Anisotropy Of Galactic Cosmic Rays Research Articles

Modulation of galactic cosmic ray anisotropy in heliomagnetosphere: Average sidereal daily variation

We formulate the modulation of galactic anisotropy of cosmic rays caused by their orbital deflection in the heliomagnetosphere. According to the formulation, the average sidereal i-th harmonic daily variation ( i = 1,2,…) produced from the anisotropy from an arbitrary direction can be expressed by a linear combination of three basic vectors for uni-directional anisotropy and five basic vectors for bi-directional anisotropy. These vectors are obtained by calculating trajectories of cosmic rays (20−10 4GV) in a model magnetosphere having Parker's Archimedian spiral structure with a flat or a wavy neutral sheet in either of two polarity states, one is called “Positive” state (away field in the northern space of the neutral sheet and toward field in the southern space) and the other is called “Negative” state (reversed state of the above). Among general characteristics of the sidereal daily variations, the most remarkable features are: (1) The observable variations in low rigidity (≲ 2000 GV) can be produced even from an uni-directional anisotropy in the direction of the Earth's rotation axis. These variations are strongly dependent on the polarity state, i.e., they are greater in the Positive state than in the Negative. (2) Those produced from the anisotropy in the Equatorial plane also show the polarity dependence but contrary to the previous case they are greater in the Negative state than in the Positive. Their magnitude in the former state is not so small even in the extremely low rigidity (∼ 100 GV) as compared with that in high rigidity region. (3) These general characteristics are not altered by the introduction of the wavy neutral sheet or the magnetic irregularities, but the variations are affected more or less, depending on the heliolatitudinal extent of the wavy sheet or the degree of cosmic ray scattering with the irregularities, (4) Sidereal daily variation for the wavy sheet shows a toward-away field dependence similar to that of Swinson-type of solar origin, but the dependence is predominant in intermediate rigidity region (∼ 500 GV), in marked contrast to that of solar origin. (5) Finally, whichever its direction may be, the uni-directional anisotropy produces the sidereal diurnal variation common to two conjugate stations in the Northern and Southern hemisphere. If there is any difference between the observed variations at the stations, it should be interpreted as being due to higher order anisotropy such as the bi-directional anisotropy.

Cosmic-Ray Confinement in the Galaxy

Phenomenological models are examined which are used to interpret the available experimental data on the spectra, composition, and anisotropy of galactic cosmic rays with energies of less than 3 million GeV. The following types of models are reviewed: closed, leaky box, nested leaky box, diffusion, dynamical halo, very inhomogeneous, continuous acceleration, and extragalactic. Consideration is given to the physical mechanisms that govern cosmic-ray confinement and escape and to the interpretation of cosmic-ray observations at earth in terms of the above models. Observations of the diffuse gamma-ray emission of the galaxy are discussed which appear to rule out the extragalactic model. The problem of very high-energy cosmic rays (higher than 3 million GeV) is briefly noted.

North-south anisotropy and radial density gradient of galactic cosmic rays.

The cosmic ray north-south anisotropy arising from the heliocentric radial density gradient of cosmic rays and the interplanetary magnetic field has been studied using data for a wide range of rigidity (15-210GV in median primary rigidity) during the period 1969-1973. Two components of the anisotropy; the north-south asymmetry and the associated sidereal diurnal variation have been analyzed to reveal a three-dimensional structure of the anisotropy in space. By means of the least-squares method and using improved coupling coefficients, the direction and certain other parameters of the anisotropy have been determined.Definite evidence has been obtained for the existence of anisotropic flow perpendicular to the ecliptic plane. The magnitude of the flow is found to be 0.081±0.021% and 0.072±0.018% at 10GV for the periods 1969-1970 and 1971-1973, respectively. It is also found that its rigidity spectrum is slightly rigidity dependent and has an upper limiting rigidity of 200GV or more. Based on the derived parameters of the anisotropy, the heliocentric radial density gradient for high rigidity cosmic rays has been estimated as a function of rigidity for 1969-1973 as (3.0±1.1)×(P/10)-(0.7±0.2)%/AU for P≤200GV (P is rigidity in GV). The diffusion coefficient is also derived, which seems to be consistent with those so far determined.

Streaming of galactic cosmic rays in the interplanetary magnetic field

We have studied in detail the correlation between the 24 hr average anisotropy of galactic cosmic rays as measured by a ring of high latitude neutron monitors and the direction of the 24 hr average interplanetary magnetic field. From the two year data period 1967–68, we have selected events showing instances of marked non-equilibrium anisotropy (i.e. large departures from the average ‘corotational’ anisotropy). These include trains of spontaneously arising enhanced diurnal variation as well as Forbush decrease events showing large anisotropy in the pre-decrease, main or recovery phase. We find that the observed anisotropies are well fitted by the assumption that the streaming of galactic cosmic rays is composed of two components (1) radial streaming away from the Sun with a velocity of ≈400 km/s and (2) diffusive streaming along the direction of the interplanetary magnetic field. The diffusive streaming is mostly towards the Sun. This description is also applicable to periods characteristic of the quiet time diurnal anisotropy. These results are analogous to those obtained by McCracken, Rao and Ness for 7–45 MeV solar flare particles.

Anisotropies of galactic cosmic rays in the solar system

At least four processes appear capable of contributing to the creation of an anisotropy of galactic cosmic rays in interplanetary space. All need not necessarily be active on a particular day to the same extent. The processes are (1) azimuthal streaming, (2) streaming due to non-uniform diffusion in a longitudinal sector structure, (3) the scattering at irregularities along the interplanetary magnetic field, and (4) latitudinal gradients in a relatively smooth magnetic field. The characteristics of the daily variation which must be expected from each process are quantitatively evaluated. Observational evidence derived from measurements of the daily variation from a network of super neutron monitors is used to derive, with precision, the spectrum of variation, the amplitude and the principal direction of maximum and minimum intensity in interplanetary space, on each day during 1964–65. The experimental results are compared with predictions to derive conclusions as to the process which operates on individual days. It is demonstrated that the diurnal and the semi-diurnal components of the anisotropy have characteristically different energy spectra of variation. The predominant process responsible for the diurnal component is the azimuthal streaming while the semidiurnal component appears to be due to scattering at magnetic field irregularities and latitudinal gradients. The manner in which the results can be used to derive the threshold energy below which isotropic diffusion seems to occur on individual days in the inner solar system is demonstrated. The results of the study strongly support the postulate of a gradient of galactic cosmic rays in the solar system perpendicular to the ecliptic.

Gradient of galactic cosmic rays normal to the solar equatorial plane

Predictions concerning the anisotropy of galactic cosmic rays due to a gradient of cosmic-ray density perpendicular to the solar equatorial plane have been verified experimentally as follows. (1) The energy spectrum of the variation of the semidiurnal component has a positive exponent. (2) The diurnal and the semidiurnal components are oriented with respect to the interplanetary magnetic field. (3) A deficiency of cosmic-ray intensity, Tmin, is observed along the direction of the interplanetary magnetic field on days when the energy spectrum of the diurnal variation has an exponent different from zero.

Anisotropy of Galactic Cosmic Rays and the Interplanetary Magnetic Field

IT has been demonstrated1,2, using data from high counting rate instruments, that the daily variation of secondary cosmic ray intensity measured on the ground can in general be related on each day to an anisotropy of primary galactic cosmic rays. The anisotropy can vary from day to day in magnitude, in the directions with respect to the Earth–Sun line of maximum intensity Tmax and of minimum intensity Tmin outside the influence of the geomagnetic field, and in the energy spectrum of variation. Tmax and Tmin are usually separated by 8–10 h. This has led Rao and Sarabhai to suggest that the anisotropy appears generally to be due to two principal processes, one a modulation equivalent to a pair of oppositely directed positive and negative sources, and the other a negative virtual source approximately oriented along the garden hose direction towards the Sun.

Probing interplanetary plasma and magnetic fields with galactic cosmic rays

The results of the day-to-day analysis of cosmic ray daily variation with a very high counting rate mu-meson detector at MIT are presented. Comparison of these with the daily variation observed by the neutron monitor at Deep River provides a basis for deriving information concerning the position and the energy spectrum of variation of the anisotropy of galactic cosmic rays on each day. The significance of the peak-to-peak amplitude, the time of maximum ( T max ) and the time of minimum ( T min ) of the daily variation in relation to the anisotropy is demonstrated and evidence presented to indicate that from time series of T max and T min it is possible to follow advancing fronts of interplanetary plasma and magnetic fields. The nature of the anisotropy is analysed and arguments advanced for the view that consideration in terms of a single virtual source is inadequate to explain the results. Having demonstrated the day-to-day changes of energy spectrum of anisotropy and the level of isotropic galactic intensity, an attempt is made to identify distinct physical states of inter-planetary space in terms of a model involving a solar wind, but with nonuniformity of velocity over different regions of the Sun, with a possibility of a blast wave propagated through it due to occasional sudden release of energy. The classification of days as belonging to one or the other of four states is performed by using geomagnetic data and the day-to-day modulation of isotropic galactic intensity. It is then found that the anisotropy associated with different states is quite distinctive and is consistent with what may be expected according to the model. It is suggested that using data on geomagnetic disturbances and galactic cosmic rays as probes, it is possible to identify distinct electromagnetic states of interplanetary space.