The apparent sidereal daily variation of cosmic-ray intensity at 42 m w. e. underground at Hobart, Tasmania

Abstract

The apparent sidereal daily variation of cosmic ray intensity underground at Hobart during 1958 is examined. There is a large annual mean apparent sidereal effect and a comparable anti-sidereal effect, associated with pronounced month by month changes in phase and amplitude of the solar daily variation. A technique is devised which permits the daily variation (d.v.) in sidereal time to be examined month by month, uncomplicated by the presence of certain background components, including for instance, a solar daily variation of constant phase and amplitude. An attempt is made to interpret the apparent sidereal effect on the assumption:a) that it is entirely spurious, arising from a solartime modulation of the solar daily variation;b) that there is a genuine part to the effect, consisting of a sidereal d.v. of constant phase and amplitude, and a spurious part, arising from an annual variation of amplitude of the solar d.v., produced by a solar-time mechanism;c) that the effect is entirely genuine, arising from a sidereal-time modulated sidereal daily variation. The theory for each case is tested against the observations by means of models. When various simple possibilities are considered, it is found that the evidence clearly favours assumptionc). The hypothesis which gives best agreement with the observations is that a solar d.v. of constant amplitude and phase is accompanied by a sidereal d.v. with semi-annual variation of amplitude and phase. In that case, the unmodulated sidereal d.v. would have amplitude 0.09% and RA∼0700 h, suggesting that the source of the anisotropy may be in the direction looking outward along the proposed spiral arm magnetic field.