Pitch angle transport of electrons due to cyclotron interactions with the coherent chorus subelements

Abstract

Chorus is a right‐hand, circularly‐polarized electromagnetic plane wave. Dayside chorus is a bursty emission composed of rising frequency “elements” with duration of ∼0.1 to 1.0 s. Each element is composed of coherent subelements with durations of ∼1 to 100 ms or more. Due to the coherent nature of the chorus subelements/wave packets, energetic electrons with pitch angles near the loss cone may stay in resonance with the waves for more than one wave cycle. The electrons could therefore be “transported” in pitch across a relatively large angle from a single wave‐particle interaction. Here we study the cyclotron resonance of the energetic electron with the coherent chorus subelements. We consider a Gaussian distribution for the time duration of the chorus subelements and derive an expression for the pitch angle transport due to this interaction. For typical chorus subelement parameters, the average pitch angle diffusion coefficients ∼(0.5–8.5) s−1 are found. Such rapid pitch angle scattering may provide an explanation for the ionospheric microbursts of ∼0.1 to 0.5 s in bremsstrahlung x‐rays formed by ∼10–100 keV precipitating electrons. The model is applicable to the cases when R = ttr /Dt = [(w + W/2) ttr /wt] > 1and inhomogeneity factor S = ttr 2 /tinh 2 < 1, where W is the electron cyclotron frequency in the ambient magnetic field, B0, w is the frequency of chorus, ttr is the trapping time (or phase oscillation period), tinh is the time for the passage through the resonance in the inhomogeneous magnetic field, and t is the duration of the chorus subelement. For the typical parameters at L = 5, the energetic electrons having pitch angles of a ≤ p/3 can satisfy both the condition R > 1 and S < 1 for a range of chorus wave amplitudes.