This paper analyzes the radial, latitudinal, and temporal variations of the magnetic field in the outer heliosphere, using the observations from Voyagers 1 and 2 and Pioneers 10 and 11. The radial variation of the magnetic field strength measured during 1973--1989 from 1 to 19 AU is compared with that predicted by Parker's spiral field model. The speed deficit (V-V1)/V1 between 1 and 19 AU is (+0.0027¿0.0026)/AU for the Voyager data and (0.0099¿0.0024)/AU for the Pioneer 11 data. Thus the Voyager data are consistent with no radial variation of the speed, as assumed in deriving Parker's equations for the spiral field, but the Pioneer data are not. The magnetic flux deficit in the outer heliosphere relative to 1 AU is measured by the quantity D≡(A-A1)/A1 versus R. For the Voyager data the 1 AU intercept of the best fit line to these data is (0.029¿0.031), which is consistent with zero, as required by the definition of D. for the Pioneer 11 data the corresponding 1 AU intercept is (-0.068¿0.044), which implies a flux deficit of (7%¿4%) at 1 AU, suggesting either a systematic error in the data or statistical uncertainties greater than we estimated. The magnetic flux deficit is given by the slope the best fit line to D versus R.

The Voyager data give a slope -(0.0027¿0.0036)/AU, which is consistent with zero, indicating no flux deficit in the Voyager data. The Pioneer data give a slope (0.0040¿0.0042)/AU, which is also consistent with zero. Since the best fit line to the Voyager observations of D versus R gives a value of D at 1 AU consistent with zero, we may set D(1)=0 and obtain D(R)=[(0.0001¿0.0021)/AU>¿(R-1)(AU), consistent with no flux deficit between 1 and 19 AU from 1977 through 1985. For the years 1986 and 1987 the Voyager data give a nonzero flux deficit equal to (-0.264¿0.075)/AU and (-0.171¿0.070)/AU, respectively. The Pioneer 11 data show no significant flux deficit during 1986 (+0.19¿0.22)/AU. The flux deficit observed by Voyager 2 during these 2 years near solar minimum might be related to the presence of a heliospheric vortex street at this time. The magnetic field strength measured by Voyager 1 at high latitudes is smaller than the magnetic field strength measured by Voyager 2 near the heliographic equator beyond 21 AU. During 1986, when the heliospheric vortex street wrapped field lines in spirals around the axes of the vortices, the source magnetic field strength at 27 ¿N was greater than the source field strength at ~1 ¿N. During 1988 and 1989, when the vortex street was no longer present, the source field strength at ~30 ¿N was less than that at ~3 ¿N to 4 ¿N. The topology of the spiral interplanetary magnetic field is determined by a defect in the heliosphere, the rotating Sun. The language of group theory can describe this and other more complex defects and vector fields in the heliosphere. ¿ American Geophysical Union 1993