Corona discharges are luminous signs of strong local electric fields allowing a continuous discharge into the surrounding atmosphere. They commonly occur at the ends of conductors at high voltage. Here we report the observation of a faint glow surrounding an insulating cm-sized box filled with mm-sized basalt beads. At an ambient pressure in the mbar range two light bands occur as soon as and only if the box is vibrated and only if it is filled with a granular medium. In addition, a glow also occurs at the inside of the box. We measured periodic electric fields at the outside of the box with spatial peaks at the positions of the light bands. The period correlates to the vibration frequency. These observations imply strong alternating fields beyond atmospheric breakdown, which are generated inside and also emerge at the outside of the insulating box. The observations can be explained by tribocharging and periodic displacement of charges between grains and the inside walls of the box.