  Analysis of the solar corona structure during the periods of minimum solar
activity from 1867 till 2006 has been carried out. A new flattening index for
the large coronal streamers has been proposed. It has been shown that the index
has been smoothly changing during the last 140 years. The minimal value of the
index occurred during activity cycles 17--19; this was the period when the
solar corona most of all corresponded to the dipole configuration of the global
magnetic field of the Sun. At the beginning of the 20th and the 21st centuries,
the corona structure corresponded to the quadrupole configuration. The reasons
for the variations in the solar corona structure and its relation to
geomagnetic activity are discussed.
