Exposed to the solar wind, the surface of the Moon is at the mercy of a powerful and constant stream of plasma moving radially out from the Sun. At certain points, such as crater walls and mountains at the poles, the flow of plasma is near horizontal. The obstruction causes a sort of wake to form on the sheltered side, over which some plasma tumbles.
The solar wind is composed of ions and electrons. The ions are a couple of thousand times more massive than the electrons and so take more energy to be displaced from their intended direction. The result is more electrons than positive ions tumble into sheltered areas. This creates an imbalance that has to then be shorted out somehow. The researchers have suggested a mechanism by which negatively charged dust rises from the surface to repel negatively charged electrons coming down. The Apollo astronauts in command modules above the surface of the Moon reported possible sightings of sun beams near crater edges at sunrise, suggesting some kind of particulate pollution in places. Further studies will be undertaken into this as the potential for electric discharges could damage equipment sent to study the Moon.
More detail with explanatory videos is here.