Corotating Solar Wind Structures and Recurrent Trains of Enhanced Diurnal Variation in Galactic Cosmic Rays

Abstract

Data from the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand, with a vertical cutoff rigidity of 16.8 GV, were utilized to determine the diurnal anisotropy (DA) of Galactic cosmic rays (GCRs) near Earth during solar minimum conditions between 2007 November and 2010 November. We identified trains of enhanced DA over several days, which often recur after a solar rotation period (∼27 days). By investigating solar coronal holes as identified from synoptic maps and solar wind parameters, we found that the intensity and anisotropy of cosmic rays are associated with the high-speed streams (HSSs) in the solar wind, which are in turn related to the structure and evolution of coronal holes. An enhanced DA was observed after the onset of some, but not all, HSSs. During time periods of recurrent trains, the DA was often enhanced or suppressed according to the sign of the interplanetary magnetic field B, which suggests a contribution from a mechanism involving a southward gradient in the GCR density, n, and a gradient anisotropy along B × ∇n. In one non-recurrent and one recurrent sequence, an HSS from an equatorial coronal hole was merged with that from a trailing mid-latitude extension of a polar coronal hole, and the slanted HSS structure in space with suppressed GCR density can account for the southward GCR gradient. We conclude that the gradient anisotropy is a source of temporary changes in the GCR DA under solar minimum conditions, and that the latitudinal GCR gradient can sometimes be explained by the coronal hole morphology.

Energy Spectrum of the Recurrent Variation of Galactic Cosmic Rays During the Solar Minimum of Cycles 23/24

“…The average mass and kinetic energy of coronal mass ejections were around five times lower in comparison with the previous minimum (Vourlidas et al, 2010), diminishing one of the barriers for entrance of GCR particles into the inner heliosphere (Mewaldt, 2013). Zhao et al (2014) also stated that a low magnetic turbulence, increasing the parallel diffusion and reducing the perpendicular diffusion in the polar direction, might be another phenomenon giving rise to the observed higher GCR intensity.…”

“…Moreover, the heliosphere changes over time, and those changes influence the GCR stream on different time scales (e.g. Kudela, 2013;Yeeram et al, 2014). The recurrent variation of the galactic cosmic-ray intensity was described for the (Forbush, 1938) and is still of interest.…”

Measurement and simulation of neutron monitor count rate dependence on surrounding structure

“…A neutron monitor is a standard ground-based instrument for measuring time variations in the flux of galactic cosmic rays (GCRs) by detecting secondary particles (mostly neutrons) from their atmospheric showers. Ground-based measurements of GCR flux variations over 22 year solar magnetic cycles, 11 year sunspot cycles, and the Sun's 27 day rotation period, as well as the GCR anisotropy from diurnal count rate variations over the Earth's 1 day rotation period, provide unique and important information about the state of the heliosphere [Okazaki et al, 2008;Yeeram et al, 2014;Nuntiyakul et al, 2014]. Cosmic rays can reach a given location on Earth only if their rigidity (related to momentum per charge) exceeds the local geomagnetic cutoff rigidity.…”

Extended Cosmic Ray Decreases with Strong Anisotropy after Passage of Interplanetary Shocks

“…More generally, the anisotropy, which relates to the flow of GCRs in space, has long been known to vary with the ∼11 yr solar activity cycle, and provided the first indication of effects of the ∼22 yr solar magnetic cycle on cosmic rays (Thambyahpillai & Elliot 1953). The GCR flux and anisotropy at ∼1-100 GeV are also known to vary with the 27 day solar rotation period in association with high speed solar wind streams and reversals of the interplanetary magnetic field (Okazaki et al 2008;Yeeram et al 2014;Modzelewska & Alania 2018).…”