[1] We analyze the response of relativistic electrons to the 276 moderate and intense geomagnetic storms spanning the 11 years from 1989 through 2000. We find that geomagnetic storms can either increase or decrease the fluxes of relativistic electrons in the radiation belts. Surprisingly, only about half of all storms increased the fluxes of relativistic electrons, one quarter decreased the fluxes, and one quarter produced little or no change in the fluxes. We also found that the pre-storm and post-storm fluxes were highly uncorrelated suggesting that storms do not simply ''pump up'' the radiation belts. We found that these conclusions were independent of the strength of the storm (minimum Dst) and independent of L-shell. In contrast, we found that higher solar wind velocities increase the probability of a large flux increase. However, for all solar wind velocities both increases and decreases were still observed. Our analysis suggests that the effect of geomagnetic storms on radiation belt fluxes are a delicate and complicated balance between the effects of particle acceleration and loss.
Abstract.Short-duration, narrow-bandwidth, HF chirps occur at slowly drifting frequencies above fpe when fpe > fee . We put forth a model of the HF chirp emissions as quasitrapped eigenmodes in a density depletion perpendicular to the ambient magnetic field. The escaping waves retain the frequency structure of the eigenmodes and are observed as HF chirps. We show that this model is quantitatively consistent with observed characteristics of HF chirps, most notably the frequency spacing of the chirp emissions (0.1-4 kHz), the fact that they are equally spaced independent of mode number, and the number of modes predicted for a given density cavity. Several mechanisms for the escape of the waves from the cavity are suggested.
Abstract. Using data from the PHAZE II sounding rocket, launched from PokerFlat, Alaska, we present high-resolution observations of structure in auroral HF waves at and below the local plasma frequency. These observations were made in the altitude range of 390-945 km where the local plasma frequency is below the electron cyclotron frequency. We observe monochromatic, long-lived, narrowband emissions occuring below the local plasma frequency during times of intense HF wave emission. We have termed these emissions "HF bands" due to their appearance in spectrogram images. These emissions are probably identical to the "spike" emissions identified by previous observers using lower time resolution data from the AUREOL/ARCAD3 satellite which showed a narrow peak spectra below the local plasma frequency. HF bands often occur when the local plasma density is varying and are associated with regions of intense Langmuir wave generation. We investigate the hypothesis that the HF bands are created when a Langmuir wave propagates from a low-density region into a higher density region. The wave moves onto the whistler mode branch and propagates as an HF band. Theoretical calculations of propagation times of whistler mode waves support this hypothesis.
Abstract.We report high resolution measurements of the frequency-time structure of HF waves just above the electron plasma frequency in overdense conditions in the auroral ionosphere.
Abstract. We present a compilation of observations of relativistic radiation belt electrons during the four Geospace Environment Modeling (GEM) storms from instruments on 10 separate spacecraft. While these four magnetic storms have very different solar wind and magnetospheric conditions, there are several characteristics of the relativistic electron response which are applicable to all four storms. We find that although the evolution of the spectral shape of the electrons at a specific L shell does not vary from storm to storm, the evolution is very different at L = 4.2 and L = 6.6. Calculations of the phase space density (PSD) show that the evolution of the PSD depends on both radial position and the value of the first adiabatic invariant. The evolution of the greater than 1-MeV electron flux at L = 4.2 and L = 6.6 for the four storms is consistent with the findings of Reeves et al. [1998c]. The flux at L = 4.2 peaks quickly after the storm (12 hours), while the fluxes at geosynchronous altitudes take several days to rise. We suggest that the common characteristics identified in the four storms that are the subject of this paper can provide a useful basis for comparisons with other storms and for development of a more complete theoretical description of relativistic electron events in general.
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