Aurora are a frequent visitor to the dark skies in the high northern and southern latitudes. The winter skies in Alaska, Norway, Iceland, and Siberia experience aurora borealis, whereas down above the opposite hemisphere in Antarctica and far southern New Zealand are found the aurora australis. In the long, cold winter months of both hemipsheres, skies erupt with glimmering curtains of colorful light.
What causes aurora? Why are some of them green, some blue/yellow, and some red? Which are the most common, and which are rare? How come some places see them and some don’t, except under very unusual circumstances? When and where are the best times to see them? Let’s dig into auroras and find out.
Aurora are the result of high-energy charged particles bombarding and reacting with atoms in our upper atmosphere (specifically the ionosphere) at elevations ranging from 100-400 km (60-250 miles). These particles stream out from the sun constantly as what is known as “solar wind.” During periods of heightened solar activity called “coronal mass ejections” (CMEs or solar storms), the solar wind blows a tempest, hurling giant waves of charged particles larger than the Earth itself. The polar regions catch the brunt of the solar winds due to the converging lines of the earth’s magnetic field at the poles.
Travelling at around 1,400 miles per second, the solar wind smashes into nitrogen and oxygen atoms, causing them to convert the energy of the collisions into light, shedding photons which we see as aurora. Oxygen atoms emitting photons have a yellow-to-green tint, whereas nitrogen atoms create blue light if the atoms re-capture the electrons after ionization, and red if it returns to a normal, grounded state.
If we had neon atoms in our upper atmosphere, we would have orange auroras. Light elements such as hydrogen and helium from blue and purple auroras, but our eyes cannot typically perceive these colors against the dark night sky. The reddish ionoshpere aurora are rarest, forming when oxygen atoms at 400 km above the earth shed light.
Think of the different colored aurora like different neon lights in a store window sign. Those lights work by sending high voltage electricity (electrons) through a glass vacuum tube filled with atoms of specific elements. Each element, when energized by the electricity, lets off light of a different color. So when you see “live nude girls” on your way into a strip club in Vegas and each word is a separate color, you know that each one of the tubes that forms the letters of each word has a different element inside of it.
Earth just suffered through a fairly intense solar storm the week before last. During what was the largest solar storm in 7 years, magnificent auroras were seen by skywatchers in places that are known for auroral displays such as Norway, Iceland, and Alaska. What was unusual however, was that that aurora were observed as far south as New York.
Auroras are typically found within 10-20 degrees of latitude from the earth’s poles. During solar storms, like the one we experienced last week, the electromagnetic effects reach much father south (or north in the case of aurora australis) as the intense solar wind showers the lower regions with charged ions and protons that flock to the magnetic flux lines which emanate from the poles.
CMEs cause more than just auroras in unusual locations. Satellites have to be re-positioned and powered down to protect sensitive on-board electronics. High-frequency radio communication is disrupted,and airlines re-route flights that typically traverse the earth’s north pole in an effort to both protect sensitive radio equipment on their jetliners, and also to reduce the amount of potentially cancer-causing radiation experienced by crew and passengers.
Our sun averages around 2,000 solar storms in every 11-year long solar cycle. These cycles are measured by observations of sunspots, which are regions of the sun where magnetic activity erupts and disturbs the photosphere of the sun, showering earth with aurora-causing ions. Single CME events can shoot jets of plasma four or more hundred thousand miles from the sun.
The solar cycle was first discovered by Samuel Heinrich Schwabe in 1843. In 1859 during what is now called the Carrington event, a solar flare erupted on September 1st, which interrupted telegraph service, and caused auroras as far south as Cuba and Hawai`i. In the early 2000s, the sun has been experiencing what is known as deep solar minimum, with no visible sunspots on approximately 73% of the days in 2008. We are now ramping back up into the highest part of a solar maximum, as evidenced by the storm we saw in late January.
Solar cycles have been used as evidence for and against man-made climate change by both sides of the argument. Observations do show that the global average temperature can rise as much as 0.1 degree C as a direct result of solar variability. If you want to learn more about the solar cycle / climate link, start with this informative article over at Science Daily.
If you really wanted to see some auroras, where should you go? Well, being a former volcanologist, the first place on my list is of course Iceland. This video by Norwegian photographer Terje Sørgjerd is more or less one of the most amazing, incredible pieces of timelapse photography ever done. Watch it full-screen a few times and see if you disagree.
At least, until you see this one, which is even more mind-bending, and gives a great perspective on the sheer scale of auroras.
Other than flying to Reykjavik, northern Canada and Alaska are also good locations to go aurora hunting. Target your trips for the winter months if you head north, since the nights are long. Since we are in a solar maximum for the next year or so, conditions are ripe for auroras. Just don’t count on your GPS or Sat phone to save you if you take it deep into the dark backcountry – these communications tend to be the first to break down during intense aurora-causing solar storms. If you salivate for auroroa photos, check out the work of Terje Sørgjerd, Kiddi Krisjians, and Ole Salomonsen - they are all masterful aurora shooters. Happy hunting!