Annual Variations in the Near-Earth Solar Wind

Abstract

Earth’s orbit and rotation introduces systematic annual variations in geomagnetic activity, most notably via the changing orientation of the dayside magnetospheric magnetic field with respect to the heliospheric magnetic field (HMF). However, aside from these geometric effects, it is generally assumed that the solar wind is randomly sampled throughout the year. But systematic changes in the intrinsic solar wind conditions in near-Earth space could arise due to the variation in Earth heliocentric distance and heliographic latitude over the year. In this study we use 24 years of Advanced Composition Explorer (ACE) data to investigate the annual variations in the scalar properties of the solar wind, namely the solar wind proton density, the radial solar wind speed and the HMF intensity. All parameters do show some degree of systematic annual variation, with amplitudes of around 10 to 20%. For HMF intensity, the variation is in phase with the Earth’s heliocentric distance variation, and scaling observations for distance largely removes the variation. For proton density and solar wind speed, however, scaling for distance does not affect the variation and the annual phase is inconsistent with Earth’s heliocentric distance variation. Instead we attribute the annual variations to Earth’s heliographic latitude variation and systematic sampling of higher speed solar wind at higher latitude. These variations are most strongly ordered at solar minimum. Conversely, combining scalar solar wind parameters to produce dynamic pressure and potential power input to the magnetosphere estimates results in solar maximum exhibiting a greater annual variation, with an amplitude of around 40%. This suggests Earth’s position in the heliosphere makes a significant contribution to space weather, in addition to the well studied geometric effects.