Eigenfrequencies of geomagnetic field lines and implications for plasma-density modeling

Details Eigenfrequencies of geomagnetic field lines and implications for plasma-density modeling Eigenfrequencies of geomagnetic field lines and implications for plasma-density modeling

Aug 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996jgr...10117385s&link_type=abstract

Journal of Geophysical Research, Volume 101, Issue A8, p. 17385-17398

Physics

Plasma Physics

24

Magnetospheric Physics: Mhd Waves And Instabilities, Magnetospheric Physics: Plasmasphere, Space Plasma Physics: Experimental And Mathematical Techniques, Space Plasma Physics: Waves And Instabilities

Scientific paper

Toroidal eigenfrequencies ωn/2π (with harmonic numbers n>=1) of dipolar magnetospheric B field lines are well approximated (within a few percent at most for L>=2) by the linear expression ωn~(3πcA0/8La)cscΛ+(n-3/4)Δω, if the plasma density ρ is proportional to (La/r)m along the magnetic field line of interest (r being the geocentric distance, a being the planetary radius, and subscript 0 signifying evaluation at the magnetic equator). The spacings Δω/2π between consecutive eigenfrequencies are nearly equal and well approximated by the reciprocal of ∮ (ds/cA), where cA is the local Alfvén speed and s is the coordinate that measures arc length along the field line. Poloidal eigenfrequencies ωn/2π with harmonic numbers n>=2 are equally well approximated by the same expression for ωn. This means that observed pulsation periodicities can be identified with their respective harmonic numbers by plotting the corresponding frequencies on a rectangular grid against possible harmonic numbers to see which reasonable identifications produce the best straight line. (Identification could alternatively be achieved, without plotting, through a modified form of linear regression.) Extrapolated to n=3/4, the linear fit to an observed eigenfrequency spectrum (toroidal with n>=1 and/or poloidal with n>=2) would then yield a good estimate for the equatorial plasma density ρ0. The slope of this same linear fit would lead (when divided by ω0.75/2π) to a good estimate for the corresponding value of m, which is the exponent of La/r in the modeled plasma density distribution.

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