ULF waves observed during substorms in the solar wind and on the ground

Abstract

We present results of analysis of ultra-low-frequency (ULF) waves observed by the Advanced Composite Explorer (ACE) satellite at the Earth's L1 Lagrangian point (in the solar wind) and the fluxgate magnetometer at the Poker Flat ground station in Alaska, USA. We considered 75 high-intensity substorm events occurring between October 2015 and April 2017. Our goal is to investigate possible correlation between spectral characteristics of the waves observed in the solar wind and on the ground. Our findings show that: 1) ULF waves with the same frequencies are frequently observed at both locations, in particular, about 30% of the cases yield very good correlation between power spectral density of corresponding signals; 2) the waves with frequencies of 0.60 mHz, 0.70–0.75 mHz are most often seen at both locations; 3) the trend of repeating dominant frequencies is consistent throughout the whole period of observation of about a year and a half. There are also some cases of either mismatch of expectation and occurrence of substorm or a poor correlation between the frequencies of ULF waves detected in space and on the ground. Our findings suggest that the variations of plasma density or the magnetic field in the solar wind indeed serves as a driver for the ULF waves in the Earth's magnetosphere-ionosphere system. This system works like a resonator, and when the frequency of the driver matches one of the eigenfrequencies of the resonator, the large amplitude waves are observed on the ground. When the frequency of the driver mismatch the eigenfrequency, the amplitude of the ground oscillations are much smaller.