Solar Cycle
The phenomena being studied by the Ulysses mission are strongly influenced by both the 11 year solar activity cycle, and the 22 year Hale solar magnetic cycle. The first and second polar passes occurred during the descending phase of solar activity cycle 22, close to solar minimum. The third and fourth polar passes, on the other hand, took place in 2000 and 2001, close to solar maximum.
The polarity reversal of the Sun's polar cap magnetic fields occurred in 2000-2001, leading to a global reconfiguration of the heliospheric magnetic field. With the third mission extension approved, the mission end date is now set for March 2008. This allows high-latitude measurements to be taken under near-minimum solar activity conditions, but with the opposite magnetic field polarity compared with 1994/1995. Such measurements are crucial to understand in detail the propagation of cosmic rays and solar energetic particles in the complex heliospheric magnetic field, for example.