Jl Bureau o Lbrary, fl.w. Bit

MAY 3 1 1962

jBoulder laboratories

PB 151399 - 13

^o. 40 - 13

MEAN ELECTRON DENSITY VARIATIONS

OF THE QUIET IONOSPHERE

NO. 13, SUMMARY OF ONE YEAR OF DATA

MAY 1959 - APRIL I960

BY J. W. WRIGHT

I. S. DEPARTMENT OF COMMERCE

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40-13

April 15, 1962

MEAN ELECTRON DENSITY VARIATIONS OF THE QUIET IONOSPHERE

NO. 13; SUMMARY OF ONE YEAR OF DATA MAY 1959 - APRIL I960

by

J. W. Wright

Central Radio Propagation Laboratory

National Bureau of Standards

Boulder, Colorado

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Price $ 1. 50

MEAN ELECTRON DENSITY VARIATIONS OF THE QUIET IONOSPHERE

NO. 13; SUMMARY OF ONE YEAR OF DATA MAY 1959 - APRIL I960

by

J. W. Wright

Central Radio Propagation Laboratory

National Bureau of Standards

Boulder, Colorado

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. This report is the last of a series which has

illustrated the electron density variations in the mean quiet ionosphere between latitudes 15 N and 50 N along the 75 W meridian.

It contains a summary of phenomena evident in the individual

reports of the series, and a summary presentation of the year's

data.

(This study performed under contract NTF-84 with the National

Aeronautics and Space Administration. )

Table of Contents

Page

Abstract

Table of Contents

1

.

Introduction 1

2. Calculation of the Electron Density Data 3

(1) Electron density at fixed heights 4

(2) Height and characteristic thickness at the F2 peak ... 4

(3) Extrapolation of profiles above hmaxF2 5

(4) Subpeak electron content 6

(5) Mean electron density profile data 6

3. Typical Examples of TN 40 - Series Representations 6

(1) Vertical cross sections 7

(2) Electron density maps in latitude vs. local time 9

(3) Latitude vs. time variation of hmaxF2 „ 12

(4) Latitude vs. time variation of F2 layer thickness .... 12

(5) Latitude vs. time variation of subpeak electron

content 12

(6) Diurnal variation of electron density at fixed

heights 17

4. Diurnal and Seasonal Variations 17

( 1

)

Height of the F2 peak 18

(2) Quarter thickness of the F2 peak 24

(3) F2 maximum electron density 24

(4) Subpeak electron content 35

Appendix: A Note on the Day -to -Day Variability of the

Ionosphere 41

( 1 ) Variability of hmax . 42

(2) Relative variability of Scat 42

(3) Relative variability of total electron content 45

(4) Relative variability of maximum electron density ... 45

Acknowledgements • References

- 1 - MEAN ELECTRON DENSITY VARIATIONS OF THE QUIET IONOSPHERE

NO. 13; SUMMARY OF ONE YEAR OF DATA MAY 1959 - APRIL I960

by

J. W. Wright

Central Radio Propagation LaboratoryNational Bureau of Standards

Boulder , Colorado

1. INTRODUCTION

By the end of the IGY, the number of ionospheric soundings

reduced to electron density profiles was comparatively small,

measured in terms of the very large vertical soundings program

then underway. The orginial difficulties stemmed from the considerable labor necessary for manual computation of single profiles,

but by the time of the IGY, the availability of electronic computers

had largely eliminated this. The remaining limitation was the

rather formidable amount of scaled data necessary for each computation. The IGY electron density profile surveys conducted by

Schmerling [1957] and Thomas and Vickers [Thomas, 1959] for

selected stations on International Quiet or Regular World Days were

limited in their scope by the large amount of data preparation

necessary.

When the National Bureau of Standards embarked upon a similar program early in the IGC (1959), part of the effort was

devoted to developing techniques by which the valuable manpower

resources of the network of U. S. and associated vertical soundings

stations -- already the mainstay of the conventional ionospheric

soundings data published by NBS -- could be applied to numerical

reduction of their ionograms for N(h) analyses. Simple methods

were developed for this purpose, permitting the station scientist to

derive and tabulate data for subsequent N(h) analysis by the central

laboratory's computer. These methods, applied to the hourly ionograms of a day's observations, require about the same effort as the

preparation of a quarter -hourly f-plot.
- 2 - Since May 1959, the NBS has conducted an hourly electron density

profile survey, using data provided by the ionospheric stations, from

a current total of 11 stations. These stations, their affiliation, and

their initial date of participation in the grogram are given in table I.

Station

Puerto Rico

Grand Bahama Is

Fort Monmouth

White Sands,N.M

St. Johns, Newf.

Adak, Alaska

Okinawa , Ryukus

Thule , Greenland

Huancayo,Peru

Talara, Peru

Baguio , Philippine s

Table I

Affiliation Latitude Longitude Mag. Dip Began N(h)

NBS 18o 30'N 67°12'W 51.5°N Jan. 1959

USASigC. 26°40'N 78°22'W 59.5°N Feb. 1959

USASigC. 40°15»N 74°01»W 51.7°N Feb. 1959

USASigC. 32°24«N 106°52'W 60° N Mar. 1959

DRTE 47°33'N 52°40'W 72° N June 1959

USASigC. 51°54'N 176°39'W 63° N June 1959

USASigC. 26°30'N 128° W 36.5°N June 1959

USASigC. 76°31'N 68°50 f W 86. 2°N July 1959

NBS 12°03'S 75°20'W 0.5°N Jan. I960

NBS 4°34'S 81°15'W 13° N Jan. I960

NBS 16°25'N 120°36I E 17° N Feb. I960

Our first objective in the study of these data has been to classify

and describe the mean quiet geographical, temporal, and height structure

of the northern mid-latitude ionosphere. For this purpose, five of these

stations (Newfoundland, Ft. Monmouth, White Sands, Grand Bahama

Island and Puerto Rico) are close enough together geographically to

permit the delineation of the structure of the mean quiet ionosphere

between geographic latitudes of 15 to 50°N, and geomagnetic latitudes

of 30 N to 59 N, with some confidence. The results from these stations

for one year of data (March 1959 - April I960) are issued by the NBS in a series of Technical Notes [Wright et al. , 1959-1961]. It is the purpose

of this summary report to review this considerable volume of data, to

portray it in a different summary form, and to discuss certain other

results which assist in the interpretation of the shole.
- 3 -

2. CALCULATION OF THE ELECTRON DENSITY DATA

For the data discussed here, the well known matrix method of

Budden [1955] has been used to derive the electron density profiles.

It is unnecessary to review the details of the primary virtual heightto-true height conversion, since in most respects the procedure has

corresponded exactly to Budden' s. However, it should be observed

that this method has several shortcoming which lead to errors in the

N(h) data. The shortcomings are nearly as well known as the method

itself and do not need detailed discussion here; it suffices to itemize

them: (a) The electron density is assumed to be a monotonic function

of height; if a valley exists there will be a height error which diminishes

from a value equal to the valley width just above the valley, to smaller

values at greater frequencies. (b) The shape of the profile is somewhat inaccurately determined in regions of strong curvature, because

of a convenient but inappropriate assumption made about the variation

of electron density within small intervals [Paul, I960]. (c) Because

the ionogram observations themselves do not ordinarily contain

information about electron densities below about 10 /cm , there is

usually an error at night due to neglect of retardation in this ionization.

While one or more of these may be serious for individual profiles,

and while their resultant may give mean profile parameters which

contain net errors of perhaps 10%, these errors are small compared

with the total variations discussed here.

The result of the primary matrix multiplication is a table of true

heights at particular plasma frequencies. It is more convenient to derive from these data certain other quantities amenable to convenient physical interpretation. Our primary true height data has

therefore been put through a secondary calculation process in which

the following parameters are derived:
- 4 -

(1) Electron density at fixed heights : The plasma frequencies

o

(ffsf) are converted to electron densities by the relation N(electrons/cm ) =

2 2

12,400 f

KT (Mc/s) . A linear interpolation among the corresponding

true heights then provides the electron density at 10 km height intervals throughout the observable portion of the profile.

(2) Height and characteristic thickness at the F2 peak: Ionograms contain no direct information from a layer peak itself, since

the virtual height of a radio frequency penetrating just to this level is

immeasurable. Nevertheless, this level is of unique interest, and it

is essential to devise means for its description. A practical method

is to fit the portion of the true height profile near the peak with a

suitable curve, and to determine the parameters of the peak from this

curve. The parabola is the simplest curve for this purpose, and has

the additional merit of closely approximating the peak of a "Chapman"

distribution.

The parabola is given by

* T f I hmax -hi \ N = Nmax Jl- ^ Ym I /, (1)

Where hmax is the height of the layer peak, Nmax the peak electron

density corresponding to the critical frequency, and Ym is a parameter

characteristic of the thickness of the layer. This curve has been fit

to the highest portion of our true height curves, using the measured

critical frequency and two true heights: the highest and the fourth

from highest. The accuracy with which the parabola may be fit to

these data depends slightly upon the spacing of the two heights and

rather critically upon the accuracy of determination of Nmax. In the

data described here, various checks have been applied to eliminate

extreme errors due to the latter cause. The frequency spacing of the

two true heights used for fitting the parabola is a compromise between

a narrow spacing (sensitive to small relative errors in the data) and a wide spacing (sensitive to real departures from the parabola). In

practice, these points are separated by about 0. 8 Mc/s.
- 5 - In addition to the height of the peak, the quantity Ym/2, which we

call " Scat" is determined. This is the quarter -thickness of the parabola

and is taken as a measure of the scale height of the atomic species at

the level hmaxFZ, in what follows. The reasons for this interpretation

are included in the following section.

(3) Extrapolation of profiles above hmaxF2: For sunspot

maximum conditions it was previously shown [Wright, I960] that an

electron distribution above the F2 peak of the form

.. i f , hmax - h hma: N = Nmax exp j? ^ 1 - — - exp ( - — > j (2)

agreed fairly well with available rocket and other data, for a neutral

particle scale height of H = 100 km. This is, of course, the wellknown equation derived by Chapman for the equilibrium distribution

due to electron production and recombination-type loss, but it is also

the equilibrium form theoretically expected of a layer under the combined influences of diffusion and attachment -like loss, as has been

shown by Hirono [1955], among others. Since these latter processes

are thought to be effective in determining the F2 peak [Ratcliffe et al. , 1956], the use of equation (2) for extrapolation above the F2 peak has

some theoretical justification. Since equation (2) approximates to

the parabola of equation (1) with Ym = 2H, the interpretation of Scat

in terms of the scale height receives a similar degree of theoretical

justification. This interpretation is quite important to the following

sections of this report.

Each of the hourly profiles used in this study has been extrapolated

above hmaxF2 by equation (2), with H = 100 km. The total electron

content, which we term Shinf, is given by adding to the subpeak content Shmax (vide infra), the quantity 2.82 HNmax, as may be shown

easily by integration of (2) between limits hmax and infinity.
-6-

(4) Subpeak electron content: The total number of electrons

in a unit column extending from the height of the lowest observable

ionization (usually less than Ne = 10 /cm ) to hmax, is termed Shmax.

It has been obtained in the course of our secondary calculations by

numerical integration of each profile between these two limits.

(5) Mean electron density profile data : The systematic calculation of hourly profiles provides a great quantity of data containing,

among other inhomogeneities , various degrees of ionospheric disturbance. Our first objective has been to obtain the mean quiet variations

from these data. This has been done by eliminating profiles at those

hours for which the magnetic character figure Kp exceeds 4 +; from

the remaining data, the monthly mean values of electron density at

10 km height intervals, and similar averages of the other special

quantities are obtained for each hour. Generally, data from about 20

profiles comprise such a mean.

At the same time, the standard deviation of the data entering

each mean is obtained. For some of the parameters a more useful

measure of the variability is the relative standard deviation or percentage variability of the quantity. This is obtained by dividing the

standard deviation by the mean. From the program described here,

some features of the behavior of a hmax, (a Scat)/Scat, ( a Shmax)/

Shmax, and (aNmax)/Nmax are discussed in an appendix to this

report.

3. TYPICAL EXAMPLES OF TN 40 -SERIES REPRESENTATIONS

The mean values of the quiet-day electron density profiles and

their derived parameters, for each month of the year March 1959 - February I960, are portrayed in a variety of representations in the

NBS Technical Note series 40 - 1 , 2, 3, . . . . etc. Typical diagrams from this series will first be illustrated and discussed as an

introduction to the parameters and as a brief review of the variety

of information available. While comments are offered regarding the

significance and interpretation of specific phenomena evident in these
- 7 - diagrams, no attempt is made here to discuss their generality; rather,

the aim is to point out several of the special properties of each form

of representation. Throughout, it should be borne in mind that our

data and conclusions pertain to a period near the maximum of the

solar cycle. In the following sections the entire body of data will

then be represented in a series of month-by-hour diagrams (for a year, running from May 1959 through April I960), which summarize

the diurnal and seasonal variations in the northern mid -latitude

ionosphere.

(1) Vertical cross sections : Figure 1 (from NBS Technical

Note 40-6) illustrates a vertical cross section of the ionosphere,

nominally above the 75°W meridian between geographic latitudes of

15° and 50°N, for 1700 75°W time, August 1959- Contours of plasma

frequency fN in Mc/s, related to electron density N by 12,400 f =

3 N electron/cm , represent the true heights of reflection vertically

incident radio waves. The height of maximum electron density is

represented by the dashed line, and electron densities above this

level are the result of an extrapolation according to the model discussed in Section 1 (3). Lines of the geomagnetic field would be very

nearly vertical within this diagram, and are therefore omitted. Note

that the vertical scale is expanded relative to the horizontal by a factor of about 5.5.

Near the level of maximum density a latitudinal gradient of

about 0. 0015 Mc/s per kilometer or 0. 16 Mc/s per degree of latitude

may be seen, and this is a fairly typical maximum value at this latitude. It becomes rapidly smaller at heights below the maximum, and

not so rapidly smaller at heights above the maximum. It is interesting to note that the latitudinal gradient considerably exceeds the

longitudinal sunrise gradient. A typical value of the sunrise gradient

is 0. 0007 Mc/s per kilometer or 0. 07 Mc/s per degree of longitude

near the F2 layer maximum.
AUGUST 1959

1700 75° W TIME

800

700

600

CO 500

oeUi

h-UJz

3

* 400

X

e>

UIX 300

200

100

NEWFOUNDLAND

I FORI MUNMOUTH

I

WHITE SANDS

I i_

GRAND BAHAMA _] .

PUERTO RICO

_l_J

50° 45° 40° 35" 30« 25° 20c !5 e

NORTH GEOGRAPHIC LATITUDE

Figure 1 Vertical cross section along 75 W meridian. Contours

of plasma frequency in Mc/s. Dashed line represents

height of F2 peak. Contours above this level obtained

from Chapman model with H= 100 km.
- 9 -

(2) Electron density maps in latitude versus time : Contours

of electron density at constant heights of 200 and 300 km are shown in

figures 2 and 3, for the months June 1959 and January I960, respectively. In these diagrams, the contours are given at intervals of

5 3

10 electrons/cm . In addition to illustrating the diurnal variation of

electron density at a fixed height versus latitude, these diagrams may

also be considered as representing the mean longitudinal variations

when it is noon over the 75 W meridian. For this purpose, the latitude

scale is expanded by a factor of 10.76 relative to the "longitude" (time)

scale.

At 200 km (fig. 2), the almost exclusively solar control of

the ionization densities is obvious: the electron densities rise rapidly

4 3

at sunrise from values less than 10 /cm (the minimum detectable

with present observing devices) and decrease below this limit a little

more slowly at sunset. A small latitudinal gradient may be seen

corresponding to the greater solar zenith angles nearer the equator.

Note, however, that at this season one of the anomalies of the ionosphere at higher altitudes is reversed at 200 km: the maximum

electron density is reached before noon, at least at the higher latitudes. This diurnal asymmetry has been noted by Croom et al [ 1959]-

At 300 km (fig. 3) strong solar control is also evident, but

it is clear that other factors are relatively more important than at

lower altitudes. The initial increase of electron density at sunrise is

nearly the same at all latitudes, although the later midmorning values

are considerably greater at low latitudes. There is also a peak above

45°N, which is partially explained by the lower altitude of the entire

layer there. The curious irregularity in the midday contours, which

appears to "move" to later hours at lower latitudes is explained by the

latitude/time variations of the height of maximum density: hmaxFZ is

nearly 300 km and varies with time above and below 300 km in the

vicinity of this irregularity.
10

ELECTRON DENSITY AT 200 KILOMETERS

JUNE 1959

75° W TIME CONTOURS--^ X 10"

Figure 2 Latitude versus local-time isoionic map at 200 km.

Contours in electrons per crrH x 10"^.
11

ELECTRON DENSITY AT 300 KILOMETERS

JANUARY I960

75° W TIME

oo

CONTOURS'^ X I0"

s

Figure 3 Latitude versus local-time isoionic map at 300 km,

Contours in electrons per cm-3 x 1U .
- 12-

(3) Latitude versus time variation of hmaxF2 : The height of

the F2 peak is illustrated for March 1959, in figure 4. This is one

of the simplest of similar diagrams for other months. Its most

obvious feature is that the F2 peak falls at sunrise from about 380 km

to about 300 km uniformily at all latitudes. The daytime variation is

characterized by a gradual rise at all latitudes, but this proceeds more

rapidly, and reaches a higher daytime altitude, at the lower latitudes.

It continues to increase until sunrise at middle and higher latitudes,

but decreases somewhat before midnight at low latitudes.

( 4) Latitude versus time variation of F2 layer thickness : The

"thickness" of the F region is characterized by our quantity "Scat,"

the quarter -thickness of a parabola fit to the F2 peak. Its latitudinal

and time variations are illustrated in figure 5 for the month of July

1959. This parameter is of special interest since it is probably an

approximate measure of the neutral particle scale height -- and hence

the temperature --at the F2 peak. It is at once clear that the thickness is strongly under solar control , increasing at sunrise from a nighttime value of about 50 km to about 7 km during daytime. In a

significant departure from solar control, however, the higher latitudes

have a thicker daytime F region than the lower latitudes.

( 5) Latitude versus time variations of sub-peak electron

content: The number of electrons in a unit column below the F2 peak

is designated Shmax; its latitudinal versus time variations for May

1959 are illustrated in figure 6. The behavior of the electron content is extremely simple, closely following the solar zenith angle.

Nevertheless, certain anomalies are evident. At low latitudes, the

maximum value of Shmax follows noon by 1 ^ - 2 hours, rather in

accord with simple theory, while in middle latitudes, the maximum

is distinctly at noon. It appears that a higher latitudes the maximum

again shifts to post-noon. Also the latitudinal midday gradient in

Shmax is very much larger than would be expected from the latitudinal dependence of the midday solar zenith angle.
- 13 -

HEIGHT OF MAXIMUM ELECTRON DENSITY

MARCH 1959

GBI

75°W TIME IN HOURS

Figure 4 Latitude versus local-time map of the height of the F2 peak.

Contours in kilometers.
- 14

QUARTER THICKNESS OF F-REGION PEAK

SCAT

JULY 1959

75° W TIME

oo

CONTOURS =Km

Figure 5 Latitude versus local -time map of the quarter -thickness of

the F2 peak. Contours in kilometers.
- 15 -

N° 50

GBI

ELECTRON DENSITY INTEGRATED TO

HEIGHT OF MAXIMUM ELECTRON DENSITY

MAY 1959

75° W TIME

CONTOURS^"51-, col XIO" cm*

Figure 6 Latitude versus local -time map of the subpeak electron

content. Contours in electrons per cm2 column x 10~12
- 16

N(t) PUERTO RICO APRIL 1959

FIXED HEIGHTS IN Km
- 1 7-

(6) Diurnal variation of electron density at fixed heights: The

diurnal variation of electron density at a given height is at first sight

the most natural representation of ionospheric variations, and is in

any case the'data necessary for a full numerical analysis of ionospheric

variations. An example is shown in figure 7 for Puerto Rico, April 1959

When the height of maximum falls below the fixed heights represented

here, the corresponding curves are dashed. Such electron densities

are the result of the topside model discussed above. The envelope of

these curves corresponds to the maximum density.

It is interesting to note the progressive departure from simple

solar control at successively greater altitudes. While the sunrise

period (and until 1000) is characterized by a rapid increase of electron

density at all heights, there then occurs a general decrease, most

marked at heights above 250 km. "* That at least part of this behavior

might be attributable to movement of the layer, is suggested by the

post-sunset increases in electron density which occur at successively

later times at lower altitudes.

4. DIURNAL AND SEASONAL VARIATIONS

A great deal of the basic information describing the F region of

the ionosphere is contained in the parameters hmaxF, Scat, Nmax,

and Shmax, which measure the height, thickness, and electron density

at the peak, and the sub-peak electron content, respectively. These

parameters have been displayed for separate months in the latitude

local time plane, in the NBS Technical Note Series 40; examples of them

have been discussed above.

This height cannot be taken as accurately determined; it represents a probable lower limit because of the possibility of a "valley

error" as mentioned in Section 2.
_ 18 _ With one year of data available, it becomes practical to display

this large quantity of information in very compact form. For each

of these four parameters, and each of the five stations, we have prepared contour "maps" showing the diurnal vs. seasonal variation of

these parameters. Figures 8 through 27 may be referred to in the

discussion which follows.

We shall discuss the various parameters individually from a phenomenological point of view, drawing attention to their characteristic diurnal and seasonal features and to the latitude variation evident

from the five stations' data.

(1) Height of the F2 peak (hmaxF2, figures 8, 9, 10, 11, 12):

Characteristically, hmaxF2 is lowest at the time immediately following sunrise, as low as 250 km in the winter and at higher latitudes.

It rises throughout the daylight period and generally continues to rise

after sunset and throughout the nighttime period. The total diurnal

excursion of hmaxFZ is surprisingly constant (100 km), whatever the

season or latitude, except that at mid-latitudes the variation is somewhat less in winter. There is a distinct similarity between the daytime

300 km contours at the station pairs White Sands/Grand Bahama, and

Fort Monmouth/Newfoundland. These pairs of stations have nearly the

same magnetic dip (60 and 72 , respectively), although their geographic latitudes are rather dissimilar.

The largest values of hmaxF2 occur in the summer nighttime,

and are typically 400 km. At all but the lowest latitude studied, these

high values are centered on local midnight; at Puerto Rico, they occur

before midnight, centered on 2100 local time in the summer months.

It is interesting to note that this tendency is entirely absent at Grand

Bahama, which is farther north by only 8° (geographic, geomagnetic,

or dip). On the other hand, the daytime similarity noted above between

stations with similar magnetic dip, is also evident in the diurnal/ seasonal

patterns of the nighttime 400 km contour.
- 19

H max NEWFOUNDLAND

Figure 8 Diurnal and seasonal variations of hmax - Newfoundland.

Contours in kilometers.
20

H max FORT MONMOUTH

Figure 9 Diurnal and seasonal variations of hmax - Ft. Monmouth.

Contours in kilometers.
21 -

H max WHITE SANDS

oo

CONTOURS : km Figure 10 Diurnal and seasonal variations of hmax - White Sands,

New Mexico. Contours in kilometers.
22

H max GRAND BAHAMA

1959 moo

CONTOURS : km

Figure 11 Diurnal and seasonal variations of hmax - Grand Bahama

Island. Contours in kilometers.
- 23 -

H max PUERTO RICO

I960 J -

CONTOURS: km

Figure 12 Diurnal and seasonal variations of hmax - Puerto Rico.

Contours in kilometers.
-24 -

(2) Quarter -thickness of the F2 peak (Scat, figures 13 - 17):

The possibility of interpreting this quantity in terms of the (neutral

particle) scale height at the F2 peak (see Section 2(3) ) suggests that

"Scat" may be an important indicator of the temperature in the F region.

We shall keep this possibility in mind, as the phenomenology of Scat

is discussed.

Scat shows a diurnal variation at all latitudes and seasons,

and this is most marked in the high latitude summer. Noon values of

Scat approach 100 km in the high latitude summer, but the nighttime

values are typically 50 km or less. At Newfoundland, the highest

latitude, the sense of the diurnal variation is reversed in the winter:

maximum values then occur in the pre -dawn hours, and generally

occur earlier in the equinox months than at the winter solstice. At

all other latitudes, the daytime values are distinctly larger than at

night. As with hmaxF2 there is somewhat more similarity in the

total pattern between stations of similar magnetic dip, than among

those of similar geographic latitude.

There is only a small seasonal variation at the lowest latitude (Puerto Rico), with minimum values in the winter and maximum

in the summer. The total change is less than at high latitudes, but

the smallest values (30 km, winter pre -dawn) are as small as the

minima elsewhere.

There is a suggestion of a semi-diurnal variation in the midlatitude winter, with maxima at 0200 and 1400, minima at 0730 and

1930.

(*) F2 maximum electron density (NmaxF2, figures 18-22):

Although the phenomenology of this parameter is well-known from its

direct relationship with foF2, we shall review its phenomenology here

in order to complete our picture of variations at the F2 peak. It will

also prove interesting to compare NmaxF2 with the F2 subpeak electron

content. The very great range of NmaxF2 (through a factor of ten at all

latitudes) renders the maxima very prominent, and the minima only

slightly less so.
25

SCAT NEWFOUNDLAND

oo

CONTOURS : Km

Figure 13 Diurnal and seasonal variations of Scat - Newfoundland.

Contours in kilometers.
26

SCAT FORT MONMOUTH

oo

CONTOURS.' km

Figure 14 Diurnal and seasonal variations of Scat - Ft. Monmouth.

Contours in kilometer s.
- 27 -

SCAT WHITE SANDS

Figure 15

00

CONTOURS : km

Diurnal and seasonal variations of Scat - White Sands,

New Mexico. Contours in kilometers.
28 -

SCAT GRAND BAHAMA

oo

CONTOURS i km

Figure 16 Diurnal and seasonal variations of Scat - Grand Bahama

Island. Contours in kilometers.
- 29 -

SCAT PUERTO RICO

1 r oo

CONTOURS: km

Figure 17 Diurnal and seasonal variations of Scat - Puerto Rico.

Contours in kilometers.
- 30 -

N max NEWFOUNDLAND

CONTOURS xio5

: electrons/cm3

Figure 18 Diurnal and seasonal variations of NmaxF2 - Newfoundland.

Contours in (electrons/cm^) x 10~-> .
- 31

Nmax FORT MONMOUTH

Figure 19

1 1 I I 8 00

CONTOURS x I0

5

: elec»rons/cm3

Diurnal and seasonal variations of NmaxF2 - Ft. Monmouth.

Contours in (electrons/cm^) x 10;5 .
- 32 -

N max WHITE SANDS

1959 M12 LST

18 00

CONTOURS XIO5 elactrons/cm3 Figure 20 Diurnal and seasonal variations of NmaxF2 - White Sands,

New Mexico. Contours in (electrons/cm3) x 10-5 .
33 -

N max GRAND BAHAMA

CONTOURS x I05

: electrons /cm3

Figure 21 Diurnal and seasonal variations of NmaxFZ - Grand Bahama

Island. Contours in (electrons/cm 3

) x 10" 5

.
34 -

N max PUERTO RICO

Figure 22

I 18 00

CONTOURS x |05: electrons /cm 3 Diurnal and seasonal variations of NmaxF2 - Puerto Rico.

Contours in (electrons/cm 3

) x 10" 5 m
-35 -

In the daytime, high latitudes show a single winter post-noon

maximum which shifts wildly to late afternoon in summer, while at

lower latitudes the post-noon maximum is between 1300 - 1400,

nearly independent of latitude or season. The similarity between the

late afternoon summer maxima at Ft. Monmouth and Newfoundland

suggests magnetic rather than solar control for this anomaly.

Low latitudes show distinct equinoctial maxima, with minima

in the summer and winter. At intermediate latitudes, the daytime

seasonal variation is complicated by the transition between these two

clear patterns. The equinox noon maxima at low latitude provide the

very highest values of Nmax. At higher latitudes, the highest values

are nearly independent of latitude, but possess the diurnal and seasonal

pattern just mentioned.

At night, the minimum values always occur in winter, and the

largest values occur in summer, at all latitudes. The minimum value

of Nmax is nearly the same at all latitudes, and occurs in the predawn hours. The period of time occupied by the minimum (seasonally

centered on December and diurnally centered on about 0500), becomes

larger with increasing latitude.

(4) Subpeak electron content (Shmax, figures 23 - 27): The

detailed variations of Shmax show the influence of each of the preceding

parameters. Again considering first the daytime periods, it is clear that

the pattern of the maxima is the same as for Nmax: high latitudes display

a winter maximum which "divides" into two equinoctial maxima at lower

latitudes. The Maxima are generally post noon, but not as late as those

of Nmax. The pre -noon increase in Shmax is more uniform with season

and with latitude than is the case with Nmax. The maximum values of

Shmax decrease with increasing latitudes.

At night, the minimum values of Shmax occur in the pre -dawn

period, and are lowest in winter. However, there is a tendency, also

suggested in the Nmax data, for the lowest values to occur before the

winter solstice, rather than at it. The minimum values of Shmax

decrease with increasing latitude.
36

Shmax NEWFOUNDLAND

8 00

CONTOURS x I0

12 elect rons/cm2

col Figure 23 Diurnal and seasonal variations of Shmax - Newfoundland.

Contours in (electrons/cm^col) x 10~12 .
- 37

Shmax FORT MONMOUTH

CONTOURS XIO

12

: electron s/cmZ

col

Figure 24 Diurnal and seasonal variations of Shmax - Ft. Monmouth.

Contours in (electrons/cm 2 col) x 10" 12 .
38

Shmax WHITE SANDS

Figure 25

8 00

CONTOURS x I0

12

: electrons /cm2col Diurnal and seasonal variations of Shmax - White Sands,

New Mexico. Contours in (electrons/cm^col) x lO" 1 ^ .
39

S h max GRAND BAHAMA

2

LST

18 00

CONTOURS x I0

12

: electrons/cm

2

col

Figure 26 Diurnal and seasonal variations of Shmax - Grand Bahama

Island. Contours in (electrons/cm^col) x 10-12.
40

S hmax PUERTO RICO

8 00

CONTOURS XIO 12

: electrons /cm*col Diurnal and seasonal variations of Shmax - Puerto Rico.

Contours in (electrons/cm^col) x 10" 1 -.
- 41 -

APPENDIX

A Note on the Day-to-Day Variability of the Ionosphere

As was mentioned earlier in the discussion of the averaging

process, the standard deviations (a ) and relative standard deviations ((7/ mean) of the profile data are also obtained. These parameters give a quantitative measure of the day-to-day variability of the

quiet ionosphere, and permit some interesting conclusions regarding

the degree to which various external influences control the ionosphere.

The standard deviation of the (roughly 20) values of hmax, Shmax,

Scat, and Nmax entering their respective means have been determined

hourly for each month of the year May 1959 -April I960. In the case

of the electron density parameters Shmax and Nmax, it is immediately

evident that the standard deviations are closely proportional to the

mean values themselves, and that a clearer picture of the variability

of the quantity is obtained if the standard deviation is expressed in

percent of the mean value. This is also meaningful for the quarter

thickness, Scat, but has little meaning for hmax. Accordingly, our

figures and remarks. will concern thepercentage variability of Shmax,

Nmax and Scat, but will deal with the true variability of hmax, in

kilometers.

From the one -year's data available to this study, it has not proven

possible to observe any important seasonal variation in these variability data. There is a slight tendency for winter daytime values to

be smaller (especially for Nmax and Hmax) than at other times and

seasons, but this has not been explored further.

Because of the relatively minor seasonal dependence of the variability parameters, all months of the year's data have been averaged

together for the preparation of the latitude versus local time maps of

figures 28, 29, 31, 32. These maps show contours of the true variability of hmax (in km) and the percent variability of Scat, Shmax, and

Nmax, respectively, between latitudes of 15 N to 50 N versus 75 W

time.
-42-

1. Variability of hmax: As shown in figure 28, hmaxF2 has

about twice the variability at night as in the daytime, and is generally

larger at lower latitudes. The minimum variability is at midday,

but the maximum variability is generally post-midnight -- markedly

so, at lower latitudes.

2. Relative variability of Scat: The percent variability of

the F2 layer quarter -thickness is shown in figure 29. Scat has been

interpreted as a measure of the neutral scale height and hence the

temperature at the F2 peak. Its variability is generally larger at high

latitudes and in the early morning or sunrise periods, ranging from

10% to 30% of its mean value. The greater variability at high latitudes may be related to corpuscular heating at latitudes approaching

the auroral zones; it is reasonable that any corpuscular heating of the

atmosphere will have more day-to-day variability than the heating due

to solar radiation.

We should also recall the remarks made concerning accuracies

of these data in the introduction. The determination of Scat depends

rather delicately upon the accuracy of Nmax. Almost certainly some

of the variability of Scat is attributable to this cause. It is clear,

however, that the variability due to errors cannot exceed 10%, the

minimum variability observed.

It is of interest to inquire to what extent the variabilities of

the height of the F2 peak and its characteristic thickness are related.

It might be imagined, for example, that all the variability of hmax

might be accounted for by variability in the thickness of the layer. This

would be the case if the lower edge of the layer were essentially fixed

in position and merely varied in thickness from day to day. Alternatively, it might be imagined that the whole region is located differently

from day to day, producing a variation in hmax that is essentially

independent of variations in the layer thickness.
- 43 -

DIURNAL, LATITUDINAL VARIABILITY OF HEIGHT

OF F2 PEAK

QUIET DAYS, MAY 1959 APRIL I960

75° W TIME

18 00

CTh max, km

Figure 28 Diurnal, latitudinal variability of hmaxF2; quiet mean

values averaged over May 1959 - April I960. Contours

in kilometers.
- 44 -

DIURNAL, LATITUDINAL RELATIVE VARIABILITY OF

THICKNESS OF F2 PEAK

QUIET DAYS, MAY 1959 -APRIL I960

GBI

Figure 29

75° W TIME (XsCQt

scat

°/

Diurnal, latitudinal relative variability of Scat; quiet

mean values averaged over May 1959 - April I960.

Contours in percent.
- 45 - To examine this point, in figure 30 we have plotted diurnally

the standard deviations (in km) of hmaxF2 and Scat averaged over the

entire year's data and for the two stations Newfoundland and Puerto

Rico. The shaded area shows in each case the part of the variability

of hmax which cannot be accounted for directly by the variability in

Scat, or vice versa.

First, one notes that generally hmax varies more than Scat,

except during high latitude daytime, when the two are essentially the

same. There is a great difference between the two at night, indicating that the layer is found at considerably different positions (altitudes)

from day to day with comparatively little change in thickness. There

is, however, some indication of a correlation between the magnitudes

of the two parameters.

3. Relative variability of total electron content: This parameter

(fig. 31) has a very regular seasonal and time dependence. It is nearly

constant (30%) at night, independent of latitude, and appears to depend

very simply upon the solar zenith angle, being smallest during daytime

at low latitude. The diurnal minimum value occurs, however, from

one to three hours post-noon.

4. Relative variability of maximum electron density : In contrast with the variability of Shmax, the variability of the maximum

electron density (fig. 32) suggests a mixture of latitudinal and solar

control. The maximum values (40%) occur in the pre -dawn period at

high latitudes. The minimum values occur towards lower latitudes in

the daytime, and these become equal to the relative variability in subpeak electron content discussed in paragraph 3. This is consistent

with the Chapman-like behavior of the low latitude daytime ionosphere

employed in section 1 (3): If the F region possesses a Chapman-like

r behavior, the electron density at all altitudes varies in proportion to

the maximum density, and hence the electron content does also. This

behavior is somewhat evident at all latitudes near midday, but is

definitely not the case at night -- especially at higher latitudes.
46

CO

q:

uj

ILlS

^

X

O

E

b

o

o

35

30

25

20

15

10

5

35

30

25

20

15

10

5

—t—i—r—i—i i i

t—i—r—l—i

i i i—i

i i—i—i—i—

VARIABILITY OF HEIGHT AND THICKNESS

QP F2-PEAK

MEDIANS, MAY 1959 -APRIL I960

NEW F0UNDLAND

PUERTO RICO

1 1

I I

1

I I I I I I I I I I I I I I I I I 12 2 8 10 12 2 4

L ST

8 10 12

Figure 30 Comparative variability of hmaxF2 and Scat versus time,

at Newfoundland and Puerto Rico, averaged over May 1959

April I960.
- 47

CBI

DIURNAL, LATITUDINAL RELATIVE VARIABILITY OF

SUB PEAK ELECTRON CONTENT

QUIET DAYS, MAY 1959 -APRIL I960

75° W TIME

18 00

CTshmax

s hmax , %

Figure 31 Diurnal, latitudinal relative variability of Shmax; quiet

mean values averaged over May 1959 - April I960.

Contours in percent.
48 -

DIURNAL, LATITUDINAL RELATIVE VARIABILITY OF

F2 MAXIMUM ELECTRON DENSITY

QUIET DAYS, MAY 1959 -APRIL I960

75° W TIME Q~N max

Nmax , /o

Figure 32 Diurnal, latitudinal relative variability of NmaxF2; quiet

mean values averaged over May 1959 - April I960.

Contours in percent.
- 49 -

However, we may note that the high latitude pre -dawn peak variability of Shmax occurs at nearly the same time as the peak variability in layer thickness (vide supra). The behavior of the variabilities

of Nrnax, Shmax, and Scat at high latitudes during nighttime are again

consistent with corpuscular heating of the atmosphere: As the day-today changes in temperature modulate the thickness of the nighttime

F region, its maximum density varies in inverse fashion without

appreciable change in the electron content.

Summary: The day-to-day variability of an ionospheric parameter,

expressed as a percentage of the monthly mean, has been shown to be

subject to interpretation consistent with the physical processes controlling the layer. The variability of a given parameter shows

properties relatable to certain physical influences which may be

obscured in the real values of the parameters themselves. In particular, the variability of the high latitude nighttime ionosphere is

consistent with a variable corpuscular influence (probably corpuscular

heating) not evident at lower latitudes.

ACKNOWLEDGEMENTS

As implied in the introduction, this study has been possible

only because of the assistance rendered at the data's primary

source, the ionospheric field stations; I wish to express our sincere

appreciation for the efforts of the many station scientists involved.

The NBS N(h) group led by G. H. Stonehocker, has rendered a service that is second only in chronology. Similarly invaluable has been

the programing of the work for NBS1 high speed computer by Dr. H.

H. Howe.
- 50 - REFERENCES

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

density with height (N(z) curves) from curves of equivalent height

against frequency ((h 1 , f) curves); from the Physics of the Ionosphere, 332-339 (The Physical Society, London, 1955).

S. Croom, A. Robbins, and J. O. Thomas, Two anomalies in the

behavior of the F2 layer of the ionosphere, Nature 184, p. 2003

(1959).

M. Hirono, Effect of gravity and pressure gradient or vertical drift

in the F2 region. Rep. Iono. Research Japan 9_, p. 95 (1955).

A. K. Paul, Bestimmung der wahren aus der scheinbaren

reflexionshbhe, Arch. Elekt. Ubertrag. 1_4, p. 468 (I960).

E. R. Schmerling, Penn. State Univ. Iono. Res. Lab. Sci.

Reports 105 and 118 (1958 - 1959).

J. O. Thomas, The distribution of electrons in the ionosphere,

Proc. IRE 47, p. 162 (1959).

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

Mean electron density variations of the quiet ionosphere, NBS

Technical Note 40-1, 2, 3, 4, 5, 6, issued to date (1959-1961).

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

Research 65, p. 185 (I960).
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