Aurora! – Earth’s Amazing Polar Lights

Few sky sights have attracted as much attention as the colorful, undulating lights known as the aurora. They have mystified, mesmerized and moved people to speculate about their origin, behavior and effects for centuries. Driven by curiosity, scientists have gradually discovered the causes of the aurora and learned to predict their appearances.

If you have seen an aurora, you know the power they possess to want such an experience them again. If you have never observed an aurora, it is certain that having the opportunity to see them is on your bucket list! The opportunity to see auroras this year are excellent—and heightened auroral activity is expected to continue for at least another year or so!

What Are Auroras and Where Do They Come From?

Auroras are a permanent feature of Earth. They exist as a pair of luminous halos, located high above our planet’s surface in the polar regions. Earth’s twin “halos” are called the “auroral ovals. Scientists refer to the northern halo as the “Aurora Borealis” or “Northern Lights,” and its southern counterpart, as the “Auroral Australis” or “Southern Lights.”

The source of the energy that powers the northern and southern lights is the Sun. A stream of radiation and electrically charged particles, or “ions” called the “Solar Wind,” constantly flows from our nearby star. The electrically charged “ions” in the Solar Wind may be attracted or repelled by Earth’s magnetic field. Ions captured by our planet’s magnetic field, plow into the upper layers of our atmosphere, colliding with oxygen and nitrogen molecules. Reactions between solar radiation, solar wind ions and the pair of atmospheric gasses result in the emission of the aurora’s colorful glow. The production of auroral light is similar to the process that occurs in fluorescent lights.

The Solar Cycle and Northern Lights

Our daytime star, the Sun, has a magnetic personality! Approximately every 11 years, the Sun’s undergoes a cycle of internal magnetic field reversals. This “Solar Cycle” has been observed by scientists since 1755. At the start and end of each 11-year Solar Cycle, there are few sunspots and solar activity is least. These times in the solar cycle are known as “Solar Minimum.” During the middle of each Solar Cycle, the number of sunspots is greatest and other kinds of solar activity also peak. The time when the Sun is most active is called “Solar Maximum.” Knowing the timing of Solar Cycles allows scientists to forecast the Sun’s activity level—and predict auroral
displays!

During Solar Maximum, increases in the speed of the solar wind and reversals in the electrical polarity of solar ions produces solar storms. Exceptionally powerful gusts called “Solar Flares are one example of solar storms. They are localized, intense bursts of radiation blasted from the surface of the Sun. Solar Flares propelled in Earth’s direction can result in brief, intense northern light displays. “Coronal Mass Ejections” (CMEs) are much larger solar storms that hurl massive clouds of highly-charged ions into space. They produce some of the most powerful Solar Wind gusts, which in turn, result in auroral displays that can last two or more days.

Monitoring the Solar Cycle helps scientists predict the likelihood of auroral activity. Since the numbers of sunspots, solar flares and CMEs peak at the time of Solar Maximum, the two-year period surrounding Solar Maximum is the most favorable time to see auroras. This year is expected to be the time of Solar Maximum for the current Solar Cycle.

Why Are Northern Lights So Colorful and Why Do They Dance?

The color of the Northern Lights depends on the gases involved and the height at which the interactions between solar radiation and solar wind ions occurs. The most frequent color seen is green. It is produced by the ionization of oxygen molecules at altitudes between 60 and 200 miles. Red is seen less frequently because it is more difficult for us to see. It is caused by reactions involving oxygen atoms at heights of 180 to 250 miles. Blue and purple appear during times of high solar activity and result from reactions involving nitrogen at altitudes of about 60 miles.

Northern lights sometimes appear as motionless patches, although flowing, rippling and rapidly pulsating displays can also be seen when interactions between Earth’s atmosphere/magnetic field and the solar wind are particularly strong. The energy input and transfer involved in an average northern light display is incredible. Instrumented rocket flights into auroras have measured atmospheric currents as high as 50,000 volts and 20 million amperes! So, it’s not surprising that, given the right conditions, northern lights can literally “dance the night away.”

Can We See Northern Lights in Minnesota?

Although Minnesota lies 1,000 miles south of the usual haunts of the northern auroral oval, our geographic location is one of the best for seeing northern light displays in the lower 48 states! This circumstance results from the fact that the Earth’s auroral ovals are centered on our planet’s geomagnetic poles, rather than Earth’s geographic poles. The North Geomagnetic Pole is located a little over 1,000 miles from the North Geographic Pole—on Baffin Island in northern Canada. Since the location of Earth’s auroral ovals are constrained by our planet’s magnetic field, this offset means that the Twin Cities lie at a geomagnetic latitude of 53 degrees! This gives us an advantage for seeing northern lights—particularly when solar storms caused by solar flares or CMEs occur. When these storms are powerful enough, the size of the auroral oval becomes brighter, wider, and most importantly—it moves southward—closer to Minnesota!

Research conducted during the past 75 years has shown that the best times to see the northern lights are the two months closest to the spring and fall equinoxes (i.e., March/April and September/October). Auroral displays appear twice as frequently during these months compared to months near the solstices (June/July and December/January).

 

Resources

Northern Light Observing Tips
● Check aurora prediction websites including the Alaska Geophysical Institute’s Aurora Forecast, Space Weather Prediction Center, and the Planetary K-Index for expert information about incoming solar storms, the level of activity in Earth’s magnetic field, and maps showing where northern lights are expected to be visible.

● When northern lights are expected to appear, check the weather forecast to be sure the sky conditions will be favorable; then choose a dark, rural site for your observing location.

● Use your eyes to observe the northern lights. They provide superb, wide-angle views of the heavens needed to take in frequently expansive northern light displays.

● The best times to look for northern lights are usually the two-hour blocks before and after midnight. (That’s 11 pm to 3 am when we’re using daylight saving time and 10 pm to 2 am when we’re on standard time.)

● Plan to observe for at least one hour. Solar storms often arrive in alternating waves of exciting activity and quieter periods.

 

Aurora Websites
● Alaska Geophysical Institute’s Aurora Forecast

● Space Weather Prediction Center – Auroral Oval Maps

● Planetary K-Index (K-Indices above “6” are often visible at the latitude of the Twin Cities)

● Aurora Borealis Forecast & Alerts (Aurora Prediction App for Android and Apple SmartPhones)

 

Information and Graphics: Rod Nerdahl, Bell Museum Astronomy Program Volunteer