Anomalous enhancement of ionospheric electron content in the Asian-Australian region during a geomagnetically quiet day

We investigate an event of anomalous enhancement in ionospheric electron content on 28 June 2002 from a network of 70 GPS receivers and a chain of ionosondes distributed around the Asian-Australian sector. After local sunset, the low-latitude peak electron density of the F-2 layer (NmF2) was observed to increase by 200% compared to the 27-day median value. Geomagnetic environment was quiet during 27-28 June as the SYM-H index was larger than -20 nT and AE was less than 200 nT under conditions of a constant northward interplanetary magnetic field Bz. A continuously enhanced eastward electric field has been identified as the main cause of this effect, though it is not fully consistent with the prediction of Scherliess and Fejer (1997) for wind disturbance dynamo model. Our observations show that the ionosphere was very disturbed in the Northern Hemisphere during summer even under low geomagnetic activity. The large ionospheric disturbance may be caused by the combination of the effects due to auroral activity and tides, a combination that still needs further investigation. This work suggests that studying very large day-to-day variability under quiet geomagnetic conditions could reveal more about the coupling processes between the ionosphere and lower atmosphere.