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<p>We investigated daily variations in the particle count rate recorded by the ground-level Tanca Cherenkov detector, which contains 11,400 liters of water and is located in the central region of the South Atlantic Anomaly (SAA). The dataset analyzed corresponds to the year 2017 and reveals two distinct types of modulation in the daily count rate. In this study, we focus on the first and smaller modulation, which occurs during the early morning hours. We show that this modulation can be attributed to Earth’s orbital motion around the Sun, known as the Compton–Getting (C–G) effect. Seasonal variations in the Compton–Getting (C–G) effect were identified, with an annual mean amplitude of (0.049&#xa0;±&#xa0;0.005)% and a phase at 03:37&#xa0;LT, about 3.5&#xa0;hours earlier than the expected 06:00&#xa0;LT. This phase advance is attributed to the East–West asymmetry in the primary cosmic-ray flux, as <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\sim \)</EquationSource> </InlineEquation>90% of the secondary particles detected by Tanca originate from primaries with energies up to 2&#xa0;TeV, still influenced by Earth’s magnetic field. A secondary modulation, observed in the afternoon, corresponds to the Solar Diurnal (SD) variation. These results provide a detailed characterization of the C–G modulation under the low-rigidity conditions of the South Atlantic Anomaly (SAA).</p>

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Compton–Getting Anisotropy in the South Atlantic Anomaly: Ground-Level Observations

  • 期刊论文

  • 发表日期：

    2025年12月12日

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  • R. de Aguiar,
  • A. C. Fauth,
  • C. E. Navia,
  • A. A. Nepomuceno,
  • M. N. de Oliveira

摘要

We investigated daily variations in the particle count rate recorded by the ground-level Tanca Cherenkov detector, which contains 11,400 liters of water and is located in the central region of the South Atlantic Anomaly (SAA). The dataset analyzed corresponds to the year 2017 and reveals two distinct types of modulation in the daily count rate. In this study, we focus on the first and smaller modulation, which occurs during the early morning hours. We show that this modulation can be attributed to Earth’s orbital motion around the Sun, known as the Compton–Getting (C–G) effect. Seasonal variations in the Compton–Getting (C–G) effect were identified, with an annual mean amplitude of (0.049 ± 0.005)% and a phase at 03:37 LT, about 3.5 hours earlier than the expected 06:00 LT. This phase advance is attributed to the East–West asymmetry in the primary cosmic-ray flux, as \(\sim \) 90% of the secondary particles detected by Tanca originate from primaries with energies up to 2 TeV, still influenced by Earth’s magnetic field. A secondary modulation, observed in the afternoon, corresponds to the Solar Diurnal (SD) variation. These results provide a detailed characterization of the C–G modulation under the low-rigidity conditions of the South Atlantic Anomaly (SAA).