Australian Cosmic Ray Modulation Research
M. L. Duldig , PASA, 18 (1), in press.
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Sidereal Anisotropies

Jacklyn (1986) presented an excellent review of galactic anisotropies observed by ground based and shallow underground instruments. In that paper Jacklyn describes the two types of sidereal anisotropy, the uni-directional or streaming anisotropy and the bi-directional or pitch angle anisotropy. Jacklyn summarised the observations from 1958 to 1984 that showed the existence of both a uni-directional and a bi-directional galactic anisotropy. The uni-directional anisotropy appeared to have a maximum at 3hr sidereal time. The bi-directional anisotropy has already been discussed here in relation to the sidereal component of the north-south anisotropy (see Section 5.6). During the 1980's it became increasingly apparent that there was an asymmetry in the northern to southern hemisphere sidereal response. A thorough investigation of this and other asymmetric phenomenon at muon energies was warranted. This was the principal motivation for Japanese researchers from Shinshu and Nagoya universities and Australian researchers from the University of Tasmania and the Australian Antarctic Division to install multi-directional surface and underground telescopes in Tasmania at approximately the co-latitude of similar Japanese instruments (see Sections 4.1 and 4.2). This collaboration confirmed the asymmetry for 1 TeV particles (Munakata et al. 1995).

A New Interpretation

In 1994 at the International Mini-Conference on Solar Particle Physics and Cosmic Ray Modulation held at the STE Laboratory of Nagoya University Nagashima et al. (1995a) introduced a major change in our interpretation of the sidereal daily variation. At this meeting Nagashima, Fujimoto & Jacklyn first proposed the concept of the Tail-In and Loss-Cone anisotropies as being responsible for the observed variation and hemispheric asymmetry. These ideas were further developed over the next few years (Nagashima et al. 1995b, 1995c, 1998). They proposed a galactic anisotropy, characterized by a deficit flux, centred on RA 12 hr, Dec. 20^o. In addition to this deficit anisotropy they postulated a cone of enhanced flux, of $\sim$68^o half opening angle, centred on RA 6 hr, Dec. -24^o. This source is termed the Tail-In anisotropy because of its close proximity to the possible heliomagnetic tail (RA 6.0 hr, Dec. -29.2^o) opposite to the proper motion of the solar system. It was noted that this is not opposite to the expected tail (RA 4.8 hr, Dec. 15^o-17^o) of the solar system motion relative to the neutral gas. The model also required that the Compton-Getting effect does not exist up to rigidities of $\sim$10⁴ GeV. A schematic representation of the model is shown in Figure 35.

Figure 35: The Tail-In and Loss-Cone anisotropy model. PM is the direction of proper motion of the solar system. RM is the direction of motion relative to the neutral gas. (From Nagashima et al. 1998).

Deriving the Tail-in and Loss-Cone Anisotropies

One aspect of the model is problematical. Usually the sidereal diurnal variation is analyzed harmonically. The proposed shape of the Tail-In anisotropy is not well suited to sinusoidal fits. The Japan-Australia collaboration therefore developed an alternative analysis technique in which they fitted gaussian functions of variable width and size (height or depth) to the sidereal daily variation. Their results agreed broadly with the model of Nagashima, Fujimoto & Jacklyn, the spectra and latitude distribution being consistent with the model. However, they found that the Tail-In anisotropy was asymmetric about its reference axis (Hall et al. 1998a). Their results were consistent with observed harmonic vectors derived by earlier studies. In subsequent and more complete analyses Hall et al. (1998b, 1999) covered the rigidity range 143-1400 GV and a viewing latitude range of 73^oN-76^oS. They confirmed that the Tail-In anisotropy is asymmetric about its reference axis (RA $\sim$4.7 hr, Dec. $\sim$14^oS). They also determined that this reference axis position may be rigidity dependent. The Loss cone anisotropy was found to be symmetric and centred on the celestial equator (RA $\sim$13 hr, Dec. $\sim$0^o). Figure 36 shows their determination of the two sidereal anisotropies. These positions are somewhat different from those proposed by Nagashima, Fujimoto and Jacklyn who based their model on results from earlier harmonic analyses. The technique applied by Hall et al. (1998b, 1999) is more sophisticated and has greater observational coverage. It remains to be seen if their result can be explained by heliospheric structures or interactions with the local galactic spiral arm.

Figure 36: The Tail-In and Loss-Cone anisotropies derived by (From Hall et al. 1998b, 1999).

Next Section: Looking to the Future
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Contents Page: Volume 18, Number 1