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 Published by RyeBrook Space Science Services 216 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 five-year data sample, when solar activity was completely absent from one hemisphere or the other. Whether this is scientifically significant or not is uncertain, and a further, more extended study is required in order to answer this question. 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 13 years. Some time after solar maximum, active regions abruptly migrate away from the solar equator, and thus we are able 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 five years leading up to 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. This project was begun while the author was Director of the Land s End Solar Observatory (2-214). RyeBrook Space Science Services acquired ownership of LESO in 214, and subsequently disbanded the observatory in early 215. This paper is therefore presented as the intellectual property of RyeBrook Space.

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 comprised some 7,3 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 24. These values, rounded to one decimal place were used to plot graphs for each year of the study, and a graph for all five 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 spotless days in the northern hemisphere, and those in the southern hemisphere during the five-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 Results: Solar Active Region Latitude Analysis 211 Monthly Averages January February March June August November December Figure 2: RyeBrook Space 215 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. Solar Active Region Latitude Analysis 212 Monthly Averages January February March June August November December

4 Figure 3: RyeBrook Space 215 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 213 Monthly Averages January February March June August November December Figure 4: RyeBrook Space 215 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.

5 Solar Active Region Latitude Analysis 214 Monthly Averages January February March June August November December Figure 5: RyeBrook Space 215 There appears to be a levelling-out of active region latitudes during 214, 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. Solar Active Region Latitude Analysis 215 Monthly Averages January February March June August November December Figure 6: RyeBrook Space 215 There is a much less marked trend in the data for 215, and there is certainly no dramatic convergence or divergence in the activity latitudes that would suggest an obvious solar maximum.

6 It is interesting to note though, that in the middle of January 215, 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. 211 to 215 Combined SARLA Monthly Averages Jan- 11 Jan- 12 Jan- 13 Jan- 14 Jan Figure 7: RyeBrook Space 216 Though there is a general trend in the five 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. There is a more marked trend during 211 than later years, and the southerly extent of active regions in the northern hemisphere as well as the northerly extent of solar activity in the southern hemisphere, show the most marked convergence toward the equator. This at times results in a widening of the band of activity though it is noted that this widening closes again toward the end of the sample period. 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. Periods of Sun-spotless Days: 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, spotless days by hemisphere were plotted for each month of the entire sample period to produce a graph. Values for the southern hemisphere were made negative

7 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. 5-5 Jan- 11 Jan- 12 Jan- 13 Jan- 14 Jan- 15 Spotless days northern hemisphere Spotless days southern hemisphere Figure 8: RyeBrook Space 216 A brief analysis of the resulting graph reveals a rough oscillation between spotless days 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. Conclusions: Figure 9: Progression of Solar Cycle 24

8 As can be seen in Figure 9, that the maximum of solar cycle 24 is far from distinct, with two main candidate peaks one in late 211, and another in early 214. 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 215 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 ). 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 five-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 five 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, 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 Figure : January 215 Daily SARLA Values

9 Jan- 11 North erly Extent (Nort hern Hemis phere ) Figure 11: Adjustment of the aspect ratio of the five-year 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 five-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 11). 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 215. Only a continued monitoring of SARLA data through the next 12 months will 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 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. (22) identified a periodic north-south hemisphere asymmetry of sunspot distribution from historical data covering the period 1874 to 2, 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 greater frequency of spotless days in the northern hemisphere, which tends to support this theory though it is also true to say that the southern hemisphere had longer periods of spotless days.

10 The phasing of north-south asymmetry of sunspot distribution is poorly understood, and is the subject of on-going research. It is likely to be a complex process, and has not yet been examined at five-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 sunspots used as a focus in this study. In summary, this project has shown very clearly that whilst SARLA data are a valuable diagnostic tool after the event, they probably have no viable role in real-time monitoring of solar cycle progress. 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: Li, K.J. (22) Asymmetrical Distribution of Sunspot Groups in the Solar Hemispheres, Astronomical Society of Japan, vol. 54, pp RyeBrook Space 216. All rights reserved. No part of this publication may be reproduced in any form without the explicit prior permission of the Chief Executive. This publication has been created in accordance with our Learning Difference Policy. If you have a learning difference and you require the document in different format, please contact us at theryebrook@gmail.com and we will be pleased to help in any way we can.