Physics
Scientific paper
Feb 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008jastp..70..482s&link_type=abstract
Journal of Atmospheric and Solar-Terrestrial Physics, vol. 70, issue 2-4, pp. 482-489
Physics
2
Magnetospheric Models, Flux-Tube Volume, Hamiltonian Mechanics, Guiding-Center Simulations, Particle Transport, Plasma Currents
Scientific paper
The equation of a magnetic field line (labeled L) in Dungey's model magnetosphere (dipole field plus uniform southward [Delta]B) is r=La[1+(r3/2b3)]sin2 [theta], where r denotes geocentric distance, [theta] denotes magnetic colatitude, a is the Earth's radius, and b is the radius of the field model's equatorial neutral line. This model can be generalized (e.g., to accommodate a ring current) by treating b as a function of L and [phi] (magnetic local time) rather than as a constant, so as to yield measured or calculated values of the equatorial magnetic field B0. (In this generalization the equatorial neutral line has a radius b*([phi])=(3a/2)L*([phi]) for some particular [phi]-dependent value of L called L*.) This approach yields an estimate for how a specified distortion of equatorial B0 might map to higher latitudes. It also allows for analytical calculation of the current density J=(c/4[pi]) [backward difference]×B at arbitrary latitude. Since charged particles (of scalar momentum p) scattered strongly in pitch angle satisfy an adiabatic invariant [Lambda]=p3[Psi], where [Psi] is the flux-tube volume (per unit magnetic flux), it is of interest to approximate (as well as possible) the flux-tube volume [Psi] as a function of L and [phi]. By generalizing the calculation of Schulz [1998a. Particle drift and loss rates under strong pitch angle diffusion in Dungey's model magnetosphere. Journal of Geophysical Research 103, 61-67], we have found such an analytical approximation of [Psi] for arbitrarily non-constant b and are using it in bounce-averaged transport simulations of diffuse auroral electrons described by a Hamiltonian function in which the kinetic energy is given by [([Lambda]/[Psi])2/3c2+(m0c2)2]1/2-m0c2, where m0 is the rest mass and c is the speed of light.
Chen Margaret W.
Schulz Michael
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