Lightning In Brown Dwarfs?

Observations have shown that continuous radio emission and also sporadic Ha and Xray emission are prominent in objects at the substellar limit. These activity signatures are interpreted as being caused by coupling of an ionized atmosphere to the stellar magnetic field. Current theory of brown dwarf atmospheres, however, states that the ionization level of these atmospheres is too low for magnetic coupling. Motivated by this contradiction, we investigate if the frictional energy released during gravitational settling of dust grains in brown dwarf atmospheres can enhance the ionization of the gas up to a level sufficient for magnetic coupling. As a first result we find that both the dust and the gas phase can be affected by collisional ionization of the dust phase. An enhanced electron fraction in the gas phase would be in favor for possible magnetic coupling. On the other hand, electric fields could be created by charge separation, causing electrostatic phenomena to occur such as discharging (lightning) known from the Earth's atmosphere. It remains to be investigated if these effects are strong enough to produce a detectable level of X-ray and/or radio emission.