Electron beams are identifiable groups of electrons that occasionally are seen streaming along the magnetic field over active auroral arcs. High-energy beams appear at the outer edges of the plateaus in distribution functions. Low-energy beams are imbedded well within a broad distribution function and usually are seen when the rocket is near the edge of an arc. The present analysis shows that the observed distribution functions containing low-energy beams are unstable to upper hybrid waves in the immediate vicinity of the rocket. The most unstable waves have growth lengths and perpendicular wavelengths of 0.3 km. Electrostatic whistlers with wavelengths and growth lengths of several kilometers may grow well above the rocket. These growth lengths are too long to destroy the observed beams through quasilinear or nonlinear processes. It is concluded that electrons are accelerated in a parallel electric field associated with closed or U-shaped equipotentials. The electrons become unstable during or after acceleration and form plateaus plus residual high-energy beams. Low-energy beams involve acceleration of cold ionospheric electrons that enter the acceleration region above open equipotentials at the edge of an arc.