11.5.4 Summary Of Ionospheric Variations
The ionosphere suffers a number of variations that have a significant effect on sky-wave communications.
- Daily. Diurnal variations must be accounted for in prediction charts, as ionization occurs during the day, while recombination occurs at night. The critical frequency is therefore higher during the day than at night.
- Geographical. It is important to ensure that ionospheric prediction charts are for the correct area since there is significant geographical variation in the intensity of ultra-violet radiation. Consequently, the ionosphere is very different at the equator than at the poles and there is an appreciable east-west variation.
- Seasonal. Predictions also need to be for the appropriate month. As illustrated in Figure 11.16, operating frequencies vary in winter and summer due to variations in the heights of the layers that result from different quantities of ultra-violet radiation as well as the expansion and contraction of the atmosphere.
- Sunspot cycle. Sunspots are darker (cooler) patches on the surface of the sun that are indicators of the sun’s activity. During periods of large numbers of sunspots, the sun emits great quantities of ultra-violet radiation, which leads to higher ionization densities and higher critical frequencies and MUFs. Sunspots increase and decrease in a regular cycle, with sunspot maxima occurring every 11 years approximately. Frequency prediction charts take account of the sunspot cycle as well as a shorter, more regular cycle that occurs every 27 days as the sun rotates.
- Solar flares. Solar flares are large electromagnetic disturbances that travel away from the sun and can cause severe HF blackout (short-wave fadeout) due to intense D-layer absorption. Flares are more prevalent in high sunspot years and can occur very suddenly (although obviously only during the day), blacking out sky wave for several hours. Warning of unusual solar activity is provided by national prediction authorities.
- Magnetic storms. Magnetic storms are also more prevalent in years of high sunspot activity and can create turbulence in the ionosphere, which leads to severe fading on sky-wave links. Magnetic storms can last for several days and can occur during the day or night.
- Sporadic 'E'. F-layer communications can suddenly be interrupted by sporadic ‘E’ disturbances which result when clouds of dense (F-layer-like) ionization occur in the E layer and severely disrupt communications.