Low currents are due to ionization by external agents such as cosmic rays, high energy photons, etc.
In this region the current is subject to statistical fluctuation. In this region the discharge is self-sustained, meaning if external agents are removed we still get the same current. This is the breakdown voltage of the device. Abnormal Glow Region. The current then increases to a value C which is largely determined by the impedance of the external circuit, and the voltage across the neon lamp reduces to a lower value D, known as the maintaining voltage.