^ecUnic&l v^ote 40-10

MEAN ELECTRON DENSITY VARIATIONS

OF THE QUIET IONOSPHERE

NO. 10- DECEMBER 1959

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

CecAmcaJ 92ote 4Q-\0

ISSUED MARCH 24, 1963

MEAN ELECTRON DENSITY VARIATIONS

OF THE QUIET IONOSPHERE

NO. 10- DECEMBER 1959

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

Central Radio Propagation Laboratory

NBS Boulder Laboratories

Boulder, Colorado

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.

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. - Price 35 cents

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

Acknowledgments , 9

References 10

Hourly Vertical Cross Sections Along

75°W Geographic Meridian 12

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

Nmax, hmax, Scat, Shmax, Shinf) 36

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

(Newfoundland, Ft. Monmouth, White

Sands, Grand Bahama, Puerto Rico) 47

in
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 December are illustrated

graphically. This report is the tenth of a series illustrating the electron density variations in the mean quiet

ionospher<

meridian.

ionosphere between latitudes 15 N and 50 N along the 75 W

IV
MEAN ELECTRON DENSITY VARIATIONS

OF THE QUIET IONOSPHERE No. 10 - December 1959

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.

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 be 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.
2

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 (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 (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)

Huancayo, Peru (January I9 60)

Talara, Peru (January I960)

Baguio, 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

fH Lat. Dip

St. Johns, Newf. 1.38Mc/s 58. 5°N 72°N 47°33'N 52°40'W

Ft. Monmouth, N. J. 1.46 51.7°N 71.5°N 40°15'N 74°01'W

White Sands, N. M. 1.30 41.2°N 60°N 32°24'N 106°52'W

Grand Bahama Island 1.30 37.9°N 59.5°N 26°40*N 78°22'W

Puerto Rico 1.15 30°N 51.5°N 18°30'N 67°12'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 (1954); this

method need not be described here, but it should be pointed 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 Decqrrber data, is given on Page 11.

The table on the following page identifies the quantities appearing on

the tabulation.
Quantity Units Remarks

Average Electron Density (N)

3 3 xlO = electron/cm

_5

(10 on maps)

Body of table; given at each

10 km of height from the

lowest hmin to 950 km.

NMAX xlO^ = electron/cm

( 10 ~5 on maps) The mean value of Nmax.

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.

HMAX Kilometers The average height of maximum electron density, deter

mined by fitting a parabola

to the upper portion of the

individual profiles.

SCAT Kilometers 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.

SHMAX 10 xlO = electrons/

cm column

(10 on maps) 12

Obtained by averaging the

integration of the individual

profiles between the limits

HMIN and HMAX.

SHINF xlO = electrons/

(10

13

cm column

on maps)

The average total number

of electrons in a column to

infinity obtained by extrapolation of observed profiles

into the region above HMAX.

(See text. )
5

The following particular features of the tabulated data should be

noted:

A. Averaging process . Each hour of each day is identified with

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

which Kp is less than 4+ 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

disturbed 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 hmax F2 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, I960). 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 numerical 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 evidence (Wright, I960) indicates H = 100 km, is an

acceptable choice.
III. Description of Graphical Representation

The relative smoothness of mean quiet ionosphere data lends

itself to various kinds of graphical presentation which offer convenient 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 December

1959. Contours are parametric in "plasma" frequency (f]\f) related to

electron density N by

12,400 f 2 ,3

N

(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 anomalies imposed by White Sands on

these contours are the result of the well-known longitudinal asymmetry

of the ionospher >.

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 45.

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

400 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; its characteristic thickness, Scat; 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 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

result 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 effects in satellite tracking requires

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.

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.
8

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 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.
Acknowledgm ents

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. Stone hocker.

The cooperation of the U. S. Army Signal Radio Propagation

Agency 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 1954, Pub. by the Physical Society

(1955).

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

in the ionosphere," Aust. J. Phys. 9_, pp 1 61 - 165, March 1956.

Hirono, M. ,

Smith, N. ,

Wright, J. W.

"Effect of gravity and pressure gradient on

vertical drift in the FZ region," Rpt. on Iono.

Res. in Japan , 9, No. 2, pp 95-104, (1955).

"The relation of Radio Skywave Transmission

to Ionosphere Measurements," Proc. IRE, 27,

No. 5, pp 332-347, (1939).

"A model of the F region above hmaxF2,"

Journal of Geophysical Research, 65_, pp

185-191, January I960.
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DEC. 1959

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OF MAXIMUM ELECTRON DENSITY

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DEC. 1959

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20

300
51

* U. S. GOVERNMENT PRINTING OFFICE : 1963 O - 681816

U. 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, D.C., 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, U. C.

Electricity. Resistance and Reactance. Electrochemistry. Electrical Instruments. Magnetic Measurements

Dielectrics. High Voltage.

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. Statistical Physics.

Radiation Physics. X-ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological Equipment.

Nucleonic Instrumentation. Neutron Physics.

Analytical and Inorganic Chemistry. Pure Substances. Spectrochemistry. Solution Chemistry. Standard Refer- ence Materials. Applied Analytical Research, Crystal Chemistry.

Mechanics. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Rheology. Combustion

Controls. Polymers. Macromolecules: Synthesis and Structure. Polymer Chemistry. Polymer Physics. Polymer Characterization. Polymer Evaluation and Testing. Applied Polymer Standards and Research. Dental Research.

Metallurgy. Engineering Metallurgy. Microscopy and Diffraction. Metal Reactions. Metal Physics. Electrolysis and Metal Deposition.

Inorganic Solids. Engineering Ceramics. Glass. Solid State Chemistry. Crystal Growth. Physical Properties.

Crystallography.

Building Research. Structural Engineering. Fire Research. Mechanical Systems. Organic Building Materials. Codes and Safety Standards. Heat Transfer. Inorganic Building Materials. Metallic Building Materials. Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics. Operations Research. Data Processing Systems. Components and Techniques. Computer Technology. Measurements Automation.

Engineering Applications. Systems Analysis.

Atomic Physics. Spectroscopy. Infrared Spectroscopy. Far Ultraviolet Physics. Solid State Physics. Electron Physics. Atomic Physics. Plasma Spectroscopy.

Instrumentation. Engineering Electronics. Electron Devices. Electronic -Instrumentation. Mechanical Instru- ments. Basic Instrumentation. Physical Chemistry. Thermochemistry. Surface Chemistry. Organic Chemistry. Molecular Spectroscopy. Ele- mentary Processes. Mass Spectrometry. Photochemistry and Radiation Chemistry.

Office of Weights and Measures. BOULDER, COLO.

Cryogenic Engineering Laboratory. Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Cryogenic Technical Services. CENTRAL RADIO PROPAGATION LABORATORY

Ionosphere Research and Propagation. Low Frequency and Very Low Frequency Research. Ionosphere Research. Prediction Services. Sun-Earth Relationships. Field Engineering. Radio Warning Services. Vertical Soundings Research.

Radio Propagation Engineering. Data Reduction Instrumentation. Radio Noise. Tropospheric Measurements.

Tropospheric Analysis. Propagation-Terrain Effects. Radio-Meteorology. Lower Atmosphere Physics.

Radio Systems. Applied Electromagnetic Theory. High Frequency and Very High Frequency Research. Fre- quency Utilization. Modulation Research. Antenna Research. Radiodetermination.

Upper Atmosphere and Space Physics. Upper Atmosphere and Plasma Physics. High Latitude Ionosphere

Physics. Ionosphere and Exosphere Scatter. Airglow and Aurora. Ionospheric Radio Astronomy. H

RADIO STANDARDS LABORATORY

IRadio Physics. Radio Broadcast Service. Radio and Microwave Materials. Atomic Frequency and Time-Interval

Standards. Radio Plasma. Millimeter-Wave Research.

Circuit Standards. High Frequency Electrical Standards. High Frequency Calibration Services. High Frequency

Impedance Standards. Microwave Calibration Services. Microwave Circuit Standards. Low Frequency Calibration

Services.
NBS