Charge and mass transfer in ice-ice collisions: Experimental observations of a mechanism in thunderstorm electrification

A microphysical mechanism that may be important in the electrification of t hunderstorms is probed in an experimental study of collisional charging of ice surfaces. The study examined mass and charge transfer in single collisi ons between two ice samples and their dependence on growth, temperature, im pact strength, and time delay between impacts. The polarity is consistent w ith earlier results in that the more rapidly growing particle charges posit ively. We find that the magnitudes of charge and mass transfer increase wit h growth rate over an appreciable range, but the charge tends to saturate a t high rates. The systematic dependence indicates that the charge transfer is more closely related to the difference in growth rate between the contac ting surfaces rather than to their temperature difference. This result prov ides quantitative confirmation of earlier researchers' work. An appreciable time delay is required for the magnitude of the charge to recover from a p revious collision. The magnitude of charge transfer cannot be explained by the thermoelectric effect or by any other mechanism proposed to date. It ap pears that the charge is carried in the mass that is transferred between th e colliding surfaces, as predicted by Baker and Dash, [1989, 1994] but the magnitude of the mass transfer is much greater than can be due to ordinary surface melting as proposed.