Asymmetry of solar active prominences separately at low and high latitudes from 1957 to 1998

New Astronomy 8 (2003) 655–664 www.elsevier.com / locate / newast

Asymmetry of solar active prominences separately at low and high latitudes from 1957 to 1998 K.J. Li a , *, X.H. Liu b , L.S. Zhan a,c,d , H.F. Liang a , H.J. Zhao a , S.H. Zhong a a

National Astronomical Observatories, Yunnan Observatory, CAS, Yunnan, China b Department of Physics, Jiaozuo Educational College, Henan, China c School of Graduates, CAS, Beijing, China d Department of Physics, Jingdezhen Comprehensive College, Jiangxi, China

Received 29 July 2002; received in revised form 1 March 2003; accepted 3 March 2003 Communicated by W. Soon

Abstract The paper presents the results of a study of the asymmetry of the solar active prominences (SAP) at low ( # 408) and high ( $ 508) latitudes, respectively, from 1957 through 1998 (solar cycles 19–22). A quantitative analysis of the hemispheric distribution of the SAP is given. We found that the annual hemispheric asymmetry indeed exists at low latitudes, but strangely, a similar asymmetry does not seem to occur for SAPs at high latitudes. We found that the north–south (N–S) asymmetry of the solar active prominences at high latitudes is always north dominated during solar cycles 19–22 while the N–S asymmetry of the SAPs at low latitudes is shifted to a dominance in the southern hemisphere for solar cycle 21 and remains south dominated even in cycle 22. Thus, the hemispheric asymmetry of the solar active prominences at high latitudes in a cycle appears to have little connection with the asymmetry of the solar activity at low latitudes.  2003 Elsevier B.V. All rights reserved. PACS: 95.10.-j; 96.60-a; 96.60-se Keywords: Sun: prominences; Sun: activity; Sun: general

1. Introduction Solar activity indices vary unevenly over the disk and solar active phenomena are not uniformly distributed between the two solar hemispheres. Various solar activity parameters have been found to be not symmetric between the northern and southern hemispheres (Atac and Oguc, 1996; Carbonell et al., 1993; Li et al., 2000; Temmer et al., 2002; Verma,

*Corresponding author. E-mail address: [email protected] (K.J. Li).

1987; Vizoso and Ballester, 1987). Long-term observations of solar activities indicate that the hemispheric behavior of the solar activities is asymmetric (White and Trotter, 1977; Garcia, 1990; Oliver and Ballester, 1994; Atac and Oguc, 1998; Duchlev, 2001). The north–south (N–S) asymmetry of the solar activities has been well known for a long time from many studies of its different manifestations (Newton and Milson, 1955; Dodson and Hedeman, 1971, 1975, 1981; Roy, 1977; Swinson et al., 1986; Verma, 1992, 1993; Verma et al., 1987; Zhong, 1995). Statistical analyses indicate that the N–S asymmetry is statistically significant, namely, it is a

1384-1076 / 03 / $ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016 / S1384-1076(03)00053-8

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real physical phenomenon and not a random mathematical fluctuation (Carbonell et al., 1993; Li et al., 2002a,b,c). The phenomenon of N–S asymmetry may yields important, detailed nature of the solar dynamo action. This is why interest in the understanding of N–S asymmetry of any activity manifestation has grown considerably, especially in the modern study of nonlinear dynamo models during the last few years (Brandenburg and Tuominen, 1990; Pulkkinen et al., 1999; Duchlev, 2001). However, up till now, the N–S asymmetry of solar activity has been studied almost exclusively adopting solar activity indices at low latitudes ( # 408), and seldom is the phenomenon examined using solar activity indicators relevant for higher latitudes ( $ 508). The solar activity at high latitudes has a different cyclical behavior from the cycle of the solar activity at low latitudes (i.e., most commonly represented by the sunspot cycles) and it is seemingly in complete antiphase with the sunspot cycles (Sheeley, 1964, 1991; Saito and Tanaka, 1960; Wilson et al., 1988; Makarov and Makarova, 1987, 1996; Makarova et al., 1989; Tanaka, 1964; Sakurai, 1998; Riehokainen et al., 2001; Li et al., 2002b). Verma (2000) investigated the N–S and west–east distributions and asymmetries of the solar active prominence (SAP) events for the whole disk for the period 1957–1998 in considerable details. He gave some important results on the asymmetry of solar active prominences when averaged over the whole solar disk. In Verma’s study, the phase relation between the activities of the solar active prominences is not investigated separately for low and high latitudes. Li et al. (2002d) studied the phase relation and found that the cycle of the solar active prominences at high latitudes (larger than 508) lead the sunspot cycle and the corresponding cycle of the solar active prominences at low latitudes (less than 408) by about 4 years. Namely, the former is anticorrelated with the latter two solar activity indicators. In this paper, we will investigate the N–S distribution and asymmetry of the solar active prominence events separately at low and high latitudes, and detailed discussion especially about the asymmetry of solar activity at high latitudes will be given.

2. Asymmetry of the solar active prominences at low and high latitudes respectively The original data of the SAP in the period 1957– 1998 came from the web site of the National Oceanic and Atmospheric Administration, Boulder CO, USA. The URL address of this web site is: ftp: / / ftp. ngdc.noaa.gov / STP/ SOLAR]DATA / FILAMENTS. Verma (2000) downloaded the data and then gave in Tables III and IV of his paper the yearly numbers of the SAP events at nine latitude bands (for ten-degree bands from 08 to 908) in the northern and southern hemispheres, respectively. In his study, the SAP events included events in an active surge region, active prominences, active dark filaments, disappearing filaments, mound prominences, bright surges on the limb, eruptive prominences on limb, loops, sprays, arch filament systems, dark surges on disk, bright surges on disk, solar sector boundaries, coronal rains and cap prominences (Verma, 2000). We had previously examined the phase relation between the activities of the SAP at low and high latitudes (Li et al., 2002d) and found that the cycle of the SAP at high latitudes (larger than 508) lead both the sunspot cycle and the corresponding cycle of the SAP at low latitudes (less than 408) by about 4 years. In other words, the cycle of the SAP at high latitudes is almost in antiphase with the sunspot cycle (Li et al., 2002d). Based on Tables III and IV in Verma (2000), we counted the yearly numbers of the SAP events respectively at high (larger than 508) and low latitudes (less than 408) in each of the northern and southern hemispheres. Table 1 gives the yearly numbers of the SAP events at low and high latitudes, respectively. The numerical values in Table 1 are also represented as Fig. 1. Fig. 1 shows that both the SAP events at high and low latitudes are not uniformly distributed in the northern and southern hemispheres. The N–S asymmetry of the yearly numbers of the SAP events at low and high latitudes, respectively, is calculated by means of the formula Asymmetry 5 (NON 2 NOS ) /(NON 1 NOS ). The result is shown in Fig. 2, where NON and NOS stand for the annual numbers of the SAP in the northern and southern hemispheres, respectively. In the figure, the mini-

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Table 1 Hemispheric distribution of the SAP (see discussion in the text for the meaning of the probability measure) Year

At low latitudes NON

1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

1225 2767 3088 2385 1127 888 1040 1020 1656 4657 4492 3614 1226 2219 2191 2760 1904 357 1093 534 554 1136 959 1022 808 715 310 550 517 1542 1489 3925 4602 4485 3742 3701 4364 2597 1651 937 1422 319

At high latitudes NOS 1165 2764 1334 1299 665 447 316 369 344 545 2355 2488 408 1509 2622 3985 2096 634 886 602 352 728 791 1044 787 587 677 853 658 1087 2583 3628 4242 4546 5395 5351 4190 3031 1999 1126 1002 665

Probability 201

1.09 3 10 4.83 3 10 201 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 ¯ 0.0 1.19 3 10 203 ¯ 0.0 1.59 3 10 206 2.17 3 10 202 ¯ 0.0 ¯ 0.0 2.90 3 10 205 3.14 3 10 201 2.99 3 10 201 1.91 3 10 204 ¯ 0.0 ¯ 0.0 1.89 3 10 205 ¯ 0.0 ¯ 0.0 3.15 3 10 204 6.43 3 10 205 2.60 3 10 201 ¯ 0.0 ¯ 0.0 2.99 3 10 202 3.55 3 10 209 4.07 3 10 209 1.55 3 10 205 ¯ 0.0 ¯ 0.0

mum times of sunspot cycles 17–23 (Li et al., 2002e) are marked to show feature of the N–S asymmetry in terms of plausible cycles. Also plotted in the figure is the N–S asymmetry of the sudden disappearances of solar prominences in the period 1931–1985 (Vizoso and Ballester, 1987). The figure

NON

NOS

Probability

76 266 69 42 31 49 36 148 77 128 102 42 5 24 35 93 113 0 253 269 125 142 48 48 12 44 49 45 195 123 94 182 357 60 14 13 150 39 160 46 12 1

51 94 29 36 28 19 7 47 51 31 95 60 1 8 35 66 85 0 231 225 108 125 57 35 8 28 40 98 216 103 61 175 175 48 14 7 168 37 89 48 7 18

1.27 3 10 202 ¯ 0.0 1.90 3 10 205 2.47 3 10 201 3.47 3 10 201 9.71 3 10 205 1.41 3 10 206 5.41 3 10 214 1.03 3 10 202 ¯ 0.0 3.08 3 10 201 3.63 3 10 202 3.12 3 10 202 1.66 3 10 203 5.00 3 10 201 1.56 3 10 202 2.28 3 10 202 1. 1.58 3 10 201 2.37 3 10 202 1.32 3 10 201 1.48 3 10 201 1.88 3 10 201 7.53 3 10 202 1.79 3 10 201 2.84 3 10 202 1.68 3 10 201 3.40 3 10 206 1.49 3 10 201 9.11 3 10 202 3.81 3 10 203 3.55 3 10 201 ¯ 0.0 1.22 3 10 201 5.00 3 10 201 8.35 3 10 202 1.56 3 10 201 4.08 3 10 201 2.83 3 10 206 4.17 3 10 201 1.18 3 10 201 3.81 3 10 206

shows that before solar cycle 21, the N–S asymmetry for the SAP events at low latitudes is similar to that for the SAP events at high latitudes, but the N–S asymmetry behavior changed starting solar cycle 21. An obvious feature is that both the asymmetries of the SAP events separately at high

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Fig. 1. Top panel: the yearly number of the SAP events at low latitudes in the northern hemisphere (the solid line) and that in the southern hemisphere (the dotted line); Lower panel: the yearly number of the SAP events at high latitudes in the northern hemisphere (the solid line) and that in the southern hemisphere (the dotted line).

and low latitudes are larger before solar cycle 21 than those during and after solar cycle 21. For the SAP events at high latitudes, its asymmetry behaves almost in the same way as the asymmetry of the solar activity at low latitudes before the year 1970, but after 1970 in an obviously different way. And most surprisingly, the high-latitude SAP events have predominant locations in the northern hemisphere from cycles 19 to 22 without a shift. It is both interesting and strange that the asymmetry of the solar activity at high latitudes is not the same as that of the solar activity at low latitudes. To be sure of whether these asymmetry values of the SAP events at low and high latitudes are of statistical significance or not, we calculate the actual probability of generating this N–S distribution. Con-

sidering a distribution of n objects in two classes, we use the following binomial formula to calculate the probability P(k) of getting k objects in class 1, and (n 2 k) objects in class 2 (Vizoso and Ballester, 1990; Li and Gu, 2000; Carbonell et al., 1993): n! 1 p(k) 5 ]]] ]n . (n 2 k)!k! 2 The probability of obtaining more than d objects in class 1 is

O p(k). n

P( $ d) 5

k 5d

Among the annual N–S distributions of the SAP events at low latitudes within 42 years (the years

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Fig. 2. The N–S asymmetry of the yearly numbers of the SAP events at low latitudes (the solid line) and that at high latitudes (the dotted line). Upward arrows indicate the normal solar activity minima.

1957 to 1998), the N–S asymmetry is highly significant (P( $ d) , 5%) in 37 cases, and P( $ d) . 10% in 5 cases. This result means that the N–S asymmetry of the SAP events at low latitudes is a real physical phenomenon and not a random mathematical fluctuation. However, among the 42 years of annual N–S distributions of the SAP events at high latitudes, the N–S asymmetry is significant (P( $ d) , 10%) in 23 cases, but insignificant (P( $ d) . 10%) in 19 cases. So, it is not possible to suggest that the N–S asymmetry of the SAP events

at high latitudes is a real physical phenomenon. Again, the asymmetry of the solar activity at high latitudes shows, perhaps, manifest difference from that of the solar activity at low latitudes. We have counted the total number of the SAP events in each of the solar cycles from 19 to 22 at low and high latitudes, respectively, and in the northern and southern hemispheres. The result is given in Table 2. We then calculated the actual probability of producing such a N–S distribution, which is also given in the table, whereby the

Table 2 Asymmetrical distribution of the SAP in sunspot cycles 19–22 Cycle

19 20 21 22

At low latitudes

At high latitudes

NON

NOS

Probability

Dominance

NON

NOS

Probability

Dominance

15 196 28 277 10 136 34 457

8703 19 195 10 749 38 180

¯ 0.0 ¯ 0.0 9.45 3 10 202 5.98 3 10 206

N N S S

794 1414 1194 1250

362 1043 1104 932

¯ 0.0 ¯ 0.0 3.01 3 10 202 ¯ 0.0

N N N N

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dominant hemisphere in each cycle is obtained and listed in the table. All probability values are less than 10% implying that the dominant hemispheres given in Table 2 are of statistical significance. The table shows that in each of solar cycles 19 and 20, the solar activity at low latitudes and high latitudes are more dominantly distributed in the northern hemisphere. However, in cycles 21 and 22, the dominant hemisphere of the solar activity at high latitudes is surprisingly different from that at low latitudes. The N–S asymmetry of the solar active prominences at high latitudes is north dominated throughout solar cycles 19–22 while the N–S asymmetry of the solar active prominences at low latitudes is shifted to southern hemisphere in solar cycle 21 and remains south dominated even in cycle 22. The N–S asymmetry of the total number of the SAP events at low and high latitudes, respectively, in cycles 19–22 is calculated and then plotted in Fig. 3. We have also plotted in Fig. 3 the N–S asymmetry indices of the

sudden disappearances of prominences (Vizoso and Ballester, 1987) versus solar cycle numbers. Fig. 3 shows that the asymmetry of the SAP events at low latitudes coincides with the asymmetry of the sudden disappearances of solar prominences. And for the solar activity at low latitudes (both the sudden disappearances of solar prominences and the SAP events at low latitudes), there is a shift of the N–S asymmetry indices from the dominance in the northern hemisphere for cycles 18–20 to the dominance in the southern hemisphere for cycles 21 and 22. This fact supports and extends early results (Verma, 1987; Li and Gu, 2000; Li et al., 2002a). The above result indicates that the asymmetry of the solar activity at high latitudes is clearly different from that of the solar activity at low latitudes. It raises several questions. Does this mean that the asymmetry of the activity at high latitudes really differs from that at low latitudes, or is the difference just caused by the fact that the solar activity at high

Fig. 3. Plot of the N–S asymmetry of the SAP events versus solar cycle number.

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latitudes is being assigned the same time interval as the solar activity at low latitudes does? The possibility exists that the asymmetry of the activity at high and low latitudes should be the same in their own cyclical intervals, but due to the assignment of the same time interval for their events, the asymmetries at high and low latitudes differ from each other. In order to clarify this question, we try to count the total number of the SAP events at high latitudes in their own cyclical intervals. Note however that, as shown in Fig. 1, the beginning and end times of a cycle of the SAP events at high latitudes are rather imprecisely defined and thus difficult to quantify. The cycle of the SAP at high latitudes, which is called the cycle of high-latitude activity here, should lead the sunspot cycle and the corresponding cycle of the SAP at low latitudes by 4 years (Li et al., 2002d). Thus we roughly regarded here the minimum of a sunspot cycle minus 4 years as the minimum time of the corresponding cycle of the SAP at high latitudes. Table 3 gives the asymmetrical distribution of the SAP defined in term of the high-latitude SAP cycles. The table also shows that the asymmetry of the solar activity at high latitudes is different from that of the solar activity at low latitudes. Thus, we may rule out the possibility that the asymmetry of the activity at low and high latitudes should be the same in their own cyclical intervals. The asymmetry of the SAP activity at high latitudes is really different from that at lower latitudes. Li et al. (2002a) fitted a straight line to the yearly values of the asymmetry of sunspot groups, for each of cycles 8–23 separately, starting each cycle with the year of the minimum between two consecutive cycles. They found that from cycles 12 to 23, the slope of the straight line changes its sign every four sunspot cycles, which could taken to suggest a

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periodic behavior in the N–S asymmetry. We also fitted a straight line to the yearly values of the asymmetry of the SAP events at low and high latitudes, respectively, for cycles 19–22. The result is shown in Fig. 4. Fig. 4 indicates that the slopes of the four straight lines for the SAP events at high latitudes have the same signs as those for the SAP events at low latitudes. Thus the SAP results are in agreement with the results derived for sunspot groups in Li et al. (2002a). Fig. 5 shows the results when we fit a straight line to the yearly values of the asymmetry of the SAP events at low and high latitudes, respectively, for cycles 20–22 but now according to the cycles defined by high-latitude activity which leads the sunspot cycles by 4 years. The slopes of the straight lines for the SAP events at high latitudes still have the same signs separately as those for the SAP events at low latitudes in each individual high-latitude cycles, but the pattern now violates the regularity of sign-switching every four sunspot cycles (or lowlatitude cycles) as noted in Li et al. (2002a). Table 4 gives the asymmetrical distribution of the SAP events averaged over the whole disk, where, ‘–’ means there is no dominant hemisphere (or we cannot show the asymmetry to be statistically significant). In cycle 21, the total number of the SAP in the southern hemisphere is slightly larger than that in the northern hemisphere, however, the difference between them is insignificant, because the probability for disproving the null-hypothesis (that the two hemispheric distribution is the same) is larger than 10%. Thus, we cannot say that SAP activity for cycle 21 either has a southern dominance or northern dominance (see also Li and Gu, 2000). There is no dominant hemisphere with solar activity equivalent for the two hemispheres in that cycle. We further note that such a case or scenario of no hemispheric

Table 3 Asymmetrical distribution of the SAPs redefined in terms of the cycles of high-latitude activity Cycle

19 20 21 22

At low latitudes

At high latitudes

NON

NOS

Probability

Dominance

NON

NOS

Probability

Dominance

10 592 31 179 12 152 29 942

7227 19 448 13 169 33 797

¯ 0.0 ¯ 0.0 2.34 3 10 22 7.02 3 10 27

N N S S

484 925 1196 1326

238 569 1008 1133

¯ 0.0 ¯ 0.0 3.07 3 10 25 4.92 3 10 25

N N N N

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Fig. 4. The fit of a regression line to the yearly values of the N–S asymmetry of the SAP events respectively at high latitudes (the lower panel) and low latitudes (the upper panel) in each of cycles 19–22.

dominance has been identified for solar cycles 14, 17, and 18 (Li et al., 2002a). As already hinted in Figs. 2 and 3, such a case did occurred in cycle 18 using the activity indicators like the sudden disappearances of solar prominences. Table 4 shows that a shift of the N–S asymmetry to a more decisive dominance in the southern hemisphere did occurred in cycle 22. This fact agrees with the early findings by Verma (1987, 2000) and by Li et al. (2002a).

3. Conclusions The SAPs in the period 1957–1998 are used to

study the N–S asymmetry of the solar activity respectively at low ( # 408) and high ( $ 508) latitudes, and the results obtained are as follows: 1. The annual hemispheric asymmetry does indeed exist at low latitudes, but strangely, a similar asymmetry does not seem to exist at high latitudes. 2. The N–S asymmetry of the solar active prominences at high latitudes is always north dominated during solar cycles 19–22 while the N–S asymmetry of the solar active prominences at low latitudes is shifted to southern hemisphere in solar cycle 21 and remains south dominated even in cycle 22.

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Fig. 5. The fit of a regression line to the yearly values of the N–S asymmetry of the SAP events at high latitudes (the lower panel) and low latitudes (the upper panel), respectively, in from cycles 20 through 22, where the cycles are defined according to the timing of high-latitude SAP events.

3. The N–S asymmetry for the SAP at low latitudes follows the trends of the N–S asymmetry of the sudden disappearances of solar prominences (Vizoso and Ballester, 1987), and shows a cyclical

Table 4 Asymmetrical distribution of the SAP events averaged over the whole disk in cycles 19–22 Cycle

NON

NOS

Probability

Dominance

19 20 21 22

16 752 30 550 11 840 36 489

9483 20 787 12 377 39 916

¯ 0.0 ¯ 0.0 1.41 3 10 201 4.55 3 10 203

N N – S

variation with solar cycles (Li et al., 2002a), but the SAP at high latitudes does not show the cyclical variation. 4. The N–S asymmetry for the SAP at the whole disk shows a cyclic variation, supporting the results given by Verma (1987, 1993, 2000) and Li et al. (2002a). Based on the cyclic variation, a southern dominance is inferred to occur in the present cycle as reported earlier by Verma (1992).

Acknowledgements This work is supported by the 973 project

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(NKBRSF G20000784), the National Science Foundation of China (10073019), the Chinese Academy of Sciences grant for the Most-Promising Research Program in Western China, and the Chinese Academy of Sciences.

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