On the Relationship Between Solar Wind High-Speed Stream Waves and Geomagnetic Pc5 Activity From Mid to High Latitudes: A Case Study on January 5-12 2008

Abstract

This study aims to investigate the possible influence of Alfvénic and compressive low-frequency fluctuations in a solar wind corotating high-speed stream on the geomagnetic activity from mid to high latitudes. We selected a stream in the declining phase of solar cycle 23, impinging on Earth's magnetosphere on 5–12 January 2008. To study solar wind fluctuations, we rotated velocity and magnetic field components into a reference frame aligned with the main magnetic field. This approach allowed us to define the fluctuation power along the direction aligned with the main magnetic field, associated with purely compressive fluctuations, and the power within the plane orthogonal to the main field, containing Alfvénic fluctuations. The Alfvénicity has been studied through the normalized cross helicity and residual energy, which describe the degree of correlation between kinetic and magnetic fluctuations and their energy balance. We used ground data from seven geomagnetic observatories from mid to high latitudes, removing well-known Sun-Earth-related periodicities during quiet periods. We analyzed the dependence of the average geomagnetic power on the down-dusk interplanetary electric field component $E_y$, by distinguishing the more and less Alfvénic populations. For the more Alfvénic population, we observe an increase in average geomagnetic power for positive $E_y$ values, corresponding to open magnetosphere conditions, particularly at auroral and sub-auroral latitudes. Additionally, we find that geomagnetic power fluctuations are coherent with those of the interplanetary medium; the coherence reaches higher values with a greater persistence for the orthogonal direction, associated with Alfven waves, with respect to the parallel one.