Fig. 5. Ionospheric data for a storm with a Dst excursion of300 nT (top panel) in March 1991. Peak height (hm F2) of the F2-layer, base of the F-region height (h'f) andfo F2 are shown for the disturbed period (broken curve) along with monthy median values (continuous curve). The data pertains to Kodaikamal. Very large plasma depletions similar to those discussed above, but at the peak of the F-region, have been observed in the nocturnal equatorial ionophere during severe magnetic disturbances. A large number of such severe storms with reference to their effects on the equatorial ionosphere were studied recently [Lakshmi et al., 1996, also see Lakshmi et al., 1991]. A single representative case will be described here as an example. Fig. 5 shows the details for a severe storm that occurred at 0300UT on 24 March 1991 with a well defined SC and with the maximum negative excursion of Dst reaching upto 300nT (top panel of Fig. 5). The other parameters shown are hmF2 (the peak height of the F2 layer), h'F (the base height of the F-layer) and f()F2. These data pertain to Kodaikanal (Geographical Lat. 10° 14' and dip lat 0.6° N). The most striking feature to be observed is the dramatic collapse of F-region ionisation as evidenced by the foF2 values (broken curve) on the night of 24-25 March 1991 during the main phase of the storm. The sharp fall in f0F2 values during 0100-0500 LT was rapid, with fgF2 values collapsing from 9.5 MHz to 3.0 MHz while the peak heights (hmF2) in general were higher than the monthly median values by as much as 50-100 km. Lakshmi eta!.,  did consider the possible effects of enhanced neutral temperatures to explain such ionisation collapses, but the results were not convincing. It is in this context that Lakshmi et al.,  based their arguments on meridional winds [Krishna Murthy et al., 1990; Krishna Murthy and Hari, 1992] sweeping the plasma from north to south (Kodaikanal geographically is well in the northern hemisphere and equatorward winds can be expected even in Equinox). In fact, storm simulations using the UCL coupled ionosphere-thermosphere model showed cross equatorial winds driven by Joule heating under equinox conditions [Fuller-Rowell et al., 1994], Such meridional winds can drive the plasma along the near-horizontal magnetic field and physically transport it to points south of Kodaikanal. When such forced migration takes place, the depleted plasma can not be replenished either from below or from the top because the magnetic field lines are horizontal and there is no production, it being night time. Thus, as the plasma is swept over from Kodaikanal towards south, the base of the F-region (h'F) is also subjected to this sweep, in fact more effectively, because the ion-neutral coupling is all the more stronger in the lower F-region than at the peak. This should be the cause for the increase in h'F as observed from the ionogram.
5 Radio communications in the tropics 5.1 Ionosphere and the radio spectrum
Quite contrary to common belief, ionosphere exercises its influences on essentially all bands of Radio spectrum from the Extreme low frequencies (ELF) to as high as 10 GHz or even higher. ELF, VLF and LF used in Navigation, Maritime Communications, Time and Frequency Transmissions etc. propagate mainly as ground waves, but also through ionospheric reflection under certain conditions. MF used for short and medium range broadcasting gets extended range of coverage in the night time through ionospheric propagation. The HF spectrum which has been the prima donna until the sixties for long distance broadcasts and point-to-point links depends entirely on ionospheric support. The VHF spectrum is used mostly in line of sight (LOS) modes which include TV broadcasts and certain satellite services-except some scatter systems using either the troposheric irregutarities [Reddy, 1987], meteor trails or irregularities in the D-region of the ionosphere. Otherwise, ionosphere is a major nuisance to VHF systems. One is through the F-region irregutarities which degrade performance of satellite-based systems through the so called scintillations. The other is in TV broadcasts where the ionosphere, especially the sporadic E-layer, can support TV signal propagation to unintended target zones in far off areas, causing interference and serious problems in spectrum management. Satellite-based radio systems in UHF (300 MHz to 3000 MHz) aud even in SHF (3GHz to 30GHz) are not immune to ionospheric degradation.
The use of HF spectrum with ionospheric support for broadcasting and point-to-point links is blessed with a number of positive factors in the tropics. Because of low solar zenith angles, we have simply greater solar ionising radiations per unit area leading to greater electron production. So we have higher electron densities facilitating higher operating frequencies with lesser ionospheric absorption.
Higher frequencies also yield better signal-to-noise ratio because of lower Atmospheric Radio Noise. Antenna dimensions and real estate requirements will also be modest at higher frequencies. The low latitude ionosphere is well protected by the terrestrial magnetic field from the invasion of solar particles which cause a host of particle effects including the Polar Cap Absorption events at high latitudes. Even the delayed effects of these particle events, like the ionospheric storms, do not cause major problems in low latitudes, except when they are very severe disturbances as discussed in section 4. HF services in the high and mid latitude zones are seriously dislocated during the main phase of the storm as the MUF values plunge far below the predicted medians. Some of the recent broad-casting systems can use real time data provided by state-of-the-art ionosonde networks [Wilkinson et al., 1992] in frequency-agility links; but the main problem will remain spectrum management. However, there are also some specific problems peculiar to low latitudes. The spatial gradients associated with the Equatorial Anomaly can pose a variety of problems including asymmetric ray paths, deviation from great circle propagation etc. [Oyinloye, 1987; CCIR, 1986]. In addition, a peculiar situation of one way communication can arise as it happened for Delhi-Madras links, caused by these large horizontal gradients in electron densities [Lakshmi et al., 1979], Fig.6 shows how the MUF values depend upon the horizontal electron density gradients; the situation is worse for long distance circuits which will have to operate with large angles of incidence. For example, if the point of reflection of a north-south HF link is equatorward of the anomaly peak in the northern hemisphere, the radio waves incident on the ionosphere for the northward circuit will continuously encounter along the path increasing levels of electron density both due to vertical as well as horizontal gradients; however, for the southward path incident at the same point, the sense of the horizontal gradient is reversed. As a consequence, the real MUF values in the two opposite directions can vary by a large margin causing communication failure in one direction.
Fig. 6. Effects of horizontal electron density gradients on the maximum useable frequencies in HF
The low latitude ionosphere also exhibits steep temporal gradients, particularly during the dawn and dusk transition periods. Fig.7 shows the pre-dawn collapse and the post dawn appearance of the F-region at Jicamarca and Kodaikanal [Lakshmi et al., 1980], It may be noted that the transitions are much more rapid and steep at Kodaikanal than at Jicamarca. Such steep pre-dawn changes at Kodaikanal have been attributed to a significant increase in the eastward ionospheric drift in the predawn hours before the morning reversal \Chandra and Rastogi, 1970; Rastogi, 1988]. This rather large eastward drift (or west ward electric field) would push down the F-layer, causing its collapse. Situation at places away from the dip equator is quite different, as discussed in section 2.2., especially if they are abetted by thermospheric winds. The consequence of such collapse of the layer is almost a failure of long distance HF links; this problem is particularly serious in the low solar epoch.
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