Studies of the sidereal daily variation of cosmic ray intensity with particular reference to observations underground
Creators: name: Jacklyn, RM
Publisher: name: University Of Tasmania
Year: 2023

description: 1.1 Introductory Remarks There is now considerable evidence that the charged primary cosmic radiation to which detectors at the earth respond is essentially isotropic in free space, at least in the seven decades of rigidity from 10\\(^{10}\\) v to 10\\(^{17}\\) v. In fact, many careful investigations over some thirty years have failed to produce convincing evidence for even a small excess of charged particle flux from any fixed galactic direction. More particularly, if the amplitude of the observed anisotropy is specified as A = I\\(_{max}\\) - I\\(_{min}\\) / I\\(_{max}\\) + I\\(_{min}\\) , there would appear to be zero anisotropy, with a margin of uncertainty of amplitude varying from less than 0.2% at the lowest primary rigidities to about 3% at the highest rigidities. Nevertheless, small anisotropies have been predicted, of special interest being those that should be associated with the propagation of cosmic rays through the galaxy in the presence of large-scale magnetic fields. In the first part of this thesis, up to the end of Chapter 5, evidence is given that there is indeed a small sidereal anisotropy, such as would be expected to be associated specifically with a relatively uniform local galactic magnetic field (see section 2 below). The evidence derives from daily variations of intensity, obtained in the main from meson telescopes located at depths of approximately 40 m.w.e. underground, where it is believed most of the response would be due to primary protons in the rigidity range 5 x 10\\(^{10}\\) v - 10\\(^{12}\\) v. The observations differ in a number of respects from those obtained at ground level. Of particular note is the fact that the amplitude of the solar daily variation is much smaller, and that the characteristics of the apparent sidereal effect reproduce themselves remarkably well from year to year. The initial problem is to determine to what extent the observed sidereal effect is due to a sidereal anisotropy, extracting, if necessary, spurious contributions that may have been generated by systematic modulation of solar components of the daily variation. If, as a result, there is significant evidence for a genuine sidereal component, it should be possible to ascertain the general character of the anisotropy, provided that observations can be Obtained from several well-separated latitudes of viewing, preferably spanning both hemispheres. The remaining chapters lead to estimates of the mean direction of the anisotropy in free space beyond the earth's field region, using assumed values for the index of the variation spectrum and for the expected cut-off rigidity due to the interplanetary magnetic field. While the time of maximum of the observed sidereal effect, after removal of any spurious sidereal components, gives some indication of the right ascension, the declination is not approximated by the latitude of maximum amplitude that might be obtained from a latitude survey, being in fact, independent of it. The determination of declination has to be made in three stages. Using observations from a number of detectors, scanning different strips of latitude, transformations are effected from the observed sidereal daily variations to the corresponding daily variations in free space, using the technique of the asymptotic cone of response. Independently of these calculations, a model that is appropriate to the type of anisotropy is formulated, yielding a free-space sidereal daily variation and an average intensity term. The amplitude of the harmonics of the daily variation term depends in part on the latitude of observation and on the declination of the anisotropy. Using the expressions for the harmonics, a value of the declination can be so determined as to give a best fit agreement between the free-space amplitudes of the model and the amplitudes derived from the observations. It will be shown that the estimated directions of the intensity maxima in free-space, when taken together with the type of two-way anisotropy that is indicated by the free-space harmonics, suggest that the primary cosmic radiation may be propagating along a rather uniform local galactic magnetic field.