The regimes of the east-west and the radial anisotropies of cosmic rays in the heliosphere

Abstract

The observations obtained over the last 23 y suggest that there are two distinct physical states of the heliosphere. One state covers the period 1957–1970 when the diurnal anisotropy consists of the azimuthal component only. One may define this period as the regime of the East-West ( co-rotation) anisotropy. The period 1971–1979 is characterized by the appearance of a radial anisotropy, which attains a maximum amplitude in 1976, when the solar activity is minimum. There appears to exist an inverse correlation between the amplitude of the radial anisotropy and solar activity. The amplitude of the EastWest anisotropy varies with time during this latter period and may also be rigidity-dependent. In 1976 the amplitude of the East-West anisotropy is zero for the underground muon data obtained at Embudo and has a lower value for the neutron monitor data obtained at Deep River. On the other hand, the amplitude of the radial anisotropy depends weakly upon the primary rigidity. The period 1971–1979 thus defines the regime of the radial anisotropy. The physical state of the heliosphere is very stable during the regime of the East-West anisotropy and extremely dynamic during the regime of the radial anisotropy. The heliosphere appears to switch from one physical state to another following the onset of the solar polar field reversal.