A case study of postmidnight enhancement in F‐layer electron density over Sanya of China

Abstract

[1] The electron density in the ionospheric F region occasionally stops its decay and rises pronouncedly during night hours, which are termed ionospheric nighttime enhancements. In this case study, we analyzed the manually scaled ionogram records measured by a Lowell DPS-4D ionosonde operated at Sanya (18.3°N, 109.6°E), China, to explore postmidnight enhancement events occurred in 2012, a year of moderate solar activities. Common features in these cases illustrate that, accompanying nighttime rises in peak electron density of F2 layer (NmF2), the height of F2 layer is depressed significantly, and the ionogram-derived electron density height profiles become thinner. There are time shifts in the development of electron density enhancements in the F layer; that is, enhancement develops earlier and reaches peaks earlier at higher altitudes than at lower altitudes. Meanwhile, plasma drift is detected downward under such events, revealing the essential role of the westward electric field in forming the postmidnight enhancements in electron density of ionospheric F layer at such low latitudes.

Comparisons of the variation of the ionospheric TEC with NmF2 over China

“…It is also shown that the peak of NmF2 is usually delay 1 or 2 hours compare to the peak of TEC in the diurnal variation. This is possibly attributed to the westward electric field in F2 layer in the afternoon, which causes the E×B driven downward plasma movement from the plasmasphere and enhance the electron density in F2 layer [11]. …”

Effects of the midnight temperature maximum observed in the thermosphere–ionosphere over the northeast of Brazil

“…This behavior is to be expected because the rate of loss of electronic density is high for altitudes between 200 and 300 km (Liu et al, 2013). On the other hand, from September to December and in February there was a small decrease in foF2 in the PRE period, which, for a short period of time, returned to growth.…”

“…On the other hand, from September to December and in February there was a small decrease in foF2 in the PRE period, which, for a short period of time, returned to growth. As suggested by Liu et al (2013), this behavior during the PRE period is due to the upward vertical drift that pushes the plasma to high altitudes where the recombination rate is insignificant, whereas in low altitudes the electronic density decreases significantly.…”

“…Liu et al (2013) associated the increase in foF2 after the PRE period to the westward electric field inducing vertical drift, concluding that downward drift is essential for the increase in the second peak of foF2. This increase occurs af- ter midnight at Sanya, China (18.24 • N; 109.50 • E), and at Eusébio it occurs before midnight.…”

Dusk-to-nighttime enhancement of mid-latitude <i>Nm</i>F2 in local summer: inter-hemispheric asymmetry and solar activity dependence

“…Thus, diurnal variation feature of the F layer electron density sometimes deviates from the diurnal trend of the Chapman theory owing to the effect of these factors. A most significant deviation from the Chapman theory is the nighttime enhancement of the electron density (e.g., Dabas and Kersley, 2003;Davies et al, 1979;Essex and Klobuchar, 1980;Evans, 1965;Farelo et al, 2002;Liu et al, 2013;Luan et al, 2008;Tsagouri and Belehaki, 2002). The nighttime enhancement refers to the phenomenon 1959 and 1970-1985 that the electron density increases during the local nighttime hours.…”