Voyager 1 and 2 observations of magnetic fields and associated cosmic-ray variations from 2000 through 2001: 60-87 AU

We consider the cosmic and heliospheric magnetic field observations made during the years 2000 and 2001 when Voyager 1 (V1) moved from 76.1 to 87 AU at a heliographic latitude of approximately 34.degrees5 and Voyager 2 (V2) moved from 59.8 to 69 AU at a heliographic latitude of approximately -25.degrees5. A global merged interaction region (GMIR) containing intense fluctuating magnetic fields was observed by V2 and V1 at 61 and 78 AU, respectively, during 2000. This GMIR produced the first steplike decrease in the cosmic-ray intensity observed in the distant heliosphere during solar cycle 23. The GMIR and the steplike decrease in cosmic-ray intensity occurred later at V1 than at V2, the time delay being approximately the propagation time of the GMIR from V2 to V1 at the solar wind speed. A spherically symmetric MHD model, with data from 1 AU as input, predicts the GMIR that caused the step decrease at V2 during the year 2000, consistent with a view that the GMIR had the form of a quasi-spherical shell. The radial extent of the GMIR was 13 20 AU at V2 and 28 AU at V1, indicating a deviation of the shape of the GMIR from spherical symmetry. It is possible that, when the trailing edge of the GMIR moved past V1, the leading part of the GMIR was in the heliosheath beyond the termination shock, if the termination shock was between 80 and 100 AU. The GMIR consisted of a cluster of magnetic field strength enhancements by a factor of approximate to2 on average. Decreases in the cosmic-ray intensity were observed by V2 and by V1 (starting at the end of 2000 and the beginning of 2001, respectively), and regions containing intense magnetic fields, which were probably parts of a second GMIR, caused them. Throughout the years 2000 and 2001, both V1 and V2 observed a correlation between changes in the cosmic-ray intensity and the magnetic field strength that are qualitatively described by the CR-B relation. Thus, the GMIRs remain effective modulation agents as they travel through the most distant regions observed to date.