NORTH–SOUTH ASYMMETRY OF DIFFERENT SOLAR ACTIVITY
FEATURES DURING SOLAR CYCLE 23
Neeraj Singh Bankoti1*, Navin Chandra Joshi1, Seema Pande2, Bimal Pande1, Kavita
Pandey1
1
Department of Physics, D. S. B. Campus, Kumaun University, Nainital, Uttarakhand,
India
2
Depertment of Physics, M.B.P.G. College, Haldwani, Kumaun University, Nainital,
Uttarakhand, India
*E-mail [email protected]
Abstract
A study on North-South (N-S) asymmetry of different solar activity features (DSAF)
such as solar proton events, solar active prominences, H flare index, soft X-ray flares,
monthly mean sunspot area and monthly mean sunspot number were carried out from
1996-2008. It is found in our result that solar cycle 23 is magnetically weak compared to
solar cycle 22. Study shows the Southern dominance of DSAF during the time period of
study. During the rising phase of the cycle the numbers of DSAF approximately equal on
the North and South Hemisphere. However, these activities tend to shift from Northern
Hemisphere to Southern Hemisphere in between year 1998-1999. The statistical
significance of the asymmetry time series using a  2 - test of goodness of fit indicates
that in most of the cases the asymmetry is highly significant, i.e., the asymmetry is a real
feature in the N-S distribution of DSAF.
Key words: Sun: asymmetry - Sun: activity - Sun: sunspot number - Sun: Soft X-ray
flare - Sun: prominences - Sun: solar proton events
1. Introduction
The occurrence of various solar activity features shows non-uniformity over the solar
disk. It has also been noticed that more activity features occur in one part of the solar
Hemisphere than the other at one time. When these features are examined with respect to
equator of the Sun, they are referred to as the North-South (N-S) asymmetry. The N-S
1
asymmetry of several solar activity features viz relative sunspot numbers (SN), sunspot
groups, sunspot areas (SA), flares, Hα flare indices (Q), solar active prominences (SAP),
coronal mass ejections (CME), radio bursts, solar gamma ray bursts, coronal holes,
faculae, green corona, corona’s ionization temperatures and magnetic fluxes have been
investigated by various authors (Maunder, 1904; Howard, 1974; Roy, 1977; Vizoso and
Ballester, 1987; Bai, 1990; Oliver and Ballester, 1994; Joshi, 1995; Ataç and Özgüç,
1996; Verma, 2000; Temmer et al., 2001, 2002; Joshi and Joshi, 2004; Joshi and Pant,
2005; Atac and Özgüç, 2006; Gao, Li and Zhong, 2007; Chang, 2008; Li et al., 2009;
Joshi et al., 2009).
Bell (1962) has found long term N-S asymmetry in the SA data for SC 8-18.
Carbonell et al. (1993) presented a thorough study of the N-S asymmetry of daily SA,
since these are good indicators of magnetic activity and found the N-S asymmetry is
statistically highly significant using  2 - test of goodness of fit. A detailed study has been
carried out by Verma (2000) to decipher N-S asymmetry of SAP during 1957-1998. The
existence of the real N-S asymmetry of SXR flare index (which is strengthened during
solar minimum) during solar cycle 21, 22 and 23 has been analyzed (Joshi and Joshi,
2004). Joshi et al. (2005) also investigated the N-S asymmetry of H flare events during
the solar cycle 23. Zaatri et al. (2006) studied the N-S asymmetry of meridional and zonal
components of horizontal, solar subsurface flows during the years 2001–2004, which
cover the declining phase of solar cycle 23. More recently, asymmetry of solar activity in
cycle 23 has been studied by Li et al. (2009) using sunspot groups and SA from May
1996 to February 2007 and find the solar activity dominance in the Southern Hemisphere
for cycle 23. Joshi et al. (2009) studied the asymmetry of SAP and groups of different
limb and disk features of SAP for solar cycle 23 and presented a comparison among the
solar cycles 21, 22 and 23.
In the past, the N-S asymmetries of several solar activity features on the entire solar
disk and atmosphere have been studied by various researchers (Waldmeier, 1971; Verma,
1987; 1992; 1993; Ballester et al., 2005). Waldmeier (1971) studied asymmetry of most
outstanding feature of solar activity in the decade 1959 -1969 on the two Hemispheres
and found that on the Northern Hemisphere spots, faculae and prominences were more
numerous and the white light corona was brighter than on the Southern Hemisphere. He
also found that the green coronal line was brighter on the Northern Hemisphere, but the
2
intensity of the red line was asymmetric in the opposite sense. Further, a comprehensive
study was carried out by Verma (1987) on N-S asymmetry for major flares (solar cycles
19 and 20), type II radio bursts (solar cycles 19, 20 and 21), white light flares (solar
cycles 19, 20 and 21), gamma ray bursts, hard X-ray bursts and CME (SC 21) and the
results were compared with the found asymmetry in favor of the Northern Hemisphere
during solar cycle 19 and 20 in favor of the Southern Hemisphere during solar cycle 21.
Verma (1992) also predicted that the N-S asymmetry in solar cycles 22, 23 and 24 may
be Southern dominated which will shift to Northern Hemisphere in solar cycle 25. Verma
(1993) studied various solar phenomena occurring in both Northern and Southern
Hemisphere of the Sun for solar cycles 8-22, calculated the N-S asymmetry indices for
several solar phenomena and plotted them against the number of solar cycles. Similarly,
Ballester et al. (2005) have studied N-S asymmetry using different phenomena of solar
activity. Summaries of the studies of Hemispherical asymmetries of solar activity have
been included in the work of Vizoso and Ballester, 1990; Li et al., 1998 and Li et al.,
2002.
The aim of our work is to make a detailed study of N-S asymmetry of DSAF (SN,
SA, SXR, Hα flare index, SAP, SPE) from 1996 to 2008 (solar cycle 23). Section 2
shows the observational data and statistical analysis. Section 3 represents a brief
description of N-S asymmetry of DSAF during the considered time period. Section 4
contains discussions and results.
2. Observational Data and Statistical Analysis:
The different solar activity features analyzed in our study have been downloaded
form following various web sites:

The monthly North and South SNs (1996-2008), from
ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SUNSPOT_NUMBERS/RNRS with
682 data points.

The monthly North and South number of SAPs (1996-2008), obtained from
ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SOLAR_FILAMENTS with 8539 data
points. After excluding the events which lie on equator, a total number of 8426 events
have been selected to study latitude and longitude distribution.
3

The monthly North and South SXR flares (1996-2008), detected by the GOES
satellites, which can be downloaded from
ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SOLAR_FLARE/XRAY_FLARES with
23335 data points. Out of this 12127 flare events for which heliographic coordinates
are given, are selected.

The monthly number of North and South flare index (1996-2007) compiled in
the Kandilli Observatory, from
ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SOLAR_FLARES/INDEX.

The monthly North and South SA (1996-2008) compiled by D.Hathaway, from
http://solarscience.msfc.nasa.gov/greenwch.shtml.

The North and South monthly number of solar proton events (SPE) (1996-2008),
from http://www.swpc.noaa.gov/ftpdir/indices/SPE.txt with 72 data points.
We compare the amplitudes of solar cycle 22 and 23 by using DSAF. In all activity
indices the amplitudes of solar cycle 23 are distinctively weaker than the solar cycle 22,
as can be seen from Fig. 1 and Table 1. First column of Table 1 represents the DSAF for
cycle 22 and 23. 2nd, 3rd, 4th and 5th columns represent the average values of the DSAF as
well as their standard deviations within brackets during the solar maximum and minimum
phases of the solar cycles 22 and 23. The last two columns illustrate the variability of
DSAF during rising phase and decay phase between the above two successive cycles.
The N-S asymmetry of solar activity phenomena has been calculated using the
formula:
ANS 
N S
NS
(1)
Here, ANS is the N-S asymmetry index, N is the number of solar activity phenomena in
Northern Hemisphere and S is the number of solar activity phenomena in Southern
Hemisphere. Thus, if ANS > 0, the activity in the Northern Hemisphere dominates and if
ANS < 0, the reverse is true. To verify the reliability of the observed N-S asymmetry
values, a  2 - test is applied with Equation (2) given below.
 NS 
2( N  S )
(N  S)

2 ANS
ANS
(2)
4
If ANS  ANS , ANS  ANS  2ANS and ANS  2ANS , the
probability
that
N-S
asymmetry exceeds the dispersion value is p  84% , 84%  p  99.5% and p  99.5%
respectively. Here the first, second and third limits imply statistically insignificant,
significant and highly significant values respectively (Joshi and Joshi, 2004; Joshi et al.,
2009).
3. N-S Asymmetry of Sunspot Numbers, Sunspot Area, SXR Flares, Hα Flare
Index, Solar Active Prominences and Solar Proton Events
To investigate the behavior in variation of DSAF in the North and South
Hemispheres, activities were counted for each Hemisphere separately and are shown in
Table 2 & Figure 2. In Table 2, first and second column represent DSAF and total
number of events reported from different ground and space based observatories and
satellites respectively. Column 3rd represents events for which the heliographic
coordinates are given and column 4th and 5th represent the total number of events in
Northern and Southern Hemisphere respectively during the considered time span. All
parameters shown in Fig 2 show nearly the same monthly variation in the North-South
Hemisphere except SAP. Further, SAP shows an abnormal behavior as it is active from
1996-1998. On the other hand, for the same period, other activities are almost dormant.
For more clear results smoothened monthly means of the DSAF indices in the Northern
and Southern Hemispheres are shown in Fig. 3 (left panel). In the temporal variation of
SA, SXR flares, SAP and SPE the strength of the activity of two Hemispheres is
different. The strength dominates in Northern Hemisphere for flare index and SAP
whereas it dominates in Southern Hemisphere for SA, SXR and SPE. Additional plot of
the smoothed asymmetry for the monthly mean is given for each solar active feature in
Fig.3 (right panel). It is clear that the dominance of activity at the beginning of the cycle
started to alternate between the two Hemispheres during the rising phase (0 – 48 months).
However during the maximum phase the N-S asymmetry dropped nearly to zero and
remained very low (48 – 96 months) and after 96 months of the considered time it is in
favor of Southern Hemisphere.
All these results are confirmed by Fig. 4 in which we have shown the cumulative
counts of the monthly DSAF for the Northern (black line) and Southern Hemispheres
(red line) respectively. The vertical spacing between the two lines is a measure of the
5
Northern/Southern excess of DSAF at that time which shows Southern dominance of SN,
SA, SXR, Q and SAP excluding SPE. In order to characterize the N-S asymmetry of
DSAF, the asymmetry values of the monthly DSAF numbers in the Northern and
Southern hemispheres were calculated, and fitted into a straight line (Fig. 5). Here,
interestingly, we notice that, during considered time period, the behavior of asymmetry
changes from North to South hemisphere in the year 1998-99 for SN, SA, Q and SXR
whereas asymmetry of SAP changes from North to South in the year 1997. From Fig. 5 it
is clearly evident that the slopes are negative for each activity which again supports our
previous observation that the activities are Southern Hemisphere dominant. To
understand the behavior of DSAF with respect to SN, correlations were obtained between
DSAF and SN (Fig. 6) and following points were noticed; (i) Higher correlation
(positive) for SA and Q (ii) Moderate correlation (positive) for SXR and SPE and (iii)
Poor correlation (negative) for SAP. In Fig. 7 we have plotted the annual asymmetry
index of DSAF. Yearly N-S asymmetry of DSAF where highly significant, significant
and insignificant values are marked with filled square (■), circle (●) and triangle ()
respectively, using  2 - test (equation 2). Significance level of N-S asymmetry is also
represented in Table 3. SXR flare, SA and SAP have larger number of highly significant
values as compared to SN and Q.
4. Discussion and Results:
We have presented a comparative study of activity between cycles 22, 23 using
DSAF. In Table 1 we have represented the minimum to maximum variability of the
DSAF during cycle 22 and 23. In column 5 of this table all the features except SAP show
values greater than 1, which means that the activity in rising phase of solar cycle 22 is
higher than the solar cycle 23. The SAP shows negative value because there are very less
number of SAP observed in maximum phase compared to minimum phase of solar cycle
23. In column 6 all the features show values greater than 1, which means that the activity
in solar cycle 22 is higher than solar cycle 23. The number of SAP events is 27.47 times
larger in decay phase of cycle 22 than solar cycle 23. During the time of increased
amplitude of solar activity we would expect short-period cycles and during the times of
decreased amplitude of solar activity we would expect loner-period cycles as in cycle 23
(Hathaway et al., 2002). There is enough evidence to support that the chosen cycle 23 is
6
magnetically weak. This peculiarity is clearly evident from comparison of DSAF during
cycle 22 and 23 (Fig. 1). Even-Odd (or Gnevyshev, G-O) effect (Gnevyshev and Ohl,
1948) is seen in the monthly averaged or smoothed wolf sunspot numbers (1749-2003)
where the odd-numbered cycle amplitudes are compared with the amplitudes of the
preceding even-numbered cycles. Our results show the violation of G-O rule in cycle 23
for DSAF.
We have also presented the N-S asymmetry of DSAF for solar cycle 23 and the
obtained results are discussed below. Earlier studies by Verma (1992, 93); Ataç and
Özgüç, (1996, 2006) and Li et al. (2002) gave the dominant Hemisphere of solar activity
in each of solar cycles 12 to 22 and inferred that a Southern dominance of solar activity
should occur in solar cycle 23. Recently Li et al. (2009) and Joshi et al. (2009) found out
the Southern dominances of SA, grouped SN and different groups of SAP respectively.
Our results support the predictions. This finding is further confirmed in the present study
by investigation of the cumulative counts of DSAF number for the Northern and
Southern Hemispheres during considered time. It is found that all the features except SPE
and Q show a small spacing during the rising and maximum phase whereas a
considerable spacing (Southern dominances) between the two cumulative lines during the
end phase of solar cycle 23 (Fig. 4). Vizoso and Ballester, 1990; Ataç and Özgüç, 1996;
Li et al., 2002 and Joshi and Joshi, 2004; found regularity in their studies of different
activity parameters that for each four cycles the slope of the fitting straight lines of the NS asymmetry values changes its sign. The solar activity of cycle 23 is very important for
demonstrating the existence of these regularities. We have also demonstrated the
Southern dominance of solar activity in cycle 23 with the help of linear fitting of monthly
asymmetry index values of DSAF (see Fig. 5), which is similar to solar cycles 20, 21 and
22 as the obtained slopes are negative. Yearly N-S asymmetry of DSAF shows similar
type of variation during cycle 23 (Fig. 7). In our study, we observe the DSAF show
periodic behavior form 1996 to 2001. N-S asymmetry for SA, SXR, Q and SAP favors
Southern Hemisphere in the year 1996. In 1997, 1999 and 2000 all the activity
asymmetries move to the Northern Hemisphere. These shift to South again in 1998 and
beyond 2001, where these strongly favor the Southern Hemisphere. It also has been
found that the variation of N-S asymmetry of SPE has an alternating nature during whole
cycle. Temmer et al. (2001) and Joshi et al. (2009) analyzed the N-S asymmetry of Hα
7
flares and SAP and found high statistical significance of N-S asymmetry. We have also
found highly significant values of all DSAF (Table 3), which is in agreement with
previous studies (Temmer et al., 2001; Joshi and pant, 2005).
The physical interpretation of the asymmetry in the solar activity has not been given
clearly so far. However, Hathaway (1996), argued that the fluctuations in the meridian
flow, believed to be a product of turbulent convection and variations in the gradient of the
rotation rate, contributed to the cycle amplitude variations. The Sun’s magnetic activity is
generally believed to be supplied by a hydro magnetic dynamo operating either in or at
the base of the solar convective zone (Pulkkine et al. 1999). Zhao and Kosovichev (2004)
have found out that meridian flows vary with the location of the solar activity belts. By
applying time-distance helioseismology measurements, it is demonstrated that the activity
belts which were located towards the pole side of the faster zonal bands migrated together
towards the solar equator around the maximum phase of the solar cycle 23 and their
synoptic maps show unsymmetrical zonal flows in the Northern and Southern
Hemispheres (Haber et al. 2002). Zaatri et al. (2006) also found the zonal flow is larger in
the Southern Hemisphere than the Northern one, and N-S asymmetry increases with
depth. These explanations are only suggestive and more observations of local
helioseismology are needed to understand the N-S asymmetry of DSAF.
Acknowledgement
Authors, NSB and NCJ are thankful to UGC, New Delhi, India for financial assistance
under RFSMS (Research Fellowship in Science for meritorious students) scheme.
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9
Tables
Table 1: Rising and decay phase variability of DSAF during solar cycle 22 and 23.
Rising
Decay
DSAF: Cycles 22
1986
1989-1991
1996
2000-2002
2008
phase
phase
and 23
Minimum
Maximum
Minimum
Maximum
Minimum
cycle22/
cycle22/
cycle23
cycle23
1.31
1.29
1.37
1.35
2.32
2.48
1.82
1.84
1.01
1.003
-29.33
27.47
13.4
148.6
8.6
111.5
2.9
(9.9)
(23.2)
(5.3)
(19.4)
(2.4)
124.7
2366.0
81.9
1716.8
22.78
(134.7)
(654.8)
(80.1)
(562.1)
(39.6)
1.2
14.9
0.4
6.3
0.46
(1.4)
(4.6)
(0.4)
(4.3)
(0.7)
730
20816
280
11294
125
(54.6)
(117.8)
(18.1)
(146.6)
(30.6)
916
8538
515
8085
86
(62.8)
(58.5)
(30.1)
(53.6)
(10.2)
Solar active
3095
33891
2213
1163
10
Prominence
(127.1)
(130.28)
(63.8)
(24.7)
(1.2)
Sunspot Number
Sunspot Area
Flare Index
H flare
x-ray flare
10
Table 2: Total number of DSAF in Northern and Southern Hemisphere during solar cycle
23.
Total
Solar Active
Total
Feature
Number
SAP
8539
8426
SXR
23,335
12,127
SPE
72
71
-
682
-
10091.16
-
443.53
SN
(monthly mean)
SA
(monthly mean)
Q
(yearly)
Number
(N+S)
Total North
Total South
Dominant
Number (%)
Number (%)
Hemisphere
4018
4408
(47.69%)
(52.31%)
5131
6996
(42.31%)
(56.69%)
36
35
(50.7%)
(49.3%)
322.1
359.9
(47.22%)
(52.78%)
4690.1
5401.06
(46.48%)
(53.52%)
212.95
230.58
(50.26%)
(49.74%)
S
S
S
S
-
11
Table 3: Significance level of DSAF during solar cycle 23 using  2 -test.
Solar Active
Highly
Feature
Significant
SAP
Significant
Insignificant
8
1
4
SXR
11
2
0
SPE
4
5
4
5
3
5
10
2
1
6
2
4
SN
(monthly mean)
SA
(monthly mean)
Q
(yearly)
12
300
SC 22
SC 23
SUNSPOT AREA
X-RAY FLARE
150
0
24
48
72
96
120
100
0
24
48
72
96
120
144
600
SC 22
SC 23
H FLARE
SUNSPOT NUMBER
0
144
150
SC 22
SC 23
400
200
50
0
1500
750
75
0
SC 22
SC 23
2250
225
0
24
48
72
96
120
0
144
0
24
48
72
96
120
144
20
SC 22
SC 23
FLARE INDEX
15
10
5
0
0
24
48
72
96 120 144
Time (11-Monthly running mean)
SOLAR ACTIVE PROMINENCES
1000
SC 22
SC 23
750
500
250
0
0
24
48
72
96 120 144
Time (11-Monthly running mean)
Figure 1: Comparison of the monthly mean of the SXR, SA, SN, Hα flare, Q and SAP
for the solar cycle 22 and solar cycle 23.
13
SPE
2
1
SAP
0
300
150
Q
0
8
4
0
SN
SA
SXR
150
75
0
2000
1000
0
80
40
0
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008
Time (Years)
Figure 2: Monthly number of solar proton events (SPE), solar active prominences (SAP),
Flare index (Q), Hα flare, soft X-ray flare (SXR), monthly mean sunspot area (SA) and
monthly mean sunspot number (SN) respectively in the Northern (the black line) and
Southern (the red line) Hemispheres from 1996-2008 (from top to bottom ).
14
20
0
36
40
20
0
36
72
108
5
4
3
2
1
0
150
36
72
108
100
50
0
36
72
108
0.5
0.0
0
36
72
108
144
Time (11-Month running mean)
24
48
72
96
120
144
168
SUNSPOT AREA
0.5
0.0
-0.5
-1.0
1.0 0
24
48
72
96
120
144
168
X-RAY FLARE
0.5
0.0
-0.5
0
24
48
72
96
120
0.5
144
168
FLARE INDEX
0.0
-0.5
0
24
48
72
0.5
96
120
144
168
SOLAR ACTIVE PROMINENCES
0.0
-0.5
0
144
NORTH
SOUTH
1.0
-0.5
144
NORTH
SOUTH
0
0.0
-1.0
144
NORTH
SOUTH
0
N-S ASYMMETRY
144
N-S ASYMMETRY
X-RAY FLARE
108
60
6
FLARE INDEX
72
SUNSPOT NUMBER
0.5
-1.0
0
1.0
144
NORTH
SOUTH
80
0
PROMINENCES
108
NORTH
SOUTH
100
SOLAR PROTON EVENT
72
N-S ASYMMETRY
36
N-S ASYMMETRY
1200
1000
800
600
400
200
0
0
N-S ASYMMETRY
SUNSPOT NUMBER
40
0
SUNSPOT AREA
NORTH
SOUTH
N-S ASYMMETRY
1.0
60
24
48
72
96
120
144
168
SOLAR PROTON EVENTS
0.5
0.0
-0.5
0
24
48
72
96
120
Time (11-Month running mean)
144
168
Figure 3. Monthly plots of N-S asymmetry of the SN, SA, SXR, Q, SAP and SPE and
Gnevyshev gap time occurrence.
15
6000
300
4500
200
3000
SA
SN
400
100
north (47.22%)
south (52.78%)
0
8000
1500
4000
Q
SXR
6000
2000
north (42.31%)
south (56.69%)
0
north (50.26%)
south (49.74%)
27
3000
SPE
SAP
250
200
150
100
50
0
36
4000
2000
1000
1995
0
north (46.48%)
south (53.52%)
north (47.69%)
south (52.31%)
1998
2001 2004
Time (Years)
2007
18
9
north (50.7%)
south (49.3%)
0
1995
1998
2001 2004
Time (Years)
2007
Figure 4: Cumulative monthly mean sunspot number (SN), monthly mean sunspot area
(SA) number of soft X-ray flare (SXR), Hα flare index (Q), solar active prominences
(SAP) and solar proton events (SPE) respectively in the Northern Hemisphere (the black
lines) and the Southern Hemisphere (the red lines) from 1996-2008.
16
0.5
0.5
0.0
0.0
SA
1.0
-0.5
-0.5
-1.0
1.0
1.0
0.5
0.5
0.0
0.0
Q
-1.0
-0.5
-0.5
-1.0
-1.0
1.0
1.0
0.5
0.5
0.0
0.0
SPE
SN
SXR
SAP
1.0
-0.5
-1.0
1995
-0.5
-1.0
1998
2001
2004
2007
1995
1998
Time (Years)
2001
2004
2007
Time (Years)
Figure 5: Fit of a regression straight line to the monthly values of the North-South
asymmetry of monthly mean sunspot number (SN), monthly mean sunspot area (SA)
number of soft X-ray flare (SXR), Hα flare index (Q), solar active prominences (SAP)
and solar proton events (SPE)respectively from 1996-2008.
17
2250
150
SXR
225
1500
r = 0.92
Q
750
0
400
9
300
6
r = 0.73
r = 0.60
r = - 0.13
200
100
3
0
0
1995 1998 2001 2004 2007 2010
4
SN
3
SPE
75
12
SAP
SA
3000
r = 0.47
2
1
0
1995
1998
2001
2004
SN
2007
2010
Figure 6: Correlation plot of the monthly mean sunspot area (SA), soft X-ray flare
(SXR), Hα flare index (Q), solar active prominences (SAP) and solar proton events (SPE)
with monthly mean sunspot number (SN); r indicates the correlation coefficient.
18
1.2
SN
SA
XR
FI
SAP
SPE
N-S Asymmetry
0.8
0.4
0.0
-0.4
-0.8
-1.2
1996
1998
2000
2002
2004
2006
2008
Time (Years)
Figure 7: Plot of N-S asymmetry of Various Solar Active Phenomena versus years.
Highly significant, significant and insignificant values marked with filled square (■),
circle (●) and triangle () respectively from 1996-2008.
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19