Space Weather

1. What is the earlier prediction for Solar Cycle 24? 
2. What is the latest prediction for Solar Cycle 24?
3. Are there any evidences supporting the need for revising the earlier prediction of Solar Cycle 24?
4. Are the intensity levels of sunspot activity related between the solar cycles?  Will there be a "grand minimum" of solar activity at the end of Solar Cycle 24? 

1. What is the earlier prediction for Solar Cycle 24? 
   

   The prediction of Solar Cycle 24 is based on the assessment and analyses made by a panel of experts led by the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA) of the United States.
   
   One solar cycle lasts for about 11 years. The numbering scheme of solar cycles, established by Rudolf Wolf, began with the 1755 - 1766 cycle.
   
   As early as 2006, the panel had predicted that during the solar maximum (i.e. the peak) of Cycle 24, the sunspot number would be as high as 160, making it the strongest solar cycle in the past 50 years and one of the strongest since records (i.e. Inter-hour Variability Index or IHV derived from magnetometer data) began in 1868.  The prediction made then (Fig.1) suggested that Solar Cycle 24 would be very strong and would peak between 2010 and 2011.

   
   
    
   Fig.1   The sunspot numbers of Solar Cycle 24 as predicted in March 2006 (image source: courtesy of NASA). Note: The predictions of International Sunspot Numbers are usually "smoothed" over time periods of about one year or more, with the dotted lines in the diagram indicating the upper and lower bounds of the expected range of monthly sunspot numbers.
   
   
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2. What is the latest prediction for Solar Cycle 24?
   

   In April 2007, opinions in the panel became split in the prediction of Solar Cycle 24, with half favouring a strong Cycle 24 and the other half going for a weak Cycle 24. By May 2009, the panel had amended its prediction for Cycle 24, saying that the intensity of solar activity would be lower than average, with an expected maximum sunspot number of about 90 and the solar maximum to occur in 2013. Observations in recent years and the latest prediction (Fig.2) both suggest that the solar activity of Cycle 24 will be weak.
   

    
   Fig.2   As in Fig.1, but for sunspot numbers of Solar Cycle 24 as predicted in December 2011 (image source: courtesy of NASA). The expected intensity of this sunspot cycle as currently predicted would be the smallest in the past 80 years.  
   
   
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3. Are there any evidences supporting the need for revising the earlier prediction of Solar Cycle 24? 
   

   Comparing Fig.1 and Fig.2, it can be seen that the solar minimum of Cycle 23 actually occurred in 2008 (Fig.2), two years later than the previous estimate of 2006 (Fig.1) and an obvious deviation from the earlier prediction made in 2006. Furthermore, considering the actual situation, it is also rather unlikely that the solar maximum of Cycle 24 would occur in the years around 2010 (Fig.1). As such, the need for revising and updating the prediction is understandable.

   
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4. Are the intensity levels of sunspot activity related between the solar cycles?  Will there be a "grand minimum" of solar activity at the end of Solar Cycle 24? 
   

   There is a prevailing view that a long and extended solar minimum is likely to be followed by a weak solar cycle. Nevertheless, even though the solar minimum of Cycle 23 lasted longer than normal, findings in the study by Dikpati et al. concluded that the peak of the next solar cycle following an extended solar minimum was not necessarily stronger or weaker than the peak of the previous cycle.
   
   In the comparison of "butterfly" patterns or amplitudes of various solar cycles in the past 130 years (Fig.3), a solar cycle with "big butterfly" pattern has more sunspots and a solar cycle with "small butterfly" pattern has fewer sunspots. The amplitude of the current butterfly pattern is smaller than those in the past several decades and is similar to the "small butterfly" patterns in the decades around 1900. Furthermore, the current solar activity in Cycle 24 remains relatively weak. Although the solar maximum of Cycle 24 is yet to be reached, the overall solar activity of Cycle 24 is very likely to be weaker if the current trend of "small butterfly" pattern continues.
   

    
   Fig. 3 - The upper panel (a butterfly diagram as at December 2011) shows the distribution of sunspot positions and occurrences on the solar surface since May 1874. The lower panel shows the extent of sunspot occurrences in each solar cycle, as indicated by the percentage of sunspot area coverage on the solar surface.  (Image source: courtesy of NASA)
   
   
   As reported in the annual meeting of the Solar Physics Division of the American Astronomical Society held in June 2011, there were indications that an extended period of extremely low solar activity ("grand minimum") might occur at the end of the current Solar Cycle 24. There will be a follow-up report to this article introducing some recent scientific findings related to the "grand minimum" of solar activity.
   
   
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   References:

1. "Scientists Predict Big Solar Cycle", Science News of 21 Dec 2006, National Aeronautics and Space Administration (NASA).
2. "Scientists Gaze Inside Sun, Predict Next Solar Cycle (Press Kits of 6 March 2006)", NASA.
3. "Solar Cycle Prediction (Updated 2011/12/16)", Space Weather Prediction Center, NASA.
4. "The Sunspot Cycle (Updated 2011/12/16)", Marshall Space Flight Center, NASA.
5. "Length of a minimum as predictor of next solar cycle's strength", by Mausumi Dikpati, Peter A. Gilman and Rajaram P. Kane, Geophysical Research Letters, Vol. 37, L06104, 4 pp., 2010.
6. "Next Solar Storm Cycle Will Start Late, Experts Split Over Intensity", News of NOAA, National Oceanic & Atmospheric Administration, 25 April 2007.
7. "News from the Solar Cycle 24 Prediction Panel", NOAA, 13 October 2006.
8. "Solar Cycle 24 Prediction Updated May 2009", NOAA, 8 May 2009.
9. American Astronomical Society (AAS) press release of 14 June 2011.
10. "When will solar cycle 23 start? Will the activities be strong?", author: Zhang Guiqing, Science Museums of China.
11. "Solar cycle", Wikipedia.

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