Characteristics of the streamer stem are essential for the study on the stable leader inception. Experiments are conducted to observe the re-illumination of streamer stems under either rising or non-changing positive electric fields in long air gaps, preceding the inception of a stable leader. Based on the simultaneously measured applied voltage, discharge current, direct images, and schlieren images, two typical types of stem re-illuminations are identified and show great differences. For the stem re-illumination under the rising electric field, it is accompanied by an abrupt elongation of stem channel, intense optical radiation, and a current superposed on at least two asynchronous current pulses, while for that subjected to the nearly non-changing electric field, there are no elongation of stem channel, a much weaker light emission, and a single, double exponential current pulse. Moreover, the latter case shows obvious periodicity. A comparative analysis of experimental results shows that factors triggering these two types of stem re-illuminations may be also different in which not only the electric field but also its time derivatives play an important role. These findings can further deepen the understanding of the streamer stem before the leader inception.

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