Harmonic Structure of Heliospheric Magnetic Field Polarity: Evidence for a Non-sinusoidal Hale Cycle

We investigate the temporal structure of heliospheric magnetic field (HMF) polarity using a monthly polarity index derived from IMF observations over 1963-2026. Spectral and time–frequency analyses, together with autocorrelation, consistently reveal a non-stationary signal dominated by the 22-year Hale cycle, accompanied by intermittent intermediate periodicities near 7.5 years and 3 years. We show that these timescales can be interpreted as arising from the harmonic structure of a non-sinusoidal Hale-cycle waveform. Using a synthetic quasi-square signal with Hale-cycle periodicity, we demonstrate that higher-order odd harmonics, particularly the third harmonic (about 7.45 years), naturally emerge from rapid polarity reversals. The variability of these harmonic components is consistent with observed cycle-to-cycle differences in polarity reversal structure. These results suggest that the heliospheric magnetic polarity exhibits a coherent but non-sinusoidal temporal structure.