Research Article

Solar cycle evolution of ULF wave power in solar wind and on ground

¹ Department of Electronics and Nanoengineering, Aalto University of Technology, 00076 Aalto, Finland
² Sodankylä Geophysical Observatory, University of Oulu, 99600 Sodankylä, Finland
³ Dipartimento di Scienze Fisiche e Chimiche, Università dell’Aquila, Via Vetoio, Coppito, 67100 L’Aquila, Italy

^* Corresponding author: reko.hynonen@aalto.fi

Received: 8 April 2019
Accepted: 17 August 2020

Abstract

The solar cycle evolution of the ultra-low frequency (ULF) power was studied in solar wind and on ground. We aim finding out how the ULF power in interplanetary and on ground magnetic field evolves over the solar cycle 23 (SC23) and how well do they follow each other in monthly time scales. The hourly power of the ULF waves was computed in the Pc5 frequency range 2–7 mHz for years 1998–2008. The highest wave power in SC23 is found to occur in late 2003 and the lowest at the solar minimum. Ground ULF power follows the IMF power and solar wind speed, particularly well during declining phase. The ULF power in winter exceeds the ULF power in other seasons during the declining phase of SC23, while equinoxes dominate in the ascending phase and the solar maximum. The ground ULF power was found to rise with magnetic latitude from 54° to 73°, after which Pc5 power decreases towards the polar cap. The Pc5 power in the auroral zone is larger in the nightside than the dayside due to substorm activity implying that magnetotail processes are an important contributor to the nightside ULF power.