North–south component of galactic cosmic ray anisotropy at 1 AU

Abstract

The galactic cosmic ray (GCR) solar diurnal anisotropy (SDA) may be represented by a vector (A; Ar, Aφ, Aθ) in a spherical polar coordinate system centered on the sun. We reported (elsewhere) the results of a detailed study of time variations of yearly radial (Ar) and east–west (Aφ) components recorded by the global network of the neutron monitors (NMs) with a long track record for 1963–2013, for four sunspot number (SSN) cycles (20–23) and the rising phase of cycle 24. A powerful new technique is used to compute and study time variations of the transverse component (Aθ) due to off-ecliptic GCR contributions, with the same NM data; GCR radial particle density gradient (Gr) drives all three components. The north–south anisotropy (Aθ) is computed from yearly NM data (Gϕ=0), a flat heliospheric current sheet (HCS) model and the concept of GCR isotropic hard sphere scattering in the solar wind plasma. Relationships to SSN, rigidity and solar polarity intervals are studied. For a positive (p-) polarity the solar magnetic field in the northern hemisphere points outward and GCRs drift from polar regions toward equatorial plane and out along HCS, setting up a symmetric gradient (Gθs) pointing away from HCS (there is a local GCR density minimum on HCS); for n-polarity interval Gθs points towards HCS (there is a local GCR density maximum on HCS). Also, there exists a heliospheric asymmetric density gradient (Gθa) perpendicular to the ecliptic plane, it is the main contributor to Aθ for the period of our analysis. This is the most interesting and significant insight.