Longitudinal Asymmetry of Solar Activity and the Gnevyshev-Ohl Rule

Abstract

The vector representation of sunspots is used to study the nonaxisymmetric features of the solar activity distribution (sunspot data from Greenwich-USAF/NOAA, 1874-2016.The vector of the longitudinal asymmetry is defined for each Carrington rotation; its modulus characterizes the magnitude of the asymmetry, while its phase points to the active longitude. These characteristics are to a large extent free from the influence of a stochastic component and emphasize the deviations from the axisymmetry. For the sunspot area, the modulus of the vector of the longitudinal asymmetry changes with the 11-year period; however, in contrast to the solar activity, the amplitudes of the asymmetry cycles obey a special scheme. Each pair of cycles from 12 to 23 follows in turn the Gnevyshev-Ohl rule (an even solar cycle is lower than the following odd cycle) or the "anti-Gnevyshev-Ohl rule" (an odd solar cycle is lower than the preceding even cycle). This effect is observed in the longitudinal asymmetry of the whole disk and the southern hemisphere. Possibly, this effect is a manifestation of the 44-year structure in the activity of the Sun. Northern hemisphere follows the Gnevyshev-Ohl rule in Solar Cycles 12-17, while in Cycles 18-23 the anti-rule is observed. Phase of the longitudinal asymmetry vector points to the dominating (active) longitude. Distribution of the phase over the longitude was studied for two periods of the solar cycle, ascent-maximum and descent-minimum, separately. The longitudinal distribution displays maximum at the longitude ∼ 180 • during ascent-maximum and at ∼ 0 • /360 • during descent-minimum. The active longitude changes with the reversals of polarity of the local (minimum of solar activity) and global magnetic fields (reversal of polar magnetic field).