Long-Term Variations in Solar Wind Parameters, Magnetopause Location, and Geomagnetic Activity Over the Last Five Solar Cycles
Abstract
We use both solar wind observations and empirical magnetopause models to reconstruct time series of the magnetopause standoff distance for nearly five solar cycles. Since the average annual interplanetary magnetic field (IMF) B-z is about zero, and the annual IMF cone angle varies between 54.0 degrees and 61.2 degrees, the magnetopause standoff distance on this timescale depends mostly on the solar wind dynamic pressure.
The annual IMF magnitude well correlates with the sunspot number (SSN) with a zero time lag, while the annual solar wind dynamic pressure (Pdyn) correlates reasonably well with the SSN but with 3-year time lag. At the same time, we find an anticorrelation between Pdyn and SSN in cycles 20-21 and a correlation in cycles 22-24 with 2-year time lag.
Both the annual solar wind density and velocity well correlate with the dynamic pressure, but the correlation coefficient is higher for density than for velocity. The 11-year solar cycles in the dynamic pressure variations are superimposed by an increasing trend before 1991 and a decreasing trend between 1991 and 2009.
The average annual solar wind dynamic pressure decreases by a factor of 3 from 1991 to 2009. Correspondingly, the predicted standoff distance in Lin et al.'s (2010, https://doi.org/10.1029/2009JA014235) magnetopause model increases from 9.7 R-E in 1991 to 11.6 R-E in 2009.
The annual SSN, IMF magnitude, and magnetospheric geomagnetic activity indices display the same trends as the dynamic pressure. We calculate extreme solar wind parameters and magnetopause standoff distance in each year using daily values and find that both extremely small and large standoff distances during a solar cycle preferably occur at solar maximum rather than at solar minimum.