WDS'11 Proceedings of Contributed Papers, Part II, 67–72, 2011.
ISBN 978-80-7378-185-9 © MATFYZPRESS
Influence of Non-vertical Echoes to Ionogram Scaling
D. Kouba1,2 and P. Koucka Knizova2
1
Charles University in Prague, Faculty of Mathematics and Physics, Prague, Czech Republic,
Institute of Atmospheric Physics, Academy of Sciences of the Czech Republic, Czech Republic.
2
Abstract. Digisonde DPS 4, which replaced the older ionosonde IPS 42 KEL Aerospace,
started regular ionospheric soundings in the Observatory Pruhonice in January 2004. The
Digisonde DPS 4 records not only ionograms, but it measures also other properties of the
electromagnetic signals reflected from ionosphere (time of flight, wave polarization,
amplitude, phase, angle of arrival etc.), and it provides automatic scaling of ionograms and
other characteristics of the ionosphere. Additional information about the wave polarization
enables us to determine and to distinguish exactly between ordinary and extraordinary
wave traces on the ionogram records. This new ability basically changes scaling of the
ionograms comparing to the ionograms obtained by classical ionosondes that do not
provide any information about arriving wave polarization. Digisonde measurements show
that in some cases the interpretation of ionograms based on classical ionospheric soundings
may lead to the systematic errors, which affect classical ionograms interpretation. Here we
have collected the representative data set in order to demonstrate possible differences in
scaling when some complicated ionospheric situations occur. We demonstrate possible
significant misinterpretations of the ionograms obtained using the classical equipment that
is not able to distinguish between ordinary and extraordinary modes and records only time
of flight of the electromagnetic sounding pulse. The aim of the contribution is to present
comparison of the data sets measured by conventional ionosondes and by new digisondes.
Errors in scaling parameters could reach several tens of MHz in frequency and several tens
of km in height. We show here the necessity to be careful in using an old ionosonde
measurements and scaled data for further analysis.
Introduction
Ionosphere is highly variable system that varies in a broad range of scales. There are fluctuations
detected at scales of seconds as well as fluctuations corresponding to the solar cycle periods. Correct
and accurate knowledge of the state of the ionosphere is a key issue in such situation.
Since middle of 20th century have been ionosondes widely used for the monitoring of state of the
ionosphere. An ionogram is a graph of the data produced by an ionosonde. It is a graph of the virtual
height of the reflection point in the ionosphere against sounding frequency.
Due to the Earth’s magnetic field, a radio wave incident on the bottom of the ionosphere is
divided into two waves which propagate independently through the ionosphere. By analogy with the
optics, one is called the ordinary wave and the other, the extraordinary wave. Critical frequency of
ordinary wave for each layer and height of the layer are typical parameters which are scaled from the
ionogram. The ionogram scaling is a complex problem which includes recognize and identification of
unusual features, developing a consistent interpretation of the ionogram.
In this paper we discuss and compare ionograms where complication in precise scaling may occur
due to the method used for ionospheric monitoring. Propagation of acoustic-gravity waves (AGW)
represents a situation where precise scaling of the ionogram traces is important for the determination
of the electron concentration profile N(h). Inaccuracy may have a significant effect on the detection
and parametrization analysis of the propagating AGW. On the other hand, there is an increasing
interest of the scientific society whether any long term trends exist in the ionosphere. Such an analysis
requires long time series of precisely scaled ionospheric parameters. Long time series data usually
contains data scaled by different operators. Besides that, from time to time the old ionosonde is
replaced by new equipment. Replacement of the old equipment by a new one of the same type is a
potential introduction of the systematic error to the data. Even more serious problem is change of the
instrument to a different type which is based on completely different principles. Such a change
introduces very serious inconsistency into the data.
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KOUBA ET AL.: INFLUENCE OF NON-VERTICAL ECHOES TO IONOGRAM SCALING
Data
The ionospheric station Pruhonice was established during International Geophysical Year in
1957–1958. First equipment operated at the station was a Russian analog ionosonde AIS. Its antenna
system consisted of two rhombic transmitting/receiving antennas of 26m height. The ionosonde
provided vertical ionospheric sounding (measured ionograms) with 15 minutes cadence at sounding
frequencies 1–16 MHz. Ionograms were displayed on an oscilloscope screen which was further
photographed by a camera (see Fig. 1a). Film records were manually interpreted/scaled and hourly
values of standard parameters (foF2, h’F, FoE, h’E, etc.) were collected and stored in the international
data centers (since 1970 ursigrams, geoalerts). For later scientific use there are available one-hour
scaled characteristics only. The original ionograms are stored on camera films which are waiting for
digitalization. The confrontation of the scaled characteristics with the original ionograms is now
practically impossible.
In 1984, the Russian ionosonde AIS was replaced by Australian ionosonde IPS-42 KEL
Aerospace. The existing rhombic antenna was used as a transmitting antenna. Additional 36 m height
double delta antenna was built as a receiving antenna. The ionosonde recorded ionograms with
15 minutes cadence in the frequency range 1–22.6 MHz. The ionosonde IPS-42 worked in the
observatory till early January 2004. IPS-42 ionograms were saved in digital format and subsequently
stored on magnetic tapes.
Unfortunately, due to the cost of the tapes, trade embargo, etc. it was not possible to archive all
the registered data in a digital format. Ionograms were printed and stored in paper version. Only some
analyzed events were stored in a digital format also. Example of ionogram measured by the IPS-42 is
presented in Fig. 1b. Since 90’s the database of raw digital ionograms has been built up. In addition to
the standard parameters an evaluation the ionospheric E/F traces was possible identified/recognized
using special software from digital ionograms. The electron density profiles were calculated by
POLynomial ANalysis method POLAN from the identified traces. [Titheridge, 1985].
There are one-hour scaled ionospheric characteristics available for the era of ionosonde IPS-42
sounding. For a plenty of measurements it is possible to compare the raw digital ionogram with scaled
values stored in databases.
In January 2004, a new digital ionosonde (Digital Portable Sounder 4, DPS-4) with four crossloop receiving antennas was installed in the observatory. The Digisonde operates on frequencies 1–
20 MHz. The Digisonde DPS 4 provides routine vertical ionospheric sounding with time resolution
15 minutes. An ionogram autoscaling process ARTIST [Galkin et al., 2008] automatically finds
standard ionospheric characteristics. In addition to the vertical ionogram sounding, drift measurements
are performed in both F and E regions. Data and autoscaled characteristics are distributed to the data
centers and presented in real-time on web.
In January 2010 we upgraded the Digisonde to version DPS-4D. The new equipment allows a
wide range of new possibilities for ionospheric measurements. The Digisonde Portable Sounder model
DPS-4D is the latest digital ionosonde developed by the University of Massachusetts Lowell, Center
for Atmospheric Research (UMLCAR). The letter “D” in the new model refers to the digital
transmitters and receivers in the DPS-4D that implement the classic functions of radio transmitters and
receivers by numeric techniques. It applies also new software solutions for data acquisition, hardware
control, user commanding and data processing [Reinisch et al., 2008]. Novelty lays in the possibility
of a wide range of programs and schedules. DPS-4D can work in PGH mode (Precision Group Height)
with precise detection of reflection height. Example of ionogram measured by the DPS-4D in the PGH
mode is presented in Fig. 1c.
Ionogram interpretation
Correct ionogram interpretation is crucial point for correct and accurate ionogram scaling. In the
paper we compare ionograms which are measured by Digisonde DPS4 and historical ionograms
measured by ionosonde IPS-42. Especially, we concentrate on special cases where probability of
misinterpretation is higher.
The rules of ionogram scaling are discussed and demonstrated on the examples in the handbooks
(for example [Wakai et al., 1986]). Usually, the measured ionogram is compared with a given example
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KOUBA ET AL.: INFLUENCE OF NON-VERTICAL ECHOES TO IONOGRAM SCALING
(a)
(b)
(c)
Figure 1. Examples of day-time ionograms measured (a) by ionosonde AIS in 1972, (b) by ionosonde
IPS-42 in 1986 and (c) by Digisonde DPS4 in 2010. Ionogram scaling in (a) is difficult and requires
long-time experience (frequency axis is non-linear and without hold ticks). Scaling accuracy is low in
comparison with the instruments used later. Ionogram in (b) can be scaled with better accuracy on
computer with special software. Present ionograms (c) are in the digital format, providing pairs of
frequency and reflection height, hence all the characteristics can be scaled with high accuracy. DPS
autoscaling software provides the first estimation of ionospheric characteristics. Interpretation of the
DPS ionograms is easier in comparison to the older measurements as there are clear differences in
vertical and oblique reflections, ordinary and extraordinary signals, etc. In the figure, ordinary and
extraordinary reflections are visualized as a dark and light tones, respectively.
and scaled accordingly. In the process of scaling classical black and white (BW) ionograms it is
necessary to follow the instructions how to identify the ordinary and extraordinary traces. In the BW
plot it is not always evident whether the reflection trace corresponds to the ordinary trace therefore a
rule for trace selection must be applied. Measurement that records time of flight and also polarity and
azimuth allows clear identification of the proper reflection trace.
However, more than one interpretation of BW ionogram often exists. Without additional
information it is impossible to decide which interpretation is correct. Therefore, ionospheric
characteristics scaled from old data can contain some errors and misinterpretations.
Spread F
Spread echo occurs on the ionogram due to special conditions in the ionosphere. Instead of simple
ordinary and extraordinary traces in the ionogram there is more complicated structure developed. On
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KOUBA ET AL.: INFLUENCE OF NON-VERTICAL ECHOES TO IONOGRAM SCALING
the ionogram additional traces can be visible (Fig. 2a) or a trace may look like a horizontal band
(Fig. 2b) across the ionogram. For standard interpretation there are basic F-spread types defined in
ionogram handbooks [Wakai et al., 1986 ]:
(a) Frequency spread—the traces near the critical frequencies are broadened in frequency. This is the
most common type of spread F at most latitudes.
(b) Range spread—the traces away the critical frequency show broadening in range.
(c) Mixed spread—the traces are broadened in both range and frequency.
(d) Spur—includes all other types of spread. It indicates a presence of traces from an oblique
reflecting region which usually reflects to a considerably higher frequency than the F layer
nearest overhead.
On the following figures we demonstrate the risk of misinterpretation of historical ionograms.
Fig. 3a shows an ionogram measured by Digisonde DPS-4 on March 22, 2004. Plots are given in BW
without discrimination of wave direction, polarity, etc. in the reciever. It corresponds to the
measurements with older ionosondes. According to the rules this kind of ionograms is interpreted as
ionogram with wide frequency spread. The critical frequency (Fig. 3a) can be analyzed as
approximately 4.2 MHz for the extraordinary mode fxF2 and approximately 3.5 MHz for ordinary
mode (according to fxF2-half of the gyrofrequency).
However, Fig. 3b indicates completely different interpretation of this measurement. The
Digisonde DPS4 can recognize ordinary and extraordinary signal, and also direction of incoming
signal. On the panel b) there are displayed signals registered in vertical direction only (incidence angle
< 20 deg.) and ordinary/extraordinary signal is clearly distinguishable. Spread F in Fig. 3b is slight if
any. There are both ordinary and extraordinary traces visible. Critical frequency for extraordinary
trace is approx. 3.2 MHz (compare with Fig. 3a!). We cannot scale critical frequency of ordinary trace
directly due to the trace vanishing on higher frequencies. It is evident that wide spread in Fig. 3a is
caused by oblique signal registered by the receiving antenna system. Vertical ionogram shows practically no spread and critical frequency scaling produce significantly lower value (1 MHz difference).
In Fig. 4 there is again ionogram with spread F condition registered by Digisonde DPS4. In this
situation the spread F occurs on both all-directions and vertical ionograms. However, pattern of spread
is different. Ordinary and extraordinary traces on the ionogram can be easily determined, interpreted
and scaled. Oblique signal on all-direction ionogram occurs above critical frequency for ordinary
signal. For this reason foF2 can be misinterpreted and error may occur in foF2 evaluation.
The third example of ionograms with spread F occurrence in Fig. 5 shows the same pattern for alltraces ionogram and vertical ionogram. However, in Fig. 5b ordinary and extraordinary traces are
easily to be recognized. In this case there is not a big difference between classical (based on manuals)
and modern (based on additional information) interpretation and scaling. However, Fig. 5b allows easy
and accurate scaling.
(a)
(b)
Figure 2. Examples of ionograms with F-spread (measured by ionosonde IPS42) in the station
Pruhonice (1996). Ordinary and extraordinary traces are hardly distinguishable in BW figures and
ionospheric characteristics can be wrongly scaled.
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KOUBA ET AL.: INFLUENCE OF NON-VERTICAL ECHOES TO IONOGRAM SCALING
Figure 3. Ionogram measured on March 22, 2004 by DPS-4. Panel (a) shows all-directions echo
ionogram like in case of measurements with the older equipment. Panel (b) shows only vertical
ordinary (dark tones) and extraordinary (light tones) signals.
(a)
(b)
Figure 4. Ionogram measured on December 18, 2005 by DPS-4. Panel (a) shows all-directions
ionogram and panel (b) shows only vertical signals.
(a)
(b)
Figure 5. Ionogram measured on December 2, 2005 by DPS-4. Panel (a) shows all-directions
ionogram and panel (b) shows only vertical signals.
Z-mode on ionogram
F-trace is typically split into two branches on the ionogram. Radio wave splits into ordinary and
extraordinary waves due to the earth’s magnetic field that introduces anisotropy into the ionospheric
plasma. The critical frequency of the extraordinary wave is half gyrofrequency higher than the critical
frequency of the ordinary wave. In high-latitude region magnetic field lines point nearly vertical and
the Appleton-Hartree equation describes the third observed branch — z-mode. O and Z modes have
left hand and the X mode right hand polarization with respect to the magnetic field direction. The
difference between fxF2 and fzF2 is equal to the gyrofrequency (Rydbeck, 1950). Ordinary trace lays
between x and z traces and foF2 value occurs in about middle of both components. Except at high
latitudes the Z mode is not commonly observed on ionograms (Davies, 1990).
Two ionograms measured by Digisonde DPS4 are presented in Fig. 6. The F-trace is split into
three branches on both examples. Both ionograms can be scaled as ionogram with z-mode according
to classical interpretation. Critical frequency foF2 is scaled from middle branch. However, the middle
trace in Fig. 6b cannot be ordinary trace due to polarization. Hence classical interpretation introduces
error in foF2 value of half gyrofrequency.
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KOUBA ET AL.: INFLUENCE OF NON-VERTICAL ECHOES TO IONOGRAM SCALING
(a)
(b)
Figure 6. Ionograms measured on January 23, 2004 (a) and on January 22, 2004 (B) by DPS-4. Ftrace is split to three branches in both cases. Ordinary trace (dark) is the middle trace on panel (a) and
left trace on panel (b). In case of BW figure it is impossible to choose the correct way.
Conclusions
Sources of the uncertainties in scaling are of several origins. (1) Uncertainty of the arrival
directions of the reflected signals. In ionograms from DPS-4 we can eliminate signal from oblique
reflections and pick up the vertical reflection information (corresponding to the URSI definition of
critical frequencies of the ionospheric layers). (2) Uncertainty in sensing of the ordinary and
extraordinary mode. In some cases (mostly spread F), it is hard to estimate boundary between ordinary
and extraordinary modes from IPS 42 ionograms. Using the new equipment we are able to distinguish
between reflected waves that have origin in non-vertical transmission of the antenna, non-horizontal
stratification of the ionosphere and non-parabolic ionospheric electron density profile.
In this paper we show a special cases for which are high probability of ionogram
misinterpretation. Of course, probability of misinterpretation in case of typical day/night/winter/summer midlatitudal ionograms in quiet conditions is low. However, in disturbed conditions, in high solar
activity, during geomagnetic storms, etc. unusual features appear on the ionogram more often and
probability of misinterpretation increasing.
In case of Pruhonice observatory is presented example of z-mode ionogram (both real z-mode and
misinterpreted) observed rare. However, ionograms with spread F are often registered there during
summer season. It means that misinterpretation of spread F ionograms can be a real problem which
can affect statistical studies of ionospheric parameters obtained from historical ionograms.
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