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Comparison of foF2 characteristics at similar low latitudes but different longitudes in the South American region
Comparison offoF2 characteristics at similar low latitudes but different longitudes in the South American region R. I?. KAX;~:and E. R. PAULA lnstituto
de Pesquisas
Espaciais.-INPE
Conselho National de Desenvolvimento SZo Jo& dos Campos, S.P., Brasil
Cientifco
e Tecnoiitgicn
- CNPq
12200
A comparison of the foF2 diurnal variation characteristics at Huancayo (I 2’ S, 75”W. dip angle at Fortaleza. In + 2 ) and Fortaleza (4”S,38”W, dip angle - _7’ in 1976) showed in general larger middayfoF2 contrast,acomparisonoffoF2atTucuman(27’S,65 W.dipangle -22 )andSBo Paulo(24”S,47”W,dipangle - 22 in ICY period) showed smaller middavfuF2 at SBo P&o. Thus. a weaker fountain effect in the eastern ,. part of the South American region is indicated.
Abstract-
In low latitudes. the ionosphericliF2 shows a steady increase from sunrise to about midday, followed by a gradual fall up to midnight. The daytime value shows a trough at the magnetic equator and crests at about + 30’ magnetic dip. This ‘equatorial anomaly’ (APPIXTON,1946) is caused (MARTYN, 1955 : DUNCAN, 1960) by the ‘fountain effect’, viz. lifting of equatorial ionization by E x B vertical drifts and the eventual diffusion ofthe ionization along the magnetic field lines, to pile up at about ~fr30 ’ dip. Equatorial anomaly has somewhat different features in different longitude zones. as shown by LYONand THOMAS(1963), RAO and MALHOTRA (1964) and THOMAS (1968) for the American, African and Asian sectors for the IGY period. A~~~R~~ (1973) examined theoretically the effect of longitudinal changes in the (a) geomagnetic field configuration, (b) neutral winds and (c) E x S drifts, and concluded that the longitudinal differences were mainly due to differences in E x B drift characteristics. In the South American region, the geomagnetic field configuration is anomalous. The magnetic dip equator on the west coast has remained at about 13”s for the last few decades, but on the east coast, it was at about S’S during ICY. Does this make any difference to thejbF2 characteristics at the equator on the west and east coast? Figure 1 shows for I976 the seasonal medianfitF2 daily variation at Huancayo (geographic latitude 12”S, longitude 75”W, dip angle -t 2”) and Fortaleza (geographic latitude 45, longitude 38”W, dip angle -2“). In D months (summer months), the solar zenith angle is lesser for Huancayo and the middayfoF2 at Huancayo is slightly larger than at Fortaleza. But in J and E months,foF’2 at Fortaleza is larger. A similar plot for I@, showed that larger $oF2 at Fortaleza was associated with lower heights of the F2 layer.
Two possibilities arise: (i) the,/ijF2 in longitudes on the east coast may be larger at all latitudes, or (ii) the fountain effect may be weaker on the east coast. The
DIF = HU
FO
Fig. 1. Upper half: Diurnal patterns of seasonal averages of monthly median&F2 at Huancayo (full lines) and Fortaleza (crosses) for J, D, E months of 1976. Lower half: The difference loF2 (Huancayo minus Fortaleza). 9
R.P.
KANE~~
E.R,PAuLA
AV 1t957-631
Fig. P. Diurnal
AV (1957.63’
ofmonthly mtdianfi~F2 at Sk Pauio (full lines) and Tucuman {dashes and crosses) for J. D. E months for 1957-1963.
patterns &seasonal averages
~mpiications forfoR at few Iatitudes would be different lbr these two cases. For exampfe, for the southern Ioeations Tucuman (27”5,65”W geographic) and S&I Paula f24”S, 47”W geographic), both ofwhich had a dip angle of about -22” during the ICY, the foF2 at Tucuman would be smaller than that at Stio Paula in case (i) and larger in case (ii). Figure 2 shows the plots for the years 1957 to 1963, In dmost all cases, $oF2 at Tucuman 4s larger than that at S&I Paufo. Thus, on the eastern coast&F2 is larger at the equator as compared to the western coast, while at latitudes away from the equator, the eastern longitude shows smallerfaF2 than at western longitude. This is consistent with case (ii), viz, a weaker fountain effect at the eastern coast. The total magnetic field at Huancayo and Fortafeza
is roughly the same. However, the d~li~atjo~ at Huancayo is about 6°F and, at Fortaleza, about 2O”W. Whether such a twist in the magnetic field can cause a substantial change in the JbF2 characteristics needs theoretical investigation. Also, the strength of the fountain effect is generally related to the strength of the equatorial electrojet. A comparison of the daily variation of H at Huancayo and Fortaleza should prove useful. Work in this direction is in progress.
Ar’knowleBg~~ents-Thanks are due to Dr. NELXINf)E&sus PARAUA,Director of INPE, for support, This work was partially supported by FN’DCT, Brazil under Contract FTNPE-537,iCT.
< omparison
offoF
characteristics
at similar low latitudes
but different longitudes
in the South American
region
II
REFERENCES ANDERSOND. N. APPLETON E. V. DUNCAN R. A. LYON A. J. and THOMAS L. MARTYN D. F.
1973 1946 1960 I963 1955
RAO C. S. R. and MALHOTRA P. L. TMMAS L.
1964
1968
Planet. Space Sci. 21,421. Nature (Lond.) 157, 691. J. atmos.terr. Phys. 18, 89. .I. atmos. terr. Phys. 25, 373. The Physics ofthe Ionosphere, London. J. utmos. terr. Phys. 26, 1075. .I. atmos. terr. Phys. 30, 1631.
p. 254, Physical
Society,
de Pesquisas
Espaciais.-INPE
Conselho National de Desenvolvimento SZo Jo& dos Campos, S.P., Brasil
Cientifco
e Tecnoiitgicn
- CNPq
12200
A comparison of the foF2 diurnal variation characteristics at Huancayo (I 2’ S, 75”W. dip angle at Fortaleza. In + 2 ) and Fortaleza (4”S,38”W, dip angle - _7’ in 1976) showed in general larger middayfoF2 contrast,acomparisonoffoF2atTucuman(27’S,65 W.dipangle -22 )andSBo Paulo(24”S,47”W,dipangle - 22 in ICY period) showed smaller middavfuF2 at SBo P&o. Thus. a weaker fountain effect in the eastern ,. part of the South American region is indicated.
Abstract-
In low latitudes. the ionosphericliF2 shows a steady increase from sunrise to about midday, followed by a gradual fall up to midnight. The daytime value shows a trough at the magnetic equator and crests at about + 30’ magnetic dip. This ‘equatorial anomaly’ (APPIXTON,1946) is caused (MARTYN, 1955 : DUNCAN, 1960) by the ‘fountain effect’, viz. lifting of equatorial ionization by E x B vertical drifts and the eventual diffusion ofthe ionization along the magnetic field lines, to pile up at about ~fr30 ’ dip. Equatorial anomaly has somewhat different features in different longitude zones. as shown by LYONand THOMAS(1963), RAO and MALHOTRA (1964) and THOMAS (1968) for the American, African and Asian sectors for the IGY period. A~~~R~~ (1973) examined theoretically the effect of longitudinal changes in the (a) geomagnetic field configuration, (b) neutral winds and (c) E x S drifts, and concluded that the longitudinal differences were mainly due to differences in E x B drift characteristics. In the South American region, the geomagnetic field configuration is anomalous. The magnetic dip equator on the west coast has remained at about 13”s for the last few decades, but on the east coast, it was at about S’S during ICY. Does this make any difference to thejbF2 characteristics at the equator on the west and east coast? Figure 1 shows for I976 the seasonal medianfitF2 daily variation at Huancayo (geographic latitude 12”S, longitude 75”W, dip angle -t 2”) and Fortaleza (geographic latitude 45, longitude 38”W, dip angle -2“). In D months (summer months), the solar zenith angle is lesser for Huancayo and the middayfoF2 at Huancayo is slightly larger than at Fortaleza. But in J and E months,foF’2 at Fortaleza is larger. A similar plot for I@, showed that larger $oF2 at Fortaleza was associated with lower heights of the F2 layer.
Two possibilities arise: (i) the,/ijF2 in longitudes on the east coast may be larger at all latitudes, or (ii) the fountain effect may be weaker on the east coast. The
DIF = HU
FO
Fig. 1. Upper half: Diurnal patterns of seasonal averages of monthly median&F2 at Huancayo (full lines) and Fortaleza (crosses) for J, D, E months of 1976. Lower half: The difference loF2 (Huancayo minus Fortaleza). 9
R.P.
KANE~~
E.R,PAuLA
AV 1t957-631
Fig. P. Diurnal
AV (1957.63’
ofmonthly mtdianfi~F2 at Sk Pauio (full lines) and Tucuman {dashes and crosses) for J. D. E months for 1957-1963.
patterns &seasonal averages
~mpiications forfoR at few Iatitudes would be different lbr these two cases. For exampfe, for the southern Ioeations Tucuman (27”5,65”W geographic) and S&I Paula f24”S, 47”W geographic), both ofwhich had a dip angle of about -22” during the ICY, the foF2 at Tucuman would be smaller than that at Stio Paula in case (i) and larger in case (ii). Figure 2 shows the plots for the years 1957 to 1963, In dmost all cases, $oF2 at Tucuman 4s larger than that at S&I Paufo. Thus, on the eastern coast&F2 is larger at the equator as compared to the western coast, while at latitudes away from the equator, the eastern longitude shows smallerfaF2 than at western longitude. This is consistent with case (ii), viz, a weaker fountain effect at the eastern coast. The total magnetic field at Huancayo and Fortafeza
is roughly the same. However, the d~li~atjo~ at Huancayo is about 6°F and, at Fortaleza, about 2O”W. Whether such a twist in the magnetic field can cause a substantial change in the JbF2 characteristics needs theoretical investigation. Also, the strength of the fountain effect is generally related to the strength of the equatorial electrojet. A comparison of the daily variation of H at Huancayo and Fortaleza should prove useful. Work in this direction is in progress.
Ar’knowleBg~~ents-Thanks are due to Dr. NELXINf)E&sus PARAUA,Director of INPE, for support, This work was partially supported by FN’DCT, Brazil under Contract FTNPE-537,iCT.
< omparison
offoF
characteristics
at similar low latitudes
but different longitudes
in the South American
region
II
REFERENCES ANDERSOND. N. APPLETON E. V. DUNCAN R. A. LYON A. J. and THOMAS L. MARTYN D. F.
1973 1946 1960 I963 1955
RAO C. S. R. and MALHOTRA P. L. TMMAS L.
1964
1968
Planet. Space Sci. 21,421. Nature (Lond.) 157, 691. J. atmos.terr. Phys. 18, 89. .I. atmos. terr. Phys. 25, 373. The Physics ofthe Ionosphere, London. J. utmos. terr. Phys. 26, 1075. .I. atmos. terr. Phys. 30, 1631.
p. 254, Physical
Society,