Being an Assessment of Historical Data Related to
The Solar Magnetic Ap Index.
Ap Index 1932 – 2017
“aa” Index 1844 – 1997 and Ap index 1991 – 2020
Additional Relevant Data
BUTTERFLY CHARTS 21-24 and 23-24
Ap ‘Leading Edge’ Delay
Associated with Spörer’s law
If we examine the historical data over the years for which we have reasonable records, (recording Ap started in 1932) we can identify a cyclic period of a little over thirty years, superimposed over, and in addition to, the cycle coinciding roughly with the eleven year sunspot cycle.
In doing this, it becomes starkly apparent that the decline in solar activity since the peak of 2003 (culminating in what became known as the ‘Halloween Super Storm’) has been both sharper and deeper than would be implied by examination of the sunspot cycle alone. What should have been a peak of activity associated with cycle 24 basically failed to materialise and what was previously considered the floor of activity of Ap=5, broken first in 2009, appears likely to be regularly replaced by a new floor level around Ap=2/3. This represents a significant decline in overall solar activity, indeed it may be inferred from the downshift occuring in October 2005 that this represented an actual ‘step change’ downwards in activity with all of the primary measurements now operating around new median levels. It may, indeed, be inferred from examination of the data that the ‘Fall-Off’ commenced in 2003, following the explosive outburst of that year. It must also be remembered that these indices are a measure of the geo-effective impacts of solar activity rather than being a direct measurement of that activity, nonetheless they are a useful indication of actual solar behaviour and give us a useable measure of that behaviour.
To extend our understanding of this aspect of solar influence backwards beyond 1935 we need to make use of the ‘aa’ index which has been recorded since 1844. This bears a very close relationship with the ‘Ap’ index and follows it closely although there are differences. We can however continue to observe the thirty year cycle.
One of the problems experienced in assessing Solar/Climate interaction based on sunspots alone is that the existence of a sunspot does not imply a geo-effective impact; similarly the absence of spots does not imply the absence of incoming material. This is, perhaps, the root of the ‘It is/It isn’t’ argument in this area; use of the Ap index gives us a more realistic overall assessment of Sun/Earth interaction and our growing knowledge in this area enables greater understanding and greater ability to identify cause and effect.
Examination of all of the relevant data leaves little doubt that the sun has undergone a step downwards in activity levels – this is evident in several of the data sources in addition to that noted above.
We are also increasingly gaining an understanding of the correlation between Sunspot activity and Coronal Hole activity, which latter tends to increase as spots decline; if in the light of this, we examine the combined ‘Ap/Spot’ chart we begin to see that the sharpest dip in ‘Ap’ is commonly coincident with the leading edge of each ‘Sunspot’ cycle. (see ‘Ap Leading Edge Delay’ – sketch above) It would appear that, as spots start to return, blank period CH activity declines, yet at that point spot related terrestrial impacts are still too small, too infrequent, to have a significant effect on the terrestrial geomagnetic environment and the terrestrial atmosphere for the first few years of the cycle. This effect will, inevitably, have its effect on surface climate at that point in time. (Refer: “Solar Activity and Surface Climate”). The principle reason for this ‘Leading Edge’ effect is best illustrated by reference to the ‘Butterfly Diagram’; at the start of each sunspot cycle, spots occur more heavily in the higher solar latitudes (Spörer’s law), ejecta from these spots is therefore more likely to be directed off the ecliptic plane and less likely to be earth directed. This corrects as the cycle progresses and Earth may receive more of any ejecta mass and it is common to see that the peak of ‘Ap’ activity is commonly associated with the trailing edge of the cycle.
In assessing the data related to the ‘Butterfly Chart’ it is relevant to observe the ‘Zero to Low’ gap between spot cycles. The gaps between cycles from 21 to 24 is clearly expanding and thinning, that between 23 and 24 is visibly clear of spots for a significant period whilst other cycles tend to show a significant overlap. It may best be visualised as the cycles shrinking – which would be in line with the fall off over the last few cycles – and pulling apart. It is necessary to go back to the early 1900’s to observe a gap of that nature between cycles. Under such conditions, other influences on the sun-earth interaction rate become more significant so that a dip associated with Russell-MacPherron effect, which may otherwise be overwhelmed, may become dominant with the inevitable effects on surface climate. In ‘busier’ inter-cycle periods this dip would be swamped by other activity and the earthly climate remain more highly elevated.
If we now view the ‘Butterfly Chart’ overall, it is possible to visualise a slow curving cycle of intensity variation over the period displayed, with a minima in the mid 1800’s, slowly rising to a maxima around 1960/1980, then staring to decline to a possible minima which would be around 2050. This, however, must be viewed as being highly speculative.
It is disappointing to note that NOAA/SWPC, Boulder Co., are to discontinue – with effect from 1st July 2020 – the older, legacy, versions of the Solar Progression charts. The “Ap” progression chart was particularly valuable in assessing any Solar/Terrestrial interaction. Best remaining data source is probably :-
although this does not give long term historical data in the way that the Boulder charts did.
Sunspot and 10.7 Flux charts have their value in this area but as has been said, the existence of a sunspot does not imply any terrestrial impact or influence, just as the absence of spots does not imply the absence of impact. The “Ap” is a far better indicator of that which is hitting the planet and it is often possible to cross reference a surface weather condition or event with “Ap” activity.
The “Ap” index was only started in 1932, to go back further we need to use the “aa” index: The weather events of 1901/2 and 1912/13 are of particular interest and we can see from the chart that these were coincident with a deep dip in “aa” values.
Of particular interest is the sudden anomalous dip in 2009 which has been noted as coincident with a very similar dip in upper atmosphere values.
If we examine the more recent charts, we begin to see some remarkable correspondences with the infamous “European heat wave of 2003” and the severe European winters of 2009/12 and also with the severe hurricane season of 2017 being of particular interest. The downshift of October 2005 is clearly apparent as is the shift in apparent median values.
Record‐low thermospheric density during the 2008 solar minimum
Daily global‐average mass log‐density (seasonal and geomagnetic activity effects removed) at a fiducial altitude of 400 km. The dotted horizontal line indicates the minimum overall density. (b) The daily 10.7 cm solar radio flux (F 10.7) normalized to 1 AU, in solar flux units (sfu, 10−22 W m−2 Hz−1). (c) Same as Figure 1a, but showing the running 81‐day average log‐densities. The green dashed line shows the solar minimum density trend reported by. (d) Same as Figure 1b, but showing the 81‐day average F 10.7 (10.7). The horizontal dotted lines in Figures 1c and 1d illustrate the differences between the cycle 23/24 and cycle 22/23 minima.
This data shows identical variation to that of the “Ap index shown in the initial chart.
*Acknowledgements : Charts used are publicly available from the relevant sources. Credit is given to the original compilers of the core data.