Auroras explained

Here at Iceland Aurora, we love enjoying the incredible visual displays in the night skies, but understanding how auroras happen makes us even more thankful to experience such a show. We hope these diagrams will help you understand a bit better and hopefuly enjoy the film with an increased sense of awe.

Sunspots Frequencies

Diagram showing the sunspots frequency over last couple of years

The solar magnetic activity cycle is the periodic change in the Sun’s activity and appearance. It has been observed by changes in the sun’s appearance and by changes seen on Earth, such as the Auroras. In the 19th century astronomers started noticing a periodic variation in the average number of sunspots seen from year to year on the solar disk. Recent researches have determined as having average length of 10.66 years. Cycles as short as 8 years and as long as 14 years have been observed. Significant variations in amplitude also occur. Solar maximum and solar minimum refer respectively to epochs of maximum and minimum sunspot counts. Individual sunspot cycles are partitioned from one minimum to the next.

Sunspots frequencies and the solar cycle determine the quantity of Aurora seen on earth. Since recordings began in 1755 there have been 24 solar cycles and we are now witnessing the second phase of the 24th solar cycle which began on January 4, 2008.

In the period between 1645 to 1715 there was an unusually long period of low sunspot frequency. This time period is known as the Maunder Minimum. Many astronomers have predicted that we are now heading to such an event as the solar cycles have been decreasing in sunspot activity since its peak in 1906.

Solar wind

Diagram showing the path of solar winds

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity forming areas of reduced surface temperature. They usually appear as pairs, with each sunspot having the opposite magnetic pole to the other. Although they are at temperatures of roughly 2,700–4,200 °C, the contrast with the surrounding material at about 5,500 °C leaves them clearly visible as dark spots. Sunspots expand and contract as they move across the surface of the Sun and can be as small as 16 kilometers and as large as 160,000 kilometers in diameter, making the larger ones visible from Earth without the aid of a telescope.

Solar flares occur when accelerated charged particles, mainly electrons, interact with plasma on the suns surface. The energy released in these events can reach the equivalent up to 160,000,000,000 megatons of TNT.

The solar flares eject clouds of electrons, ions and atoms which, if they have enough energy, escape the sun’s gravity and flow outward supersonically to great distances. These clouds are called solar winds and they typically reach Earth a day or two after the event traveling at average speeds of 400 km/s and fast solar winds can reach average speeds of 750 km/s.

Earth’s magnetic field

Diagram showing Earth's magnetic field

Earth’s magnetic field, also known as the geomagnetic field, creates a so called magnetosphere that extends from the Earth’s interior to a region above the ionosphere and extends several tens of thousands of kilometers into space. It protecting the Earth from the charged particles (electrons) of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that protects the Earth from harmful ultraviolet radiation.
The magnetic field is believed to be generated by electric currents in the conductive material of Earths core, created by convection currents due to heat escaping from the core. The geomagnetic poles (north and south) are antipodal points where the axis of a best-fitting dipole intersects the Earth’s surface.

It is at the geomagnetic poles, predominantly high latitude, Arctic (north) and Antarctic (south) regions of the earth where plasma (electrons) from the solar winds, trapped in the magnetosphere, travels down along the Earth’s magnetic field lines, loses energy and enters the atmosphere. This creates the conditions known as auroras.

In northern latitudes the effect is known as the aurora borealis, named by Galileo after the greek name for the northern wind, Boreas. The Aurora Borealis are also known as the Northern Lights and they are visible in most of Alaska, northern parts of Canada, the southern half of Greenland, Iceland, Northern Norway, Sweden and Finland. As well as the western half of the Russian north.

Its southern counterpart, the aurora australis also known as the southern lights, has features that are almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone. It is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia.

 Colors of the Auroras

Diagram showing the ionisation and excitation of atmospheric constituents

An aurora is a natural light display in the sky, named from the latin word “sunrise” and the Roman goddess of dawn. They are caused by charged particles, mainly electrons and protons, entering the atmosphere from above colliding with oxygen atoms and nitrogen based molecules. These particles get ionized or excited by the collision and as they flow down to earths atmosphere they take on a multitude of color hues differential to altitude.

At high altitude, above 200 km, oxygen red dominates. At altitudes between 100 – 200 km, nitrogen blue/red and oxygen green are visible which is the most common of all Auroras. Below 100 km, nitrogen blue/red are emitted which at the lowest altitudes turns into pink with a mixture of light green and red. The brightness and visibility of these hues are dependent on the amount of energy absorbed from the electrons provided by the solar winds.

Auroras take on many different forms. The most distinctive and brightest are the curtain-like auroral arcs. They eventually fragment or ‘break-up’ into separate, and rapidly changing, often rayed features which may fill the whole sky.
Most Auroras occur in a band known as the “Auroral Zone”, a ring-shaped region with a radius of approximately 2500 km around Earth’s magnetic pole. They are most clearly seen at night against a dark sky. A geomagnetic storm can cause the auroral ovals (north and south) to expand, and bring the aurora to lower latitudes.