1. What do sunspots tell us about recent and past trends in solar activity ?
Frédéric Clette & Laure Lefèvre
Royal Observatory of Belgium
WDC - SILSO
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2. SN and GN recalibration: early preview
Ongoing revision of inhomogeneities in the Sunspot Number (WDC-SILSO) and Group Number series (Hoyt & Schatten 1994, 1998) SN Workshops,
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3. SN and GN recalibration: early preview
4 main corrections
10 to 40%
Obtained independently
Based only on sunspot data
SN and GN agree within uncertainties back to the Dalton Minimum
Clette, Svalgaard, Vaquero, Cliver, 2014
Space Science Reviews, Aug. 2014, Springer Online First , 69 pages
DOI /s
Arxiv:
Specola
drift SN
weighting
RGO trend
RGO/
SOON
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4. Implications: low secular trends
Trend +15%/century Trend +40%/century
SN correction: SN /1.20 after GN correction: GN * 1.37 before 1880
Corrected SN & GN series agree
Secular trend is largely eliminated < 5% / century
Soon after the Maunder Minimum, solar activity was similar to present levels
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5. Open solar flux (geomagnetic indices)
Similar conclusions: recent open magnetic field reconstructions show only a weak trend over last 180 years (Lockwood, Living Reviews SP, 9/2013)
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6. Implications: recent cycles
New SN series (red): correction of SN scale drifts due to the Locarno pilot station Changes in cycle 22
Maximum 22 second peak is higher, almost equal to first peak
Cycle 22
Slightly decreased (- 5 to10%)
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7. Implications: recent cycles
Changes in cycle 23
Maximum 23 second peak becomes higher than the first peak.
New maximum in 2002 instead of 2000
Cycle 23
Slightly increased (+ 5 to 10%)
Cycle 23 decline raised by about 20%
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8. Implications: recent cycles
New SN series (red): correction of SN scale drifts due to the Locarno pilot station Deviation between Ri and F10.7 in cycle 23
Still significant disagreement during the decline of cycle 23
Indication of a real solar change
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9. Implications: a variable number of spots/group (SN/GN ratio)
SONNE SSN
Reconstructed GN series (SILSO, SONNE):
Both indices SN and GN calculated from the same data set
Changing SN/GN ratio :
Stable over cycles 19 to 22
Decline in cycle 23 and 24
Decrease of the average number of spots per group by ~30%
Locarno SSN
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10. Implications: a variable number of spots/group (SN/GN ratio)
Apparent secular variations of the ratio SN/GN (Tlatov 2013):
NS/NG increase for stronger cycles ?
Ratio of the original SN/GN series: different sets of observations.
SN and GN contain a different information about the solar cycle
A probe for past changes in the solar dynamo ?
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Tlatov 2013
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11. Vanishing small spots
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12. Cycle 23 small-spot deficit
Exploitation of detailed sunspot catalogs: DPD, Debrecen; NOAA/SOON (Lefèvre & Clette 2011, 2012, 2013)
Scale-dependent small-spot deficit in cycle 23:
Deficit of small groups (A & B types): Ratio cycle 23 / cycle 22 ~ 50%
Deficit of small spots inside all groups: Ratio cycle 23 / cycle 22 < 75%
Starts in 1998, significant only after 2000
Small vs Large spots
Small vs Large groups
Clette & Lefèvre 2012
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13. Cycle 23 small-spot deficit
A,B C
Similar size-dependent trends found by Kilcik et al. 2013:
Upgrade of Kilcik et al results
Based on Learmonth data
Small spots A,B types: factor ~2 deficit
Intermediate C type: moderate decrease
Large D,E,F,H groups: no difference between SC23 and 24.
D,E,F H
_____ Sunspot counts
Sunspot group counts
_____ International SN Ri
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14. Cycle 23 small-spot deficit
Similar results from Nagovitsyn et al. 2013
4 classes based on their area (multimodal distribution)
Only the smallest spots (A<17 msh) show a decline in SC23
Number of largest spots increases
Intermediate sizes: no change
SS: small < 17 msh
SL: 17 msh<S<58 msh
LS: 58 msh<S<174 msh
LL: > 174 msh
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15. New catalog validation
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16. Sunspot catalog cross-validation
Need to assess the homogeneity of the primary DPD sunspot catalog:
Several data sources: ground based stations (80% in Hungary)
Resolution: 1 to 2”/pixel (instrument, seeing)
Sunspot groups and individual sunspots (unique identification)
Cross-analysis with the new STARA MDI sunspot catalog (F.Watson, NSO):
SOHO MDI continuum images
Resolution: 2”/pixel
Individual sunspots (not tracked)
Two versions built separately:
Whole spot (penumbral area)
Separate umbral areas
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17. Main Sources of mismatch
DPD spot classification:
“Too” detailed: sunspot classes without equivalent in other catalogs:
Penumbrae without umbrae
Small umbral kernels inside common penumbrae
Dropped in sunspot area comparisons
Time difference between daily DPD and STARA observations:
STARA version 1: mean ∆t=10 h
Image closest to daily magnetogram
Poor match for small spots due to spot evolution over 10 hours.
STARA version 2:mean ∆t= 2-3 h
We use only version 2 N N
∆t DPD-STARA (hours)
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18. Sunspot group matching: movie
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19. Matching in distance and size
Sunspot position: very accurate match
Mean difference: 0.35°
max. ~5° (due to splitting of large complex spots)
Sunspot areas (umbra+penumbra): good match
No bias: mean ratio = 1
RMS dispersion increases for small sunspot areas:
10msh<A<100msh: σ= 20%
Main causes of larger differences in small spot areas:
DPD-STARA time difference: small spots evolve faster
MDI lower spatial resolution (2 “): pixel quantization
All
UP > 10msh
UP> 100 msh
UP> 500 msh
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20. Catalog matching: conclusions
Total number of individual spots: 57500
Matching in DPD & STARA: 93 %
Non-matching: % (mainly small short-lived spots)
Sunspot areas: the accuracy of DPD sunspot areas is the best available among existing catalogs
The comparison with STARA confirms the accuracy of areas for A > 30msh
For A < 30msh, only DPD: results still rest on the intrinsic stability of the DPD catalog construction
Sunspot counts: no systematic variation over time found in the DPD catalog (T. Baranyi, private communication)
The in/exclusion of DPD-specific classes does not influence the time variations found in the small-spot population.
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21. Variations of Other sunspot properties
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22. Decline of core magnetic fields
Average core magnetic field in umbra (FeI line: 1565 nm, Kitt Peak)
Solar cycle modulation (Nagovitsyn et al., 2012)
NB: only the strongest field each day.
Linear decline -40 G/year (Penn & Livingston 2011, 2013)
Most recent data (2014): BABO (Penn & Livingston), MDI/HMI (F. Watson)
Decline has stopped in cycle 24, but no solar cycle modulation
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23. Growth/decay rates of active regions
Study of group growth and decline (Javaraiah 2011):
Data: Greenwich photographic catalog, USAF/SOON catalog
In cycle 23:
Lower growth rate
Decay rate increasing
Scarcity of groups with A < 37 msh (Javaraiah 2013)
Coherent with sunspot deficit
Weaker growth rates similar to moderate cycles 12 to14
Growth
Decay
Javaraiah 2011
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24. Implications: a shallow dynamo?
Solar-cycle modulation of high-frequency p-modes (Basu et al. 2013): BISON data
Top layers (r > rʘ): deviate after 1998
Deeper layer: deviation during entire cycle 23
Thinning of the subsurface magnetic field layer (< km)
Dynamo models: Are there two dynamo components, deep and shallow ? Does the near-surface shear layer play an independant role?
Babcock-Leighton near-surface flux diffusion mechanism (Muñoz-Jaramillo et al. 2010, 2011)
Role of a near-surface shear layer (A. Brandenburg 2005, Brnadenburg et al., 2013)
Near-surface magnetic flux aggregation mechanism (K. Schatten 2009, Rempel, et al 2009)
Basu et al
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25. Implications for the TSI – SSI reconstructions
Vanishing small spots = irradiance excess ?
Less sunspots (B < 1500 G spot formation threshold)
Lower sunspot blocking (visible + IR)
Additional contribution to plage and network
Excess in near-UV, microwaves (and solar wind ?)
Possible cause of the divergence between sunspot indices and solar proxies
Can this “reversal” process reduce the effective irradiance decrease expected at low solar activity? (Grand Minima?)
Base level in solar flux close to the last SC23-24 minimum (Schrijver et al. 2011)
Proxies cannot be based on a simple linear extrapolation of recent high solar cycles (scale-dependant, lifetimes)
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26. Conclusions
Multiple evidence of a global change in small-scale sunspot magnetic fields
Still unclear if the current change is:
A steady evolution towards a new activity regime
A larger deviation in a global solar cycle modulation
Long-term variations of the number of spots per group
Results supported by
In-depth validation DPD sunspot catalog versus the MDI/STARA catalog
Recalibrated SN and GN series
Limited rise of average solar activity since the Maunder minimum
Concept of a Grand Maximum in the 20th century is questioned
Release of new the SN and GN series by mid-2015
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27. More information available at …
WDC – SILSO Sunspot Index
and Long-term Solar Observations
Sunspot Number Workshops
Historical Archive of Sunspot Observations
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28. 21/11/2014
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29. Long-term integration: a moderate Modern Maximum ?
Time-integrated responses to the solar input:
Cosmogenic isotopes: deposition processes (ice, sediments)
Earth climate: thermal inertia of oceans
Gaussian running mean over 22 years (2 solar cycles):
Original series
Ratio Max cycles 3-19 = 1.27
Ratio 22-yr envelope = 1.30
Corrected series
Ratio Max cycles 3-19 = 1.08
Ratio 22-yr envelope = 1.17
The clustering of high solar cycles reduces the time-integrated effects of the corrections by 50%
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30. Sunspot areas versus F10.7
F10.7 vs SSN
F10.7 vs sunspot area
Same F10.7 excess versus SOON sunspot areas in the late part of cycle 23 (Hathaway 2010, 2013)
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31. Cycle 23 small-spot deficit
Contradictory results from de Toma et al. (2013):
Based on San Fernando Obs. Images
Small spots: no decline
Large spots: decrease in cycle 23
but
Spatial resolution of the CFDT1 instrument is too low for analysis of the smallest spots (>5”/pixel).
Smallest spots <30 msh are not properly detected.
De Toma et al., 2013
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32. DPD: an example A very detailed dataset
Master Spot corresponding to this common penumbra
A very detailed dataset
Common penumbra
Penumbra with no umbra (U=0)
Not very contrasted
NOAA 7815
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33. Area differences
The main difference between MDI images and DPD images is due to the lower resolution of MDI images
Area difference of two circles with radius r and r+dr is dA/A≈2dr/r
Bias appears when A is close or below the pixels size.
(SOHO-DPD)/DPD 100 points
(STARA- DPD)/DPD
50000 points
Gyori et al., 2004
Confirmed by this study:
much larger statistics ( spots instead of < 100)

34. Sunspot number versus other solar indices and fluxes
Very high correlation with photospheric parameters (R2 >0.95): RG, RA, RBoulder, Area, Mx
(Bachmann et al. 2004, Rybansky et al. 2005, Wilson and Hathaway 2006, Tapping et al. 2007, Bertello et al. 2010, Hempelmann and Weber 2012)
Measure of the global emergence rate of (toroidal) magnetic flux (Petrovay 2010, Stenflo 2012)
Chromospheric and mixed indices (TSI, CaII-K, MgII):
Good but lower correlations:
Non-linear relation
Time lags (magn. flux dispersion)
Different physics !
Mean mag. flux
Sunspot Nb.
Stenflo, 2012
Solanki & Fligge 1999
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35. Sunspot number versus other solar indices and fluxes
Blind Source Separation applied solar radio and UV indices:
Method: Bayesian positive source separation (Moussaoui et al. 2006)
Chromosphere
Corona
Chromosphere
Photosphere
Active regions
Radio gyroresonance
Network, plages
T. Dudok de Wit, SSN2 Workshop, 2012
3 clusters of indices, each dominated by one of 3 sources :
Photosphere (SN, GN)
Chromospheric (DSA, MgII, Lyα, radio λ > 10cm)
Coronal (radio λ < 10cm)
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36. Cycle 24 in the long-term SN perspective
Cycle 24 is among the late cycles
Tie-point (Ri=13) at the end of preceding cycle
2 main cycle “families”:
Steep – strong (max > 130)
Slow – weak (max < 80)
Best fit with cycles 12, 14, 15, 16
Tie-point (Ri=40) in the rising phase of the cycles
Return to an average activity regime like in the late 19th and early 20th century
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37. Cycle 24 in the long-term SN perspective
Cycle 24 best matches:
Cycle 14 [ ]: Rmax = 64.2
Cycle 15 [ ]: Rmax = !
3 features of moderate cycles:
Plateau (duration up to 3 years)
Multiple sharp peaks
Late maximum (~highest random peak)
Cycle decline by mid-2015
Next minimum in
Cycle 14
Cycle 15
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