Power spectral signatures of interplanetary corotating and transient flows

Abstract

Recent studies of the time behavior of the galactic cosmic ray intensity have concluded that long term decreases in the intensity are generally associated with systems of interplanetary flows that contain flare-generated shock waves, magnetic clouds, and other transient phenomena. In this paper the magnetic field power spectral signatures of such flow systems are compared to power spectra obtained during times when the solar wind is dominated by stable corotating streams that do not usually produce long-lived reductions in the cosmic ray intensity. We find that the spectral signatures of these two types of regimes (transient and corotating) are distinct. However, the distinguishing features are not the same throughout the heliosphere. The transient flows at 1 AU tend to have smaller correlation lengths and larger magnetic helicity scale lengths than do the corotating flows. In data collected beyond 1 AU, the primary differences are in the power spectra of the magnitude of the magnetic field rather than in the power in the field components. Consequently, decreases in cosmic ray intensity are very likely due to magnetic mirror forces and gradient drifts rather than to pitch angle scattering.