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Field line distribution of density at L=4.8 inferred from observations by CLUSTER

Abstract : For two events observed by the CLUSTER spacecraft, the field line distribution of mass density rho was inferred from Alfven wave harmonic frequencies and compared to the electron density n(e) from plasma wave data and the oxygen density n(O+) from the ion composition experiment. In one case, the average ion mass M equivalent to rho/n(e) was about 5 amu (28 October 2002), while in the other it was about 3 amu (10 September 2002). Both events occurred when the CLUSTER 1 (C1) spacecraft was in the plasmatrough. Nevertheless, the electron density ne was significantly lower for the first event (n(e)= 8 cm(-3)) than for the second event (n(e)= 22 cm(-3)), and this seems to be the main difference leading to a different value of M. For the first event (28 October 2002), we were able to measure the Alfven wave frequencies for eight harmonics with unprecedented precision, so that the error in the inferred mass density is probably dominated by factors other than the uncertainty in frequency (e. g., magnetic field model and theoretical wave equation). This field line distribution (at L=4.8) was very flat for magnetic latitude |MLAT|less than or similar to 20 degrees but very steeply increasing with respect to |MLAT| for |MLAT|greater than or similar to 40 degrees. The total variation in rho was about four orders of magnitude, with values at large |MLAT| roughly consistent with ionospheric values. For the second event (10 September 2002), there was a small local maximum in mass density near the magnetic equator.
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R E Denton, Pierrette Décréau, M J Engebretson, Fabien Darrouzet, J L Posch, et al.. Field line distribution of density at L=4.8 inferred from observations by CLUSTER. Annales Geophysicae, European Geosciences Union, 2009, 27 (2), pp.705-724. ⟨10.5194/angeo-27-705-2009⟩. ⟨insu-02871269⟩

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