Enhancement and HF Doppler observations of sporadic-E during the solar eclipse of 22 July 2009

The sporadic E (Es) frequently emerging in midlatitude during summer is a very special layer in the ionosphere, and its formation mechanism is different to from that of other layers. The total solar eclipse of 22 July 2009 provided a very unique opportunity to study the relationship of Es and solar radiation. During the solar eclipse day and the days before and after, the vertical incidence ionosonde was located in Wuhan to record the ionograms for this event. Two oblique incidence high-frequency radio systems were used to record the waves from Wuhan to Suzhou and from Wuhan to Huaian. The enhancement of Es during the eclipse period was observed in the vertical and oblique incidence ionograms. The quasi-periodic fluctuations in the critical frequency and Doppler frequency shift curves indicated the possible existence of the gravity waves, which may be responsible for the Es enhancement. However, we find that the enhancement occurred earlier than the appearance of gravity waves and consider that there may be other mechanisms which contribute to the observed enhancement in the ionosphere. A hypothesis is put forward that the cooling effect of the lunar shadow induced powerful airflow from the northern and southern limits of the shadow toward its center, which accelerated the irregularities in Es to produce the large-scale Doppler shift in the reflected waves and form the meridional windshear. Both the windshear and the gravity waves may affect the Es layer and increase the electron concentration. Many observed phenomena are in accordance with this.