SOLAR ORIGIN OF THE 26-DAY PERIODICITY OBSERVED BY ULYSSES

The Ulysses spacecraft discovered that the interplanetary magnetic sec tor structure went through a major restructuring in mid-1992. The obse rved recurrence period changed from about 25.4 days to about 26.2 days . Another interesting discovery is that the solar wind speed, energeti c particle fluxes, and interplanetary magnetic field all varied quasip eriodically with a similar 26.2-day period during Ulysses' midlatitude passage south of the ecliptic in 1992-1993. In order to find the sola r origins of these interplanetary phenomena, we compared Ulysses obser vations with relevant solar data. According to our study the global pa ttern of the open magnetic field lines originating in the photosphere changed drastically in June 1992, and this resulted in a major restruc turing of the interplanetary sector structure. After that time the mag netic field pattern in the midlatitude and high-latitude zones of the southern hemisphere was dominated by two large unipolar regions (cover ing the entire longitude interval) that rotated with a synodic period of about 28.5 days until mid-1993. Because the heliographic longitude of the spacecraft remained the same while it approached the Sun, the 2 6.2-day period seen by Ulysses is equivalent to the terrestrial synodi c period of 28.5 days. By analyzing soft X ray data observed by the Yo hkoh satellite we confirm the existence of a stable lobe protruding fr om the polar coronal hole. This protrusion persisted from 1992 until t he end of the study in mid-1995.