Impulsive events marked by durations ranging from 5 to 15 min and peak‐to‐peak variations in the H component exceeding 40 nT are common in observations by high‐latitude day side ground magnetometers. We survey observations by a global network of high‐latitude ground magnetometers to distinguish between their proposed causes. Peak event amplitudes and occurrence rates occur prior to local noon in the range of geomagnetic latitudes from 70° to 76°. Although most events can be associated with abrupt changes in the interplanetary magnetic field (IMF) orientation, few can be associated with abrupt changes in the solar wind dynamic pressure. Many events can be observed during intervals of steadily northward IMF orientation, but only a few can be observed during intervals of steadily southward IMF orientation. We suggest an interpretation of the events in terms of a model in which changes in the IMF orientation modulate the fraction of the solar wind pressure applied to the magnetosphere.

Abstract. We present THEMIS-A low-and high-energy plasma, magnetic field, and energetic particle observations of long period (11-36 min) irregular compressional pulsations in the dawnside magnetosphere from 08:00 to 12:24 UT on 7 November 2007. We demonstrate that the pulsations maintain thermal and magnetic pressure balance, then employ finite gyroradius techniques to determine wave properties from the gyrophase distributions of 5-10 keV ions. The waves generally move sunward at velocities ∼10 km s −1 with the background plasma convection flow. Wavelengths range from 6700 to 23 300 km, corresponding to azimuthal wavenumbers m from 18 to 76. Wave periods decrease with increasing radial distance. Having determined the parameters describing the waves, we consider three previously proposed explanations: generation by substorm injection, generation by bounce or drift-bounce instabilities, and generation by the drift-mirror instability. The interval was quiet geomagnetically, arguing against any relationship to substorm injections. We found that ions with low energies of 69-628 eV or high energies of 28-615 keV would have been required to account for drift-bounce resonance during this interval, but inspection reveals ion fluxes at these energies near background levels during the time period considered. On the other hand, the criteria for the drift mirror instability are marginally satisfied. As predicted for the drift mirror instability, particle distributions peak more sharply near 90 • pitch angles during magnetic field strength enhancements than during strength depressions. At this point we therefore interpret the compressional pulsations observed by THEMIS A in terms of the drift mirror instability.

We present a case study of the magnetospheric and ionospheric response to a series of repetitive (7–8 min) solar wind dynamic pressure and interplanetary magnetic field (IMF) strength variations generated at the bow shock. During a period of duskward and antisunward IMF orientation, the magnetospheric and ionospheric observations indicate transient events moving dawnward and duskward away from a point of origin at or after local noon. In contrast, during a subsequent period of dawnward and antisunward IMF orientation, the observations indicate events moving duskward away from a point of origin at or prior to 0900 LT. We consider explanations in terms of pressure pulse driven riplets on the magnetopause or directly driven bursts of antiparallel merging.

[1] We reexamine a long-period (10-18 min) poloidal pulsation observed by THEMIS-A in the outer dawn magnetosphere from 10:00 to 12:30 UT on 7 November 2007. The interval was originally reported by Korotova et al. (2009). Although the nonlinear compressional plasma and magnetic field perturbations observed by THEMIS-A during this interval agree well with model predictions for the linear perturbations associated with antisymmetric waves generated by the ballooning-mirror mode instability, the phase relationships between these perturbations indicates a complex frequency rather than the purely imaginary frequency that theory predicts for the outer dawnside magnetosphere. Variations in the radial plasma velocity confirm that a phase-locked north/south oscillation in the equatorial line of nodes associated with the ballooning-mirror mode waves doubles the frequency of the compressional component of the magnetic field during these pulsations. The same velocity and magnetic field perturbations exclude explanations for the frequency doubling in terms of spatial gradients sweeping back and forth across the spacecraft or drift-bounce resonances. Azimuthal electric fields associated with the pulsations generate field-aligned anisotropies in the pitch angle distributions that become more prominent with increasing ion energy due to the presence of drift-shell splitting and radial flux gradients that steepen with increasing energy. Although there was no evidence for drift-bounce interactions during this event, the role of such events in ion energization in other events and at other locations remains to be evaluated.