A measurement of the solar and sidereal cosmic-ray anisotropy at e₀ ∼ 10¹⁴ eV

The results of the measurement of the cosmic-ray solar and sidereal anisotropies at primary energy E₀ ≈ 10¹⁴ eV performed by the EAS-TOP Extensive Air Shower array (Campo Imperatore, National Gran Sasso Laboratories, 2005 m above sea level, latitude 42°.5 N are presented. The measurement includes 4 years of data taking (1990, 1992, 1993, 1994) for a total of 1.3 × 10⁹ events and is performed at two different mean primary energies: E₀^v ≈ 1.5 × 10¹⁴ eV and E₀ⁱ ≈ 2.5 × 10¹⁴ eV. The two results are compatible (within 2 σ) and can therefore be combined. The obtained amplitude and phase of the first harmonic in sidereal time are (in the equatorial plane) A_{sid, δ=0}, (E₀ ≈ 2 × 10¹⁴ eV) = (3.73 ± 0.57) × 10^-4 and _sid = 1.82 ± 0.49 hr local sidereal time, with significance 6.5 a. The amplitude of the anisotropy exhibits the expected cos δ dependence. A first harmonic in solar time compatible with the expected Compton-Getting effect due to the motion of revolution of the Earth around the Sun is observed with significance 7.3 σ. The corresponding measured amplitude and phase (also in the equatorial plane) are A_{sol, δ=0°} = (4.06 ± 0.55) × 10^-4 and _sol = 4.92 ± 0.53 hr, the expected values being 4.7 × 10^-4 and 6.0 hr. Different checks of stability of the detectors and consistency of the data are presented.