The Communication/Navigation Outage Forecasting System (C/NOFS) satellite was launched in 2008, during solar minimum conditions. An unexpected feature in the C/NOFS plasma density data is the presence of deep plasma depletions observed at sunrise at all satellite altitudes. Ionospheric irregularities are often embedded within these dawn depletions. Their frequencies strongly depend on longitude and season. Dawn depletions are also observed in coincident satellite passes such as DMSP and CHAMP. In one example the depletion extended 50° × 14° in the N‐S and E‐W directions, respectively. These depletions are caused by upward plasma drifts observed in C/NOFS and ground‐based measurements. The reason for these upward drifts is still unresolved. We discuss the roles of dynamo electric fields, over‐shielding, and tidal effects as sources for the reported depletions.

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It has been proposed that ionosondes can be used to measure vectorial nighttime ionospheric drifts at F region altitudes. These measurements have been validated at mid and high magnetic latitudes on campaign basis. Here we report concurrent F‐region drift measurements made at Jicamarca (11.95°S, 76.87°W), under the magnetic equator, using the main Incoherent scatter radar (ISR) and a digisonde portable ionosonde (DPS). As far as the vertical drift measurements, we show a fair agreement between the two techniques at periods when convection dominates other factors (e.g., around pre‐reversal enhancement). At other times, production and recombination dominate, and DPS vertical drifts are not reliable. For the horizontal drift component, we limited our measurements to the zonal component. We find poor agreement, being worse at times when the E region electron density is high (e.g., during the day). The amplitude of DPS zonal velocities are significantly larger than ISR zonal drifts. During the day, we find that the DPS zonal drifts are in better agreement with the drift velocity of the long wavelength equatorial electrojet (EEJ) instabilities. This is to be expected since, the diffraction pattern on the ground – to which any reflection HF drift technique will be sensitive to – is mainly, if not solely, dependent on the electron density structure at EEJ heights, where the electron irregularity density is sufficiently high to diffract the phase front of the F region reflected wave.

Abstract.In November 2004, a large and variable interplanetary electric field (IEF) was felt in the reference frame of the Earth. This electric field penetrated to the magnetic equator and, when the Jicamarca Radio Observatory (JRO) was in the dusk sector, resulted in a reversal of the normal zonal component of the field. In turn, this caused a counter-electrojet (CEJ), a westward current rather than the usual eastward current. At the time of the normal pre-reversal enhancement (PRE) of the eastward field, the Jicamarca incoherent scatter radar (ISR) observed that the westward component became even more westward. Two of the three current explanations for the PRE depend on the neutral wind patterns. However, this unique event was such that the neutral wind-driven dynamos could not have changed. The implication is that the Haerendel-Eccles mechanism, which involves partial closure of the equatorial electrojet (EEJ) after sunset, must be the dominant mechanism for the PRE.