Properties of dayside outer zone chorus during HILDCAA events: Loss of energetic electrons

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Abstract

[1] The dayside outer zone (DOZ) portion of the magnetosphere is a region where chorus intensities are statistically found to be the most intense. In this study, DOZ chorus have been examined using OGO-5 plasma wave and GEOTAIL plasma wave, magnetic field and energetic particle data. Dayside chorus is noted to be composed of ∼0.1 to 0.5 s rising-tone emissions called “elements.” The duration of the elements and their frequency-time characteristics are repeatable throughout the chorus event (lasting from tens of minutes to hours), but may differ from event to event. Chorus is a right-hand, circularly polarized electromagnetic plane wave. Waves are detected propagating from along the ambient magnetic field, Bo, to oblique angles near the Gendrin angle, θGendrin. All waves, independent of wave direction of propagation relative to Bo, are found to be circularly polarized, to first order. Chorus rising-tone elements are composed of coherent “subelements” or “packets” with durations of ∼5 to 10 ms. Consecutive subelements/packets step in frequency with time to form the elements. The peak amplitudes within a packet can be ∼0.2 nT or greater. The subelement or packet amplitudes are at least an order of magnitude larger than previously estimated chorus amplitudes obtained by power spectral measurements. This discrepancy is due to the presence of interspacings between chorus elements, the interspacings between subelements/packets within an element, and the different frequencies of subelements/packets within a rising-tone. DOZ chorus studied here were found to be consistent with generation via the loss cone instability of substorm-injected temperature-anisotropic (equation image/equation image > 1) E = 5 to 40 keV electrons drifting from the midnight sector to the DOZ region. Using a large amplitude subelement/packet wave magnetic field amplitude of ∼0.2 nT, it is shown that the instantaneous Kennel-Petschek pitch angle diffusion rate Dαα is ∼5 × 10−2 s−1. This latter quantity is based on incoherent waves. If energetic electrons stay in cyclotron resonance throughout their interaction with a coherent subelement of duration 10 ms, they may be “pitch angle transported” by ∼5°. Therefore electrons within 5° of the loss cone can be lost in a single wave-particle interaction. Several such interactions as the electrons traverse the wave region can lead to much larger pitch angle transport angles. The similar time-scales of chorus elements and bremsstrahlung X-ray microbursts (∼0.5 s) can be explained by the “pitch angle transport” mechanism described above. Increasing and decreasing pitch angle transport via this mechanism will lead to much higher pitch angle diffusion or “super diffusion” rates. Isotropic unpolarized noise of ∼20 pT peak amplitude has also been detected. The noise is well above instrument noise levels and is speculated to be remnants of chorus or hiss.

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