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Comparison between d- and lower e-region electron density profiles and IRI-79
Adv.
Space Res.
Printed
in Great
Vol. 5, Britain.
No. 10, pp. All rights
95-98, 1985 reserved.
0273-1177185 $0.00 + .50 Copyright 0 COSPAR
COMPARISON BETWEEN D- AND LOWER E-REGION ELECTRON DENSITY PROFILES AND IRI-79 G. A. Moraitis Ionospheric Institute, National Observatory of Athens, GR-11810, P. 0. B. 20048, Greece
ABSTRACT The D and E-region electron density profiles obtained by different techniques are compared with the IRI-79 model to see how they fit. The rocket data showed good agreement. However discrepancies between the observed and model values were found especially for solar zenith angle greater than 50 degrees. INTRODUCTION D-region electron densities profiles measurements were made during the International Quiet Sun Year (1963-1965) in Crete by the Norvegian Group and the Ionospheric Institute of Athens. Two techniques were used to measure the hight distribution of D-region electron density: the partial reflections and the cross modulation technique. During the above time period intensive measurements were made in Mallia on the island Crete. This-experiment was part of a bigger one covering stations located from 69.5'N to 19.2'S and from ll.lOE to 25.L"E. The geographic-coordinates of Mallia station are: 35.'4oN, 25.,!+oE. Dip angle: 50°, Modip: 44.3O. EXPERIMENTAL RESULTS Observations of ionospheric cross modulation were made on Crete continuously from sunrise to sunset during the periods 20 September - 3 October 1964 and August - September 1965. Cross modulation measurements show a large spread in each data set. The data set takenseparately showed a range of electron density distributions. If however, the additional assumption is made that the electron density at each height must have a smooth diurnal variation, the range of possible models is restricted. Figure 1. shows the profiles that give the best fit with the experimental data. The measurements give no detailed information about the electron density distribution at night or below about 65Xm during the day. However, it is possible to set upper limits to the electron densities in these cases. Pig. 1 also shows the upper limits for the electron densities below 85Km when solar zenith angle x>aoO. As reported earlier /l/ between 65 and 55Km the electron concentration at any time of the day must not exceed 50 electrons cm-3. This deduction is based on the fact that no significant negative cross modulation has been observed. The uniqueness of the electron density distributions presented heee has been tested to see whether a different set of profiles can explain the observations equally well, and to try to estimate the accuracy of the method. It was found that each of the hourly data sets can be explained by a one layer model, but if a smooth and consistent diurnal variation of electron density is required, the two-layer structure gives the best fit. The sensitivity of the experiment will vary with height and with time of the day, but in general it is possible to state that the electron density at any height in the range 65-8oKm cannot change by more than 30 per cent from the models without spoiling the agreement between measured and computed values of cross modulation.
95
~z?i:
96
C. A. Moraitis
SO
K4~
10
~
10 N4~W •
;‘
‘I’
‘—3—
Fig. 1. Comparison of results obtained on Crete from partial reflections in Aug.—Sep. 1965 and by cross modulation in Sep.—Oct. 1964 /1/. Measurements of partial reflection were carried out in the period 30 August— 11 September 1965. Electron density distributions obtained within zenith angle intervals of 100 were averaged and the average diurnal variation of the electron density thus obtained is compared with the results derived from the cross modulation measurements figure 1. Some profiles obtained during one single afternoon are shown in figure 2. ~1--
..i
70 AFTERNOON
Fig. 2. Electron density profiles obtained during the afternoon of 11 Sep. 1965. /1/. ROCKET MEASUREMENT During the 15 May, 1966 a Sparrow/Arcas sounding rocket was launched by ESRO from a provisional launching site established at Karystos, 70Km east of Athens. The payload was instrumented for studying the effects in the lower ionosphere caused by the annular solar eclipse present on 20 May. The results were obtained by studying the Faraday rotation of the plane of polarization and the differential absorption effects on propagation waves. The first rocket under the code AEO1 were launched on the 15 May for test and calibration. Figure 3 shows the electron density profile obtained from AEO1 /2/.
D— and Lower E—Region Profiles
100
97
I,
Eo11i5.L2~I.:79.
io2
1o~
1o~
Fig. 3. Electron density profile obtained from AEO1 /2/ and from IRI—79 Model. IRI
-
MODEL
The model giving the electron density profiles in the D—region, as used in IRI—79 /3/ is based on experimental data. The model gives typical electron density profiles as a function of geographic coordinates, time, solar zenith angle and solar activity. The mathematical formulae, the computer program and some examples for selected places are reported in /3/. Electron density profiles are calculated using the above computer program for the following time periods: September — October 1964 During the above time period the sunspot number has a mean value R=5.4. Figure 4 shows separately the electron density distribution deduced from cross modulation measurements. The eJ.ectron density distribution after IRI — model for solar zenith angles 35~ 70°and800 is interposed in the same figure as dotted lines. As it is mentioned before for this case Thrane proposes two possible explanations: the one layer model and the two—layer structure. One layer structure Observations and IRI—model have some characteristic discrepancies. Visually the observations have their inflection point about 10 Km lower than the model
~oo~
90 9
0
~: ~: ~
800700
~
~
AFTORNOON
~
60~ 300
_‘400
60
70
350
,o2
:
—
,~3
10~
3
N/cm
Fig. 4. Electron density profiles obtained by cross modulation technique and from IRI—79.
102
1o~
1o~
Fig. 5. Partial reflections
si~~3
98
G. A. Moraitis
By replacing the inflection point (HMD) interval from 81—88 Km to 71—78 Km and by inserting the new values in the IRI—79 model a displacent of the profile for approximately the same magnitude is presented. In that case the calculated D—region maximum electron density is greater than the observed. (See Table). TABLE Solar Zenith Angle
35° 370
400 500 600 700 800
Inflecti n Point Experiment IRI—79 Model 73—75 Kin 71 72 “ 72—73 “ 73~74 “
76 78—82
~ “
83.3 Km 83.3 “ 83.5 U 83.5
83.6
“
83.7 U 84 ~
Two layer structure As mentioned earlier if a smooth and consistent diurnal variation of electron density is required, the two layer structure gives the best fit. In this case the maximum of ionization that occurs between 65 and 70 Km may be a stratification of ionization. The experimental results show only the low part of the profile. The hypothesis of two layers structure can be suported looking to the partial reflection data for August—September 1965. During the above period the sunspot number have a mean value R=9.5 (Figure 5). It may be noted that measurements taken in the same place, but with different techniques and with time difference of one year agree rather well in the height range 75—80 Km. DISCUSSION It appears that more than one ground—based technique is required to study adequately the entire D—region. As reported in /4/, a combination of VLF, partial reflection or wave interaction and incoherent scatter techniques should be considered for continuous ground based measurements in the D—region. The low values of electron density in the lower D—region could be derived from VLF measurements, the incoherent scatter technique could furnish accurate electron densities down to the ledge, and the partial reflection and wave interaction experiments could yield reasonably accurate electron densities in the intermediate region. Looked from this point of view, the IRI—79 model fits quite well in this limited height range. On the other hand IRI—79 model fits well with profiles obtained from rocket experiment in the whole height range and for local noon period. Discrepencies are observed for solar zenith angles>50°. REFERENCES 1. 2.
3. 4.
E.V. Thrane, A. Hang, B. Bjelland, M. Anastassiades and E. Tsagakis, 3. Atm. Terr. Phys. 30, 135—150 (1968). M. Jespersen and B. Moller Pedersen, 3. Atm. Terr. Phys. 32, 1859—1863 (1970). K. Rawer (chinn), J.V. Lincdn and R.Q. Conkright (eds), International Reference Ionosphere — IRI 79, Report UAG—82, World data Center — A for solar — terrestrial physics, Boulder, Co., USA (Nov. 1981). C. F. Sechrist, Jr. Radio Science 9,
# 2, 137—149 (1974).
Space Res.
Printed
in Great
Vol. 5, Britain.
No. 10, pp. All rights
95-98, 1985 reserved.
0273-1177185 $0.00 + .50 Copyright 0 COSPAR
COMPARISON BETWEEN D- AND LOWER E-REGION ELECTRON DENSITY PROFILES AND IRI-79 G. A. Moraitis Ionospheric Institute, National Observatory of Athens, GR-11810, P. 0. B. 20048, Greece
ABSTRACT The D and E-region electron density profiles obtained by different techniques are compared with the IRI-79 model to see how they fit. The rocket data showed good agreement. However discrepancies between the observed and model values were found especially for solar zenith angle greater than 50 degrees. INTRODUCTION D-region electron densities profiles measurements were made during the International Quiet Sun Year (1963-1965) in Crete by the Norvegian Group and the Ionospheric Institute of Athens. Two techniques were used to measure the hight distribution of D-region electron density: the partial reflections and the cross modulation technique. During the above time period intensive measurements were made in Mallia on the island Crete. This-experiment was part of a bigger one covering stations located from 69.5'N to 19.2'S and from ll.lOE to 25.L"E. The geographic-coordinates of Mallia station are: 35.'4oN, 25.,!+oE. Dip angle: 50°, Modip: 44.3O. EXPERIMENTAL RESULTS Observations of ionospheric cross modulation were made on Crete continuously from sunrise to sunset during the periods 20 September - 3 October 1964 and August - September 1965. Cross modulation measurements show a large spread in each data set. The data set takenseparately showed a range of electron density distributions. If however, the additional assumption is made that the electron density at each height must have a smooth diurnal variation, the range of possible models is restricted. Figure 1. shows the profiles that give the best fit with the experimental data. The measurements give no detailed information about the electron density distribution at night or below about 65Xm during the day. However, it is possible to set upper limits to the electron densities in these cases. Pig. 1 also shows the upper limits for the electron densities below 85Km when solar zenith angle x>aoO. As reported earlier /l/ between 65 and 55Km the electron concentration at any time of the day must not exceed 50 electrons cm-3. This deduction is based on the fact that no significant negative cross modulation has been observed. The uniqueness of the electron density distributions presented heee has been tested to see whether a different set of profiles can explain the observations equally well, and to try to estimate the accuracy of the method. It was found that each of the hourly data sets can be explained by a one layer model, but if a smooth and consistent diurnal variation of electron density is required, the two-layer structure gives the best fit. The sensitivity of the experiment will vary with height and with time of the day, but in general it is possible to state that the electron density at any height in the range 65-8oKm cannot change by more than 30 per cent from the models without spoiling the agreement between measured and computed values of cross modulation.
95
~z?i:
96
C. A. Moraitis
SO
K4~
10
~
10 N4~W •
;‘
‘I’
‘—3—
Fig. 1. Comparison of results obtained on Crete from partial reflections in Aug.—Sep. 1965 and by cross modulation in Sep.—Oct. 1964 /1/. Measurements of partial reflection were carried out in the period 30 August— 11 September 1965. Electron density distributions obtained within zenith angle intervals of 100 were averaged and the average diurnal variation of the electron density thus obtained is compared with the results derived from the cross modulation measurements figure 1. Some profiles obtained during one single afternoon are shown in figure 2. ~1--
..i
70 AFTERNOON
Fig. 2. Electron density profiles obtained during the afternoon of 11 Sep. 1965. /1/. ROCKET MEASUREMENT During the 15 May, 1966 a Sparrow/Arcas sounding rocket was launched by ESRO from a provisional launching site established at Karystos, 70Km east of Athens. The payload was instrumented for studying the effects in the lower ionosphere caused by the annular solar eclipse present on 20 May. The results were obtained by studying the Faraday rotation of the plane of polarization and the differential absorption effects on propagation waves. The first rocket under the code AEO1 were launched on the 15 May for test and calibration. Figure 3 shows the electron density profile obtained from AEO1 /2/.
D— and Lower E—Region Profiles
100
97
I,
Eo11i5.L2~I.:79.
io2
1o~
1o~
Fig. 3. Electron density profile obtained from AEO1 /2/ and from IRI—79 Model. IRI
-
MODEL
The model giving the electron density profiles in the D—region, as used in IRI—79 /3/ is based on experimental data. The model gives typical electron density profiles as a function of geographic coordinates, time, solar zenith angle and solar activity. The mathematical formulae, the computer program and some examples for selected places are reported in /3/. Electron density profiles are calculated using the above computer program for the following time periods: September — October 1964 During the above time period the sunspot number has a mean value R=5.4. Figure 4 shows separately the electron density distribution deduced from cross modulation measurements. The eJ.ectron density distribution after IRI — model for solar zenith angles 35~ 70°and800 is interposed in the same figure as dotted lines. As it is mentioned before for this case Thrane proposes two possible explanations: the one layer model and the two—layer structure. One layer structure Observations and IRI—model have some characteristic discrepancies. Visually the observations have their inflection point about 10 Km lower than the model
~oo~
90 9
0
~: ~: ~
800700
~
~
AFTORNOON
~
60~ 300
_‘400
60
70
350
,o2
:
—
,~3
10~
3
N/cm
Fig. 4. Electron density profiles obtained by cross modulation technique and from IRI—79.
102
1o~
1o~
Fig. 5. Partial reflections
si~~3
98
G. A. Moraitis
By replacing the inflection point (HMD) interval from 81—88 Km to 71—78 Km and by inserting the new values in the IRI—79 model a displacent of the profile for approximately the same magnitude is presented. In that case the calculated D—region maximum electron density is greater than the observed. (See Table). TABLE Solar Zenith Angle
35° 370
400 500 600 700 800
Inflecti n Point Experiment IRI—79 Model 73—75 Kin 71 72 “ 72—73 “ 73~74 “
76 78—82
~ “
83.3 Km 83.3 “ 83.5 U 83.5
83.6
“
83.7 U 84 ~
Two layer structure As mentioned earlier if a smooth and consistent diurnal variation of electron density is required, the two layer structure gives the best fit. In this case the maximum of ionization that occurs between 65 and 70 Km may be a stratification of ionization. The experimental results show only the low part of the profile. The hypothesis of two layers structure can be suported looking to the partial reflection data for August—September 1965. During the above period the sunspot number have a mean value R=9.5 (Figure 5). It may be noted that measurements taken in the same place, but with different techniques and with time difference of one year agree rather well in the height range 75—80 Km. DISCUSSION It appears that more than one ground—based technique is required to study adequately the entire D—region. As reported in /4/, a combination of VLF, partial reflection or wave interaction and incoherent scatter techniques should be considered for continuous ground based measurements in the D—region. The low values of electron density in the lower D—region could be derived from VLF measurements, the incoherent scatter technique could furnish accurate electron densities down to the ledge, and the partial reflection and wave interaction experiments could yield reasonably accurate electron densities in the intermediate region. Looked from this point of view, the IRI—79 model fits quite well in this limited height range. On the other hand IRI—79 model fits well with profiles obtained from rocket experiment in the whole height range and for local noon period. Discrepencies are observed for solar zenith angles>50°. REFERENCES 1. 2.
3. 4.
E.V. Thrane, A. Hang, B. Bjelland, M. Anastassiades and E. Tsagakis, 3. Atm. Terr. Phys. 30, 135—150 (1968). M. Jespersen and B. Moller Pedersen, 3. Atm. Terr. Phys. 32, 1859—1863 (1970). K. Rawer (chinn), J.V. Lincdn and R.Q. Conkright (eds), International Reference Ionosphere — IRI 79, Report UAG—82, World data Center — A for solar — terrestrial physics, Boulder, Co., USA (Nov. 1981). C. F. Sechrist, Jr. Radio Science 9,
# 2, 137—149 (1974).