Middle-latitudinal band structure observed in the nighttime ionosphere

In this study, the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) electron densities (Ne) from 200 to 800 km during 2006-2017 are used to investigate the middle-latitudinal band structure of the nighttime ionosphere. The main results are as follows. (1) In the nighttime, the Ne at about ±40° geomagnetic latitudes is generally greater, especially in the topside ionosphere. (2) Double bands of relatively high Ne are seen at all longitudes during the equinoxes, while only one latitudinal band is clearly visible at specific longitudes in the winter hemisphere during the solstices. (3) The middle-latitudinal band structure of the topside ionosphere is more pronounced at later local times (LTs) at night and under lower solar activity. (4) The middle-latitudinal band structure mainly occurs at 23-05 LT. (5) The seasonal/longitudinal variations of the middle-latitudinal band structure are consistent with those of the plasmaspheric total electron content (TEC) at low latitudes and upward field-aligned winds at middle latitudes. (6) At the middle-latitudinal bands, the downward plasma diffusion from the plasmasphere provides stable plasma source to the topside ionosphere, while the upward plasma motion due to the neutral winds is the primary source for the F₂ peak region. (7) The plasma sources for the middle-latitudinal band structures at different altitude regions are probably different, so that the resulting measurements might be due to a coalescing of these two completely different mechanisms. (8) The modulated recombination rates due to the thermospheric dynamics could also reinforce this structure.

Figure 1. Variations of COSMIC electron densities as a function of geographic longitude and latitude (upper) and as a function of geomagnetic latitude (bottom) during March equinox and under low solar activity. The left panels show the results at 800 km and 03 LT (middle-latitudinal band), and the right panels are at 300 km and 19 LT (equatorial ionization anomaly). The contour lines of the apex geomagnetic latitudes at 0° and ±40° (gray dash line) are also plotted in the upper panel for reference. The red line in the bottom panel denotes the median values.