Merged interaction regions and large-scale fluctuations observed by voyagers 1 and 2 in the distant heliosphere

Merged Interaction Regions (MIRs) and large-scale fluctuations of the heliospheric magnetic field play a major role in the structure and dynamics of the solar wind. We compare the Voyager 1 and 2 (V1 and V2) observations made near the maximum of solar activity (1989) with those made during the declining phase of solar activity (1983 and 1994). Global MIRs (GMIRs) with strong magnetic fields, preceded by a strong shock, were observed near solar maximum. During the declining phase of the previous solar cycle (in 1983), corotating MIRs (CMIRs) with a period of $∼26$ days were observed at 13–15 AU by both V1 and V2. During the current declining phase (in 1994), MIRs were observed by V1 at 54–58 AU and by V2 at 42–45 AU, but they were not periodic and their amplitudes were smaller than those observed in 1983. The absence of CMIRs at Voyager 2 during 1994 suggests a possible reconfiguration of CMIRs between 15 and 42 AU resulting in an absence of any apparent periodicity and a decreasing amplitude. These changes may be due to the increasing distance from the Sun, the departure of V1 from the equatorial zone of sector structure, temporal changes at the source, or a combination of these 3 effects.