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

Pioneer 6 and 7 space probes carried charged‐particle telescopes that measured for the first time both the direction of arrival and differential energy spectra of protons and alpha particles over the energy range 0.6 to ∼100 Mev/nucleon. The background counting rate is reduced by using an anticoincidence shield around the telescope so that proton flux changes as small as ∼10 −3 particles/cm² sec ster are detected with a time resolution of ∼one hour. Near the beginning of the new solar activity cycle (no. 20) over the period December 1965‐September 1966, proton flux increases have been associated unambiguously with specific centers of solar activity. The origin and propagation of these protons in the corona and interplanetary space have been investigated for this period. Enhanced proton fluxes in the energy range 0.6–13 Mev have been observed to come from the sun continuously over heliocentric longitude ranges as great as 180° in association with specific active regions. The enhanced fluxes exhibit definite onsets, when the active centers reach ∼60–70° east of central meridian, and cutoffs, when the center reaches ∼100–130° west. The onset and cutoff, together with the modulation of galactic cosmic rays, point to the existence of a corotating magnetic region associated with the active center. Superposed on the 0.6‐Mev flux level are occasional large and discrete flare‐produced intensity increases from these centers extending in energy to more than 50 Mev for protons. The solar electrons that appear during the initial phase of some solar flare events have been excluded by energy loss versus range measurements. The protons from flares over a longitude range of >60° are observed initially to propagate rapidly through the corona to the interplanetary field. The short transit and rise times are not explained by isotropic diffusion from the flare site across coronal magnetic fields. From these observations, a quasi‐stationary model is proposed for the distribution of magnetic fields above the active center. In this model, some field lines rooted in or near the active center are spread out in the corona over a range of ∼100–180° longitude (possibly because of the presence of super‐heated coronal plasma) and then extended into interplanetary space by the solar wind. Observations prove that both the enhanced and discrete flare proton fluxes propagate along the spiral interplanetary magnetic field from the western hemisphere of the sun. Present evidence supports the view that the origin of the >0.6‐Mev proton flux is in flare‐like processes continually occurring in the solar active center. The existence of strong unidirectional anisotropies in the initial phases of discrete flare proton events implies that little scattering occurs in the magnetic fields between the sun and spacecraft. However, the gradual approach to an isotropic flux at late times indicates that the decay phase is controlled by the interplanetary magnetic field beyond ∼1 AU.