>rary, M.W. Bldg

JUN A 1982

j5oulder laboratories

PB 161647

92o. 746

ANALYSIS OF IONOSPHERIC VERTICAL

SOUNDINGS FOR ELECTRON DENSITY

PROFILE DATA

III. PROCEDURES FOR OBTAINING MONTHLY

SUMMARY VIRTUAL HEIGHT CURVES FOR

N(h) ANALYSIS

(COMPOSITE VIRTUAL HEIGHT CURVES)

BY J. W. WRIGHT

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March 3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent

with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials, devices, and structures; advisory services to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; and the development of standard practices, codes, and specifications. The work includes basic

and applied research, development, engineering, instrumentation, testing, evaluation, calibration services,

and various consultation and information services. Research projects are also performed for other government

agencies when the work relates to and supplements the basic program of the Bureau or when the Bureau's

unique competence is required. The scope of activities is suggested by the listing of divisions and sections on the inside of the back cover.

Publications

The results of the Bureau's research are published either in the Bureau's own series of publications or

in the journals of professional and scientific societies. The Bureau itself publishes three periodicals avail- able from the Government Printing Office: The Journal of Research, published in four separate sections,

presents complete scientific and technical papers; the Technical News Bulletin presents summary and pre- liminary reports on work in progress; and Basic Radio Propagation Predictions provides data for determining

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technical Vtote

1 46

May 1, 1962

ANALYSIS OF IONOSPHERIC VERTICAL SOUNDINGS

FOR ELECTRON DENSITY PROFILE DATA

III. Procedures for Obtaining Monthly Summary

Virtual Height Curves for N(h) Analysis

(Composite Virtual Height Curves)

by

J. W. Wright

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED KY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price $.75

ANALYSIS OF IONOSPHERIC VERTICAL SOUNDINGS

FOR ELECTRON DENSITY PROFILE DATA

III. Procedures for Obtaining Monthly Summary

Virtual Height Curves for N(h) Analysis

(Composite Virtual Height Curves)

by

J. W. Wright

Abstract

Procedures are described for use at ionospheric sounding

stations for obtaining a median virtual height curve from a number

of individual ionospheric vertical soundings. This median curve

may then be analyzed to find an electron density profile representative of the individual observations. The method is advocated as an

economical way to obtain a world N(h) morphology and to obtain

control data for studies using more detailed and accurate profiles.

Foreword

This series of reports concerns methods in use or available

at the National Bureau of Standards for analysis of ionospheric

vertical soundings to obtain electron density profile data. Previous

reports have described overlays for convenient manual analysis of

ionograms (Wright and Norton, 1959), and for extrapolation of

N(h) profiles above hmax F2 (Wright, 1959). The present report

describes a procedure for obtaining a median virtual height curve

from a superposition of many curves, from which an N(h) profile

representative of the constituent observations can be calculated. , The basic concept described here was first suggested by Schmerling

and Thomas (1956), was more fully described by King (I960), and was

tested in comparison with other methods by Wright (I960).

ANALYSIS OF IONOSPHERIC VERTICAL SOUNDINGS

FOR ELECTRON DENSITY PROFILE DATA

III. Procedures for Obtaining Monthly Summary

Virtual Height Curves for N(h) Analysis

(Composite Virtual Height Curves)

by

J. W. Wright

1. Introduction

As techniques continue to be developed for obtaining electron

density profiles from ionograms, it is becoming clear that several

different kinds of such data may usefully be obtained from the same

ionograms. On the one hand, individual ionograms or groups of

ionograms may be analyzed to obtain individual electron density

profiles for special studies; usually great detail and accuracy is

desirable in such studies, requiring corresponding care and precision in preparation of the "input" virtual height data. On the other

hand, a great need exists for a broad survey of the average height

structure of the ionosphere, in all its geographic, diurnal, seasonal

and solar variations. It is clear that such average profiles (for example,

averages of all the quiet days in a month at a particular hour) will not

show all the fine detail of an accurate individual profile, since these

fine details may not be consistently present on each virtual height

curve. Consequently, for the purposes of obtaining average profiles

it is not necessary to employ scaling or calculation methods which

require extremely great care and precision in preparing the input data.

Until recently, the accuracy of calculated electron density

profiles was limited by the mathematical techniques available, provided reasonable care had been exercised in scaling the ionograms.

In fact, even with the best ionograms, there was little hope of obtaining
2

much more detail on an individual profile than in a month's average.

However, there has been great progress recently in the development of

techniques; methods are now available which can give highly detailed

and accurate N(h) profiles, provided the ionograms are of excellent

quality, and provided they are very carefully and meticulously scaled.

Such scaling is very laborious and is not well suited to routine calculation of large numbers of profiles. The older, rather crude methods

are still suitable for obtaining average profiles, and it therefore

becomes desirable to employ an ionogram scaling method of appropriate simplicity. Obviously, if the virtual height curves may be averaged

together in a simple way, much labor in scaling and computing from the

individual curves may be saved.

This note describes an efficient process for obtaining and

scaling an "average" virtual height curve. The process has been

tested at a small number of ionosphere stations for two years, and

has been shown to be a very convenient way of obtaining median N(h)

profiles which correspond with acceptable accuracy to the "mean-quiet

profile" obtainable by averaging individual profiles (Wright, I960).

The method was recommended for international systematic use by the

URSI World-Wide Soundings Committee at its 1961 meeting in Nice.

II. Basic Principle

Each of the ordinary-ray virtual height curves over which the

average is to be taken is traced onto the same sheet of standard "Ionogram Tracing" paper. A "median virtual height curve" is obtained by

a simple counting process. This median curve is then scaled for the

numerical values of virtual heights required for calculation of the

median electron density profile. An example of the composite virtual

height curves, obtained in a month's scaling at 2100 hours local time,

is shown in figure 1.
8 9 10

Figure 1

III. Recommended Program; Facilities Needed

The standard program recommended by the URSl/WWSC calls

for determination of monthly median virtual height curves at hourly

intervals. Thus, in the course of a month's operation, twenty-four

sheets (one for each hour) of standard Ionogram Tracing paper would

accumulate tracings of the entire month's hourly virtual height curves.

These tracings are most easily made by using a "front projection" type

scaling table ( fig. 2) which can project the ionogram onto the

"Ionogram Tracing" form with proper optical magnification to fit

the height/frequency coordinates of the form. A copy of the form

currently in use for C-model ionosondes is shown in figure 3. The

single curve is intended to illustrate numerical scaling methods, and

will be discussed in Section X.
Figure 2
J4L

statimi BOULDER

TIME ZONF 105° W mtf 29 FEB 1962

0.7 0.8

timf 1200

0.9 1.0

INITIALS JIW_

MB

I]ml leher

A 103 9

1 8

2 7

3 6

4 5

BI03 9

1 8

2 7

3 6

4 5

CI05 9

1 8

2 7

3 6

4 5

D 113 9

1 8

2 7

3 6

« 5

EII9 9

1 8 268

2 7 172

3 6 150

4 5 140

F237

I 230

9 4

259

6

5 240 9 Figure

6 348

5 348

H33I 9

1 8

2 7

3 6

4 5 323

1324 9

1 8

2 7

3 6

4 5 340

J 371 9

1 8

2 7 508

3 402 6 470

4 5441

K 9

1 8

2 7

3 6

4 5

L 9

1 8

2 7

3 6

4 5

M 9

1 8

2 7

3 6

4 5

6

It is strongly urged that scalers trace the various curves onto

the form lightly with a sharp, hard pencil. When 30 curves have been

accumulated on the form, the advantages of this will be apparent.

Special pencils, with replaceable leads, erasers, and sharpeners are

available to NBS and associated stations as "Ionogram Tracing Pencils."

IV. Precautions: Ionogram Scales

It is absolutely essential that the ionosonde be adjusted so that

the ionogram height and frequency markers fit the scaling form exactly.

This should be carefully and critically checked at least once per day,

and the ionosonde adjusted accordingly. The procedure for this adjustment on C -model ionosondes is as follows:

a) The scaling table should first be adjusted, without

film, so that the light strikes the scaling surface perpendicularly at the center of the projected area. This may be

easily checked by viewing the shadow cast by a righttriangle held on edge against the center of the scaling

surface.

b) It is assumed that the VFO and camera motors

run at their nominal (marked) speeds and that the correct

gear positions are used. If there is any doubt regarding

motor speeds, they should be checked against an accurate

clock. Defective motors should be replaced.

c) Without regard to the height markers , the scaling

table magnification should be adjusted so that the frequency markers of an ionogram exactly fit the frequency

scale of the tracing form. Note that the beginning and end

of the ionogram also effectively represent frequency markers.
7

When the scaling table has received these adjustments, it

should never be necessary to change them again. All further

adjustments should be done on the ionosonde. Note : If alignment

of all of the frequency markers cannot be achieved whatever the

adjustment of the scaling table, then the VFO cam and/or trimmers

are not correctly adjusted, and the markers (observed with earphones)

will not occur at their proper places on the frequency dial. See the

ionosonde manual for the proper alignment procedure. Before

deciding to re -cut the VFO cam, please inform the Equipment

Development Group, NBS.

d) When alignment of the frequency markers

with the standard form is satisfactory, examine the

height markers and determine -whether they are

expanded or compressed. Judge the amount of

expansion or compression, in percent, by counting ionogram markers vs. height lines on the form. Adjust

the height scale expansion contr ol of the ionosonde to

achieve the proper expansion or compression of the

markers. If necessary, make several test ionograms

around the approximate position, and mark the control

for each one. Compare the ionograms, in the scaling table,

with the standard form, and adjust the ionosonde again if

necessary. If the height markers are non-linear, adjust

the linearity control for best linearity.

Do not be satisfied with poor height marker alignment 1

It is very important to your own scaling and to those who

use the ionograms later that the markers fit the standard

form exactly . If you find difficulties you cannot deal with

alone, ask the Equipment Development Group at NBS for

assistance.
V. Obtaining the Median Virtual Height Curve

There is a unique median virtual height curve defined by the

composite virtual height curves. This is easy to see in the simplified

diagram of figure 4:

Figure 4

In this diagram, the curves 1,2, 3 6 represent parts of

various virtual height curves. The point M represents a point on the

median curve, which is to be found. What we wish to show is that

for any line, AB, which passes through M and which crosses each of

the curves 1, 2 , 3 , 6 once and only once, M is the median of

the points of intersection of A B with these curves. This is obvious

from figure 4. It is clear that the angle AB makes with the horizontal

is unimportant (compare A' B"). It is not even necessary that the line

A B be straight, provided it crosses each virtual height curve once and

only once.
Therefore, various points, M,on the median curve can be found

by first drawing a number of lines AB through the composite virtual

height curves. Then, knowing the number of virtual height curves in

the composite (the "median count"), one can count the intersection

points from either side along the lines, AB, until the middle number

of the count is reached. For an even count, the median is half-way

between the points of the two middle values. This process is illustrated for a simple case of median count 6 in figure 5.

Figure 5

Dots in this figure show points on the median curve found by

counting to midway between the 3rd and 4th intersection of the lines

AB. Where the curves are so closely packed as to make counting

impossible, the median may easily be drawn through the middle of
10

the pack, without counting. Account of the number of curves traced

is kept in a block in the lower right corner of the form. It will frequently be necessary to count the number of intersection points when

partial h'(f) curves have been traced. Not very many lines, AB, are

necessary to define the general course of the median curve. These

lines should be placed wisely, to identify important features of the

majority of individual curves. It is important that each of the lines,

A B, cross each of the virtual height curves once and only once.

A median curve may then be drawn through the M-points.

The final median virtual height curve should resemble the majority

of individual curves. For example, the highest F2 virtual height

should be about as great as the average height of this point on the

individual curves. If there is an Fl cusp on most of the curves, give

the median curve an approximately average shape and height at this

cusp. Make the foE cusp (if any) and the surrounding virtual heights

of about average shape and height. Always use the local count (the

number of intersection points) if this is less than the total number of

curves traced.

VI. Treatment of Sporadic E Echoes

Es echoes are not used for analysis of N(h) profiles. If c, h,

q, 1, f types (see IGY Annals, Vol. Ill, Pt. 1, Wright, Knecht, Davies,

1957) of Es are present, they should be ignored. Only what can be

seen of the ordinary-ray reflections from the E, Fl, and F2 layers,

complete with any stratifications which may be present, should be

traced. In the Arctic, or during auroral or disturbed conditions, if a nighttime E layer is visible, its ordinary -ray should be included

in the composite tracings. This applies also to some "retardation

Es" echoes which are clearly observed to retard the F region echo.

Account should be kept of the number of blanketing Es echoes (occulting

any large portion of the F region echoes) in a space at the left end of

the form.
11

VII. Oblique Echoes

Always attempt to identify the "overhead" echo. This will

usually be the lowest ordinary -ray echo at each frequency, or the

echo with the most nearly correct multiple echo. If the overhead

echo cannot be identified throughout the frequency range , one may

leave out the mysterious parts, and trace the reliable parts.

VIII. Spread Echoes

Trace any well-defined ordinary-ray echo which shows

through the spread echo, and which, by continuity with surrounding ionograms, seems to be representative of the general criticalfrequency variation. In severe cases of spread echo, either or

both the inner and outer limits of the spread may be traced, if they

are well defined. In counting the median, each traced curve must

counted; thus the final count can exceed the number of days in the

month, if spread echo is observed. Account should be kept of the

number of "spread" ionograms in the space F at the left end of the

form.

IX. The Extraordinary -Ray

The extraordinary -ray should not be confused with the

ordinary-ray. The only part of the extraordinary -ray required

at present on the composite tracings is any observable portion

of the X-ray rising to the gyro -frequency at night. See figure 6.
12

Figure 6

Only the downcoming portion of the X-ray is required. It is advisable

to trace this in a contrasting color (blue) to avoid confusion with the

composite ordinary-ray traces.

X. Scaling the Median Virtual Height Curve

It is now assumed that a satisfactory median virtual height

curve has been obtained by the methods outlined above. This median

curve should be drawn carefully with a hard red pencil.

Now, ignoring the surrounding maze of individual curves,

enough virtual heights are to be scaled from the median curve so as

to represent it numerically in some detail. Note that the frequency

scale of the tracing is divided in equal intervals of logf. A numerical
13

log f scale is found across the top of the graph. Blocks at the

bottom of the paper provide spaces for each of the log f lines in the

graph immediately above. The first space in each block (with an

alphabetic letter) corresponds to the left-hand heavy log f line

immediately above. These heavy log f lines are at intervals of

0. 1 logf.

The virtual height at each point where the median curve

crosses a 0. 1 log f line is to be scaled to the nearest kilometer

and written beside the letter in the corresponding space below.

Next, virtual heights should be scaled at as many 0.01 log f lines

as necessary to describe fairly completely the variation in the

virtual height curve within each 0. 1 logf interval. It is not necessary to scale every 0. 01 logf value. The examples shown in

figure 3 should give an idea of the amount of extra points required.

To decide how many 0. 01 logf values are needed in a given

interval, try to imagine a straight line connecting any two consecutive

scaled points. This is roughly how the computer "sees" the numerical

data. Add extra scaled values wherever the real curve "bends" away

from the imagined straight line by more than about two millimeters

(or about the distance .of 10 km on the tracing form).

Each value of virtual height should be scaled to the nearest

kilometer. The heights should be written into the appropriate box

beside the number telling at which 0.01 logf line it was scaled.

Simply leave unused boxes blank. The computer finds the proper

frequency to associate with each height by the position in the

tabulation of each value, so it is very important to put the heights

in the correct box.
14

XI. Scaling foF2

The median value of foF2 should be scaled from the median

virtual height curve. Instead of scaling foF2 in Mc/s, it is more

accurate to scale log foF2 in units of 0. 001 log f from the scale at

the top of the graph. It should be written in the space provided at

the right-hand end of the form.

XII. Use of Standard Scaling Letters

Spaces have been provided at the left end of the form for

those letters which describe conditions of special interest to this

method of analysis. The special cases for Spread Echo and for

blanketing Es have already been noted. Other spaces are provided

for noise (S), loss of observations (C), or blanketing Sporadic E (B).

Account should be kept in these spaces of the number of ionograms

affected by these causes. At the end of the month a single letter may

be entered in the space provided beside the time box (see below). A

letter should only be added if it occurs in more than l/2 the count.

XIII. Recording the Date and Time

The name of the ionosphere station, its standard time zone,

the month and whole -hour are to be written in the appropriate

spaces at the top of each of the 24 forms used for a month's composite

virtual height curves. Numbers designating the year, month and

whole -hour of the tracings should also be written in the small boxes

provided for the convenience of the punched-card machine operators.
15

The spaces for "Day" are to be filled in with the total number

of day's tracings which appear on the forms. * As remarked earlier,

a small tally written in the appropriate box on the form, to which one

mark is added as each tracing is rm.de, will help to keep account of

the number of tracings; it should be kept in the space provided.

Thus ^WrTT 111 represents an accumulated tally of 8 tracings.

The minutes -blocks beside "Time" should be filled in with

zeros.* Therefore, the completed Date-Time blocks for the month

of February 1962 at 0900 containing a total of 26 tracings would look

like this:

1 YR. MO. DA. 6 2 2 2 6

£ 9

5Em

QUAL LETTER

IV. Mailing Address

Scaled median virtual height data and composite tracings

should be sent on a monthly basis to

National Bureau of Standards

Central Radio Propagation Laboratory

82. 70 Vertical Soundings Research Section

Boulder, Colorado, U.S.A.

from those stations participating in the "Composite Virtual Height"

program of the National Bureau of Standards.

* Spaces for "Day" and "Minutes" are shown on these forms to

permit their use in other specialized programs.
16

References

King, G. A. M. "A convenient method of getting representative

Ionospheric heights" J. Geophys. Research, 65, 1623-1624

(1960).

Schmerling, E. R. and J. O. Thomas "The distribution of

electrons in the undisturbed F2 layer of the Ionosphere",

Phil. Trans. Roy. Soc. Series A, 248,609-620 (1956).

Wright, J. W. , "A test of a procedure for easy estimation of

representative monthly electron density profiles for the

Ionosphere", J. Geophys. Res. 65_, 3215-3217 (I960).

Wright, J. W. and R. B. Norton "Analysis of Ionospheric

Vertical Soundings for Electron Density Profile Data.

I. Facilities for convenient manual reduction of ionograms",

NBS Technical Note No. 14, July 1959.

Wright, J. W. "Analysis of Ionospheric Vertical Soundings for

Electron Density Profile Data. II. Extrapolation of

observed electron density profiles above hmaxFZ",

NBS Technical Note No. 19, June 1959.

T). S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, IXC, and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineering in the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

front cover.

WASHINGTON, D.C.

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