It is one of nature’s most breathtaking displays and it has been visible from Polson several times already this year, but scientists with the National Oceanic and Atmospheric Administration’s (NOAA) Space Weather Prediction Center say the best shows of 2017 may be yet to come.
“I would anticipate that we will see a few more auroral displays during this cycle that will be big enough to be seen from the lower 48 states,” NOAA Space Weather Prediction Testbed Director Dr. Rodney Viereck said. “We are in the waning cycle, which is the time when some of the biggest storms occur. This, however, has been a small solar cycle compared to the previous four or five. The really big storms seem to occur during the declining phase of the cycle. In fact, if you go back through history, the four or five biggest storms we have on record (going back into the 1800’s), have all been during moderate to small size solar cycles. We could, at any time during the current small solar cycle, have one of the biggest storms we have ever observed.”
According to Viereck, the sun is currently at the point in its 11-year cycle where the solar activity is winding down, but strong auroras are still possible. So far this year, Earth has seen several geomagnetic storm events, including two in April that have been visible in Polson and further south in the northern hemisphere.
“The sun goes through an 11-year solar cycle. The really big events happen at the late part of the solar maximum, which is where we are now,” he said. “We are in the declining phase of the current solar cycle, which is a good time to see Coronal Mass Ejections and aurora activity. That’s why we have been seeing more recently and we have had a fair amount of activity over the last few years.”
The solar magnetic activity cycle is the periodic change in the sun’s activity and appearance. It has been observed since the 19th century, when scientists noticed a periodic variation in the number of sunspots visible from year to year. Recent research has determined the sun has an average solar cycle of 10.66 years, though cycles as short as eight years and as long as 14 years have been observed.
Solar maximum and solar minimum are terms that refer to the periods of the most and the fewest sunspots being visible during a solar cycle.
Sunspot frequencies and the solar cycle determine the quantity of aurora seen on Earth. Since recording began in 1755, there have been 24 solar cycles. We are currently in the second phase of the 24th recorded solar cycle, which began in January 2004.
According to Viereck, aurora activity begins with events on the sun. He says that aurora activity has two causes: charged particles launched towards Earth during a Coronal Mass Ejection (CME) or unusually strong solar winds. Viereck said that Earth has seen aurora activity caused by both sources this year.
When the super-charged particles from either source reach the Earth’s magnetosphere (CME particles can reach earth in less than a day, but usually take 1-2 days on average), that is when the light show begins.
“The particles that get accelerated out on the edges of the magnetic field get brought in towards the poles. The electrons get funneled down to the poles, and when they hit the atmosphere, they create the aurora,” Viereck said. “The bigger the storm, the brighter the aurora, which can then be seen further from the poles. The stronger storms deform the magnetic field around the poles and that’s why it can be seen further from the poles. If you have a particularly bright aurora display, you can see it 600-700 miles away.”
The magnetic activity around Earth is measured by the Kp index, which takes current electromagnetic observations from points around the Earth and compares them with that point’s historic averages. The index ranges from Kp 0 (very little activity) to Kp 9 or stronger (extreme geomagnetic storm). On average, the Earth sees 900 days where the Kp reaches 5 (minor storm) every solar cycle and four days where it reaches a 9 (extreme storm). The photos published with this article were all taken during storms with Kp levels above 6.
The different colors visible with the aurora are caused by variations in the chemistry of the atmosphere at different altitudes. The color that is visible when the aurora occurs depends on which gas the charged particles are interacting with.
At high altitude, above 200 km, the high oxygen content creates a red hue. At altitudes between 100-200 km, the combination of nitrogen and oxygen creates a green aurora, which is the most common. Auroras that occur below 100 km will be primarily blue due to the higher concentration of nitrogen.
The brightness of the aurora depends on the amount of energy being carried by the charged particles colliding with atmosphere. The more energy, the brighter the aurora and the higher the Kp index reading.
“The bigger storms tend to have a broader spectrum of particle energy, so you will get a lot of the reds. The stronger storms, that are visible further south, would have a lot of the red, tall, rays at the top,” Viereck said. “When you are further south, sometimes you can see the aurora when you look to the northern horizon and the tall, red auroral rays are what you will see.”
Viereck said the most powerful storm in recent history occurred at the end of October, 2003. Known as the Halloween 2003 storm, the Kp index moved above 9 several times over a three-day period and the aurora borealis was seen as far south as Texas and Florida and the aurora Australis was seen as far north as New Zealand. The magnetic disturbances and radiation associated with the storm knocked out satellites, disrupted power grids and prompted astronauts to seek cover in the more shielded sections of the International Space Station.
The most powerful storm on record occurred in early September, 1859, when a large solar flare caused the aurora to be visible as far south as Mexico and the Caribbean in the northern hemisphere and as far north as Papua New Guinea south of the equator. Known as the Carrington Event, it is theorized that it might have been possible for someone at high altitude near the equator to view both the aurora borealis and Australis simultaneously. The storm disrupted telegraph communications for days and even allowed some telegraph operators to continue sending and receiving messages after their equipment had been disconnected from power supplies.
As for this year’s storms, they have been moderate at best, but NOAA says the best may be yet to come.
“We have had some good storms in this cycle, even before this year, but the really big storms would be much, much, much bigger than what we have seen this year,” Viereck said. “We have been having moderate storms so far, but you never know what the next one is going to bring.”