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The Solar Cycle

The Solar Cycle. David H. Hathaway NASA/NSSTC AAS/SPD 2007 May 30. Outline. Significance of the Solar Cycle Characteristics of the Sunspot Cycle Characteristics of the Magnetic Cycle Predictions for Solar Cycle 24 Conclusions. Significance of the Solar Cycle.

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The Solar Cycle

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  1. The Solar Cycle David H. Hathaway NASA/NSSTC AAS/SPD 2007 May 30

  2. Outline • Significance of the Solar Cycle • Characteristics of the Sunspot Cycle • Characteristics of the Magnetic Cycle • Predictions for Solar Cycle 24 • Conclusions

  3. Significance of the Solar Cycle

  4. Solar Activity: The Bastille Day Event Solar activity (including: flares, prominence eruptions, coronal mass ejections, solar energetic particle events, and high-speed solar wind streams from coronal holes) varies in frequency with the solar cycle.

  5. Solar Activity Affects…

  6. Activity Cycles on Other Stars Activity cycles are also seen on other stars by measuring photo-metric changes in Ca II K-line emission. Cycle periods vary with the stellar rotation rate relative to the convection timescale. Radick, R. R. (2000) Adv. Space Res. 26, 1739-1745

  7. Characteristics of the Sunspot Cycle

  8. Sunspot Number and Solar Activity Sunspot Area 10.7cm Radio Flux GOES X-Ray Flares Total Irradiance Geomagnetic aa index Climax Cosmic-Ray Flux Sunspot number is well correlated with solar activity. The 400-year length of the sunspot number record helps to characterize the solar cycle. The connection with cosmic rays leaves even longer records of solar activity in tree rings (14C) and ice cores (10Be).

  9. The 11-year Sunspot or Wolf Cycle [Schwabe, 1844] Heinrich Schwabe, a Swiss apothecary, reported a cycle of about 10-years length in the number of sunspot groups and spotless days from 18 years of his own observations. Rudolf Wolf then initiated daily counts of sunspots and attempted to extend the count back to 1749.

  10. The Shape of the Solar Cycle The sunspot cycles are asymmetric in shape with rapid rises to maximum and slower declines to minimum. Bigger cycles take less time to reach maximum than do smaller cycles (The Waldmeier Effect). Cycle periods have been normally distributed about a mean of about 131 months.

  11. Sunspot Latitude Drift – Spoerer’s Law [Carrington, 1858] Sunspots appear in two bands on either side of the equator. These bands spread in latitude and migrate toward the equator as the cycle progresses. Cycles often overlap at minimum.

  12. Dalton Minimum The Maunder Minimum [Maunder, 1894] Sunspot cycles vary widely in amplitude with occasional periods of inactivity like the Maunder Minimum (1645-1715).

  13. Multi-Cycle Variability After removing the secular trend, there is little evidence for any significant periodic behavior with periods of 2-cycles (Gnevyshev-Ohl) or 3-cycles (Ahluwalia), and only weak evidence for variability over 7- to 10-cycles (Gleissberg).

  14. Short-Term Variability A number of short-term periodicities have been reported for solar activity. In most, if not all cases, the period and the phase of the oscillations vary with time. Empirical Mode Decomposition Monthly sunspot numbers 20 21 22 Decadal time scale variations (The solar cycle) Bi-annual/Annual time scale variations (Double peaks for some cycles depending on phase) Annual/Semi-annual time scale variations (152d periodicity from 1980 to mid-1983) Monthly time scale variations (Largely noise)

  15. Hemispheric Differences The hemispheres display distinct asymmetries (North - South) that are evident in a variety of indicators. Yet, both hemispheres are more-or-less locked in phase.

  16. Magnetic Cycle Characteristics

  17. Active Region Tilt: Joy’s Law [Hale et al., 1919] Active regions (sunspot groups) are tilted so that the following polarity spots are slightly poleward of the preceding polarity spots. This tilt increases with latitude. Howard (1991)

  18. 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.

  19. The Sun’s Magnetic Cycle Magnetic field erupts through the surface as tilted bipoles in two bands on either side of the equator. Individual regions decay by spreading out over the surface. Remnant magnetic elements are sheared apart by differential rotation and carried poleward by a meridional flow.

  20. Polar Field Reversals [Babcock, 1959] The polarity of the polar magnetic fields reverses at about the time of the solar activity maximum.

  21. Solar Cycle Predictions

  22. Dikpati & Charbonneau Dynamo This is a 2D kinematic dynamo which uses the observed internal differential rotation, a realistic meridional circulation, a reasonable diffusivity, and a parameterized α-effect. It produces a reversing magnetic field configuration with a 22-year period and an equator-ward propagation of active zones. In/CCW Out/CW

  23. A Dynamo Prediction Dikpati, de Toma & Gilman (2006) have fed sunspot areas and positions into their numerical model for the Sun’s dynamo and reproduced the amplitudes of the last eight cycles with unprecedented accuracy (RMS error < 10). Recent results for each hemisphere shows similar accuracy. Cycle 24 Prediction ~ 160 ± 15

  24. Precursor Predictions Precursor techniques use aspects of the Sun and solar activity prior to the start of a cycle to predict the size of the next cycle. The two leading contenders are: 1) geomagnetic activity from high-speed solar wind streams prior to cycle minimum and 2) polar field strength near cycle minimum. Geomagnetic Prediction ~ 160 ± 25 (Hathaway & Wilson 2006) Polar Field Prediction ~ 75 ± 8 (Svalgaard, Cliver, Kamide 2005)

  25. Cycle 24 Predictions Based on the appearance of the remains of a new cycle spot at 33°N in late November 2006, we expect to go through minimum in the spring of 2008. A large cycle (sunspot number ~ 160) would then peak in late 2011 while a small cycle (sunspot number ~ 75) would peak in late 2012.

  26. Conclusions • The solar cycle displays numerous significant characteristics which should be reproduced by dynamo models. • Explicit Dynamo modeling of the cycles since 1880 using sunspot areas as input predicts the amplitudes of the last eight cycles with better accuracy than any other method and predicts a larger than average cycle 24 (sunspot number ~ 160). • Geomagnetic precursor activity also indicates a much larger than average cycle 24 (sunspot number ~ 160). • Polar field strength indicates a smaller than average cycle 24 (sunspot number ~ 75). See: http://solarscience.msfc.nasa.gov/

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