  Solar coronal holes are sources of high-speed solar wind streams, which cause
persistent geomagnetic activity especially at high latitudes. Here we estimate
seasonal solar wind speeds at 1 AU for the last 100 years using high-latitude
geomagnetic measurements and show that they give information on the long-term
evolution of important structures of the solar large-scale magnetic field, such
as persistent coronal holes. We find that the centennial evolution of solar
wind speed at 1 AU is different for equinoxes and solstices, reflecting
differences in the evolution of polar coronal hole extensions and isolated
low-latitude coronal holes. Equinoctial solar wind speeds had their centennial
maximum in 1952, during the declining phase of solar cycle 18, verifying that
polar coronal holes had exceptionally persistent extensions just before the
peak of the Grand Modern Maximum of solar activity. On the other hand, solstice
speeds had their centennial maximum during the declining phase of solar cycle
23 due to large low-latitude coronal holes. A similar configuration of seasonal
speeds as in cycle 23 was not found earlier, not even during the less active
cycles of early 20th century. Therefore the exceptional occurrence of
persistent, isolated low-latitude coronal holes in cycle 23 is not related to
the absolute level of sunspot activity but, most likely, to the demise of the
Grand Modern Maximum.
