Solar Cycle Number 22 (1986 - 1996) in Review
For solar observers and those in the business of forecasting the solar-terrestrial environment, the end of a solar cycle and the anticipation of a new one brings a feeling of excitement along with some regret at seeing the end of an era. Part of the excitement comes from the host of questions which arise about the nature of the new cycle. Will the cycle be large or small? Will there be intervals of intense flare activity? Will there be large magnetic disturbances and interesting effects such as during March 1989? The last cycle, denoted Cycle 22, appears to be over and a new solar cycle is underway. It is therefore an opportune time to review the events of the last cycle.
Cycle 22 certainly provided us with many highlights. Early in the cycle the smoothed sunspot number (determined by the number of sunspots visible on the sun and used as the traditional measure of the cycle) climbed rapidly; in fact more rapidly than for any previously recorded cycle. This caused many to predict that it would eclipse Cycle 19 (peak sunspot number of 201) as the highest cycle on record. This was not to be as the sunspot number ceased climbing in early 1989 and reached a maximum in July of that year. Whilst not of record amplitude, Cycle 22 still rated as 4th of the recorded cycles and continued the run of recent large solar cycles. (Cycles 18, 19 and 21 were all exceptional!) A very notable feature of Cycle 22 was that it had the shortest rise from minimum to maximum of any recorded cycle.
|Sunspot Cycle Number||Year of Minimum||Minimum Sunspot Number||Year of Maximum||Maximum Sunspot Number||Rise to Max (yrs)||Fall to Min (yrs)||Cycle Length (yrs)|
The maximum phase brought some extraordinary intervals of activity. Prime amongst these was the March 1989 period which started on March 6th with the appearance of a large sunspot region on the eastern edge of the sun. The next 14 days produced 11 "X class" flares (the largest category in X-ray emission) and 48 "M class" (still very large flares). However, the most outstanding feature of the interval occurred on March 13-14 with one of the largest geomagnetic storms in the last 50 years. This storm had an amazing list of effects on earth and in space. Power systems in Canada and Sweden failed as large electric currents were induced in power lines and tripped protective relays. Increased atmospheric drag, resulting from the expansion of the earth's outer atmosphere during the disturbance, altered the orbits of many satellites with the result that NASA lost track of some of them for a short period. Satellite navigation systems failed to operate and High Frequency (HF) communication systems were also out of action. Aurorae were sighted at quite equatorial latitudes. The southern regions of Australia were under cloud but numerous sightings were made into Queensland and even at Exmouth in Western Australia (which is north of the tropic of Capricorn). It was indeed an extraordinary period of time!
October 1989 was another interesting interval with a large proton event as a result of solar flares. These are periods of increased flux of charged particles from the sun and they could have drastic consequences for astronauts away from the protective covering of the earth's atmosphere. After October 1989 there was much speculation about the effects of a similar event during a manned mission to Mars and this has resulted in increased efforts to predict these events. The period also brought us a large (but not comparable with March 1989) geomagnetic storm again with sightings of aurorae. For IPS Radio and Space Services the period was especially memorable as it coincided with a gathering of many of the world's leading solar-terrestrial forecasters at a workshop organised by IPS. As might be expected the large proton event gave the discussions a strong focus and an urgency not seen at previous meetings.
June 1991 was the most outstanding solar flare activity of this cycle, and probably the largest since the peak of the record Cycle 19. The interval from June 1st to June 17th was filled with numerous intense solar flares. An interesting indicator of the strength of this activity came from the US GOES satellites which measure the X-ray output of flares. X-ray detectors on GOES saturated during 5 flares over the period. Other flares have occasionally saturated the detectors but it was a unique event for more than one flare from a single region to achieve this feat. If saturation of the GOES detectors is a measure of strength as in old-time carnivals, then solar activity in June 1991 "rang the bell" like never before! In Australia, we were well placed to see the fireworks because, by pure chance, many of the large flares occurred during the middle of our day. This meant that local HF communications were badly disrupted by short-wave fadeouts which cut circuits for many hours. An interesting side effect were the occurrence of several magnetic crochets during the month. These are abrupt changes in the terrestrial magnetic field which arise because of the rapid ionisation of the D-layer of the ionosphere during a large solar flare (this is the same effect which causes short-wave fadeouts when HF signals are absorbed in the highly ionised D-layer). Magnetic crochets are rarely observed because a flare must be both energetic in X-rays and also occur close to local midday. In Sydney at the IPS Space Forecast Centre we saw three in one week. June 1991 was quite remarkable and very memorable!
The maximum phase of the cycle appeared to end rather abruptly in early 1992 when monthly values of sunspot number dropped significantly. The decline of Cycle 22 to its minimum in May 1996 was also remarkable because of the lack of major flare activity. The cycle had a multiple personality - malevolent in its first half and quite benign in its decline. This behaviour contrasts with Cycle 21 which was more active in its decline than during its rise or even its maximum.
The cycle was less than 10 years in duration - a fair bit shorter than the "traditional" eleven year cycle. However, it is an interesting fact that all but one of the last seven cycles have been less than eleven years (Cycle 20 was the exception). We are in an era not only of large solar cycles but also short ones!
There is a old saying that there is "nothing new under the sun" - but this does not apply to the sun itself! Cycle 22 proved to be a remarkable cycle by any measure. While we can not be certain what precisely Cycle 23 has in store for us, knowing the sun, we can be sure that it will be full of interest and excitement.
This comparison indicates that Region 6659 in June 1991 easily heads the list of solar regions, surpassing even the March 1989 region. The present solar cycle (Number 22) is well represented with 6 out of the 15 flares in the list, including the top two flares!
Material prepared by Richard Thompson