Spectral content of fluctuations in the geomagnetic field during the course of March 21–23, 2017 geospace storms

Abstract

Relevance. Magnetic storms have been studied for a long time, with particular attention being paid to unique storms, which occur only 1 or 3 times per solar cycle. The occurrence rates of moderate storms are much higher. The magnetic storms have been determined to exhibit high variability, with each storm having a particular history. Therefore, the study of any magnetic storm in detail is of interest.

The aim of this work is to present the spectral analysis of the geomagnetic field fluctuations observed to occur during the geospace storm of March 21–23, 2017 and on neighboring days.

Methods and Methodology. The database from the fluxmeter magnetometer at the V. N. Karazin Kharkiv National University Magnetometer Observatory (49°38' N, 36°56' E geographic coordinates) has been used to analyze the fluctuations in the horizontal components of the geomagnetic field in the 1–1,000-s period range. The temporal variations in the horizontal components are subjected to the systems spectral analysis that includes mutually complementary the wavelet transform, the short-time Fourier transform, and the Fourier transform in a sliding window with a width adjusted to be equal to a fixed number of harmonic periods. The Fourier transform in a sliding window provides the highest resolution for periods.

Results. The systems spectral analysis of the horizontal components of the geomagnetic field has been carried out both in the 1–1,000-s period range and in the three separate period subranges of 1–50 s, 50–200 s, and 200–1,000 s. The component intensities are estimated to be the largest in the 200–1,000-s period subrange. On the reference days, the level of fluctuations is usually observed to not exceed ±(0.2–0.5) nT, whereas it shows an increase of ±(4–6) nT during the course of the first magnetic storm, and of ±2 nT during the second magnetic storm. In the course of the sudden commencement of the first storm, the period of the oscillation predominant in the spectrum is estimated to be about 350 and 600 s, whereas during the course of the main phase of the magnetic storm, the period of the oscillation predominant in the spectrum is estimated to be 800–900 s. During the recovery phase of the magnetic storm, the level of fluctuations in the geomagnetic field is estimated to not usually exceed ±(1–2) nT, whereas the period of the predominant oscillation is observed to be 700–900 s. The dawn and dusk terminators are accompanied by an increase in the level of fluctuations in the geomagnetic field.

Conclusions. The temporal variations parameters in the spectral components of the geomagnetic field have been determined during the moderate magnetic storm.