What is Solar Minimum and Why Do We Care?

1. What is Solar Minimum andWhy Do We Care? W. Dean Pesnell NASA, Goddard Space Flight Center

2. How Does the Sun Look? Summer Solstice at Stonehenge, 2005 Here are two views of the Sun on 29 September 2009. Visible spots on the left correspond to magnetic regions on the right.

3. What is Solar Minimum? How can we define solar minimum? The traditional emphasis on sunspot number (minimum in Rz, maximum in number of spotless days) New cycle activity exceeds old cycle activity Corona shape and alignment Maximum in polar field Others? Is this minimum weird? What are the interesting features of this minimum? What will Solar Cycle 24 look like?

4. What is Solar Minimum? Solar minimum is the set of contiguous months during the solar cycle when the 12-month mean of monthly average sunspot numbers is the smallest. Solar minimum is defined only in terms of sunspot number. The Sun is still emitting light, particles, and magnetic field. It is simply a different state.

5. What is Solar Minimum? The solar minimum between Solar Cycles 22 and 23 could be May 1996 (by the minimum definition) or October 1996 if more information, such as the number of old vs. new spots, is used. Could a quantity that peaks at solar minimum be used to define the instant of solar minimum? (Is there a conjugate variable?)

6. WSO Polar Magnetic Field Polar field has peak at minimum, sign change at maximum We have three successively smaller minima (1.3, 1.0, 0.6)! Data available at http://wso.stanford.edu

7. The Polar Magnetic Field Peaks Near Solar Minimum

8. So we have an example of a secondary variable, but it is not possible to use it to define the instant of solar minimum Is solar minimum late? No, a little long and deviates from recent past but within observed range If minimum was in December 2008 SC 23 was about 12.6 years long, one standard deviation from the mean length, longest since SC 6 We have have been spoiled with fairly regular cycles for 50 years Is it weird? Not really The number of spotless days (>729 since 2006) is large compared to the average of 485 It is also extremely well-observed by many observatories But, there are some interesting features of this minimum Oddities in this Minimum

9. Sunspot and F10.7 Residual of the fit between RZ and F10.7 (in blue.) The North-South asymmetry in the number of active regions is shown as a solid red line. The tendency of RZ to be smaller than F10.7 may be increasing with time.

10. Are Sunspots Fading Away? Livingston and Penn have been studying the magnetic field and umbra/photosphere contrast ratio in sunspots since the maximum of Solar Cycle 22. The magnetic field in the umbral region has been decreasing while the umbra is also fading into the photosphere. Umbra Penumbra Photosphere

11. The Sun Today As we move thru solar minimum the large loops have faded and the coronal holes at the poles (PCHs) and equator are the featured presentation. At this time in the solar cycle our Space Weather is dominated by the high-speed streams coming from the now-fading equatorial coronal holes. The magnetic field near the pole (in the PCH) is 40% smaller this minimum. What about the PCH area? EIT Fe XII 195 Å (roughly 1.5 million K) 2009/10/08 01:13

12. Polar Coronal Hole Same Size Kirk, Pesnell, Young ,and Hess Webber, 2009, Solar Physics, 257, 99-111.

13. Notables in this Minimum • Solar EUV spectral irradiance • Helioseismology • Response of the magnetosphere • Orbital debris October 8, 2009, another spotless day on the Sun (MDI)

14. EUV: The Heartbeat of SWx • The solar EUV spectral irradiance causes much of what we call space weather • This irradiance has been reported by SEE on TIMED as daily and orbital values • Identifying the sources of this irradiance is a major goal of the EVE instrument on SDO

15. Ultrasound of the Sun Helioseismology compares how sound travels between different parts of the Sun to see into and through the Sun. Here we see that bands of faster rotating material (jet streams) appear to determine where sunspots appear (GONG and MDI). But we only have two points.

16. Ultrasound of the Sun Helioseismology compares how sound travels between different parts of the Sun to see into and through the Sun. Here we see that bands of faster rotating material (jet streams) appear to determine where sunspots appear (GONG and MDI). But we only have two points.

17. Ultrasound of the Sun Drawing a line at 25˚ shows how the zonal band moves thru the same latitude at about the time sunspots appear. Does this show how the Sun times solar activity?

18. State of the Magnetosphere The Dst index has dropped to levels not encountered before in the measured record. Only quicklook data is available in 2007 and 2008, but trend is present in the second half of 2006. Shown in black +’s is the number of days in each Carrington rotation with a daily average Dst < -25 (at least small storms) and sunspot number in red The trend after 2006 is the most interesting feature

19. A Bad Time for Satellite Breakups Deliberate and accidental satellite breakups have increased the amount of tracked orbital debris by 20-40%! Without an dramatic increase in solar activity we may have to live with this debris for a century.

20. How Active Will Solar Cycle 24 Be? Our lack of knowledge about the dynamo is summarized by the spread of predictions for Solar Cycle 24.

21. How Active Will Solar Cycle 24 Be? Blue = predicted Red = F10.7 (annual) +++ = F10.7 (monthly) … = date of prediction Solar activity predictions by Schatten et al., have used the polar magnetic field to predict 3 cycles and predict a low Cycle 24.

22. What Do We Learn FromSolar Minimum? The Sun’s magnetic field is created by an internal dynamo. We accept that major variations in that dynamo have occurred in the past. What could the present minimum teach us about the solar dynamo? Will sunspots continue to be a good indicator of solar activity? F10.7? For satellite drag the EUV spectral irradiance will tell us which works. Changes in the convection zone may be all that is necessary (magnetic field vs. sunspot formation) How does the magnetic field measured outside the Sun reflect what is happening inside the Sun? Solar dynamo is constantly running. Solar minimum is just as important as solar maximum. Does the lack of symmetry between the northern and southern hemispheres give us a clue? How far are we from physically consistent models of the solar dynamo?

23. Obrigado! The Solar Dynamics Observatory, the first mission of Living With a Star, will provide the data needed to understand the solar convection zone and how magnetic field is assembled and dissipated in the solar atmosphere. SDO is at the Cape, ready to GO! (more tomorrow) http://sdo.gsfc.nasa.gov

24. Summary • We need a physical definition of solar minimum, perhaps the competition between the 2 cycles, because we use the instant of minimum to set the timing of predictions for upcoming solar cycle. Best definition would be a dynamo model. • Solar minimum is a good time to study the Sun as a Star • Basal level of emission, effect on climate and evolution • Studies of isolated active regions that grow in coronal holes • Helioseismology without sunspots • Spectral irradiance levels • Relative importance of photons, particles, and magnetic field • Simple configuration of magnetic field and current sheet • Galactic cosmic rays are most dangerous during minimum and may be more so in the upcoming 11 years • Better predictions of solar activity would be useful

25. Magnetic Field • Three sunspot cycles are covered with regular LOS magnetograms • Shows the butterfly diagram plus the polar contribution and field surges that are not seen in the sunspot record