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The Aurora Borealis Explained

Guest astronomer, Professor Don Kurtz, retells his journey aboard the Golden Eagle Arctic Explorer journey earlier this year and answers the question, “what causes the Aurora Borealis?” in his fascinating synopsis of the Sun-Earth connection, the Earth’s special place in the solar system and the beauty of galaxies and planets.

St Petersburg

St Petersburg: what an evocative name! My wife, June, and I had wanted for decades to see this fabled city, so when Golden Eagle Luxury Trains invited me to be the astronomical guide for the “Arctic Explorer – Quest for the Aurora Borealis” journey in February 2017, I said yes with alacrity. We dreamed of the classic architecture of St Petersburg, a performance at the Mariinsky Theatre, the spectacle of the northern lights, days gazing at the snowy lands north of the Arctic Circle, dog sleds and huskies, a drink in the Ice Bar of the famous Snow Hotel in Kirkenes, and of Moscow and the Kremlin. The Golden Eagle experience was way beyond our expectations, Russia and the Russians were charming eye-openers for us, and the fine dining was of the highest standard throughout, both on and off the train.

The only disappointment was that the cloudy skies that produced such beautiful snow flurries for us kept the aurora borealis hidden throughout our time north of the arctic circle, although we did see the green glow of an aurora low on the northern horizon one night in a thin gap in the clouds. On the first day on the train (between St Petersburg and the arctic circle), I gave a lecture on how the Sun and Earth interact to produce aurorae. Here is a synopsis for reference for the coming few years for this journey in search of the northern lights.

Spit of Vasilyevsky Island in St. Petersburg, Russia
Spit of Vasilyevsky Island in St. Petersburg, Russia

The Sun today: Hot with storms — The Sun-Earth Connection

Magnetic fields play a large role in stars. In our own Sun “weather” twists and churns the magnetic field to release stupendous amounts of energy, guiding the solar wind, and occasionally blasting Earth with mass ejections. In this first talk we looked at the surface of our Sun in detail to see sunspots, prominences, flares, coronal mass ejections and other magnetic phenomena: Solar weather. The wind from the Sun and ejections from its surface have significant impact on Earth. A big flare on the Sun produces more power than 10 million volcanic explosions the size of Tambora or Krakatoa. Rare blasts of material can bombard the Earth producing power failures, radio communication disruption and spectacular aurorae. They are potential death for astronauts on missions beyond the van Allen belts, e.g. on the way to the Moon or Mars.

When the Sun is not blasting the Earth with a flare or coronal mass ejection, there is still a variable wind, mostly of hydrogen, blowing off the surface of the Sun. That is trapped in the Earth’s magnetic field as charged particles – protons and electrons – that are moving so fast that the electrons bounce back-and-forth between the Earth’s northern and southern magnetic poles in only one second! The wind from the Sun drags the Earth’s magnetic field out into a long tail in the direction opposite the Sun (which is the Earth’s night side). Occasionally, as the magnetic field lines blow in the wind from the Sun, they “reconnect”; this is similar to a short-circuit in electrical wires. The magnetic reconnection is like a released spring; a huge amount of energy accelerates the electrons, which then ram into the high atmosphere of the Earth causing the most common gases, Nitrogen and Oxygen, to glow. That is what produces most aurorae.

During the Golden Eagle journey I kept a close watch on the space weather predictions for the aurorae. There are often 10 to 20 gigawatts of power in the particles trapped in the Earth’s magnetic field. That is equivalent to the power produced by 20 to 40 coal-fired power plants, which typically generate 500 megawatts (or half a gigawatt) each. The state of the Earth’s magnetic field and the likelihood of auroral displays is monitored by a number called the planetary Kp index. When that is 4 or greater there is a good chance of auroral displays. The US National Oceanographic and Atmospheric Administration (NOAA) has a web site which gives the values of the Kp index for 3 days on the main page, and for up to 27 days under its forecasts, so that the chances for viewing the northern lights can be estimated. There is even a good animation of the best chances for aurorae.

See: http://www.swpc.noaa.gov

Click on the box that says aurora, which shows the green auroral oval surrounding the Earth’s north magnetic pole, then start the animation with the arrow and it is easy to see what the prediction is for the aurora over the coming day. The far northern reaches of Russia near Murmansk, and Norway near Kirkenes, are some of the best places to see the northern lights. Of course, it is also necessary for the sky to be clear, since the aurorae are typically formed hundreds of kilometres up in the Earth’s atmosphere, way above any clouds, which therefore block the view when they are around.

Kirkenes – in search of the aurora borealis

While we did not see the northern lights this time, the attractions of Kirkenes kept everyone enthralled. While some of the party were out in the night for hours on dog sleds with beautiful snow flurries, many of the rest of us were toasting each other with Russian vodka in the Ice Bar of the Snow Hotel, which is kept at a constant -5 C to preserve the sculptures and the ‘furniture’, all of which are carved from snow and ice. We were pleased to have furs on the ice chairs, and for those who stayed overnight there were warm sleeping bags on mattresses in rooms that were also sculpted of ice and snow.

Professor Donald Kurtz and his wife
Professor Donald Kurtz and his wife at the Kirkenes Snow Hotel

Between Fire and Ice – the Earth’s Special Place

For my second lecture at our hotel in Kirkenes we looked at the Earth’s perfect position in our solar system, caught between the inferno of the Sun and the frozen moons and dwarf planets out at Pluto and beyond. We looked at monster hurricanes, canyons and volcanoes, weird and wonderful moons, crashing comets, and I talked about the death of the dinosaurs and the ultimate “heat death” of the Earth as the Sun swells to be a  “Red Giant” star billions of years in the future. On occasion, the clouds permitted us to see Venus at its most brilliant in the evening sky, with much fainter red Mars not far away.


Murmansk was surprising for June and me. During the Soviet era this was a closed city, not just to tourists, but to most Russians, too. For us the very name suggests an inaccessible place of mystery. Because of its remoteness and the harshness of the winters, there is a large turnover in the population each year, with perhaps 100,000 people moving in and out. We had a tour of the first nuclear-powered icebreaker, the Lenin, and our guide was outstanding (as were all the guides throughout the journey). This gave a glimpse of the world of sailors who smash through sea ice up to 3 metres thick with commercial ships following in their wake, keeping the arctic sea lanes open through the long far-northern winter. There were moving references to the part played by Allied forces in keeping the food supply lines open to Murmansk during World War II through the Arctic Convoys.Murmansk Bay

From a hill above the city we visited the monumental Alyosha, a 35-m high statue of a “Defender of the Soviet Arctic during the Great Patriotic War (WWII)”. From there we looked down onto the partially frozen sea to a lovely “glitter path” of sunlight across the bay, seen in this picture.

If the sea were perfectly flat, it would act as a mirror, and one would see a single image of the Sun reflected. The path of light across the water that we do see is caused by the myriad waves that tip up at different angles, with each wave providing a reflection of the Sun, if the tip is just right. The path is wider for choppier seas, and narrower for calm ones. So we can see in the picture that on our day in Murmansk, there were plenty of waves out in the sea.

The far north offers excellent chances to see sun dogs and ice crystal haloes, sun pillars, glories, fogbows, moonbows and many other atmospheric phenomena. We even watched for the “green flash” on the sunset from the church of the Saviour-on-the-Water. While no green flash appeared that day, the extreme slowness of the sunset at such high latitude was evident; with so many layers in the atmosphere the sunset tantalisingly kept popping back up again during the last long minute, while we waited for a possible green flash, but in vain.

The Beauty of Galaxies

For my third lecture on the train, I showed the most beautiful pictures of the Universe taken with giant ground-based telescopes, from the Hubble Space Telescope, and from the infrared Herschel Space Telescope – mind-boggling pictures that are stunning natural works of art. We looked at the birth and death of stars, stars exploding, swirling gas clouds, stars made of solid “diamond”, galaxies in unprecedented detail, gas jets shooting out of giant black holes at nearly the speed of light, crashing and cannibalistic galaxies, and the deepest-space, most distant photograph ever taken, looking back more than 10 billion years in time, nearly to the birth of the Universe.

With clear night-time skies in the far north, the stars hardly rise or set – most of them just go in circles about the sky. Polaris – the north star – is always at an angle to the horizon equal to the latitude of the observer, so for us near 70 degrees north latitude, Polaris was high in the sky, along with the familiar “Big Dipper” or Plough, the constellation of Ursa Major, the Great Bear, a potent symbol of Russia. By the way, much to our surprise, one of our guides explained to us that the bear is seen as a somewhat genial animal in Russia, with many folk tales of its bumbling behaviour. This is very different from the western view we were raised with of the Russian Bear as a symbol of strength and power. Likewise, we were intrigued to be told by our guides that the “red” of Red Square, the Red Star of Russia, the Red Army, is a colour symbolising beauty. In Russian, red means beautiful.


Planets and Pulsations: The New Keplerian Revolution

One of the biggest questions humans can ask is, “Are we alone?” Does Earth harbour the only life in the universe? Nearly everyone has an opinion on this question, but, as a scientist, I want to know. For my final lecture on the train I talked about our first step in finding the answer to this deep question – our search for other planets like the Earth, planets with rocky surfaces and liquid water where conditions are similar to our home. The Kepler Space Mission has done this. With the discovery of nearly 6000 probable exoplanets, and 3600 certain planets, Kepler has revolutionised our view of what solar systems look like. It has found 600 entire solar systems orbiting other stars and it has found planets orbiting double stars: Yes, Luke Skywalker’s fictional home planet Tatooine with its double sun really does exist out there. The Kepler mission measured the brightnesses of nearly 200,000 stars for four years, giving us a view of the stars 100 times more precise than is possible from the ground. As a result, a jewel box of exotic stars has been discovered: Heartbeat stars; beaming binaries; stars with killer storms; and a symphony of sounds in stars that let us see to the nuclear furnaces in their very cores. The technique to do that is a new research field now known as “asteroseismology”, and I am a co-author of the fundamental textbook in this field, so for the last lecture I was also discussing my own research. The Kepler space telescope has been so successful, that work I did for over 20 years, with more than 2000 nights at the telescope, is now obsolete. I could not be happier. Almost every star we look at with Kepler and other space missions gives new discoveries.

I had to hold my tongue for this last lecture. I was fully aware that NASA was about to announce the discovery of a cool red dwarf star, Trappist-1, with seven Earth-like planets orbiting it, three of which are in the “habitable zone” where water may be liquid. Here is a NASA artist’s impression of the possible view from the outermost planet. There are thousands of news reports all around the world on this discovery.

Red dwarf star
Photo credit: NASA – Red Dwarf Star


This great discovery was under a news embargo, which is why I could not mention it on the train, just a few days before the announcement. I knew about it because one of the authors of the discovery paper, Dr Daniel Holdsworth, currently holds a postdoctoral fellowship with me at the University of Central Lancashire. He, along with many of the other authors, spent the day of the press release giving TV and radio interviews. I hope all of the passengers from our Golden Eagle journey have seen this news. It even generated a Google opening page cartoon.


Petrozavodsk, Moscow, Vladimir, Suzdal

Our journey took us to villages and cities of beauty and history. We had special concerts and folk performances, a sleigh ride, and, as always, gourmet meals. My memory from childhood, growing up in the US in the 50s and 60s, was of images of Red Square with ballistic missiles and strutting soldiers marching in parade – a show of the power of the Soviet Union. What a delight to get to Red Square to see the unreal beauty of St Basil’s Cathedral, and to find not soldiers and missiles on the square, but the lights of the architectural gem of GUM, and on the square … a funfair and ice-skating rink!

For June and me, this Golden Eagle journey opened a new view on parts of the world that previously were mysterious to us. We found all the passengers congenial and interesting, and we dined with nearly everyone for at least one meal. The tour guide, Tatiana, was a real star. Any mention of a desire within her earshot, and it happened quickly – except for the northern lights, which were beyond even her ability to make things happen!!! The aurorae would have been a bonus; but this was a “quest”, and, as is often the case, the journey and the people were the rewards.

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