Transcript Slide 1

The Mean Field of the Sun
Leif Svalgaard
Stanford University
Sept. 2, 2011
1
Crimean Astrophysical
Observatory [since 1968]
Mount Wilson Observatory
[1970-1982]
2
Wilcox Solar Observatory
[since 1976]
3
How the Mean Field is measured
The MF is taken to be the difference between
the magnetic signal in λ525.0 nm and the nonmagnetic line λ512.4 nm.
4
The Early Interpretation
MF as measured at CrAO
1968
BINT = 7.2×10-5 BMF (for
2.0 R☼). “Thus there is a
very direct relationship in
polarity and in magnitude
between the mean solar
field and the observed
interplanetary field with a
4½ day delay” (Schatten,
1970)
5
The Polarity Relationship [then]
WSO 5 days before
6
The Polarity Relationship [now]
WSO 5 days before
IMF
7
Same Recurrence Periods over
Time for MF and HMF [IMF]
HMF
19262011
8
Rotation Plots of
the Sector Polarity
CH
CH
1
27
Bartels Rotations
Skylab Workshop, 1976
9
1976
12
The Potential Field Source
Surface Model Illustrates
Many First-Order Effects
4
Simplification and Flattening with Height
“Domes of closed field lines”
Flattening with Polar Fields
10
The Heliospheric Current Sheet through the
Solar cycle
Artist: Werner Heil
Cosmic Ray
Modulation
caused by
latitudinal
variation of
HCS and CIRs
Svalgaard & Wilcox, Nature, 1976
11
The MF at the Start of Cycle 24
12
And in Detail
Note the detailed 27-day recurrence in sign and magnitude of the MF
13
Different Observatories Agree on the
Polarity, but NOT on the Magnitude
Roughly: WSO = 2, CrAO = 1, MWO = 4, and SOLIS = 1
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MWO Anomaly
Solar Mean Field Normalized to SOLIS
160
uT
140
WSO
120
2.5 MWO disk avg
Average MF
100
80
60
40
CrAO
MWO
20
MWO disk avg
0
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
200
MF Oct 2003 - Jan 2007
SOLIS
150
100
50
WSO
0
-100
-50
0
50
-50
-100
-150
-200
SOLIS = 1.85 WSO
R2 = 0.80
100
Using the regression factors for each observatory
we can bring them all onto the same scale and
compute the yearly average of the magnitude
After the upgrade of MWO their MF is much too small
15
Evolution of the MF Since 2003
16
Getting Smaller
17
And Smaller
18
And Smaller, but still matching HMF polarity
19
The MF is Riding on a Background
HMF that does not Fall Below ~4 nT
20
The Importance of the Polar Fields [?]
Even with all the sophistication of current models of
the Corona and HMF they are hostage to the correct
value of the solar polar fields, which may be different
at the two poles and even have longitudinal structure
within the polar caps.
This is particularly important at solar minimum when
the HCS is largely flat.
Pneuman & Kopp, 1971
MHD
Vseskhsvjatsky, 1963
1954 Eclipse
21
Conclusion
• The Solar Mean Field continues to track the
polarity of the HMF
• The magnitude of the MF does not reflect that of
the HMF, but rides on top of a fixed [?]
background [i.e. that does not track the polar
fields]
• The MF can be used to monitor the calibration of
magnetographs
• We should calculate the MF from HMI as a
product.
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