Long-term Solar Synoptic Measurements with Implications for the Solar Cycle

1

Long-term Solar Synoptic Measurements with Implications

for the Solar Cycle

Leif SvalgaardStanford University

23 April 2013

2

Dimensions of Synoptic Observations

• Time Dimension (long-term data sets)• Spatial Dimension (distribution over the

disk of activity, synoptic maps, height, depth)

Most efficient are visualizations combining the two dimensions, e.g.

• Movies, Butterfly diagram, Rotation sequences, …

3

Classes of Synoptic Observations• Single Point (e.g. Center Disk spectra)• Full-Disk (e.g. Sunspot Number, F10.7 flux,

Mean Field, Total Solar Irradiance)• Image-based (e.g. Sunspot Area, Ca K line

index, Magnetograms, Synoptic charts)

And aggregate views derived from images, e.g.• Polar Field Evolution• Hemispheric Asymmetries• Meridional and ‘Torsional’ circulations

4

Synoptic Observations are Important for Understanding and Predicting the Solar Cycle

• With limited time for this presentation Leif can only touch upon some aspects that have been important for his own research

• It should, however, be clear how those relate to the wider issues, like constraining dynamo theories, forming inner boundary conditions for space weather, affecting the environment of the Earth and our space assets and technological infrastructure.

5

Predicting Solar Activity

Sunspots

Polar Fields

Solar Prediction Panel, 2006

Observations and theory suggest that the magnetic field at the poles of the Sun at solar minimum is a good predictor of the next solar cycle.

Theory:Deep circu-lation?

The low polar fields at the recent solar minimum predicted a small cycle 24

-400

-300

-200

-100

0

100

200

300

400

1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015

MSO* WSO

North - South Solar Polar fields [microTesla]

Sunspots 24232221

MWO* WSO

?

WSO

Leif

Measurements of the Polar Fields [within 30 degrees of the pole] seems to be a good predictor of solar activity for the next cycle. The Cycle 24 Prediction Panel did not quite 'go all the way' and predicrt the 'smallest cycle in a 100 years'.

6

050

100150200250300350400450500

1980 1985 1990 1995 2000 2005 2010 2015 2020

232221

Active Region Count

24

Numbered Active Regions per Month

How is Cycle 24 Evolving? As predicted 9 years ago using the polar field precursor method

D. Hathaway

Prediction

14

24

Cycle 24 is beginning to look like Cycle 14

Lowest in a 100 years

SIDC

Leif

Cycle 24 is progressing as indicated by the Polar Field Precursor, although a bit too low so far.Cycle 24 is beginning to look like cycle 14 a century ago.

7

A different view of polar fields: Nobeyama Image of 17GHz Emission

ν17 GHz = λ1.76 cmνe = B (Tesla) × 28 GHz

Beam width 10”

1. General Limb brightening: Bremsstralung (free-free) from hot atmosphere [10,000 – 13,000 K]

2. Active regions bright: Gyro-resonance from strong fields

Leif

The 17GHz microwave emission mapped at Nobeyama shows distinct regions of enhanced emission in the polar regions [if there are magnetic field and coronal holes present]

8

Coronal Holes Everywhere Show Same Behavior as the Polar Holes

3 Days later

When a coronal hole is at the limb, the bright 17GHz patches appear, otherwise not

Quantifying the Brightening:

Leif

The brightness patches are present in every coronal hole when at the limb, but hardly on the disk.To quantify the emission we can calculate the brightness temperature of segments of a ring just inside the limb, then move the segment around the limb to get the brightness temperature as a function of position angle.

9

Evolution of Patches over the Cycle

Reversal

Reversal

E

W

W

S

N

Pol

es

Leif

In addition to the obvious 'butterfly diagram' shown by the active regions when crossing the limb, the polar regions stand out at position angles 90 and 270 degrees. The annual variation is clearly seen as well as the disappearance of emission when the polar fields reverse.

10

Excess Tb over 10,800K, signed according to WSO polar field sign

WSO Sensitivity in 2001

Leif

Plotting daily values of the brightness temperature in excess of the average 10,800K but with a sign corresponding to that of the polar fields measured at WSO shows a clear relationship between excess and the magnetic polar fields, including the annual variation.This gives us an alternative way of measuring the elusive polar fields.

11

Using Polar Faculae Count to determine Polar Magnetic Flux and HMF Field Strength

1905-2011

Andrés Muñoz-Jaramillo, Neil Sheeley, et al., 2013

Leif

The number of white-light polar faculae is also a proxy for the polar fields. More than a century of data is available.The [scaled] sunspot are plus a polar field contribution yeilds yet another proxy for the heliospheric magnetic field matching well the observed field and the two geomagnetic proxies

12

Observed Polar Field Reversals

MWO: Roger Ulrich, 2012

Supersynoptic charts MWO

Leif

The super-synoptic maps from MWO shows the drift of active region fields to the poles and the ensuing reversals of the polar fields [ovals]. The two hemispheres reverse at slightly different times.

13

And the ‘Rush to the Pole’ of Coronal Emissions

Measurements of the location of ‘peaks’ of Fe XIV coronal emission at 503 nm (the ‘Green Line Corona’) over 7 solar cycles. The plots show the probability of observing a ‘peak’ at a given latitude as a function of time.

Is there an ‘extended’ cycle of 17 years?

Leif

The Rush to the Poles can be seen in synoptic observations of the Green Corona also showing the times of reversals and a hint [?] of the 'extended cycle'.

14

Asymmetric Solar Activity Spots & Groups

18

Leif

The asymmetry in polar reversal is a simple consequence of asymmetry of activity, shown by sunspot number [top] and number of groups per month [bottom]

15

Comparing Cycles 14 and 24

Cycle 14 suggests that the activity in the South might pick up in cycle 24

Leif

Cycle 24 is beginning to look much like cycle 14 a century ago. The division of cycle into a cycle in the Northern Hemisphere and a delayed cycle in the south, may by analogy also be in store for cycle 24.

16

70-100 Year ‘Gleissberg Cycle’ in Solar Activity Asymmetry?

Zolotova et al., 2010

Is this a ‘regular’ cycle or just over-interpretation of noisy data [like Waldmeier’s]?

‘Prediction’ from this: South will lead in cycle 25 or 26 and beyond. We shall see…

Extreme Asymmetry during the Maunder Minimum…

There are various dynamo theoretical ‘explanations’ of N-S asymmetry. E.g. Pipin, 1999. I can’t judge these…

Leif

There are speculations that the asymmetry is cyclic following the ~88-year Gleissberg cycle.One must caution against over-interpretation of nolisy data, but is there is such a cycle, the South should be leading from cycle 25 on.

17

Cosmic Ray Modulation Depends on the Sign of Solar Pole Polarity

Miyahara, 2011

The shape of the modulation curve [alternating ‘peaks’ and ‘flat tops’] shows the polar field signs.

North pole

North pole

Ice cores contain a long record of 10Be atoms produced by cosmic rays. The record can be inverted to yield the cosmic ray intensity. The technique is not yet good enough to show peaks and flats, but might with time be refined to allow this.Svalgaard

& Wilcox, 1976

Leif

The polarity of the polar fields influence the modulation of the galactic cosmic ray flux. A fundamental question is whether or not the polar field always reverse in every cycle.The geomagnetic records answers that in the affirmative back to the 1830s, but we don't know if this is generally true, e.g. during the Maunder Minimum.

18

The Butterfly Diagram

1825 1867

SchwabeStaudach

Arlt

Arlt

Hathaway

Leif

The famous Butterfly Diagram is the ultimate in synoptic observations covering solar activity back well into the 18th century. A primary goal of solar physics is to explain this diagram.

19

We Observe Fewer Spots per Sunspot Group

431,000 daily obs.

There is a weak solar cycle variation on top of a general downward trend seen by all observers

As the sunspot number is primarily determined by the number of spots, the SSN will be too low as a measure of solar activity

Leif

Another fundamental question is whether the relation between sunspot groups and sunspot numbers is constant. Historically there has been ~10 spots per group, but recently we observe only about half that number. This is unprecedented in the record and may be a harbinger of solar activity entering a new 'regime' with fewer [and, perhaps, progressivly fewer] spots.

20

TSI and CME-rate no longer following the Sunspot Number

Leif

If so, the SSN is no longer a good measure of 'real' solar activity. Already it is puzzling that Total Solar Irradiance [TSI] has not followed the SSN down from cycle 23, i.e. is higher than expected from the spots observed. Also the CME-rate is higher than expected. Standard dogmas seem to be under pressure. Exciting times ahead [?]

21

Conclusions

• To enable practical prediction of solar activity synoptic observations in both space and time are indispensable

• The long-term evolution of the solar cycle can only be tracked [and eventually understood] by sustained and calibrated synoptic observations

• I must be preaching to the choir today

Documents

Long-term Solar Synoptic Measurements with Implications for the Solar Cycle