The classic view is that Dst and SYM‐H time series differ mainly due to the dissimilarity in the method to determine the base values. Dst and SYM‐H are both indices designed to measure the intensity of the storm time ring current. They are calculated in similar but not identical manners. Since SYM‐H has the distinct advantage of having 1‐min time resolution compared to the 1‐hour time resolution of Dst, it is worth determining if the differences introduced by using different ground stations and slightly different methods of baseline subtraction produce statistically significant differences in the values of the indices. We have examined data from these indices collected over more than 20 years to determine the extent to which Dst and SYM‐H are equivalent or different. We found that a simple combination of linear trends with a break at SYM‐H = −300 nT provides an excellent comparison with the Dst index. For quiet times and for small storms the deviations are typically no more than 10 nT. Moderate storms feature deviations typically only slightly more than 10 nT, and intense storms have deviations that are usually less than 20 nT. We conclude that the classic view is accurate and recommend that in future studies the SYM‐H index be used as a de facto high‐resolution Dst index.

Detrended fluctuation analysis was applied to the magnetic storm index SYM‐H for the epoch 1981–2002. The objective was to determine the characteristic fractal statistical differences, if any, between a quiet and active magnetosphere. The entire data set comprises over 11 million points that include numerous intervals that can be classified as quiet or active. For quiet intervals we required Kp ≤ 1 for 10,000 consecutive minutes. Similarly, to qualify as an active interval required Kp ≥ 4 for 10,000 consecutive minutes. All active intervals included magnetic storms. Detrended fluctuation analysis was applied to each of these intervals to obtain local scaling exponents. A clear difference in statistical behavior during quiet and active intervals is implied through analysis of the scaling exponents for the quiet and active intervals; active intervals generally have larger values of scaling exponents. This implies that although SYM‐H appears monofractal on shorter timescales, it is more properly described as a multifractional Brownian motion. An overall trend toward higher scaling exponents was also discovered for increasing magnetospheric activity, possibly implying an increase in organization with magnetospheric activity. The overall distribution of the scaling exponents for active intervals was Gaussian. For quiet intervals, however, it was bi‐Gaussian, perhaps indicative of different internal (magnetospheric) and external (solar wind) nonlinear forcings.

[1] Using a new two-dimensional nonlinear finite element model, we investigate the interaction of dispersive shear Alfvén wave (SAW) field line resonances (FLRs) and ion acoustic waves in Earth's magnetosphere. We solve the full set of nonlinear reduced MHD equations self-consistently in arbitrary geometries. Initially, a Cartesian box model is used to demonstrate the reliability of our numerical solution in determining the linear and nonlinear evolution of FLRs. Then the full reduced MHD equations with the effects of electron inertia, ion Larmor radius correction, and electron thermal pressure are solved in dipolar and stretched magnetic topologies. We show that time-dependent dispersion and density steepening lead to localization of a highly structured FLR within an ionospheric (equatorial) density cavity (bump). When nonlinear effects are accounted for, we find that FLRs preferentially form in regions of low wave dispersion. Field line stretching and ponderomotive density redistribution lead to a significant reduction in FLR eigenfrequencies, bringing them into the range of observations. Nonlinear effects also cause a rapid acceleration of the timescale over which small perpendicular spatial scales appear. In our model, it is shown that density perturbations can be comparable to the equilibrium background density.

[1] We compare open-closed field line boundary positions from the BATS-R-US Global MHD model and CANOPUS photometer measurements of red-line emissions. We choose intervals of steady interplanetary and ionospheric conditions in order to adhere to the ''steady-state'' picture that we are trying to address. Nine intervals are chosen that correspond to stable IMF and auroral conditions that can be simulated with the MHD model. We find that on average, the steady-state BATS-R-US MHD model provides an excellent estimate of the open-closed field line boundary proxy as determined by the red-line auroral emissions. Typical errors between the model calculations of the openclosed field line boundary and the observations are within the inherent error in using the red-line emissions.

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