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. What Does SC24 Look Like? 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. Minimum has passed in F10.7 F10.7 is the solar radio irradiance at 10.7 cm (2.8 GHZ), measured near Penticton, Canada. It is well-correlated with sunspot number and is often used in models of the thermosphere and ionosphere. Minimum in F10.7 occurred in late 2008.

5. 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.

6. 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?)

7. Sunspot Number The annual sunspot number has about 180 years of observations. The current minimum is remaining low for the longest period of time in about 100 years.

8. Spotless Days? The number of spotless days peaks at solar minimum. The current minimum has had more spotless days then any cycle since 1900. Before 1900 the number of spotless days is not a good index. But, this still defines minimum by a lack of spots.

9. What is Solar Minimum? A linear oscillator can be described with a pair of phase space variables. The square of the displacement can be plotted as the solar cycle (with minima shown by the vertical lines.)

10. What is Solar Minimum? Adding the squared velocity in red shows how the maxima align with the minima in a linear oscillator.

11. What is Solar Minimum? Many nonlinear oscillators are described with a pair of phase space variables. A well-known example is the Van der Pol oscillator and equation.

12. What is Solar Minimum? The time dependence of a moderately nonlinear Van der Pol oscillator ( = 0.5) looks more like the solar cycle, with a rapid rise and a more gradual decline.

13. What is Solar Minimum? But the other variable does not have to peak at the minimum of the first. A second quantity does not necessarily help define the time of minimum.

14. 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

15. The Polar Magnetic Field Peaks Near Solar Minimum

16. 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

17. WSO Current Sheet Tilt Maximum inclination of the current sheet from 1976 to mid-2009 Although tilt was high longer than the past two minima, it has since dropped Data available at http://wso.stanford.edu

18. Cosmic Rays are Maximum after Solar Minimum The cosmic ray flux continues to grow and has exceeded earlier fluctuations. McMurdo currently has the highest relative flux. The highest relative flux is at South Pole in 1965 but measurements there stopped in November 2005. Swarthmore/Newark (blue) and Thule are the other stations. (The neutron monitors of the Bartol Research Institute are supported by NSF grant ATM-0527878.)

19. Cosmic Rays are Maximum after Solar Minimum By plotting the negative of the relative fluxes we see a pattern similar to the sunspot number. Cosmic rays respond to the global field, but their timing is delayed by the propagation time.

20. So we have an example of a secondary variable, but it is not possible to use it to define the instant of solar minimum Physical models of the solar dynamo are required 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 (>740 since 2004) 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

21. Total Solar Irradiance TSI is lower this minimum than the previous two Unexpected change after a greatly disputed increase in the previous minimum Few mechanisms exist for magnetic changes in the basal solar luminosity Figure from C. Fröhlich

22. 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.

23. 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

24. Polar Coronal Holes 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) 2009/10/31 01:13

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

26. 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)

27. 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

28. 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.

29. 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.

30. 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. Active regions have continued to appear in the band as it moves toward the equator. Does this show how the Sun times solar activity?

31. p-mode Frequencies are Smaller Salabert, et al. (2009), The onset of solar cycle 24, A&A, 504, L1-L4. Low-degree p-modes in this minimum have lower frequencies than in 1996. Is there a helioseismic minimum?

32. 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

33. 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.

34. 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.

35. 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.

36. 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?

37. 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

38. What Will Watch SC24? The total mass of the spacecraft at launch is 3200 kg (payload 270 kg; fuel 1400 kg). Its overall length along the sun-pointing axis is 4.5 m, and each side is 2.22 m. The span of the extended solar panels is 6.25 m. Total available power is 1450 W from 6.5 m2 of solar arrays (efficiency of 16%). The high-gain antennas rotate once each orbit to follow the Earth. Launch is planned on an Atlas V EELV SDO will be placed into an inclined geosynchronous orbit ~36,000 km (21,000 mi) over New Mexico for a 5-year mission

39. Questions? 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! http://sdo.gsfc.nasa.gov