Even though the normal E region is Chapman-like in nature, isolated forms of ionization are often observed in the E region having a variety of shapes and sizes. These ionization forms have been termed sporadic E because they appear quasi-randomly on a day-to-day basis, and they generally defy deterministic prediction methods. Sporadic E (or Es ionization) has been observed during rocket flights and with incoherent backscatter radar, and a layer thickness of the order of 2 kilometers has been observed. They are generally large-scale structures, having horizontal dimensions of 100s of kilometers at middle latitudes. Polar and equatorial forms have different structures and causal mechanisms. Although sporadic E consists of an excess of ionization (against the normal E region background) it does not appear to be strongly tied to solar photoionization processes. Still, midlatitude Es occurs predominantly during summer days. Sporadic E does exhibit seasonal and diurnal tendencies, which have been examined statistically, and at least three different types of sporadic E ionization have been discovered with distinct geographical regimes. These are low latitude (or equatorial), midlatitude (or temperate), and high latitude ionization. Figure 3-17 depicts the probability of Es occurrence.
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