Sudden Ionospheric Disturbance (SID)
- overview of the sudden ionospheric disturbance, SID, that affects
HF radio signal propagation
Sudden Ionospheric Disturbances or SIDs are often experienced
on the HF or short wave bands where they may also be called blackouts, short
wave fades (SWF), or they are sometimes referred to as the Dellinger effect,
after the person who first noted them. As the name implies they occur suddenly,
and they cause a blackout of ionospheric radio propagation for a number of
hours. This may cover part, or even all of the HF radio spectrum.
Cause
There are a number of types of blackout that may be experienced on the HF bands.
Sudden Ionospheric Disturbances, SID, only last for a few hours, but they may be
the precursor for a longer blackout. Generally a SID is caused by a large solar
flare (M or X class - see the article on Solar Disturbances on this website).
Along with the flare there is a massive increase in the level of radiation that
is emitted by the Sun. It takes just over 8 minutes for the radiation from the
Sun to reach the Earth, at which point the effects start to be noticed and the
SID begins. As light takes the same time to reach the Earth, there is no prior
warning of an event happening.
The radiation from the Sun caused by the flare is not limited
to one form of radiation, but it includes all forms including a high level of
X-rays. The X-rays are able to penetrate through to the D layer or D region of
the ionosphere and as a result they give rise to a high level of ionisation in
the D region. This results in a very significant increase in the level of D
region attenuation.
Although the increase in the level of radiation is rapid, it
takes time for the level of ionisation to rise. As a result the lower
frequencies are affected first, and as the degree of ionisation increases, so
the higher frequencies are affected. During a normal day the D region normally
affects frequencies up to two or three MHz, but the increase in radiation from a
flare can cause much higher frequencies to be affected. Often it can result in a
complete black out of the HF bands. At other times a SID may only affect the
lower frequencies.
Other effects
There are a number of other side effects that may be noticed, especially at the
onset of a Sudden Ionospheric Disturbance, SID. The D region is affected by the
large increase in X-rays, but other forms of radiation, including ultra-violet
also increase in intensity. This gives rise to an increase in the degree of
ionisation in the higher regions of the ionosphere. This can result in an
increase in the maximum usable frequency, especially at the beginning of a radio
blackout when the D region attenuation is not as high.
Summary of the effects of solar disturbances
SID duration
A SID may last only a few hours. As recovery starts, the high frequencies are
affected first. After the flare as the level of radiation falls, so will the D
region ionisation. As its intensity falls, so it will affect the higher
frequencies less, and HF propagation conditions will slowly return to normal
unless they are part of a larger disturbance.
It is also worth noting that it is only the sunlit side of
the earth that is affected. Any areas that were in darkness when the flare
occurred escape the effects. A further effect of a flare is that at the same
time as the X-ray radiation reaches the Earth giving rise to a blackout, it is
found that noise bursts from the increased levels of radio frequency radiation
may be monitored at VHF.
|
