The Collimation of Cosmic Rays by the Interplanetary Magnetic Field

Abstract

The Pioneer 6 spacecraft was instrumented to provide detailed information on the magnetic field, plasma, and energetic particles pervading interplanetary space. Cosmic-ray and magnetic field measurements are complementary in that the cosmic rays can provide information regarding the magnetic field configuration over a large volume of space while the direct field measurements describe the local field near the spacecraft. Clearly, intercomparison of the two species of data adds considerably to the value of both types of observation. Cosmic rays released by solar flares are of particular value in such studies, for the point and time of injection of the particles into the solar system is known at least approximately, and hence a more precise specification of the magnetic field configuration traversed by the cosmic rays is possible. During the first thirty days of the flight of the Pioneer 6 spacecraft, three solar flares were observed, as detailed elsewhere [Bartley et al., 1966]. It is the purpose of this communication to compare a portion of the cosmic-ray and magnetic data observed during one of these flare events and lo demonstrate specific particle guidance properties of the interplanetary magnetic field. The data were obtained at a time when the spacecraft was 2.8 X 106 km from earth and at a sun-earth probe angle of 90 ø east of the sun and arc, therefore, unaffected by any terrestrial phenomena. It has been shown previously that both cosmic-ray observations [McCraken, 1962] and magnetic data [Ness et al., 1964] are in general agreement with the Archimedes spiral model for the interplanetary magnetic field [Parker, 1958]. In particular, if irregularities in the interplanetary field are of a scale size large compared to the cyclotron radius of the cosmic ray, the first adiabatic invariant of the particles' motion, sin g O/B, is conserved, where 0 is the pitch angle of the particles' motion. For a cosmic ray spiraling around an interplanetary line of force, B decreases by a factor of approximately 10' in traveling from the sun to the earth. Thus, pitch angles near the orbit of earth will be small (< 1ø), so that the solar cosmic radiation will be markedly anisotropic, and the particle flux will be observed streaming from a direction parallel to the interplanetary magnetic field vector. Irregularities in the magnetic field of a scale size comparable to the particle cyclotron radius will tend to scatter the cosmic rays and thereby destroy such an anisotropy.It has been shown [Bartley et al., 1966] that the solar-generated cosmic radiation arriving at the earth during the period December 29, 1965, through January 1, 1966, was markedly anisotropic. For any given period of time, the direction of the maximum cosmic-ray flux was determined (Figure 2, Bartley et al.) and was found to vary markedly with time. In particular, there were times at which the maximum cosmicray flux was arriving from directions 90 ø to 150 ø west of the earth-sun line. Such directions are not explicable on the basis of the simple Archimedes-spi...

Directional discontinuities in the interplanetary magnetic field

“…The existence of marked and abrupt changes in the direction of anisotropy of low energy (210 MeV) solar protons was demonstrated by Bartley et al (1966). A strong correlation between the direction of anisotropy of low energy solar protons and the magnetic field direction, shown by McCracken and Ness (1966), supports the inference of Bartley et al that discontinuities in the direction of anisotropy are due to discontinuities in the magnetic field direction. Observations of numerous discontinuities during the early days of the flight of Pioneer 6 have led all of the above authors to suggest that the interplanetary medium consists of numerous identifiable filaments.…”

Intermittency, nonlinear dynamics and dissipation in the solar wind and astrophysical plasmas

“…This perspective stands in contrast to a number of features of interplanetary observations that are consistent with an active turbulence cascade, including, for example, reduction of Alfvénicity, lowering of the Alfvén ratio, development of anisotropy, evolution of the correlation scale [98,99], observed plasma heating [100,101] and the direct measurement of cascade rates [18,102,103]. The recent revival of the so-called spaghetti model of solar wind flux tubes [65,104,105] treats flux tubes and their boundaries, which are static discontinuities, as inert remnants of coronal processes. The interpretation goes so far as to separate the flux tubes and discontinuities from other fluctuations, the latter regarded as 'turbulence'.…”

Protons associated with centers of solar activity and their propagation in interplanetary magnetic field regions corotating with the Sun

“…It was found that, on the average, solar-flare proton fluxes of N 10 MeV displayed large anisotropies directed primarily along the prevailing spiral angle of the interplanetary magnetic field (Fan, --2-Lamport, Sirnpson and Smith, 1966;McCracken and Ness, 1966). This very high degree of anisotropy i s explained by propagation of the particles with low pitch angles along the direction of the magnetic lines of force.…”