Anisotropy Of Cosmic Ray Intensity Research Articles

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

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.

Solar cycle and 27-day variations of the diurnal anisotropy of cosmic rays during the solar cycle 23

The diurnal anisotropy of cosmic-ray intensity observed over the period 1997–2006, which coincides with the solar cycle 23, has been analyzed using cosmic ray data from Athens and Oulu neutron monitor stations. In this analysis it was observed that the time of the diurnal variation maximum shifted to earlier hours than the corotation direction from 1997 to 2000, where the polarity state of the magnetic field was positive ( $qA > 0$ ) and to later hours from 2001 to 2006 where the polarity state was negative ( $qA < 0$ ). Moreover the 27-day variation of the cosmic-ray diurnal anisotropy in connection with the 27-day variation of the interplanetary magnetic field has been studied. In this study six groups of ten Bartel rotations each one have been analyzed (2232–2241, 2249–2258, 2275–2284, 2286–2295, 2328–2337, 2356–2365). It is remarkable that the 27-day variation of cosmic- ray intensity is characterized by the well known sector structure and it is well correlated with the $B_{XY}$ component of the interplanetary magnetic field. These findings during the last solar cycle confirm once again the close relation of the diurnal variation of cosmic ray intensity and the interplanetary magnetic field.

Galactic Anisotropy of Cosmic Ray Intensity Observed by an Air Shower Experiment

AbstractWe have monitored multi-TeV cosmic rays by a small air shower array in Tehran (35°43′ N, 51°20′ E, 1200 m = 890 g cm−2). More than 1.1 × 106 extensive air shower events were recorded. These observations enabled us to analyse sidereal variation of the galactic cosmic ray intensity. The observed sidereal daily variation is compared to the expected variation which includes the Compton–Getting effect due to the motion of the earth in the Galaxy. In addition to the Compton–Getting effect, an anisotropy has been observed which is due to a unidirectional anisotropy of cosmic ray flow along the Galactic arms.

Average anisotropy characteristics of high energy cosmic ray particles and geomagnetic disturbance index Ap

The average characteristics of the diurnal and semi-diurnal anisotropy of cosmic ray intensity at relativistic energies have been obtained by using data from the worldwide grid of neutron monitor for the period 1989 to 1996. The complex behaviour of the diurnal amplitudes and time of maxima (phase) and its association with the Ap index on a long-term and day-to-day basis have been studied. Even though the general characteristics, on a yearly average basis, have not changed significantly during this period, both the diurnal and semi-diurnal amplitudes and phases vary significantly, besides significant changes being observed for different interplanetary conditions on a short-term basis. It is found that the relationship between the Ap index and the diurnal vector is out of phase during the period 1991 to 1995. On a long-term basis, the correlation of diurnal variation with Ap index has been found to vary during the solar cycle. On a short-term basis, it has been observed that the high Ap days are usually associated with higher amplitudes with phase shifted to earlier hours.

A northern sky survey for both TeV cosmic-ray anisotropy and TeV γ-ray sources using the Tibet air shower array

Results on 2-dimensional TeV cosmic ray anisotropy and γ -ray point source search using data taken from Tibet-HD (Feb.1997-Sep.1999) and Tibet-III (Nov.1999-Oct.2001) arrays are presented. From 0° to 60° in declination, large scale anisotropy of cosmic-ray intensity at a magnitude of about 0.1% is observed. In the mean while, the well-known steady source Crab Nebula and the high state of the flare type source Mrk421 are detected.

Application of diffusion — Convection model to diurnal anisotropy data

The diurnal anisotropy of cosmic-ray intensity observed over the period 1970–1977 has been analysed using neutron-monitor data of the Athens and Deep River stations. Our results indicate that the time of the maximum of diurnal variation shows a remarkable systematic shift towards earlier hours than normally beginning in 1971. This phase shift continued until 1976, the solar activity minimum, except for a sudden shift to a later hour for one year, in 1974, the secondary maximum of solar activity. This behavior of the diurnal time of maximum has been shown to be consistent with the convective- diffusive mechanism which relates the solar diurnal anisotropy of cosmic-rays to the dynamics of the solar wind and of the interplanetary magnetic field. Once again we have confirmed the field-aligned direction of the diffusive vector independently of the interplanetary magnetic field polarity. It is also noteworthy that the diurnal phase may follow in time the variations of the size of the polar coronal holes. All these are in agreement with the drift motions of cosmic-ray particles in the interplanetarty magnetic field during this time period.

Variability of the diurnal anisotropy of the cosmic ray intensity during the interval 1973–1978

The diurnal anisotropy of cosmic ray intensity has been investigated over the interval 1973–1978 by using neutron monitor data from Calgary, Deep River, Goose Bay, and Inuvik. It is observed that in general, the 27‐day variation in the diurnal anisotropy is greater for the component perpendicular to the nominal interplanetary magnetic field (IMF) in comparison with the parallel (or field aligned) component, which has very little ‘power’. However, the direction of the average anisotropy vector is aligned almost perpendicular (within the hour) to the IMF. The results are discussed in terms of alternate possibilities: the ‘power’ with 27‐day period and its harmonics arise from fluctuations in the amplitude aligned in the direction (a) of the total vector itself (≈ 16 hours) (b) perpendicular to the IMF.

Amplitude of diurnal anisotropy of cosmic ray intensity

Cosmic ray intensity mean diurnal variation, taking into account angle between solar corotational velocity and earth equatorial plane

Semidiurnal Anisotropy of Galactic Cosmic Ray Intensity

The semidiurnal variation of galactic cosmic ray intensity is investigated using data from mainly high counting rate neutron and meson monitors during 1964–1968. It is shown that in order to explain the observed semidiurnal variation it is necessary that an anisotropy of cosmic ray intensity be present in interplanetary space. The energy spectrum and the asymptotic latitude dependence of the anisotropy are then determined. The energy spectrum has a positive exponent close to + 1 for the power law in energy. The strength of the anisotropy decreases more rapidly than cosλ with increasing asymptotic latitude λ, both cos2λ and cos3λ being acceptable. The distribution of cosmic ray intensity in the range of heliolatitudes ± 7.25° at the orbit of the earth, obtained using data from the Ottawa neutron monitor, does not support the explanation of the semidiurnal variation based on the models of Subramanian and Sarabhai or Lietti and Quenby.

Diurnal anisotropy of cosmic ray intensity

Diurnal anisotropy of cosmic ray intensity