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The Galactic cosmic ray spectrum exhibits subtle variations over the 22 yr solar magnetic cycle in addition to the more dramatic variations over the 11 yr sunspot cycle. Neutron monitors are large ground-based detectors that provide accurate measurements of variations in the cosmic ray flux at the top of the atmosphere above the detector. At any given location the magnetic field of the Earth excludes particles below a well-defined rigidity (momentum per unit charge) known as the cutoff rigidity, which can be accurately calculated using detailed models of the geomagnetic field. By carrying a neutron monitor to different locations, e.g., on a ship, the Earth itself serves as a magnet spectrometer. By repeating such latitude surveys with identical equipment, a sensitive measurement of changes in the spectrum can be made. In this work, we analyze data from the 1994 through 2007 series of latitude surveys conducted by the Bartol Research Institute, the University of Tasmania, and the Australian Antarctic Division. We confirm the curious "crossover" in spectra measured near solar minima during epochs of opposite solar magnetic polarity, and show that it is directly related to a sudden change in the spectral behavior of solar modulation at the time of the polarity reversal, as revealed from contemporaneous variations in the survey data and a fixed station. We suggest that the spectral change and crossover result from the interaction of effects due to gradient/curvature drifts with a systematic change in the interplanetary diffusion coefficient caused by turbulent magnetic helicity.

Abstract. The September 29, 1989, cosmic ray ground level enhancement had a complex intensity-time profile with two distinct peaks observed at some neutron monitors. The event was detected by surface and some underground muon telescopes indicating the presence of particles up to •30 GV. We have modeled the response of neutron monitors and surface muon telescopes at the times of both intensity peaks and during the decay phase. We have determined particle rigidity spectra and pitch angle distributions centered on apparent arrival directions. Some evidence of bidirectional flow is present during the second peak. This may be due to backscattering of particles from beyond Earth. There was a high degree of scattering from all directions resulting in a significant isotropic component. Spectra derived from ground level responses are consistent with satellite observations. The specmma between a few MeV and a few tens of GeV is consistent with shock acceleration.

Abstract. We report the first systematic survey of cosmic ray precursors of geomagnetic storms. Our data set comprises the 14 "major" geomagnetic storms (peak Kp > 8-) identified by Gosling et al. [ 1990] together with 25 large storms (peak Kp > 7-) observed from 1992 through 1998. After eliminating events for which the muon detector network had poor coverage of the sunward interplanetary magnetic field (IMF) direction, we determined that 15 of the remaining 22 events (68%) had identifiable cosmic ray precursors with typical lead times ranging from 6 to 9 hours prior to the storm sudde.n commencement (SSC). Of the 15 precursors, 10 were of the "loss cone" (LC) type which is characterized by an intensity deficit confined to a narrow pitch angle region around the sunward IMF direction. Cosmic rays in the loss cone presumably originate in the cosmic-ray-depleted region downstream of the approaching shock. The remaining five precursors were of the "enhanced variance" (EV) type which is characterized by intensity increases or decreases that do not systematically align with the IMF direction. The incidence of precursors increases with storm size; for instance, 89% of storms with peak Kp greater than or equal to 8.0 had precursors. Our results show that the muon detector network can be a useful tool in space weather forecasting. However, new detector(s) installed to fill major gaps in the present network are urgently required for better understanding the nature of precursors and for reliable space weather forecasting. In this paper we examine cosmic ray precursors of large geomagnetic storms by analyzing high-energy cosmic ray intensities observed by ground-level muon detectors. As far as we know, this is the first systematic survey of cosmic ray precursors in which event selection is determined by the occurrence of a geomagnetic storm rather than by a Forbush decrease or other cosmic ray effect. Similar analysis using the high-latitude neutron monitor network will be reported hours prior to the shock arrival, assuming a shock speed of 600 km/s. This also implies that the precursors might be observed much earlier by muon detectors than by neutron monitors, as the muon detector responds to higher-energy (> 50 GeV) cosmic rays for which •.// is expected to be much longer. In this respect, the muon observations at higher-energy regions may supply useful information for forecasting geomagnetic storms. This is a primary motivation of the present analysis. It will be demonstrated that the muon observation network could be a very good tool for space weather forecasting if it has a good coverage of a full range of pitch angles. Data Analysis MethodWe analyze the cosmic ray precursors of geomagnetic storms recorded by a network of ground-level muon detectors at "Nagoya" (Japan), "Hobart" (Australia) and "Mawson-PC" (Antarctica) (see Table 1). Each of these detectors is multidirectional, allowing us to simultaneously record the intensities in various directions of viewing. The total number of directional telescopes used in this...

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 transport equation, including diffusion, 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