Methodology and Data Sources
Chapter 2
2.1 Introduction
Solar activity rises and falls with an 11-year cycle that affects us in many ways. Increased
solar activity includes increases in extreme ultraviolet and x-ray emissions from the Sun
which produce dramatic effects in the Earth’s upper atmosphere. The associated
atmospheric heating increases both the temperature and density of the atmosphere at many
spacecraft altitudes. The increase in atmospheric drag on satellites in low Earth orbit can
dramatically shorten the lifetime of these valuable assets [1]. Increases in the number of
solar flares and coronal mass ejections (CMEs) raise the likelihood that sensitive
instruments in space will be damaged by energetic particles accelerated in these events.
These solar energetic particles (SEPs) can also threaten the health of both astronauts in
space and airline travelers in high altitude, polar routes. Solar activity apparently affects
terrestrial climate as well. Although the change in the total solar irradiance seems too small
to produce significant climatic effects, there is good evidence that, to some extent, the
Earth’s climate heats and cools as solar activity rises and falls [2]. There is little doubt that
the solar cycle is magnetic in nature and produced by dynamo processes within the Sun [3].
In the following sections we have briefly discussed the methods and techniques used for
the study and also outlined the different data sources.
2.2 The Discovery of Solar Cycle
The solar cycle commonly known as sunspot cycle is the periodic change in the Sun's
activity and changes in the number of sunspots, flares, and other visible manifestations.
Sunspots are regions on the solar surface that appear dark because they are cooler than the
surrounding photosphere, typically by about 1500 K (thus, they are still at a temperature of
about 4500 K, but this is cool compared to the rest of the photosphere). They are only dark
in a relative sense; a sunspot removed from the bright background of the Sun would glow
quite brightly. The largest sunspots observed have had diameters of about 50,000 km,
which makes them large enough to be seen with the naked eye. Sunspots often come in
groups with as many as 100 in a group, though sunspot groups with more than about 10 are
relatively rare.
Sunspots were almost certainly seen by prehistoric humans viewing the Sun through hazy
skies. The earliest actual recordings of sunspot observations were from China [4,5] over
2000 years ago. Yet, the existence of spots on the Sun came as a surprise to westerners
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when telescopes were first used to observe the Sun in the early 17th century. This is
usually attributed to western philosophy in which the heavens and the Sun were thought to
be perfect and unblemished [6,7].
2.3 Telescopic Sunspot Observations
Despite the first accurate description of sunspots made by the English astronomer Thomas
Harriot and Frisian astronomers Johannes and David Fabricius, systematic and qualified
telescopic observations of sunspots begin only at the middle of the 19th century. But strong
efforts of many scientists and particularly of Rudolf Wolf make it possible to extend the
sunspot record back to the beginning of 18th century [8]. The reliability of different parts
of the Wolf number data set, however, is different: the data are reliable since 1848, their
reliability is good during 1818–1848, it is questionable from 1749–1817 and it is poor
during 1700–1748 [9]. Recently Hoyt and Schatten [10] have finished the reconstruction of
group sunspot number (GSSN) in which they used many historical sources missed by
previous investigators. Their data set covers the time interval 1610–1995 and authors
consider GSSN data as quite reliable during 1653–1730, 1750–1788 and after 1795 [10].
Wolf numbers and GSSN show some differences in the 18th century-the values of Wolf
numbers during this period are a bit higher. These two series are the longest direct
instrumental records of SA.
2.4 Historical Sunspot Observations
The largest sunspots and sunspot groups can be seen with unaided eye at sunrise and sunset
or through smoke and haze [11]. These sunspots were observed during the pre-telescopic
age by ancient Oriental (in particularly Chinese) astronomers and now the data on ancient
sunspot observations made by naked eye (SONE) is the longest non-proxy record of
sunspot activity in the past. The most complete catalogue of SONE, covering more than 18
centuries and including more than 200 events, was collected by Wittmann and Xu [5]. The
reliability of the information on solar activity, extracted from historical chronicles, was
analysed in many works [5,12,13] and it was shown that the Oriental historic data really
reflect such features of sunspot activity as 11-year and century scale cycles, butterfly
diagrams, deep Maunder-type minima of sunspot activity. Qualification of ancient Chinese
astronomers was high, observations of many lunar and solar eclipses, novae, supernovae,
comets, meteor showers and even a possible observation of Jupiter’s satellite Ganymede
(364 B.C.) confirm their professionalism. However, the SONE record has substantial
shortcomings:
(1) The ancient astronomers often mixed real sunspots with other celestial or
meteorological phenomena. For example, Wittmann and Xu [5] compared more than 20
Oriental sunspot sightings with the data of European astronomers since 1848 and they
found that only one third of the naked-eye observations are confirmed by western
telescopic records.
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(2) Ancient sunspot observations were not systematic and so are non uniform in time. For
example, very often sunspots were detected near the day of new moon. This happened
because just in these periods observations were more frequent and intensive-determination
of the new moon date had an important calendar purpose [5]. Dating of observations is also
not always quite accurate.
(3) Some climatic effects may be present in different SONE series [13]. Nevertheless,
SONE data are rather valuable for investigation of solar variability over a long time scale
and the catalogue of Wittmann and Xu was intensively used in our work.
2.5 Schwabe’s Discovery
Although Christian Horrebow mentions this possible periodic variation in 1776 the sunspot
cycle was not truly discovered until 1844. In that year Heinrich Schwabe reported [14] that
his observations of the numbers of sunspot groups and spotless days over the previous 18
years indicated the presence of a cycle of activity with a period of about 10 years. Figure
2.1 shows his data for the number of sunspot groups observed yearly from 1826 to 1843.
Figure 2.1 Sunspot groups observed from 1826 to 1843 by Heinrich Schwabe [14]
2.6 Wolf’s Sunspot Number
The modern era of sunspot counting began in the mid-1800s with the research of Bern
Observatory director Rudolf Wolf, who introduced what he called the "Universal Sunspot
Number" as an estimate of the suns activity [8]. This investigator, motivated by H. H.
Schwabe's discovery of an apparent 10-year periodicity--"sunspot cycle"--in the frequency
with which spots seemed to appear over the years, sought to develop an index by which
long-term trends could be monitored and periodicity verified and studied.
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It is said that Wolf would have preferred to measure the areas covered by the sunspots
rather than their number but the methods and equipment of the day were not adequate for
this task. As an alternative, he developed an index based on the number of spots and spot
groups (clusters of related spots).
His “relative” sunspot number, , thus emphasized sunspot groups with
... (2.1)
R = k (10 g + n)
Where is a correction factor for the observer,
is the number of identified sunspot
groups, and is the number of individual sunspots.
These Wolf, Zurich, or International Sunspot Numbers have been obtained daily since
1849. Wolf himself was the primary observer from 1848 to 1893 and had a personal
correction factor = 1.0 [8].
2.7 Wolf’s Reconstruction of Earlier Data
Wolf extended the data back to 1749 using the primary observers along with many
secondary observers but much of that earlier data is incomplete. Wolf often filled in gaps in
the sunspot observations using geomagnetic activity measurements as proxies for the
sunspot number. It is well recognized that the sunspot numbers are quite reliable since
Wolf’s time but that earlier numbers are far less reliable.
2.8 Sunspot Numbers
The International Sunspot Number is the key indicator of solar activity. This is not because
everyone agrees that it is the best indicator but rather because of the length of the available
record [15]. Traditionally, sunspot numbers are given as daily numbers, monthly averages,
yearly averages, and smoothed numbers. The standard smoothing is a 13-month running
mean cantered on the month in question and using half weights for the months at the start
and end. Solar cycle maxima and minima are usually given in terms of these smoothed
numbers.
Additional sunspot numbers do exist. The Boulder Sunspot Number is derived from the
daily Solar Region Summary produced by the US Air Force and National Oceanic and
Atmospheric Administration (USAF/NOAA) from sunspot drawings obtained from the
Solar Optical Observing Network (SOON) sites since 1977. These summaries identify each
sunspot group and list the number of spots in each group. A fourth sunspot number is the
Group Sunspot Number,
, devised by Hoyt and Schatten [16]. This index counts only
the number of sunspot groups, averages together the observations from multiple observers
(rather than using the primary/secondary/tertiary observer system) and normalizes the
numbers to the International Sunspot Numbers using
=
.
∑
… (2.2)
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where is the number of observers,
is the -th observer’s correction factor,
is the
number of sunspot groups observed by observer , and 12.08 normalizes the number to the
International Sunspot Number. Hathaway et. al [17] found that the Group Sunspot Number
follows the International Number fairly closely but not to the extent that it should supplant
the International Number. In fact, the Group Sunspot Numbers are not readily available
after 1995. The primary utility of the Group Sunspot number is in extending the sunspot
number observations back to the earliest telescopic observations in 1610. These sunspot
numbers are available from NOAA. The International Number can be obtained monthly
directly from SIDC.
2.9 Sunspot Areas
Sunspot areas are thought to be more physical measures of solar activity. Sunspot areas
and positions were diligently recorded by the Royal Observatory; Greenwich (RGO) from
May of 1874 to the end of 1976 using measurements off of photographic plates obtained
from RGO itself and its sister observatories in Cape Town, South Africa, Kodaikanal,
India, and Mauritius. Both umbral areas and whole spot areas were measured and corrected
for foreshortening on the visible disc. Sunspot areas were given in units of millionths of a
solar hemisphere (μHem).
In 1977 NOAA began reporting much of the same sunspot area and position information in
its Solar Region Summary reports. These reports are derived from measurements taken
from sunspot drawings done at the USAF SOON sites. The sunspot areas were initially
estimated by overlaying a grid and counting the number of cells that a sunspot covered. In
late 1981 this procedure was changed to employ an overlay with a number of circles and
ellipses with different areas. The sunspot areas reported by USAF/NOAA are significantly
smaller than those from RGO [17-20].
Sunspot areas are also available from a number of solar observatories. While individual
observatories have data gaps, their data are very useful for helping to maintain consistency
over the full interval from 1874 to the present. The combined RGO USAF/NOAA datasets
are available online (RGO). These datasets have additional information that is not reflected
in sunspot numbers – positional information – both latitude and longitude. The distribution
of sunspot area with latitude (Figure 2.2) shows that sunspots appear in two bands on either
side of the Sun’s equator. The average daily sunspot area for each solar rotation since May
1874 is plotted as a function of time in the lower panel. The relative area in equal area
latitude strips is illustrated with a color code in the upper panel. Sunspots form in two
bands, one in each hemisphere, that start at about 25° from the equator at the start of a
cycle and migrate toward the equator as the cycle progresses. At the start of each cycle
spots appear at latitudes above about 20 – 25°. As the cycle progresses the range of
latitudes with sunspots broadens and the central latitude slowly drifts toward the equator,
but with a zone of avoidance near the equator. This behavior is referred to as “Sporer’s
Law of Zones” by Maunder [21] and was famously illustrated by his “Butterfly Diagram”
[22].
52
Figure 2.2 Sunspot areas as a function of latitude and time
2.10 Solar Flux at 10.7 cm
The 10.7 cm Solar Flux is the disk integrated emission from the Sun at the radio
wavelength of 10.7 cm (2800 MHz) [23,24]. This measure of solar activity has advantages
over sunspot numbers and areas in that it is completely objective and can be made under
virtually all weather conditions. This index is a measure of the noise level generated by the
sun at a wavelength of 10.7 cm at the earth's orbit. The global daily value of this index is
measured at local noon at the Penticton Radio Observatory in Canada. Historically, this
index has been used as an input to ionospheric models as a surrogate for the solar output in
wavelengths that produce photo ionization in the earth's ionosphere (in the ultraviolet
bands).
2.11 Magnetic Field
Magnetic fields on the Sun were first measured in sunspots by Hale [25]. The magnetic
nature of the solar cycle became apparent once these observations extended over more than
a single cycle [26]. A magnetogram from sunspot cycle 22 (1989 August 2) is shown on
the left with yellow denoting positive polarity and blue denoting negative polarity. A
corresponding magnetogram from sunspot cycle 23 (2000 June 26) is shown on the right.
Leading spots in one hemisphere have opposite magnetic polarity to those in the other
hemisphere and the polarities flip from one cycle to the next.
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Figure 2.3 Magnetogram from sunspot cycle 22 and cycle 23
In addition to Hale’s Polarity Laws for sunspots, it was found that the Sun’s polar fields
reverse as well. Babcock [27] noted that the polar fields reversed at about the time of
sunspot cycle maximum. The Sun’s south polar field reversed in mid-1957 while its north
polar field reversed in late-1958. The maximum for cycle 19 occurred in late-1957. The
polar fields are thus out of phase with the sunspot cycle-polar fields are at their peak near
sunspot minimum. This is also indicated by the presence of polar faculae-small bright
round patches seen in the polar regions in white light observations of the Sun-whose
number also peak at about the time of sunspot minimum [28].
2.12 Prediction of the Maximum Annual Mean Sunspot Number
Solar cycle predictions are needed to plan long-term space missions; just like weather
predictions are needed to plan the launch. Fleets of satellites circle the Earth collecting
many types of science data, protecting astronauts, and relaying information. Predictions of
the sunspot number have been made since the cycle was discovered. A few prediction
methods have been developed based on the precursor technique which is found to be
successful for forecasting the solar activity. Considering the geomagnetic activity aa
indices during the descending phase of the preceding solar cycle as the precursor, we
predict the maximum amplitude of annual mean sunspot number in cycle 24 .
Considering the geomagnetic activity aa index as the precursor, the maximum amplitude
of the solar cycle 24 is predicted using the method employed by Jain [29]. The temporal
behavior of observed annual mean sunspot number (red) and annual mean aa index (blue)
54
for solar cycles 11 to 23 considered in our investigation is shown in Figure 2.4. It is
observed from the figure that the annual mean aa index ranges from 5.7 (in 1901) to 36.6
(in 2003) which is an indicator of minimum and maximum geomagnetic activity
respectively, during the period of 1868–2008. Whereas, the annual mean sunspot number
varies between 1.4 (in 1913) and 190.2 (in 1957). Sunspot numbers rise steadily to
maximum and then fall steadily to a low level during each sunspot cycle, whereas
geomagnetic indices (Ap or aa) show two or more maxima per cycle, one near or before
the sunspot maximum and others in the declining phase, and the gap between the two
primary maxima (the Gnevyshev gap) results in the quasi-biennial and quasi-triennial
periodicities observed in the geomagnetic indices [30].
Figure 2.4 The observed annual mean aa index (blue) and annual mean sunspot number
(red) for the period of 1868–2008 indicating that the annual mean sunspot number for 2008
is 2.86.
As the annual mean sunspot number for the year 2008 is 2.86, which is within the range of
sunspot minimum value, we have considered the sunspot minimum year for solar cycle 23
to be 2008 in the present study. The annual mean aa index and annual mean sunspot
number are obtained by averaging the monthly mean of geomagnetic activity index aa and
monthly mean of sunspot number respectively for the period 1868–2008.
For nth cycle, we determined (aa* n)dsc, an average of the geomagnetic aa index, of the
year in which observed sunspot is minimum and four years preceding to it (i.e., total 5
years). Then we compared (aa* n)dsc of the nth cycle with the observed maximum annual
mean sunspot number (Rn+1)max of (n+1)th cycle and obtained a relationship between
(aa*n)dsc and (Rn+1)max which is shown in Figure 2.5.
55
Figure 2.5 Observed amplitude (Rn+1)max is plotted against (aa*n)dsc. Correlation
coefficient is found to be r = 0.85.
The best linear fit to the data with the correlation coefficient of 0.85 led us to derive an
asymptotic relation as follows:
(
)max = 6.138 (a
∗
)dsc – 1.1
… ( 2.3)
Using relation (2.3), we have obtained the maximum annual mean sunspot number for
cycles 12 to 23, which are almost in agreement with the observed values. The standard
deviation σ = ±21 is found from the difference between the calculated and observed values.
The relation (2.3) enabled us to predict the maximum annual mean sunspot number for
cycle 24 (R23+1) max to be 111 ± 21. This suggests that the maximum amplitude will be less
than that of cycles 21–22. Our prediction of the maximum amplitude is in good agreement
with the predictions made by a few earlier investigators [31-35] while in contrast to
Hathaway and Wilson [36].
2.13 N–S Asymmetry of Solar Activity
The distribution of various solar activity phenomena with respect to heliographic latitudes
as a function of time has been investigated in different studies. These activity features
include flares, magnetic flux, sunspot numbers, sunspot area etc. These studies indicate
that a solar cycle is not symmetric considering the distribution of solar activity separately
in northern and southern hemisphere [37]. This intrinsic feature (N-S asymmetry) poses a
challenge for dynamo model calculations.
56
To study the spatial distribution of solar activity (for e.g. solar flare) with respect to
heliographic latitudes, we have calculated the number of flares in the interval of 10◦
latitude for northern and southern hemispheres. In this case those events have been
excluded which occurred at 0◦ latitude. Since the number of flares above 50◦ latitude is
very small in both the hemispheres, the number of flares occurring above 50◦ latitude is
merged in one group.
It is customary to describe N–S asymmetry by an asymmetry index
… (2.4)
A=
where N and S are the yearly number of flares in the northern and southern hemisphere of
the Sun respectively. The statistical significance of the flare dominance in northern and
southern hemispheres has been assessed by using the binomial probability distribution. Let
us consider a distribution of n objects in 2 classes. The binomial formula gives us the
probability P(k) of getting k objects in class 1 and (n−k) objects in class 2, such that
P(k) =
!
!(
)!
( 1 − )
… (2.5)
and the probability to get more than d objects in class 1 is given by
P(≥ d) = ∑
( )
… (2.6)
In general, when P (≥ d) > 10%, implies a statistically insignificant result (flare activity
should be regarded as being equivalent for the two hemispheres), when 5% < P (≥ d) <
10% it is marginally significant, and when P (≥ d) < 5% we have a statistically significant
result (flare occurrence is not due to random fluctuations) [38,39].
2.14 Our Experimental Set Up
We have used Log-periodic Dipole Array (LPDA), Spectrum Analyzer and Digital Storage
Oscilloscope (DSO) for capturing solar radio bursts. The details of the analyzing methods
and utilization of the data have been discussed in the respective sections with the results
obtained. The photograph of the Log-periodic Dipole Array installed at Department of
physics, University of Kalyani, and the receiving system recording the solar burst in the
frequency range 50 MHz to 300 MHz range is shown in Figure 2.6.
Figure 2.6 The photograph shows the LPDA and the receiving system connected through LNA
57
2.15 Data Sources
The data used in this thesis are as follows:
(1) The monthly and yearly mean northern and southern hemispheric sunspot number from
http.//www.sidc.be/sunspot_data
http://sidc.oma.be/sunspots/bulletins/monthly/,
http://www.swpc.noaa.gov/Data
http://lasp.colorado.edu/sorce/tsi_data/daily/sorce_tsi_L3_c24h_latest.txt
http://www.nasa.gov/topics/solarsystem/features/spotless_sun.html
(2) The flare index was downloaded from
NOAA’s National Geophysical Data Center (NGDC)
ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA
http://solarscience.msfc.nasa.gov/greenwch/sunspot_area.txt,
http://maurol.com.ar/solar_cycle/,
http://solarscience.msfc.nasa.gov/,
http://sidc.oma.be/sunspots/bulletins/monthly/,
http://www.swpc.noaa.gov/Data
(3) The sunspot area data was downloaded from
http://solarscience.msfc.nasa.gov/greenwch/sunspot_area.txt,
http://omniweb.gsfc.nasa.gov/form/dx1.html
http://www.ngdc.noaa.gov/stp/solar/sunspotregionsdata.html
(4) The solar wind data was downloaded from
http://www.swpc.noaa.gov/Data;
http://omniweb.gsfc.nasa.gov/form/dx1.html;
http://www.ngdc.noaa.gov/stp/solar/sunspotregionsdata.html
References
[1] T. Pulkkinen, Space Weather: Terrestrial Perspectives, Living Reviews in Solar
Physics, 4, 2007
[2] J. D. Haigh, A. D. Winning, R. Toumi and J. W. Harder, An Influence Of Solar
Spectral Variations on Radiative Forcing of Climate, Nature, 467, 696-699, 2010
[3] P. Charbonneau, Dynamo Models of the Solar Cycle, Living Reviews in Solar
Physics, 2, 2005
[4] D. H. Clark and F. R. Stephenson, An Interpretation of Pre-Telescopic Records
from the Orient, Quarterly Journal of the Royal Astronomical Society, 19, 387–
410, 1978
58
[5] A. D. Wittmann and Z. T. Xu, A catalogue of sunspot observations from 165 BC
to AD 1684, Astronomy & Astrophysics Supplement, 70, 83–94, 1987
[6] R. J. Bray and R. E. Loughhead, Sunspots, John Wiley and sons, New York,
1965
[7] R.W. Noyes, The Sun, Our Star, The Harvard Books on Astronomy, Harvard
University Press, Cambridge, 1982
[8] R. Wolf, Abstract of his latest Results, Monthly Notices of the Royal
Astronomical Society, 21, 77–78, 1861
[9] J. A. Eddy, The Maunder Minimum, Science, 192, 1189–1202, 1976
[10] D. V. Hoyt and K. H Schatten, Group Sunspot Numbers: A New Solar Activity
Reconstruction, Solar Phys., 179, 189–219, 1988
[11] A. Wittmann, The Sunspot Cycle before the Maunder Minimum, Astronomy &
Astrophysics, 66, 93-97, 1978
[12] D, M. Willis, M. G. Easterbrook and F. R. Stephenson, Seasonal Variation of
Oriental Sunspot Sightings, Nature, 287, 617-619, 1980
[13] Y. A. Nagovitsyn, Solar Activity During the Last Two Millennia: Solar Patrol in
Ancient and Medieval China, International Journal of Geomagnitism and
Aeronomy, 41, 711, 2001
[14] H. Schwabe, Sonnen-Beobachtungen im Jahre 1843, Astronomishe Nachrichten,
21, 233–236, 1844
[15] A. Sen Gupta, Accumulation of Scientific Knowledge is The Password for
Success in Developing Scientific Attitude in Society - a Policy Approach, XXII
Indian Social Science Congress, 13-18, 1999
[16] D. V. Hoyt and K. H. Schatten, Group Sunspot Numbers: A New Solar Activity
Reconstruction, Solar Physics, 179, 189–219, 1988
[17] D. H. Hathaway, R. M. Wilson and E. J. Reichmann, Group Sunspot Numbers:
Sunspot Cycle Characteristics, Solar Physics, 211, 357–370, 2002
[18] M. Fligge and S.K Solanki, Inter-cycle Variations of Solar Irradiance: Sunspot
Areas as a Pointer, Solar Physics, 173, 427–439, 1997
[19] T. Baranyi, L. Gyori, A. Ludmany and H.E. Coffey, Comparison of sunspot area
data bases, Monthly Notices of the Royal Astronomical Society, 323, 223–230,
2001
[20] L. A. Balmaceda, S. K. Solanki, N.A Krivova and S. Foster, A Homogeneous
Data Base of Sunspot Areas Covering More than 130 Years, Journal of
Geophysical Research, 114, A07104, 2009
[21] E. W. Maunder, Spoerer’s Law of Zones, Observatory, 26, 329–330, 1903
[22] E. W. Maunder, Note on the Distribution of Sun-spots in Heliographic Latitude,
1874 to 1902, Mon. Not. R. Astron. Soc., 64, 747–761, 1904
[23] K. F. Tapping and D. P. Charrois, Limits to the Accuracy of the 10.7 cm Flux,
Solar Physics, 150, 305–315, 1994
[24] A. B. Bhattacharya, S. K. Kar, R. Mukhopadhyay, A. K.Goswami, A. K. Sen
and R. Bera, Observation of the Microwave Radio Sun Using L, S and C-Band
Radio Telescopes, Indian Journal Radio and Space Physics, 27, 124-129, 1998
59
[25] G. E. Hale, On the Probable Existence of a Magnetic Field in Sun-Spots,
Astrophysical Journal, 28, 315–343, 1908
[26] G. E. Hale, F., Ellerman, S. B. Nicholson and A.H Joy, The Magnetic Polarity of
Sun-Spots, Astrophysical Journal, 49, 153–178, 1919
[27] H. D. Babcock, The Sun’s Polar Magnetic Field, Astrophysical Journal, 130,
364–365, 1959
[28] N. R. Sheeley Jr, Polar Faculae, 1906–1990, Astrophys. J., 374, 386–389, 1991
[29] R. Jain, Prediction of the Amplitude in Sunspot Cycle 23, Solar Physics, 176, 2,
431–437, 1997
[30] R. P. Kane, Correction to "A Preliminary Estimate of the Size of the Coming
Solar Cycle 23, Based on Ohl's Precursor Method" by R. P. Kane, Geophysical
Research Letters, 25, 3121, 1998
[31] J. L. Wang, J. C. Gong, S. Q. Liu, G. M., Le and J. L Sun, The prediction of
Maximum Amplitudes of Solar Cycles and the Maximum Amplitude Of Solar
Cycle 24, Chinese Journal of Astronomy and Astrophysics, 2, 557–562, 2002
[32] E. Echer, N. R Rigozo, D. J. R. Nordemann and L. E. A. Vieira, Prediction of
Solar Activity on the Basis of Spectral Characteristics Of Sunspot Number,
Annals of Geophysics, 22, 2239–2243, 2004
[33] R. S. Dabas, Sharma, Kabita, Das, M. Rupesh, K. G. M. Pillai, Chopra, Parvati
and N. K. Sethi, Solar Physics, 250, 171–181, 2008
[34] K. M. Hiramath, Solar Forcing on the Changing Climate, Astrophysics and
Space Science, 314, 45–49, 2008
[35] J. Javaraiah, Predicting the Amplitude of a Solar Cycle Using The North-South
Asymmetry in the Previous Cycle: II. An improved prediction for solar cycle 24,
Solar Physics, 252, 419–439, 2008
[36] D. H. Hathaway and R. M. Wilson, Geomagnetic activity indicates large
amplitude for sunspot cycle 24, Geophysical Research Letters, 33, L18101, 2006
[37] B. Joshi, P. Pant and P. K. Manoharan, North-South Distribution of Solar Flares
during Cycle 23, Journal of Astrophysics and Astronomy, 27, 151-157, 2006
[38] M. Temmer, A. Veronig, A. Hanslmeier, W. Otruba and M. Messerotti,
Statistical Analysis of Solar H-alpha flares, Astronomy & Astrophysics, 375,
1049, 2001
[39] M. Temmer, Solar Activity Patterns–A Hemisphere Related Study, Ph. D.
Thesis, University at Graz, Austria, 2004
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