A study of the geomagnetic indices asymmetry based on the interplanetary magnetic field polarities

Abstract

Data of geomagnetic indices (aa, Kp, Ap, and Dst) recorded near 1 AU over the period 1967–2016, have been studied based on the asymmetry between the interplanetary magnetic field (IMF) directions above and below of the heliospheric current sheet (HCS). Our results led to the following conclusions: (i) Throughout the considered period, 31 random years (62%) showed apparent asymmetries between Toward ($\mathbf{T}$) and Away ($\mathbf{A}$) polarity days and 19 years (38%) exhibited nearly a symmetrical behavior. The days of $\mathbf{A}$ polarity predominated over the $\mathbf{T}$ polarity days by 4.3% during the positive magnetic polarity epoch (1991–1999). While the days of $\mathbf{T}$ polarity exceeded the days of $\mathbf{A}$ polarity by 5.8% during the negative magnetic polarity epoch (2001–2012). (ii) Considerable yearly North–South (N–S) asymmetries of geomagnetic indices observed throughout the considered period. (iii) The largest toward dominant peaks for $aa$ and $Ap$ indices occurred in 1995 near to minimum of solar activity. Moreover, the most substantial away dominant peaks for $aa$ and $Ap$ indices occurred in 2003 (during the descending phase of the solar cycle 23) and in 1991 (near the maximum of solar activity cycle) respectively. (iv) The N–S asymmetry of $Kp$ index indicated a most significant away dominant peak occurred in 2003. (v) Four of the away dominant peaks of Dst index occurred at the maxima of solar activity in the years 1980, 1990, 2000, and 2013. The largest toward dominant peak occurred in 1991 (at the reversal of IMF polarity). (vi) The geomagnetic indices (aa, Ap, and $Kp$) all have northern dominance during positive magnetic polarity epoch (1971–1979), while the asymmetries shifts to the southern solar hemisphere during negative magnetic polarity epoch (2001–2012).