Sudden Ionospheric Disturbances
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Ron Fleshman KB2VIV
Our atmosphere is composed of several layers.  The first layer is the troposphere.  It extends from the surface of the
earth up to about 15 km.  Next is the stratosphere.  It extends from about 15 km to about 50 km.  The last layer is the
ionosphere.  It extends from about 50 km on out (see fig 1).
Most of the ionosphere is electrically neutral, but when solar radiation strikes electrons are dislodged from the atoms
and molecules forming the ionospheric plasma.  Only the shorter wavelengths of solar radiation (extreme ultraviolet
and x-ray) can produce this ionization.  Historically the ionosphere has been divided into regions ( D, E, and F ), with
the term layer referring to the ionization within the region.  The lowest is the D region, it occupies the altitudes
between about 50 and 90 km.  The E region is next at altitudes of 90 and 150 km. The F region is the last with
altitudes above 150 km.  The F layer is further broken down into two sub layers, the F1 and the F2 layers.
The ionosphere varies greatly due to changes in its source of ionization.  Since it responds to solar EUV radiation, the
ionosphere varies over the 24-hour period and over the 11-year solar sunspot cycle.  At nigh when solar radiation is
greatly reduced the D and E layers disappear and the F1 and F2 layers combine.  On shorter time scales solar
radiation can increase dramatically during a solar flare.  This greatly increases the D and E region ionization.  This
dramatic change is usually referred to as a Sudden Ionospheric Disturbance (SID).
There are certain regions ( high and low latitude F region ) and certain times ( principally after sunset ) when the
ionosphere may become highly turbulent. This change in ionization and turbulence can seriously affect radio wave
propagation in the ionosphere.  The monitoring the changes in VLF radio propagation is one way these events can be
observed and recorded.
fig 1
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                                                                          VLF Software Receiver
A receiver for VLF studies can be made from a computer and a computer sound card.  A sound card will accept frequencies
from a few hertz up to about half the sampling frequency of the sound card.  All that is needed is an antenna and
appropriate software.  Computer sound cards can sample at different rates. The most common is 41000 samples per sec.
Some of the better cards can sample at 96000 samples per sec.  Nyquest theorem states that the maximum frequency is
one half the sampling rate, so frequencies up to 48000 can be possible.
There are many software applications available for this purpose.  I have chosen
Spectrum Lab by Wolfgang Buscher
(DL4YHF). The main reason I use this is because it is a spectrum analyzer program.  As such it shows the frequency and
levels of all signals.  It has a waterfall display along with plotting and screen capture options.  I have loaded Spectrum Lab
into an older computer and use it as a dedicated audio spectrum analyzer and VLF receiver.  I  use the same antenna
design that I use for the SIDs receiver.  This configuration shows activity on the whole VLF spectrum.  Data can be exported
into excel for detailed analysis.  This way I can pick the frequencies that I want to monitor with the SIDs receiver for more
detailed study.