A more thorough explanation:

Definition: Immediately; without delay; promptly.

Examples:

• The teacher announced the pop quiz, and thereupon the students quickly took out their pencils and paper.
• The judge made his ruling, and thereupon the lawyers began to prepare their appeals.

The examples illustrate how "thereupon" is used to indicate that something happens immediately or promptly after another event. In the first example, the students take out their pencils and paper without delay after the teacher announces the pop quiz. In the second example, the lawyers begin to prepare their appeals promptly after the judge makes his ruling.