1. What is an Extreme Solar Minimum?
W. Dean Pesnell
NASA, Goddard Space Flight Center

2. The Sun Has Many Cycles
Summer Solstice at Stonehenge, 2005
We used to time our years by the motion of the Sun in the sky, building large structures to anticipate that motion.

3. The Sun Has Many Cycles
Summer Solstice at Stonehenge, 2005
I would like to mention that Simon Newcomb ( ), who calculated a 5000 year catalogue of solar eclipses, was born about 150 km from here in Wallace.

4. 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?
Was solar minimum in December 2008? (SIDC ISSN continued to decrease until March 2009 but needs to be smoothed.)

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.
We define minimum different from maximum. While maximum is local, minimum is defined using a ceiling value (i.e., < 20). This implies that the Sun returns to the same base state at every minimum.

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. 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)!

8. The Polar Magnetic Field Peaks Near Solar Minimum

9. 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 Swarthmore/Newark (blue) and Thule are the other stations. (The neutron monitors of the Bartol Research Institute are supported by NSF grant ATM )

10. Cosmic Rays are Maximum after Solar Minimum
By plotting the negative of the relative fluxes we see a pattern similar to the sunspot number.

11. 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

12. Oddities in this Minimum
So we have a few examples of secondary variables, but not enough to use them to define the instant of solar minimum
Is it late?
No, a little long and deviates from recent past but well within average
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 (>670 since 2006) is large compared to the average of 485, but similar to the early part of the 20th century
It is also extremely well-observed by many observatories
But, there are some interesting features of this minimum

13. Spotless Days?
The number of spotless days has tended to go down while the total sunspot number per cycle has increased.

14. Notables in this Minimum
Solar EUV spectral irradiance
Helioseismology
White light (TSI & corona)
Response of the magnetosphere
July 27, 2009, another spotless day on the Sun (MDI)

15. 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

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
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.

18. 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?

19. Total Solar Irradiance
Figure from C. Fröhlich
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

20. 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.

21. Coronal Variations Have Not Simplified as Yet
Dissociates O2
Corona evolves from simple, dipolar-like structure aligned with the rotation axes at minimum to complex at maximum and back to simple
Current LASCO images show an aligned structure with multiple streams
Work done by Daniel Young and Chris St. Cyr shows pattern is consistent through three cycles, possible extension to include all photographed eclipses

22. 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

23. 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.

24. What Do We Learn From Solar 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 does the present minimum teach us about the solar dynamo?
Is the core a part of the variation?
Changes in the convection zone may be all that is necessary
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?

25. Predictions of Solar Cycle 24
Our lack of knowledge about the dynamo is summarized by the spread of predictions for Solar Cycle 24.

26. 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

27. Questions? SDO is at the Cape, ready to GO!
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!