How well do the Ca II K index time series correlate with the ISN?

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1 How well do the Ca II K index time series correlate with the ISN? National Solar Observatory 2nd Sunspot Number Workshop May 21-25, Brussels, Belgium

2 Outline Importance of Ca II K observations. Comparison of various Ca II K index time series: Historical data: Modern data: Present Consistency of Ca II K index time series at different time scales: Monthly averages Annual averages Comparison of the Ca II K index time series with the International Sunspot Number (ISN) time series. Conclusion: Both historical and modern Ca II K index time series correlate well with the ISN on time scales above one month.

3 Outline Importance of Ca II K observations. Comparison of various Ca II K index time series: Historical data: Modern data: Present Consistency of Ca II K index time series at different time scales: Monthly averages Annual averages Comparison of the Ca II K index time series with the International Sunspot Number (ISN) time series. Conclusion: Both historical and modern Ca II K index time series correlate well with the ISN on time scales above one month.

4 Outline Importance of Ca II K observations. Comparison of various Ca II K index time series: Historical data: Modern data: Present Consistency of Ca II K index time series at different time scales: Monthly averages Annual averages Comparison of the Ca II K index time series with the International Sunspot Number (ISN) time series. Conclusion: Both historical and modern Ca II K index time series correlate well with the ISN on time scales above one month.

5 Outline Importance of Ca II K observations. Comparison of various Ca II K index time series: Historical data: Modern data: Present Consistency of Ca II K index time series at different time scales: Monthly averages Annual averages Comparison of the Ca II K index time series with the International Sunspot Number (ISN) time series. Conclusion: Both historical and modern Ca II K index time series correlate well with the ISN on time scales above one month.

6 Outline Importance of Ca II K observations. Comparison of various Ca II K index time series: Historical data: Modern data: Present Consistency of Ca II K index time series at different time scales: Monthly averages Annual averages Comparison of the Ca II K index time series with the International Sunspot Number (ISN) time series. Conclusion: Both historical and modern Ca II K index time series correlate well with the ISN on time scales above one month.

7 Importance of Ca II K observations They provide an important measure of solar magnetic activity. In particular, solar magnetic plages and network are easily observed in this strong chromospheric line, at nm. Plages account for roughly one-half of the Sun s total magnetic flux and for most of the Sun s variability in UV flux. Ca II K observations started at the beginning of the 20th century at several observatories. These archives are of great importance for studies of the solar magnetism on time scales longer than the activity cycle.

8 Ca II K index time series Historical Data MWO-UCLA: 8/1915-7/1985 (to present via proxy) MWO-Tlatov: 8/1915-7/1985 MWO-Apn: 8/1915-7/1985 (to 4/1999 via proxy) Kodaikanal: Modern Data - Integrated Sunlight KPO (Bill Livingston): 10/ present SPO: 11/ present SOLIS-ISS: 12/ present Note: Most of these data and related information are available on line from the UCLA, NSO, and NOAA websites.

9 Ca II K index time series Historical Data MWO-UCLA: 8/1915-7/1985 (to present via proxy) MWO-Tlatov: 8/1915-7/1985 MWO-Apn: 8/1915-7/1985 (to 4/1999 via proxy) Kodaikanal: Modern Data - Integrated Sunlight KPO (Bill Livingston): 10/ present SPO: 11/ present SOLIS-ISS: 12/ present Note: Most of these data and related information are available on line from the UCLA, NSO, and NOAA websites.

10 The historical MWO data set Starting in 1915 spectroheliograms in the light of the Ca II K line were taken twice per day at the Mount Wilson Solar Observatory. About the same time "white light" direct images were taken once per day. Approximately 45,000 images in the light of the Ca II K line, acquired from August 1915 to July Approximately 30,000 broad-band images (white light) acquired over the same time period. The spectroheliograms were first digitized by Foukal (1996) using a 512-pixel format, 8-bit camera. The same images were re-digitized (starting in 2003) at UCLA using a higher resolution ( ), 12-bit digitizer.

11 Distribution of MWO Ca II K images Number of images Year

12 The Sun on Ca II K: June 12, 1958

13 Historical Ca II K index time series MWO-UCLA: The index is derived from the properties of the intensity-ratio distribution of pixels across the image. The time series is extended using the MPSI as a proxy. MWO-Tlatov: The index is defined as the fraction of the solar hemisphere occupied by the chromospheric plages + active network at any given time. MWO-Apn: The index is defined as the percentage of the solar disk covered by plages + active network. The original time series was extended using NSO/SP data. Kodaikanal (KKL): Same as MWO-Tlatov. Spectral bandpass (centered on K3): 0.35 Å for MWO observations, and 0.5 Å for KKL and NSO/SP observations.

14 The MWO-UCLA Ca II K index 1 Each image is flat-fielded using a median filter. 2 A histogram is calculated using all pixels located within 0.99 solar radii from the center of the (flat-fielded) image. 3 A four-parameter Gaussian function is used to model this distribution: y(x) = A exp( u 2 /2) + B, where u = (x x c )/σ, x is the bin value, y is the fractional number of pixels in the solar disk with value x, σ and x c are the width and the center of the distribution, respectively (x c 1). 4 The constant baseline B is defined as the Ca II K index. 5 The parameter σ is related to the plage contrast.

15 The MWO-UCLA Ca II K index (cont d.) Rejected Ca II K Index Year

16 MWO-UCLA vs. MWO-Tlatov 0.12 CaK index: monthly mean values 0.10 MWO-Tlatov MWO-UCLA Fractional area Year Fractional area Year

17 Plage contrast comparison 3.0 Yearly mean values MWO-Tlatov MWO-UCLA 0.20 Plage contrast Histogram width Year

18 MWO-UCLA Ca II K index vs. MPSI Ca K Index Index = 2.39x x10-3 arctan(0.485xmpsi) MPSI CaK Index Rescaled MPSI Ca K Index Year

19 Extended MWO-UCLA Ca II K series Original Rescaled MPSI CaK index Year

20 Historical Ca II K time series 0.12 Monthly averages (annual values for KKL) 0.10 MWO-Tlatov MWO-UCLA Apn KKL Fractional area Year Fractional area Year

21 Historical Ca II K index: Correlation matrix The correlation matrix has been computed using the yearly mean values of each time series from 1916 to Tlatov UCLA Apn KKL Tlatov UCLA Apn KKL

22 Historical Ca II K data vs. ISN 300 ISN Yearly mean values 250 MWO-UCLA (r=0.97) Apn (r=0.97) International sunspot number MWO-Tlatov (r=0.98) KKL (r=0.96) Year Note: During cycles 16 and 17 ISN underestimates the Ca II K by 30%!

23 Historical Ca II K data vs. ISN (cont d.) 300 ISN Monthly mean values 250 MWO-UCLA (r=0.92) Apn (r=0.92) MWO-Tlatov (r=0.94) International sunspot number Year

24 Historical Ca II K data vs. ISN: Correlation by cycle r (ISN,Ca II K) MWO-UCLA Apn MWO-Tlatov KKL From yearly mean values Cycle Number r (ISN,Ca II K) MWO-UCLA Apn MWO-Tlatov From monthly mean values Cycle Number

25 ISS Ca II K spectral line profile 0.30 March 15, :00 UT Scaled Intensity 0.15 Fe I Ti II Fe I 0.10 Fe I Fe I Wavelength (nm) SOLIS-ISS high spectral resolution (R = 300,000) observation of the K line. ISS data are available from:

26 Ca II K integrated sunlight vs. ISN Monthly averages 300 KPO ISS SPO ISN r(spo,kpo) = Å emission index r(spo,isn) = 0.94 r(kpo,isn) = International Sunspot Number Year

27 Ca II K integrated sunlight vs. ISN (cont d.) Annual averages 300 KPO ISS SPO ISN r(spo,kpo) = Å emission index r(spo,isn) = 0.98 r(kpo,isn) = International Sunspot Number Year

28 Ca II K integrated sunlight vs. ISN (cont d.) Monthly averages 300 KPO ISS SPO ISN Å emission index International Sunspot Number Year

29 Conclusions Overall there is a very good agreement among the different Ca II K index time series on time scales above one month. Some discrepancies are noticeable between the MWO and KKL data, particularly during cycle 18 and 19. The KPO and SPO 1-Å emission index time series show significant differences during the last three solar minima. Is data calibration a factor? Correlations between the Ca II K index time series and the ISN exceed 0.96 for all the yearly mean indices, and 0.92 for the monthly mean values. These correlations are slightly higher for modern data. During cycles 16 and 17 ISN underestimates the Ca II K by 30%.

30 Conclusions Overall there is a very good agreement among the different Ca II K index time series on time scales above one month. Some discrepancies are noticeable between the MWO and KKL data, particularly during cycle 18 and 19. The KPO and SPO 1-Å emission index time series show significant differences during the last three solar minima. Is data calibration a factor? Correlations between the Ca II K index time series and the ISN exceed 0.96 for all the yearly mean indices, and 0.92 for the monthly mean values. These correlations are slightly higher for modern data. During cycles 16 and 17 ISN underestimates the Ca II K by 30%.

31 Conclusions Overall there is a very good agreement among the different Ca II K index time series on time scales above one month. Some discrepancies are noticeable between the MWO and KKL data, particularly during cycle 18 and 19. The KPO and SPO 1-Å emission index time series show significant differences during the last three solar minima. Is data calibration a factor? Correlations between the Ca II K index time series and the ISN exceed 0.96 for all the yearly mean indices, and 0.92 for the monthly mean values. These correlations are slightly higher for modern data. During cycles 16 and 17 ISN underestimates the Ca II K by 30%.

32 Conclusions Overall there is a very good agreement among the different Ca II K index time series on time scales above one month. Some discrepancies are noticeable between the MWO and KKL data, particularly during cycle 18 and 19. The KPO and SPO 1-Å emission index time series show significant differences during the last three solar minima. Is data calibration a factor? Correlations between the Ca II K index time series and the ISN exceed 0.96 for all the yearly mean indices, and 0.92 for the monthly mean values. These correlations are slightly higher for modern data. During cycles 16 and 17 ISN underestimates the Ca II K by 30%.

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