Probing interplanetary plasma and magnetic fields with galactic cosmic rays

Abstract

The results of the day-to-day analysis of cosmic ray daily variation with a very high counting rate mu-meson detector at MIT are presented. Comparison of these with the daily variation observed by the neutron monitor at Deep River provides a basis for deriving information concerning the position and the energy spectrum of variation of the anisotropy of galactic cosmic rays on each day. The significance of the peak-to-peak amplitude, the time of maximum (T_max) and the time of minimum (T_min) of the daily variation in relation to the anisotropy is demonstrated and evidence presented to indicate that from time series of T_max and T_min it is possible to follow advancing fronts of interplanetary plasma and magnetic fields. The nature of the anisotropy is analysed and arguments advanced for the view that consideration in terms of a single virtual source is inadequate to explain the results. Having demonstrated the day-to-day changes of energy spectrum of anisotropy and the level of isotropic galactic intensity, an attempt is made to identify distinct physical states of inter-planetary space in terms of a model involving a solar wind, but with nonuniformity of velocity over different regions of the Sun, with a possibility of a blast wave propagated through it due to occasional sudden release of energy. The classification of days as belonging to one or the other of four states is performed by using geomagnetic data and the day-to-day modulation of isotropic galactic intensity. It is then found that the anisotropy associated with different states is quite distinctive and is consistent with what may be expected according to the model. It is suggested that using data on geomagnetic disturbances and galactic cosmic rays as probes, it is possible to identify distinct electromagnetic states of interplanetary space.