NASA Goddard Space Flight Center, Laboratory for Extraterrestrial Physics, Greenbelt, Maryland 20771

Observations by the Voyager 1 and 2 spacecraft of the interplanetary magnetic field between 1 and 5 AU have been used to investigate the large-scale structure of the IMF in the years 1977 to 1979, a period of increasing solar activity. This complements the Pioneer 10, 11 investigation between 1 and 8.5 AU during 1972-1976 when the sun was less active. In contrast to the good agreement of the Pioneer observations with the ideal field configuration of the Parker spiral model during near solar minimum conditions, the Voyager spacecraft found notable deviations from that configuration. We attribute these deviations both to temporal variations associated with increasing solar activity, and to the effects of fluctuations of the field in the radial direction. The amplitude of the latter fluctuations was found to be large relative to the magnitude of the radial field component itself beyond approximately 3 AU. The IMF sector structure was generally not well-developed during the period of this study. Notable differences were found between Voyager 1 and 2 observations. Differences in the region 1-2 AU are attributed to the substantially different latitudes of the two spacecraft during much of the period. Later differences are most likely associated with the fact that the Voyagers moved through the region between 4 and 5 AU at different times. Both Voyager 1 and Voyager 2 observed decreases with increasing heliocentric distance in the amplitude of 'transverse' fluctuations in B that are consistent with the presence of predominantly undamped Alfvén waves in the solar wind, although not necessarily implying the presence of them. The presence of convective structures, compressive modes, and/or a saturated instability of Alfvén waves cannot be excluded by these Voyager results. Fluctuations in the strength of B (relative to mean field strength) were found to be small in amplitude, with a RMS that is approximately one third of that for the transverse fluctuations, and they are essentially independent of distance from the sun.

J. Geophys. Res., 87, No. A6, 4345-4353, June 1982