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.
Highlights
Magnetospheric ultra-low frequency (ULF) waves are slow fluctuations of magnetic field within the Earth’s magnetospheric cavity, detected on the ground as geomagnetic pulsations
We have computed ULF wave power in solar wind based on the measurements of interplanetary magnetic field (IMF), and on ground based on the magnetic field horizontal component (BH)
We have no observations between L = 7 (68°) and L = 9 (71°), we found that the ULF power in the night sector (21–03 MLT) shows a trend consistent with Rae et al (2012) results, with the maximum in correspondence to the nightside auroral oval, i.e. at 68° or between 68° and 71°); such power is likely due to processes related with substorms occurring in the magnetotail
Summary
Magnetospheric ultra-low frequency (ULF) waves are slow fluctuations of magnetic field within the Earth’s magnetospheric cavity, detected on the ground as geomagnetic pulsations. Solar wind speed has been shown to have the largest effect to the ground ULF power, the second most important factor being Bz < 0, and the third most one being perturbations of the solar wind density (Bentley et al, 2018). Bentley et al (2018) studied ground ULF power with solar wind quantities considering their interdependencies, finding solar wind speed the most important factor, density fluctuations and southward IMF. They could not rule out the effect of IMF fluctuations completely. We conclude this paper by examining the latitudinal variability of the Pc5 power over SC23
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