Wavelet cross-spectrum analysis of the ring current using magnetic records from multiple low-latitude stations

Abstract

[1] Ground magnetic disturbances in equatorial regions are directly connected to the spatial and temporal variations of the ring currents, which have both the symmetric and asymmetric components, as well as many local-time-dependent M-I currents. In this study, we applied the wavelet analysis technique, which is especially appropriate for the data with impulsive and time-dependent spectral features, to the magnetic records from multiple low-latitude stations in order to study the dynamic behaviors of the symmetric and asymmetric parts of the ring currents for both quiet and storm conditions. First, we decomposed the magnetic records into various details in the wavelet spectral domain and systematically studied the temporal and frequency properties of the magnetic disturbances for various geomagnetic and seasonal conditions. Then we performed wavelet cross-spectrum analysis on the data from multiple stations in both UT and LT time frames in order to separate and elucidate the effects of the symmetric and asymmetric parts of the ring currents. Our results show that slow time-varying components of the ring current are largely globally symmetric, and during quiet periods, the magnetic effect of this symmetric part is comparable to that of the asymmetric (or local-time-dependent) part. During storm periods, the magnetic disturbances associated with both the symmetric and asymmetric parts of the ring currents increase significantly, but the increase of the symmetric ring current is much larger than that of the asymmetric current and it becomes dominant during storm periods. This result is based on the statistical study of multiple-day timescale variations. For shorter timescales such as the main phase of storms, the asymmetric component could be stronger than symmetric component. Our results also indicate that there are substantial residues of the magnetic effects of local-time-dependent currents left in the Dst index, and this further proves that the Dst is not an ideal index for the description of the symmetric ring current.