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| Titel |
Field line distribution of density at L=4.8 inferred from observations by CLUSTER |
| VerfasserIn |
R. E. Denton, P. Décréau, M. J. Engebretson, F. Darrouzet, J. L. Posch, C. Mouikis, L. M. Kistler, C. A. Cattell, K. Takahashi, S. Schäfer, J. Goldstein |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 27, no. 2 ; Nr. 27, no. 2 (2009-02-16), S.705-724 |
| Datensatznummer |
250016408
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| Publikation (Nr.) |
 copernicus.org/angeo-27-705-2009.pdf |
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| Zusammenfassung |
| For two events observed by the CLUSTER spacecraft, the field line
distribution of mass density ρ was inferred from Alfvén wave
harmonic frequencies and compared to the electron density ne from
plasma wave data and the oxygen density nO+ from the ion
composition experiment. In one case, the average ion mass M≈ρ/ne 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 (ne=8 cm−3) than for the second
event (ne=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
Alfvén 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|≲20° but very steeply increasing with respect
to |MLAT| for |MLAT|≳40°. The total variation in
ρ 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. The inferred mass density decreases to a minimum
23% lower than the equatorial value
at |MLAT|=15.5°, and then steeply increases as one moves along the
field line toward the ionosphere. For this event we were also able to
examine the spatial dependence of the electron density using
measurements of ne from all four CLUSTER spacecraft. Our analysis
indicates that the density varies with L at L~5 roughly like L−4, and
that ne is also locally peaked at the magnetic equator, but with a
smaller peak. The value of ne reaches a density
minimum about 6% lower than the equatorial value at |MLAT|=12.5°, and then
increases steeply at larger values of |MLAT|. This is to our knowledge
the first evidence for a local peak in bulk electron density at the magnetic equator.
Our results show that magnetoseismology can be a useful technique to determine the field
line distribution of the mass density for CLUSTER at perigee and that the
distribution of electron density can also be inferred from measurements by
multiple spacecraft. |
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