K. Fujimoto

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.

Galactic and heliotail‐in anisotropies of cosmic rays as the origin of sidereal daily variation in the energy region < 104 GeV

It is shown that the cosmic ray sidereal daily variation in the energy region <EU ∼ 104 GeV is due to two kinds of anisotropy, one is a galactic anisotropy from the direction with right ascension αG = 0 hours and declination δG = −20° and can be observed even in the energy region as low as ∼60 GeV with the same form and almost the same phase as observed by small air showers with ∼EU [Nagashima et al., 1989]. The other is a newly discovered directional excess flux confined in a narrow cone with a half opening angle of ∼68° from the direction (αT ∼ 6.0 hours; δT ∼ −24°) and observed only in the energy region less than EU with its maximum near 103 GeV. It is suggested that the excess flux is of solar origin and the direction toward it seems to coincide with the expected heliomagnetotail direction (αTP = 6.0 hours; δTP = −29.2°) opposite the proper motion of the solar system but does not coincide with the expected tail direction (αTN = 4.8 hours; δTN = 15° ∼ 17°) opposite to the relative motion of the system to the neutral gas. The flux (called the tail-in anisotropy hereafter) shows maximum at the December solstice when the Earth is closest to the magnetotail and almost disappears at the remote side of the Earths orbit from the tail at the June solstice. Owing to the discovery of the tail-in anisotropy, the observed phase shift of the sidereal diurnal (24 hours) variation from 6 to 0 hours with the increase of energy, which has been one of the unsolved problems, can be explained by the distinctive contributions from the two anisotropies. Finally, it appears that the observed sidereal variations deny the existence of the Compton-Getting effect due to the motion of the solar system at least in the energy region less than ∼EU. This implies that the solar system drags with it in its motion the surrounding interstellar magnetic field within which the cosmic rays with low energy (less than ∼EU) are isotropically confined.

Local-time-dependent pre-IMF-shock decrease and post-shock increase of cosmic rays, produced respectively by their IMF-collimated outward and inward flows across the shock responsible for Forbush decrease

Abstract The cosmic-ray storm known as “Forbush decrease” is produced generally as a result of the transient diffusion-convection of cosmic rays caused by the passage of the interplanetary magnetic field (IMF) shock wave. It is emphasized, however, that the storm is frequently accompanied by non-diffusion-convection-type phenomena. In the present paper, the authors show the existence of such phenomena, which are dependent on local time. These are: (1) the precursory decrease of cosmic-ray intensity in front of the shock, which occurs in the morning (6–12 h), having nearly the same rigidity spectrum as that of the Forbush decrease; and (2) the post-shock increase, which belongs to the daily variation in general, but bears the following anomalous characters; a steep peak as high as the pre-storm intensity level, an extremely soft rigidity spectrum and a phase of 18–24 h in space considerably later than the usual. It is concluded that the precursory decrease is produced by the IMF-collimated outward flow of the low-density cosmic rays from the inside of the shock, and that the collimation is determined by the ratio between the ordered magnetic fields at the shock front and at the observation point. Inversely, the post-shock increase is produced by the IMF-collimated inward flow of the high-density cosmic rays from the outside. As an extreme case of the above phenomena, they also point out the existence of the IMF-guided square wave of cosmic-ray intensity with 24 h periodicity, which is produced as a result of the Earths rotation in the unbalanced two-way flows along the magnetic lines of force connecting two separated regions occupied, respectively, by the high- and low-density cosmic rays. Finally, a serious influence of the precursory decrease on the determination of the commencement of Forbush decrease and also on the study of the precursory increase expected to appear in front of the shock wave, is discussed on the basis of definite examples.

Gaussian analysis of two hemisphere observations of galactic cosmic ray sidereal anisotropies

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.

Localized pits and peaks in forbush decrease, associated with stratified structure of disturbed and undisturbed magnetic fields

SummaryForbush decrease (FD) is generally interpreted as a result of diffusion-convection of cosmic rays in a disturbed interplanetary magnetic field associated with the magnetohydrodynamic shock wave caused by solar flare. In this paper, we point out that a large number of FDs contain an isolated region or regions with pit-type time profile, in which cosmic rays are not in a diffusion-convection state but in a trapped state in undisturbed, uniform and strong magnetic field perpendicular to the solar wind. The trapped state is also characterized with a large ratio of the magnetic to ion thermal energy. The median duration time of the state is about 8 hours. About half of these states are associated with the northward (or southward) magnetic field, while the other half with the eastward (or westward) magnetic field. Flares responsible for the former state seem to be concentrated in an eastward region from about 30°W on the solar disk, while those for the latter state seem rather symmetric with respect to the centre of the solar disk. It is suggested that the trapped state is produced inside a magnetic tube of force which is not of a small scale such as that of the magnetic bubble pointed out by Klein and Burlaga, but of a large scale, having a horseshoe structure with its ends supposed to be connected to somewhere in an inner region near the Sun and with its cross-section supposed to be of a thin filament with radial and transverse dimensions of ≈0.1 a.u. and ≈1.1 a.u. at the Earth’s orbit. This belt-like tube of force is supposed to be produced on the solar surface or near the Sun and to be carried out by solar wind in a frozen state, trapping in itself low-density cosmic rays near the Sun. In addition to the pits, we point out also the existence of some peaks which are observed not only in the trapped region but also in a region of extremely disturbed magnetic field neighbouring in between two trapped regions. It is suggested that cosmic rays in the region of the latter type are supposed to be guided freely (or easily) from outer space through a path with similarly disturbed magnetic state, and therefore, they could maintain their density in the region always higher than in the neighbouring regions. Two kinds of cosmic-ray-guiding mechanism in the above can be regarded as being at opposite poles.

Nature of solar-cycle and heliomagnetic-polarity dependence of cosmic rays, inferred from their correlation with heliomagnetic spherical surface harmonics in the period 1976–1985

Abstract Correlation of cosmic-ray intensity (I) with the solar magnetic field expanded into the spherical surface harmonics, B n s (n⩽ 9) , by Hoeksema and Scherrer has been studied using the following regression equation: I(t)=A 0 + ∑ i=1 3 A i X (t−τ i ) , where X i s are subgroups of B n s classified in ascending order of n, and τi is the time lag of I behind correlation coefficient between the observed and simulated intensities (Iobs, Isml) in the period 1976–1985 is ∼0.87 and considerably better than that derived from any single index of solar activity. The lag time τ3 is greater than others, indicating that the higher order magnetic disturbances effective to the cosmic-ray modulation have a longer lifetime in space than the lower order disturbances. The rigidity spectrum of the cosmic-ray intensity variation responsible for AI due to the dipole moment is harder than those for others (A2,A3), indicating that the lowest order (i.e. largest scale) magnetic disturbances can modulate cosmic rays more effectively than the higher order disturbances. As another result of the present analysis, it has been found that the intensity depends also on the polarity of the polar magnetic field of the Sun; the residual (Iobs−Isml) of the simulation changes its sign from positive to negative with a time lag (0–5 Carrington rotation periods) behind the directional change of the solar magnetic dipole moment from northward to southward, and has a softer rigidity spectrum than AiS. The dependence is consistent with the result having been obtained in the previous period, 1936–1976, by one (K.N.) of the present authors. The polarity dependence can be found also in the 22-year variation of the time lags obtained every solar cycle in the period 1936–1985. The theoretical interpretation of these polarity dependences is discussed on the basis of the diffusion-convection-drift model.

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.

Galactic cosmic-ray anisotropy and its heliospheric modulation, inferred from the sidereal semidiurnal variations observed in the rigidity range 300–600 GV with multidirectional muon telescope at Sakashita underground station

The existence of sidereal semidiurnal variation of cosmic-ray intensity in a rigidity region 102-103 GV has been reported by many researchers, but there is no consensus of opinion on its origin. In this paper, using the observed semidiurnal variations in a rigidity range (300–600 GV) with 10 directional muon telescopes at Sakashita underground station (geog. lat. = 36°, long. = 138°E, depth = 80 m.w.e.), the authors determine the magnitudes (η1, η2) and directions (a1, a2) of the first- and second-order anisotropies in the following galactic cosmic-ray intensity distribution (j) jdp = j0{1 + η1P1(cos χ1) + η2P2(cos χ2)}dp , where Pnis the nth order spherical function and χn is the pitch angle of cosmic rays with respect to an. For the determination, the influence of cosmic-rays heliomagnetospheric modulation, geomagnetic deflection and nuclear interaction with the terrestrial material and also of the geometric configuration of the telescopes are taken into account. Usually, the semidiurnal variation is produced by the second-order anisotropy. The present observation, however, requires also the first-order anisotropy which usually produces only the diurnal variation, but can produce also the semidiurnal variation as a result of the heliospheric modulation. The first- and second-order anisotropies are characterized with η1) > 0 and η2 < 0 have almost the same direction (a1 ∼ a2) specified by the right ascension (α ∼ 0.75 h) and declination (δ ∼ 50°S) and, therefore, they can be expressed, as a whole, by an axis-symmetric anisotropy of loss-cone type (i.e. deficit intensities in a cone). It is noteworthy that this anisotropy approximately coincides with that inferred from the air shower observation at Mt Norikura in the rigidity region ∼ 104 GV.

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.