The cosmic ray anisotropy below 10<sup>15</sup> eV

Abstract

Abstract. The measurement of the anisotropy in the cosmic ray (CR) arrival direction distribution provides important informations on the propagation mechanisms and on the identification of their sources. In the last decade the anisotropy came back to the attention of the scientific community, thanks to several new two-dimensional representations of the CR arrival direction distribution which clearly showed the existence of anisotropies at different angular scales in both hemispheres. The origin of the observed anisotropies is still unknown. So far, no theory of CRs in the Galaxy exists yet to explain the observations leaving the standard model of CRs and that of the local magnetic field unchanged at the same time. In this paper the observations of Galactic CR anisotropy will be briefly summarized, with particular attention to the results obtained by the ARGO-YBJ experiment in the Northern Hemisphere. IntroductionThe CR arrival direction distribution and its anisotropy has been a long-standing problem ever since the 1930s. In fact, the measurement of the anisotropy is a powerful tool to investigate the propagation mechanisms and the spatial source distribution determining the CR world as we know it.As CRs are mostly charged nuclei, their paths throughout the Galaxy are deflected and highly isotropized by the action of galactic magnetic field (GMF) they propagate through before reaching the Earth. The GMF is the superposition of a regular and a chaotic contribution. Although the strength of the non-regular component is still under debate, the local total intensity is supposed to be B = 2 ÷ 4 µG (Beck, 2001). In such a field, the gyroradius of CRs is given by r a.u. = 100 R TV , where r a.u. is in astronomic units and R TV is the particle rigidity in TeraVolt.The high degree of isotropy observed in the CR arrival direction distribution suggests that the propagation in the Galaxy of CRs trapped by the magnetic field can be described in terms of diffusion, at least up to 10 16−17 eV (Berezinskii, 1990).The measurement of the anisotropy is complementary to the study of the CR energy spectrum and elemental composition to understand the origin and propagation of the radiation and to probe the structure of the magnetic fields through which CRs travel. In fact, while the elemental composition of CRs observed at the Earth is a quantity averaged over all possible propagation trajectories and large time intervals, thus mainly probing the diffusive propagation mechanisms, the anisotropy, on the contrary, can give information on the structure of the magnetic field near the solar system.In principle any anisotropy reflects a motion. As an example, three different effects may lead to a CR anisotropy. The first effect is related to the motion of the Earth/Solar System with respect to the isotropic CRs rest frame (the so-called Compton-Getting effect, Compton and Getting, 1935). The second is due to nearby and recent CR sources (pulsars or SNRs). For an isotropic propagation, a CR source distribution in the Galaxy is expected to ...

Future Ground-based Wide Field of View Air Shower Detectors

“…The measurement of the anisotropy in the arrival direction distribution of CRs is a complementary way to understand the origin and the propagation of the radiation. In fact CR anisotropy is a fingerprint for their origin and propagation [22,23,24]. The study of the anisotropy can clarify the origin of the knee.…”

The LHAASO experiment: From Gamma-Ray Astronomy to Cosmic Rays

“…The origin of the observed anisotropies is still unknown. So far, no theory of CRs in the Galaxy exists yet to explain the observations leaving the standard model of CRs and that of the local magnetic field unchanged at the same time [9,10].…”

The ARGO-YBJ legacy to next generation wide field-of-view experiments

“…These observations rule out the hypothesis that a Compton-Getting effect due to the motion of the heliosphere with respect to the local insterstellar medium (expected as a dipole with a maximum in the direction of the Galactic Center decl. 49 • , R.A. 315 • and a larger amplitude 3.5×10 −3 ) is a major source of the anisotropy [18,19].…”