Sequences of Spotless Days on the Sun
During September and October of 1996, we experienced a sequence of 37 days in a row during which there were no spots observed on the sun. This sequence is longer than any during recent solar minima - consistent which the current minimum being "deeper" than those during recent solar cycles.
But how does this sequence compare with historical sequences from the record of sunspot observations. The following table makes this comparison for observations since 1900 table.
| Year of Sequence | Number of Spotless Days |
|---|---|
| 1913 | 92 |
| 1901 | 69 |
| 1902 | 49 |
| 1902 | 45 |
| 1912 | 43 |
| 1901-2 | 40 |
| 1924 | 39 |
| 1913 | 39 |
| 1996 | 37 |
| 1933 | 36 |
| 1944 | 36 |
The table shows that the 1996 sequence, whilst impressive, is still considerably shorter than some early in the century. However, a note of caution is required because the coverage of observations was not nearly as good early this century as it is now. A sequence of spotless days can be broken by a single day on which a small spot appears. Lack of coverage by observations could therefore be very important in determining the length of such spotless sequences.
With the above qualification, the sequence in 1996 is still the longest observed in the last 50 years during which good observations have been available.
Material prepared by Richard Thompson
