Using Solar Active Region Latitude Analysis to Monitor Solar Cycle Progress

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1 Using Solar Active Region Latitude Analysis to Monitor Solar Cycle Progress A Study Commissioned by RyeBrook Space Science Services RyeBrook Space 2017 Abstract: This paper seeks to answer the question does solar active region latitude analysis (SARLA) have value as a means of tracking solar cycle progress? Whilst the SARLA data show the expected migration of solar active regions toward the solar equator as cycle 24 progresses toward maximum the effect is rather subtler than expected, and only becomes dramatically obvious when the data are presented in a certain manner. This research demonstrates that a single point of reference in the data gives little or no indication of solar cycle progression, and that SARLA is only useful as a retrospective tool for identifying solar cycle change-over. A coincidental finding of the research is a north-south hemisphere asymmetry of active region distribution that results in extended periods during the first five-year data sample, when solar activity was completely absent from one hemisphere or the other. Whether this is scientifically significant or not was uncertain, and a further, more extended study was designed, in which a further 12 months of data were collected to determine this. Background: Since the 11-year solar activity cycle was first recognised in 1843, records have shown a repeating pattern in which solar active regions occur closer to the solar equator as the solar cycle progresses. This is evident in the well-known Butterfly Diagram (Figure1) that plots active region latitudes over the last 130 years. Some time after solar maximum active regions peter-out in what is known as Solar Minimum, before eventually appearing again at much higher latitudes in the build-up to the next solar maximum. This abrupt migration of active regions away from the solar equator allows us to assess when one cycle has ended, and the next has begun. In this paper, the author has taken data for active region latitudes for the six years leading up to and beyond the accepted maximum of solar cycle 24 a weak maximum and has arranged these data in spread sheet form for analysis. The intention of this process has been both to confirm the value of solar active region latitude analysis (SARLA) as a means of tracking solar cycle progress, and also to look for any other patterns that might be hidden in the data. The project was begun while the author was Director of the Land s End Solar Observatory ( ). After the closure of the observatory in early 2015, he continued with the project independently.

2 Figure 1: The Butterfly Diagram. Source NASA Methodology: The capture of data for this study was time-consuming and laborious. For each day in the five-year sample period, there were four data-points. These were: the northerly extent and southerly extent of solar active regions in the northern solar hemisphere, and the northerly and southerly extent of solar active regions in the southern solar hemisphere. These extents had to be manually identified and extrapolated from the daily lists of latitude co-ordinates for all active regions. The resulting datasets comprise some 8,760 data-points. Because this large number of data is too cumbersome to use for analysis, and in order to smooth any errors, the average monthly values for the four extents of solar activity were calculated, reducing the number of data-points to just 288. These values, rounded to one decimal place were used to plot graphs for each individual year of the study, as well as a combined graph for all six years. A benefit of manually capturing all of these data was that any patterns hidden in the data became very apparent to the author. In the case of an apparent north-south hemisphere asymmetry of active region distribution, it was noted that there were a significant number of periods where solar activity was absent from one hemisphere or the other for several days at a time, and it was only by manually capturing all of the data that this became obvious. The need to analyse this phenomenon to ascertain its scientific significance, necessitated manipulation of the data to show a comparison between days when there were no active regions in the northern hemisphere, and those in the southern hemisphere during the six-year sample period. This meant changing the values of southern hemisphere episode to negatives in order to show both sets of data on the same graph in a meaningful way. When viewing this graph it is important to remember that values for northern and southern hemisphere are both actually positive.

3 It is important to understand that this study deals with active regions - not sunspots - and in many cases there were active regions present when no sunspots were visible. These active regions were detectable in wavelengths other than white light, such as by magnetogram or Hydrogen-alpha imaging. In this way, a study of sunspot latitudinal distribution over the same timescale would give a very different result. Results: In a number of ways, solar activity cycle 24 was peculiar (Basu, S. 2013). Its atypical doublepeaked maximum led to the frequently asked question have we reached solar maximum yet? to which there seemed to be a conflicting consensus of opinion. As such, this study has broken new ground. Rather than being a study of latitudinal distribution of active regions during a typical solar activity cycle, it has been a close analysis of the unfolding of an unusual solar cycle, and is as such an exploration of uncharted territory. There follows a chronological presentation of the findings with a brief commentary: Solar Active Region Latitude Analysis 2011 Monthly Averages January February March April May June July August September October November December Southerly Extent Southerly Exetent Figure 2: SARLA values for the year 2011 There is a marked trend toward the solar equator in the southerly extent of active regions in the northern hemisphere. The northern extent of active regions in the northern hemisphere does not appear to shadow the southern extent, with the overall effect of a widening of the band of activity in the northern hemisphere. The extent of active regions in the southern hemisphere shows no particular trend, though there is an apparent widening of the band of activity toward the end of the year. There were occasions when solar activity was absent in the southern hemisphere for several days at a time.

4 Solar Active Region Latitude Analysis 2012 Monthly Averages January February March April May June July August September October November December Southerly Extent Southerly Exetent Figure 3: SARLA values for the year 2012 There is a marked trend toward the solar equator in the northerly extent of active regions in the northern hemisphere, as well as in the northerly extent of active regions in the southern hemisphere. The southern extent of active regions in both hemispheres does not seem to show any marked trend, with the exception of a last-minute convergence of the southern extent in the southern hemisphere, There were a significant number of occasions when solar activity was absent in the southern hemisphere for several days. This occurs much less frequently in the northern hemisphere, though more so than in the previous year. Solar Active Region Latitude Analysis 2013 Monthly Averages January February March April May June July August September October November December Southerly Extent Southerly Exetent Figure 4: SARLA values for the year 2013

5 There is a subtle general trend toward the solar equator in both hemispheres. This is most noticeable in the northern extent of the southern hemisphere active regions, Once again there were short periods when solar active regions were absent from the southern hemisphere, but more noticeably there were a significant number of periods where there was an absence of activity in the northern hemisphere. This seems to represent a change in the dynamics. Solar Active Region Latitude Analysis 2014 Monthly Averages January February March April May June July August September October November December Southerly Extent Southerly Exetent Figure 5: SARLA values for the year 2014 There appears to be a levelling-out of active region latitudes during 2014, with a significant widening of the band of activity toward the end of the year. Some short periods of an absence of activity in the northern hemisphere were noted, though less than in the previous year, and there were no periods of absence of activity in the southern hemisphere.

6 Solar Active Region Latitude Analysis 2015 Monthly Averages January February March April May June July August September October November December Southerly Extent Southerly Exetent Figure 6: SARLA values for the year 2015 There is a much less marked trend in the data for 2015, and there is certainly no dramatic convergence or divergence in the activity latitudes that would suggest an obvious solar maximum. It is interesting to note though, that in the middle of January 2015, active region 2262 came so close to the solar equator that it actually wandered between the northern and southern hemispheres for a few days. Once again it was noticed that there were periods of several days at a time during the year, when solar activity was completely absent from one hemisphere or another. It is therefore likely that these occurrences are of scientific significance, and should be studied in more detail. From 1 January 2016 to 31 December 2016 an extension of the SARLA project recorded an additional 12 months worth of data for analysis:

7 Figure 7: SARLA values for the year 2016 Perhaps disappointingly; the data for 2016 showed a confusing pattern of trend, with a marked deviation of southern hemisphere active regions toward the solar equator during the period March to August, that seems to have been reciprocated by a deviation away from the solar equator by the northern extent of northern hemisphere active regions. There is a subtle migration of the southern extent of northern hemisphere active regions away from the solar equator throughout the year. The pattern in the southern hemisphere toward the end of the year is less clear, with a dramatic narrowing of the northern and southern extent of southern hemisphere active regions that is more of a refection of the petering-out of activity in the southern hemisphere than anything else. Figure 8: SARLA values for the period 1 January 2011 to 31 December 2016 Though there is a general trend in the six years data of a convergence of solar active regions toward the solar equator there is disappointingly no dramatic marker for the point of solar maximum. What does stand out very clearly is a marked widening of the latitudinal band of activity in the southern hemisphere before a dramatic narrowing toward the end of the data sample period. This is not reciprocated in the northern hemisphere, and indicates a north-south hemisphere asymmetry of activity. From Figure 1, it is apparent that convergence of the northerly extent of solar active regions in the northern hemisphere, and the southerly extent of active regions in the southern hemisphere, continues for some time after the point of solar maximum.

8 Periods of Days without Active Regions: The significance of periods of several days at a time where solar activity was completely absent from one hemisphere or the other is uncertain. In order to try to understand any scientific significance, days without active regions by hemisphere were plotted for each month of the entire sample period to produce a graph. Values for the southern hemisphere were made negative in order that they may be plotted in a meaningful way on the graph, but it must be remembered that these are in fact positive values. It is also important to note that these occasions are not the same as days without sunspots. This study takes into account active regions that did not necessarily contain sunspots. Figure 9: Days without active regions by hemisphere 1 January 2011 to 31 December 2016 A brief analysis of the resulting graph reveals a rough oscillation between days in which there were no active regions in the northern hemisphere, and those in the southern hemisphere, with no obvious regular pattern. However, the distribution changes also do not appear entirely random. A very dramatic asymmetry is revealed in the final 12 months of the study, when there is a marked increase in the number of days without active regions in the southern hemisphere, and is an illustration of the asymmetric nature of solar activity decline toward the end of solar cycle 24.

9 Conclusions: Using Solar Active Region Latitude Analysis to Monitor Solar Cycle Progress Figure 10: Progression of Solar Cycle 24 As can be seen in Figure 10, that the maximum of solar cycle 24 is far from distinct, with two main candidate peaks one in late 2011, and another in early In the SARLA data, these two points are not clearly defined by any significant convergence of active regions toward the equator. It is interesting that around the middle of January 2015 it was noticed from the SARLA daily data, that active region 2262 came so close to the solar equator, that for several days it wandered between the northern and southern hemispheres (Figure 11). This event shows as a dramatic, if brief convergence toward the solar equator, and if it were possible to identify the moment of solar maximum based solely upon SARLA data, this would probably be it; particularly as there was a reciprocal convergence in the northerly extent of activity in the northern hemisphere, and the southerly extent of activity in the southern hemisphere at the time. This represents the closest convergence during the entire six-year sample. Of course, this is seriously at odds with the sunspot count and radio flux definitions of the solar maximum. Unfortunately, the smoothing effect of monthly averaging of the data has rendered this event invisible in the yearly and six year plots. But to answer the question posed by the background of this project does SARLA analysis have value as a means of tracking solar cycle progress? is less easy than might at first have been thought. Although the SARLA data show a clear and marked migration of solar active regions toward the solar equator as the cycle proceeds, it is not until the point of abrupt divergence away from the equator again that we can clearly determine the end of one cycle,

10 and the beginning of the next. As such, this will always be a retrospective analysis, and has no value in real-time solar cycle progression tracking Southerly Extent Southerly Exetent Figure 11: January 2015 Daily SARLA Values Figure 12: Adjustment of the aspect ratio of the fiveyear graph of SARLA values, so that the data-points become squashed together reveals the more familiar pattern that we see in the Butterfly Diagram, confirming that the convergence effect is real and measurable. When the aspect ratio of the six-year graph of daily SARLA values is adjusted so that the data-points are squashed up together, we see the reassuringly familiar pattern of the Butterfly Diagram (Figure 12). But we do not see the abrupt divergence away from the solar equator that signals the end of one solar cycle and the beginning of the next. This leads to an impression that cycle 24 had not ended by 31 December Only a continued monitoring of SARLA data through the next 12 months would be likely to reveal the critical date of solar cycle change-over. The second objective of this research project was to look for any significant patterns that may be hidden in the data. During capture of the data it became very obvious that there were a significant number of periods of several days each year when solar activity was completely absent from one hemisphere or the other, and it was wondered whether this was scientifically significant. When considering this it was important to remember that the data were only being drawn from one view of the Sun that facing the Earth - and that the situation on the far side of the Sun

11 at any given time may have been completely different i.e. there may have been no active regions in the southern hemisphere on the Earth-facing side, but there may have been on the far-side. Nonetheless, the visible portion is all that we have data for, and so the observation is made that there were a significant number of days each year, when solar active regions were absent from one hemisphere or the other on the visible side of the Sun. In their paper Asymmetrical Distribution of Sunspot Groups in the Solar Hemispheres, Li, et al. (2002) identified a periodic north-south hemisphere asymmetry of sunspot distribution from historical data covering the period 1874 to 2000, and suggested that there is a genuine periodic fluctuation that could lead to solar maximum being reached at a different time in each hemisphere (out of phase). They further suggest that this periodic phasing is likely to mean that solar cycle 24 will be south dominated. From the data gathered here, it is clear that there is a genuine north-south hemisphere asymmetry, with the number of days in which there were no active regions in the southern hemisphere increasing dramatically toward the end of the data sample. The phasing of north-south asymmetry of active region distribution is poorly understood, and is the subject of on-going research. It is likely to be a complex process, and has not yet, to the author s knowledge; been examined at six-year resolution. It is possible that continued investigation might reveal a finer periodic sub-phase within the courser 8-year phase, but this is pure conjecture. It is also important to realise that previous studies in this area have been of sunspot numbers, rather than the absences of active regions used as a focus in this study. There is an inherent weakness in studying a single solar cycle in isolation. Many solar cycles, perhaps cycle 24 as well, show atypical characteristics, and so a clearer overview is to be found in studying several cycles in combination. Nonetheless, this study has revealed some important features, not least of which is the presence of a north-south asymmetry of active region distribution. In addition, it must be said that studying the distribution of active regions rather than white light sunspot distribution, leads to vastly differing results, and is a departure from the norm of this type of study. Choosing to study active region distribution, regardless of the presence or otherwise of sunspots, was driven by a limitation in the availability of data. Similarly, differences in the methods of smoothing data will lead to variations in results, particularly in defining the point of solar cycle maximum and cycle change-over. With all of these limitations and variations taken into account, this single isolated study is rather limited in its scientific value and merit. However, it has provided an opportunity for the author to acquire an in-depth experience of a solar activity cycle from a singular perspective of the distribution of active regions, and has provided a snapshot in time of this one parameter of this single solar activity cycle, which has value in terms of understanding the symptomatic manifestations of solar activity, if not the underlying mechanisms.

12 In summary, this project has shown very clearly that whilst SARLA data can be a valuable diagnostic tool after the event, they probably have no viable role in real-time monitoring of solar cycle progress, and this was the primary enquiry of the project. SARLA data would appear to have no value in determining the point of solar maximum, but may be of use in determining the point of solar cycle change-over. The results have however, indicated very clearly that a continued period of observation is required in order to identify the point of divergence of active regions away from the solar equator that will signal that the next solar cycle has begun. Such a study would also collect further data that will enable a refined evaluation of the north-south asymmetry of active region distribution. References: Basu, S. (2013) The peculiar solar cycle 24 where do we stand?, Journal of Physics: Conference Series 440 (2013) Li, K.J. (2002) Asymmetrical Distribution of Sunspot Groups in the Solar Hemispheres, Astronomical Society of Japan, vol. 54, pp