In Situ Acceleration and Gradients of Charged Particles in the Outer Solar System Observed by the Voyager Spacecraft

Abstract

The probable connection between cosmic rays and the electromagnetic state of the interplanetary medium was recognized by Hannes Alfven as early as 1949 (Alfven, 1949, 1950); he pointed out that the properties of cosmic rays necessitate a mechanism, external to Earth but within the solar system, capable of accelerating particles to extremely high energies. In advocating the view of local origin for part of the cosmic-ray spectrum, Alfven and his colleagues developed a very general type of acceleration mechanism called ‘magnetic pumping’. The unique data set of the two Voyagers extends over an entire decade (1977–1987) and is most suitable to explore the problem of acceleration of charged particles in the heliosphere. The energy coverage of the Low Energy Charged Particle (LECP) experiment covers the range ~ 30 keV to several hundred MeV for ions and ~ 22 keV to several MeV for electrons. Selected observations of interplanetary acceleration events from ~ 1 to ~ 25 AU are presented and reviewed. These show frequent acceleration of ions to several tens of MeV in association with shocks; highest energies (≳220 MeV oxygen) were measured in the near-perpendicular (θ Bn ≃87.5°) shock of January 5, 1978 at ~1.9 AU, where electron acceleration was also observed. Examples of ion acceleration in association with corotating interaction regions are presented and discussed. It is shown that shock structures have profound effects on high-energy (≥ 70 MeV) cosmic rays, especially during solar minimum, when a negative latitudinal gradient was observed after early 1985 at all energies from ~ 70 MeV down to ~ 30 keV. By early 1987, most shock acceleration activity in the outer heliosphere (~ 25 to 30 AU) had ceased both in the ecliptic (Voyager-2) and at higher (~30°) ecliptic latitudes (Voyager-1). The totality of observations demonstrate that local acceleration to a few hundred MeV, and as high as a few GeV is continually present throughout the heliosphere. It should be noted that in 1954 when Alfven suggested local acceleration and containment of cosmic rays within the solar system, no one treated his suggestion seriously, at any energy. The observations reviewed in this paper illustrate once more Alfven’s remarkable prescience and demonstrate how unwise it is to dismiss his ideas.