"An unexpected and drastic drop in the phase velocity Vph of Farley‐Buneman (FB) waves with increasing altitude was observed in the equatorial electrojet over Jicamarca. The effect was detected with the newly employed 430‐MHz radar looking vertically. The decrease in Vph was 67 m/s and 36 m/s over 2.4 km for the FB waves moving towards and away from the radar, respectively. By contrast, the 430‐MHz data from 20° west displayed little dependence on altitude. Simultaneous observations with a 50‐MHz radar at 23° and 51° west also displayed little change of Vph with altitude. We show that electron inelastic cooling which defines gradual transition from super‐adiabatic to isothermal processes at 50 MHz (used in majority of observations), becomes unimportant at higher frequencies. The effect is evinced at radar frequencies ≥150 MHz and requires altitude resolution <2 km to be observed. Averaging over >7 km at oblique incidence masks the effect".
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