Diurnal anisotropy of polar neutron monitors: Dome C looks poleward

Abstract

Galactic cosmic rays (GCR) exhibit a small local anisotropy observed as diurnal variability of ground-based neutron monitor (NM) count rates. Since the asymptotic directions of various NMs are different, their ability to observe the GCR diurnal variation also varies. Here we show that the Dome C (DOMC) NM is hardly sensitive to the diurnal variation, with its amplitude being 0.03%, in contrast to other polar NMs whose sensitivity to the diurnal variability ranges from 0.16 to 0.4%. We argue that this is related to the fact that DOMC NM has a narrow asymptotic-direction cone looking nearly to the South pole (geographic latitude above 75°). This makes the DOMC NM a unique detector being the only existing NM accepting cosmic-ray particles originating from the off-equatorial region. This is important for detailed studies of cosmic-ray transport in the vicinity of Earth, specifically for anisotropic solar energetic particle events.