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...

The cosmic ray north-south anisotropy arising from the heliocentric radial density gradient of cosmic rays and the interplanetary magnetic field has been studied using data for a wide range of rigidity (15-210 GV in median primary rigidity) during the period 1969-1973. Two components of the anisotropy; the north-south asymmetry and the associated sidereal diurnal variation have been analyzed to reveal a three-dimensional structure of the anisotropy in space. By means of the least-squares method and using improved coupling coefficients, the direction and certain other parameters of the anisotropy have been determined. Definite evidence has been obtained for the existence of anisotropic flow perpendicular to the ecliptic plane. The magnitude of the flow is found to be 0. 0810. 021% and 0. 072+0. 018% at 10 GV for the periods 1969-1970 and 1971-1973, respectively. It is also found that its rigidity spectrum is slightly rigidity dependent and has an upper limiting rigidity of 200 GV or more. Based on the derived parameters of the anisotropy, the heliocentric radial density gradient for high rigidity cosmic rays has been estimated as a function of rigidity for 1969-1973 as (3.01.1) x (P/10)-(7.2%/AU for P<200 GV (P is rigidity in GV). The diffusion coefficient is also derived, which seems to be consistent with those so far determined.

[1] A coronal mass ejection (CME) associated with an X17 solar flare reached Earth on October 29, 2003, causing an $11% decrease in the intensity of high-energy Galactic cosmic rays recorded by muon detectors. The CME also produced a strong enhancement of the cosmic ray directional anisotropy. Based upon a simple inclined cylinder model, we use the anisotropy data to derive for the first time the three-dimensional geometry of the cosmic ray depleted region formed behind the shock in this event.We also compare the geometry derived from cosmic rays with that derived from in situ interplanetary magnetic field (IMF) observations using a Magnetic Flux Rope model.

We have analyzed the yearly averaged sidereal daily variations in the count rates of 46 underground muon telescopes by fitting Gaussian functions to the data. These functions represent the loss cone and tail‐in anisotropies of the sidereal anisotropies model proposed by Nagashima et al. [l995a, b]. The underground muon telescopes cover the median rigidity range 143–1400 GV and the viewing latitude range 73°N–76°S. From the Gaussian amplitudes and positions we have confirmed that the tail‐in anisotropy is more prominent in the southern hemisphere with its reference axis located at declination (δ) ∼14°S and right ascension (α) ∼4.7 sidereal hours. The tail‐in anisotropy is asymmetric about its reference axis, and the observed time of maximum intensity depends on the viewing latitude of the underground muon telescopes. We also find that the declination of the reference axis may be related to the rigidity of the cosmic rays. We show that the loss cone anisotropy is symmetric and has a reference axis located on the celestial equator (δ ∼ 0°) and α ∼ 13 sidereal hours. We have used the parameters of the Gaussian fits to devise an empirical model of the sidereal anisotropies. The model implies that the above characteristics of the anisotropies can explain the observed north‐south asymmetry in the amplitude of the sidereal diurnal variation. Furthermore, we find that the anisotropies should cause the phase of the sidereal semidiurnal variation of cosmic rays to be observed at later times from the northern hemisphere compared to observations from the southern hemisphere. We present these results and discuss them in relation to current models of the heliosphere.

[1] We have developed a method for determining interplanetary coronal mass ejection (ICME) geometry from galactic cosmic ray data recorded by the ground-based muon detector network. The cosmic ray density depression inside the ICME, which is associated with a Forbush decrease, is represented by an expanding cylinder that is based on a theoretical model of the cosmic ray particle diffusion. ICME geometry and orientation are deduced from observed time variations of cosmic ray density and density gradient and are compared with those deduced from a magnetic flux rope model. From March 2001 to May 2005, 11 ICME events that produced Forbush decreases >2% were observed, and clear variations of the density gradient due to ICME passage were observed in 8 of 11 events. In five of the eight events, signatures of magnetic flux rope structure (large, smooth rotation of magnetic field) were also seen, and the ICME geometry and orientation deduced from the two methods were very similar in three events. This suggests that the cosmic ray-based method can be used as a complementary method for deducing ICME geometry especially for events where a large Forbush decrease is observed.