Speedy solar wind delivers spectacular auroras across Canada

Auroras above the town of Ponoka, Alberta on the night of Nov 2-3, 2015. Credit: Tree & Dar Tanner (#TeamTanner). Used with permission.

Tuesday, November 3, 2015, 11:49 AM - Were you under clear skies Monday night? Did you manage to spot the aurora? While it happened a bit later than anticipated, some were treated to a spectacular light show due to the speedy solar wind.

Although the strong geomagnetic storm predicted by NOAA space weather forecasters didn't fully manifest on Monday night, some areas across Canada and the northern US caught some beautiful views of the Aurora Borealis.

Originally forecast as a G3-level geomagnetic storm, which was expected to peak between 4 and 7 p.m. EST, what actually developed was a bit weaker and a bit later than anticipated.

Left - probability of aurora visibility for 8:20 UTC (3:20 a.m. EST) Nov 3. Right - estimated geomagnetic storm strength, each bar in graph representing a 3-hour span. Credit: NOAA SWPC

Even though things didn't work out exactly as forecast, there were still some spectacular sightings of the Northern Lights overnight, as these images snapped from southern Ontario and central Alberta demonstrate.

What was happening here?

While the spectacular St. Patrick's Day auroras seen earlier this year were touched off by an immense coronal mass ejection (CME) - a massive eruption of charged particles from the Sun, the auroras that were visible Monday night were due to a very different reason - a fast stream of solar particles pouring out of a hole in the Sun's magnetic field, which space weather forecasters call a "Coronal Hole High-speed Solar-wind Stream" or CH HSS. 

A view of the Sun's corona from November 2, 2015. Credit: NASA SDO

As shown in the image above, the dark regions are openings in the Sun's magnetic field. While the magnetic field lines in brighter regions form into long brilliant loops that reconnect with the Sun's surface, the magnetic field lines in the dark regions stretch far out into space. This allows charged particles to stream away from the surface at high speed. This stream, which is part of the solar wind, is stretched out as the Sun rotates, forming into long curved "ribbons" as seen in the animation below:

Space weather forecast animation for Monday through Tuesday (times shown in UTC), showing the alternating "ribbons" of dense, slow-moving solar matter and diffuse, fast-moving particles. Credit: NOAA SWPC

There are two types of regions in the solar wind, which sweep around the Sun in spiral waves.

Dense regions, shown in bright regions in the upper panels of the animation, are filled with an abundance of slow moving particles that generate a combined magnetic field that typically points in the same direction as Earth's magnetic field. Diffuse regions, shown in bright regions in the lower panels of the animation, are streams of fast-moving particles zipping through largely empty space, and this stream typically has a combined magnetic field that points in the opposite direction as Earth's. When these regions sweep past Earth, each has a different effect on the planet's magnetosphere. The dense, slow moving flow is simply diverted around the planet, as the magnetic fields the particles and the planet both repel each other. The diffuse, fast-moving stream can cause some disturbances in Earth's magnetic field while we're in the flow, though, since the particles directly interact with Earth's magnetosphere and ionosphere.

It's the transition through the boundary between the two regions that tends to cause the most dramatic effects, though.

This boundary - where the flow speed and magnetic field direction both change abruptly - is known as a "corotating interaction region" or CIR. When Earth encounters a CIR, as it did on Monday night, the "shock" of the transition from one region to the other stirs up the magnetic field around the planet, causing it to fluctuate, sometimes in unpredictable ways. This can dump a significant amount of energy into Earth's magnetosphere, which can result in strong geomagnetic storms.

Typically, the stronger the geomagnetic storm, the further away from the poles the resulting auroras are visible. Thus, while minor or moderate storms keep auroras mostly confined to northern regions, a strong geomagnetic storm usually delivers auroras that are visible to most of Canada and the northern United States.

How to see the Aurora Borealis

For those regions under clear skies Monday night, or at least with enough breaks in the cloud to see some of the night sky, spotting the aurora may still take a little bit of travel.

If you are in or near a large city, even though you have clear skies and can see the moon and a few stars, the light pollution in the region will very likely be too much to see anything of the aurora.

In order to see these displays, getting away from that light pollution is a necessity.

For British Columbia, the Prairie Provinces, northern or eastern Ontario, northern Quebec and in Atlantic Canada, that may be as simple as driving a short ways outside the community.

In places like southern Ontario, with a dense cluster of large cities, all adding to the light pollution problem, a bit more travel is likely involved. Regions between Windsor and London, along the eastern shores of Lake Huron, and up through the Bruce Peninsula are probably the best regions in the southwest. Driving from the western GTA or Hamilton up past Orangeville or from eastern GTA to the Kawartha Lakes area would likely provide some good viewing potential in central Ontario.

For southern Quebec or the Ottawa area, the best option is to drive north until the skies get darker.

In all cases, looking north will give the best view, however in some regions of the central and northern Prairies and the northern territories, the auroras can appear directly overhead! In the far north, you may even need to turn around and look south to see them, since the aurora usually forms a "ring" around the Arctic, and so it can actually be south of your location!

Auroras can appear and disappear suddenly, and they can be quite faint, especially the further south you are. Once you've found a good place to watch from, allow your eyes rough 30 minutes to adjust to the dark, and avoid looking at any sources of light. Even briefly glancing at an overhead light, a set of headlights, or even your phone or camera, can spoil your nightvision and force you to wait longer until your eyes adapt again.

Repeat performance?

According to NOAA's latest forecast, the effects of this corotating interaction region were expected to continue for the rest of Tuesday and into Wednesday morning. While the geomagnetic storm did not appear to reach the hoped-for G3 strength, it maintained a fairly steady level at G1 strength (Kp=5), and it spread another brilliant display of auroras across the country.

Left - probability of aurora visibility for 10:25 UTC (5:25 a.m. EST) Nov 4. Right - estimated geomagnetic storm strength, each bar in graph representing a 3-hour span. Credit: NOAA SWPC

Sources: NOAA SWPC | NASA SDO | NOAA SWPC | TeamTanner Photography

Related Video: Watch the Aurora Borealis from Camden Lake, in Camden East, Ontario, on Wednesday, October 7, 2015.