Incoherent Thoughts: Stochastic Acceleration Robert Sheldon June 27, 2005 National Space Science & Technology Center

Sychrocyclotrons Resonant acceleration is like particle accelerators: to keep up the efficiency, the frequency must be adjusted for the accelerating particle. But, to keep the power requirements down, the power must be strongly peaked around a narrow range of frequencies. These two competing processes then require a very unusual arrangement of particles + waves: = low entropy. Low entropy  improbable

Stochastic Acceleration The other way to accelerate is high entropy:  heat No specific frequency range, power distributed over many frequencies. Therefore low power, and slow acceleration Multisteps needed to get to high energy Long time, so we need a trap. High entropy  high probability

Dipole vs Quadrupole Trap The nature of the trap determines the effectiveness of the stochastic acceleration. Fermi-acceleration has a limited lifetime trap, therefore Emax is time-limited Diffusion in dipole trap puts the highest-E at the atmosphere, so highest energies are precipitated and lost. Quadrupole trap lasts a long time, and highest energies escape. A good place for stochastic diffusion. (Lots of good properties!)

Conclusions Unless observations dictate otherwise, it is most probable to have high-entropy processes. But high-entropy processes are inefficient, so the trap becomes most important. Three (4?) traps have been found in near- Earth space: Fermi-trap, Dipole, and Quadrupole (Current Sheet?)

Incoherent Thoughts: Spatial Diffusion Coeeficients Rob Sheldon NSSTC

Transport Quasi-Linear Diffusion is a resonant transport. If first two invariants are good, then we can write a total energy hamiltonian: H = KE + PE = u Bm + q E Then particle trajectories mapped to the equatorial plane follow iso-energy contours where dH = 0. So diffusion of this iso-energy contour, is spatial transport. This is really just the “radial” diffusion coefficient, but iso-contours may not be circular.

Calculation of this “radial” transport coefficient Since dH/dt = 0, then = 0 Transport then, is <> 0 So writing the full H = uB + qE we have: = So, stochastic power, without any particular resonance at the drift period (but of course, it contributes to the averaging integral more) Note: the q/u ratio determines the effectiveness of versus. Hi u  Rad belt, Lo u  plasma