Hi,
I was in a fairly broad department in graduate school - people in it studied every thing from the Earth's atmosphere through the solar system and out into the universe. I was interested in most of those things - but you need to settle on something in particular when you write your thesis. There was a professor who was looking for a student to work with data from the Sun. I thought that would be good because it related to astronomy, which I had like since I was a kid, with something that affected life on Earth, making it more immediate. I ended up studying radio waves from solar flares.
cheers,
Terry