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Properties of the magnetospheric backward wave oscillator inferred from CLUSTER measurements of VLF chorus elements

Abstract : [1] According to the backward wave oscillator (BWO) model, a sharp gradient (or step-like deformation) on the electron distribution function is the most important factor in chorus generation, but such a feature is very difficult to observe directly. The properties of the step in the BWO model determine the dimensionless parameter q quantifying the excess of the energetic electron flux above the absolute-instability threshold. This parameter, in turn, is related to the frequency sweep rate of chorus elements, which we obtained by using data from the WBD instrument onboard the CLUSTER satellites in the equatorial region for more than 7000 chorus elements. Then, using the CLUSTER data for the plasma density and magnetic field, we calculated q assuming the validity of the BWO theory and found that the q values depend only weakly on the density; the average values of q ≈ 7 for the lower band chorus (f/f ce < 0.5) and q ≈ 13 for the upper band (f/f ce > 0.5). These q values constitute a large excess over the generation threshold (q > 3) resulting from numerical simulation of discrete elements with rising frequency and are thus consistent with the simulations. Another important feature of the q parameter is the significant scatter of its values during each Cluster passage of the generation region. Using the obtained q values we estimate the relative height of the step in the electron distribution function to lie in the range from 0.01 to 0.3. (2012), Properties of the magnetospheric backward wave oscillator inferred from CLUSTER measurements of VLF chorus elements,
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E Titova, A Demekhov, B Kozelov, O Santolík, E Macúšová, et al.. Properties of the magnetospheric backward wave oscillator inferred from CLUSTER measurements of VLF chorus elements. Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2012, 117, A08210 (10 p.). ⟨10.1029/2012JA017713⟩. ⟨insu-01179854⟩

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