Abstract
Abstract. 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.
Highlights
IntroductionThe field line dependence of magnetospheric mass density will affect the propagation of magnetosonic (fast mode) waves, the eigenmode structure of Alfven waves, and the Published by Copernicus Publications on behalf of the European Geosciences Union
We use toroidal Alfven frequencies observed by the CLUSTER spacecraft on 28 October 2002 in order to find what is probably the most precise field line distribution of mass density determined to date, one for which the errors in the inferred mass density are probably dominated by factors other than the uncertainty of the Alfven frequencies
Using the TS05 magnetic field model to map the position of the CLUSTER spacecraft to the position of maximum geocentric radius, we find that this position is offset from MLAT=0 by MLAT0 (1.63◦ for the 10 September 2002 event), and we judge MLAT=MLAT0 to be a better description of the location of the magnetic equator
Summary
The field line dependence of magnetospheric mass density will affect the propagation of magnetosonic (fast mode) waves, the eigenmode structure of Alfven waves, and the Published by Copernicus Publications on behalf of the European Geosciences Union. (Takahashi and Denton argued that both of these contribute to the range of possible solutions.) For instance, we can conclude from Takahashi and Denton’s results that for L>6 in the afternoon local time sector, there is on average a peak in mass density at the magnetic equator, but we are not sure how much variability there is in the statistical distribution, that is, to what extent the distribution can be more or less peaked For this reason, it is still of interest to look at individual events, if the frequencies can be measured with great accuracy. We use toroidal Alfven frequencies observed by the CLUSTER spacecraft on 28 October 2002 in order to find what is probably the most precise field line distribution of mass density determined to date, one for which the errors in the inferred mass density are probably dominated by factors other than the uncertainty of the Alfven frequencies (e.g., magnetic field model and theoretical wave equation).
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.