Photoelectron flux in the topside ionosphere measured by retarding potential analyzers

Satellite measurements of suprathermal electrons made at the time of conjugate sunrise indicate that they represent photoelectrons escaping from the production level of about 300 km and moving along the geomagnetic field lines into the conjugate ionosphere. The daytime maximum value of the flux is about 2.5 × 10 8 electrons cm −2 sec −1 . The spectral distribution of the flux shows a maximum around 5‐10 ev.