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What is an Extreme
Solar Minimum?
W. Dean Pesnell
NASA, Goddard Space
Flight Center
SHINE 2009 Workshop, August 2009
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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.
SHINE 2009 Workshop, August 2009
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The Sun Has Many Cycles
Summer Solstice at Stonehenge, 2005
I would like to mention that Simon Newcomb (1935-1909), who
calculated a 5000 year catalogue of solar eclipses, was born
about 150 km from here in Wallace.
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What is Solar Minimum?
How can we define solar minimum?
1.
2.
3.
4.
5.
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.)
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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.
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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?)
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WSO Polar Magnetic Field
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Polar field has peak at minimum, sign change at maximum
We have three successively smaller minima (1.3, 1.0, 0.6)!
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The Polar Magnetic Field Peaks
Near Solar Minimum
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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.)
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Cosmic Rays are Maximum
after Solar Minimum
By plotting the negative of the relative fluxes we see a pattern similar to the sunspot number.
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Magnetic Field
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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
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Oddities in this Minimum
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So we have a few examples of secondary variables, but not
enough to use them to define the instant of solar minimum
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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
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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
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Spotless Days?
The number of spotless days has tended to go down while the total sunspot
number per cycle has increased.
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Notables in this Minimum
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Solar EUV spectral irradiance
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Helioseismology
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White light (TSI & corona)
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Response of the
magnetosphere
SHINE 2009 Workshop, August 2009
July 27, 2009, another spotless
day on the Sun (MDI)
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EUV: The Heartbeat of SWx
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The solar EUV spectral irradiance causes much of what we call space weather
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This irradiance has been reported by SEE on TIMED as daily and orbital values
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Identifying the sources of this irradiance is a major goal of the EVE instrument on SDO
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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.
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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.
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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?
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Total Solar Irradiance
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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
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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.
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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
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Current LASCO images show an aligned structure with multiple streams
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Work done by Daniel Young and Chris St. Cyr shows pattern is consistent through three
cycles, possible extension to include all photographed eclipses
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State of the Magnetosphere
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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
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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.
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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?
1.
2.
3.
4.
5.
6.
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?
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Predictions of Solar Cycle 24
Our lack of knowledge about the dynamo is summarized
by the spread of predictions for Solar Cycle 24.
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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
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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
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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!
SHINE 2009 Workshop, August 2009
http://sdo.gsfc.nasa.gov
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