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^o. 40-5

MEAN ELECTRON DENSITY VARIATIONS

OF THE QUIET IONOSPHERE

No. 5 -JULY 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

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NATIONAL BUREAU OF STANDARDS

40-5

Issued

August I96I

MEAN ELECTRON DENSITY VARIATIONS

OF THE QUIET IONOSPHERE No. 5 - July 1959

by

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

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

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Price $1.50

MEAN ELECTRON DENSITY VARIATIONS OF THE QUIET IONOSPHERE

No. 5 - July 1959

*y

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 r suits of this program for

the month of June are illustrated graphically. This

report is the fourth of a scries illustrating the electron density variations in the mean quiet ionosph re

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,^00 km;

Nmax, hmax, Scat, Shmax, Shinf) 36

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

(Newfoundland, Ft. Monmouth, White

Sands, Grand Bahama, Puerto Rico)... ^7

MEAN ELECTRON DENSITY VARIATIONS OF THE QUIET IONOSPHERE

5 - JULY 1959

by

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

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 d

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 1 -ngthy 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 bh< IGY and satellite programs have m

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 those of the U. S. Army Signal Radio Propagation Agency, and

through cooperation with closely associated stations in other countries,

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 (BBS, 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, NewfoundlanI (Def. Res. Tel. Establ. Canada, June 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)

Baguio, Philippines (February 19 60)

.

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

obi,,-i i in Mi .
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.

Ft. Monmouth, N.J.

White Sands, N.M.

Grand Bahama Island

Puerto Rico

1 . 38Mc/s 58.5 N 72°N U7°33'n 52°Uo ? w

l.k6 51.7°n 71.5°N i+0°15'N 7i+°oi'w

1.30 Ul.2°N 60 N 32°2l+'N io6°52'w

1.30 37-9°n 59-5°n 26°^0 '

78°22'w

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 ( 195*0

this method need not be described here, but it should be pointod 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 Julj data is given on

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

Quantity Unit: Remarks

Average Electron _ Density (N) xlO = electron/cm Body of table; given at each

10 km of height from the lowest

(10 on maps) hmin to 950 km.

NMAX

3 o xlO = electron/cm The mean value of Umax,

(10"^ on maps)

COUNT Count of the number of profiles

entering 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.

HMAK Kilometers The average height of maximum

electron density, determined by

fitting a parabola to the upper

portion of the individual profiles.

SCAT

SHMAX

SHINF

Kilometers

xlO

10

electrons/

o cm col '.ainn 1

(10 on maps)

xlO = electrons/

cm column

(10 on maps)

One half of the half-thickness

of the parabola best fitting

the upper portion of the F- region profile. Approximates the

scale height near the true HMAX.

Obtained by averaging the integration of the individual profiles between the limits HMIN

and HMAX.

The average total number of

electrons in a column to infinity obtained by extrapolation of 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, 196-O • 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 IINmax where 11 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. Included here are three

such graphical forms 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 July 1959*

Contours are parametric in "plasma" frequency (f„) related to electron density N by

2 3

12,1+00 f (Mc/s) = N(electrons/cnr).

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

I4.5. Here, for fixed levels in the ionosphere (150,200,250,300,350,

lj-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
7

distribution of electron densities at the indicated heights when it is noon over the 75 w meridian. Similar maps for the peak density, Nma; ; its height, hrnax; its characteristic thickness, Scat; the total electron content to Nmax, S hrnax; 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 150 and 400 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 c

- suit of the extrapolations by the Chapman model discussed in Section

II C

IV Discussion 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 :ts in satellite tracking requir

a knowledge of the electron density profil< in th lir ction of the

satellite or an estimate of I h Lectron density at atellil

self. Alternatively these properties of the ionosph r . or estimates

of the total electron content in a column, may 1 by observations from rockets and satellites, or from radar moon-echoes, and com- pared with ground-based observations of the kind shown

Frequently it will be n< c< ssary to use instantaneous density data

because of the considerable variability of the ionosp: i-out tl quiet mean. In such cases, the m of value as a

"

level" in the 1 sign of th ; xample, in the cho

radio frequencies, sat heights, antenna . tc
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;

Martyn 195&) 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

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 the

ionosphere," Aust. J. Phys, 2» PP I6I-I65, March

1956.

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-lC^, (1955)-

Smith, N. , "The relation of Radio Skywave Transmission to Ionosphere measurements," Proc. IRE, 2J, No. 5, pp 332-

3^7, (1939).

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

Geophysical Research, 65 I85-I9I, January i960.
11

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N(t) NEWFOUNDLAND JULY 1959

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N(t) FORT MONMOUTH JULY 1959

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N(t) GRAND BAHAMA ISLAND JULY 1959

FIXED HEIGHTS IN Km
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N(t) PUERTO RICO JULY 1959

FIXED HEIGHTS IN Km

. DEPARTMEN1 OF COMMERCE

Luther H. Hodges, Secretary

FtEAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The 3Cope of activities of the National Bureau of Standards at its major laboratories in Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in te< In general, each section carries out specialized research, development, and engineerins in the field in'ti its title. A brief description of the activities, and of the resultant publications, appears on the inside of front cover.

WASHINGTON, D.C.

Electricity. Resistance and Reactance. Electrochemistry. Electrical Instruments. Magnet

Dielectrics. Metrology. Photometry and Colorimetry. Refractometry. Photographic Research. Length. Engineering Metrology.

Mass and Scale. Volumetry and Densimetry.

Heat. Temperature Physics. Heat Measurements. Cryogenic Physics. Equation of State. Radiation Physics. X-ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological Equipt

Nucleonic Instrumentation. Neutron Physics.

Analytical and Inorganic Chemistry. Pure Substances. Spectrochemistry. Solution Chemistn

ence Materials. Applied Analytical Research.

Mechanics. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Hheology. CombuControls. Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. I' ture. Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

trolysis and Metal Deposition.

Mineral Products. Engineering Ceramics. Glass. Refractories. Enameled Metals. Crystal <.r<> Properties. Constitution and Microstructure. Building Research. Structural Engineering. Fire Research. Mechanical Systems. Organic BuildJ Codes and Safety Standards. Heat Transfer. Inorganic Building Materials. Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Math. •rations Research. Data Processing Systems. Components and Techniques. Digital Circuitry. Di|

Applications Engineering.

Atomic Physics. Spectroscopy. Infrared Spectroscopy. Solid Slate Physics.

Instrumentation. Engineering Electronics. Electron Devices. Electronic Instrumentation, ments. Basic Instrumentation. Physical Chemistry. Thermochemistr) . Surface! smistry. Molt cular Kinetics. Mass Spectrometry.

Office of Weights and Measures. BOULDER, COLO.

Cryogenic Engineering. Cryogenic Equipment. Cryogt

Ionosphere Research and Propagation. Low I .-vices. Sun-Earth Relationship Radio H Radio Propagation Engineering. Data Reductioi

Vnalysis. Propaga rain I- f f

Radio Standards. High Frequency Electrical St,in<l.ir.< v and Time Interval Si rowave Circuit Standards. Radio Systems. High Frequency an tearch. M

Navigati' .is. Upper Atmosphere and Space Physics

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