and 2009). The maximum positive asymmetry happened in the year 1995 with a magnitude (0.625 ± 0.3 nT). In addition, there are 7 negative asymmetries showed in (1975, 1980, 1984, 1991, 1997, 2004 and 2011) with largest negative asymmetry in year 1991 (−0.837 ± 0.5 nT), and in year 2004 (−0.885 ± 0.3 nT) which occurred during the second negative polarity (2001 to the present). Moreover, the largest peaks of the asymmetry appeared around the decline phase of the solar cycle.
Tables 1-4, display the average values of the field magnitude and the geomagnetic parameters for toward and away polarity days, respectively, as well as the differences between the two groups over the epochs of positive and negative IMF polarity years during the four solar cycles (21, 22, 23 and 24). The total number of days north and south of the heliospheric current sheet as calculated from each parameter is listed in the tables.
Figure 1. The annual averages for the interplanetary magnetic field (IMF) and the geomagnetic indices aa, Ap, Kp, AE, and DST) during the 1975-2013 period.
Table 1. The average values of the North-South asymmetry for the field magnitude and the geomagnetic parameters during the +ve & −ve IMF polarity cycle for cycle 21.
Figure 2. Comparison of yearly difference of north-south asymmetries in the field magnitude B, and the geomagnetic parameters (aa, Ap, Kp, AE, and DST) over the time interval from 1975 to 2013.
Table 2. The average values of the North-South asymmetry for the field magnitude and the geomagnetic parameters during the +ve & −ve IMF polarity cycle for cycle 22.
Table 3. The average values of the North-South asymmetry for the field magnitude and the geomagnetic parameters during the −ve IMF polarity cycle for cycle 23.
Table 4. The average values of the North-South asymmetry for the field magnitude and the geomagnetic parameters during the −ve IMF polarity cycle for cycle 24.
From these tables, one can note that the grand average differences of the field magnitude north and south of the HCS are −0.03 ± 0.7 nT, −0.20 ± 0.8 nT, and −0.01 ± 0.6 nT for qA > 0 epochs during the three solar cycles 21, 22, and 23, respectively, as well as −0.16 ± 0.7 nT, 0.28 ± 0.5nT, 0.04 ± 0.7nT, and −0.03 ± 0.5 nT for qA < 0 epochs during cycles 21, 22, 23 and 24, respectively, which means that there is no remarkable N-S asymmetry in the averaged field magnitude over the former epochs and there is no magnetic solar cycle dependence is evident.
On the other side, plot 2c for the geomagnetic index (Ap) (by taking the error bar into account) shows 19 years out of 39 years with N-S asymmetry. Ten of them have a positive asymmetry and occurred in 1976, 1979, 1981, 1983, 1989, 1990, 1993, 1994, 1995 and 2010. In contrast, nine clear negative asymmetries happened in 1978, 1985, 1991, 1999, 2002, 2003, 2004, 2007 and 2008. The largest positive asymmetry placed in 1994 (6.24 ± 1.9) near to the minimum of the solar cycle, and the largest negative asymmetry placed in 2003 (−6.58 ± 1.4) in the descending phase of the solar cycle 23.
Tables 1-4 indicate that the N-S asymmetry for Ap index reaches maximum values through the positive magnetic polarity qA > 0. In contrast, the Ap for the away days south of the current sheet is larger in magnitude than those of toward polarities north of the current sheet during the −ve polarity (2001-2008) years. It is important to remember that during the positive solar magnetic polarity the away sector occurs north of the current sheet and south of it during the positive polarity. So from the four tables, it is clear to note that Ap has a northern dominance during cycles 21 and 24, and southern dominance during cycle 22 and 23. Plots 2b, 2d and 2e have the same behavior for the Ap index with different magnitudes. The asymmetry of the Disturbance Storm-Time (DST) Index during 1975-2013 is displayed in the bottom panel of Figure 2. The north-south asymmetry in DST index is confirmed over the former period. Eight years (1978, 1986, 1988, 1991, 1999, 2002, 2007 and 2008) out of 39 were appeared with negative asymmetry, and 11 years (1976, 1979, 1983, 1990, 1993, 1994, 1995, 2000, 2010, 2011 and 2013) with positive asymmetry. The largest positive asymmetry obtained in 1979 (9.34 ± 2.3), and the largest negative asymmetry obtained in 1991 (−12.71 ± 4.8). By looking at the results on the fourth tables, one can see that the average values of the N-S asymmetry for DST index reach to their maximum value during the first positive magnetic polarity epoch (1976-1978) throughout cycle 21 with a magnitude −3.39 ± 3.4. Furthermore, the significance of the asymmetry changes automatically every solar magnetic polarity cycle (qA < 0 or qA < 0) as shown in the considered tables. So the asymmetry has a southern dominance during cycles 22 and 23 and northern asymmetry during cycles 21 and 24, which means that it has the same behavior for the previous geomagnetic indices. The results shown in plot 2a for the field magnitude B are qualitatively consistent with those observed in plots 2b to 2e, which mean that the asymmetry that observed in the interplanetary parameters reflect another asymmetry in the geomagnetic indices, and the two asymmetries may provide multiple causes for producing the observed asymmetric modulations of cosmic rays.
Series of power spectrum density (PSD) have been performed on the yearly difference variations of the IMF field magnitude (B), and the geomagnetic index (Ap, and DST) between the positive and the negative polarity days (i.e., Toward and Away days). The power spectrum density (PSD) is calculated for a wide frequency range (0.0937 - 0.5 c/year), which corresponds to a range from 2 to 10.7 years.
Figure 3 displays the PSD of the asymmetry for the IMF, Ap, and DST for the period from (1975-2013). The spectrum of the asymmetry for the field magnitude B shows significant peaks at (6.4, 4, 3.3, 2.7 and 2.4 years) as shown in plot 3a. Whereas, plot 3b displays significant peaks at wave-lengths 5.3, 4, and 2.5 years for the asymmetry of the geomagnetic index Ap. Furthermore, the PSD of the DST index shows significant peaks at 8, 5.3, 3.3, and 2.7 years. From the other hand, the PSD of the asymmetry for the geomagnetic index aa, and Kp gives the same peaks that observed on the geomagnetic index Ap, for this reason aa and Kp are missing from the PSD analysis. The plots confirmed similar or identical fluctuations of the 5.3 - 6.4, 4, 3.3 and 2.5 - 27 years between the considered parameters. The 5.3 - 6.4 yr variation found in the considered parameters may be attributed to the different path of ion particles in heliosphere  . The 4 years, may be caused by the dual peak structure of geomagnetic activity or it is caused by a sector boundary of crossings  . A simple explanation for the peak at 8 year that appeared in the spectrum of DST index with lower magnitude is that it may be related to the formation rate and the magnetic structure of achieving regions in the solar southern hemisphere  . Finally it is obvious that the N-S asymmetry is appeared for the considered parameters, and the main periodicity of the asymmetry does not coincide with the 10.7 year solar cycle. But the main periodicity of the asymmetry may be the periodicity at the 5.0, 4.0 and 3.3 years that were founded in the considered parameters.
Through the years, the conflict between researchers about whether the N-S asymmetry is real or the north and south hemisphere has an identical behavior, raised many question marks. In this paper the asymmetry between the solar field north and south of the heliospheric current sheet (HCS) has been examined for the field magnitude B and the geomagnetic indices (Ap, aa, Kp, AE and DST) over the period (1975-2013), by separating the daily data of the field magnitude and the geomagnetic indices in two groups according to the field polarity sense (away or toward). Then, according to studying the N-S asymmetry as yearly difference variations between Toward and Away days for both positive and negative polarity epochs for the considered parameters, the following results were gotten:
1) The field magnitude B has only 15 of 39 years having asymmetry with magnitude greater than the estimated error, and the dependence N-S asymmetry of B upon the IMF solar polarities is statistically insignificant. Finally and according to Tables 1-4, there is no clear indication of the presence of north-south asymmetry in the grand average field magnitude during the solar cycles.
2) Most of the N-S asymmetry for the geomagnetic parameters (aa, Ap, Kp, AE and DST), occurred during the period of positive polarity epochs, near to the minimum solar cycle or around the declining phase. In addition, the asymmetry has a northern dominance during cycles 22 & 23 and southern dominance during cycles 21 & 24.
3) By comparing the north-south asymmetry for the field magnitude and geomagnetic indices, one can conclude that the north-south asymmetry of the interplanetary parameters, together with the north-south asymmetry observed in the geomagnetic indices, may provide multiple causes for producing the observed asymmetric modulations of cosmic rays.
Figure 3. The PSD of the asymmetry for (a) The IMF; (b) the geomagnetic index Ap and (c) the DST index for the period from (1975-2013).
On the other side, the power spectrum density (PSD) of the asymmetry values for the field magnitude and geomagnetic parameters gives a significant periodicity proofed the asymmetry in the considered parameters. However, the 10.7 year solar cycle is not the exit on these parameters, which means that the N-S asymmetry is independent of the solar cycle. These results are identical with   . Furthermore, the main periodicity of the asymmetry may be 5.2, 4.0 and 3.3 years that exist in the parameters with higher confidence levels.
The authors are deeply grateful to National Space Science Data Center (NSSDC)-NASA and the national geophysical data center (NGDC) for the hourly IMF data and for providing the Interplanetary and magnetic activity data online.
Cite this paper
Mohammed AliEl-Borie,Ali Abdel-MoniemAbdel-Halim,Shady YousryEl-Monier, (2016) North-South Asymmetry of the Interplanetary Magnetic Field Magnitude and the Geomagnetic Indices. International Journal of Astronomy and Astrophysics,06,14-22. doi: 10.4236/ijaa.2016.61003