| Contents | Last update |
|---|---|
| Periods with spotless days (>20 days) since 1849 | 01 September 09 |
| Number of periods with spotless days (P0) per solar cycle | 01 September 09 |
| Main characteristics per solar cycle | 01 September 09 |
| Evolution of spotless days in SC23-24 and comparison with other cycle transits | 01 September 09 |
| Monthly number of spotless days | 01 September 09 |
| Smoothed monthly Wolfnumber and smoothed monthly number of spotless days | 01 September 09 |
| Top 25 of years with most spotless days since 1849 | 01 September 09 |
Source: Daily Wolfnumbers from SIDC, RWC Belgium, World Data Center for the Sunspot Index, Royal Observatory of Belgium, 1849-2009. (Data source: SIDC)
Next update scheduled for early January 2010
Periods with spotless days (>20 days) since 1849
Perioden met vlekkenloze dagen (>20 dagen) sinds 1849
Pending further review of the preliminary daily sunspot data by SIDC, the period from 31 July 2009 till 31 August 2009 is one of the longer ones since the beginning of daily solar observations in 1849. An even longer period in recent history occured during the previous solar cycle transition, from 13 September 1996 till 24 October 1996, when the sun was spotless for 42 consecutive days. One of the longest spotless periods (since 1818) is probably from 24 October 1822 till 12 March 1823 (140 days!), but unfortunately, the series are broken on 29 December 1823 (no observation available for that day). With 9 entries so far, the current solar cycle transit ranks 4th in transits with the most of 20 or more spotless days stretches. Only the transits to cycles 14 (10), 12 (12) and 15 (17) had even more such periods.
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Number of periods with spotless days (P0) per solar cycle
Aantal perioden met vlekkenloze dagen (P0) per zonnecyclus
| SC | 10 < P0 < 15 | 15 < P0 < 20 | 20 < P0 < 30 | P0 > 30 | Total |
|---|---|---|---|---|---|
| 10 | 7 | 8 | 4 | 3 | 22 |
| 11 | 7 | 2 | 3 | 2 | 14 |
| 12 | 12 | 9 | 7 | 5 | 33 |
| 13 | 13 | 4 | 3 | 0 | 20 |
| 14 | 9 | 10 | 4 | 6 | 29 |
| 15 | 15 | 7 | 13 | 4 | 39 |
| 16 | 8 | 6 | 1 | 1 | 16 |
| 17 | 11 | 0 | 1 | 3 | 15 |
| 18 | 0 | 1 | 2 | 1 | 4 |
| 19 | 5 | 6 | 2 | 1 | 14 |
| 20 | 2 | 1 | 0 | 0 | 3 |
| 21 | 4 | 2 | 1 | 0 | 7 |
| 22 | 2 | 1 | 1 | 0 | 4 |
| 23 | 8 | 0 | 0 | 1 | 9 |
| 24 | 9 | 6 | 7 | 2 | 24 |
Above table and graph clearly show 2 groups. SC10-15 had many long periods with spotless days. SC16-23 do hardly have any such periods. Although the first spotless day for SC24 dates already back to January 2004, it is only since mid 2007 that SC24 has started producing longer stretches of spotless days. Now, in 2009, the distinction between SC24 and SC16-23 has become obvious, both in number of spotless periods as in number of periods with 20 or more spotless days.
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Main characteristics per solar cycle
Hoofdkenmerken per zonnecyclus
| SC | Month 1st Rd=0 | Month SC-min | Delta | Total Rd=0 |
|---|---|---|---|---|
| 10 | April 1850 | January 1856 | 69 | 654 |
| 11 | October 1861 | April 1867 | 66 | 406 |
| 12 | May 1873 | December 1878 | 67 | 1028 |
| 13 | January 1885 | February 1890 | 61 | 736 |
| 14 | November 1895 | September 1901 | 70 | 938 |
| 15 | March 1908 | June 1913 | 63 | 1019 |
| 16 | April 1920 | April 1923 | 36 | 534 |
| 17 | September 1930 | September 1933 | 36 | 568 |
| 18 | November 1941 | April 1944 | 29 | 269 |
| 19 | December 1950 | April 1954 | 40 | 446 |
| 20 | November 1961 | August 1964 | 33 | 227 |
| 21 | July 1973 | March 1976 | 32 | 272 |
| 22 | November 1983 | September 1986 | 34 | 273 |
| 23 | April 1994 | May 1996 | 25 | 309 |
| 24 | January 2004 | ? | 66 +/- 4 | 797 +/- 245 |
| 33 +/- 5 | 362 +/- 134 |
Table above gives for each new solar cycle the month in which the first spotless day appeared (after solar maximum...), the month in which the (Meeus smoothed) solar cycle minimum took place, the delay in months between those 2 timings, and the total number of spotless days that occured during the cycle transition. As can be seen, there is a big difference between solar cycles 10 to 15 and 16 to 23. The first group has a long delay (66 months) and many spotless days (almost 800 on the average), whereas the other group, containing the more recent cycles, has only 33 months between first spotless day and cycle minimum and not even half of the total number of spotless day of the first group (less than 400 days!). This makes a prediction for the start of SC24 very difficult. According to the second group, this minimum should occur late 2006 (March 2007 at the latest), but this has been excluded by the recent Wolfnumbers. On the other hand, if SC24 belongs in the category of the old cycles, then minimum would take place mid-2009 (March 2009 at the earliest). But then SC23 would have last over 13 years, making it one of the longest solar cycles over the last 250 years (SC04: 169 months). Though a minimum was reached in December 2008, SC24 still refuses to go into a higher drive as shown by the recent stretch of spotless days in August 2009. If the low activity persists, a new and lower minimum can not be excluded.
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Evolution of spotless days in SC23-24 and comparison with other cycle transits
Evolutie van de vlekkenloze dagen in SC23-24 en vergelijking met voorgaande cyclusovergangen
The thick green line in the graph above shows the accumulated (total number of) spotless days so far during the transit from solar cycle 23 to 24. The thick blue and red line show the same parameter, but respectively for the average of solar cycles 10 to 15 (SCo) and 16 to 23 (SCn). The thin threaded lines show the respective standard deviations (+sd and -sd). The number of spotless days of the current solar cycle transit follows very well that of the average of the group of old solar cycles. Nonetheless, it must be noted that the variation between the cycles is quite large, as can be seen from this raw graph. The pink and purple vertical lines show the average timing of the cycle minimum (and standard deviations) compared to the month with first spotless day, and this for the respective cycle groups (SCo min, SCn min). The recent cycles had already reached their minimum (pink), which certainly does not seem the case for the current cycle transit, apparently heading for a late solar cycle minimum. Based on preliminary SIDC-data, the SC-minimum may have been reached in December 2008, only 3 months shy of the lower standard deviation for the SC10-15 group. However, the timing of the minimum can still be delayed, in function of the persisting low solar activity. The current solar cycle transit has accumulated already 705 spotless days.
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Monthly number of spotless days
Maandelijks aantal vlekkenloze dagen
The two graphs on top show the monthly number of spotless days during this cycle transit (red) compared to respectively the average number during SC10 to 15 and during SC16 to 23. It seems that maximum number of spotless days is happening only now. This is more in line with the old solar cycles, and less with the more recent ones. Also, SC24 seems to have a really hard time to get going, whereas all cycles of the group SC16 to 23 were already well beyond their minimum (green asterisk).
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Smoothed monthly Wolfnumber and smoothed monthly number of spotless days
Afgeglad maandelijks Wolfgetal en afgeglad maandelijks aantal vlekkenloze dagen
This graph shows the smoothed monthly Wolfnumber (Rs) and smoothed monthly number of spotless days (SDs) since 1849. For obvious reasons, the instances of minimum Rs and maximum SDs happen for most cycles in the same month. Exceptions are SC14 (3 months later) and SC23 (4 months later). There is a strong relationship between the two parameters during solar cycle minimum: SDsmax = -1,4755 Rsmin + 27,076 (r2=0,92). The smoothed Rs and SDs both reached a minimum in December 2008 (resp. values of 1,72 and 24,2). This is comparable with the low activity transits in the early 20th-century, and certainly contrasts with the previous 4 cycle transits.
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Top 25 of years with most number of spotless days since 1849
Top 25 van jaren met het meeste aantal vlekkenloze dagen sinds 1849
This graph shows the number of spotless days per year since 1849. There are 106 years with at least 1 spotless day, of which 24 years with 150 or more spotless days. 1913 is recordholder with a staggering 311 days! Based on preliminary data, 2008 ranks fourth in years with a spotless sun (265 days; 19 July dropped off the list compared to previous update). Only 1878, 1901 and 1913 have even more such days. With 705 days in total and already 196 spotless days in 2009, the current solar cycle transit will have at least 3 years in this top 25. Compared to the solar cycle transits around the late 19th - early 20th century, the low 12th and moderate 15th solar cycle were preceded by 4 years with 140 or more spotless days. Despite the minimum, it seems that many more spotless days are on their way.
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Correction: 02 January 2009 - Both Kim Mackey and Chuck Beason spotted a missing 22-day stretch of spotless days (18 Nov 08 - 09 Dec 08). First two tables and related graph were corrected. Thanks everybody for keeping a watchful eye on this page.
Correction: 12 June 2008 - Jeff Lackey (ARRL) spotted an error in the top table (June-July 1976). As a result, the entire database was double-checked. The first two tables and graph were adapted. There is no change in the overall conclusions on the evolution of spotless days in the solar cycles considered.
Previous updates: 01 January 2008, 02 May 2008, 01 September 2008, 02 January 2009
Original: 07 October 2007