Tracing the Solar Wind Cycle at 1 AU: Variability in the Delayed Response to Solar Activity

Abstract The continuous flux of charged particles from the Sun, i.e., the solar wind, influences both planetary and circumplanetary environments. Although the precise origin of each type is still debated, the solar wind originates primarily from the expansion of the solar corona and is driven by the solar magnetic field. The cyclic 11-year variations observable in several solar activity proxies can also be traced in the average properties of the solar wind, though the relationship in terms of amplitude and phase synchronization with solar activity is not uniform. Focusing on the period 1965 – 2024, we investigate how the relationship between a chromospheric proxy, the Ca ii K index, and 1AU solar wind properties, such as speed, temperature, and dynamic pressure, has evolved over the last five solar cycles. On the one hand, variations in their relationship are found in terms of time lag, correlation coefficient, and amplitude (i.e., fit slope) in a cycle-based analysis. In particular, we find evidence consistent with a linear relationship between the time lag (in years) and the slope of the fit characterizing the dependence of solar wind properties on the intensity of the solar magnetic cycle. We also examine these variations in light of the contribution of the different solar wind flow types along individual solar cycles. On the other hand, continuous cross-correlation reveals distinct dynamical regimes in solar wind–Ca ii K lag, with stable behavior at 2 – 4 years and instability emerging at both shorter and longer lag intervals, suggesting a nonlinear bifurcation mechanism. Finally, the cycle-to-cycle variations reported can help in understanding the space climate connection between solar activity and near-Earth solar wind properties, additionally providing insight into the contribution of each solar wind flow type.