Ionospheric heating by magnetic conjugate-point photoelectrons

Regular observations of incoherent backscatter spectrums of the ionosphere at the Arecibo Ionospheric Observatory in Puerto Rico since June 1964 establish presunrise and postsunset ionospheric heating above the Fmax to be present during the winter, though less evident toward either equinox, and absent during the summer. The observed heating is interpreted in terms of a streaming of photoelectrons from the sunlit magnetic conjugate ionosphere. The detailed timing of the onset of the heating supports this, and a semiquantitative model for the heating mechanism explains further details of the observations. Photoelectrons streaming from the magnetic conjugate ionosphere are concluded to produce significant heating effects for the midlatitude upper F region.