The Aurora Borealis

Northern lights above the Sjon Mountain in Northern Norway. Photo: Petter Hamnes.

For thousands of years people in the northern part of the world have marveled at the spectacular and fearful displays that occasionally light up the night sky.

There have been hundreds of stories and theories to explain these celestial lights what we now know as the aurora borealis or northern lights. But no one until about a century ago, suspected a connection with the sun.

Every northern culture has oral legends about the aurora, passed down for generations. During the viking period, northern lights were referred to as reflections from dead maidens.

The phenomenon was often referred to as a vengeful force. In ancient times, most people were afraid of the lights. Some people would not let their children outside to play while there were auroras, fearful they could get
killed.

The aurora has many names

The sami (lapp) people called it guovssahas, the light you can hear. The eskimos in the northernmost parts of Canada believed that the northern lights were created by spirits, which, dressed in the mystical light, were having fun because the sun is away, that they were playing soccer with a walrus skull. The rapidly moving auroras were called the dance of death.

The Vikings who lived in Norway a thousand years ago, named it the northern lights. In Norway children were often told that by waving with white clothing, the intensity of waving increased the motion of the aurora!

Early science

The strong aurora on March 6, 1716 could be observed in large parts of Europe and gave birth to more modern science. Sir Edmund Halley published the first detailed description of the aurora in that year.

He lamented that at the age of 60 years he had given up on experiencing this amazing phenomenon. He suggested that “auroral rays are due to the particles, which are affected by the magnetic field; the rays are parallel to earth’s magnetic field.”

Kristian Birkeland

A major breakthrough was made by an eccentric norwegian scientist – Kristian Birkeland, 1867-1917—who had a theory that charged particles from the sun could ignite auroras. To prove his theory-which is still valid today - he built his own world in a glass box, electrified his model earth with its own magnetic field and showed how particles from the sun could ignite auroras.

Kristian Birkeland as depicted on the Norwegian 200 kroner bill.

The particles were captured by the earth’s magnetic field and channeled down towards the polar regions. He also showed that they would be identical and simultaneous at both poles. Birkeland, arguably Norway’s greatest scientist, also established the first permanent aurora observatory.

He indicated the existent of the solar wind and the earth’s magnetosphere and studied the properties of comet
tails. Many of these ideas were not confirmed before after the space age some 60 years later.

The electrical currents he described in the upper atmosphere are still called Birkeland currents.

Other pioneers

Two other noteworthy Norwegians in this area were Lars Vegard, the first scientist to map the colors of the aurora, and Carl Størmer, who continued where Birkeland left off and calculated that there is a belt-like area
around the earth in which particles are reflected to and from between the poles.

Kristian Birkeland and his terrella eksperiment at the University of Oslo.

Verification of this region came years later on the basis of satellite measurements made by the american James van Allen. Størmer also calculated the height of the northern lights to be 80-130 kilometers.Today we know it typically extends from about 80-100 km to 250 km and on rare occasions up to 500-800 km.Thus, the aurora is not a weather phenomenon, almost all weather occurs in the first 16 km of the atmosphere.

The colors of the northern lights

The brightest color in the aurora is the green or yellowish-green. However, the scientific name for the phenomena is aurora borealis, which is Latin and translates into the “red dawn of the north”. Its counterpart is the aurora australis, or southern lights.

It was the Italian scientist Galileo Galilei (1564-1642) who first used the expression. When the aurora borealis is extremely active it movesfarther south. On the latitude where Galileo was living, northern lights are mainly red in color.

Auroral light occurs when atoms and molecules of gases in the upper atmosphere are struck by high-energy electrons. Different gases give off different colors of light when excited.

The colors of the aurora come from oxygen and nitrogen gas. The aurora is made up of blue, green, and red light. The highest part of the auroral curtain is red, the middle is greenish-white and the lower edge is pink.

The Aurora - a message from the Sun

The source of the aurora is the sun. The sun provides energy to all life on earth and drives the climate system and is vital to our very existence. It powers photosynthesis in plants and is the ultimate source of all food and fossil fuel.

However, the sun gives us more than just a steady stream of warmth and light. Situated 150 million kilometers away from us, the sun is a huge thermonuclear reactor, fusing hydrogen atoms into helium and producing million-degree temperatures and intense magnetic fields.

Southern lights as seen from the space shuttle Discovery. Photo: NASA.

 Near the surface, the sun is like a pot of boiling water, with bubbles of hot electrified gas. The steady stream of particles blowing away from the sun is known as the solar wind.Blustering at 1.5 million kilometers per hour, the solar wind carries a million tons of matter into space every second (that’s the mass of Utah’s Great Salt Lake).

Solar activity

Every 11 years the sun undergoes a period of activity called the “solar maximum,” followed about five years later by a period of quiet called the “solar minimum.” During solar maximum there are many sunspots, and
during solar minimum there are few. Thus, one way of tracking solar activity is by observing
the number of sunspots.

Sunspots are dark patches like freckles on the solar surface formed when magnetic field lines just below the sun’s surface are twisted and poke through the solar surface. Sunspots can last from a few hours to several months, and a large sunspot can grow to several times the size of the earth.

Why do scientists care about sunspots? Because they are visible signs of the turmoil inside the sun that lead to space weather effects on earth.

Coranal mass ejections

Coronal mass ejections (CMEs) and solar flares are often associated with sunspot groups. The twisted magnetic field above sunspots are sites where solar flares are frequently observed to occur.

Solar flares are short, intense explosions that accelerate particles and intense X-ray radiation into space. Energy equal to a billion megatons of TNT is released. CMEs, caused by temporary breaks in the magnetic controlling
field lines, are much larger storms that thrust billions of tons of particles (a mass equal to that of 100,000 battleships) at speeds up to 8 million kilometer per hour!

A CME blasting off the Sunʼs surface in the direction of Ea CME blast and subsequent impact at Earth. Photo: NASA.

During solar maximum, CMEs and flares can occur several times per day with some of them aimed in the earth’s direction. Fortunately, our planet is protected from the harmful effects of the radiation and the hot plasmaby our atmosphere and by an invisible magnetic shell known as the magnetosphere.

Magnetic shield

Produced as a result of the Earth’s own internal magnetic field, the magnetosphere shields us from most of the sun’s particles by deflecting them around the earth.

Auroras are caused by CMEs or gusts in the solar wind that will push on our magnetosphere so that particles inside the magnetosphere are injected into the earths upper atmosphere where they collide with oxygen and
nitrogen.

These collisions, which usually take place between 60–300 km above ground, cause the oxygen and nitrogen to become electrically excited and to emit light (fluorescent lights and televisions works in much the same way). The result is a dazzling dance of green, blue, white, and red light in the sky.

Sun-Earth Connection

The auroras never hurt a sailor or a farmer. It is only with our modern electrical, electronic and space technologies that the storms from the sun become damaging, and even personally hazardous for astronauts.

The more we do in space and the more we depend on modern electronics, the more serious and potentially
costly the problems will become.

Storms on the sun can interfere with systems on earth that our society depends upon.

The Sun observed in visible light showing dark sunspots in the photosphere. Photo: SOHO/ESA/NASA.

Norway, as we have seen, has a long tradition in studying the sun and the aurora due to the fact that the sun interact with the earth system.

This field of science is often called the sunearth connection. Today, after the start of the space age, scientists are utilizing space borne instruments and cameras to study the aurora.

A sounding rocket range was established on the north Norwegian island of Andøya. It is the northernmost permanent rocket launch facility in the world and the first rocket launch there took place in 1962. More than 700 rockets have been launched, many of them NASA rockets.

Space weather and aurora forecasts

By monitoring the activity on the Sun and measuring the speed of the solar wind particles one can predict the strength and the location of the aurora.

By adding prediction of weather and clear sky it will provide a useful tool for aurora hunters and tourists.

Norway has long traditions in studies of the aurora and space wether since we are located right beneath the auroral zone. For this reason there is a large number of aurora and space weather instruments currently operated in northern Norway and on the Svalbard island in the Arctic.

Rocket launch from Andøya Rocket Range in 2006. Foto: Abrahamsen/ARR.

This includes among others the facilities at Andøya Rocket Range, the Eiscat radars in Tromsø and at Svalbard, an extensive network of magnetometers and the newly opened aurora observatory - Kjell Henriksen Observatory (KHO) at Svalbard.

Watching the space weather

For Norway a better understanding and operational monitoring of the Sun and SpaceWeather is important for many reasons. Power grid companies need to be alerted about solas storms that can induce strong currents in the power grid and cause damage to the system.

Solar storms can degrade navgation systems such as GPS. Thus, we are monitoring the ionosphere using geodetic GPS reference stations to allow for correction for solar induced erros in the GPS signal.

Norway will be especially interested in the rapid ionospheric changes affecting navigation accuracy over the large ocean areas in the Norwegian Sea and the Barents Sea. In fact

Norway has responsibility for issuing navigation accuracy warnings to seafarers in these areas. Dynamic positioning of oil drilling ships/platforms, directional drilling, radiocommunication, and helicopter operations in the polar night have especially strong needs for space weather information.

 

This article is an edited version of a much longer text published in Scandinavian Review Vol. 93 no. 3.

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About the Author:

Pål Brekke, a solar scientist, is senior adviser for space science coordination at the Norwegian Space Center. He is the former ESA Deputy Project Scientist for Solar and Heliospheric Observatory (SOHO) at NASA Goddard
Space Flight Center