We find that ions E¿B drifting through an auroral arc can undergo transverse acceleration and stochastic heating. This result is very analogous to recent work regarding similar phenomena in the magnetotail (B¿chner and Zelenyi, 1990; Chen and Palmadesso, 1986; Brittnacher and Whipple, 1991). An analytic expression for the maximum arc width for which chaotic behavior is present is derived and numerically verified. We find, for example, that a 1.5-km-thick arc at &Lgr;=65¿ requires a minimum potential drop of 3 kV for transverse ion acceleration and heating to occur. Thicker arcs require higher potential drops for stochasticity to occur. This mechanism could be a source for conic ions. ¿ American Geophysical Union 1992