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Pressure gradient effect on a particle velocity distribution
Author(s) -
Lui A. T. Y.
Publication year - 2009
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2008gl036458
Subject(s) - gyroradius , physics , velocity gradient , moment (physics) , diamagnetism , pressure gradient , thermal velocity , range (aeronautics) , particle velocity , velocity moments , population , radius , particle (ecology) , mechanics , test particle , distribution function , flow (mathematics) , classical mechanics , computational physics , magnetic field , flow velocity , materials science , geology , optics , quantum mechanics , demography , computer security , oceanography , wavefront , sociology , zernike polynomials , computer science , composite material
In space plasma missions, measurements of the three‐dimensional particle velocity distribution are used to examine the dynamical evolution of plasmas. The first velocity moment of the measured particle velocity distribution is often regarded as the bulk flow of the population. Accurate determination of bulk flows is crucial if the frozen‐in‐condition is judged by E + V × B = 0, where E is the electric field, V is the bulk flow, and B is the magnetic field. In this paper, we show that this first velocity moment computed from the measured particle velocity distribution can deviate substantially from the bulk motion of the particle population when a significant pressure gradient exists near the measurement location. The discrepancy, which arises from the diamagnetic drift with a finite Larmor radius effect, increases with increasing energy range used in the computation. The result calls for caution in the proper interpretation of this velocity moment and suggests a means to avoid error in the determination of bulk flow due to pressure gradient effects.