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The Astrophysical Journal Volume 681 Number 1 Create an alert RSS this journal

Y. Okazaki et al. 2008 ApJ 681 693 doi:10.1086/588277

Drift Effects and the Cosmic Ray Density Gradient in a Solar Rotation Period: First Observation with the Global Muon Detector Network (GMDN)

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Y. Okazaki1, A. Fushishita2, T. Narumi2, C. Kato2, S. Yasue2, T. Kuwabara3, J. W. Bieber3, P. Evenson3, M. R. Da Silva4, A. Dal Lago4, N. J. Schuch5, Z. Fujii6, M. L. Duldig7, J. E. Humble8, I. Sabbah9, J. Kóta10 and K. Munakata2

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Abstract References Cited By
We present for the first time hourly variations of the spatial density gradient of 50 GeV cosmic rays within a sample solar rotation period in 2006. By inversely solving the diffusive flux equation, including the drift, we deduce the gradient from the anisotropy that is derived from the observation made by the Global Muon Detector Network (GMDN). The anisotropy obtained by applying a new analysis method to the GMDN data is precise and free from atmospheric temperature effects on the muon count rate recorded by ground-based detectors. We find the derived north-south gradient perpendicular to the ecliptic plane is oriented toward the heliospheric current sheet (HCS; i.e., southward in the toward sector of the interplanetary magnetic field [IMF] and northward in the away sector). The orientation of the gradient component parallel to the ecliptic plane remains similar in both sectors, with an enhancement of its magnitude seen after the Earth crosses the HCS. These temporal features are interpreted in terms of a local maximum of the cosmic ray density at the HCS. This is consistent with the prediction of the drift model for the A < 0 epoch. By comparing the observed gradient with the numerical prediction of a simple drift model, we conclude that particle drifts in the large-scale magnetic field play an important role in organizing the density gradient, at least in the present A < 0 epoch. We also found that corotating interaction regions did not have such a notable effect. Observations with the GMDN provide us with a new tool for investigating cosmic-ray transport in the IMF.

Subject headings

cosmic rays; solar wind; Sun: magnetic fields


Dates

Issue 1 (2008 July 1)

Received 2007 December 26, accepted for publication 2008 March 17



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