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

Anastasia Tezari,Anastasios Kanellakopoulos,Sofia Kolovi,Christina Plainaki,Dimitrios Katsinis,Maria Andriopoulou,Helen Mavromichalaki

doi:10.5194/angeo-34-1053-2016

Abstract

Abstract. The diurnal anisotropy of cosmic ray intensity for the time period 2001 to 2014 is studied, covering the maximum and the descending phase of solar cycle 23, the minimum between solar cycles 23 and 24, and the ascending phase and maximum of solar cycle 24. Cosmic ray intensity data from 11 neutron monitor stations located at different places around the Northern Hemisphere obtained from the high-resolution Neutron Monitor Database (NMDB) were used. Special software was developed for the calculations of the amplitude and the phase of the diurnal anisotropy vectors on annual and monthly basis using Fourier analysis and for the creation of the harmonic dial diagrams. The geomagnetic bending for each station was taken into account in our calculations determined from the asymptotic cones of each station via the Tsyganenko96 (Tsyganenko and Stern, 1996) magnetospheric model. From our analysis, it was resulted that there is a different behavior of the diurnal anisotropy vectors during the different phases of the solar cycles depending on the solar magnetic field polarity. The latitudinal and longitudinal distribution of the cosmic ray diurnal anisotropy was also examined by grouping the stations according to their geographic coordinates, and it was shown that diurnal variation is modulated not only by the latitude but also by the longitude of the stations. The diurnal anisotropy during strong events of solar and/or cosmic ray activity is discussed.

Highlights

The spatial anisotropy of the galactic cosmic radiation (GCR) in the interplanetary medium is observed as the daily variation in cosmic ray (CR) intensity which is recorded by ground-based detectors
As CRs enter the heliosphere and gyrate along interplanetary magnetic field (IMF) line, various anomalies of the magnetic flux scatter them from their gyro-orbits, resulting in diffusion
At the same time the solar wind converts the CR outward, causing particles traveling along a regular portion of magnetic field lines to undergo magnetic curvature and drifts

Summary

Introduction

The spatial anisotropy of the galactic cosmic radiation (GCR) in the interplanetary medium is observed as the daily variation in cosmic ray (CR) intensity which is recorded by ground-based detectors. The detectors scan through different portions of the CR angular distribution with a 1-day period The projection of this anisotropy on the ecliptic plane may be observed as diurnal anisotropy (Yeeram and Saengdokmai, 2015). The intensity of GCR recorded by ground-based neutron monitors (NMs) shows periodic and abrupt changes as a function of space, time, and energy (Oh et al, 2010). This phenomenon, which is known as the diurnal anisotropy of CR intensity, is a localtime short-term variation (Pomerantz and Duggal, 1971; Ahluwalia, 1988). The amplitude and the time of maximum of the CR diurnal anisotropy vectors during the different phases of SCs 23 and 24 and during intense CR events are examined and discussed

Data analysis

Geomagnetic bending

Diurnal anisotropy during the solar cycle

Diurnal anisotropy during strong cosmic ray events

Findings

Discussion and conclusions