From AMS Glossary
Any net electrical charge that exists in a given region of space.
In electronics, this usually refers to the electrons in the space between the filament and plate of an electron tube. In atmospheric electricity, space charge refers to a preponderance of either negative or positive ions within any given portion of the atmosphere. A net positive space charge is found in fair weather at all altitudes in the atmosphere and is largest near the earth's surface. The general downward flux of this positive space charge is known as the air–earth conduction current.