A comparison of theoretical and experimental estimates of the solar proton diffusion coefficient during three flare events

The propagation time for solar protons observed during the events of January 24, February 25 and March 17, 1969 are compared with those estimated from numerical solutions of the Fokker-Planck transport equation, using values of the diffusion coefficient of the form K _r = K ₀ r ^b where r is radial distance from the Sun, K ₀ is obtained from the plasma-field parameters near the Earth and b varies from - 3 to + 1. K ₀ is derived either by assuming that all the magnetic fluctuation power is in small amplitude transverse waves or alternatively in discontinuous changes in ¦B¦ along the flux tube of propagation. In the first case it is found that the K ₀ values calculated require either b -3, implying a very rapid wave growth with r, or the Fokker-Planck equation reduces to the situation of purely convective transport which is at variance with the experimental observations. More reasonable results are found in the second case although even here K ₀ is probably underestimated. Alternative ways of deducing K ₀ empirically from particle anisotropy measurements are put forward and these seem to favour the discontinuity model.