MAGNETIC-FIELDS IN THE DISTANT HELIOSPHERE APPROACHING SOLAR MINIMUM - VOYAGER-1 AND VOYAGER-2 OBSERVATIONS DURING 1994

During 1994, Voyager 2 (V2) was at a heliocentric distance [R] = 43.4 AU and latitude [delta(s/c)] = 11.9 degrees S, while Voyager 1 (V1) wa s at [R] = 56.3 AU and [delta(s/c)] = 32.5 degrees N. The mean magneti c field strength observed by V1 during 1994 was 0.05 nT, the weakest a nd most distant magnetic field ever measured in situ. A bimodal distri bution of azimuthal magnetic field directions was observed by V2, and a single-peaked distribution was observed by V1. Thus V2 was in the '' sector zone'' (the range of latitudes that contains the heliospheric c urrent sheet), and V1 was in the unipolar region above it, consistent with the latitudes of the heliospheric current sheet obtained from an extrapolation of the neutral line computed from solar magnetic field o bservations. A lognormal distribution of the magnetic field strength ( hourly averages) was observed for 95% of the V1 data and most of the V 2 data. An approximately lognormal distribution of B is a general prop erty of the heliospheric magnetic field, observed out to 58.1 AU and u p to 32.6 degrees S latitude. During 1994 an exponential tail in the d istribution of the magnetic field strength was observed by both V1 and V2 for fields stronger than average. An exponential tail in the magne tic field strength distribution was observed by V1 and V2 from 1983 to 1994 at latitudes from 11 degrees S to 33 degrees N and at distances from 13 to 58 AU. Thus an exponential tail of the distribution of magn etic field strengths is a basic property of the heliosphere beyond 13 AU. There was no significant latitudinal gradient in the exponential t ail observed by V2 in the sector zone and observed by V1 above the sec tor zone during 1994. Multifractal structure in the large-scale fluctu ations of the magnetic field strength was observed by V1 for periods f rom 8 hours to 256 hours equaling 10.7 days for the magnetic fields st ronger than average. A multifractal spectrum is a general property of the large-scale magnetic field strength fluctuations, from 1983 throug h 1994 between 13 AU and 58 AU. There remains a need for a statistical theory of the solar wind that can account for these observations and for cosmic ray models that include the effect of multifractal magnetic field strength fluctuations.