SOLAR EXCURSION PHASES DURING THE LAST 14 SOLAR-CYCLES

The strong half solar rotation (about 13-14 days) periodicity in solar wind, IMF and geomagnetic activity was recently shown to arise from i ntervals during which the heliospheric current sheet is flat and tilte d. These intervals denote the excursion phases of the solar dipole. In this letter we locate the solar excursion phases for the last 14 sola r cycles using long-term observations of geomagnetic activity. This al lows us to study the evolution of the solar corona for more than 150 y ears, i.e. far longer than with any other continuous data set. In the last eight cycles, largest excursions were found in mid- to late decli ning phase of the solar cycle, in agreement with the present view on c oronal development during the solar cycle. However, in most of the ear lier cycles 9-14, excursions were less frequent and occurred close to sunspot maximum. This suggests that the solar corona was exceptionally stable during the early low-activity cycles, having open coronal hole s until close to sunspot maximum. Furthermore, we find that there is a connection between new sunspot activity and the solar dipole tilt.