On the distribution and asymmetry of solar active prominences

The paper presents the results of a study of the distribution and asymmetry of solar active prominences (SAP) for the period 1957–1998 (solar cycles 19–23). The east-west (E-W) distribution study shows that the frequency of SAP events in the 81–90° slice (in longitude) near the east and west limbs is up to 10 times greater than in the 1–10° slice near the central meridian of the Sun. The north-south (N-S) latitudinal distribution shows that the SAP events are most prolific in the 11–20° slice in the northern and southern hemispheres. Further, the E-W asymmetry of SAP events is not significant. The N-S asymmetry of SAP events is significant and it has no relation with the solar maximum year or solar minimum year during solar cycles. Further, the present study also shows that the N-S asymmetry for cycles 19–23 follows and confirms the trend of N-S asymmetry cycles as reported by Verma (1992).     

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

The paper presents the results of a study of the asymmetry of the solar active prominences (SAP) at low (≤40°) and high (≥50°) 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.     

North-South asymmetry in the solar flare index

This paper reports the results of a study of the N-S asymmetry in the flare index using the results of Knoka (1985) combined with our results for the solar cycles 17 to the current cycle 22. By comparing the time-variation of the asymmetry curve with the solar activity variation of the 11-year cycle, we have found that the flare index asymmetry curve is not in phase with the solar cycle and that the asymmetry peaks during solar minimum. A periodic behaviour in the N-S asymmetry appears: the activity in one hemisphere is more important during the ascending part of the cycle whereas during the descending part the activity becomes more important in the other hemisphere. The dominance of flare activity in the southern hemisphere continues during cycle 22 and, according to our findings, this dominance will increase gradually during the following cycle 23.     

North-South asymmetry in sudden disappearances of solar prominences

This paper presents the results of a study of the N-S asymmetry in sudden disappearances (SD) of solar prominences during solar cycles 18–21, obtained as a part of a more extensive research on SD and reappearances during years 1931–1985 (Ballester, 1984). As can be seen, the N-S SD asymmetry curve is not in phase with the solar cycle and peaks about the time of solar minimum, the asymmetry reverses in sign during the solar maximum, being, this change of sign, coincident with the reversal of the Sun's magnetic dipole. The SD asymmetry curve can be fitted by a sinusoidal function with a period of eleven years. On the other hand, the SD asymmetry curve shows a strong coincidence with the N-S asymmetries presented by other solar activity manifestations as studied by different authors.     

Asymmetric variations of the coronal green line intensity

The analysis of the daily measurements of the coronal green line intensity, which have been extensively tested for homogeneity and freedom of trends observed at the Pic-du-Midi observatory during the period 1944–1974, has revealed some characteristic asymmetric variations. A north-south asymmetry of the green line intensity is the main feature of the period 1949–1971 while a south-north one is obvious within 1972–1974 and the minor statistical significance span 1944–1948. On the other hand a significant W-E asymmetry has been confirmed in the whole period 1944–1974. It is noteworthy that the period 1949–1971, where the N-S asymmetry takes place consists a 22-yr solar cycle which starts from the epoch of the solar magnetic field inversion of the solar cycle No. 18 and terminates in the relevant epoch of the cycle No. 20.The combination of N-S and S-N asymmetry with a W-E one makes the NW solar-quarter to appear as the most active of all in the 22-yr cycle 1949–1971, while in the periods 1944–1948 and 1972–1974 the SW quarter is the most active. Finally, from the polar distribution of the green line intensity has been derived that the maximum values of the asymmetries occur in heliocentric sectors ± 10°–20° far from the solar equator on both sides of the central meridian.Physical mechanisms which could contribute to the creation of both N-S and E-W asymmetries of the solar activity and the green line intensity as an accompanied event, like different starting time of an 11-yr solar cycle in the two solar hemispheres, the motion of the Sun towards the Apex, and short-lived active solar longitudes formed by temporal clustering of solar active centers, have been discussed.     

太阳活动周的3个参量：AS,BT和BW

通过对１２－２２周（（１８７８－１９９５年）太阳大黑子群分布南北半球不对称的整体特征的研究,探讨了太阳活动周的长期演化趋势．约定Ｎ与Ｓ分别表示北南半球大黑子群数之和,ＢＮ与ＢＳ为北南半球大黑子群的纬度和．由这４个物理量定义了太阳活动周的３个参量：（１）太阳活动不对称指数ＡＳ＝（Ｎ－Ｓ）／（Ｎ＋Ｓ）;（２）平均纬度ＢＴ＝（ＢＮ＋ＢＳ）／（Ｎ＋Ｓ）,ＢＳ取负值;（３）太阳活动带的宽度ＢＷ＝ＢＮ／Ｎ－ＢＳ／Ｓ．对上述１１个活动周,得到了有关８０年周期的性质及奇偶数周大黑子群数变化的有意义的统计结果．     

North–south asymmetry of different solar activity features during solar cycle 23

A study on north–south (N–S) asymmetry of different solar activity features (DSAF) such as solar proton events, solar active prominences [total, low (?40°) and high (?50°) latitudes], Hα flare indices, soft X-ray flares, monthly mean sunspot areas and monthly mean sunspot numbers carried out from May 1996 to October 2008. Study shows a southern dominance of DSAF during this period. During the rising phase of the cycle 23 the number of DSAF approximately equals on both, the northern and the southern hemispheres. But these activities tend to shift from northern to southern hemisphere during the period 1998–1999. The statistical significance of the asymmetry time series using a χ²-test of goodness of fit indicates that in most of the cases the asymmetry is highly significant, meaning thereby that the asymmetry is a real feature in the N–S distribution of DSAF.     

North-south asymmetry in solar, interplanetary, and geomagnetic indices

Data of hourly interplanetary plasma (field magnitude, solar wind speed, and ion density), solar (sunspot number, solar radio flux), and geomagnetic indices (Kp, Ap) over the period 1970-2010, have been used to examine the asymmetry between the solar field north and south of the heliospheric current sheet (HCS). A persistent yearly north-south asymmetry of the field magnitude is clear over the considered period, and there is no magnetic solar cycle dependence. There is a weak N-S asymmetry in the averaged solar wind speed, exhibited well at times of maximum solar activities. The solar plasma is more dense north of the current sheet than south of it during the second negative solar polarity epoch (qA < 0). Moreover, the N - S asymmetry in solar activity (Rz) can be statistically highly significant. The sign of the average N - S asymmetry depends upon the solar magnetic polarity. The annual magnitudes of N - S asymmetry depend positively on the solar magnetic cycle. Most of the solar radio flux asymmetries occurred during the period of positive IMF polarity.     

Evidence for solar-cycle evolution of north-south flare asymmetry during cycles 20 and 21

The record of flare incidence from January 1969 to October 1988 indicates that the north-south (N-S) distribution of large flares is periodic and approximately in phase with the 11-year sunspot cycle. These data are based on observations of the whole-disk Sun in continuum soft X-rays which commenced in early 1969 and have proceeded without interruption to the present time. The pattern of occurrence, observed for slightly less than two sunspot cycles, is that large flares concentrate in north heliographic latitudes soon after solar minimum and then migrate gradually southward as the cycle progresses. By the end of the cycle, most large flares occur in the south. The degree of N-S asymmetry apparently is a function of the intensity of the flare; the most intense flares show the largest amount of N-S asymmetry. The data suggest that sunspots and flares may be driven by distinctly different excitation mechanisms arising at different levels in the convection zone. This conjecture is supported by recent work of Bai (1987, 1988), who has discovered that the superactive regions producing the majority of flares rotate at a speed substantially different from the Carrington rate, which is based primarily on the observed motion of sunspots.     

22-Year periodicity in the solar differential rotation

Using the data on sunspot groups compiled during 1879&amp;#x2013;1975, we determined variations in the differential rotation coefficientsA andB during the solar cycle. The variation in the equatorial rotation rateA is found to be significant only in the odd numbered cycles, with an amplitude &amp;#x223C; 0.01 &amp;#x03BC; rads^-1. There exists a good anticorrelation between the variations of the differential rotation rateB derived from the odd and even numbered cycles, suggesting existence of a &amp;#x2018;22-year&amp;#x2019; periodicity inB. The amplitude of the variation ofB is &amp;#x223C; 0.05 &amp;#x03BC; rad s^-1.     

The distribution of sunspots over the Sun

The distribution of the sunspots for the period 1967–1987 (solar cycles 20 and 21) is presented here. We find that the ±11–20° latitude belt is most prolific for the occurrence of various spot types irrespective of magnetic-field ranges. Furthermore, longitudinally sunspots occur most prolifically at six or more places on the Sun. Spatially 7–9 zones are present in each hemisphere (north or south) of the Sun where about 50% sunspots occur and occupy only about 4% area of the Sun. During the above cycles at least 5 flare zones were regularly present in each hemisphere. The existing models cannot explain these active zones on the Sun. Thus, the present analysis emphasizes the need for a new magnetic models of the Sun.     

The north–south asymmetry of filaments in solar cycles 16–21

In the present study, the north–south asymmetry of filaments in solar cycles 16–21 is investigated with the use of the solar filaments observed at the Observatoire de Paris, Section de Meudon from March 1919 to December 1989. Filament activity is found regularly dominated in each of cycles 16–21 in the same hemisphere as that inferred by sunspot activity, and it is found to run in a different asymmetrical behavior at different latitudinal bands, suggesting that the north–south asymmetry of filament activity should be a function of latitudes. The regularity on the north–south asymmetry of sunspot activity given by Li et al. (2002b) is demonstrated by filament activity. The periods in the north–south asymmetry of solar filament activity are 9.13, and 12.8 years without the solar cycle found.     

Periodicities in the north-south asymmetry of solar activity

This paper tries to cast additional evidence on the proposed periodic behaviour of the N-S asymmetry in sudden disappearances (SD) of solar prominences (Vizoso and Ballester, 1987). We have performed a Blackman-Tukey power spectrum of the values of the SD N-S asymmetry and the results shows a significant peak, above 95% confidence level, at 12.4 years, another peak at 2.3 years fails to be statistically significant. Moreover, power spectrum performed with the values of N-S asymmetry of flare number and flare index (Vizoso and Ballester (1987) display significant peaks, above 95% confidence level, around 3.1–3.2 years.     

The North—South Asymmetry of Soft X-Ray Flare Index During Solar Cycles 21, 22 and 23

In this paper the N—S asymmetry of the soft X-ray flare index (FI _SXR) during solar cycles 21, 22 and 23 has been analyzed. The results show the existence of a real N—S asymmetry which is strengthened during solar minimum. The slope of regression lines fitted to the daily values of asymmetry time series is negative in all three cycles. The yearly asymmetry curve can be fitted by a sinusoidal function with a period of eleven years. The power spectral analysis of daily asymmetry time series reveals significant periods of around 28.26 days, 550.73 days and 3.72 years.     

Confirmation of the 25.5-day fundamental period of the Sun using the north-south asymmetry of the flare index

Short-term periodicities of solar activity were studied. To perform the study, a north-south asymmetry time series was constructed by using the northern and the southern hemisphere flare index values for solar cycle 22. The statistical significance of this time series was calculated. It indicates that in most of cases the asymmetry is highly significant during cycle 22. Power spectral analysis of this time series reveals a periodicity around 25.5 days, which was announced before as a fundamental period of solar activity (Bai and Sturrock, 1991). To investigate the time agreement between the two hemispheres, the phase distribution was studied and a phase shift of about 0.5 was found. An activity trend from the north to the south was found.     

Does solar flare activity lag behind sunspot activity?

Recently, Wheatland and Litvinenko (2001) have suggested that over the solar cycle both the flaring rate and the magnetic free energy in the corona lag behind the energy supply to the system. To test this model result, we analyzed the evolution of solar flare occurrence with regard to sunspot numbers (as well as sunspot areas), using H flare data available for the period 1955–2002, and soft X-ray flare data (GOES 1–8 Å) for the period 1976–2002. For solar cycles 19, 21, and 23, we find a characteristic time lag between flare activity and sunspot activity in the range 1015 months, consistent with the model predictions by Wheatland and Litvinenko (2001). The phenomenon turns out to be more prominent for highly energetic flares. The investigation of solar activity separately for the northern and southern hemisphere allows us to exclude any bias due to overlapping effects from the activity of both hemispheres and confirms the dynamic relevance of the delay phenomenon. Yet, no characteristic time lag >0 is found for solar cycles 20 and 22. The finding that in odd-numbered cycles flare activity is statistically delayed with respect to sunspot activity, while in even-numbered cycles it is not, suggests a connection to the 22-year magnetic cycle of the Sun. Further insight into the connection to the 22-year magnetic cycle could possibly be gained when a 22-year variation in the energy supply rate is taken into account in the Wheatland and Litvinenko (2001) model. The existence of a 22-year modulation in the energy supply rate is suggested by the empirical Gnevyshev – Ohl rule, and might be caused by a relic solar field.     

Hemispheric asymmetry of sunspot area in solar cycle 23 and rising phase of solar cycle 24: Comparison of three data sets

Analysis of long-term solar data from different observatories is required to compare and confirm the various level of solar activity in depth. In this paper, we study the north–south asymmetry of monthly mean sunspot area distribution during the cycle-23 and rising phase of cycle-24 using the data from Kodaikanal Observatory (KO), Michelson Doppler Imager (MDI) and Solar Optical Observing Network (SOON). Our analysis confirmed the double peak behavior of solar cycle-23 and the dominance of southern hemisphere in all the sunspot area data obtained from three different resources. The analysis also showed that there is a 5–6 months time delay in the activity levels of two hemispheres. Furthermore, the wavelet analysis carried on the same data sets showed several known periodicities (e.g., 170–180 days, 2.1 year) in the north–south difference of sunspot area data. The temporal occurrence of these periods is also the same in all the three data sets. These results could help in understanding the underlying mechanism of north–south asymmetry of solar activity.     

Long-term cyclic variations of prominences, green and red coronae over solar cycles

Long-term cyclic variations in the distribution of prominences and intensities of green (530.3 nm) and red (637.4 nm) coronal emission lines over solar cycles 18&amp;#x2013;23 are presented. Polar prominence branches will reach the poles at different epochs in cycle 23: the north branch at the beginning in 2002 and the south branch a year later (2003), respectively. The local maxima of intensities in the green line show both poleward- and equatorward-migrating branches. The poleward branches will reach the poles around cycle maxima like prominences, while the equatorward branches show a duration of 18 years and will end in cycle minima (2007). The red corona shows mostly equatorward branches. The possibility that these branches begin to develop at high latitudes in the preceding cycles cannot be excluded.