Abstract
The ionospheric E‐region from 100 km to 150 km is typically regarded as being in photo‐chemical equilibrium (PCE) state during daytime. Incoherent scatter radar observations at Arecibo show that PCE is not always valid. During July 31–Aug. 5, 1992, the vertical ion drift was observed to increase by 50 m/s over a ten kilometer altitude range. The large divergence resulting from the steep velocity gradient reduced the electron concentration by 30%, invalidating the generally held PCE. Observations of non‐PCE allow us to estimate the chemical loss rate. By assuming that the loss rate does not have a day‐to‐day variability, the dissociative recombination rate derived from the incoherent scatter radar (ISR) observation is about 1.0 × 10−7 cm3s−1, which is significantly lower than the laboratory rate of NO+, or O2+. The most plausible explanation for the discrepancy is that the relative concentration of O2+ and NO+ changed significantly when non‐PCE occurred.
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