Conjugacy of electron microbursts and VLF chorus

Abstract

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.

Anisotropic pitch angle distribution of ~100 keV microburst electrons in the loss cone: measurements from STSAT-1

“…Chorus is a bursty whistler mode wave with elements of ∼0.1-1 s and has similar time scales as electron microbursts (Tsurutani and Smith, 1974). Moreover, the coincident occurrences between microburst and VLF chorus waves have given support to the idea that low energy microbursts are produced by wave-electron cyclotron resonance interactions (Rosenberg et al, 1981).…”

Modelling substorm chorus events in terms of dispersive azimuthal drift

“…The cyclotron wave-particle interaction results in a decrease in the pitch angle of the resonant electrons and may lead to their precipitation (Foster and Rosenberg, 1976;Rosenberg et al, 1981). Electron precipitation can be monitored using a riometer, which indicates the degree of absorption of cosmic VHF radio noise due to charged particle den- .…”

X ray microbursts and VLF chorus

“…The experimental observations included X-ray and VLF-wave measurements from a balloon near Roberval, and VLF data from a ground receiver at Siple. A detailed study of those events by Rosenberg et al (1981) concluded that the interactions were similar to those of . However, the X-ray precipitation at Siple was not measured during the events.…”

“…However, the X-ray precipitation at Siple was not measured during the events. Rosenberg et al (1981) also presented evidence of a spatial separation along the field line of the wavegrowth and pitch-angle scattering regions. Furthermore, to achieve agreement between the theoretical and observed time lags between VLF and X-ray bursts, they had to assume a non-negligible time delay in the pitch angle scattering process itself (-70 -80 milliseconds).…”