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[1] We present a detailed case study of the characteristics of auroral forms that constitute the first ionospheric signatures of substorm expansion phase onset. Analysis of the optical frequency and along-arc (azimuthal) wave number spectra provides the strongest constraint to date on the potential mechanisms and instabilities in the near-Earth magnetosphere that accompany auroral onset and which precede poleward arc expansion and auroral breakup. We evaluate the frequency and growth rates of the auroral forms as a function of azimuthal wave number to determine whether these wave characteristics are consistent with current models of the substorm onset mechanism. We find that the frequency, spatial scales, and growth rates of the auroral forms are most consistent with the cross-field current instability or a ballooning instability, most likely triggered close to the inner edge of the ion plasma sheet. This result is supportive of a near-Earth plasma sheet initiation of the substorm expansion phase. We also present evidence that the frequency and phase characteristics of the auroral undulations may be generated via resonant processes operating along the geomagnetic field. Our observations provide the most powerful constraint to date on the ionospheric manifestation of the physical processes operating during the first few minutes around auroral substorm onset.

The numerical results from a physics-based global magnetohydrodynamic (MHD) model are used to examine the effect of the interplanetary magnetic field (IMF), solar wind dynamic pressure, and dipole tilt angle on the size and shape of the magnetopause. The subsolar magnetopause is identified using the plasma velocity and density, the cusps are identified using the thermal pressure, and the whole shape of the magnetopause is determined with the three-dimensional streamlines traced through the simulation domain. The magnetopause surface obtained from the simulations is fitted with a three-dimensional surface function controlled by ten configuration parameters, which provide a description of the subsolar magnetopause, the cusp geometry, the flaring angle, the azimuthal asymmetry, the north-south asymmetry, and the twisting angle of the magnetopause. Effects of the IMF, solar wind dynamic pressure, and dipole tilt angle on the configuration parameters are analyzed and fitted by relatively simple functions. It is found that the solar wind dynamic pressure mainly affects the magnetopause size; the IMF mainly controls the magnetopause flaring angle, azimuthal asymmetry, and twisting angle; and the dipole tilt angle mainly affects the magnetopause north-south asymmetry and the cusp geometry. The model is validated by comparing with available empirical models and observational results, and it is demonstrated that the new model can describe the magnetopause for typical solar wind conditions.

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[1] The adiabatic drift-resonant interaction between relativistic, equatorially mirroring electrons and narrowband, Pc 5 ultra low frequency (ULF) waves in the magnetosphere is investigated using a time-dependent magnetohydrodynamic (MHD) wave model. Attention is focused on the effect of a ULF wave packet with finite duration on the equatorially mirroring, relativistic electron phase space density (PSD) profile. It is demonstrated that a burst of narrow band ULF waves can give rise to the growth of strong localized peaks in PSD with L-shell by nondiffusive radial transport. This contrasts with the diffusive ''external source acceleration mechanism'' described by Green and Kivelson (2004), a radial transport mechanism often attributed to ULF waves, which cannot produce peaks in PSD that increase with time. On the basis of this paradigm, observations of locally growing PSD peaks are usually attributed to very low frequency (VLF) wave acceleration by resonant interactions with lower-band chorus (e.g., Horne et al., 2005). However, we show that in situations where large amplitude, narrow bandwidth ULF waves are also observed, these time-limited coherent ULF waves can also generate growing PSD peaks and under such circumstances may offer an alternative explanation.