Photoelectron flux and protonospheric heating during the conjugate point sunrise

Selected Explorer 31 data have been studied to establish the extent of protonospheric heating by the onset of photoelectrons from the conjugate ionosphere. Observations made during low-latitude passes (1.2<L<2.5) at altitudes above 2000 km have been analyzed to determine the electron and ion temperatures and the flux of photoelectrons for the periods when one foot of the field line is in darkness and the other is undergoing sunrise. In the night hemisphere there is an increase in electron temperature from about 1500° to 3500°K and in ion temperature from about 1500° to 3000°K and a simultaneous increase in suprathermal flux from about 2×107 to 15×107 electrons cm−2 sec−1 for electrons of energies greater than 3.7 ev. Observations also show an appreciable return flow of photoelectrons back into the sunlit hemisphere from the conjugate night hemisphere. Theoretical calculations of the energy and flux degradation of the photoelectrons along the field line are made and compared with the observations. Using the photoelectron energy loss as the energy input to the ambient plasma and using the steady-state heat conduction equation, the electron temperature profile along the field line is calculated.