  In the declining phase of the solar cycle, when the new-polarity fields of
the solar poles are strengthened by the transport of same-signed magnetic flux
from lower latitudes, the polar coronal holes expand and form non-axisymmetric
extensions toward the solar equator. These extensions enhance the occurrence of
high-speed solar wind streams (HSS) and related co-rotating interaction regions
in the low-latitude heliosphere, and cause moderate, recurrent geomagnetic
activity in the near-Earth space. Here, using a novel definition of geomagnetic
activity at high (polar cap) latitudes and the longest record of magnetic
observations at a polar cap station, we calculate the annually averaged solar
wind speeds as proxies for the effective annual occurrence of HSS over the
whole Grand Modern Maximum (GMM) from 1920s onwards. We find that a period of
high annual speeds (frequent occurrence of HSS) occurs in the declining phase
of each solar cycle 16-23. For most cycles the HSS activity clearly maximizes
during one year, suggesting that typically only one strong activation leading
to a coronal hole extension is responsible for the HSS maximum. We find that
the most persistent HSS activity occurred in the declining phase of solar cycle
18. This suggests that cycle 19, which marks the sunspot maximum period of the
GMM, was preceded by exceptionally strong polar fields during the previous
sunspot minimum. This gives interesting support for the validity of solar
dynamo theory during this dramatic period of solar magnetism.
