A field-aligned electric field, ΔE∥, which is accompanied by drift Alfv¿n wave perturbations in an inhomogeneous plasma, can be expressed in simple terms as ΔE∥=Te/e k2⊥&rgr;2i ∂Δ&psgr;/∂ℓ ∂ℓnn/∂&psgr; where &psgr; and Δ&psgr; are the magnetic flux function and its perturbation, Te is the electron temperature, n(&psgr;) is the plasma density, k⊥ is the perpendicular wave number, &rgr;i is the ion Larmor radius and ℓ is the coordinate along a magnetic line of force. ¿American Geophysical Union 1990