The ionogram

An ionogram is a graph of time-of-flight against transmitted frequency. Each ionospheric layer shows up as an approximately smooth curve, separated from each other by an asymptote at the critical frequency of that layer. The upwardly curving sections at the beginning of each layer are due to the transmitted wave being slowed by, but not reflected from, underlying ionisation which has a plasma frequency close to, but not equaling the transmitted frequency. The critical frequency of each layer is scaled from the asymptote, and the virtual height of each layer is scaled from the lowest point on each curve.

An ionogram can be much more complicated than just two layers. There can also be such phenomenon as;

• The F1 layer. An additional layer which appears in the F region, between the two existing peaks. To tell the two Flayers apart, the upper layer is referred to as the F2 layer, and the lower layer the F1 layer.
• Sporadic E, Es. This layer is a patchy, very dense layer sometimes exceeding 16 Mhz (3.1 x 10^11 /m^3). Despite their intensity, these layers do not extend over a large height range, and so do not exhibit an asymptote at the critical frequency, as the transition is too sudden. They appear on an ionogram as a narrow horizontal line at around 100km. An intense Es layer can prevent any echoes from reaching the upper layers This is known as blanketing.
• Multiple hops The return signal can skip from the Earth to the ionosphere and back again, sometimes several times before it is attenuated. These multiple echoes appear on an ionogram at multiples of the original virtual height.
• D-region Absorption. This is caused by ionisation in the D-region that absorbs the transmitted wave before it can return to the ground. This absorption is characterised by no echoes being received from the low frequency end of an ionogram.
• Lacuna. When turbulence occurs (as the result of large electric fields for example), the stratified nature of the ionosphere gives way to a more complex structure. Under such conditions, the reflected signal may not reach the receivers, and so the height range at which the turbulence occurs is lost on the ionosonde trace. Such gaps are termed Lacuna and their position on an ionogram gives some indication as to the height at which the turbulence is occurring.
• Spread-F. With an ionosonde, echos are received from any portion of the ionosphere where the electron density gradient is perpendicular to the transmitted wave. This most often happens overhead, but occasionally conditions exist such that echoes from other regions of the sky return to the ionosonde. If the electron concentration in these regions differs from the ionosphere overhead, two traces are observed. For a given angle from the zenith, the horizontal separation is greatest in the F-region, and so differences in ionospheric conditions are most likely to be observed in the F-region. If the geometry is right for echoes to be received from a whole range of locations and the ionospheric conditions vary over that range (such as when a trough is overhead) multiple traces will appear on an ionogram, and the F trace is said to be 'spread'. With a digital ionosonde, such as the Dynasonde, these traces can be resolved by considering the horizontal position of each echo.

Ionosondes | WDC