The F2 Region

The F2 region is the most prominent layer in the ionosphere, and this significance arises as a result of its median height (the highest of all the component layers) and of course its dominant electron density. It is also characterized by large ensembles of irregularity scales {AL} and temporal variations {AT}. The F2 region is a vast zone which eludes prediction on the microscale (A L < 1 Km) and mesoscale (1 < AL < 1000 Km) levels, and even provides challenges to forecasters for global and macroscale (AL > 1000 Km) variations. This is largely because of the elusive transport term in the continuity equation. There are also a host of so-called anomalous variations to consider, and these are the subjects of a succeeding section.

As in the E and F1 regions, we may conveniently specify the behavior of the F2 region in terms of equivalent plasma frequency rather than the electron density. For the peak of ionization we have:

where foF2 is the ordinary ray critical frequency.

While foF2 exhibits solar zenith angle, sunspot number, and geomagnetic latitude dependencies, simple algebraic algorithms do not characterize these relationships. As a consequence, mapping methods are used to describe the F2 region electron density patterns.

The CCIR published its CCIR Global Atlas of Ionospheric Characteristics, which includes global maps of F2 layer properties for sunspot numbers of 0 and 100, for every month, and for every even hour of Universal Time [CCIR, 1966], Figure 3-10 is an illustration of the global distribution of foF2 for a sunspot number of 100. Such maps are derived from coefficients based upon data obtained from a number of ionosonde stations for the years 1954-1958 as well as for the year 1964. This set of coefficients is sometimes identified by an ITS prefix but is known more generally as the CCIR coefficients. Because of the paucity of data over oceanic areas, a method for improving the basic set of coefficients by adding theoretically derived data points was developed. As a result, a new set of coefficients has been sanctioned by URSI and this is termed the URSI coefficient set. Many communication prediction codes, which require ionospheric sub-models, allow selection of either set of ionospheric coefficients.

3.4.6 Anomalous Features of the Ionospheric F Region

The F2 layer of the ionosphere is probably the most important region for many radiowave systems. Unfortunately the F2 layer exhibits the greatest degree of unpredictable variability because of the transport term in the continuity equation. As indicated previously, this term represents the influences of ionospheric winds, diffusion, and dynamical forces. The Chapman description for ionospheric behavior depends critically upon unimportance of the transport function. Consequently many of the attractive, and intuitive, features of the Chapman model are not observed in the F2 region. The differences between actual observations and predictions derived on the basis of a hypothetical Chapman description have been termed anomalies. In many instances, this non-Chapmanlike behavior is not anomalous at all, but rather typical.

The following subsections indicate the major forms of anomalous behavior in the F2 layer: diurnal, Appleton, December, winter, and the F region trough. A few comments are provided for each major form.

copyright RPSI, 2000-2003

Fignre 3-10: Map of foF2 showing the worldwide distribution under the following conditions: 15 November, Sunspot Number = 135, Time = 0000 UTC. The contours offoF2 are developed using the URSI set of ionospheric coefficients. Curves similar to this are found in the Atlas of Ionospheric Coefficients fCCIR, 1966], Used by permission. Radio Propagation Services.

3.4.6.1 Diurnal Anomaly.

The diurnal anomaly refers to the situation in which the maximum value of ionization in the F2 layer will occur at a time other than at local noon as predicted by Chapman theory. On a statistical basis, the actual maximum occurs typically in the temporal neighborhood of 1300 to 1500 LMT. Furthermore there is a semidiurnal component which produces secondary maxima at approximately 1000-1100 LMT and 2200-2300 LMT. Two daytime maxima are sometimes observed (one near 1000 and the other near 1400), and these may give the appearance of a minimum at local noon. This feature, when observed, is called the midday "biteout".

3.4.6.2 Appleton Anomaly.

This feature is symmetric about the geomagnetic equator and goes by a number of names including: the geographic anomaly, the geomagnetic anomaly and the Appleton anomaly, as well as the equatorial anomaly. The Appleton anomaly is associated with the significant departure in the latitudinal distribution of the maximum electron concentration within 20 to 30 degrees on either side of the geomagnetic equator. Early in the morning a single ionization peak is observed over the magnetic equator. However, after a few hours the equatorial F-region is characterized by two distinct crests of ionization that increase in electron density as they migrate poleward The phenomenon is described as an equatorial fountain initiated by an E x B plasma drift (termed a Hall drift), where E is the equatorial electrojet electric field and B is the geomagnetic field vector. This drift is upwards during the day since the equatorial electric field E is eastward at that time. As the electrojet decays, the displaced plasma is now subject to downward diffusion when the atmosphere begins to cool. This diffusion is constrained along paths parallel to B, which maps to either side of the geomagnetic equator. The poleward extent of the anomaly crests is increased if initial Hall drift amplitude is large. This anomalous behavior accounts for the valley in the EJF(zero)F2 parameter (with peaks on either side) seen at the geomagnetic equator in Figure 3-10. There are significant day-to-day, seasonal and solar controlled variations in the onset, magnitude and position of the anomaly. There are also asymmetries in the anomaly crest position and electron density. Asymmetries in the electron density in the anomaly crests appear to be the result of thermospheric winds that blow across the equator from the subsolar point. The effect of magnetic activity on the anomaly is to constrain the electron density and latitudinal separation of the crests. Magnetic activity is monitored worldwide, and the quasi-logarithmic index Kp is used to represent the level of worldwide activity [Mayaud, 1980], There have been suggestions that when Kp is > 5, the anomaly disappears. But other observations have shown an increased separation of the anomaly crests, a fact that may lead some stations to observe a reduced foF2 value.

3.4.6.3 December Anomaly.

This phenomenon refers to the fact that the electron density at the F2 peak over the entire earth is 20% higher in December than in June, even though the solar flux change due to earth eccentricity is only 5% (with the maximum in January).

3.4.6.4 Winter (Seasonal) Anomaly.

In this case, the noontime peak electron densities are higher in the wintertime than in the summertime despite the fact that solar zenith angle is smaller in the summer than it is in the winter. This effect is modulated by the 11 -year solar cycle and virtually disappears at solar minimum.

3.4.6.5 The F-Region (High Latitude) Trough.

This is representative of a number of anomalous features that are associated with various circumpolar phenomena including particle precipitation, the auroral arc formations, etc. The high latitude trough is a depression in ionization, occurring mainly in the nighttime sector, and it is most evident in the upper F-region [Muldrew, 1965]. It extends from 2 to 10 degrees equatorward of the auroral oval, an annular region of enhanced ionization associated with optical aurora (see Section 3-8). The trough region is associated with a mapping of the plasmapause onto the ionosphere along geomagnetic field lines (see Figure 3-18). The low electron density within the trough results from a lack of replenishment through candidate processes such as antisunward drift, particle precipitation, or the storage effect of closed field lines. The latitudinal boundaries of the trough may be sharp, especially the poleward boundary with the auroral oval. A model of the trough is due to Halcrow and Nisbet [1977],

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