An isolated conducting sphere of radius \(R\) carries an initial positive charge \(Q_0\) and gradually loses charge over time to the surrounding humid air. A fixed probe holding a small positive test charge \(q_0\) is stationed at a distance \(r = 3R\) from the center of the sphere to measure the magnitude of the electrostatic force \(F_E\) exerted on it as a function of time \(t\), yielding the graph shown. Which of the following statements correctly relates a physical property of the sphere to the features of this graph?