The ultimate guide to observing the aurora

All you need to know about one of the night sky’s most spectacular sights
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Image Credit: 
Paul Money

All you need to know about one of the night sky’s most spectacular sights



There’s something about the Northern Lights that inspires a sense of wonder in anyone. In February 2007, I accompanied a group of people on a short trip to Tromsø in northern Norway to experience a day in the Arctic. The highlight was being taken to the top of Mount Storsteinen just outside the city, to a height of 420m. Here, standing in a –21°C wind-chilled snow field under clear skies, I found myself in a huddled group of people waiting excitedly to see if the Northern Lights would grace us with their presence.

We didn’t have long to wait. Three bright main bands of aurora appeared: one north, one overhead and one south, unceremoniously slicing Orion in two. Like giant swirls of green-tinted cloud, the aurora snaked its way across the star-laden sky. Then, in an instant, the whole display came alive, dancing and writhing so fast it was hard to keep up with it. A reddish hue appeared to line the edge of some of the swirls, and other structures such as rays and curtains could be seen. The display was truly awesome and quite humbling. The Northern Lights, also called the Aurora Borealis, occurs when the Earth’s magnetic field interacts with the solar wind – a constant stream of charged particles emanating from the Sun. The Sun has a magnetic field and the state of this field – its strength and north/south polarity – is effectively ‘locked into’ the solar wind as it sets off. The state of this interplanetary magnetic field (IMF) in part determines whether or not you’ll see the aurora.

Green is the colour

The solar wind has an average speed of 400km/s, and it squashes the Earth’s magnetic field on the sunward side. On the other side, the magnetic field is elongated, forming a huge magnetotail. Within the magnetotail lies a giant sheet of plasma (ionised gas), which is constantly being topped up with particles from the solar wind. Interactions between the Earth’s magnetic field and the solar wind cause particles from the plasma sheet to accelerate back towards the Earth. They head down converging lines of magnetic force at the magnetic poles.

These lines pass through our atmosphere in a ring around each magnetic pole. But as the Earth’s magnetic field is deformed, each ring appears as an oval, elongated away from the Sun. The plasma interacts with the atoms in our atmosphere, causing them to emit radiation, and the visible radiation is seen from the ground as the aurora. The main colours seen are green and, to a lesser extent, red, both due to the excitation of atmospheric oxygen. Blues and purples, caused by the excitation of ionised nitrogen, also occur.

The Earth’s two auroral ovals produce the Aurora Borealis in the northern hemisphere and the Aurora Australis in the southern hemisphere. Of the two, the Aurora Borealis is better known simply because it passes over more populated areas. If penguins could talk then the Aurora Australis would be just as famous. Under certain conditions, the ever-present auroral ovals can be enhanced. If the IMF’s vertical magnetic field component (Bz) points south, reconnection with the Earth’s magnetic field can occur, which can lead to auroral substorms. These typically show enhanced activity lasting anywhere from 1-3 hours and occur four or five times a day.

Shock tactics

Eruptions of activity can also occur if active regions on the Sun result in coronal mass ejections (CMEs) – vast clouds of plasma that head towards Earth. The CME shockwave arrives some hours or days later, and when it reaches the edge of Earth’s magnetic field a ‘sudden storm commencement’ (SSC) occurs (and is usually detected by the world’s magnetic observatories within four minutes). If Bz points south for at least six hours following the SSC, a geomagnetic storm may occur, resulting in a dramatic aurora. This may last for several days, over which time the auroral ovals may broaden and extend towards the equator. If this happens, the aurora may be visible from the entire UK.

If it looks like there’s going to be a sighting, one tip is to aim a digital camera at a dark northern horizon, put it on maximum sensitivity (high ISO and maximum aperture) and take some long exposures, lasting for several seconds or even minutes. If you catch anything with a greenish tint then it may be worth heading for the darkest site you can get to. It’s also worth noting that the optimum time to see the aurora is local midnight (ignoring daylight saving).

Geomagnetic storms generally occur in the most active years of the Sun’s 11-year activity cycle. At the moment (March 2008) we are at solar minimum and the probability of such storms is low. But this is set to change as we head towards the next maximum in 2011 or 2012. At the time of solar minimum, the predominant feed for the aurora comes from coronal holes – these are gaps in the Sun’s magnetic field where the field lines are open, allowing outpourings of plasma into the Solar System. In addition, around the time of the equinoxes (late March and September), the Earth’s magnetic field lines up particularly well with Bz. If Bz turns south at these times, the door really opens and strong displays are likely to occur.

The best time of year to see the Northern Lights is late autumn to early spring, because this is when the higher latitudes experience darkness. Unless you’re prepared to wait for a geomagnetic storm, you’ll need to head north to get close to the auroral oval. It’s a good idea to check out the most likely weather for your chosen destination and organise your trip for when the skies are likely to be dark and clear, and the Moon out of the way. The chances of seeing an impressive display are better during solar maximum, but auroral substorms or coronal holes may still produce dramatic aurorae during solar minimum. The biggest enemy is the weather, though, because a thick blanket of cloud will hide everything.

See for yourself

Chartered flights to see the Northern Lights offer the advantage of getting close to the auroral oval without clouds interfering with the view. On board a blacked-out plane you’ll get to see the sky as you’ve probably never seen it before. You’ll also have the advantage of seeing a large expanse of the auroral oval on the horizon.

As the aurora is a natural phenomenon, there is no guarantee of seeing it. Even if you’re located in an optimal location and the sky is dark and clear, you may still be unlucky and see nothing at all. Sometimes it appears as a quiescent band glowing in the sky, quite devoid of any structure; at other times, all hell breaks loose and the display can be breathtaking. Even under a blank sky it’s worth being patient, as the aurora can often appear in an instant… and disappear just as fast. Both cloud and a bright Moon will affect a display’s visibility, but a bright aurora can overcome even these.

If this feature has whetted your appetite to see the aurora, there’s still time to do so this year. Northern Lights flights or day trips to Tromsø may still be available (see below for details), although they do book up fast. If this is too short notice for you, then the next aurora chasing season will begin in the late autumn 2008 through to spring 2009.
For me, the memory of the brilliant aurora seen above Tromsø was unforgettable. I may have seen an awesome display once, but I can’t wait to see it again.

The four types of aurora

Arc
This is a glowing east-west arc with a defined lower edge. A ‘rayed arc’ is an arc containing rays

Band
Similar to an arc, but lacking the regular shape.
A ‘rayed band’ (seen here) is a band containing rays

Patch

Patches are bright areas of aurora, without any distinct edges or clearly defined shape

Rays
Auroral rays resemble searchlight beams. They can appear from below to converge, forming a corona

How to predict the aurora

Step 1
Get an overview of current solar and auroral activity by visiting Spaceweather (www.spaceweather.com). Here you’ll find the state of the auroral oval (red is good), the value of the worldwide geomagnetic activity index (Kp) and the state of Bz. A Kp value of more than five indicates storm conditions, and with Bz pointing south, things can get interesting.

Step 2
Now go to the University of Alaska Fairbanks’ Geophysical Institute (www.gedds.alaska.edu/AuroraForecast/) website to see the latest forecast – click the maps to change location. The site  often gives forecasts for future activity for several weeks ahead, and gives a clear 10-point activity scale.

Step 3
At the Polar Operational Environmental Satellite site (www.swpc.noaa.gov/pmap/), close to real-time activity plots indicate the current state of the auroral oval. These give you the satellite’s interpretation of the position and intensity of the auroral oval over a map of Earth’s high latitude regions.

Three ways to see the aurora

Northern Lights flights
Flights operated by Omega Holidays
(www.omega-holidays.com) are available in the winter and spring months from various location around the UK. The format of the flights includes an hour-long pre-flight presentation on the night sky and the aurora, followed by a three-hour flight that normally takes you up to the border of Icelandic airspace. You can see examples of the view from these flights at www.auroraflights.co.uk.

Short breaks
Numerous companies offer ground-based trips to locations where you may see the aurora. Arctic Discovery (www.arcticdiscovery.com/
northern-lights.php) offers four-day (three-night) trips to Swedish Lapland while Omega Holidays (www.omega-holidays.com) provides two-day (one-night) trips to Finnish Lapland and day trips to Tromsø. If you fancy going further afield, Taber Holidays (www.taber.co.uk) offers longer trips to Greenland.

Cruises
Probably the best-known Northern Lights cruise operator is Hurtigruten (www.hurtigruten.co.uk/northern_lights.asp), which provides astronomy cruises around the fjords of northern Norway, which last for several days. The usual itinerary involves a return flight to your cruise departure port, which is included in the price of the package. You can also book cruises with Blue Water Holidays (www.cruisingholidays.co.uk/norway/).

Find out more

BAA Aurora Section
www.britastro.org/aurora/

AuroraWatch
www.dcs.lancs.ac.uk/iono/aurorawatch/

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