Abstract
Abstract. Several in situ measurements performed in the solar wind evidenced that solar type III radio bursts were some-times associated with locally excited Langmuir waves, high-energy electron fluxes and low-frequency electrostatic and electromagnetic waves; moreover, in some cases, the simultaneous identification of energetic electron fluxes, Langmuir and whistler waves was performed. This paper shows how whistlers can be excited in the disturbed solar wind through the so-called "fan instability" by interacting with energetic electrons at the anomalous Doppler resonance. This instability process, which is driven by the anisotropy in the energetic electron velocity distribution along the ambient magnetic field, does not require any positive slope in the suprathermal electron tail and thus can account for physical situations where plateaued reduced electron velocity distributions were observed in solar wind plasmas in association with Langmuir and whistler waves. Owing to linear calculations of growth rates, we show that for disturbed solar wind conditions (that is, when suprathermal particle fluxes propagate along the ambient magnetic field), the fan instability can excite VLF waves (whistlers and lower hybrid waves) with characteristics close to those observed in space experiments.Key words. Space plasma physics (waves and instabilities) – Radio Science (waves in plasma) – Solar physics, astrophysics and astronomy (radio emissions)
Highlights
Solar type III radio bursts are generated by energetic electron beams – sometimes associated with solar flares – which originate from the solar corona and travel along open magnetic lines toward the interplanetary space (Zaitsev et al, 1972, 1974; Melrose, 1974; Gurnett and Anderson, 1976)
“fan instability” by interacting with the energetic electrons at the anomalous Doppler resonance. This instability does not require any positive slope in the suprathermal electron tail and can account for physical situations where plateaued reduced electron velocity distributions were observed in association with Langmuir and whistler waves; in this case, whistlers can use the free energy from the beam that is not available for Langmuir waves
Owing to linear calculations of growth rates, we demonstrate in this paper that, for disturbed solar wind conditions, the fan instability can excite VLF waves with characteristics close to those observed in the solar wind
Summary
Whistlers observed in the solar wind are usually believed to be generated by some instability caused by the distortion of the electron velocity distribution from the maxwellian one. The following mechanism was proposed to describe one possible source of growth for VLF waves: after the reduced electron velocity distribution is plateaued by the Langmuir oscillations, it can remain unstable to electrostatic whistlers with a finite perpendicular wave number that can interact at the Landau resonance with field-aligned electrons of energies ranging from 100 eV to 3 keV On another hand, reporting on lowfrequency magnetic field fluctuations that are enhanced behind interplanetary shocks, Corotini et al (1982) presented indirect evidence that whistlers propagating at very large normal angles (above 70 degrees) and with wavelengths of the order of c/ωp may be generated in the solar wind during disturbed times. “fan instability” by interacting with the energetic electrons at the anomalous Doppler resonance This instability does not require any positive slope in the suprathermal electron tail and can account for physical situations where plateaued reduced electron velocity distributions were observed in association with Langmuir and whistler waves; in this case, whistlers can use the free energy from the beam that is not available for Langmuir waves. Considering two of the examples cited above, that is the cases of auroral solar wind and type III solar radio burst plasma conditions, calculations show that oblique whistlers can be excited through the mechanism of fan instability at the same frequencies as those observed in the space experiments
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