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An ionospheric trough is a region of decreased F layer plasma density. At these altitudes the plasma depletion is due to decreased concentration of O+, i.e., it is different from the 'light ion trough' in the topside ionosphere. Note that the word trough is also used with the region of low cold plasma densities just outside the plasmapause in the magnetosphere; also there the depletion of lighter ions is responsible for the feature.
There are two ways to categorise the troughs. It is quite typical to make a distinction between the sub-auroral troughs at mid-latitudes, and high-latitude troughs within the auroral region or polar cap. Another possibility is to consider the formation mechanism operating. It seems that ion convection plays a crucial role in the trough development. However, there are two different ways for it to operate (e.g., Rodgers et al.,1992):
The mid-latitude trough (sometimes also called as the main ionospheric trough) is typical for the sub-auroral ionosphere (see, e.g., Moffett and Quegan, 1983; Roger et al., 1992). It has been explained with the stagnation model: the plasma observed has been convecting through nightside in the absence of ionisation sources, and thus 'decayed'.
The trough is primarily a nightside phenomenon, though it has been observed at all local times. It is azimuthally extended but only a few degrees wide. When observed in the dusk sector, the latitude decreases with local time from about 75° to 55° (L = 15 - 3): thus many dayside troughs belong often to this 'mid-latitude' category (e.g., Whalen, 1987, 1989; Mallis and Essex, 1993) in spite of their high latitude. The poleward, field-aligned edge of the trough lies close to the equatorward boundary of the diffuse aurora (region 2 field-aligned current system). As expected, the trough moves to lower latitudes with increasing geomagnetic activity (see, e.g., Collis and Häggström, 1988). Mid-latitude troughs are also characterised by high electron temperatures because of reduced cooling (proportional to electron density) and increased soft precipitation.
The relationship between the mid-latitude trough and the trough near the Harang discontinuity (Williams and Jain, 1986; Winser et. al., 1986) seems somewhat unclear (at least to the present author). Relationships to such features as plasmapause and SAR-arcs may also be worth more studies.
The SAID troughs are a subset of mid-latitude troughs. They are associated with large, short-lived electric fields observed immediately equatorward of the auroral oval and related to enhanced geomagnetic activity. Rodger et al. (1992) define SAID as an event where an ion velocity of > 1 km/s occurs. At this limit the effective O+ recombination rates (dominated by O+ + N2 reaction) become enhanced by a factor > 2. This, together with frictional heating induced upward ion flows, creates the trough. Note that the trough can be embedded in a stagnation mid-latitude trough.
The best known stagnation-type high-latitude trough is the 'ionisation hole' or 'polar hole', so far observed only in the southern polar cap (Brinton et al., 1978). For review of other high-latitude trough observations, see Rodger et al. (1992). In short, they are formed by frictional heating mechanism in the auroral oval. The electric fields needed can be found, e.g., at the edges of auroral arcs and at the poleward edge of the auroral oval.