Hi Jennifer,
That is a great question! It took many different scientists centuries to work out that the Sun's magnetic field and solar activity affected the Earth's magnetosphere. One of the first people to study the Earth's magnetic field was an English scientist named William Gilbert who lived from 1544 to 1603. When William Gilbert was alive, people already knew about the magnetic properties of iron ore (lodestone) and used compasses in navigation. However, they did not really understand the Earth's global magnetic field. William Gilbert was one of the first to study the Earth's magnetic field using a round lodestone he called a terella to simulate the Earth. He also established a lot of the magnetic terminology we use today. Years later, around 1612-13, Galileo made observations of sunspots. However it would still be years before people understood the connection between the Earth's magnetic field and solar activity. A Norwegian scientist named Kristian Birkeland was the first to discover that the aurora borealis or northern lights were related to the Earth's magnetic field in the early 1900s. Birkeland was an adventurer who made this discovery through his polar expeditions in 1902-03 and laboratory experiments using terellas similar to the ones that Gilbert used. In 1908, an American astronomer named George Ellery Hale realized that sunspots had magnetic structure. A few years later, Birkeland proposed the existence of a solar wind as well as field-aligned currents in the Earth's magnetosphere, but his ideas were controversial for a long time because they could not be verified using ground-based measurements. It was not until the first satellites were launched in the late 1950s that scientists were able to confirm Birkeland's theories about the connection between the Sun and the aurora.
You can read more about Galileo and William Gilbert here-
http://www.magnet.fsu.edu...ioneers/gilbert.html
http://galileo.rice.edu/s...unspot_drawings.html
And about George Hale and Kristian Birkeland here -
http://physics.usc.edu/solar/history.html
http://en.wikipedia.org/wiki/Kristian_Birkeland
Kris