Solar-Flare Cosmic Rays at and Beyond the Modulation Boundary

In this letter we consider the characteristics of solar flare cosmic ray events observed beyond the postulated modulation boundary, should such a boundary exist. We point out that observations of solar flare particles at large distances from the sun provide a sensitive and precise diagnostic of any such boundary. The present consensus based primarily on indirect inferences from cosmic ray data appears to be that cosmic ray scattering effectively ceases within some 5 to 10 AU [see, e.g., Burlaga, 1967; O'Gallagher and Simpson, 1967; Ng and Gleeson, 1971; and Garrard, 1972]. Since it is expected that the solar wind continues to flow out to perhaps 50 AU [Axford, 1971], this view implies a large region of flow in which one has the classical large-scale magnetic field [Parker, 1963], but an effective absence of irregularities, which scatters the particles. Dynamic models of the solar wind producing such phenomena have been considered by Jokipii and Davis [1969] and Jokipii [1973].