On the time lag between solar wind dynamic parameters and solar activity UV proxies

TL;DR

This study examines the variable time lag between solar activity indicators and solar wind parameters over multiple solar cycles, revealing non-constant lags and periodicities around 10.2 years, and establishes empirical relations linking solar activity to solar wind properties.

Contribution

It provides a detailed analysis of the evolving time lag behavior between solar activity proxies and solar wind parameters over five solar cycles, highlighting non-constant lags and their relation to solar periodicities.

Findings

Lag between Ca II K index and solar wind speed varies from 6 to 1 year.

Periodicities of ~10.2 years found for solar wind parameters.

Empirical relations established linking solar activity to wind parameters.

Abstract

The solar activity displays variability and periodic behaviours over a wide range of timescales, with the presence of a most prominent cycle with a mean length of 11 years. Such variability is transported within the heliosphere by solar wind, radiation and other processes, affecting the properties of the interplanetary medium. The presence of solar activity-related periodicities is well visible in different solar wind and geomagnetic indices, although with time lags with respect to the solar one, leading to hysteresis cycles. Here, we investigate the time lag behaviour between a physical proxy of the solar activity, the Ca II K index, and two solar wind parameters (speed and dynamic pressure), studying how their pairwise relative lags vary over almost five solar cycles. We find that the lag between Ca II K index and solar wind speed is not constant over the whole time interval investigated, with values ranging from 6 years to 1 year (average 3.2 years). A similar behaviour is found also for the solar wind dynamic pressure. Then, by using a Lomb-Scargle periodogram analysis we obtain a 10.21-year mean periodicity for the speed and 10.30-year for the dynamic pressure. We speculate that the different periodicities of the solar wind parameters with respect to the solar 11-year cycle may be related to the overall observed temporal evolution of the time lags. Finally, by accounting for them, we obtain empirical relations that link the amplitude of the Ca II K index to the two solar wind parameters.