Cosmic ray solar semidiurnal anisotropy: 2. Heliospheric relationships of anisotropy parameters

Abstract

We have used the corrected experimental semidiurnal variation observations described at length in Paper 1, to compute the amplitude and direction of semidiurnal anisotropy in free space for the period 1965 to 1992. The time interval covers several epochs of solar activity maxima, minima, and polar field reversals. We find that the amplitude of anisotropy fluctuates between 0.05% and 0.10%, except for the period 1971 to 1977 when it acquires a value as large as 0.25% at high primary rigidities (Rm > 16 GV). The physical cause of this anomalous behavior is not identified yet. The asymptotic time stays close to 0300 LT, independent of the primary rigidity (R). This direction is perpendicular to the nominal direction of interplanetary magnetic field (IMF) at Earth's orbit. The polarity of IMF has no effect on the semidiurnal anisotropy parameters. The physical significance of our results is discussed in the context of the theoretical ideas available in the literature. The contributions from the symmetric transverse gradients to semidiurnal anisotropy appear to be minimal, but those from pitch angle scattering may be appreciable.

Harmonics of exceptionaly low amplitude anisotropic wave train events in cosmic ray intensity

“…The variation is due to the second order anisotropy produced by the diffusion-convection of cosmic rays in interplanetary space [2,3]. Studies of the solar semi-diurnal variation have been made by many authors [4,5] to obtain information about solar modulation in various conditions of the heliosphere. Mori et al [6] and Nagashima et al [7] have investigated the existence of the tridiurnal variation, i.e., the third harmonic of daily variation in the recorded cosmic ray intensity.…”

A study of daily variation in cosmic ray intensity during high/low amplitude days

“…Recent data acquired by the Ulysses spacecraft during its fast heliolatitude scan shows that the latitude distribution of GCR has both symmetric and asymmetric components [10,11]. According to Ahluwalia and Fikani [12,13], the contribution of the symmetric transverse gradient to semi-diurnal anisotropy is minimal and the larger contribution comes from some other source(s). Nagashima's treatment also implies that semi-diurnal and tri-diurnal anisotropies have common features, and has been observed by many workers [14][15][16].…”

Influence of solar heliospheric parameters on cosmic rays anisotropy

“…Subramanian and Sarabhai [5] and Quenby and Lietti [6] both attributed the origin of semi-diurnal anisotropy to symmetric latitudinal cosmic ray density gradient in the heliosphere with particle density rising on both sides of the equatorial plane [7]. According to Ahluwalia and Fikani [11,12] the contribution of the symmetric transverse gradient to semi-diurnal anisotropy is minimal and the larger contribution comes from some other source(s). Recent data acquired by the Ulysses spacecraft during its fast heliolatitude scan shows that the latitude distribution of GCR has both symmetric and asymmetric components [9,10].…”