What do sunspots tell us about recent and past trends in solar activity ? Frédéric Clette & Laure Lefèvre Royal Observatory of Belgium WDC - SILSO ESWW11 – Nov 2014

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, 2011-2015 21/11/2014 ESWW11 – Nov. 2014

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 10.1007/s11214-014-0074-2 Arxiv: http://arxiv.org/abs/1407.3231 Specola drift SN weighting RGO trend RGO/ SOON 21/11/2014 ESWW11 – Nov. 2014

Implications: low secular trends Trend +15%/century Trend +40%/century SN correction: SN /1.20 after 1947 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 21/11/2014 ESWW11 – Nov. 2014

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) 21/11/2014 ESWW11 – Nov. 2014

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%) 21/11/2014 ESWW11 – Nov. 2014

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% 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

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 ? 21/11/2014 Tlatov 2013 ESWW11 – Nov. 2014

Vanishing small spots 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

Cycle 23 small-spot deficit A,B C Similar size-dependent trends found by Kilcik et al. 2013: Upgrade of Kilcik et al. 2011 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 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

New catalog validation 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

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 -20 -10 0 10 20 ∆t DPD-STARA (hours) 21/11/2014 ESWW11 – Nov. 2014

Sunspot group matching: movie 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

Catalog matching: conclusions Total number of individual spots: 57500 Matching in DPD & STARA: 93 % Non-matching: 7 % (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. 21/11/2014 ESWW11 – Nov. 2014

Variations of Other sunspot properties 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

Implications: a shallow dynamo? Solar-cycle modulation of high-frequency p-modes (Basu et al. 2013): BISON data Top layers (r > 0.997 rʘ): deviate after 1998 Deeper layer: deviation during entire cycle 23 Thinning of the subsurface magnetic field layer (< 20000 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. 2013 21/11/2014 ESWW11 – Nov. 2014

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) 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

More information available at … WDC – SILSO Sunspot Index and Long-term Solar Observations http://sidc.be/silso http://ssnworkshop.wikia.com/wiki/Home Sunspot Number Workshops Historical Archive of Sunspot Observations http://haso.unex.es/ 21/11/2014 ESWW11 – Nov. 2014

21/11/2014 ESWW11 – Nov. 2014

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% 21/11/2014 ESWW11 – Nov. 2014

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) 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

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 (50000 spots instead of < 100)

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 21/11/2014 ESWW11 – Nov. 2014

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) 21/11/2014 ESWW11 – Nov. 2014

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 21/11/2014 ESWW11 – Nov. 2014

Cycle 24 in the long-term SN perspective Cycle 24 best matches: Cycle 14 [1902-1913]: Rmax = 64.2 Cycle 15 [1913-1923]: Rmax = 105.4 ! 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 2019-2020 Cycle 14 Cycle 15 21/11/2014 ESWW11 – Nov. 2014