1. NASA MSFC Space Science Office
Solar Cycle Update
David H. Hathaway
NASA MSFC Space Science Office
2013 August 17
Huntsville Hamfest

2. Don’t shoot the messenger!

3. Outline The Sun and Solar Activity The Sun and the Ionosphere
The Solar Cycle
The Sun’s Magnetic Dynamo
Solar Cycle Predictions

4. The Sun and Solar Activity

5. The Sun Radius = 109 REarth Mass = 333,000 MEarth
Surface Temp = 9,930 °F
Surface Density = Air/5000
Core Temp = 28 million °F
Core Density = Gold × 8
Composition: 70% H
28 % He
2% (C, N, O…)

6. Sunspots
Sunspots are dark (and cooler) regions on the surface of the Sun. They have a darker inner region (the Umbra) surrounded by a lighter ring (the Penumbra).
Sunspots usually appear in groups that form over hours or days and last for days or weeks.
The earliest sunspot observations indicated that the Sun rotates once in about 27 days.

7. Sunspot Structure
Sunspots are regions where intense magnetic fields break through the surface of the Sun. The magnetic field strengths are typically about 6000 times stronger than the Earth’s magnetic field.
Magnetic fields and the ionized gases within the Sun are intimately tied together. Where magnetic pressure dominates – the gas follows the magnetic field. Where gas pressure dominates – the magnetic field follows the gas. In sunspots the magnetic pressure dominates – this inhibits the convective transport of heat and makes sunspots cooler.

8. Solar Magnetism – The Key

9. Explosive Events
Solar flares – X excess in X-rays and Extreme Ultraviolet (EUV)
Coronal Mass Ejections (CMEs) – magnetic clouds blasted off the Sun
Solar Energetic Particles – relativistic particles from flares and CMEs

10. CME Impact on Earth
Magnetized clouds of plasma blasted off of the Sun as CMEs can impact the Earth’s environment – distorting the magnetic field surrounding the Earth and producing energetic particles that stream into the Earth’s atmosphere to create aurorae.

11. Space Weather
Space weather refers to conditions on the Sun and in the space environment that can influence the performance and reliability of space-borne and ground-based technological systems, and can endanger human life or health.

12. The Sun and the Ionosphere

13. The Basics
Extreme Ultraviolet (EUV) and X-rays from the Sun ionize the gases in the Earth’s upper atmosphere. At night the ions and electrons recombine at the denser lower altitudes (the D and E layer) where collisions are more frequent. The D layer disappears at night. The E layer (or patches of the E layer) can survive the night if the solar EUV is strong. Solar EUV varies with the sunspot cycle – strong at max.

14. The solar spectrum from X-rays to Microwaves
Solar ultraviolet light (UV) is energetic enough to dissociate O2 molecules and form O3 (ozone) in the stratosphere. It takes Extreme Ultraviolet (EUV and XUV) to ionize atoms and molecules in the ionosphere.
N2 -> N2+ + e- for λ < 80 nm
O -> O+ + e- for λ < 91 nm
O2 -> O2+ + e- for λ < 103 nm
NO -> NO+ + e- for λ < 134 nm
EUV produces the F-region
XUV (+ Ly β) produces the E-region
X-rays (+ Ly α) produce the D-Region
Ly β = nm, Ly α = nm
Ly α
He 30.4
The solar spectrum from X-rays to Microwaves

15. Solar Cycle EUV Variability
Measurements of the flux of radio waves at 10.7 cm (2.8 GHz) have been made three times daily since 1947 from sites in Canada. This is often used as a proxy for solar EUV to characterize the ionospheric forcing. However, measurements of the actual EUV emissions indicate some deviations – in particular at the last cycle minimum.
30.4 nm
Araujo-Pradere et al. (2011)

16. Wavelength coverage (2004)
TIMED SEE
The Solar EUV Experiment (SEE) on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission was designed to fill in this “EUV Hole”. TIMED was launched in 2002 and is still operating.
Wavelength coverage (2004)
XUV Variability

17. SORCE SSI

18. MUF and TEC
The Maximum Usable Frequency (MUF) for radio waves reflected off of the ionosphere is directly related to the Total Electron Content (TEC) of the ionosphere. The TEC depends upon both the solar EUV and X-ray irradiance and geomagnetic activity. We now have direct observations of solar EUV/XUV/X-ray emissions – no need for proxies!

19. The Solar Cycle

20. Sunspot Cycle Discovery
Astronomers had been observing sunspots for over 230 years before Heinrich Schwabe, an amateur astronomer in Dessau, Germany, discovered in 1844 that the number of sunspot groups and the number of days without sunspots increased and decreased in cycles of about 10-years.
Schwabe’s data for 1826 to 1843
Number of Sunspot Groups per Year
Number of Spotless Days

21. 23 Full Cycles
Shortly after Schawbe discovery Rudolf Wolf proposed using a “Relative” Sunspot Number count. While there were many days without observations prior to 1849, sunspots have been counted on every day since. To this day we continue to use Wolf’s Relative Sunspot Number and his cycle numbering.
The average cycle lasts about 11 years, but with a range from 9 to 14.
The average amplitude is about 100, but with a range from 50 to 200.

22. Sunspot Latitudes
Sunspots appear in two bands on either side of the equator. These bands drift toward the equator as the cycle progresses. Big cycles have wider bands that extend to higher latitudes. Cycles overlap by 2-3 years.

23. Hale’s Magnetic Polarity Law
In 1919 Hale (along with Ellerman, Nicholson, and Joy) found that the magnetic field in sunspots followed a definite law, “Hale’s Law” such that: “…the preceding and following spots … are of opposite polarity, and that the corresponding spots of such groups in the Northern and Southern hemispheres are also opposite in sign. Furthermore, the spots of the present cycle are opposite in polarity to those of the last cycle”.

24. Active Region Tilt- Joy’s Law
In that same 1919 paper Joy noted that sunspot groups are tilted with the leading spots closer to the equator than the following spots. This tilt increases with latitude.

25. Three Solar Cycles in Action
In addition to these magnetic polarity changes and the equatorward drift of the sunspot latitudes, there are important flows on the surface and within the Sun: Differential Rotation – faster at the equator, slower near the poles; and Meridional Flow – flow from the equator toward the poles along the surface.

26. Polar Field Reversals
In 1958 Babcock and Livingston noted that the magnetic polarities of the Sun’s weak polar fields also reverse from one cycle to the next, and that this reversal happens at about the time of sunspot cycle maximum (the South reversed in 1957 the North in 1958).

27. The Sun’s Magnetic Dynamo

28. Babcock’s Dynamo (1961)
Dynamo models have been developed to explain the sunspot cycle.
a) Dipolar field at cycle minimum threads through a shallow layer below the surface.
b) Differential rotation shears out this poloidal field to produce a strong toroidal field (first at the mid-latitudes then progressively lower latitudes).
c) Buoyant fields erupt through the photosphere giving Hale’s polarity law and Joy’s Law.
d) Meridional transport cancels preceding polarities across the equator and carries following polarity to the poles.

29. Polar Fields – Seeds for Cycles
The Sun’s polar fields are the seeds of the next solar cycle in these dynamo models. We have direct observations for the last three cycles and a proxy (polar faculae) for the last 10 cycles.
Muñoz-Jaramillo et al. (2013)

30. Polar Faculae Intensity Magnetic Field
Polar faculae are small bright features seen in the polar regions in intensity images. They are associated with small magnetic features and are produced by the fact that these small magnetic tubes allow us to see into the Sun where it is hotter and brighter.
Keller et al. (2004)

31. Polar Fields as Predictors
Muñoz-Jaramillo et al. (2013) find a strong correlation between polar fields and the amplitude of the next solar cycle (hemisphere by hemisphere in terms of peak sunspot area).
North – Blue squares
South – Red circles
Cycles are from direct magnetic measurements.
Earlier cycles are from counting polar faculae.

32. Polar Field Prediction for Cycle 24
Small Cycle 24 predicted by Polar Fields (Svalgaard, Cliver, & Kamide, 2005)

33. Four Years Ago - Minimum
Small Cycle 24 predicted.

34. This Year - Maximum This is the Maximum of Cycle 24!
The polar fields built up during Cycle 23 were a good predictor.

35. How are Polar Fields Made?
Back to Babcock (1961)
a) Dipolar field at cycle minimum threads through a shallow layer below the surface.
b) Differential rotation shears out this poloidal field to produce a strong toroidal field (first at the mid-latitudes then progressively lower latitudes).
c) Buoyant fields erupt through the photosphere giving Hale’s polarity law and Joy’s Law.
d) Meridional transport cancels preceding polarities across the equator and carries following polarity to the poles.

36. Polar Field Production
Variations in sources: more sunspots – more field
Variations in source details: more tilt – more field
Variations in poleward transport: meridional flow variations

37. Polar Field Production I
Cycles with more sunspots should produce stronger polar fields which leads to another strong cycle … some other variation is needed to modulate/regulate this process. Otherwise the cycles would either grow without limit or decay to nothing.

38. Polar Field Production II
Dasi-Espuig et al. (2010) found that the average tilt, <α>, of sunspot groups changed with cycle amplitude. Small tilt in big Cycle 19 would counteract the large number of sunspots and modulate the polar field production. Problems: results from different observatories are wildly different and big tilt in big Cycle 21.
Solid line - <α> from Mt. Wilson Observatory
Dash-Dot line - <α> from Kodaikanal Observatory
Dashed line – sunspot area from Greenwich Observatory

39. Polar Field Production III
We have made measurements of the Meridional Flow of magnetic elements since 1996 and find significant variations.
The Meridional flow is fast at cycle minima and slow at cycle maxima.
The slow-down at cycle maxima depends on cycle strength – much slower in Cycle 23 than in Cycle 24.
The slow down occurs at latitudes just above the sunspot zones (red dots).
This would transport less following polarity magnetic field to the poles in big cycles.

40. The Prognosis for Cycle 25
Cycle 24 has far fewer (and smaller) sunspots than Cycles 22 and 23. Although the faster Meridional flow should help overcome this deficit, the slow rate of change in the polar fields strongly suggests that polar fields that will build up over the rest of Cycle 24 will still be very weak.

41. The Incredible Shrinking Sunspot Cycle

42. Conclusions Solar activity impacts modern technology
Solar EUV/XUV emissions and magnetic disturbances control the ionosphere
We are making progress in understanding the Solar Cycle
Cycle 24 is at its peak – the smallest in 100 years.
Cycle 25 may be even smaller yet!