Unusual Enhancement of Ionospheric F2 Layer Critical Frequency Before the 23 August 2011 Virginia (USA) Earthquake

A case study was made of the variations of the ionospheric F2 layer critical frequency (foF2) before the M = 5.8 earthquake that struck Virginia (USA) on 23 August 2011. The data collected at 9 mid-latitude ionosonde stations during 22-23 August 2011 have been examined. The results demonstrate an unusual enhancement in foF2 observed about 14 h prior to the shock at Wallops Is. ionosonde, which is close to the earthquake epicenter. Meanwhile, no perturbations in foF2 were detected at the other ionosondes located far from the earthquake epicenter. We suggest that the enhancement in foF2 over Wallops Is. is likely associated with some earthquake preparatory processes.