Solar modulation of galactic cosmic radiation

Abstract

In this review an attempt is made to present an integrated view of the solar modulation process that cause time variation of cosmic ray particles. After briefly surveying the relevant large and small scale properties of the interplanetary magnetic fields and plasma, the motion of cosmic ray particles in the disordered interplanetary magnetic fields is discussed. The experimentally observed long term variations of different species of cosmic ray particles are summarised and compared with the theoretical predictions from the diffusion-convection model. The effect of the energy losses due to decelaration in the expanding solar wind are clearly brought out. The radial density gradient, the modulation parameter and their long term variation are discussed to understand the dynamics of the modulating region. The cosmic ray anisotropy measurements at different energies are summarised. At high energies (E> 1 GeV), the average diurnal anisotropy is shown to be energy independent and along the 18.00 h direction consistent with their undergoing partial corotation with the sun. The average semi-diurnal anisotropy seems to vary with energy as E +1 and incident from a direction perpendicular to the interplanetary field line, consistent with the semi-diurnal component being produced by latitudinal gradients. Both the diurnal and semi-diurnal components are shown to be practically time invariant. On a day to day basis, however, the anisotropy characteristics such as the exponent of variation, the amplitude and the phase show very high variability which are interpreted in terms of convection and variable field aligned diffusion due to the redistribution of the galactic cosmic ray density following transient changes in the interplanetary medium. The anisotropy observation at low energies (E< 100 MeV) are, however, not explained by the theory.The rigidity dependence and the anisotropies during short term variations such as Forbush decreases are discussed in terms of the proposed field models for the interplanetary field structure and are compared with the observed rigidity dependence of long term variations. The data pertaining to the 27 day corotating Forbush decreases and their association with enhanced diurnal variation are also presented. The relationship between the energetic storm particle events which are caused by the acceleration of particles in the shock fronts and the Forbush decreases which are caused by the exclusion of galactic particles by the enhanced field structure in the same fronts are clearly brought out. Thus the recurrent increases at low energies and recurrent decreases at high energies may both be caused by the field structure in the shock front. In conclusion, the properties of the very short period fluctuations (18-25 cph) are summarised.

Interplanetary flow systems associated with cosmic ray modulation in 1977–1980

“…One class of models considers that the basic process is scattering of energetic particles by random, small-scale disturbances in the interplanetary magnetic field (see the reviews by Fisk, 1980;Rao, 1972;and Quenby, 1983). Another class of models considers that the basic motions aro drifts in the ordered large-scale magnetic field (see Joki.pii et al, 1977;Jokipii and Thomas, 1981) .…”

Cosmic Ray Associated with Coronal Index and Solar Flare Index during Solar Cycle 22 - 23

“…The numbers of sunspots were generally used as a reliable parameter to measure solar activity [3] [4] [5]. The 11-year's variation in the intensity of cosmic ray observed on the The solar modulation of cosmic ray intensity has been studied in recent decades using data from ground based detectors, in combination with various solar activity and solar-terrestrial relations [7] [8] [9] [10] [11]. Galactic cosmic rays in the energy range from several hundreds MeV to tens of GeV are subjected to heliospheric modulation, under the influence of solar output and its variation.…”

Latitudinal and longitudinal dependence of the cosmic ray diurnal anisotropy during 2001–2014

“…The diurnal variation is due to complex phenomena, deriving from the convective-diffusive theory, which involves the radial convection of GCR flux by the solar wind and the inward diffusion along the interplanetary magnetic field (IMF) (Parker, 1964;Rao, 1972;Forman and Gleeson, 1975;Sabbah, 2013). An energy-independent anisotropic flow of CR particles in the 18 h co-rotational direction is generated due to the equilibrium between the convection and diffusion mechanisms (Krymsky, 1964;Rao, 1972;Mishra and Mishra, 2008).…”

“…An energy-independent anisotropic flow of CR particles in the 18 h co-rotational direction is generated due to the equilibrium between the convection and diffusion mechanisms (Krymsky, 1964;Rao, 1972;Mishra and Mishra, 2008). This can explain the long-term average, but not short-term variations in diurnal anisotropy (Yeeram and Saengdokmai, 2015).…”