Magnetic fields in the distant heliosphere approaching solar minimum: Voyager 1 and 2 observations during 1994

Abstract

During 1994, Voyager 2 (V2) was at a heliocentric distance 〈R〉 = 43.4 AU and latitude 〈δs/c〉 = 11.9°S, while Voyager 1 (Vl) was at 〈R〉 = 56.3 AU and 〈δs/c〉 = 32.5°N. The mean magnetic field strength observed by Vl during 1994 was 0.05 nT, the weakest and most distant magnetic field ever measured in situ. A bimodal distribution of azimuthal magnetic field directions was observed by V2, and a single‐peaked distribution was observed by Vl. Thus V2 was in the “sector zone” (the range of latitudes that contains the heliospheric current sheet), and Vl 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 observations. A lognormal distribution of the magnetic field strength (hourly averages) was observed for 95% of the Vl data and most of the V2 data. An approximately lognormal distribution of B is a general property of the heliospheric magnetic field, observed out to 58.1 AU and up to 32.6°S latitude. During 1994 an exponential tail in the distribution of the magnetic field strength was observed by both Vl and V2 for fields stronger than average. An exponential tail in the magnetic field strength distribution was observed by Vl and V2 from 1983 to 1994 at latitudes from 11°S to 33°N and at distances from 13 to 58 AU. Thus an exponential tail of the distribution of magnetic field strengths is a basic property pf the heliosphere beyond 13 AU. There was no significant latitudinal gradient in the exponential tail observed by V2 in the sector zone and observed by Vl above the sector zone during 1994. Multifractal structure in the large‐scale fluctuations of the magnetic field strength was observed by Vl for periods from 8 hours to 256 hours equaling 10.7 days for the magnetic fields stronger than average. A multiffactal spectrum is a general property of the large‐scale magnetic field strength fluctuations, from 1983 through 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.