The name aurora comes from the Latin word meaning sunrise and was the name of the Roman goddess of the dawn. In the northern latitudes, an aurora is called the aurora borealis or the Northern Lights. Boreas is the Greek name for the north wind.
In the southern latitudes, an aurora is called the aurora australis or the Southern Lights. Australis means “of the south” in Latin.
The plural of aurora is aurorae.
Aurorae are either diffuse or discrete.
A diffuse aurora is a glow in the sky that cannot usually be seen by people.
A discrete aurora appears within the area of the diffuse aurora and varies in brightness.
Sometimes the lights of the discrete aurora are so bright that you could read a book by them at night time.
A discrete aurora looks like curtains of light that form in an east-west direction – sometimes they change from second to second, sometimes they stay the same for hours on end. The curtains are formed by parallel rays of light lined up in the direction of the magnetic field.
The lights are mostly bright green or red.
For many years stories were told of sounds coming from aurorae but scientists did not believe this to be true.
However very recent research recorded clapping sounds coming from an aurora and this noise was thought to be caused by solar particles.
This is a very rare occurrence, which would only be possible to hear on a windless night and away from all other background noise.
Sunspots are dark spots that appear from time to time on the surface of the Sun.
These dark spots are caused by extreme magnetic activity. During this activity the Sun’s magnetic field loops out into space instead of looping back into the Sun. This is called a coronal mass ejection.
The amount of sunspot activity varies from year to year and appears to be in an eleven-year cycle. The time when there is the most sunspot activity is called the solar maximum and when there is the lowest activity it is called the solar minimum.
Ions are particles that have an electrical charge.
The solar wind is a continuous flow of ions that are released from the Sun during a coronal mass ejection.
The solar wind usually reaches Earth at a speed of 400km per second but during a magnetic storm the solar wind can be faster.
The charged ions are blown towards the Earth by the solar wind. The Earth’s magnetic field turns most of them away.
However, the Earth’s magnetic field is weaker at the poles and so some ions enter the Earth’s atmosphere and collide with atoms of gas.
These collisions in Earth’s high atmosphere cause energy to be released in the form of auroral light.
As the ions enter the Earth’s high atmosphere, they bump into atoms of oxygen and nitrogen. The colour of the resulting aurora depends on which atom the ions hit and where they meet.
Auroral displays appear in many colours but shades of green are the most common. These are produced by the collision with oxygen atoms about 95km (60 miles) above the Earth.
Other reported colours include red, blue, yellow and purple.
Most aurorae occur in a band known as the auroral zone which is 3° to 6° wide and 10° to 20° from the magnetic poles. Places like Canada, Finland, Norway, Sweden and Scotland are some of the best places to view the Northern Lights.
It is harder to see the Southern lights because there is less land near to the South Pole but the Antarctic, southern Argentina, New Zealand or the island of South Georgia are all places where this phenomenon can be witnessed.
When there are magnetic storms, aurorae can be seen in lower latitudes.
The aurora can be seen best at magnetic midnight which only occurs when the magnetic pole, the observer and the Sun are all aligned. It is best to view these displays in winter because nights tend to be clearer and the hours of darkness are longer.
Large magnetic storms mean that the aurorae can be seen at lower latitudes and these are most common during the solar maximum, which is the peak of the eleven-year sunspot cycle, and for three years after that peak.
Magnetic storms also happen more often during the months around an equinox. An equinox occurs in March and September, when day and night are of equal length.
In most cases the Northern and Southern Lights are mirror images of each other that occur at the same time and have similar colours and shapes.
In 1859 it is thought that the Sun released one of the biggest coronal mass ejections ever. This created a series of huge magnetic storms in August and September of that year.
The aurorae these storms produced were extremely bright and very widespread, being seen at much lower latitudes than normal. It is said that in Boston, USA it was so bright at 1am in the morning that you could read a newspaper.
The storms disrupted many telegraph lines but those that were in line with the magnetic field of the aurora on the night in question continued to work even when the batteries were turned off.
This helped scientists to understand the connection between aurorae and electricity.
Aurora in folklore
Many native people thought that the lights they saw were spirits of the animals that they hunted. Some believed that the lights were the spirits of their own people. In medieval times it was thought that seeing an aurora meant that war or famine were just around the corner.
Of course, other planets have magnetic poles and so they also have aurorae. The Hubble Space Telescope has observed aurorae on Jupiter and Saturn. Other instruments have recorded aurorae on Venus, Mars, Uranus and Neptune as well as Jupiter’s moons Io, Europa and Ganymede.
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