Conjugacy of electron microbursts and VLF chorus

Information on the location of microburst source regions is limited. One measurement at L ≈ 8.5 placed the source within 4 RE of the ionosphere. Measurements at 5≲ L ≲6, though less conclusive, suggested that source regions may be located either near the equatorial plane or at higher magnetic latitudes along the field line. This paper reports simultaneous observations of bremsstrahlung X rays and VLF radiowave emissions that reveal a detailed correlation between electron microbursts precipitated in one hemisphere and chorus elements of rising frequency recorded at the conjugate point. The measurements were made at Roberval, Canada, and Siple Station, Antarctica (L ≈ 4.1), during magnetic substorms on July 9 and 15, 1975. The relationship between electron energy (50 ≲ E ≲ 200 keV) and wave frequency ( ≲ f ≲4 kHz), and the measured time difference (0.01 s ≤Δt ≤0.13 s) between detection of the electrons and waves at ionospheric conjugate points are consistent with near-equatorial cyclotron resonance interactions occurring outside the plasmasphere. In both cases, the observations could be accounted for if a diffusive-equilibrium distribution of electron density along the field line was assumed. The so-called ‘collisionless’ (or R−4) model of electron density was not in accord with the observations. Some evidence is found for a separation of the wave growth and electron scattering regions. Evidence is also found indicating that the process of electron scattering requires a finite time, up to ∼80 ms under the conditions of these observations. The present results suggest that microburst generation regions are located within 20° of the equator on subauroral field lines.

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