The ionosphere is not a uniform set of layers, or even regions. The level of ionisation can vary considerably in ionisation intensity and the overall structure of the region.
One particular form of irregularity within the F region is called spread F and this can impact HF radio communication links.
Spread F is an interesting phenomenon that can occur in the F region at some times and it is more common in some places than others.
What is Spread F
One of the easiest methods of detecting the actual state of the ionosphere above a given point and deducing the effect this ,ay have on HF radio communication is to use an item called an ionosonde. This is effectively aHF pulsed radar system that sees the reflections from the ionosphere.
Spread F is normally defined in terms of the reflections received by an ionosonde and displayed on an ionogram.An ionosonde transmits pulses up towards the ionosphere and normally an echo is returned and this has approximately the same length as the pulse that has been sent out. A small elongation of the pulse may be seen because the pulse is reflected back from a "spread" of different heights in the ionosphere.
When Spread F is present, echoes which are received back from ionosonde soundings that indicate that there are irregularities in the F layer. Instead of receiving a defined echo to give the effective height of the ionosphere at that frequency, a diffused or fuzzy echo is received. Often the echo of the transmitted pulse received back at the ionosonde can be ten times the length of the transmitted pulse.
There are two main types of spread that are seen on the ionograms when Spread F is present:
- Range spread: This form of Spread F refers to the diffuseness on the horizontal part of the ionogram trace.
- Frequency spread: This refers to diffuseness near the critical frequency of the F2 region.
When spread F occurs the F region or F layer does not consist of a layer with a uniformly varying level of ionisation as normally seen. Instead it appears as though turbulence has broken it up into a variety of areas of differing levels of ionisation. It can be described as plasma irregularities in the F region.
The different ionised clouds or areas each reflect signals giving a variety of paths that the signal can take. The signal takes slightly different amounts of time to travel the different paths. This means that the signal at the receiver is a combination of a variety of components that have taken different paths. This results in distortion and the flutter that is characteristic of signals that have travelled via a polar route. Signal levels are also lower because this method of reflection is not as efficient as normal F layer reflections.
Occurrence of Spread F
It is found that Spread F occurs far more in some regions than in others:
- Equatorial regions: Around the equator between ±20°, Equatorial spread F occurs very frequently. In these regions, equatorial spread F is an evening and night time occurrence. It can appear first near sunset, but it is most frequently observed between 2100 and 0100 local time, although it can appear earlier, especially towards the sunspot maximum. The appearance of equatorial spread F seems to correlate to the evening rise in the height of the F2 region.
- Above latitudes of 40° : The second is at much higher latitudes above 40°, increasing with latitude to the extent that it is almost permanent in winter. This occurs mainly at night time, but also sometimes in the day. Close to the magnetic pole, there is a region of almost permanent Spread F - during the summer months, Spread F occurs between 90% and 100% of the time during the night time and more than 50% of the time during the day. During the summer months, Spread F occurs almost 100% of the time during both day and night.
There is little evidence of spread F occurring between latitudes of 20° and 40° - equatorial spread F and polar spread F are the only two occurrences of the phenomenon.
The regions affected by spread F may occur in patches as wide as several hundred kilometres. The patches drift horizontally at speeds of around 100 metres per second, always drifting eastwards in equatorial regions.
Interestingly the occurrence of spread F differs between the two regions. In equatorial regions spread F occurs on magnetically quiet days, and disappears with the occurrence of a magnetic storm. However at higher latitudes it is linked with magnetic activity.
Spread F is an interesting ionospheric phenomenon. It explains the very watery sounding signals that are often present on polar radio communication propagation paths and it can also be used to good effect to ensure radio communication can be established over polar paths where it might not otherwise be possible.