Experimental observations of the correspondence of Alfvénic and compressional waves in the Solar Wind high-speed streams with the geomagnetic activity at high latitudes in the Ultra Low-Frequency range

The solar wind(SW) speed and the interplanetary magnetic field(IMF) exhibit wide Alfvénic fluctuations, particularly within high-speed streams(HSS), which may influence geomagnetic activity. Alfvén waves are ubiquitous in the SW, with a high occurrence in HSS emanating from coronal holes.These waves play a crucial role in the stability, heating, and transport of magnetized plasmas, making the investigation of their effects on the geomagnetic field highly significant.This study aims to investigate the effects of Alfvénic and compressive fluctuations within SW corotating streams at 1 AU on geomagnetic activity in the frequency range 1-7 mHz,as observed at high-latitude geomagnetic observatories in both hemispheres.Several corotating streams were analyzed, together with the corresponding geomagnetic field data recorded at ground.For each observatory, a long-term Composite Quiet Background(CQB) of the geomagnetic power has been estimated in correspondence with quiet geomagnetic periods; then, the geomagnetic power of each event was scaled to the CQB to better highlight the geomagnetic variations during the selected events.The IMF and SW velocity components at 1AU have been rotated into the Mean ElectroMagnetic Field Aligned(MEMFA) reference frame to identify fluctuation along two main directions: one aligned and one orthogonal to the ambient IMF. For this purpose, the first part of the work was devoted to testing the rotation procedure from the Heliocentric Earth Ecliptic to the MEMFA reference frame; the main field-aligned direction has been identified by applying the Empirical Mode Decomposition method to SW velocity and IMF components at 1 AU.The method's reliability and its limitations have been checked in identifying Alfvénic fluctuations through the spectral analysis of time series in the MEMFA reference frame.With this procedure, the fluctuations in the directions aligned and orthogonal to the main field are studied.I simulated the magnetic and velocity fields of a typical corotating stream to highlight the peculiarities of each case study and be able to better identify Alfvén waves when applying this procedure to real data.The procedure has been tested in several cases, by adding the presence of Alfvén waves and noise.Then the spectral analysis of the MEMFA components of both magnetic and velocity fields has been performed to define the power related to the components aligned and orthogonal to the ambient IMF.The efficiency of the procedure and the result’s reliability are supported by MC tests.The method has been also applied to a real case represented by a selected corotating SW stream.The spectral power in the two directions provides information on the Alfvénicity of SW fluctuations which were found very similar to those obtained using the Elsässer variables through two quadratic invariants used to describe MHD turbulence: the normalized cross-helicity and the normalized residual energy, which indicate the level of correlation and energy balance between SW velocity and magnetic field fluctuations, respectively.These combined methods help distinguish between compressive and Alfvénic fluctuations, which can have a different impact on low-frequency geomagnetic variations.Finally, a joint analysis was conducted between the SW data in the MEMFA reference frame and the ground-based geomagnetic data, scaled to the CQB.The joint analysis was performed through correlation analysis, joint distribution analysis, and cross-coherence between fluctuations power in the corotating streams and ground-based power.The results show that ground-based power at the different latitudes is better correlated with the SW power in the orthogonal direction with respect to the aligned direction.Additionally, an increase in ground-based power is observed corresponding to high values of high values of normalized cross-helicity, and various coherence peaks are identified between SW and IMF fluctuations power and geomagnetic power.