M. Koyama

Precursors of geomagnetic storms observed by the muon detector network

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 sudden 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.

Geometry of an interplanetary CME on October 29, 2003 deduced from cosmic rays

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 rime 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. Copyright 2004 by the American Geophysical Union.

Preliminary analysis of two‐hemisphere observations of sidereal anisotropies of galactic cosmic rays

By using the two-hemisphere network of underground muon telescopes we have examined the average sidereal daily variations in the count rates recorded by 48-component muon telescopes. The telescopes respond to primary cosmic rays with rigidities between ∼140 and 1700 GV and view almost the entire celestial sphere. We have modeled the data by using Gaussian functions, and we have related the Gaussian parameters to the recent tail-in and loss cone anisotropy model proposed by Nagashima et al. [1995a, b] to explain the sidereal daily variations. We have used the model parameters to derive the rigidity and latitude spectra of the galactic anisotropies and find them to be qualitatively in agreement with Nagashima et al.s predictions. The results indicate, however, that the tail-in anisotropy is asymmetric about its reference axis, whereas the loss cone anisotropy is more symmetric. We show that these characteristics of the galactic anisotropies may explain the north–south asymmetry observed in the amplitude of the sidereal diurnal variation derived from Fourier analysis techniques.

LONG TERM VARIATION OF COSMIC RAY LATITUDE GRADIENT IN THE HELIOSPHERE

Abstract We examine the long-term change in the unidirectional latitude gradient ( G θ ) of galactic cosmic-rays in the heliosphere, by analyzing the “Toward-Away” solar diurnal variation (SDV) of cosmic-ray intensity recorded by a network of Japanese multi-directional muon telescopes during 18 years from 1978 to 1995. In our analysis, we take into account not only the north-south (NS) symmetric SDV ( S sym ) but also the NS anti-symmetric SDV ( S anti - sym ), which was first observed by the Nagoya surface muon telescope in 1971–1979 and well confirmed by the two hemisphere observations at Nagoya and Hobart in 1992–1995. The phase of the yearly mean S sym in space is found at ∼0500 or ∼1700 hours local solar time depending on the year, while the phase of S anti - sym is always found at ∼1700 hours in the northern hemisphere. G θ derived from the component of S sym perpendicular to the interplanetary magnetic field shows no clear variation related to the 11-year solar activity- or 22-year solar magnetic-cycles, but it remains positive after the late 80′s implying a higher density of cosmic-rays in the southern hemisphere below the heliospheric current sheet.

Enhanced sidereal diurnal variation of galactic cosmic rays observed by the two-hemisphere network of surface level muon telescopes

Significant enhancements of the cosmic ray sidereal diurnal variation were observed during the period 1992–1995 by the two-hemisphere network of surface-level multidirectional muon telescopes at Hobart (Tasmania, Australia) and Nagoya (Aichi, Japan). The telescopes cover the primary cosmic ray rigidity range of 50–120 GV. Since the enhancement is less prominent in the higher rigidity range (150–550 GV) covered by the shallow underground observations at Misato and Sakashita, it is concluded that the enhancement was caused by significant solar modulation in the lower energy region. Observed sidereal diurnal variations, corrected for spurious variations by a procedure proposed by Nagashima, give a space harmonic vector with amplitude of 0.104 ± 0.008% at 60 GV and maximum at 6.9 ± 0.3 hour local sidereal time. The time of maximum is consistent with northward streaming of cosmic rays perpendicular to the ecliptic plane. Such a north–south anisotropy is expected from cross-field ξNS = − λ⊥ Gθ diffusion if both the cross-field mean-free-path λ⊥ and the southward directed unidirectional latitudinal density gradient Gθ have large enough magnitudes. It is shown that the sector-dependent solar diurnal variations are also enhanced in the period, consistent with Gθ being directed south of the ecliptic plane. Magnitudes of Gθ and λ⊥ derived from the observations are discussed.

SOLAR CYCLE VARIATIONS OF MODULATION PARAMETERS OF GALACTIC COSMIC-RAYS IN THE HELIOSPHERE

Abstract Solar cycle variations of modulation parameters are derived from cosmic-ray anisotropy observed by a network of multidirectional muon telescopes. The network covers wide ranges of median rigidity of primary cosmic-rays and effective latitude of viewing. It was found that the radial density gradient varies with a good correlation with the solar activity, while the parallel mean-free-path of the cosmic-ray diffusion varies with an anti-correlation with the solar activity. These features are both in accord with the conventional modulation theory incorporating convection and diffusion processes. The correlation coefficients of yearly mean values of radial density gradient and parallel mean-free-path with the sunspot number were respectively 0.7 and 0.6. The bi-directional latitudinal gradient showed a clear 22-year solar magnetic cycle as predicted by the drift model for the cosmic-ray transport in the heliosphere. The unidirectional latitudinal gradient, on the other hand, showed no clear variation related to the 11-year solar activity or 22-year solar magnetic-cycles, but it remains positive after the late 80s implying a higher density of cosmic-rays in the southern hemisphere below the heliospheric current sheet. We also analyze temporal variations of modulation parameters derived from neutron monitor observations at ∼10 GV. By comparing with those obtained from muon observations at 60 GV, we discuss the rigidity dependence of temporal variations of modulation parameters.

Solar semidiurnal anisotropy of galactic cosmic ray intensity observed by the two‐hemisphere network of surface‐level muon telescopes

Observations made by the two-hemisphere network of surface-level, multidirectional muon telescopes at Hobart (Tasmania, Australia) and Nagoya (Aichi, Japan) are used to examine the origin of the solar semidiurnal variation in cosmic ray intensity. The network allows us to precisely determine the asymmetry of the variation across both hemispheres. It is shown that the variation is consistent with the north–south (NS) symmetric distribution of cosmic ray intensity in space. The phase of the space harmonic vector responsible for the variation is consistent with both the second-order anisotropy expected from a bidirectional latitudinal density gradient (type I) and also one arising from pitch angle scattering (type II). The network also observed a purely NS antisymmetric, antisidereal diurnal variation with the maximum phases differing by 12 hours between the two hemispheres. This is consistent with an antisidereal diurnal variation arising from annual modulation of the solar diurnal variation produced by a second-order anisotropy. The phase of the space harmonic vector responsible for the antisidereal diurnal variation is consistent with the phases predicted from both type I and type II anisotropies. It is shown, however, that the ratio of the amplitude of the space harmonic vector of the antisidereal diurnal variation to that of the solar semidiurnal variations is consistent with the type II anisotropy but not with the type I anisotropy. This result implies that the solar semidiurnal variation and the antisidereal diurnal variation observed during the period 1992–1995 mainly arise from the type II anisotropy and cannot be explained solely as arising from the type I anisotropy.

Sidereal daily variation of galactic cosmic rays

Abstract We have analyzed the yearly averaged sidereal daily variations in the count rates of 44 underground muon telescopes by fitting Gaussian functions to the data. These functions represent the Loss-cone (LC) and Tail-in (TI) anisotropies proposed by Nagashima et al. . The telescopes cover the median rigidity range 143GV–1400GV and the viewing latitude range 73°N–76°S. We find that the TI anisotropy has its reference axis located at declination ( δ ) ≈ 14°S and right ascension ( α ) ≈ 4.7 sidereal hours. We show that the TI anisotropy is asymmetric about the reference axis and its observed α depends on the viewing latitude of the telescopes. We also show that the LC anisotropy is symmetric and has a referenceaxis located at the celestial equator and α ≈ 13 sidereal hours. From the parameters of the Gaussian fits we devise an empirical model of the sidereal anisotropies which implies that the above characteristics of the anisotropies can explain the north-south asymmetry in the amplitude of the sidereal diurnal variation. Furthermore, we find that the phase of the sidereal semi-diurnal variation of cosmic rays should be recorded at later times when measured from the northern hemisphere compared to observations made from the southern hemisphere.