Soon after 1843, when Heinrich Schwabe discovered what appeared to be an 11-year periodicity in the number of sunspots on the solar disc, scientists began to pore over the records of their predecessors and put some order into these occurrences.
They defined a sunspot cycle as beginning at the time when the number of sunspots was at a minimum, and lasting, through the maximum, until the next minimum approximately 11 years later. They called the cycle beginning in 1749 "cycle zero", and numbered the rest sequentially from there; we are now approaching the maximum of cycle 23, with the number of sunspots expected to peak this June or July. A prominent side-effect of a solar sunspot maximum is a marked enhancement of the frequency and brilliance of aurora borealis, the northern lights.
The aurorae originate in the stream of electrically charged particles speeding earthwards from the sun. As these particles, mainly hydrogen ions and free electrons, approach Earth, the planet's magnetic field acts like a gigantic cathode ray tube, the central core of every television set; the charged particles are channelled into beams, deflected towards the poles and focused on to Earth's upper atmosphere, which acts in this respect like a fluorescent screen.
This fluorescent luminosity provides the range of red, green, pink and blue lights which the aurorae emit to provide the brilliant displays which decorate the northern sky. The aurorae are most noticeable at times of very high solar activity, during the months when the number of sunspots is at a maximum and the stream of charged particles from the sun is most prolific.
The phenomenon has also been observed to have a 27-day periodicity, which coincides with the period of rotation of the sun with respect to Earth moving in its orbit.
In the auroral zone of maximum frequency, a region which extends from the north of Norway, south of Iceland and Greenland, over northern Canada to the north of Siberia, the phenomenon can be seen almost every night the sky is clear as a brilliant spectacle of restlessly moving coloured streamers.
At the latitude of Ireland and Britain, however, the northern lights are seen only on about 5 per cent of occasions when conditions otherwise are suitable, appearing low in the sky near the northern horizon as a kind of grey-white glow with a sharp lower border.
However, with the approach of the solar sunspot maximum, spectacular displays even at these latitudes become an occasional possibility. As this spring waxes into summer, a nightly eye to the north might, now and then, be rewarded by a truly awesome sight.