vol 5 - cont j. applierd sciecnces 2

8/9/2019 Vol 5 - Cont J. Applierd Sciecnces 2

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Continental J. Applied Sciences 5:31 - 43, 2010

Wilolud Journals, 2010.

2foF VARIABILITY AND IRI MODEL FOR AN EQUATORIAL STATION DURING LOW AND HIGHSOLAR ACTIVITY PERIODS.

Nzekwe N. M1,2

, Joshua E. O1

and Imafidon L. O3

1Physics Department, University of Ibadan, Ibadanm Oyo State, Nigeria.

2,3Physical Science Department, Yaba

College of Technology, Lagos State, Nigeria

ABSTRACT

The critical frequency of the F2, Layer ( 2foF ) of the ionosphere for an equatorial station,

Ouagadougou Burkina Faso (geomag. Lat. 16.40, geomag. Long. 71.1

0, dip 1.5

0W) obtained

from an ionosonde measurements were compared with those of IRI-2001 model predictions.

The radio International Telecommunication Union (ITU-R), option of the IRI model values

showed much agreement with 2foF data during low solar activity periods at all times of the

day. For high solar activity periods (of 1989 and 1990) slight disparities occurred at 10 to 24

LT. The interquartile deviations from the median and standard deviations from the mean were

employed for the determination of the variability parameter for the station. The data sets of

2foF were found close to a normal distribution, hence the standard deviation from the mean

about the observed 2foF was used as a fairly good index to describe the variability of this

ionospheric parameter. Some of the present results of this work are in agreement with other

works carried out at the same and different equatorial stations.

KEYWORDS: Variability, equatorial ionosphere, ionosonde data, solar activity

INTRODUCTION

The ionosphere exerts a great influence on the propagation of radio signals. Transmitted signals to the

ionosphere are refracted and sent back to the Earth, making it possible for radio reception at different distances forwaves traveling along the surface of the Earth. The amount of refraction in the ionosphere decreases with an

increase in frequency and for a very high frequency is almost nonexistent. These frequencies are properly defined by

the critical frequencies of the various layers. In radio propagation by way of the ionosphere, the critical frequency

(fo) at the vertical incidence is the limiting frequency at or below which incidence, the wave component is reflected

by and above which it penetrates through an ionospheric layer.

The ionosphere affects our modern society in many ways. International broadcasters such as the voice of American

(VOA) and the British Broadcasting Corporation (BBC) still use the ionosphere to reflect radio signals back toward

the Earth so that their entertainment and information programmes can be heard around the world. The ionosphere

provides long range capabilities for commercial ship to-shore communications, trans-oceanic air craft and

surveillance systems. The sun has a dominant effect on the ionosphere and solar events such as flares or coronal

mass ejections can lead to worldwide communication blackouts on the short wave bands.

Furthermore, the ionosphere has exerted a great deal of significances in modern day academic researches such asexplaining the reasons for the global reduction in the height of the atmosphere, possibility and evidence for anadditional F3 layer in the ionosphere [Balan et al., 1998] space modeling, predictions and so on.

The IRI and the variations of 2foF

The International Reference Ionosphere (IRI) is one of the most widely used empirical models [Rawer and Bilitza,

1989, 1990]. Several authors [ Alazo et al., 2003; Lazo et al., 2003; Mosert et al., 2003, Adeniyi and Radicella,

2003] have estimated and analyzed the deviations from the monthly mean for specified conditions to model the

variability of 2foF .

Studies on the subject of ionospheres variability vary from those that analyze specific parameters on a large

geographical area to those that are limited to a few stations or to one station. Such studies for example include those


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Nzekwe N. M et al.,: Continental J. Applied Sciences 5:31 - 43, 2010

of [kouris and Fotiadis., 2002; Gulyaeva et al., 1998; Leitinger and Hochegger, 1999; Gulyaeva and Mahajam,2001; Rawer et al., 2003; Sethi et al., 2003; Zhang et al., 2004 and Gordienko et al., 2005]. Various variability

indices have been adopted in the study of the variability of 2foF , common methods adopted in the references cited

above are quartile deviation from median and standard deviations from mean. Bilititza et al. [2004], indicated that

standard deviation from mean is a good measure for describing variability but difficult to interpret, since one may

not be sure that the distribution of the data is Gaussian.

Davis and Groome, [1964] provided values of the fractional decile deviations of 2foF from its monthly median

values, based on measurements for the American longitudinal chain. Further regional and global statisticalvariability analyses were reported by [Sowers and Pokempner, 1989], physical sources of variability have been

examined by [Forbes et al., 2000] and by [ Rishbeth and Mendillo, 2001]. Day to day changes in experimental

electron density and variability of the IRI electron density profile have being examined by [Amarante et al., 2004;

Bradley 2000; Bradley and Cander, 2002;Bradley et al 2004; Mikhailov and Mikhailov, 1995]. [Radicella, 2002,

2003], used vertical incidence measurement of data from a range of different stations at various locations, includinglow latitudes to establish the form of variations in 2foF .

Dependence of Critical Frequency on Maximum Electron Density

The critical frequencyfo of an Ionospheric layer is related to the maximum electron density in an Ionospheric layer

by;

max9 Nfo (1)

WhereNmax is the maximum electron density of an Ionospheric layer.

Data and method of Analysis

The parameter used for this study is 2foF . The data is from Ouagadougou, Burkina Faso (Geo lat. 12.40N, Geo

long. 1.50W; Dip 5.90). All the available data for 1986 and 1987 periods of low solar activity, 1989 and 1990 periods

of high solar activity were used for the analysis. The monthly hourly average for each month of the year was

calculated for the 24 hours. These were used as the observed 2foF data. From the monthly hourly averages the

upper (75 %), median (50 %) and lower (25 %) quartiles were obtained. Hence the monthly variability for therepresentative months of the various seasons was obtained, based on the IRI recommendations. The use of relative

inter-quartile range and standard deviation gave the measure of variability, v as

)%100*(2

131

Q

QQv

= (2).

and, )%100*( 12

=v . (3)

where 1v and 2v are measures of variability, 1Q , 2Q and 3Q are the lower, median and upper quartiles,

respectively. 1 is the standard deviation from the mean and is the mean.

Furthermore, seasonal grouping was done by combining the hourly values of 2foF for all days of the month of

December (December Solstice) represents November, December, January and February. The month of March(March equinox) is represented by March and April, and the month of June (June solstice) is represented by May,

June, July and August. September (September equinox) is represented by September and October. Hourly averages

and their corresponding standard deviations were calculated for each of the season of the year under consideration.


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The frequency distribution table of 2foF with class intervals of 0.5 MHz was prepared for each hour for 1987 and1989, so as to show the comparism histograms of occurrence frequencies of the observed data. For the validation of

IRI, the IRI 2001 code predictions was used to generate 2foF values for each hour of the 15th (middle day) of each

month of the year . These hourly values were taken to be representative of the monthly hourly averages for the days

of that month. These monthly averages were used to compute the seasonal averages. This was done for each of the

year considered. The hourly averages obtained in this way were compared with the set of prediction coefficients of

the Radio section of the international Telecommunication Union (ITU-R).

RESULTS

0 5 10 15 20 25

2

4

6

8

(a) JAN 1986

foF2(MHz)

LT (HRS)

0 5 10 15 20 25

2

4

6

8

(d) JAN 1987

foF2(MHz)

LT (HRS)

0 5 10 15 20 25

2

4

6

8

10(b) MAR 1986

foF2(MHz)

LT (HRS)

0 5 10 15 20 25

2

4

6

8

10

(e) MAR 1987

foF2(MHz)

LT (HRS)

0 5 10 15 20 25

2

4

6

8

(c) MAY 1986

foF2(MHz)

LT (HRS)

0 5 10 15 20 25

2

4

6

8

10(f) MAY 1987

foF2(MHz)

LT (HRS)


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Figure 1 median local time variations of foF2 ionosonde data versus IRI-2001 model for Ouagadougou duringDecember solstice (January), March solstice (March) and June solstice (May) of 1986 and 1987 low solar activity

periods. The curves with closed circles is the ITU-R predicted values while open triangles is the values of the

observed data.

0 5 10 15 20 254

6

8

10

12

(a) JAN 1989

foF2(MHz)

LT(HRS)

0 5 10 15 20 25

6

8

10

12

(e) JAN 1990

foF2(MHz)

LT (HRS)

0 5 10 15 20 25

6

8

10

12

14

(b) MAR 1989

foF2

(MHz)

LT (HRS)

0 5 10 15 20 25

6

8

10

12

14

(f) MAR 1990

foF2

(MHz)

LT (HRS)

0 5 10 15 20 254

6

8

10

12

(c) MAY 1989

foF2(MH

z)

LT (HRS)

0 5 10 15 20 254

6

8

10

12

14

(g) MAY 1990

foF2(MHz)

LT (HRS)

Figure 2 The same as Figure 1, except for Jan., Mar., and May, 1989 and 1990 high solar activity periods.


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2 4 6 8 100

5

10%STDEV = 31.28(i) DEC 1987

Freq.ofoccurrence

foF2 (MHz)8 9 10 11 12 13

0

5

10

15%STEDEV = 13.76(iv) DEC 1989

Freq.ofOccurrence

foF2 (MHz)

2 4 6 8 100

2

4

6

8

%STDEV = 27.31(ii) SEP 1987

Freq.ofOccurrence

foF2 (MHz)8 10 12 14 16

0

2

4

6

8

10

%STDEV = 17.78(v) SEP 1989

Freq.ofoccurence

foF2 (MHz)

2 3 4 5 6

3

6

9

12% STDEV = 28.97

(iii) JUN 1987

freq.ofoccurrence

foF2 (MHz)

6 7 8 90

1

2

foF2 (MHz)

% STDEV = 12.94

(vi) JUN 1989

freq.ofoccurence

Figure 3 Comparism of histogram of occurrence frequencyof observed foF2 at midnight (00 LT) for Ouagadougou at low

(1987) and high (1989) solar activity periods, for December

solstices, September equinoxes and June solstices. Percentage

standard deviation are indicated on each histogram


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2 3 4 5 60

10

20

30

40 %STDEV = 43.20(i) DEC 1987)

Freq.ofoccurrence

foF2 (MHz)2 4 6 8 10

0

5

10

15%STDEV=23.72(iv) DEC 1989

Freq.o

fOccurrence

foF2 (MHZ)

3 4 5 6 7 8

0

10

20

%STDEV = 23.71

(ii) SEP 1987

Freq.ofOccurrence

foF2 (MHz)4 6 8 10 12

0

5

10

15

%STDEV = 20.69

(v) SEP 1989

Freq.ofoccurrence

foF2 (MHz)

4 5 6 70

20

40%STDEV= 23.17

(iii) JUN1987

Freq.

Ofoccurrence

foF2 (MHz)6 7 8 9

0

10

20

30

40%STDEV=10.21

(vi) JUN1989

Freq.ofOccurrence

foF2 (MHz)

Figure 4 Same as Figure 3, but for 06 LT.


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6 7 8 9 10110

10

20

(i) DEC1987

Freq.ofOccurrence

f0F2 (MHz)

%STDEV= 15.67

8 10 12 140

10

20 %STDEV= 12.73

Freq.ofOccurrenc

e

foF2 (MHz)

(iv) DEC1989

7 8 9 100

10

20

30

Freq.ofOccurrence

foF2 (MHz)

%STEDEV=10.84

(ii) SEP1987

10 12 14 16

0

10

20

30

foF2 (MHz)

Freq.ofoccurence %STDEV= 12.05%

(v) SEP1989

5 6 7 8 9 100

10

20

30

foF2 (MHz)

Freq.O

foccurrence

%STDEV = 12.96

(iii) JUN 1987

8 10 12 14 160

10

20

%STDEV= 16.48

(vi) JUN1989

Freq.ofOccurrence

foF2 (MHz)

Figure 5 Same as Figure 3, but for midday.


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8 10 120

10

20

30%STDEV = 12.30(i) DEC 1987

Freq.ofOccurrence

foF2 (MHz)

8 10 12 140

10

20

30

40%STDEV= 12.14(iv) DEC1989

Freq.ofOccurrence

foF2 (MHz)

8 10 120

10

20

30%STDEV= 12.35

(ii) SEP1987

Freq.ofOccurrence

foF2 (MHz)8 10 12 14

010

20

30

%STDEV = 12.13

(v) SEP 19 89

Freq.of

occurrence

f0F2 (MHz)

7 8 9 1011120

10

20

30%STDEV=11.86

(iii) JUN1987

Fre

q.ofOccurrence

foF2 (MHz)

8 9 10 11 12 130

10

20

30

40%STDEV = 9.63

(vi) JUN 1989

Freq.ofoccurrence

foF2 (MHz)

Figure 6 Same as Figure 3, but for 18 LT.

DISCUSSION.

Comparison

2foF with IRI-2001 model prediction

Figure 1, shows the comparison of observed 2foF with those generated from the IRI 2001-model predictions at

low solar epochs of 1986 and 1987. The left side column of the plots is for low solar activity periods of 1986 while


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the right side column of the plots is those of the low solar activity periods of 1987. A pre-sunrise minimum of2foF occurred at 05 LT and a sunrise minimum at 06 LT, which is in agreement with those of IRI model

predictions, except for the months of May where a pre-sunrise minimum occurred between 04 and 05 LT. A sunset

peak occurred at 17 LT and 18 LT for all the representative months. For January 1986 at 18 to 24 LT, the ITU-R

underestimated the predicted values of 2foF . The ITU-R values were underestimated for March 1986 between the

hours of 21 to 24 LT, while those of March 1987 were overestimated between the hours of 12 to 19 LT. Also, the

model underestimated the values of 2foF for May 1986 between the hours of 06 and 18 LT, while that of May

1987 is between 06 and 21 LT. On a general note for Figure 1, The ITU-R options of the IRI Model showed much

agreement with the observed 2foF for the different local time at low solar activity periods, during the different

seasons. This is in line with the results of Adeniyi et al. [2003], who used the data from the same station under

investigation but different set of data for high and low solar activity periods, and found that at low solar activity

periods, the CCIR (now ITU-R) option gave a better representation.

In Figure 2, the median local time variation of 2foF ionosonde data together with the IRI- model predictions were

compared during the 1989 and 1990 periods of high solar activity for the different seasons. The left side column are

plots for the months of January, March and May representing the different seasons for 1989 solar activity period.

The right side column is the plots of the corresponding months for January, March and May representing thedifferent seasons for 1990 period of high solar activity. The IRI 2001 model showed much agreement for the hours

of 06 to 09 LT for all the representative seasons. A sunrise minimum occurred at about 05 LT and 06 LT for the

different representative months of the seasons. A morning peak occurred at 09 LT, also for the different months. The

model overestimated the predicted values of 2foF for 1989 during the hours of 10-24 LT with that of May 1989,

showing much disagreement between 10 LT and 24 LT. The model overestimated the values of predicted 2foF

between 18-24 LT for 1990 months. Disparities observed in this high solar activity periods are in line with the

results ofAdeniyi et al., [2005], who found that the ITU-R values does not agree much with the observed 2foF at

high solar activity periods. These disparities can be adduced to the fast vertical motion of the F2-layer at high solaractivity. A night minimum occurred at 20 and 21 LT, except for January where it occurred at 10 LT. The different

values of 2foF at different local time coupled with the shape of the curves in Figures 3.1 and 3.2, are indication

that the critical frequency of the F2-layer ( 2foF ) changes with time.

Variations of 2foF at various local time

Figure 3, shows the comparism of histogram of the occurrence frequencies of observed 2foF at midnight (00 LT)

at low and high solar activity periods of 1987 and 1989, respectively. The left side column shows the histograms of

occurrence frequency for 1987 low solar activity and the right side column shows corresponding histograms of

occurrence frequency for 1989 high solar activity for December solstices, September equinoxes and June solstices,

with their percentage standard deviation from their mean indicated on each histogram. December 1987 has a

maximum frequency of occurrence at 5.0 MHz and minimum frequency of occurrence of 9.0 and 11.0 MHz

corresponding to a standard deviation from the mean of 31.28 % while December 1989 has maximum frequency ofoccurrence of 9.0 MHz and a minimum frequency of occurrence of 10.5, 12.0 and 13.0 MHz. Corresponding to a

standard deviation from the mean of 13.76 %. Similarly for September equinoxes, 1987 has a maximum frequency

of occurrence of 7.0 MHz and a minimum of 2.5, 3.5, 4.5, 7.5 and 8.5 MHz corresponding to a standard deviation of

27.31 %, while 1989 has a maximum frequency of occurrence of 9.0 MHz and a minimum of 7.0 and 8.0 MHz

corresponding to a standard deviation of 17.78 %. June solstices, 1987 has a maximum frequency of occurrence of

3.0 MHz and a minimum of 2.0 and 5.5 MHz corresponding to a standard deviation from the mean of 28.97 % whileJune 1989 has a maximum frequency of occurrence of 6.9 and 8.4 MHz and a minimum frequency of occurrence of

9.0 MHz corresponding to a standard deviation from the mean of 12.94 %, even though the data were scanty. From

Figure 3, it can be seen that the variability is greater than the variability at high solar activity, for each corresponding

representative months, as indicated in each histogram by their percentage standard deviation from the mean.


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06 LT variation of 2foF is shown in Figure 4. The plots on the left side column is for low solar activity periods of1987 for months of December, September and June representing December solstice, September equinox and Junesolstice respectively. This corresponds to the right hand side column of the plots representing high solar activity

periods of 1989. For December solstice, the maximum frequency of occurrence for 1987 occurred at 1.0 MHz and

the minimum at 2.0 MHz corresponding to a standard deviation from the mean of 43.20 %, while the maximum

frequency of occurrence for 1989 (December) occurred at 6.0 MHz and the minimum occurred at 3.0, 8.5, 10.0 and

11.0 MHz corresponds to a standard deviation of 23.72 % since the data sets were evenly distributed than those of

December 1987. Still on 06 LT, September 1987 has a minimum of 5.0 and 8.0 MHz corresponding to standard

deviations from the mean of 23.71 %. September 1989 has a maximum frequency of occurrence at 6.5, and 7.5

MHz, with a minimum of 3.9, 11.5 and 12.0 MHz corresponding to a standard deviation of 20.69 %. June solstice of

1987 has a maximum frequency of occurrence of 4.0 MHz and a minimum of about 2.4 MHz corresponding to a

standard deviation of 23.17 % while June solstices of 1989 has a maximum frequency of occurrence of 6.8 MHz

and a minimum of 1.25 MHz corresponding to a standard deviation of 10.21 %. Also, the variability at low solar

activity is 2 times greater than those at high solar activity, except for the month of September for the equinoxes werethe spread or variability is slightly different compare to those of the solstices (December and June).

12 LT histogram of occurrence frequency of 2foF variations is shown in Figure 5. The left side column is the

representative months for low solar activity periods of 1987 while the right side column is the representative months

for the seasons of high solar activity periods. The values of the percentage standard deviation from the mean

(STDEV) indicated on each histogram, shows the measure of variability or spread for each of the season. The spread

at June 1989 is largest, having a maximum frequency of occurrence of 9.0 and 11.5 MHz and a minimum of 15.0

and 16.0 MHz with a corresponding standard deviation from the mean of 16.48 %. This is followed by that of

December 1987 which has a maximum frequency of occurrence of 7.0 MHz and minimum of 10.5 and 11.5 MHz

corresponding to a standard deviation from the mean of 15.67 %. Differences in the measure of spread or variability

are seen in each month of low solar activity periods indicated by their percentage standard deviations from the mean

on each histogram. Here the variability is more at high solar activity than at low solar period except for December

solstices, where low solar period at 12 LT exceeds that of high solar period.

Figure 6, shows the examples of histogram of occurrence frequency for 1987 periods of low solar activity on the left

side column while the right side column shows 1989 periods of high solar activity for 18 LT, indicating the various

seasons. The measure of variability for the corresponding months are slightly the same having almost the same

maximum and minimum frequency of occurrence, corresponding slightly to the same percentage standard

deviations from their mean. This suggest that the features of the various seasons (December solstices and September

equinoxes for 1987 and 1989 solar activity periods) are the same with respect to the variations in 2foF at 18 LT.

Except at June solstices where there is a slight differences in the measure of variability. The maximum frequency of

occurrence occurred at 9.0 MHz and a minimum of 7.0, 11.5 and 12.0 MHz corresponding to a standard deviation of

11.86 % for June 1987, whereas for June 1989, the maximum frequency of occurrence occurred at 9.0 MHz and the

minimum at 13.0 MHz corresponding to a standard deviation of 9.63 %.

Comparing Figure 3 and 5, the variability of the ionospheric parameter ( 2foF ) at midnight (00 LT) is greater thanthe variability at midday (12 LT), except for June 1989 where the variability at midday slightly exceed that at

midnight (00 LT). The observations suggest that the electron density at midday at the bottom side of the F2, layer is

higher than the electron density at midnight (00 LT), since 2foF depends on electron density.

Generally, from Figures 3, 4, 5, and 6 the histograms having percentage standard deviations from the mean (V 2)

about any value between 11.5 and 13.0 inclusive indicate that the 2foF data were normally distributed. The

remaining data set, were found to be close to normal distribution, hence the percentage standard deviation from the

mean (V2) was taken as a fairly good index for describing the variability of 2foF . This result is in agreement with

the work ofAdeniyi et al. [2005], who carried out variability studies for the same station but for different set of data

at low, moderate and high solar activities of 1985, 1990 and 1993 respectively. They tested for the normality of the


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data set and concluded that most of the data sets were normally distributed and few were close to normaldistribution. The rising and falling of the histogram bars is an indication and a prove that the critical frequency of the

F2 layer ( 2foF ), varies with time.

CONCLUSION.

The results of this work are summarized as follows:

1. The ITU-R Options of the IRI model is in much agreement with the observed values of 2foF at low solar

epoch than at high solar epoch.

2. Minimum values of 2foF occurred at 06 LT for January of all the years and 05 LT for March and May

during low solar activity periods.

3. A pre-noon peak occurred at 09 LT and a pre-sunset peak at17 LT at low solar activity periods.

4. A morning minimum of observed 2foF occurred at 06 LT except for May where it occurred at 05 LT.,

during high solar activity periods.5. A morning peak occurred at 10 LT except for September where it occurred at 09 LT., for high solar activity

periods of 1989 and 1990.

6. A night minimum of observed 2foF occurred at 21 LT during high solar activity periods which is in

agreement with the ITU-R values.7. Furthermore, the rising and falling of the histogram bars is an indication

and a prove that the critical frequency of the F2 layer ( 2foF ) varies with time, hence a measure of variability was

obtained for the various months. These further showed that the hourly values of 2foF have a distribution close to

a normal one for most of the times, irrespective of time of the day, season or solar cycle period. Hence the standarddeviation from the mean may be taken as a fairly good index for describing the variability of this ionospheric

parameter.

8. Finally our result will help in improving the accuracy of the IRI Model prediction of 2foF .

ACKNOWLEDGEMENT

Our appreciation is due to Professor J.O. Adeniyi of the University of Ilorin, a member of the IRI task force, for

releasing the ionosonde data used in this work. My regard and appreciation goes also to Dr. O.K Obrou a member of

the IRI, task force of laboratoire de physique de l atmosphere, Universite de Cocody, Cote-divore for his assistance

in getting across to us the IRI 2001 empirical model software.

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Received for Publication: 17/06/10

Accepted for Publication: 09/07/10

Corresponding author

Nzekwe N. M

Physics Department, University of Ibadan, Ibadanm Oyo State, NigeriaEmail: [email protected]

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