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Clear skies on Wednesday night? Head outside and away from bright city lights and you may just catch a spectacular view of the Northern Lights.
OUT OF THIS WORLD | Earth, Space And The Stuff In Between - a daily journey through weather, space and science with meteorologist/science writer Scott Sutherland

Solar wind set to spark impressive auroras Wednesday night

Scott Sutherland
Meteorologist/Science Writer

Tuesday, October 6, 2015, 5:05 PM - Clear skies on Wednesday night? Head outside and away from bright city lights and you may just catch a spectacular view of the Northern Lights.

Back on September 12, the solar wind set off a brilliant display of the aurora borealis that was seen in many regions across Canada. Now, that same streamer of the solar wind is sweeping past Earth again.

According to NOAA's Space Weather Prediction Center, this actually touched off a moderate (G2) geomagnetic storm earlier than originally expected. The storm, along with the accompanying auroral display, spread across most of Canada early Wednesday morning rather than waiting until Wednesday evening.

Projected aurora conditions for 3:45 a.m. ET, Wednesday, October 7, 2015. Credit: NOAA's Space Weather Prediction Center

The updated forecast is now calling for a minor, G1, geomagnetic storm starting around 8 p.m. ET Wednesday, which will ramp up to a moderate (G2) storm by 11 p.m. ET, then subside to a G1 storm again from around 2 a.m. to 5 a.m. ET Thursday.

That favours the eastern half of Canada for the best viewing during the strongest parts of the geomagnetic storm, however, some of the best auroral displays seen in the past have actually occurred after the peak of the storm. Thus, viewers in the Prairies, western Canada and northern Canada - those will clear skies, at least - should certainly not let this opportunity pass by without checking it out for themselves.

Current weather systems, as of Wednesday morning, across Canada.

What's going on here?

The face of the Sun is pretty quiet right now, with just one - albeit Earth-sized - sunspot tracking towards its western limb. Otherwise all seems calm, however, appearances can be deceiving, as other views of the Sun, courtesy NASA's Solar Dynamics Observatory, reveal (below):

An HMI Intensitygram (left), AIA 193 Angstrom filter image (centre) and AIA 211, 193, 171 Angstrom composite (right), from around 3 p.m. ET, Tuesday, Oct 6, 2015. Credit: NASA SDO

The left image, above, shows the Sun's outermost shell, the photosphere. This is useful when tracking sunspots, which travel from left to right across this view. The centre image is taken using a filter that captures the sun's hot corona, and the right image is a composite that combines the centre view with two other filtered images, to further highlight activity in the corona.

The photosphere is showing very little activity, making it unlikely that we'll have any major solar flares, and thus coronal mass ejections. These coronal mass ejections (CMEs) are one of the main causes of geomagnetic storms, as the thick ribbons of solar matter that get thrown out into space wash across Earth's magnetic field. Plenty of that solar matter gets trapped in the field lines, building up until it causes a backlash, sending those particles streaming down into the atmosphere near the north and south poles.

However, while density of solar particles is certainly one way to touch off a geomagnetic storm, it's not the only way. Speed can make up for density.

Thus, when a coronal hole opens up on the Sun, as shown by the dark regions in the centre and right images above, the fast-moving charged particles that streaming out of those holes speed along on the journey between the Sun and Earth. When they reach Earth, they hit the planet's magnetic field with enough velocity that they still transfer a lot of energy, as much or possibly even more than what's delivered by a CME impact, and thus the geomagnetic storm is sparked.

Solar wind prediction animation, showing the Sun (yellow dot), Earth (green dot), and the STEREO spacecraft (red and blue dots). The circular fields are viewing the system from "top down" perspective, and the slices view from the trailing position in Earth's orbit. Times in UTC.

Credit: NOAA Space Weather Prediction Center. 

As the top field in the animation shows, the Earth (the green dot) is in a region of very low density plasma, so space around the planet is nearly empty of solar particles. However, as the bottom field reveals, the particles that are there are moving outward from the Sun very fast.

Note, as well, the much more orange/red region sweeping through on the other side of the Sun from the Earth. That stream of the solar wind was the same one that delivered the strong (G3) geomagnetic storm back around September 20, along with the resulting displays of the northern lights. So, stay tuned later this month, as that particular band of the solar wind comes back around for another pass.

Best viewing conditions

To view the aurora borealis, it's best to be under clear, dark skies.

However, "clear, dark skies" means not only having no cloud above to block the view, but also being as far away from city lights as possible.

Unfortunately, the light pollution thrown off by large cities - Toronto, Hamilton, Montreal, etc - simply overwhelms these sometimes subtle displays and can even wash out more vibrant ones as well.

Anyone wishing to check out the show for themselves can try one of the various Dark Sky Preserves across the country, however simply piling in the car and driving a distance out of the city can still yield results.

Sources: NOAA SWPC | RASC | Darksitefinder.com

To see the effects of light pollution, and possibly pick a good place to drive to see the show, check out the embedded interactive Dark Site Finder map below:

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