What the Long-Term Sunspot Record Tells Us About Space Climate

1. What the Long-Term Sunspot Record Tells Us About Space Climate David H. Hathaway NASA/MSFC National Space Science and Technology Center Huntsville, AL, USA 2004 June 21 Space Climate Symposium Oulu, Finland

2. Outline • Sunspot Data • Sunspot Number and Solar Activity • Sunspot Cycle Characteristics • Conclusions

3. Sunspot Data Sunspot Number International Sunspot Number 1749-Present (Complete from 1849) R = 10 G + N (G = # of Groups, N = # of spots) Group (Hoyt and Schatten) Sunspot Number 1610-1995 GSSN = 12.08 G Sunspot Areas and Positions Royal Greenwich Observatory 1874-1976 NOAA/USAF 1969-Present Mount Wilson Observatory 1917-1985

4. Sunspot Number and Solar Activity Sunspot number is well correlated with other measures of solar activity. The long record of sunspot numbers helps to characterize the solar cycle. Sunspot Area 10.7cm Radio Flux GOES X-Ray Flares Total Irradiance Geomagnetic aa index Climax Cosmic-Ray Flux

5. The “11-year” Sunspot or Wolf Cycle [Schwabe, 1844] Cycle periods are normally distributed with a mean period of 131 months and a standard deviation of 14 months.

6. Sunspot Latitude Drift: Spörer’s Law [Carrington, 1858] Sunspots appear in two bands on either side of the equator. These bands spread in latitude and then migrate toward the equator as the cycle progresses. Cycles often overlap around the time of minimum.

7. Active Region Tilt: Joy’s Law [Hale et al., 1919] Active regions are tilted so that the following polarity spots are slightly poleward of the preceding polarity spots. This tilt increases with latitude. Howard, R.F., Solar Phys. 136, 251-262 (1991)

8. Hale’s Polarity Law [Hale, 1924] The polarity of the preceding spots in the northern hemisphere is opposite to the polarity of the preceding spots in the southern hemisphere. The polarities reverse from one cycle to the next.

9. Polar Field Reversal at Cycle Maximum [Babcock, 1959] The polarity of the polar magnetic fields reverses at about the time of the solar activity maximum.

10. The Sun’s Magnetic Cycle This movie shows Spörer’s Law, Joy’s Law, and Hale’s Law. It also reveals the differential rotation and poleward meridional flow.

11. The Waldmeier Effect The sunspot cycles are asymmetric – the time to rise to maximum is less than the time to fall to minimum. Furthermore, large amplitude cycles take less time to reach maximum than do small amplitude cycles.

12. Amplitude-Period Effect The amplitude of a cycle is weakly correlated to its period. A stronger, and more significant correlation is between the amplitude of a cycle and the period of the previous cycle.

13. Amplitude-Minimum Effect The amplitude of a cycle is correlated with the size of the minimum that precedes the cycle.

14. Individual Cycle Characteristics Big cycles start early and grow fast. This directly produces the Waldmeier Effect. By doing so they cut short the previous cycle (Amplitude-Period Effect) and produce a higher minimum due to the overlap (Amplitude-Minimum Effect).

15. Hemispheric Differences The hemispheres display distinct asymmetries (North - South) that are evident in a variety of indicators.

16. North-South Asymmetry The association of this asymmetry with a dynamo mechanism is still unclear. Finding the answer to why the activity is asymmetric may tell us more about the dynamo mechanisms.

17. Latitude Drift of Sunspot Zones We examined the latitude drift of the sunspot zones by first separating the cycles where they overlap at minimum. We then calculated the centroid position of the daily sunspot area averaged over solar rotations for each hemisphere. [Hathaway, Nandy, Wilson, & Reichmann, ApJ 589. 665-670 2003 & ApJ 602, 543-543 2004]

18. Centroid Position vs. Time The centroid of the sunspot area drifts toward the equator and slows to a stop at a latitude of about 8°. There is no indication of a poleward moving branch at higher latitudes.

19. Drift Rate vs. Latitude The drift rate in each hemisphere and for each cycle (with one exception) slows as the activity approaches the equator.

20. Drift Rate – Period Anti-correlation The sunspot cycle period is anti-correlated with the drift velocity at cycle maximum. The faster the drift rate the shorter the period. R=-0.5 95% Significant

21. Drift Rate – Amplitude Correlations The drift velocity at cycle maximum is correlated to the cycle amplitude but a stronger and more significant correlation is with the amplitude of the second following (N+2) cycle. This may help to explain some of the hemispheric asymmetry. R=0.5 98% Significant R=0.7 99% Significant

22. Dynamos with Meridional Flow These observations strongly support dynamo models that incorporate a deep meridional flow to transport magnetic flux toward the equator at the base of the convection zone. Dikpati and Charbonneau, ApJ 518, 508-520, 1999

23. Long-Term Variability The Group Sunspot Number of Hoyt and Schatten closely follows the International Sunspot number as well as other indicators of solar activity. It has the advantage of extending back in time through the Maunder Minimum to 1610.

24. Secular Trend Since Maunder Minimum The Group Sunspot Number shows a significant secular increase in cycle amplitude since the Maunder Minimum.

25. Multi-Cycle Periodicities? After removing the secular trend, there is little evidence for any significant periodic behavior with periods of 2-cycles (Gnevyshev-Ohl), 3-cycles (Ahluwalia), or 7- to 10-cycles (Gleissberg). This does not preclude quasi-periodic variations on these or longer time scales.

26. Conclusions • Sunspot numbers are well correlated with other activity indicators – and should be good indicators of space climate • Cycle periods are normally distributed (131±14 months) • Big cycles start early and grow rapidly • There are real asymmetries between the hemispheres • The equatorward drift of the activity bands favors dynamos with deep meridional flows • There has been a secular increase in cycle amplitude since the Maunder Minimum • Evidence for multi-cycle periodicities is weak in the limited length of the sunspot record • These conclusions are subject to revision