Radial dependences of the interplanetary magnetic field between 1 and 5 AU: Pioneer 10

The radial dependences of the distant interplanetary magnetic field (IMF) between 1 and 5 AU have been investigated by using Pioneer 10 vector helium magnetometer data acquired in 1972-1973 during Bartels solar rotations 1896-1918. Least squares fits were made of the radial dependences of the averages of the magnitudes of IMF components and total field and plane projections. Radial fits were also made of the standard deviations of these variables over the solar rotation, 1 day, and 3-hour intervals. Results which emerge from the study include the following: (1) Weighted averages of the radial component of the field are found to vary with heliocentric distance R and R/sup -2.10plus-or-minus0.030,/ in significant agreement with the Parker model. (2) For the entire data set the tangential component of the field is found to vary as R/sup -1.29plus-or-minus0.06./ (3) Further analysis was carried out on a subset of the data for which the average solar wind velocity was relatively low. Within this subset of data obtained during solar rotation periods with low average daily ΣKp the tangential field is found to decrease as R/sup -1.23plus-or-minus0.05/ and thus to deviate significantly from the Parker model. (4) The deviation between the tangential field variation observed within this subset of the data and the predictions of the model appears to be associated with compressional interaction regions at the leading edges of high-velocity streams; outside these regions the variations are approximately in accord with the Parker model. (5) The radial decrease of the IMF composite variance (the sum of the variances for all three field components) in a subset of the data obtained during times when the average solar wind velocity was low is consistent with the theory that nonlocally generated Alfven waves are being swept out from the sun undamped in the interval 1.0-3.3 AU. (6) The power spectrum in the frequency range 1-8 cycles/d is found to have a slope of approximately 3/2