The ionosphere is called that because it is a very thin plasma of electrically charged atoms (ions) and electrons, due to the ultraviolet (UV) radiation from the Sun. Technically it extends from about 50km up to over 1000km (thanks Wikipedia!), but LEO starts at 150km—below that, you can’t maintain a stable orbit. The ionosphere, as mentioned, is driven by solar activity. The portion facing the Sun has more ionization; also, solar activity can drive its behavior strongly. There are also dips in the magnetic field line, leading to radiation increases at lower altitudes. We’ve mentioned the poles, and regions such as the South Atlantic Anomaly (SAA) also have field lines that dip lower.
Having stated you can fly anything, obviously you want to choose what you fly to achieve your goals. For example, if you’re sending up sensors, you’ll want to ensure a couple things: