to the spectacle of an aurora borealis (Northern Lights). It’s rare for this phenomenon to be visible at our low latitude (average 35°), and when it does appear it’s not the moving curtains of multiple colors that recently made the news. Rather, it a diffuse red glow only seen in dark locations away from cities. The photo above was taken in Arizona by a long time friend and colleague who graciously allowed me to use it in this post.

Of course I had my camera on a tripod, programmed for long exposures and ready for my own attempt — the event was predicted days in advance (more about that later). But I’m located 3000 ft lower in altitude than my friend, and 65 miles farther south in latitude. And that means 65 miles closer to the Phoenix metro area. Three nights in a row I stayed up till midnight hoping to get a shot, but no dice.

In the 45 years I’ve lived in Arizona I’ve only seen an aurora twice, and both were that diminutive red glow. When I lived in Wisconsin I saw many more, including the moving curtains with multiple colors. But Wisconsin has an average latitude of 45° and that extra 10° of latitude makes a huge difference. To understand that difference we need to understand the physical mechanisms that underlie aurorae.

The solar wind is a stream of charged particles (electrons, protons, and alpha particle) flowing outward from the Sun. It’s always there, but varies on an 11 year cycle with solar activity and sunspots. The solar wind interacts with Earth’s magnetosphere (magnetic field) to channel charged particles toward the north and south poles, as shown in the graphic below. And it’s this constant rain of particles near the poles the creates the frequent auroral displays at high latitudes. You aren’t seeing the particles themselves, you’re seeing molecules of oxygen and nitrogen that have been hit by a particle, excited to a higher energy state, and decayed back to a lower energy state by emitting a green, blue, or red photon. Space.com has an excellent explanation of the details.

But to get auroras at lower latitudes like Arizona you need more than just the typical solar wind — you need something called a coronal mass ejected (CME) that just happens to be aimed at Earth. The video below shows a CME erupting from the surface of the Sun. It’s moving at around 900 miles per second. This was in August of 2012, and not related to the recent activity, but it shows the scale of a CME relative to the size of the Sun. It was captured by the joint ESA/NASA Solar Heliospheric Observatory (SOHO):

For a further sense of CME size, check out this frame grab with Earth added to scale:

That CME wasn’t aimed at Earth, as is apparent from the side perspective. But the Sun rotates once every 27 days, so an active region on its surface does occasionally point straight at us. When that happens we get aurora displays like the one this month. The number of charged particles streaming into Earth’s magnetosphere increases dramatically, resulting in extreme far south aurorae. The cool thing is that we can now predict when this will happen. That’s because of the emerging science of space weather.

A suite of satellites from various organizations are continuously monitoring the Sun and will detect any CMEs aimed at Earth. It takes the fastest CMEs around 15–18 hours to reach Earth, so we have ample warning. This is what the aurora forecast was around the time I was watching for it:

You can see the forecast for Arizona wasn’t that good, but these forecasts are uncertain, as with surface meteorological forecasts. Despite the forecast, some Arizonans saw an aurora. Alas, I did not.

However, that sunspot complex will be rotating around the Sun and be pointed back at Earth next week. If it stays active, and throws another CME at Earth, we may be treated to another extreme southerly aurora. You can bet I’ll have my camera ready again. Stay tuned for a sequel.

Next Week in Sky Lights ⇒ How Satellites Point Themselves

Blue Sunset on the Red Planet
Q&A: How Satellites Point Themselves
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