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Galactic cosmic-ray anisotropy and its modulation in the heliomagnetosphere, inferred from air shower observation at Mt. Norikura

  • Published:
Il Nuovo Cimento C

Summary

Extensive air showers (EAS) with median primary energy (E m) of (1013÷1015) eV have been observed since 1970 at Mt. Norikura (2770 m above sea level; geographic latitude 36.1°N, longitude 137.6°E) in order to study a cosmic-ray sidereal daily variation of galactic origin. We report here a summary of the observed results. EAS shows a significant sidereal diurnal variation with an amplitude (0.060±0.003)% and a phase (0.8±0.3) h sidereal local time forE m≈1.5·1013eV. Sidereal semi- and tridiurnal variations also are statistically significant. These variations are proved to be of galactic origin by a method which uses the difference of two directional (eastward and westward) air shower observations. It is found that these variations are subject, as predicted by Nagashimaet al., to the annual variation due to the heliomagnetospheric modulation of the galactic anisotropy, which dominates in the rigidity region ≈1012V, and further that the annual variation changes its phase due to the polarity reversal of the polar magnetic field of the Sun, which occurs at the period of the maximum solar activity. This indicates that the anisotropy is produced by the charged cosmic rays, contrary to the expectation from the γ-ray origin hypothesis suggested by Alexeenko and Navara. With the increase ofE m, the anisotropy seems to be reduced as we have not been able to detect any significant sidereal variation in EAS withE m≈2·1014eV. This seems contradictory to the conventional conclusion that the energy spectrum of the anisotropy is flat or slightly increases with energy.

Riassunto

Sono stati osservati sciami cosmici estensivi (EAS) con energia primaria mediana (E m) di (1013⋎1015) eV dal 1970 a Mt. Norikura (2770m, latitudine geografica 36.1° N, longitudine 137.6° E) per studiare una variazione gionarliera siderale dei raggi cosmici di origine galattica. Si riporta una sintesi dei risultati osservati. EAS mostra una significativa variazione siderale diurna con ampiezza (0.060±0.03)% e fase (0.8±0.3)h in tempo locale siderale perE m∼1.5·1013eV. Le variazioni siderali semi e tri-diurne sono risultate anch'esse statisticamente significative. Si prova che queste variazioni sono d'origine galattica con un metodo che utilizza la differenza tra due osservazioni di sciami cosmici direzionali (est e ovest). Si trova che queste variazioni sono soggette, come previsto da Nagashimaet al., alla variazione annuale dovuta alla modulazione eliomagnetica dell'anisotropia galattica, che domina nella regione di rigidità ∼1012V e inoltre che la variazione annuale cambia fase a causa dell'inversione di polarità del campo magnetico polare del Sole, chesi verifica nel periodo di massima attività solare. Ciò indica che l'anisotropia è prodotta da raggi cosmici carichi, contrariamente a quanto previsto dall'ipotesi sull'origine da raggi gamma, suggerita da Alexeenko e Navara. Con l'aumento diE m, l'anisotropia sembra ridursi in quanto non siamo riusciti a rilevare una variazione siderale significativa in EAS conE m∼2·1014 eV. Questo risultato sembra in contraddizione con la conclusione convenzionale che lo spettro di energia dell'anisotropia è piatto o cresce appena con l'energia.

Резюме

Широкие атмосферные ливни со средней первичной энергией (E m), равной (1013⋎1015) зВ, наблюдались с 1970 года на г. Нарикура (2770 м над уровнем моря; географическая широта 36.1°N, долгота 137.6°E), с целяю исследовать суточное изменение сидерических космических лучей галактической природы. Предлагается обзор полученных результатов. Широкие атмосферные ливни обнаруживают значительное сидерическое суточное изменение с амплитудой (0.060±0.003)% и фазой 0.8±0.3 часа сидерического времени дляE m∼1.5·1013 эВ. Сидерические полу-и трех-суточные изменения являются также статистически заметными. Доказывается, что эти изменения имеют галактическое происхождение. Этот результат получен с помощью метода, который использует разности при наблюдениях атмосферных ливней в двух направлениях (в восточном и западном направлениях). Получено, что эти вариации подчиняются, как было предсказано Нагашима и др., годовому изменению, обусловленному гелиомагнитосферной модуляцией галактической анизотропии, которая доминирует в области жесткости ∼1012 D, и кроме того, годовая вариация изменяет фазу, в связи с реверсированием полярности магнитного поля Солнца, котороепроисходит в период максимума солнечной активности. Этот реультат показывает, что возникает анизотропия, вызванная заряженными космическими лучами, в противоположность рредсказанию Алексеенко и Навара, связанному с гипотезой происхождения γ-лучей. При увеличенииE m анизотропия, повидимому, уменьшается, так как мы не смогли зарегистрировать какого-либо существенного сидерического изменения в пироких атмосферных ливнях в случаеE m∼2·1014эВ. Это противоречит общепринятому заключению, что энергетический спектр анизотропии является пологим или слегка увеличивается с энергией.

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Author notes

  1. I. Morishita

    Present address: School of Liberal Arts, Asahi University Hozumi, 501-02, Gifu, Japan

Authors and Affiliations

  1. Cosmic-Ray Research Laboratory, Faculty of Science, Nagoya University, 464, Nagoya, Japan

    K. Nagashima, K. Fujimoto, S. Sakakibara, Z. Fujii, H. Ueno, I. Morishita & K. Murakami

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Nagashima, K., Fujimoto, K., Sakakibara, S. et al. Galactic cosmic-ray anisotropy and its modulation in the heliomagnetosphere, inferred from air shower observation at Mt. Norikura. Il Nuovo Cimento C 12, 695–749 (1989). https://doi.org/10.1007/BF02511970

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