Cosmic ray modulation and the distant heliospheric magnetic field: Voyager 1 and 2 observations from 1986 to 1989

Abstract

The changes in the cosmic ray intensity >70 MeV/nucleon observed by Voyager 1 (hereinafter called V1) near 30° heliographic latitude and by Voyager 2 (hereinafter called V2) near 0° heliographic latitude from 1986 to 1990 were closely related to the large‐scale fluctuations in the magnetic field strength at V1 and V2, respectively. The decreases in the cosmic ray intensity observed by V1 and V2 from 1986 to 1990 were generally related to merged interaction regions (MIRs). An exception to this relation was observed from about day 45 to day 110, 1987 at V1. The MIRs are of three general types: “global MIRs” (GMIRs), which extend around the Sun and to high latitudes, whose effects persist over several solar rotations; “corotating MIRs” (CMIRs), which are quasi‐periodic structures generated by the interaction of corotating streams lasting several solar rotations; and “local MIRs” (LMIRs), which are nonperiodic and have limited latitudinal and/or longitudinal extent, whose effects last less than one solar rotation. The cosmic ray intensity shows a net increase with time when the MIRs are weak and the magnetic field strength is relatively low, for example, from ∼day 120 to ∼day 45, 1987 at V1 and from ∼120, 1986 to ∼195, 1987 at V2. The cosmic ray intensity fluctuates about a constant level when CMIRs and rarefaction regions are dominant. This is the case from ∼230, 1987, to ∼275, 1988, at V1 and during some intervals from ∼245, 1987, to ∼60, 1989, at V2. The cosmic ray intensity decreases locally during the passage of a LMIR. Such a local decrease was observed at both V1 and V2 near day 100, 1986. Local decreases in the cosmic ray intensity were observed at V2 but not V1 beginning near days 240 and 320, 1987, and near day 275, 1988. A global, broad, steplike change in the cosmic ray intensity was observed at all of the spacecraft during the passage of a GMIR, beginning approximately near day 125, 1989, at V1 and near day 65, 1989, at V2 and extending for approximately 100 to 150 days, respectively.