Equinoctial asymmetry of a low-latitude ionosphere-thermosphere system and equatorial irregularities: evidence for meridional wind control

摘要

Nocturnal ionospheric height variations were analyzed along the meridian of100° E by using ionosonde data. Two ionosondes were installed near themagnetic conjugate points at low latitudes, and the third station wassituated near the magnetic equator. Ionospheric virtual heights were scaledevery 15 min and vertical × drift velocitieswere inferred from the equatorial station. By incorporating the inferredequatorial vertical drift velocity, ionospheric bottom heights with theabsence of wind were modeled for the two low-latitude conjugate stations,and the deviation in heights from the model outputs was used to infer thetransequatorial meridional thermospheric winds. The results obtained for theSeptember and March equinoxes of years 2004 and 2005, respectively, werecompared, and a significant difference in the meridional wind was found. Anoscillation with a period of approximately 7 h of the meridional windexisted in both the equinoxes, but its amplitude was larger in September ascompared to that in March. When the equatorial height reached the maximumlevel due to the evening enhancement of the zonal electric field, thetransequatorial meridional wind velocity reached approximately 10 and 40 m/sfor the March and September equinoxes, respectively. This asymmetry of theionosphere-thermosphere system was found to be associated with thepreviously reported equinoctial asymmetry of equatorial ionosphericirregularities; the probability for equatorial irregularities to occur ishigher in March as compared to that in September at the Indian to WesternPacific longitudes. Numerical simulations of plasma bubble developments wereconducted by incorporating the transequatorial neutral wind effect, and theresults showed that the growth time (e-folding time) of the bubble washalved when the wind velocity changed from 10 to 40 m/s.