The occurennce of the northern (and southern) lights should increase over the next several months. This is because the Sun’s surface is becoming more active, and it is eruptions on the Sun’s surface which cause these wonderful phenomena.
The 11-year sunspot cycle
Sunspots have been studied in detail since the early 1600s. After several decades it was realised that the number of sunspots on the Sun’s surface goes through a regular cycle.
Although the strength of each sunspot maximum varies, this plot clearly shows that for over 400 years the number of sunspots increases and decreases with a period which averages to 11 years. The only exception to this is the so called Maunder minimum, which occurred in the second half of the 1600s.
In the last decade we have developed a pretty good understanding of what causes sunspots, but we still do not know why they come and go with this 11-year cycle.
What are sunspots?
In 1908 George Ellery Hale discovered that sunspots are areas of intense magnetic field. In fact, they occur in pairs, with one spot being the north pole of the magnet and the other spot being the south pole.
Coronal mass ejections (CMEs)
The magnetic field lines between a sunspot pair pull material off of the surface of the Sun, as shown in this photograph.
Most of the time this material flows along the field lines, but when the magnetic field strength becomes large enough the field lines can break, and when this happens a coronal mass ejection occurs. A CME is essentially an expulsion of charged hydrogen gas from the surface of the Sun. Hydrogen gas is just made up of protons and electrons, so a ball of protons and electrons hurtles away from the Sun’s surface.
This huge ball of gas spreads out into the Solar System. Because it is an eruption from a particular point on the Sun’s surface, the charged particles travel in a particular direction. They typically travel at speeds of about 500 km/s, and so if they are coming towards Earth they take about 3-4 days to do so once they leave the Sun.
The northern (and southern) lights
If the charged particles in the CME hit the Earth they get directed by the Earth’s magnetic field to travel along the magnetic field lines. The field lines of course run from the magnetic north pole of the Earth to its magnetic south pole. As the charged particles travel towards one or other magnetic poles they pass down through the Earth’s atmosphere.
As they pass through the atmosphere the charged electrons can excite atoms in the Earth’s atmosphere, and when these excited atoms de-excite, they emit light. This is exactly the same physics as happens inside e.g. a fluorescent light tube. Different elements emit different colours, and even the same element can emit different colours depending on how it is excited. This is what leads to the wonderful light show we see in the northern (and southern) lights.
Here is a wonderful time-lapse video of the northern lights.
As the photograph and video show, the atmospheric light-show that is the northern (or southern) lights is incredibly beautiful. So, with the sunspot cycle coming back to its maximum, the next several months provides you with a good chance to see them. It will be a sight you will never forget.
I found this graphic which I thought was quite good.