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 (WDCSILSO) and Group
Number series
(Hoyt & Schatten 1994,
1998)
SN Workshops,
2011-2015
21/11/2014
ESWW11 – Nov. 2014
2
SN and GN recalibration: early preview
• 4 main
corrections
– 10 to 40%
– Obtained
independently
– Based only on
sunspot data
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
• SN and GN
agree within
uncertainties
back to the
Dalton
Minimum
21/11/2014
Specola
drift
SN
weighting
RGO trend
RGO/
SOON
ESWW11 – Nov. 2014
3
Implications: low secular trends
Trend +15%/century
SN correction: SN /1.20 after 1947
Trend +40%/century
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
4
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
5
Implications: recent cycles
New SN
series (red):
correction of
SN scale
drifts due to
the Locarno
pilot station
Maximum 22
second peak is
higher, almost
equal to first peak
Changes in
cycle 22
Cycle 22
Slightly decreased
(- 5 to10%)
21/11/2014
ESWW11 – Nov. 2014
6
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%)
21/11/2014
ESWW11 – Nov. 2014
Cycle 23 decline
raised by about 20%
7
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
8
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
19
14
20
Ri/Rg
21
22
Tlatov
23
24
SONNE
13
12
11
10
9
21/11/2014
ESWW11 – Nov. 2014
9
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
10
VANISHING SMALL SPOTS
21/11/2014
ESWW11 – Nov. 2014
11
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
12
Cycle 23 small-spot deficit
A,B
D,E,F
• Similar size-dependent trends
found by Kilcik et al. 2013:
• Upgrade of Kilcik et al.
2011 results
• Based on Learmonth data
C
• 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.
H
_____ Sunspot counts
--------- Sunspot group counts
_____ International SN Ri
21/11/2014
ESWW11 – Nov. 2014
13
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
14
NEW CATALOG VALIDATION
21/11/2014
ESWW11 – Nov. 2014
15
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
16
Main Sources of mismatch
1.
DPD spot classification:
N
– “Too” detailed: sunspot classes
without equivalent in other catalogs:
• Penumbrae without umbrae
• Small umbral kernels inside common
penumbrae
– Dropped in sunspot area
comparisons
2.
Time difference between daily
DPD and STARA observations:
N
– 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
21/11/2014
ESWW11 – Nov. 2014
-20
-10
0
10
∆t DPD-STARA (hours)
17
20
Sunspot group matching: movie
21/11/2014
ESWW11 – Nov. 2014
18
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:
All
UP > 10msh
UP> 100 msh
UP> 500 msh
– DPD-STARA time difference:
small spots evolve faster
– MDI lower spatial resolution (2 “):
pixel quantization
21/11/2014
ESWW11 – Nov. 2014
19
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
20
VARIATIONS OF OTHER
SUNSPOT PROPERTIES
21/11/2014
ESWW11 – Nov. 2014
21
Decline of core magnetic fields
Average core magnetic field in umbra (FeI line: 1565 nm, Kitt Peak)
Linear decline -40 G/year
(Penn & Livingston 2011, 2013)
Solar cycle modulation
(Nagovitsyn et al., 2012)
NB: only the strongest field each day.
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
22
Growth/decay rates of active regions
• Study of group growth and
decline (Javaraiah 2011):
Growth
– 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)
Decay
• Coherent with sunspot deficit
• Weaker growth rates similar to
moderate cycles 12 to14
Javaraiah 2011
21/11/2014
ESWW11 – Nov. 2014
23
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)
Basu et al.
• 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)
21/11/2014
ESWW11 – Nov. 2014
24
2013
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
25
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
26
More information available at …
Sunspot Number Workshops
Historical Archive of Sunspot Observations
21/11/2014
ESWW11 – Nov. 2014
27
21/11/2014
ESWW11 – Nov. 2014
28
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
29
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
30
Cycle 23 small-spot deficit
• Contradictory results from de
Toma et al. (2013):
• Based on San Fernando Obs.
Images
De Toma et al., 2013
• 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.
21/11/2014
ESWW11 – Nov. 2014
31
DPD: an example
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
32
Area differences
• The main difference between MDI images and
DPD images is due to the lower resolution of
MDI images
(SOHO-DPD)/DPD
100 points
• 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.
(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
•
Mean mag. flux
Sunspot Nb.
(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)
Stenflo, 2012
• Chromospheric and mixed indices
(TSI, CaII-K, MgII):
– Good but lower correlations:
• Non-linear relation
• Time lags (magn. flux dispersion)
• Different physics !
Solanki & Fligge 1999
21/11/2014
ESWW11 – Nov. 2014
34
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
35
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
36
Cycle 24 in the long-term SN
perspective
• Cycle 24 best matches:
Cycle 14
– 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 15
• Cycle decline by mid-2015
• Next minimum in 2019-2020
21/11/2014
ESWW11 – Nov. 2014
37