Source regions of banded chorus

Abstract

ELF/VLF chorus emissions are very intense electromagnetic plasma waves that are naturally and spontaneously excited near the magnetic equatorial plane outside the plasmasphere during periods of magnetic disturbance. These emissions are believed to play an important role in the acceleration of 10 to 100 keV radiation belt electrons to MeV energies during the disturbed time periods. Spacecraft observations near the magnetic equatorial plane in the regions where chorus emissions are generated show that the chorus often appears in two distinct frequency bands, one band below fce/2 and one band above fce/2, where fce is the local electron gyrofrequency. This configuration is known as banded chorus. In the present paper we show that this type of configuration can be readily explained if it is assumed that the chorus is excited in ducts of either enhanced or depleted cold plasma density.

Electron hybrid code simulation of whistler-mode chorus generation with real parameters in the Earth’s inner magnetosphere

“…On the other hand, we did not find a gap at half the gyrofrequency in both Elements 1 and 2, which is evident in the observation. This difference implies that the mechanism of a gap at 0.5 Ω e0 is governed by the physical process that is not described by the spatially one-dimensional system treating purely parallel propagating electromagnetic waves, such as the nonlinear damping through Landau resonance Yagitani et al 2014) and/or properties of obliquely propagating whistler-mode waves (e.g., Bell et al 2009;Katoh 2014;Nunn and Omura 2015). Macúšová et al (2010) analyzed 13 events of whistlermode chorus observed by the Cluster spacecraft during different levels of geomagnetic activity and showed that the frequency sweep rate decreases with the decreasing background plasma density.…”

“…The typical frequency range of chorus is from 0.2 to 0.8 Ω e0 , where Ω e0 is the electron gyrofrequency at the magnetic equator. Chorus are often classified into the lower band (0.2-0.5 Ω e0 ) and the upper band (0.5-0.8 Ω e0 ) because of the different propagation properties and characteristics appeared in the spectra accompanied with a distinct gap at half the gyrofrequency (e.g., Bell et al 2009). Since whistler-mode waves satisfy the cyclotron resonance condition with electrons in the wide range of kinetic energy and pitch angle, resonant scattering of energetic electrons in the magnetosphere has been extensively discussed by various approaches: by a diffusion code based on the quasi-linear theory (e.g., Thorne et al 2010), by a test-particle analysis (e.g., Albert 2001;Bortnik et al 2008), and by an analytic approach (e.g., Lakhina et al 2010).…”

A simulation study of the propagation of whistler-mode chorus in the Earth’s inner magnetosphere

“…The propagation of whistler-mode waves along a field line of the density enhancement or density decrease has been studied for decades (e.g., Smith et al 1960). The propagation properties of whistler-mode chorus have also been discussed by theories (e.g., Bell et al 2009) and observations (e.g., Haque et al 2011) for the explanation of the spectral gap at half the gyrofrequency dividing chorus emissions into upper band and lower band. Figure 6 shows the radial distribution of the background cold electrons at the magnetic equator of the simulation system.…”

“…The propagation properties of chorus are governed by the spatial distribution of ambient cold electrons controlling the dispersion relation of plasma waves. Previous studies have revealed conditions of trapping of whistler-mode waves within enhanced or depleted plasma density (Bell et al 2009). …”

Multipoint observations of plasma phenomena made in space by Cluster

“…This is banded chorus and its existence has been known since 1969 but until recently there has been no satisfactory explanation of the phenomenon. Using WBD, Bell et al (2009) showed that banded chorus can be explained if chorus is excited in ducts of either enhanced or depleted cold plasma. The result was later verified by Haque et al (2011Haque et al ( , 2012.…”