Aurora have been observed for thousands of years and they are the most dramatic indications of solar activity. They are produced when flows of energetic charged particles collide with the upper atmosphere.
The brilliant colors from reds to purples indicate atoms of oxygen and nitrogen being stimulated by these collisions to give off specific wavelengths of light. They are produced at elevations from 65 kilometers to 1000 kilometers, under conditions where the atmosphere is a better vacuum than you would find inside a TV picture tube. Because of the specific way in which the light is produced, it is impossible for aurora to happen in the higher-density layers of the atmosphere below 50 kilometers. Despite the appearances to casual observers, the aurora never reach the ground.
Auroral activity is most intense during times when solar activity is the highest and the Coronal Mass Ejections make their way to Earth to impact the magnetosphere. They can also be produced as various parts of the magnetosphere rearrange in the so-called geotail region, which extends millions of miles into space on the opposite 'night time' side of the earth from the sun.
The ionosphere is a narrow zone of charged particles in the earth's atmosphere. It was not discovered until 'wireless' radio communication was invented around the turn of the century. It has an average density of about 1O electrons per cubic centimeter, but can be 10 to 100 times as 'charged' during solar storms.
At low frequencies below 10 megacycles, the ionosphere acts like a mirror and allows ground to ground signals to be 'bounced' long distances around the earth. At higher frequencies the ionosphere becomes transparent so that communication via ionosphere bounce becomes impossible. Instead, we must rely on satellite communication to relay signals from point to point on the earth.
The properties of the ionosphere change with the time of day, the season, and especially with the level of solar activity. In the later case, solar flares can cause radio signal 'fade outs' which are well-known to amateur radio operators.