Abstract
Abstract. Discrete ELF/VLF chorus emissions, the most intense electromagnetic plasma waves observed in the Earth's radiation belts and outer magnetosphere, are thought to propagate roughly along magnetic field lines from a localized source region near the magnetic equator towards the magnetic poles. THEMIS project Electric Field Instrument (EFI) and Search Coil Magnetometer (SCM) measurements were used to determine the spatial scale of the chorus source localization region on the day side of the Earth's outer magnetosphere. We present simultaneous observations of the same chorus elements registered onboard several THEMIS spacecraft in 2007 when all the spacecraft were in the same orbit. Discrete chorus elements were observed at 0.15–0.25 of the local electron gyrofrequency, which is typical for the outer magnetosphere. We evaluated the Poynting flux and wave vector distribution and obtained chorus wave packet quasi-parallel propagation to the local magnetic field. Amplitude and phase correlation data analysis allowed us to estimate the characteristic spatial correlation scale transverse to the local magnetic field to be in the 2800–3200 km range.
Highlights
Discrete ELF/VLF chorus emissions, the most intense electromagnetic plasma waves observed in the Earth’s radiation belts and outer magnetosphere, have received increased at-tention in the past several years
Chorus emissions, which propagate in the whistler mode, usually consist of two narrow frequency range bands centered around onehalf the electron gyrofrequency at the geomagnetic equator of the magnetic field line on which the waves are observed (Tsurutani and Smith, 1974)
The upper band exists in the frequency range of ω/ωce≈0.5–0.75 and contains discrete chorus elements rising at a few kHz/s
Summary
Discrete ELF/VLF chorus emissions, the most intense electromagnetic plasma waves observed in the Earth’s radiation belts and outer magnetosphere, have received increased at-. A number of ELF/VLF chorus emissions have been observed near the magnetic equatorial plane in the dayside outer magnetosphere. The GEOTAIL spacecraft has observed many ELF/VLF chorus emissions in the Earth’s outer magnetosphere with the Plasma Wave Instrument (PWI) (Nagano et al, 1996). The radiation belt chorus generation region scales were estimated theoretically (Helliwell, 1967; Trakhtengerts, 1999) and experimentally (Santolik and Gurnett, 2003; Breneman et al, 2009) using data from coordinated CLUSTER observations. Santolik and Gurnett (2003), and Breneman et al (2009) determined the correlation lengths of chorustype whistler waves based on multi-point Cluster WBD measurements near the chorus source region during the magnetic storm of 18 April 2002.
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