Characteristics of quiet as well as enhanced diurnal anisotropy of cosmic radiation

Abstract

It is shown that the model which has been successful in explaining the anisotropy of low energy cosmic radiation of solar origin in terms of simple convection and field aligned diffusion can also be extended to explain both the quiet and enhanced diurnal variation of cosmic radiation at higher energies. The enhanced diurnal variation which shows a maximum around ~2000 hr is shown to be caused by the superposition of normal convection and enhanced field aligned diffusion due to an enhanced positive density gradient of approximately ~10 per cent/AU. The enhanced gradient in the early part of the event is caused by a depletion of cosmic ray intensity along the garden-hose direction and later in the event (T ≥ 4 days) is caused by an excess flux from the anti-garden-hose direction. The quiet day average diurnal anisotropy of cosmic radiation can be understood as due to an equilibrium between the convective and diffusive flow, resulting in a corotation vector along the 1800 hr direction. The observed amplitude of the annual mean diffusive vector is consistent with a positive radial density gradient of ~4.5 per cent/AU, which agrees with other radial gradient measurements. The diffusive vector during both quiet and enhanced anisotropy periods is shown to be along the interplanetary field direction. The ratio of perpendicular to parallel diffusion coefficient under normal conditions is derived to be ≤ 0.05 indicating that the transverse gradients, in general, are negligible.