Fig. 1.2.1. The observed CR spectrum broken into five energy ranges. The shaded area shows the region subjected to solar modulation. According to Dorman (1977a).

The upper boundary of interval 1 should be determined by CR interactions with the 2.7 °K relict microwave radiation in case of meta-galactic origin of the highest energy CR; the numerous available EAS experimental data are indicative of the existence of the particles with energies more than 1020 eV in the primary CR, but not more than 1021 - 1022 eV.

The boundary between intervals 1 and 2 is characterized by the jump change in the power exponent of the CR differential spectrum from 3.2-3.5 to 2.7 from interval 1 to interval 2 (this fact was first established on the basis of EAS measurements (e.g., Khristiansen, M1974).

The boundary between intervals 2 and 3 has particular meaning in the case of observations inside the solar system and corresponds to the upper energy boundary of CR modulation in the interplanetary space established on the basis of the data of many years of underground and ground based observations (Bishara and Dorman, 1973a,b,c, 1974a,b, 1975).

The chemical and isotopic composition and the regularities of the CR modulation by solar wind in the energy range 3 have been sufficiently studied and it is undoubted that interval 3 is completely of galactic origin.

The boundary between intervals 3 and 4 corresponds to the minimum of the CR spectrum in kinetic energy/nucleon and is probably somewhat variable with solar activity. This boundary separates the energy range of explicitly galactic origin (interval 3) from range 4 whose origin is being extensively discussed and has not become clear as yet. The problem of CR origin for interval 4 is discussed in detail in Dorman (1974, 1977d,e) where the following possible alternatives are treated: the solar (generated in solar flare acceleration processes and trapped for some time in the solar corona and in the Heliosphere), anomalous CR formed by ionization of interstellar atoms penetrating into interplanetary space and then accelerated in the vicinity of terminal shock wave, and galactic origin (small energy galactic CR so called sub-CR penetrating from interstellar space into Heliosphere along the magnetic channels).

The boundary between energy ranges 4 and 5 is somewhat artificial, though it was assumed in Dorman (1977a) to be about 1 MeV/ nucleon. As the solar activity changes, this boundary may shift to both sides and the displacement may be from several tenths of an MeV/nucleon to several MeV/nucleon. The physical meaning of this boundary is that interval 5 is markedly different in the chemical composition, form of energy spectrum, and mode of temporal variations from interval 4. This fact is undoubtedly indicative of the different origin of CR in intervals 4 and 5. It is not excluded that the relative importance of various sources of interval 5 (low energy CR generation in solar corona in connection with chromospheric flares and during the quiet Sun; acceleration by the interplanetary shock waves and other disturbances in solar wind; generation and escaping from magnetospheres of rotating planets with a large magnetic field such as Jupiter, Saturn, and even the Earth; low energy particle generation in the transient layer between the solar wind and galactic magnetic field) varies markedly in time thereby resulting in the shift of the boundary between intervals 4 and 5.

The lower boundary of interval 5 extends, according to numerous works up to energies of ~ 0.01 MeV/nucleon and, perhaps, even lower, essentially coinciding with the upper energy boundary of the solar wind particles (let us note that these very low energy CR particles may have their origin from acceleration of background plasma particles in planetary magnetospheres and in the interplanetary space). Thus the observed CR spectrum is extended from ~ 104 eV/nucleon to ~ 1021 eV (since the super-high energy particles are exclusively detected with EAS arrays, only the total energy can be determined), within ~ 17 orders.

1.2.4. Main CR properties and origin of CR in the interval 1

In various years, and up to recently, many researchers were of the opinion that the CR particles in the super-high energy interval 1 (1021 eV > Ek > 3 x1015 eV according to the classification in Section 1.2.3) were mainly of metagalactic origin (Cocconi, 1960; Oda, 1961; Fichtel, 1963; Laster, 1964; Johnson, 1970; Berezinsky et al., 1974; Hillas, 1975; Colgate, 1975a). This hypothesis was critically analyzed by Ginzburg and Syrovatsky (M1963). The following arguments favoring the metagalactic origin of the interval 1 (or its highest energy side) were considered: (i) the absence of the known sources of such high energies (up to about1021 eV) in the Galaxy, (ii) the serious difficulties associated with the retention of the particles of very high energies in the Galaxy. The discovery and the study of the pulsars, however, have made it possible to suggest highly probable mechanisms of particle acceleration in the Galaxy up to ~ 1020 eV (Ginzburg, 1969; Gunn and Ostriker, 1969; Silvestro, 1969; Colgate, 1975b,c). In particular, it was argued in Silvestro (1969) that the pulsars were capable of accelerating also the very heavy nuclei up to super-high energies. It is not excluded, either, that the particles of such high energies are generated in powerful processes taking place in the vicinities of the galactic center (Dorman, 1969). A serious argument favoring the galactic origin of the super-high energy CR is the absence of the spectrum cut-off at the high-energy side up to ~ 1020 eV. Such cut-off should necessarily take place in the case of metagalactic (or extragalactic) origin owed to interactions with the 2.7 °K relict microwave radiation (Zatsepin and Kuzmin, 1966; Greisen, 1966; Hillas, 1968; Prilutsky and Rozental, 1969). Ginzburg (1968) presents a number of additional arguments against the hypothesis of the metagalactic origin of main part of observed CR, and Syrovatsky (1971) argues that the CR up to the highest observable energies may be of galactic origin.

1.2.5. The anisotropy in energy intervals 1 and 2

The anisotropy and mode of propagation in the Galaxy of super-high energy CR are of special interest in connection with the examined problem of their origin. The published data of the measurements of 84 largest size of EAS with four EAS arrays at Sydney, Volcano Ranch, Haverah Park, and Yakutsk have been used by Hillas and Ouldridge (1975) to study the distribution of the arrival of the >

2 x1019 eV CR particles to the Earth. The search for sidereal anisotropy on the basis of the above data has given a value of ~ 60% for the amplitudes of the first and second harmonics. The possibility was analyzed in Hillas and Ouldridge (1975) that the obtained results were owed to the particles' arrival from the galactic clusters or super-clusters. The estimates of Hillas and Ouldridge (1975), show however, that, if the galactic super-clusters contain from 5 x103 to 104 galaxies of the type of our Galaxy, the flux of the super-high energy particles expected from such super-clusters proves to be at least 400 times as small as the flux observed on the basis of EAS measurements. The data of measurements in the lower energy range also analyzed in Hillas and Ouldridge (1975) show that the amplitude of the sidereal anisotropy (see Fig. 1.2.2) in the 3x1015 eV > Ek > 1011 eV energy range (interval 2 according to the classification given in Section 1.2.3) varies very little with energy and remains constant (~ 0.1%) with the peak near 19h of sidereal time, which corresponds to an inconsiderable flux of CR along the force lines of the galactic spiral field.

Fig. 1.2.2. The amplitude of the sidereal CR anisotropy as a function of energy Ek in the range 1014 - 1020eV (the points with the vertical bars denoting the measurement errors). The solid line shows the dependence of 4dd from Ek according to Eq. 1.2.1 and the dependence E-25 ¡D{Ek) by taking into account of Eq. 1.2.2.

Ek teV]

Fig. 1.2.2. The amplitude of the sidereal CR anisotropy as a function of energy Ek in the range 1014 - 1020eV (the points with the vertical bars denoting the measurement errors). The solid line shows the dependence of 4dd from Ek according to Eq. 1.2.1 and the dependence E-25 ¡D{Ek) by taking into account of Eq. 1.2.2.

The data of the various observations displayed in Fig. l.2.2 show that the best agreement with the experimental data can be obtained on the assumption that the amplitude of the sidereal anisotropy 4sid in the energy range

3 x 1015 eV > Ek > 1011 eV is not constant but increases approximately as

Was this article helpful?

0 0

Post a comment