S. Sakakibara

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.

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.

The insensitivity of the cosmic ray galactic anisotropy to heliomagnetic polarity reversals

Abstract Using the cosmic ray sidereal and anti-sidereal diurnal variations observed underground in London and Hobart during the period 1958–1983, it is demonstrated that: (1) the phase changes of the apparent sidereal diurnal variation observed only in the Northern Hemisphere cannot be attributed to the change of the heliomagnetospheric modulation of galactic cosmic ray anisotropy caused by the polarity reversal of the solar magnetic field, but that they are due to the fluctuation of the spurious sidereal variation produced from the anisotropy responsible for the solar semi-diurnal variation; (2) the spurious sidereal variation can be eliminated from the apparent variation by using the observed anti-sidereal diurnal variation; and (3) after the elimination, the sidereal diurnal variations in the Northern and Southern Hemispheres almost coincide with each other and are stationary throughout the period, regardless of the polarity reversal of the heliomagnetosphere. The origin of the corrected sidereal variation is discussed.

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.

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 flare and neutron telescope

For the solar neutron event of June 3rd,1982, a new interpretation is given on ion acceleration mechanisms at the solar surface. The event can be explained by the impulsive acceleration of ions just after electron acceleration. For the June 4th,1991 solar neutron event, discrepancies in the absolute solar neutron fluxes measured by three different detectors are investigated using neutron beams at RCNP, Osaka university. We propose here an interpretation which explains all data consistently based on the premise that solar neutrons generate many low energy neutrons in the atmosphere by internuclear cascade processes.