Signature of Alfvénic and compressive waves in corotating Solar Wind high-speed streams on low-frequency geomagnetic activity at high latitudes

In the Solar Wind (SW), when a fast stream overtakes a slower one, it forms a corotating high-speed stream (HSS) with an upstream corotating interaction region (CIR) and a downstream rarefaction region (RR). The SW speed and the interplanetary magnetic field (IMF) exhibit wide fluctuations within CIRs and HSSs, that may have effects on the geomagnetic activity. Our study aims to investigate the effects of Alfvénic and compressive low-frequency fluctuations within SW corotating streams at 1 AU on geomagnetic activity in the low-frequency range (Pc5, 1-7 mHz), as observed at high-latitude geomagnetic observatories in both hemispheres. We analyzed several corotating streams, which were analyzed together with the corresponding geomagnetic field data recorded at ground observatories at high latitudes. For each observatory, we estimated a long-term geomagnetic power background in correspondence with quiet geomagnetic activity periods (low Kp index); then, we re-scaled the power to the background to better highlight the geomagnetic variations during the selected events. For SW data, we rotated the IMF and SW velocity components at 1AU into the Mean ElectroMagnetic Field Aligned (MEMFA) reference frame to identify fluctuation along two main directions: one aligned and one orthogonal to the ambient magnetic field. We compared the re-scaled geomagnetic field power with SW parameters, such as the power of IMF and SW velocity along the two directions, as well as with two quadratic invariants used to describe MHD turbulence: the normalized cross-helicity and the normalized residual energy. This combined method helps distinguish between compressive and Alfvénic fluctuations, providing insights into their impact on low-frequency geomagnetic variations.