Hi,
The first person to actually measure the magnetic field of the Sun was George Ellery Hale, who measured the strong magnetic fields in sunspots from Mt. Wilson observatory in 1907-1908.

People had noticed before that a link to the solar cycle and geomagnetic activity, though,
which would make them wonder about the magnetic properties of the Sun.

See
https://www2.hao.ucar.edu...l-timeline-1800-1999
https://www.nps.gov/parkh...utowsky5/astro4a.htm

Terry