- Email: [email protected]
Rocket and ground-based electron density soundings versus IRI representation
Available online at www.sciencedirect.com Pergamon www.elsevier.com/locate/asr
SCIRNCm
DIRECT.
doi: 10.1016/SO273-1177(03)00033-4
ROCKET AND GROUND-BASED ELECTRON DENSITY SOUNDINGS VERSUS IRI REPRESENTATION J. H. A. Sobral’, M. A. Abdu’, P. Muralikrishna’, J. LaBelle’, Vivian M. Castilho’, and C. J. Zamlutti’ ‘institute National de Pesquisas Espaciasis, INPE, Caixa Postal 51.5. SGo Jest dos Campos. 12201-970, Sa’o Paulo, Brazil 2Durtmouth College, Hanover, NH 03755. U.S.A.
ABSTRACT Monthly averages of F-region cut-off frequency foF2 and peak height hmF2 data obtained during the solar maximum years 1978 and 2000 through the use of ionosondes located at equatorial and low latitude regions in Brazil are compared with the RI representations. The equatorial stations are Fortaleza (3”53’S, 38”25’W), and SZLO Ltis (2*20’S, 44’12’W), and the low latitude station was Cachoeira Paulista (22”41’S, 4S”OO’W).The ionosonde data analyzed here were taken at each 15 minutes, in the months of February, May, August and November, of 1978 and 2000. In a second part of this study, electron density profiles obtained by three rocket flights in the Brazilian equatorial region are also compared with the IRI representations. The rocket experiments were carried out on July 7, 1984, December 11, 1985 and October 14, 1994. The first and the second experiments were performed at the subequatorial station Natal (35’14’W, 5’55%) and the third one at the equatorial station Alc&ntara (2’19’S, 44’22W). The results clearly show that lRI significantly underestimates the equatorial hmF2 during the pre-midnight period. On the other hand, the RI representation for hmF2 over offequatorial station Cachoeira Paulista is in rather better agreement with the experimental density profile. The JRI representation of the foF2 parameter is in reasonable agreement with the data for both the equatorial region and Cachoeira Paul&a, although the model appears to systematically overestimate foF2 in August for both 1978 and 2000. The rocket electron density profiles show that the highly variable height distribution of the electron density during ionospheric plasma bubble events may always prevent the correct representation of electron density profiles in the equatorial region during the occurrence of those events. 0 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
INTRODUCTION The purpose of the International Reference Ionosphere-RI is to provide standard values of relevant ionospheric parameters such as electron density, electron temperature, ion composition, ion temperatures and ion drifts. This work presents a comparison between RI model predictions of F-region electron density peak height hmF2 and the F-region cutoff &equency foF2 and measurements of these quantities with ionosondes at Fortaleza, SBo Luis and Cachoeira Paulista. The ionosonde data presented here were obtained in 1978 and 2000. The magnetic equator in the Brazilian region has drifted considerably in the period 1978-2000. For example, the dip angle over the Sb Luis/Al&tara region at h=300 km moved from 6.4N in 1978 to 1.3s in 2000. The other dip angles at h=300 km concerned in this work are the following ones: Fortaleza 2.58/1978, 10.6S/2000, Cachoeira Paulista 25.5S/1978, 31.58/2000, Natal 11.7SI1984, 12.1Y1985. The RI model used here was obtained from the website http://nssdc.gsfc. nasa.gov/space/model/models/iri. html. In a second part of this study, electron density profiles obtained by three rocket flights in the Brazilian equatorial region are presented and compared with the RI representation. The first and the second rocket experiments involved Brazilian-made Sonda III rockets and were carried out at the Brazilian rocket launching facility pp.569-575, 2003 8 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/03 $30.00 + 0.00
Adv. Space Res. Vol. 31, No. 3,
at Natal on July 7. 1984 and December 11, 1985, and the third experiment was carried out using a Black Brant XI at the Brazilian rocket launching facility Alctitara on October 14, 1994. EXPERIMENTAL RESULTS Ionosonde and Digisoode Data
Figures 1 to 4 show monthly averages at each 15min of the experimental and RI ionospheric parameters foF2 and F-region peak height for the months of February, May, August and November, which were chosen as to approximately represent the four seasons. The bars on RI and experimental curves of Figures 1 to 4 are variance bars of the monthly averages of RI and experimental foF2 and F-region peak height data. Figures 1 and 2 (1978) were obtained through the use of an ionosonde and Figures 3 and 4 (2000) were obtained through a Digisonde. In the case of the ionosonde data the F-region peak height is calculated as the height parameter hpF2 which is the height read from the ionogram F layer trace at a frequency f = 0.83 f0F2. The hpF2 parameter provides a reasonable estimate of the F-layer peak height over the Brazilian low-latitude region especially at night-time according to a detailed study conducted by Batista et al. (1991) and Sobral et al. (2001). The Digisonde has built-in software that calculates automatically the F layer peak height h,F2. Rocket Data
The Natal experiments were made using a high frequency capacitance probe flown on board Brazilian Sonda III rockets and the Alclntara experiment with a plasma frequency probe and a high frequency capacitance probe and a Langmuir probe on board a Black Brant XI rocket during the Guard campaign (LaBelle et al., 1997; Pfaff et al., 1997). The high frequency capacitance probe used in these measurements were developed and built by INPE’s Aeronomy Division. Figure 5 show electron density profiles from these three rocket experiments. DISCUSSION
Figure 1 shows that the F-region critical frequency values over Fortaleza 1978 are in general rather close to RI predictions in the four months considered. The monthly daily sum of Kp in the four months analyzed are close to each other. The IRI model appears to underestimate the cutoff frequency foF2 between sunset and midnight in February, but this effect may result from the low number of experimental data. Otherwise, the discrepancies in prediction versus experimental determination of foF2 are rather small, as for example during nighttime in May and August. On the other hand, the mode1 does a much poorer job predicting the parameter hpF2, for example between sunset and midnight in February and May, when the discrepancy ranged up to 180 km in May around the time of prereversal zonal electric field (vertical drift) enhancement at - 20:OOLT. In August and November such discrepancy tended to be smaller. Figure 2 shows good agreement between the RI and measured foF2 values over Cachoeira Paulista during daytime for February, May and November, 1978, but less agreement in August, for which the model clearly overestimates the experimental results. The hpF2 shows reasonably good agreement with IRI prediction during nighttime but not during day-time, when experimental values considerably exceed those of RI. Batista et al. (1991), compared measurements of hpF2 with values derived from the POLAN code real height calculation program. They concluded that hpF2 was found to underestimate the F-layer peak height as compared to the hmF2 derived by POLAN during day-time but matched reasonably well during night-time. Their study included the sites of Cachoeira Paulista and Fortaleza. At Fortaleza between 06:OOLT and 18:OOLT,they found a mean positive deviation of 55 km of hpF2 with respect to the POLAN hmF2. Therefore the present results showing the difference [hpF2 - IRIhmF2] ranging up to over 1OOkmaround 11LT in February suggest that the W does underestimate the h,,,F2 at these hours. The comparison of ionospheric parameters foF2 and hmF2 shown in Figures 3 and 4 are based on Digisonde data. Figure 3 shows that model foF2 matches closely the measured values during daytime at Sgo Luis 2000. After 1800LT in May and August, there is a mismatch between model and experimental values of foF2, consistent with the foF2 results from Fortaleza (Figure 1). The F-region peak heights hmF2 predicted by IRI differ significantly from the measured values during night-time, especially after sunset in all four months. This disagreement between the IRI and observed values is so large that the model and experimental variance bars do not even overlap. The discrepancy maximizes around the pre-reversal enhancement peak time 1900-2000LT when the difference [hmF2 - IRIhmFZ]
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Fig. 1. Mean local time variation of hpF2 and foF2 ionosonde data versus IRI representation at Fortaleza, as indicated.
Cachoeira Paulista 1978
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Fig. 2. The same as Fig. 1 except site is Cachoeira Paulista.
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Fig. 3. Mean local time variation of hmF2 and foF2 Digisonde data versus IRI representation at SBo Luis.
Cachoeira Paulista 2000 Month)vmeans: 600 ~
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Fig. 4. Same as Fig. 3 except site is Cachoeira Paulista
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0 t- ’ lE+4
“111’’1’ “1”“’ ’ “111” ’’ “““” ’“1’111’ ’“““I’’““M ’ ““‘ d “llw ’ll’d lE+5
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Electron density (cms3)
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Fig. 5. Experimental rocket and RI electron density profiles reaches a magnitude of about 150 km. Such large difference between the IRI predicted h,,,F2 and their observed values have been pointed out by Abdu et al., (1996). During 0000-0600LT the disagreement (RI underestimated) is considerably larger in May and August than in February and November. Figure 4 shows the behavior of the ionospheric parameters at Cachoeira Paulista under the condition of higher average sunspot numbers but with approximately equivalent daily sum of Kp magnitude, as compared with the Kp values of Figure 2. It shows that foF2 matches model predictions during &y-time at Cachoeira Paulista, but a discrepancy between model predictions and experimental measurements arises during night-time (from 18:OOto 05:OOLT). In the pre-midnight period the IRI foF2 exceeds average experimental values, and the opposite situation occurs in February and November. The IRI peak height representation is very close to the experimental average values at Cachoeira Paulista between 06:OOLT and -03:OOLT in all four months. In May, August and November between 03:OOLT and 06:OOLT, IRI peak height representation tends to be higher than the experimental average (Figure 4). Figure 5 shows the electron density profiles from three rocket experiments carried out in the equatorial region. The first plot on the left hand side corresponds to the Gua~$ experiment carried out at Alc&ara. As previously shown in Figures 1 and 3, respectively, the local time 19:55 LT is in the time frame in which the discrepancy between hpF2 and hmF2 maximizes with respect to IRI prediction. The rocket electron density profiles of Al&tam confii such a discrepancy by showing that the rocket F2 peak height of approximately 400 km lies above the peak height predicted by RI. Because of its high apogee altitude this Guar& rocket experiment provided a unique opportunity to verify the IRI electron density profile at high altitudes. It was found that at high altitudes there is a significant difference in the shape of the measured profile compared with the shape of the IRI profile. That is, the experimental profile presented considerably higher electron density gradients with altitude. The Natal December 1985 profile shown in the middle of the panel shows the same effect, with a difference in peak height of the order of 50 km, which is also consistent with the Digisonde results for the equatorial station Sgo Luis in November (Figure 3). The July 1984 rocket electron density profile (right hand side of Figure 5) matches closely to the IRI prediction, which is consistent with the close match between IRI predictions and measurements of hoF2 2000 around 1505LT at Sb Luis in August 2000 (Figure 3). Unfortunately rocket plasma data are missing between approximately 270 km and 450 km in this last profile due to technical problems with the payload instrumentation. The large amplitude time- and space-variations of the ionospheric plasma bubble bite-out signatures shown in the Alcbtara and Natal profiles shown in the left-hand and middle panels of Figure 5, respectively, show the
s74
J. H. A. Sobral
<‘Ia/.
difficulty that models such as IRI face in representing the electron density profiles in the equatorial region during the occurrence of the ionospheric bubbles. CONCLUSIONS Monthly averages at each 15 minutes of the F-region peak height hmF2 and cut-off frequency foF2 obtained from ionosonde data from equatorial (Fortaleza-Fz and !%o Luis-SL) and low latitude (Cachoeira Paulista-CP) sites in Brazil are compared with the IRI predictions for the summer, fall, winter and spring months of February, May, August and November, respectively. The comparison was made for the solar maximum epoch years 1978 and 2000. The main conclusions of this study are: I. The IRI significantly underestimates hmF2 in all four months examined for the equatorial stations Fortaleza and SBo Luis (Figures 1 and 3) during the pre-midnight period, with maximum mismatch occurring in the time frame 20:OOLT in which the peak of the pre-reversal enhancement occurs. The average discrepancy may reach up to 180 km. The variance bars of the IRI and experimental hmF2 are considerably apart from each other for the equatorial stations Fz (Figure 1) and SL (Figure 3) from sunset to midnight, suggesting that the discrepancies are not caused by random factors, but by underestimation in the IRI prediction of the equatorial hmF2. 2. In general, at the equator, the IRI appears to do a much better job predicting foF2 than it does at predicting Fregion peak height. 3. The IRI predictions of hmF2 are close to the experimental values at Cachoeira Paulista between 06:OOLT and -03:OOLT in all four months (Figure 4). However, between 03:OOLT and 06:OOLT, IRI predictions of F-peak height exceed the average experimental values. 4. The IRI F-region peak height representation is, in general, closer to the low-latitude CP data than in the case of Fz and SL. 5. In both May 1978 and May 2000, IRI predictions of foF2 for Fz and SL between sunset and midnight (Figures 1 and 3) are clearly smaller than average experimental foF2. 6. In August 1978 (Figure 2), August 2000 (Figure 4) and May 2ooO (Figure 4) the IRI representation of foF2 for CP exceeds the experimental data at all local times. 7. The IRI and experimental (Digisonde) values of the parameter hmF2 in the year 2000 are in close agreement with each other for CP, especially during daytime. 8. The measured parameter hpF2 at Cachoeira Paulista exceeds the IRI predicted hmF2 during day-time in 1978. The rocket experiments have yielded the following results: 1. A close electron density profile agreement between IRI and experimental results for the I l/12/1985 and 28/07/ 1984 and poor agreement in the 14/l O/l 994 experiment, especially at high altitudes in the latter case. 2. The highly variable height distribution of the electron density may always prevent the correct representation of electron density profiles in the equatorial region during the occurrence of the ionospheric bubbles. 3- The electron density slope of the topside ionosphere near 800-900 km as qbserved by rocket in situ measurement shows a fast decrease with height as compared to the much slower rate of decrease predicted by the IRI model. ACKNOWLEDGEMENTS This work was partially supported by the Fundacgo de Amparo a Pesquisa do Estado de Sb Paulo-FAPESP Grant no. 1999/00437-O, by the Conselho National de Desenvolvimento Cientitico e Tecnologico-CNPq Grants 522919/96-O and 520185/95. The authors are thankful to Maria Goreti de Aquino for partial support in the data processing. REFERENCES
Abdu, M. A., I. S. Batista and J. R. de Souza, An Overview of W-Observational Data Comparison in American (Brazilian) Sector Low Latitude Ionosphere, Adv. Space Res., 18, 13-22, 1996. Batista, I. S., E. R. de Paula, M. A. Abdu and I. J. Kantor, Comparative Study of Ionogram F2 Peak Height from Different Techniques, Geojisica International, 30,249-252, 1991.
Rocket and Ground Observations
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LaBelle, J., J. -M. Jahn, R. F. Pfaff, W. E. Swartz, J. H. A. Sobral, M. A. Abdu, P. Muralikrishna and E. R. de Paula, The Brazil/Guara Campaign High-Altitude Equatorial Spread-F Campaign: Results of Large-Scale Measurements, Geophys. Res. Lett., 24,169 1- 1694,1997. Pfaff, R. F., J. H. A. Sobral, M. A. Abdu, W. Swartz, J. LaBelle, M. Larsen, R. Goldberg and F. J. Schmidlin, The Guam Campaign: A series of rocket-radar investigations of the Earth’s Upper Atmosphere at the Magnetic Equator, Geophys. Res. Lett., 24, 1663-1666, 1997. Sobral, J. H. A. , M. A. Abdu, Cristina S. Yamashita, W. D. Gonzalez, Alicia C. de Gonzalez, In&z S. Batista, C. J. Zamlutti and B. T. Tsurutani, Responses of the Low-Latitude Ionosphere to very Intense Geomagnetic Storms, J. Atmos. Solar-Terr. Phys., 63,965974,200l.
SCIRNCm
DIRECT.
doi: 10.1016/SO273-1177(03)00033-4
ROCKET AND GROUND-BASED ELECTRON DENSITY SOUNDINGS VERSUS IRI REPRESENTATION J. H. A. Sobral’, M. A. Abdu’, P. Muralikrishna’, J. LaBelle’, Vivian M. Castilho’, and C. J. Zamlutti’ ‘institute National de Pesquisas Espaciasis, INPE, Caixa Postal 51.5. SGo Jest dos Campos. 12201-970, Sa’o Paulo, Brazil 2Durtmouth College, Hanover, NH 03755. U.S.A.
ABSTRACT Monthly averages of F-region cut-off frequency foF2 and peak height hmF2 data obtained during the solar maximum years 1978 and 2000 through the use of ionosondes located at equatorial and low latitude regions in Brazil are compared with the RI representations. The equatorial stations are Fortaleza (3”53’S, 38”25’W), and SZLO Ltis (2*20’S, 44’12’W), and the low latitude station was Cachoeira Paulista (22”41’S, 4S”OO’W).The ionosonde data analyzed here were taken at each 15 minutes, in the months of February, May, August and November, of 1978 and 2000. In a second part of this study, electron density profiles obtained by three rocket flights in the Brazilian equatorial region are also compared with the IRI representations. The rocket experiments were carried out on July 7, 1984, December 11, 1985 and October 14, 1994. The first and the second experiments were performed at the subequatorial station Natal (35’14’W, 5’55%) and the third one at the equatorial station Alc&ntara (2’19’S, 44’22W). The results clearly show that lRI significantly underestimates the equatorial hmF2 during the pre-midnight period. On the other hand, the RI representation for hmF2 over offequatorial station Cachoeira Paulista is in rather better agreement with the experimental density profile. The JRI representation of the foF2 parameter is in reasonable agreement with the data for both the equatorial region and Cachoeira Paul&a, although the model appears to systematically overestimate foF2 in August for both 1978 and 2000. The rocket electron density profiles show that the highly variable height distribution of the electron density during ionospheric plasma bubble events may always prevent the correct representation of electron density profiles in the equatorial region during the occurrence of those events. 0 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
INTRODUCTION The purpose of the International Reference Ionosphere-RI is to provide standard values of relevant ionospheric parameters such as electron density, electron temperature, ion composition, ion temperatures and ion drifts. This work presents a comparison between RI model predictions of F-region electron density peak height hmF2 and the F-region cutoff &equency foF2 and measurements of these quantities with ionosondes at Fortaleza, SBo Luis and Cachoeira Paulista. The ionosonde data presented here were obtained in 1978 and 2000. The magnetic equator in the Brazilian region has drifted considerably in the period 1978-2000. For example, the dip angle over the Sb Luis/Al&tara region at h=300 km moved from 6.4N in 1978 to 1.3s in 2000. The other dip angles at h=300 km concerned in this work are the following ones: Fortaleza 2.58/1978, 10.6S/2000, Cachoeira Paulista 25.5S/1978, 31.58/2000, Natal 11.7SI1984, 12.1Y1985. The RI model used here was obtained from the website http://nssdc.gsfc. nasa.gov/space/model/models/iri. html. In a second part of this study, electron density profiles obtained by three rocket flights in the Brazilian equatorial region are presented and compared with the RI representation. The first and the second rocket experiments involved Brazilian-made Sonda III rockets and were carried out at the Brazilian rocket launching facility pp.569-575, 2003 8 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/03 $30.00 + 0.00
Adv. Space Res. Vol. 31, No. 3,
at Natal on July 7. 1984 and December 11, 1985, and the third experiment was carried out using a Black Brant XI at the Brazilian rocket launching facility Alctitara on October 14, 1994. EXPERIMENTAL RESULTS Ionosonde and Digisoode Data
Figures 1 to 4 show monthly averages at each 15min of the experimental and RI ionospheric parameters foF2 and F-region peak height for the months of February, May, August and November, which were chosen as to approximately represent the four seasons. The bars on RI and experimental curves of Figures 1 to 4 are variance bars of the monthly averages of RI and experimental foF2 and F-region peak height data. Figures 1 and 2 (1978) were obtained through the use of an ionosonde and Figures 3 and 4 (2000) were obtained through a Digisonde. In the case of the ionosonde data the F-region peak height is calculated as the height parameter hpF2 which is the height read from the ionogram F layer trace at a frequency f = 0.83 f0F2. The hpF2 parameter provides a reasonable estimate of the F-layer peak height over the Brazilian low-latitude region especially at night-time according to a detailed study conducted by Batista et al. (1991) and Sobral et al. (2001). The Digisonde has built-in software that calculates automatically the F layer peak height h,F2. Rocket Data
The Natal experiments were made using a high frequency capacitance probe flown on board Brazilian Sonda III rockets and the Alclntara experiment with a plasma frequency probe and a high frequency capacitance probe and a Langmuir probe on board a Black Brant XI rocket during the Guard campaign (LaBelle et al., 1997; Pfaff et al., 1997). The high frequency capacitance probe used in these measurements were developed and built by INPE’s Aeronomy Division. Figure 5 show electron density profiles from these three rocket experiments. DISCUSSION
Figure 1 shows that the F-region critical frequency values over Fortaleza 1978 are in general rather close to RI predictions in the four months considered. The monthly daily sum of Kp in the four months analyzed are close to each other. The IRI model appears to underestimate the cutoff frequency foF2 between sunset and midnight in February, but this effect may result from the low number of experimental data. Otherwise, the discrepancies in prediction versus experimental determination of foF2 are rather small, as for example during nighttime in May and August. On the other hand, the mode1 does a much poorer job predicting the parameter hpF2, for example between sunset and midnight in February and May, when the discrepancy ranged up to 180 km in May around the time of prereversal zonal electric field (vertical drift) enhancement at - 20:OOLT. In August and November such discrepancy tended to be smaller. Figure 2 shows good agreement between the RI and measured foF2 values over Cachoeira Paulista during daytime for February, May and November, 1978, but less agreement in August, for which the model clearly overestimates the experimental results. The hpF2 shows reasonably good agreement with IRI prediction during nighttime but not during day-time, when experimental values considerably exceed those of RI. Batista et al. (1991), compared measurements of hpF2 with values derived from the POLAN code real height calculation program. They concluded that hpF2 was found to underestimate the F-layer peak height as compared to the hmF2 derived by POLAN during day-time but matched reasonably well during night-time. Their study included the sites of Cachoeira Paulista and Fortaleza. At Fortaleza between 06:OOLT and 18:OOLT,they found a mean positive deviation of 55 km of hpF2 with respect to the POLAN hmF2. Therefore the present results showing the difference [hpF2 - IRIhmF2] ranging up to over 1OOkmaround 11LT in February suggest that the W does underestimate the h,,,F2 at these hours. The comparison of ionospheric parameters foF2 and hmF2 shown in Figures 3 and 4 are based on Digisonde data. Figure 3 shows that model foF2 matches closely the measured values during daytime at Sgo Luis 2000. After 1800LT in May and August, there is a mismatch between model and experimental values of foF2, consistent with the foF2 results from Fortaleza (Figure 1). The F-region peak heights hmF2 predicted by IRI differ significantly from the measured values during night-time, especially after sunset in all four months. This disagreement between the IRI and observed values is so large that the model and experimental variance bars do not even overlap. The discrepancy maximizes around the pre-reversal enhancement peak time 1900-2000LT when the difference [hmF2 - IRIhmFZ]
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Fig. 1. Mean local time variation of hpF2 and foF2 ionosonde data versus IRI representation at Fortaleza, as indicated.
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2c-d 0
LocalTime
Fig. 2. The same as Fig. 1 except site is Cachoeira Paulista.
LocalTime
*
J. H.A. Sobral tf (11
572
saoLuis
Monthly means:
2000
,6,Kp=21-
600 -500
5 -400 2 E ml 0
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=2001
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.,......... 2 4 6 6
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E -400 2 Em
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=
6
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200
= 19
2+.......-. 0 2 4 ,I(_Kp=20-
i
21 0
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--NovSL 14
LocelTime
LocalTime
Fig. 3. Mean local time variation of hmF2 and foF2 Digisonde data versus IRI representation at SBo Luis.
Cachoeira Paulista 2000 Month)vmeans: 600 ~
,500
FeblRl
-FebCP
e400 cf 5 300 200
0
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-AqIRI -AugCP
F500
$400
km =
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0
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IRI
-Nov -NovCP
A
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LocalTime
Fig. 4. Same as Fig. 3 except site is Cachoeira Paulista
LocalTime
Rocket and Ground Observations
1000
lE+2
lE+3
I ,11,,, I I ,I ,111I I111111 , ,llllll
lE+4
I ,,,1,,,
vs IRI
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Natal26107l1964 15:05L.T.
V
Natal 11/12/1965 21:30 L.T.
200 a
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“111’’1’ “1”“’ ’ “111” ’’ “““” ’“1’111’ ’“““I’’““M ’ ““‘ d “llw ’ll’d lE+5
lE+6
lE+7
Electron density (cms3)
lE+4
lE+5
’“‘lull
lE+6
lE+7
Fig. 5. Experimental rocket and RI electron density profiles reaches a magnitude of about 150 km. Such large difference between the IRI predicted h,,,F2 and their observed values have been pointed out by Abdu et al., (1996). During 0000-0600LT the disagreement (RI underestimated) is considerably larger in May and August than in February and November. Figure 4 shows the behavior of the ionospheric parameters at Cachoeira Paulista under the condition of higher average sunspot numbers but with approximately equivalent daily sum of Kp magnitude, as compared with the Kp values of Figure 2. It shows that foF2 matches model predictions during &y-time at Cachoeira Paulista, but a discrepancy between model predictions and experimental measurements arises during night-time (from 18:OOto 05:OOLT). In the pre-midnight period the IRI foF2 exceeds average experimental values, and the opposite situation occurs in February and November. The IRI peak height representation is very close to the experimental average values at Cachoeira Paulista between 06:OOLT and -03:OOLT in all four months. In May, August and November between 03:OOLT and 06:OOLT, IRI peak height representation tends to be higher than the experimental average (Figure 4). Figure 5 shows the electron density profiles from three rocket experiments carried out in the equatorial region. The first plot on the left hand side corresponds to the Gua~$ experiment carried out at Alc&ara. As previously shown in Figures 1 and 3, respectively, the local time 19:55 LT is in the time frame in which the discrepancy between hpF2 and hmF2 maximizes with respect to IRI prediction. The rocket electron density profiles of Al&tam confii such a discrepancy by showing that the rocket F2 peak height of approximately 400 km lies above the peak height predicted by RI. Because of its high apogee altitude this Guar& rocket experiment provided a unique opportunity to verify the IRI electron density profile at high altitudes. It was found that at high altitudes there is a significant difference in the shape of the measured profile compared with the shape of the IRI profile. That is, the experimental profile presented considerably higher electron density gradients with altitude. The Natal December 1985 profile shown in the middle of the panel shows the same effect, with a difference in peak height of the order of 50 km, which is also consistent with the Digisonde results for the equatorial station Sgo Luis in November (Figure 3). The July 1984 rocket electron density profile (right hand side of Figure 5) matches closely to the IRI prediction, which is consistent with the close match between IRI predictions and measurements of hoF2 2000 around 1505LT at Sb Luis in August 2000 (Figure 3). Unfortunately rocket plasma data are missing between approximately 270 km and 450 km in this last profile due to technical problems with the payload instrumentation. The large amplitude time- and space-variations of the ionospheric plasma bubble bite-out signatures shown in the Alcbtara and Natal profiles shown in the left-hand and middle panels of Figure 5, respectively, show the
s74
J. H. A. Sobral
<‘Ia/.
difficulty that models such as IRI face in representing the electron density profiles in the equatorial region during the occurrence of the ionospheric bubbles. CONCLUSIONS Monthly averages at each 15 minutes of the F-region peak height hmF2 and cut-off frequency foF2 obtained from ionosonde data from equatorial (Fortaleza-Fz and !%o Luis-SL) and low latitude (Cachoeira Paulista-CP) sites in Brazil are compared with the IRI predictions for the summer, fall, winter and spring months of February, May, August and November, respectively. The comparison was made for the solar maximum epoch years 1978 and 2000. The main conclusions of this study are: I. The IRI significantly underestimates hmF2 in all four months examined for the equatorial stations Fortaleza and SBo Luis (Figures 1 and 3) during the pre-midnight period, with maximum mismatch occurring in the time frame 20:OOLT in which the peak of the pre-reversal enhancement occurs. The average discrepancy may reach up to 180 km. The variance bars of the IRI and experimental hmF2 are considerably apart from each other for the equatorial stations Fz (Figure 1) and SL (Figure 3) from sunset to midnight, suggesting that the discrepancies are not caused by random factors, but by underestimation in the IRI prediction of the equatorial hmF2. 2. In general, at the equator, the IRI appears to do a much better job predicting foF2 than it does at predicting Fregion peak height. 3. The IRI predictions of hmF2 are close to the experimental values at Cachoeira Paulista between 06:OOLT and -03:OOLT in all four months (Figure 4). However, between 03:OOLT and 06:OOLT, IRI predictions of F-peak height exceed the average experimental values. 4. The IRI F-region peak height representation is, in general, closer to the low-latitude CP data than in the case of Fz and SL. 5. In both May 1978 and May 2000, IRI predictions of foF2 for Fz and SL between sunset and midnight (Figures 1 and 3) are clearly smaller than average experimental foF2. 6. In August 1978 (Figure 2), August 2000 (Figure 4) and May 2ooO (Figure 4) the IRI representation of foF2 for CP exceeds the experimental data at all local times. 7. The IRI and experimental (Digisonde) values of the parameter hmF2 in the year 2000 are in close agreement with each other for CP, especially during daytime. 8. The measured parameter hpF2 at Cachoeira Paulista exceeds the IRI predicted hmF2 during day-time in 1978. The rocket experiments have yielded the following results: 1. A close electron density profile agreement between IRI and experimental results for the I l/12/1985 and 28/07/ 1984 and poor agreement in the 14/l O/l 994 experiment, especially at high altitudes in the latter case. 2. The highly variable height distribution of the electron density may always prevent the correct representation of electron density profiles in the equatorial region during the occurrence of the ionospheric bubbles. 3- The electron density slope of the topside ionosphere near 800-900 km as qbserved by rocket in situ measurement shows a fast decrease with height as compared to the much slower rate of decrease predicted by the IRI model. ACKNOWLEDGEMENTS This work was partially supported by the Fundacgo de Amparo a Pesquisa do Estado de Sb Paulo-FAPESP Grant no. 1999/00437-O, by the Conselho National de Desenvolvimento Cientitico e Tecnologico-CNPq Grants 522919/96-O and 520185/95. The authors are thankful to Maria Goreti de Aquino for partial support in the data processing. REFERENCES
Abdu, M. A., I. S. Batista and J. R. de Souza, An Overview of W-Observational Data Comparison in American (Brazilian) Sector Low Latitude Ionosphere, Adv. Space Res., 18, 13-22, 1996. Batista, I. S., E. R. de Paula, M. A. Abdu and I. J. Kantor, Comparative Study of Ionogram F2 Peak Height from Different Techniques, Geojisica International, 30,249-252, 1991.
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LaBelle, J., J. -M. Jahn, R. F. Pfaff, W. E. Swartz, J. H. A. Sobral, M. A. Abdu, P. Muralikrishna and E. R. de Paula, The Brazil/Guara Campaign High-Altitude Equatorial Spread-F Campaign: Results of Large-Scale Measurements, Geophys. Res. Lett., 24,169 1- 1694,1997. Pfaff, R. F., J. H. A. Sobral, M. A. Abdu, W. Swartz, J. LaBelle, M. Larsen, R. Goldberg and F. J. Schmidlin, The Guam Campaign: A series of rocket-radar investigations of the Earth’s Upper Atmosphere at the Magnetic Equator, Geophys. Res. Lett., 24, 1663-1666, 1997. Sobral, J. H. A. , M. A. Abdu, Cristina S. Yamashita, W. D. Gonzalez, Alicia C. de Gonzalez, In&z S. Batista, C. J. Zamlutti and B. T. Tsurutani, Responses of the Low-Latitude Ionosphere to very Intense Geomagnetic Storms, J. Atmos. Solar-Terr. Phys., 63,965974,200l.