J/ApJ/793/L4 Polar network index for the solar cycle studies (Priyal+, 2014)
Polar network index as a magnetic proxy for the solar cycle studies.
Priyal M., Banerjee D., Karak B.B., Munoz-Jaramillo A., Ravindra B.,
Choudhuri A.R., Singh J.
<Astrophys. J., 793, L4 (2014)>
=2014ApJ...793L...4P 2014ApJ...793L...4P (SIMBAD/NED BibCode)
ADC_Keywords: Sun ; Magnetic fields ; Solar system
Keywords: magnetic fields - Sun: activity - Sun: faculae, plages
Abstract:
The Sun has a polar magnetic field which oscillates with the 11 yr
sunspot cycle. This polar magnetic field is an important component of
the dynamo process which operates in the solar convection zone and
produces the sunspot cycle. We have direct systematic measurements of
the Sun's polar magnetic field only from about the mid-1970s. There
are, however, indirect proxies which give us information about this
field at earlier times. The Ca-K spectroheliograms taken at the
Kodaikanal Solar Observatory during 1904-2007 have now been digitized
with 4kx4k CCD and have higher resolution (∼0.86 arcsec) than the
other available historical data sets. From these Ca-K
spectroheliograms, we have developed a completely new proxy (polar
network index, hereafter PNI) for the Sun's polar magnetic field. We
calculate PNI from the digitized images using an automated algorithm
and calibrate our measured PNI against the polar field as measured by
the Wilcox Solar Observatory for the period 1976-1990. This
calibration allows us to estimate the polar fields for the earlier
period up to 1904. The dynamo calculations performed with this proxy
as input data reproduce reasonably well the Sun's magnetic behavior
for the past century.
Description:
The spatial resolution of the Ca K spectroheliograms taken at
Kodaikanal (hereafter KKL) is about 2 arcsec and the exit slit of the
spectroheliograph yields a spectral window of 0.5 Å centered at the
Ca-K line at 3933.67 Å. Ermoli et al. (2009ApJ...698.1000E 2009ApJ...698.1000E) pointed
out that the Kodaikanal archive hosts the longest homogeneous record,
with fewer variations in spatial resolution. The earlier version of
the 8 bit data at Kodaikanal is sufficient to study those plage area
with high intensity contrast, but does not provide the required
photometric accuracy to properly identify the network structures
because of the small intensity contrast of these features. Therefore,
we have designed and developed two digitizer units, using a 1 m
labsphere with an exit port of 350 mm which provides a stable and
uniform source of light with less than 1% variation from the center to
the edge of the light source. The CCD camera with 4kx4k format, a
pixel size of 15 u square, and a 16 bit read out, operating at
temperature of -100°C, was used to digitize the images. The Ca-K
network can be clearly seen because of the high spatial resolution of
digitization (0.86 arcsec).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
fig2.dat 87 14 Scatter plot of KKL PNI values with the Wilcox
Solar Observatory (WSO) polar field
measurements for the period of 1977-1990.
fig3ab.dat 61 82 PNI and MWO facular counts for the years
1909-1990.
fig3c.dat 25 8 A correlation plot between the strengths of
solar cycles and the PNI values at the
preceding minima.
fig4.dat 57 8 PNI asymmetry of the nth cycle minimum phase is
plotted against the sunspot area asymmetry of
the (n+1)th cycle.
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See also:
J/ApJS/169/439 : Models of solar magnetic structures (Socas-Navarro, 2007)
J/A+A/539/A13 : Solar bipolar magnetic fields (Yamamoto, 2012)
J/ApJ/752/126 : Modeling the magnetic field in the solar corona
(Yamamoto+, 2012)
Byte-by-byte Description of file: fig2.dat
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Bytes Format Units Label Explanations
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1- 4 I4 yr Obs.Y Year of the observation
5 A1 --- f_Obs.Y [m] Flag on Obs.Y (1)
7- 15 F9.3 --- KKL-PNI-N KKL Polar Network Index North
17- 23 F7.3 --- e_KKL-PNI-N Error in KKL-PNI-N
25- 32 F8.3 --- KKL-PNI-S KKL Polar Network Index South
34- 39 F6.3 --- e_KKL-PNI-S Error in KKL-PNI-S
41- 51 F11.8 --- WSO-PF-N WSO Polar Network Index North
53- 63 F11.9 --- e_WSO-PF-N Error in WSO-PF-N
65- 75 F11.8 --- WSO-PF-S WSO Polar Network Index South
77- 87 F11.9 --- e_WSO-PF-S Error in WSO-PF-S
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Note (1): Flag as follows:
m = Indicates a solar minimum year, red symbols in the figure.
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Byte-by-byte Description of file: fig3ab.dat
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Bytes Format Units Label Explanations
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1- 4 I4 yr Obs.Y Year of the observation
6- 14 F9.3 --- KKL-N KKL Polar Network Index North
16- 24 F9.3 --- KKL-S KKL Polar Network Index South
26- 34 F9.5 --- MWO-N MWO polar facular count North
36- 43 F8.4 --- MWO-S ? MWO polar facular count South (1)
45- 52 F8.3 --- SA-N Northern hemisphere sunspot area
54- 61 F8.3 --- SA-S Southern hemisphere sunspot area
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Note (1): Blank values indicate NaN values.
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Byte-by-byte Description of file: fig3c.dat
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Bytes Format Units Label Explanations
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1- 6 F6.1 yr Obs.Y Year of the solar minima (1)
8- 12 F5.1 --- Rmax Maximum sunspot number of the cycle n+1
14- 22 F9.4 --- PNI KKL Polar Network Index value at end of cycle n
24- 25 I2 --- Cycle [15/22] Solar cycle number
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Note (1): The last data point corresponding to solar cycle 22 was not included
in the figure as the analysis was restricted up to 1990 and cycle 22 includes
the years 1985-1995.
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Byte-by-byte Description of file: fig4.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- Cycle [15/22] Solar cycle number
4- 11 F8.4 --- SA-N Northern hemisphere sunspot area
13- 20 F8.4 --- SA-S Southern hemisphere sunspot area
22- 29 F8.5 --- SAA Sunspot area asymmetry (1)
31- 39 F9.4 --- KKL-PNI-N ? KKL Polar Network Index North
41- 48 F8.3 --- KKL-PNI-S ? KKL Polar Network Index South
50- 57 F8.5 --- KKL-A ? KKL Polar Network Index asymmetry (2)
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Note (1): Calculated as (SA-N - SA-S)/(SA-N + SA-S).
Note (2): Calculated as (KKL-PNI-N - KKL-PNI-S)/(KKL-PNI-N + KKL-PNI-S).
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History:
From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 21-Apr-2017