Spatiotemporal scaling properties of the ground geomagnetic field variations

摘要

The spatiotemporal scaling properties of the auroral region ground horizontal magnetic field fluctuations are investigated in terms of structure function analysis. First, the distorting effects of the continuation of the magnetic field from the ionosphere to the ground level and the effects of the geomagnetic induction on the derived scalings are studied. It is found that the spatial fluctuations of the magnetic field and its time derivative are distorted by the field continuation in the ranges of scales of the data, i.e., 100 < Δr < 2400 km (above ～1000 km, analysis is spatially one-dimensional) and 10 < τ < 104 s. Specifically, the extent of the distortion is found to be dependent on the original ionospheric level scaling of the fluctuations. The effect of the geomagnetic induction is found to be negligible in the ranges of scales of the data. The main findings of the study are (1) there is a significant change in the dynamics of the field fluctuations in the range 80 < τ < 100 s where the time derivative of the magnetic field undergoes a transition from correlated to uncorrelated temporal behavior, (2) the spatiotemporal behavior of the time derivative of the magnetic field above temporal scales of 100 s resembles that of uncorrelated white noise, (3) the spatial symmetry of the field fluctuations increases during substorms, indicating the presence of spatially less ordered ionospheric equivalent currents, and (4) the spatial scaling properties of the field fluctuations may explain why the magnetotelluric sounding method works better than expected in auroral regions.