Ulysses Calculating Anisotropies

If the number of ion counts in the energy channel we are interested is sufficiently high to be unaffected by Poisson statistics [i.e. 1/root(no. of counts)] then we deduce the anisotropy present in the ion flux distribution in the following way. The ion counts are converted into fluxes and then we use the 16 measurements from the 2 telescope 8 sector system to make a spherical harmonic fit to the flux distribution using the equation below.
ln J(E,v)=ln J0(E)+A1.v+A2((3cos2psi+1)/4)

Where J is the differential flux, J0 the omnidirectional flux, E the ion energy, v the ion unit velocity vector, A1 the first order anisotropy vector, A2 the second order anisotropy magnitude and psi the particle pitch angle.

The fit above yields the first order anisotropy vector which can be resolved into field aligned and field perpendicular components. Field perpendicular components can arise either due to a bulk flow of the plasma or due to a spatial gradient in the ion flux, or as a combination of the two effects. Anisotropies parallel to the magnetic field need not be simply related to the thermal flow of the plasma and generally pertain to the nature of the ions from which they were derived. The parallel anisotropy component tells us about the asymmetry of the particle distribution in the bulk flow frame.

You can find out about our results and progress using the ATs anisotropy data.

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Written by Nick Laxton. Last changed 24th July 1997