Simultaneous observations of F_2 layer stratification and spread F at postmidnight over a northern equatorial anomaly region

摘要

Simultaneous observations of F_2 layer stratification and spread F at postmidnight (00:00 LT to 05:00 LT) were carried out on 22, 23, and 28 November 2013, using ionosondes distributed over a northern equatorial anomaly region at three specific locations, i.e., Puer (PUR, 22.7°N, 101.05°E, dip latitude 12.9°N), Chiang Mai (CMU, 18.8°N, 98.9°E, dip latitude 9.04°N), and Chumphon (CPN, 10.7°N, 99.4°E, dip latitude 0.93°N). The results show that both the PUR and CMU stations observed the F_2 layer stratification at postmidnight in the Northern Hemisphere, frequently accompanied with gravity waves (the periods~30–100min). It is reported that F_2 layer stratification at postmidnight can be observed in the Northern Hemisphere for the first time. It is suggested that the thermospheric neutral wind triggered by gravity waves strongly contribute to the altitude dependence of the combined vertical plasma velocity, which consequently poses significant impacts on the occurrence of the low-latitude F_2 layer stratification at postmidnight. In addition, the spread F other than F_2 layer stratification was observed at the CPN station located at the geomagnetic equator, suggesting that smaller geomagnetic inclination tend to inhibit the postmidnight F_2 layer stratification in the equatorial region. Furthermore, on 23 November 2013 a good correlation was identified between the F_2 layer stratification at PUR and the spread F at both CMU and CPN, possibly due to that the large-scale gravity waves originating atmiddle latitudes contribute to the nighttime spread F observed in the low-latitude and equatorial regions.