Power-law dependence of the wavelet spectrum of ground magnetic variations during magnetic storms

Abstract

We show that time variations in the ring current and auroral electrojets in 2015, during which three major magnetic storms (MSs) occurred, are recorded in several specific ways by ground magnetometers. Specifically, we show that the time variations in magnetic field intensity have power-law wavelet spectra. The variations in ring current intensity are detected in geomagnetic field measurements of ground stations at magnetic mid-latitudes between λ=-50° and +50°. We confirmed the results of Balasis et al. (2006), who had shown that the Dst index wavelet spectrum follows a power law of the form f-β, where f is the frequency, and β is the exponent of the power law, by applying the same wavelet analysis to the SYM-H index version of the Dst index, and also to ground magnetic field measurements. Balasis et al. (2006) also showed that the critical exponent β of the power law, obtains values greater than 2 (or equivalently that the Hurst exponent H>0.5), shortly before the onset and during the main phase of the storms. Here, using wavelet transforms on the Earth’s magnetic field as measured at different magnetic latitudes (MLAT), we showed that at mid-geomagnetic latitudes, where the Earth’s magnetic field is determined mainly by the ring current, the resulting Hurst exponent is greater than 0.5 several hours before and during magnetic storms. Geomagnetic field variation at higher magnetic latitudes between λ=+50° and +73°, also show power-law wavelet spectra. However, here we find a more complex dependence with two power-law exponents. We interpret this double power law as the effect of both the ring current and the auroral electrojets. Finally, for polar latitudes, between λ=+73° and +85°, we find the same complex dependence with two power-law exponents. Our results show that the geomagnetic field at auroral latitudes has much lower power and very different wavelet spectra compared to the geomagnetic field at ranges of lower magnetic latitude. Our results show how wavelet-based measures of ground geomagnetic variations reflect the time evolution of several magnetospheric current systems.