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

• Published in: Journal of geophysical research. Space physics Vol. 130; no. 4
• Main Authors: Carnevale, G., Regi, M., Lepidi, S., Francia, P.
• Format: Journal Article
• Language: English
• Published: 01.04.2025
• Subjects: Alfvéni waves; geomagnetism; Pc5; solar wind
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2024JA033645

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 Ey ${E}_{y}$, by distinguishing the more and less Alfvénic populations. For the more Alfvénic population, we observe an increase in average geomagnetic power for positive Ey ${E}_{y}$ 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. Plain Language Summary This study explores how the fast solar wind influences Earth's magnetic field, especially at high latitudes. The solar wind carries different types of waves. These include compressive waves, which move in any direction, and Alfvén waves, which are guided by the main magnetic field direction. By selecting an event observed in January 2008, we examined how these waves interact with Earth's magnetic field in a specific low‐frequency range. Using data both from space and Earth‐based magnetic observatories, we found that Alfvén waves are linked to greater geomagnetic activity at auroral and sub‐auroral latitudes when the solar wind creates conditions favorable for energy transfer into Earth's magnetosphere. Our results also show that these Alfvén waves may be linked to more persistent energy transfer in the Earth's magnetic field. These findings improve our understanding of how the solar wind influences space weather and its effects on the planet. Key Points Solar wind fluctuations in corotating high‐speed streams affect geomagnetic activity from mid to high latitudes Geomagnetic Pc5 power increases in open magnetosphere conditions at auroral and sub‐auroral latitudes, mainly for the Alfvénic solar wind Geomagnetic power fluctuations are persistently coherent with Alfvénic fluctuations present in the interplanetary medium