Letter to the Editor
A global small sunspot deficit at the base of the index anomalies of solar cycle 23
L. Lefèvre and F. Clette
Royal Observatory of Belgium (ROB), Circulaire 3, 1180 Bruxelles, Belgium
e-mail: firstname.lastname@example.org; email@example.com
Received: 6 September 2011
Accepted: 16 November 2011
Context. The variability of the 11-year cycle of solar activity on secular timescales is well established through the sunspot record, but it remains unpredictable. Indeed, the duration of the last solar cycle 23 was exceptionally long and took the solar physics community by surprise. The long minimum was marked by particularly low and often unprecedented levels of the international sunspot index Ri and most other solar indices. Earlier in the course of cycle 23, discrepancies appeared between several of those indices, raising a new issue: is there something wrong in the recent index values or is a real physical change occurring inside the Sun?
Aims. By exploiting detailed sunspot information, we look for independent evidence of a concrete and significant global change in sunspot properties appearing in the course of cycle 23.
Methods. To achieve this goal, we compared existing sunspot indices, such as the international sunspot number (Ri), and exploited the most complete information currently available on individual sunspots, obtained by merging two recent and complementary catalogs. Detailed statistics were obtained according to group type and spot size.
Results. We find that the Sun has shown an important deficit in small spots since the last activity maximum around 2000. While the number of large-scale spots remained largely unaffected, the occurrence rate of the smallest sunspots, and among them the ones with the shortest lifetimes, was more than halved during cycle 23. This explains the divergence between indices, weighted in favor of the largest active regions/magnetic structures, and sunspot counts that do not include such a weighting. It also confirms an actual intrinsic transition in the magnetic field generation inside the Sun, arising years before the exceptional activity minimum.
Conclusions. Our results thus reveal the potential of such detailed sunspot analyses for understanding and predicting future trends in the solar cycle. The change found here in the small individual sunspots suggests that solar and solar-terrestrial proxies should be redefined for the current state of the Sun, replacing the present ones. This scale-dependent change also provides support to dynamo models involving the coexistence of a deep and a superficial dynamo.
Key words: methods: data analysis / Sun: activity / sunspots / catalogs / Sun: dynamo
© ESO, 2011