Abstract
Abstract. We compare the probability distributions of substorm magnetic bay magnitudes from observations and a minimal substorm model. The observed distribution was derived previously and independently using the IL index from the IMAGE magnetometer network. The model distribution is derived from a synthetic AL index time series created using real solar wind data and a minimal substorm model, which was previously shown to reproduce observed substorm waiting times. There are two free parameters in the model which scale the contributions to AL from the directly-driven DP2 electrojet and loading-unloading DP1 electrojet, respectively. In a limited region of the 2-D parameter space of the model, the probability distribution of modelled substorm bay magnitudes is not significantly different to the observed distribution. The ranges of the two parameters giving acceptable (95% confidence level) agreement are consistent with expectations using results from other studies. The approximately linear relationship between the two free parameters over these ranges implies that the substorm magnitude simply scales linearly with the solar wind power input at the time of substorm onset.
Highlights
Since the introduction of the substorm concept as a sequence of events observed in the aurora (Akasofu, 1964), much of the focus in the literature has been on the phenomenology of the substorm
It comprises a latitudinal chain of magnetometers and is less prone to measurement errors in the magnetic bay magnitude caused by variations in the latitude of the auroral electrojet
Since the substorm magnetic bay magnitude is equal to the minimum value of the synthetic IL index during the expansion phase it is reasonable to assume that: (a) the contributing station is between 20:29 MLT and 01:15 MLT and at λ=70◦; (b) the polar cap boundary is at the same magnetic latitude, in order that the contributing station is at the latitude where the eastward equivalent current minimises www.ann-geophys.net/25/2427/2007/
Summary
Since the introduction of the substorm concept as a sequence of events observed in the aurora (Akasofu, 1964), much of the focus in the literature has been on the phenomenology of the substorm. Global MHD models solve the fundamental physical equations of large-scale plasma dynamics in a realistic magnetospheric geometry but do not adequately capture processes on the kinetic scale that can couple to the large scale They have had limited success in reproducing substorm behaviour (see Raeder and Maynard, 2001, and other papers in that issue). Intermediate complexity models have been designed that are less computationally expensive and which arguably try to improve the representation of kinetic-scale processes appropriate to the substorm, at the expense of simplified physics and a simplified magnetospheric geometry (Klimas et al, 1992, 1994; Horton and Doxas, 1998; Klimas et al, 2004) The dynamics of these models can be analysed statistically but is complicated by the number of free parameters. We repeat their analysis using the synthetic AL index and compare the results
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