Electric fields have been observed at the westward edge of the Arecibo HF heater beam, at the F region reflection altitude, by a sounding rocket. We first show that the floating double probe used for these measurements is functioning reliably. The data are shown to be free of sheath rectification effects within the heated volume. Away from the heater beam the double probe data show primarily motional emf, consistent with Earth frame fields of less than 2 mV m-1 (typical, on average, of summer post-midnight conditions over Arecibo), while near the heater beam, significant Earth frame fields are observed. The convective drift due to these fields is consistent with the apparent drift of simultaneously measured heater-induced field-aligned irregularities. The measured convection electric field magnitude is used to estimate the strength of the current maintaining it against the background conductivity, yielding 5 mA m-1. Estimates of the current due to ponderomotive force are lower than this value, unless one invokes electrostatic waves with electric fields roughly 8 times that of the pump field. Temperature gradient currents are estimated as well and are a possible source. The observed shear in the convection electric field is used to estimate a field-aligned current density of 0.5 &mgr;A m-2. We conclude that the heater is indeed driving convection on the scale size of the heated volume but are unable to definitively determine how the corresponding electric fields are being produced. ¿ 2001 American Geophysical Union