Convection electric field observations near the Arecibo HF heater beam

Electric fields have been observed at the westward edge of the Arecibo HF h eater beam, at the F region reflection altitude, by a sounding rocket. We f irst show that the floating double probe used for these measurements is fun ctioning reliably. The data are shown to be free of sheath rectification ef fects within the heated volume. Away from the heater beam the double probe data show primarily motional emf, consistent with Earth frame fields of les s than 2 mV m(-1) (typical, on average, of summer post-midnight conditions over Arecibo), while near the heater beam, significant Earth frame fields a re observed. The convective drift due to these fields is consistent with th e apparent drift of simultaneously measured heater-induced field-aligned ir regularities. The measured convection electric field magnitude is used to e stimate the strength of the current maintaining it against the background c onductivity, yielding 5 mA m(-1). Estimates of the current due to ponderomo tive force are lower than this value, unless one invokes electrostatic wave s with electric fields roughly 8 times that of the pump field. Temperature gradient currents are estimated as well and are a possible source. The obse rved shear in the convection electric field is used to estimate a field-ali gned current density of 0.5 muA m(-2). We conclude that the heater is indee d driving convection on the scale size of the heated volume but are unable to definitively determine how the corresponding electric fields are being p roduced.