Abstract
Abstract. After reviewing briefly the present state of knowledge about chorus-like emissions, we present an overview of Magion 5 satellite observations of these emissions in the inner magnetosphere of the Earth. From the extensive VLF data recorded on board the Magion 5 satellite, we show examples of different types of discrete elements, representing rising and falling tones, and discuss their spectral properties, such as the bandwidth and the characteristic frequency as compared to the equatorial electron gyrofrequency. We analyse the possibility of satellite observation of discrete elements, assuming nonducted wave propagation from the source. As for the characteristic dimension of the generation region, we apply the figures obtained from the recently published correlation analysis of chorus emission recorded by four satellites in the Cluster experiment. We conclude that different frequencies in the chorus element should be emitted in a certain span of wave normal angles, so that the whole element could be observed far from the generation region.Key words. Magnetospheric physics (plasmasphere; plasma waves and instabilities) – Space plasma physics (wave-particle interactions) – Ionosphere (wave propagation)
Highlights
Chorus is an electromagnetic whistler mode emission that consists of narrow-band tones usually rising in frequency on the time scale of several tenths of a second (Sazhin and Hayakawa, 1992)
We conclude that different frequencies in the chorus element should be emitted in a certain span of wave normal angles, so that the whole element could be observed far from the generation region
We have presented a variety of spectrograms recorded on the Magion 5 satellite, illustrating different features of choruslike emissions, and discussed them in the light of recent findings from other satellites like Polar, Cluster and Geotail
Summary
Chorus is an electromagnetic whistler mode emission that consists of narrow-band tones usually rising (sometimes falling) in frequency on the time scale of several tenths of a second (Sazhin and Hayakawa, 1992). Trakhtengerts (1995, 1999) introduced a theory of the backward wave oscillator in ELF/VLF frequency band According to this theory, a step-like electron distribution function, which is formed as a result of the development of cyclotron instability, leads to an absolute instability and the wave generation in the form of discrete elements with rising frequency. Recent measurements of Poynting flux in chorus emission performed on the Polar satellite (LeDocq et al, 1998) provided an experimental proof that chorus is generated in the region close to the magnetic equator, as had been supposed by early works (Helliwell, 1969). The observations did not show evidences of the chorus magnetospheric reflections that often occur for lightning-generated whistlers inside the plasmasphere These findings were confirmed by the first results from Cluster wideband plasma wave measurements presented by Gurnett et al (2001). We present a variety of chorus-like emissions registered on board the Magion 5 satellite
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