Heliospheric magnetic field strength out to 66 AU: Voyager 1, 1978–1996

We discuss Voyager 1 (V1) observations of the heliospheric magnetic field strength from 1978 through 1996. During this period the distance of V1 from the Sun increased from ≈3 AU to 66 AU and its heliographic latitude increased from ≈5°S to 33°N. The magnetic field strength profile observed by V1 is consistent with Parker's spiral field model when one considers (1) the solar cycle variation of the observed magnetic field strength at 1 AU, B1(t) (which is a measure of the source field strength) and (2) the latitudinal and solar cycle variations of the solar wind speed,V(t, θ). Both B1(t) and V(t, θ) make significant contributions to the variation of the magnetic field strength variations observed by V1. There is no evidence for a “magnetic flux deficit” increasing with distance from the Sun. There is a solar cycle variation of the magnetic field strength in the outer heliosphere, which will affect the modulation of cosmic rays.