Irregularities in the Ionosphere

In addition to the various anomalous features of the ionosphere, irregularities in the electron density distribution may be observed throughout the ionosphere. The size, intensity, and location of these irregular formations are dependent upon a number of factors including: geographical area, season, time-of-day, and the levels of solar and magnetic activity. The Traveling

Ionospheric Disturbance (TID, see Section 3.5,2 and Section 3.13) belongs to a special class of irregular formations that are generally associated with significant changes in the electron density (e.g., > a few per cent) over large distances (e.g., > 10 Km). The remaining irregularities, loosely termed ionospheric inhomogeneities, typically develop as the result of ionospheric instability processes and are not directly associated with TIDs. On the other hand, TIDs have been shown to be a possible catalyst in the formation of ionospheric inhomogeneities, especially in the vicinity of the Appleton anomaly. Relatively small-scale ionospheric inhomogeneities are important since they are responsible for the rapid fading (i.e., scintillation) of radio signals from satellite communication and navigation systems. Such effects may introduce performance degradations or outages on systems operating at frequencies between 100 MHz and several GHz. Models of radiowave scintillation have been developed, and these are based upon a basic understanding of the global morphology of ionospheric inhomogeneities.

There are inhomogeneities in all regions of the ionosphere, but the equatorial and high latitude regions are the most significant sources. Hunsucker and Greenwald [1983] have reviewed irregularities in the high latitude ionosphere while Aarons [1977] has examined the equatorial environment. Refer to Section 3-8 for more on high latitude phenomena. Specifically we shall describe the phenomenon of polar patches and blobs.

Equatorial inhomogeneities tend to develop following sunset and may persist throughout the evening but with decreased intensity after local midnight. The irregularities are thought to be the result of an instability process brought about by a dramatic change in F-region height at the magnetic equator following sunset. The scale lengths of the irregularities may range between roughly a meter and several kilometers, and the spectrum of the irregularities has been observed to exhibit a power law distribution. There is a tendency for the irregularities to be field-aligned with an axial ratio of roughly 20 to 1. In addition, the irregularities are organized in distended patches. While the situation is variable, the patch sizes range between -100 Km and several thousand kilometers in the upper F-region, and have a mean dimension of - 100 Km in the lower F region. The equatorial irregularities tend to be more intense and widespread at the equinoxes and at solar maximum, but magnetic activity tends to suppress the growth of the irregularities.

High latitude irregularities exist within the polar cap and the auroral zone, with the latter being primarily associated with the bottomside F-region. The high latitude F-region is quite variable, and unlike midlatitudes, it may have an electron density that is less than the E-region during nocturnal hours. In the wintertime, structured auroral arcs may migrate within the polar cap and the electron density enhancements within these formations may be several orders of magnitude greater than the normal background, especially during elevated solar activity conditions. During disturbed geomagnetic conditions, structured electron density patches have been observed to travel across the polar cap in the anti-sunward direction. These irregularities may have a significant effect on communication systems. For both the auroral zone and the polar cap, increased geomagnetic activity has a dramatic influence on the growth of irregular ionospheric formations. Moreover, for large and sustained values of Kp, it has been observed that the high latitude irregularity patterns tend to migrate equatorward replacing the background midlatitude properties.

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