Revised 10/02/97

Executive Summary

Based on recent observations of solar activity, Solar Cycle 23 has begun and is expected to reach a maximum smoothed monthly sunspot number near 160 (between the values of 130 and 190) near March, 2000 (between June 1999 and January 2001). The corresponding prediction for 10.7 cm solar flux is approximately 205 solar flux units (sfu; 10^-22 W/m²), between 175 and 235 sfu.

This result reaffirms the consensus prediction made in September, 1996, for a solar cycle which is comparable to the last two cycles, but is unlikely to exceed Cycle 19, the largest cycle on record.

Based on traditional smoothed monthly sunspot numbers and 10.7 cm solar flux, the minimum level of solar activity between Cycles 22 and 23 occurred in May, 1996. However, based on additional factors including the number of spotless days and the dominance of new cycle active regions, the Solar Cycle 23 Panel recommends that October, 1996, be used as the effective onset of Cycle 23.

The date of geomagnetic minimum is not yet certain; therefore the panel provisionally retained their previous prediction which calls for intermittent storm activity especially between 1999 and 2005 with storm frequencies and intensities similar to that experienced during Cycle 22.

Background

In September, 1996, in advance of knowledge of the end of Solar Cycle 22, an international panel of 12 scientists met in Boulder, Colorado to consider 28 collected predictions of the profile and amplitude of Cycle 23 solar and geomagnetic activity. They concluded that a reasonable forecast was a new cycle comparable to old Cycle 22. In particular, the maximum smoothed sunspot count was predicted to be between 130 and 190, with 160 as a representative value (the maximum smoothed Cycle 22 value was 159). This is a large cycle, but is unlikely to exceed Cycle 19, the largest known solar cycle, which reached a smoothed monthly maximum of 201 in March 1958. The panel further estimated that the maximum would occur between January 1999 and June 2001, with March 2000 as a good "middle-ground"; estimate. The wide spread in possible dates for the month of maximum was considered necessary because the date of the onset of Cycle 23 was unknown. This prediction was disseminated on the NOAA Space Environment Center Internet home page in October, 1996, and a summary was published [Joselyn et al. 1997].

In the year since that meeting, there has been a slow but steady increase in sunspot numbers and monthly 10.7 cm flux, and a rapid upturn in activity beginning in late August, 1997. The minimum in smoothed monthly sunspot numbers (weighted 13-month running average) is May, 1996, with a value of 8.0. The panel reconvened to review their solar activity prediction in light of these new data as well as several new forecasts recently published.

Solar Activity

The most problematic point discussed was the time of the cycle minimum. While the traditional numerical prescription as well as other measures of solar irradiance and activity agree that May, 1996, was the minimum smoothed month of the cycle, Karen Harvey noted that there are several factors that argue that this date is misleading as a fiducial for Cycle 23 onset. In particular, no new cycle spots were observed before this month - a situation never before recorded. The date of minimum is expected to represent that time when new cycle activity becomes dominant; that is, the new cycle should have been in progress as the old cycle declined, the minimum then marking the crossover. But for Cycle 23, new cycle regions did not outnumber old cycle regions until December, 1996. The resurgence of activity in the months following May is due to old cycle regions (low latitude spot groups with the appropriate magnetic polarity of old cycle regions). Another factor that indicates May is a misleading date is that the maximum number of spotless days occurred in September and October, 1996. Finally, the Panel noted that the epoch tables specifying cycle maxima and minima included in Waldmeier [1961] are not consistent with a strict determination based on smoothed sunspot number.

The Panel then reviewed the published definition of sunspot extrema [McKinnon 1987] which includes 5 criteria:

When observations permit, a date selected as either a cycle minimum or maximum is based in part on an average of the times when extremes are reached (1) in the monthly mean sunspot number, (2) in the smoothed monthly mean sunspot number, and (3) in the monthly mean number of spot groups alone. Two more measures are used at time of sunspot minimum: (4) the number of spotless days and (5) the frequency of occurrence of "old" and "new" cycle spot groups.

Because the smallest monthly mean sunspot numbers were achieved in September (1.6) and October (0.9), 1996, and the most spotless days occurred in October, the Panel agreed that October, 1996, was the effective onset of Cycle 23.

There are significant implications for this choice of Cycle 23 onset, especially for statistical prediction techniques that key on the date of minimum such as the McNish-Lincoln method. Since this method is inherently short-range, McNish and Lincoln [1949] recommended that it not be used for making projections longer than one year in advance. Modifications of this method, such as that developed by the NASA Marshall Space Flight Center (MSFC) [Niehuss et al. 1996], can be used with 13-month smoothed data to provide estimates over an entire current solar cycle. Jerry Owens showed results for this method provisionally initialized using the May 1996 minimum, which are also reported in the September 1997 MSFC Solar Memo. These results indicate a mean (50th percentile) maximum 10-cm solar flux of 149 sfu (sunspot number of about 103) and a 95th percentile value of 224 sfu (sunspot number of about 180). While the 50th percentile value is less than the Panel's expected lower bound of 175 sfu, MSFC routinely recommends use of the 95th percentile values for spacecraft and space vehicle applications, within the range of 175 to 235 agreed by the Panel. He also showed results demonstrating that use of a later date to initialize the calculation results in a profile with faster rise time and higher peak value. By means of a similar modified McNish-Lincoln technique, Alan Thomson used the method of Kerridge et al. [1989], based on Holland and Vaughan [1984], to calculate a mean (50th percentile) maximum 10.7 cm flux of 175.5 sfu (± 45 for upper and lower 95th percentile values) in March 2001 with an earlier peak of 173.9 in December 1999. The latter calculation uses only cycles 18-22, whereas the MSFC calculation uses solar cycles 1-22.

Richard Thompson presented an analysis of historical data that indicates that the early rise of a solar cycle is a poor indicator of the eventual amplitude of that cycle. This is an important limitation in the use of projection techniques such as McNish-Lincoln in determining the amplitude of the cycle.

Werner Mende continued his work with spectral filtering, which accurately predicted the date of smoothed minimum approximately 2 years in advance. His research isolated a band of filters of approximately 4 years in length; these filters remove multiple maxima but do not alter cycle length. He scaled available cycles to unit amplitude, and found that the inflection point (where the derivative of the slope changes sign) on the increasing flank of the cycle profile was the most repeatable characteristic from one cycle to the next. His estimate for the date of the peak of the new cycle is the sum of 2 years from the numerical minimum to the inflection point, and then 1.9 years more to maximum. He expects a duration of 5.4 years from maximum to the end of the cycle. Using his filtered date of minimum, the inflection point is in mid-1998 and the resulting maximum date and amplitude are in close agreement with the Panel consensus.

The Panel also considered 10 new and two updated predictions of Solar Cycle 23 behavior. All of the new submissions used techniques which were already represented among the predictions that were evaluated in 1996. Most of these newly furnished forecasts fell within the Panel's range of maximum values. Those that predicted an average or low cycle were discounted for the same reasons that such predictions were discounted previously: they each relied on the entire record of cycle activity. The Panel believes that recent high activity cycles and the precursor techniques, successful in predicting previous cycles, should be more heavily weighted. Ultimately, the Panel found no compelling reason to alter the previous estimate of the amplitude or shape of Cycle 23.

However, because the onset of Cycle 23 is now established, the Panel agreed that the range of possible dates of maximum could be narrowed. Based on an historical range of rise times for cycles of the expected amplitude, the month of smoothed maximum should lie between June 1999 and January 2001, with March 2000 as a likely mid-range estimate. Figure 1 illustrates the mid-range prediction for 10.7 cm flux solar initialized to October, 1996, along with the data observed through September, 1997 (smoothed Cycle 23 data are shown, except for the last 6 months which are the monthly mean values). The monthly mean values for Cycle 22, matched so that Cycle 22 minimum, September 1986, is aligned with October, 1996, are shown for comparison.

Figure 1: Actual smoothed or monthly mean values (last 6 months) of 10.7 cm solar flux and the predicted profile of 10.7 cm solar flux for Cycle 23, along with the observed values for Cycle 22 aligned so the predicted cycle and Cycle 22 begin in October, 1996.

Geomagnetic Activity

The record of geomagnetic aa indices, beginning in 1868, shows that the minimum in geomagnetic activity typically lags the minimum in solar activity. That behavior appears to be continuing for this cycle. The annual average aa for 1996 was 18.6; thus far for 1997 (January through August), the annual average aa is 15.9. Until the date and level of geomagnetic minimum is firmly established, the Panel predictions made in 1996 for Cycle 23 geomagnetic activity will be retained. These are repeated in Table 1. The precursor approach is based on the same data and analysis used by Richard Thompson for his solar cycle precursor prediction, and the climatological approach is based on the median behavior of the activity observed in the record of odd-numbered cycles of geomagnetic aa index [Joselyn et al. 1997].

Because of the relevance of geomagnetic activity for upper atmospheric density calculations, NASA MSFC predicts the geomagnetic A_p index with the same modified McNish-Lincoln algorithm used for the solar flux prediction; 13-month smoothed A_p data are substituted for the solar flux data [Niehuss et al. 1996]. That calculation, initialized in November, 1996, and reported in the September 1997 MSFC Solar Memo, predicts a maximum smoothed monthly A_p of 24.9 (95th percentile, approximately equivalent to an aa of 31) in August of 2004. This value is consistent with the upper envelope of the Panel prediction made one year ago. The newest British Geological Survey McNish-Lincoln result using the aa index, also based on a preliminary initialization date of November 1996, is for a 50th percentile maximum aa of 28.2 nT in mid-2002. This is slightly less than the aa of 29.2 nT used to estimate the climatological number of occurrences shown in Table 1.

The Panel continues to recommend that more research is necessary in the area of long-term geomagnetic activity predictions.

References:

Holland, R.L. and W.W. Vaughan, Lagrangian Least-Squares Prediction of Solar Flux (F10.7), J. Geophys. Res., 89, 11-16, 1984.

Joselyn, J.A., J.B. Anderson, H. Coffey, K. Harvey, D. Hathaway, G. Heckman, E. Hildner, W. Mende, K. Schatten, R. Thompson, A.W.P. Thomson, and O.R. White, Panel achieves consensus prediction of Solar Cycle 23, EOS, Trans. Amer. Geophys. Union, 78, pages 205, 211-212, 1997.

Kerridge, D.J., V. Carlaw, and D. Beamish, A Review of Methods for Solar and Geomagnetic Activity Forecasting for Application in Space Missions Planning, BGS Technical Report, WM/89/14C (62 pp.) 1989.

McKinnon, J.A., Sunspot Numbers: 1610-1985, Report UAG-95, World Data Center A for Solar-Terrestrial Physics, NOAA, Boulder, CO, January 1987.

McNish, A.G., and J.V. Lincoln, Prediction of Sunspot Numbers, Eos, Trans. Amer. Geophys. Union, 30, p. 673, 1949.

Niehuss, K.O., H.C. Euler, Jr., and W.W. Vaughan, Statistical Technique for Intermediate and Long-Range Estimation of 13-Month Smoothed Solar Flux and Geomagnetic Index, NASA TM-4759, Washington, DC, September, 1996.

Waldmeier, M., The Sunspot Activity in the Years 1610-1960, Zurich Schultess and Company, Switzerland, 1961.

Acknowledgments

The NOAA Space Environment Center (SEC) convened the Solar Cycle 23 Project Panel with the support of the NASA Office of Space Science and the respective institutions of the panel members. The Panel Members are Dr. Jo Ann Joselyn (Chair, NOAA Space Environment Center, Boulder, CO); Helen Coffey (NOAA National Geophysical Data Center, Boulder, CO); Dr. Karen Harvey (Solar Physics Research Corp, Tucson AZ); Dr. David Hathaway (NASA Marshall Space Flight Center, Huntsville, AL); Gary Heckman (NOAA Space Environment Center, Boulder, CO); Dr. Ernie Hildner (NOAA Space Environment Center, Boulder, CO); Dr. Werner Mende (Institute of Meteorology, Free University of Berlin, GERMANY); Dr. Jerry Owens (NASA Marshall Space Flight Center, Huntsville, AL); Dr. Kenneth Schatten (NSF/Upper Atmospheric Division, Arlington, VA); Dr. Richard Thompson (IPS Radio and Space Services, NSW, AUSTRALIA); Dr. Alan W.P. Thomson (British Geological Survey, Edinburgh, Scotland, UK); Dr. Oran R. White (NCAR High Altitude Observatory, Boulder, CO).