Particle lifetimes in strong diffusion
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The mean lifetime τ of a particle distribution, driven to isotropy by intense pitch-angle diffusion, is calculated by analytical means for conditions applicable to the Earth's magnetosphere. The resulting algebraic expressions reduce to τ≈[64La/35vα c 2 (1−η)] in the limit of a small equatorial loss cone (half-angle α c ), wherev is the particle speed,L is the magnetic shell parameter,a is the radius of the Earth, and η is the particle albedo from the atmosphere at either foot of the field line. Distinction is made in the full expressions for τ between complete isotropy (caused by strong pitch-angle diffusion all along the field line) and incomplete isotropy (caused by strong diffusion that is localized at the magnetic equator) over the upward hemisphere in velocity space.
KeywordsIsotropy Field Line Particle Distribution Velocity Space Algebraic Expression
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