Probing the ionosphere with controlled radio signals to determine its
condition began more or less in the early 1930's. The same basic method
used then is currently employed in what are known as vertical incidence
(V.I.) ionosondes. These transmitter-receiver installations operate much
like a radar set, only the frequency is swept from about 1 to 30 MHz while
the pulses are being emitted and (if reflected) received. Pulse duration
and sweep times vary with the models, with the fastest sweeps used being
about 15 seconds. Peak power outputs (ERP) are on the order of 30 kw.
The displays of the ''echoes'' are made on a cathode ray tube, usually
with provisions for filming the complete sweep (ionogram). This may be
done automatically at preset intervals. A somewhat idealized ionogram
is shown below in Figure 1. h', the vertical scale, is known as the
virtual height since the pulse is actually gradually slowed down before
reflection and the distance is thus exaggerated on the scope trace. The
horizontal scale for frequency may be either linear or logarithmic. The
display is what one expect at mid-latitudes on a quiet spring day with
a moderate sunspot level.
Now for the terminology lesson. You'll note that the trace is split in
two (shown above in different weight for clarity - original ionograms are
not always that considerate). This is caused by the earth's magnetic field
and is known as birefringence or magneto-ionic splitting. The two rays the
signal is split into are called the ordinary (o-ray) and extraordinary
(x-ray). In very high geomagnetic latitudes there may even be a third trace
called the z-ray. The x-ray is the higher frequency reflected. foE, fxE,
foF1, fxF1, foF2, and fxF2 are the so-called critical frequencies for the
o and x rays for the various ionospheric layers.
Moving along to our main object of concern, Sporadic E. The phenomena
were noted in the mid-1930's on ionograms, and it received a variety of
names such as abnormal E, night E, et al. The termination of it as Es
(E for the layer height it occured at, and s for sporadic) came in the
1940's. In the late 1930's it was noted that the unexpected DX openings
on the then 5-meter ham band matched up very well with these lonosonde
observations of Es.
There are two types of Es easily distinguished on ionograms: blanketing
and non-blanketing (or transparent). Figures 2 and 3 show the respective
varieties (with only the o-ray shown for clarity). Some new terms now come
up: foEs, fxEs, fobEs, and fxbEs. fobEs is the lowest frequency at which
a higher layer can be 'seen' thru the Es, and is called the blanketing
frequency. Non-blanketing Es is more commonly observed in the equatorial
For reasons to be explained later (Part II), some workers prefer the use
of fEs for the highest Es reflected signal and fbEs for the blanket cut-off.
Figure 2 Figure 3
Blanketing Es Non-Blanketing Es
By now you may be wondering why the F-layer traces have the curving
shapes. It is not strictly an indication of height changes. The normal
E layer, even though it is being penetrated by the pulses, is still
slowing down the signal, less so as the frequency rises. Thus the F1
trace appears to lower as the frequency increases (less 'holding back'
by the E layer) to a point (usually noted as h' min F1). The rise then
noted is due to the signal penetrating deeper into the layer until the
critical frequency is reached. The F1 layer then similarly affects the
F2 trace causing the curves to it.
In most cases of Es (except the auroral-zone variety) this slowing
down effect (group retardation) is virtually absent, indicating a very
thin layer of ionization. Note the F1 trace in Figure 2 at fbEs does not
appear to fall.
In blanketing Es, fEs traces may extend up to several times the
frequency of fbEs. If the fbEs extends above fxF2 the condition is known
as total blanketing. With fbEs lower than fxE, and h' Es greater than
h' E, the Es will be essentially transparent.
________ _ __________
There are other types of ionospheric probing besides vertical incidence.
One popular type is known as backscatter radar, where a narrow beam (on a
fixed or swept frequency) is sent out. See Figure 4. The returning echoes
are then examined on either a PPI (classical radar screen display) or a
range-sector plot. Resolution in angle may be as good as 2 or 3 degrees
to half-power points on the antenna array (achieved by using up to a
dozen nicely spaced, vertical and/or horizontal spacing, multi-element
Yagis). Commonly used frequencies range from the mid h.f. to 50 MHz plus.
With vertical stacking, elevation angle scans are possible. Provisions
for automatically changing the frequency (in steps) may be used, with
Figure 5 illustrating this effect on an Es patch.
10 MHz 20 MHz 30 MHz
As one might expect, time lapse photography techniques vividly display
the movements of Es patches etc., just as those of weather radars show
storm developments. Many of the more recently made this way are from the
1970 ESSA Summer Es experiment (May 1970 CQ, p. 31).
Other probing methods include one-way oblique paths where a receiver
is fix-tuned to the frequency of a transmitter at a desired distance-
direction, analogous to leaving your set on a vacant low-band TV channel
and waiting. A more elegant refinement of this is to use a transponder
(e.g., 1970 ESSA on 49.6 MHz between Boulder, CO and Seminary, MS) which
will send back its own signal if the path is open and it detects an
All of these methods have their high-points and their limitations.
Vertical incidence is always plagued by the problem of reflections coming
from directions other than straight up (mainly due to tilted layers).
Multiple reflections (see Figure 6) occur from the signal literally
vibrating between the ground and the ionosphere (E or F regions). Another
serious problem is interference on the ionogram due to the signals of
other services operating in the h.f. bands (you can easily spot the WWV
outlets and KOA on Boulder ionograms).
Figure 6 h' |
Multiple | _____ f4Es
Reflections | ________ f3Es
| _____________ f2Es
| ____________________ fEs
Backscatter radar suffers from the problem that the distant ground
(or water) reflection coefficients vary with the terrain and angle. This
produces a high degree on non-uniformity with direction and distance.
Cases of the V.I. ionosonde at Stillwater, OK in July 1970 detecting Es
when the Boulder h.f. backscatter radar revealed none are good examples.
Using fixed oblique circuits severely limits the amount of data since
until Es reaches close enough to the midpoint to 'open' the path, the
system is ignorant of its existence.
The possibility of cloud-to-cloud paths (see Mel Wilson's excellent
articles in Dec 1970 and Mar 1971 QST) can cause endless perplexions.
And with all that, there often comes about a distinct lack of
agreement in what radio methods find vs. rocket probes, the subject of
Part II of this series.
_______________ _ ____________
For those desiring a more in-depth mathematical discussion on
electromagnetic wave behavior in a plasma (which is what the ionosphere
amounts to) National Bureau of Standards Monograph 80, Ionospheric
Radio Propagation, by Kenneth Davies will amply supply you a few chapters
and many other references as well. Government Printing Office sells it at
a very bargain price of $ 2.75.
NBS Circular 582, by Dr. E.K. Smith, though over 15 years old remains
a classic in the Es field. The GPO offers this volume at $ 3.25.
In the more expensive arena, a collection of papers entitled Ionospheric
Sporadic E was put out by Pergamon Press for $15.50. Issued in 1962 some
of it is a bit dated now.
Page last modified March 5, 1999