Edward Appleton won a Nobel Prize for his confirmation of the existence of this region of the Earth's atmosphere. This region of the atmosphere encompasses other regions like the exosphere and thermosphere, and forms the inner portion of the magnetosphere. This region's name comes from the fact that atoms and molecules here are positively charged by ultraviolet radiation from the Sun, and a solar flare can momentarily increase this charge. For 10 points, name this region of Earth's atmosphere where radio propagation takes place.
a) Mesosphere
b) Exosphere
c) Stratosphere
d) Ionosphere