Inner properties of seasonal topside ionospheric structures determined via DEMETER ion perturbations

In this investigation, 117718 ionospheric perturbations with a space size t = 20–300 s, but without a limit on amplitude (A), were automatically searched globally via software using ion density data measured by the Detection of Electromagnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite for approximately six years. After eliminating 18169 perturbations (PERs) that occurred in perturbed time (Kp ≥ 3) in order to avoid the global effects of geomagnetic storms, 99549 PERs were left. These PERs were divided into six groups with respect to local time and season, but only the summer and winter PERs were kept for good comparison. Each group of PERs was distributed on a map in terms of amplitude and space size. The distributions of all groups of plasma PERs showed that the equatorial ionization anomaly (EIA) structure developed well during the winter daytime. This structure is characterized by a low plasma PER density (Np) on both sides of the magnetic equator at low latitudes and a sudden enhancement of Np at a magnetic latitude of 15° at both sides of the magnetic latitudes. At the same time, this daytime anomaly was also clearly presented with a simultaneous enhancement in both Np and space size, even beyond 200 s along its boundary line. From the viewpoint of space size, there is a demarcation point, t = 120 s; the occurrence probabilities of day PERs are always higher than those at night before this point, whereas this result is reversed after this point. Another key point is that large spatial-scale perturbations (t > 200 s) present their major significance during daytime, especially during summer daytime, probably due to ionization under strong sunlight. The WSA (Wedell Sea Anomaly) phenomenon shows a clear configuration at all times except on the summer day. The topside ionosphere remained calm during the day in summer, and no positive PERs with a magnitude of more than 50% or a negative magnitude of more than 100% occurred in this WSA area. In winter nighttime, this WSA structure was characterized by major large positive PERs with a magnitude of more than 100%.