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MEAN ELECTRON DENSITY VARIATIONS

OF THE QUIET IONOSPHERE

3 -MAY 1959

BY J.W. WRIGHT, L.R. WESCOTT AND D.J. BROWN

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

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properties of materials: the development of methods and instruments for testing materials, devices,

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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.

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NATIONAL BUREAU OF STANDARDS

technical <=YLote

40-3

March i960

MEAN ELECTRON DENSITY VARIATIONS

OF THE QUIET IONOSPHERE 3 - MAY 1959

lay

J. W. Wright, L. R. Wescott and D. J. Brown

Central Radio Propagation Laboratory

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

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MEM ELECTRON DENSITY VARIATIONS OF THE QUIET IONOSPHERE

3 - MAY 1959

ty

J. W. Wright, L. R. Wescott and D. J. Brown

Central Radio Propagation Laboratory

ABSTRACT

The CRPL has initiated a program for large-scale

computation of electron density profiles from ionospheric vertical soundings. Scaling is performed at

field stations permitting computation of hourly profiles at the Central laboratory. These profiles are combined to form hourly mean quiet profiles for each

station and month. The results of this program for

the month of May are illustrated graphically. This

report is the third of a series illustrating the electron density variations in the mean quiet ionosphere

between latitudes 15 N and 50°N along the 75 w meridian.

11

TABLE OF CONTENTS

PAGE

I Introduction 1

II Description of Basic Data 3

III Description of Graphical Representation.. 6

IV Discussion and Applications of the Data.. 7

References 10

Hourly Vertical Cross sections along 75°W

Geographic Meridian 12

Isoionic Maps (150,200, 250, 300, 350,400 km;

Nmax, hmax, Shmax, Shlnf) 36

Electron Density vs. Time Curves (N(t)),

(Newfoundland, Ft. Monmouth, White

Sands, Grand Bahama, Puerto Rico)...

^

MEAN ELECTRON DENSITY VARIATIONS OF THE QUIET IONOSPHERE

3 - MAY 1959

by

J. W. Wright, L. R. Wescott and D. J. Brown

Central Radio Propagation Laboratory

I Introduction

Part of the "basic responsibility of the Central Radio Propagation Laboratory is to gather ionospheric, solar, or other geophysical

data necessary in the pursuit of research leading to improvements in

radio communication and knowledge of the earth's upper atmosphere.

The existing network of ionospheric vertical sounding stations is an important source of such data. Typically, the radio sounding data

directly provide observations of peak electron densities (corresponding to "critical" frequencies foE, foFl, foF2), data on Sporadic E,

and certain derived propagation data such as maximum usable frequencies or MUF factors (Smith, 1939).

In fact, the vertical sounding is potentially capable of providing the complete electron density profile of the underside of the

Ionosphere (i.e., below hmaxF2 )

, and of providing a basis for extrapolation to much greater altitudes. However, because a lengthy and

difficult calculation Is required, little of this work had been done

until quite recently when techniques and computers have become available, and the exigencies of the IGY and satellite programs have made

it imperative.
2

In the course of its development of facilities for electron

density profile calculations, the CRPL has succeeded in devising

means "by which basic data for this purpose may he scaled by the

individual field stations, thereby decentralizing and simplifying

the most onerous part of the work. Through Its own station network and In cooperation with the U. S. Army Signal Ionosphere

Stations of the Signal Radio Propagation Agency, the CRPL has

initiated an extensive systematic data reduction program, from

which hourly electron density profiles are being computed for

the following stations:

Puerto Rico (NBS, January 1959)

Grand Bahama Island (U. S. Army Signal Corps, February 1959)

Fort Monmouth, New Jersey (u. S. Army Signal Corps, Feb. 1959)

White Sands, New Mexico (U. S. Army Signal Corps, March 1959)

St. Johns, Newfoundland (U. S. Army Signal Corps, April 1959)

Adak, Alaska (U. S. Army Signal Corps, June 1959)

Okinawa, Ryukus (U. S. Army Signal Corps, June 1959)

Thule, Greenland (U. S. Army Signal Corps, July 1959)

Bogota, Colombia (January i960)

Huancayo, Peru (January i960)

Talara, Peru (January i960)

Bagulo, Philippines (February i960)

.

Affiliation and approximate date of initial participation in this

program are given in parentheses.

The hourly electron density profiles are extensively used in

the research programs of CRPL, and are supplied directly to certain

other agencies as part of various research and practical activities.

However, in this comparatively early stage, broad dissemination of

the computed data is a somewhat difficult problem.

In an attempt to make at least a summary of the results of

the program widely and rapidly available, the CRPL has initiated

the present series of reports. These provide graphical representations of the monthly mean quiet hourly electron densities

from certain of the stations in the preceding list, from which

a fairly clear picture of the local ionosphere morphology may be

obtained

.
These reports contain N(h) data for the stations at Newfoundland, Fort Monmouth, Grand Bahama Island, White Sands, and Puerto

Rico. Pertinent facts concerning these stations are given in the

following table:

Table 1

Geomag. Coordinates Geog. Coordinates

Station fH Lat. Dip

St. Johns, Newf. 1.38Mc/s n O

58.5 N 72°N 47°33 , N 52°Uo ! w

Ft. Monmouth, N.J. 1A6 51.7°N 71.5°N i4-0°I5'N 74°oi*w

White Sands, N.M. 1.30 1U.2°N 60 N 32°2^ , N I06°52'w

Grand Bahama Island 1.30 3T.9°N 59-5°N 26%) 'N 78°22'W

Puerto Rico 1.15 30°N 51.5 N 18°30 , N 67°i2'w

II Description of Basic Data

True heights of reflection of radio waves are calculated from

the observed or "virtual" heights by the method of Budden (l^^k);

this method need not be described here except to point out that

full correction for geomagnetic field effects is made and that the

usual restrictions to monotonic profiles apply.

Tabulations of the mean electron density data at 10 km intervals and certain related quantities are the bases for the graphs

and charts. A sample for the Puerto Rico May data is given on

Page 11. The table on the following page identifies the quantities appearing on the tabulation.
Table 2

Quantity Units Remarks

Average Electron

Density (n) :10 = electron/cm Body of table; given at

each 10 km of height

from the lowest hmin to

950 km.

NMAX

3 3 xlO = electron/cm The mean value of Nmax.

COUNT Count of the number of

profiles entering the

the mean.

HMIN Kilometers The average height of

zero or very low electron density, obtained

by linear extrapolation

of the electron density

of the individual profiles.

HMAX Kilometers

SHMAX xlO = electrons/

cm^ column

- 12

( 10 " on maps)

SHINF xlO = electrons/

cm2 column

.10

(10 on maps)

The average height of

maximum electron density, determined by

fitting a parabola to

the upper portion of

the individual profiles.

Obtained by averaging

the integration of the

individual profiles

between the limits

HMIN and HMAX.

The average total num- ber of electrons in a column to infinity

obtained by extrapolaof observed profiles

into the region above

HMAX. (See text)
The following particular features of the tabulated data should

be noted:

A. Averaging process . Eachhour of each day is identified with

its magnetic character figure, Kp„ For each hour, those days for

which Kp is less than k+ are included in the "quiet" average. The

other days are similarly combined to form a "disturbed" average;

however, this rarely has physical significance because the number

of distrubed periods is usually small and the behavior of the ionosphere during disturbed hours is not consistent. Thus graphs for

these latter averages are not presented here.

B. Determination of hmax . The nature of the "true height"

process is such that no direct measure of hmaxF2 is obtained, the

virtual height at Nmax being immeasurable. A useful procedure is

to fit the portion near hmaxF2 with a suitable curve and to determine hmax from this curve. A parabola is quite satisfactory for

this purpose; it is fitted to two of the highest true heights and

the measured value of Nmax(foF2).

C. Extrapolation of profiles above hmax . Before averaging,

the individual profiles are extrapolated above hmax by a Chapman

distribution of 100 km scale height. This assumed model seems to

agree well with the few published measurements dealing with the

profile of the F region above hmax (Wright, 1960)- Extrapolation

is necessary in order to calculate homogeneous averages near hmax

and the average profiles are, in fact, given to 950 km*

D. Integrated Electron Densities , The total number of electrons in a unit column of the ionosphere between the effective

bottom (hmin) and hmax is called Shmax; it is obtained by num- erical integration of the observed profiles between these

limits. An estimate of the total electron content to infinity,

based upon the Chapman model, is called Shinf, It is obtained by

adding to Shmax the quantity N = 2.82 HNmax where H is an (assumed

constant) scale height for the region above hmax. Current evident
(Wright, i960) indicates H = 100 km, is an acceptable choice.

Ill Description of Graphical Representation

The relative smoothness of mean quiet ionosphere data lends

itself to various kinds of graphical presentation which offer con- venient aids to the visualization of the height, geographic, and

temporal variability of the ionosphere. Three such graphical forms

are included here, prepared from the tabulations described in Section II.

A. Vertical cross sections of the ionosphere along the 75 W

meridian . Pages 12 through 35 give for each hour, a vertical cross

section of the mean quiet ionosphere along a meridian section,

nominally the 75 W geographic meridian, for the month of May 1959*

Contours are parametric in "plasma" frequency (fN

) related to electron density N by

12,1*00 f

N

2

(Mc/s) = N( electrons/cm^).

The height of maximum electron density is represented by a dashed

line. Note that the vertical scale is expanded about 5.5 times.

With the exception of White Sands, each of these stations is

reasonably close to the 75 W meridian (see Table 1). There is the

possibility that some of the anomolies imposed by White Sands on

these contours are the result of the well-known longitudinal

asymmetry of the ionosphere.

B « Local time vs Latitude Maps . Another form of presentation,

useful for indicating the two-dimensional geographic variations of

the ionosphere, is that illustrated by the maps of pages 36 through

1*5 . Here, for fixed levels in the ionosphere (150,200,250,300,350,

1*00 km), contours of electron density are drawn in the latitude

vs. local time plane. To the extent to which longitude anomalies

are negligible, these maps give also the longitude vs. latitude
distribution of electron densities at the indicated heights when it

is noon over the 75 W meridian. Similar maps for the peak density,

Nmax; its height, hmax; the total electron content to Nmax, Shmax;

and the estimated total electron content, Shinf, are also shown.

C. Electron density vs. time curves (N(t) curves) . The mean

quiet diurnal variation of electron density between I50 and ^00 km

heights above the sounding stations is illustrated by these curves. Dashed lines are used wherever the electron density at any height

falls below the peak density (i.e. when the height hmaxF2 falls

below the height for the curve in question.) Such electron densities are the result of the extrapolations by the Chapman model

discussed in Section II C.

IV DisQussion and Applications of the Data

Mean quiet electron density data over wide geographic areas

are essential for application to numerous problems of both practical and scientific interest,, For example, the assessment of the

radio refraction errors and Faraday rotation effects in satellite

tracking require a knowledge of the electron density profile in

the direction of the satellite or an estimate of the electron

density at the satellite itself. Alternatively these properties

of the ionosphere, or estimates of the total electron content in

a column, may be measured by observations from rockets and satellites, or from radar moon-echoes, and compared with ground-based

observations of the kind shown here.

Frequently it will be necessary to use instantaneous density

data because of the considerable variability of the ionosphere

about the quiet mean. In such cases, the mean data are of value

as a "reference level" in the design of the experiment; for example,

in the choice of radio frequencies, satellite heights, antennas,

etc.
8

Since electron density data represent virtually our only source

of continuous, widespread data on the earth's upper atmosphere, such

data also are essential to geophysical problems involving the electrical, neutral, and magnetic properties of the atmosphere.

For example, data on the vertical cross sections may be compared

with theoretical expectations for the meridional height dependence

of the quiet ionosphere. Several workers, (Hirono, 1955) > anc^ Martyn (1956), have investigated the equilibrium height (of hmax)

for an ionosphere controlled by diffusion, height-varying electron

loss rate, and uniform (or zero) vertical electron drift. The

results are qualitatively in agreement with these observations in

predicting a rise of hmax towards the equator in daytime and a reversal of this tendency at night

.

The N(t) curves are perhaps the most physically significant

representation of electron density data. The strong solar control

at all levels and the important perturbations of this solar control

in the upper F region may be easily seen„
Acknowledgment s

We are indebted to Mr. A. H. Shapley for guidance In the

initiation of this work, and to Dr. H. H. Howe for the development of the computer programs upon which the whole system

heavily depends. This series of reports results from the com- bined efforts of the NBS Electron Density Profile Group, including at various times L. R. Wescott, L. Hayden, D. J. Brown,

F. J. Burmont, I. Ford, N. Moore, M. Durham, G. Lira, under the

direct supervision of G. H. Stonehocker. The cooperation of the

U. S. Army Signal Radio Propagation Agency, in particular the

assistance of Mr. F. H. Dickson (Chief), is gratefully acknowledged
10

References

Budden, K. G. , "A method for determining the variation of Electron

Density with height (n(z) curves) from curves of

equivalent height against frequency (h'(f) curves).

Proc. Cambridge Conf. on the Physics of the Ionosphere, pp 332-339, September l%k, Pub. by the

Physical Society (1955).

Martyn, D. F., "Processes controlling ionization distribution in thi

ionosphere," Aust„ J„ Phys, 2> PP I6I-I65, March

1956o

Hirono, M. , "Effect of gravity and pressure gradient on vertical

drift in the F2 region," Rpt. on Iono. Res. in

Japan, _£, No, 2, p 95-1C4, (1955)-

Smith, N. , "The relation of Radio Skywave Transmission to Ionosphere measurements," Proc IRE, _2J, No. 5, pp 332-

3^7, (l939)o

Wright, J„ W. , "A model of the F region above hmaxF2," Journal of

Geophysical Research, 6^ I85-I9I, January i960.
11

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18 00

APARTMENT OF COMMERCE

Frederick H. Mueller, Secretary

ATIONAL BUREAU OF STANDARDS

A. V. Astin, Direc

THE NATIONAL BUREAU OF STANDARDS

of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

ler, Colo., is suggested in the following listing of the divisions and sectioi linical v. ach section carries out specialized research, development, and engineering in the field indii description of the activities, and of the resultant publications, appears on the inside of tin

Electrochemistry. Electrical Instruments. Magii.

Photographic Research.

cular K:

IINGTON, D.C. TRICITY. Resistance and Reactance.

lies. IGY. Photometry and Colorimetry. Refractomel

nd Scale. Volumetry and Densimetry.

.attire Physics. Heat Measurements. Cryogenic Physics. Uheology.

Jadica! ch. Equation of State. Statistical Physics. Molecular Spectroscopy.

IADIATION PHYSICS. X-Ray. Radioactivity. Radiation Theory. High Em

Equipment. Nucleonic Instrumentation. Neutron Physics.

Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganii

dar Structure and Properties of Gases. Physical Chemistry. Thermochen

ices. LECI Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. I A) FIBROUS MATERIALS. Rubber. Textiles. Paper. Leather. Testing an

Plastics. Dental Research. URGY. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallui: RODUCTS. Engineering Ceramics. Glass. Refractori

ructure. \P< II. Structural Engineering. Fire Research. Mechanical Sj anic Build

I Pat Transfer. Inorganic Buildir ials. [ATICS. Numerical Analysis. Computation. Statistical

iponentB and Techniqui uitry. I ing.

NTATION.

I ion.

I Measures.

\PKKPHigh frequi

Airglow and Aurora. I
NBS