Imagine that the Earth's magnetic field is like a harp. When a string is plucked it causes a vibration - we hear this vibration as a sound. Plucking a different string produces a different sound. A gentle pluck will produce a quiet sound while a strong pluck will make a loud sound.
The Earth's magnetic field is constantly being plucked by particles in the solar wind. As a result, it is full of waves. These appear at different times and places depending on what event on the Sun injected the particles into the solar wind - a coronal mass ejection, a solar flare or whatever.