- Email: [email protected]
Two cases of ionospheric storms at the equatorial anomaly
Advances in Space Research 39 (2007) 668–673 www.elsevier.com/locate/asr
Two cases of ionospheric storms at the equatorial anomaly G.A. Mansilla a
a,*
, M. Mosert
b
Laboratorio de Ionosfera, Departamento de Fı´sica, Universidad Nacional de Tucuma´n – CONICET, Av. Independencia 1800, 4000 San Miguel de Tucuma´n, Argentina b CASLEO, Av. Espan˜a 1512 (sur), CP 5400, San Juan, Argentina Received 15 December 2005; received in revised form 26 June 2006; accepted 24 October 2006
Abstract This paper presents an evaluation of the IRI-2000 model, which includes the storm-time correction (STORM model), to predict foF2 during two intense geomagnetic storms (peak Dst < 100 nT) in the equatorial anomaly region. To this end, both IRI model outputs (with the STORM model included and without the STORM model) are compared with ionosonde data from a chain of stations located in the 278°–295° longitudinal range. The results show that the STORM model, even though it is not designed for low latitudes in general captures the variations of the measured values. But there is an overestimation of the foF2 data during the main phase and first stage of the recovery of the storms. Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Geomagnetic storm; STORM model; Equatorial anomaly
1. Introduction The knowledge of the critical frequency of the F2-layer, foF2 (proportional to the square root of the peak electronic density of F2-layer, NmF2) is essential for the prediction of ionospheric behaviour. Several empirical and semi-empirical models (e.g., Barghausen et al., 1969; Anderson, 1973; Llewellyn and Bent, 1973; Bent et al., 1976; Anderson et al., 1987) have been developed to predict ionospheric parameters during quiet conditions. One of the most widely empirical models used to describe the ionosphere during magnetically quiet conditions is the International Reference Ionosphere, IRI (Bilitza, 1986, 1990, 2001) which provides median values of electron density, electron temperature, and ion composition as a function of height for a given location, time and sunspot number. This model is being continuously revised and updated through an international cooperative effort sponsored by the Committee on Space Research and the International Union of Radio Science. *
Corresponding author. E-mail addresses: [email protected] (G.A. Mansilla), [email protected] (M. Mosert).
The latest version of the IRI contains the empirical storm-time ionospheric correction (STORM model) designed by Fuller-Rowell et al. (2000, 2001). It dependents on the intensity of the storm, and is a function of latitude and season (Araujo-Pradere et al., 2002a,b). A number of recent studies have compared the IRISTORM predictions with ionosonde data at mid-latitudes during storm time periods (Araujo-Pradere and FullerRowell, 2003; Araujo-Pradere et al., 2004; Mansilla et al., 2004). In general, the results indicate a significant improvement of IRI-2000 with the STORM model over IRI-2000 without the STORM model. In this paper, the performance of IRI-2000 with the correction of the STORM model included is evaluated in the equatorial anomaly (EA) region during two intense geomagnetic storms (peak Dst < 100 nT) by comparing ionosonde data with predictions obtained from the IRI model. The database used in this analysis, provided by the Space Physics Interactive Data Resource (SPIDR), are hourly values of foF2 from a chain of stations located in the 278°–295° longitudinal sector. The stations and their locations are listed in Table 1.
0273-1177/$30 Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2006.10.022
G.A. Mansilla, M. Mosert / Advances in Space Research 39 (2007) 668–673
669
Table 1 Station list Station
Geographic latitude
Geographic longitude
Geomagnetic latitude
Geomagnetic longitude
Bogota Talara Huancayo La Paz Tucuma´n
4.5 4.6 12.0 16.5 26.9
285.8E 278.6E 284.6E 292.8E 294.6E
15.9 6.6 0.6 5.0 15.5
4.9W 11.9W 5.7W 1.3E 3.8E
The sudden commencements (SC) of the selected geomagnetic storms occurred at 0818 UT on March 15, 1960 (storm 1) and at 1312 UT on April 17, 1965 (storm 2). Storm 1 occurred during descending phase of solar cycle and storm 2 under low solar activity. These events were selected because no recent simultaneous measurements from an American chain located at equatorial and low latitudes are available. Although some stations present data gaps, nevertheless they are useful to investigate how well the IRI-STORM model is able to represent foF2 observations in the EA region. 2. Results The variation of the Dst geomagnetic index during the March 15–17 1960 storm period (storm 1) is shown at the top of Fig. 1. The SC occurred in predawn hours in this longitudinal sector. The main phase continued for about 24 h until around 07–08 UT (02–03 LT) on March 16 followed by a long-duration recovery. The lower part of Fig. 1 shows the diurnal behaviour of foF2 over equatorial and low latitude ionosonde stations during the storm period (solid circles) and superposed the prediction of IRI-2000 with the STORM model included (solid line). There are no data for Bogota´. It can be seen that the STORM model overestimates foF2 a few hours after SC at all the stations in the daytime hours on March 15 (15–20 UT). During the recovery phase predicted values capture the variation of measured values mainly at Talara (geomagnetic latitude = 6.6°) and Huancayo (geomagnetic latitude = 0.6°) while at La Paz (geomagnetic latitude = 5.0°) and Tucuma´n (geomagnetic latitude = 15.5°) the STORM model significantly underestimates the observations. To quantify the improvement of IRI-2000 with the STORM model included over IRI-2000 without the STORM model, the differences for both the IRI predictions from the measured foF2 values, in percentage, were calculated. These differences are given by: DfoF2 ð%Þ ¼ ½ðfoF2 foF2Þ=foF2 100 where foF2* is the modelled critical frequency (with the STORM model ‘‘turn on’’ or ‘‘turn off’’) and foF2 is the corresponding measured hourly critical frequency. Fig. 2 shows these relative differences for the storm on 15–17 March 1960. It can be seen that no appreciable improvement is obtained by using the STORM model in IRI because the STORM model introduces only a very
small change. The initial (09–12 UT on March 15) overestimation is between 60% and 90%. During the recovery phase the relative differences are in general lower than 30%. The strong disagreement among predicted and measured frequencies observed at Tucuma´n during this stage of the storm is possibly because this station is located below the south crest of the equatorial anomaly where local effects are produced. In order to illustrate the actual and modelled equatorial anomaly structure Fig. 3 shows the latitudinal variation of foF2 data and STORM-model predictions at specified times during daytime on March 15, 16 and 17. It can be seen that the equatorial anomaly arch is present during the storm period. The model predictions follow the variation of the foF2 data. As was already mentioned, IRI prediction with the STORM model overestimate foF2 measurements during the main phase, but there is evidently good agreement during the recovery phase of the storm mainly at equatorial latitudes. The upper panel of Fig. 4 presents the variation of Dst for the period April 17–19, 1965 (storm 2). The SC occurred during the sunrise hours in the longitude sector under consideration. The main phase remained until around 09–10 UT on April 18 in the predawn hours followed by a slow recovery. The time variations of foF2 data and IRI predictions with the STORM model at equatorial and low latitudes are also shown. No data were available for Tucuma´n. In general the STORM model predictions follow the variation of the data in the daytime hours during the initial stage of the storm. At Bogota (geomagnetic latitude = 15.9°) and Talara (geomagnetic latitude = 6.6°) there is a small overestimation of the data (the trend of available data at La Paz shows also an overestimation) whereas at Huancayo (geomagnetic latitude = 0.6°) the overestimation is greater. During the recovery phase, at Bogota and Talara the predictions of the model are closer to the observations than at Huancayo and La Paz, where the STORM model significantly underestimates the foF2 data. Fig. 5 presents the relative differences between the IRI2000 outputs (with and without the STORM model) and foF2 measurements. In general, there is not a considerable difference between the IRI-2000 predictions with and without STORM model. An improvement can be seen with the storm-time ionospheric correction model at Huancayo (smaller relative differences) during the end of the main phase and early stage of the recovery. During the main
670
G.A. Mansilla, M. Mosert / Advances in Space Research 39 (2007) 668–673
March 15-17, 1960
March 15-17, 1960 50
0
Dst [nT]
Dst [nT]
50 -50 -100 -150
0 -50 -100 -150 -200
-200 0
12
0
12
0
0
12
12
0
UT [hours]
0
12
Talara
Talara 20
DfoF2 (%)
foF2 (MHz)
12
UT [hours]
15 10 5
100 0 0
12
0
12
0
12
-100 -200
0 0
12
0
12
0
UT [hours]
12
UT [hours]
storm ON
Huancayo
storm OFF
Huancayo DfoF2 (%)
foF2 (MHz)
20 15 10 5 0
100 0 0
12
0
12
0
12
0
12
12
UT [hours]
UT [hours]
storm ON
La Paz
storm OFF
La Paz DfoF2 (%)
20
foF2 (MHz)
0
-200 0
15 10 5
100 0 0
12
0
12
0
12
-100 -200
0 0
12
0
12
0
UT [hours]
12
UT [hours]
storm ON
Tucuman
storm OFF
Tucuman DfoF2 (%)
20
foF2 (MHz)
12
-100
15 10 5
100 0 -100
0
12
0
12
0
12
-200
0 0
12
0
12
0
12
UT [hours]
UT [hours] storm ON
storm OFF
Fig. 1. Temporal variation of Dst geomagnetic index (upper panel), foF2 data (solid circles) and output of the IRI-2000 model with the STORM model (thin lines) in lower panels at equatorial and low latitude stations for the March 15–17, 1960 storm period.
Fig. 2. Temporal variation of Dst (upper panel) and relative deviations for both IRI outputs (with and without the STORM model) from foF2 measurements for the March 15–17, 1960 storm period.
phase the relative differences are in general lower than 50% at all the stations including Huancayo, which overall shows the greatest disagreement with the model. During recovery
phase the underestimation of the model does not exceed 50% and the overestimation at Bogota´ is also smaller than 50%.
G.A. Mansilla, M. Mosert / Advances in Space Research 39 (2007) 668–673 March 15
April 17-19, 1965 17UT (12LT)
15
19UT (14LT)
10
17UT (12LT)-IRI
5
19UT (14LT)-IRI
0 -5
0
5
10
15
Dst (nT)
foF2 (MHz)
20
-20 -15 -10
671
20
50 0 -50 -100 -150 -200
Geomagnetic latitude (degrees)
0
12
0
12
0
12
0
12
0
12
0
12
UT (hours) March 16
Bogota 17UT (12LT)
15
20
19UT (14LT)
10
17UT (12LT)-IRI
5
19UT (14LT)-IRI
0 -20 -15 -10
-5
0
5
10
15
20
foF2 (MHz)
foF2 (MHz)
20
15 10 5 0
Geomagnetic latitude (degrees)
0
12
0
12
UT (hours)
March 17
Talara
17UT (12LT)
15
19UT (14LT)
10
17UT (12LT)-IRI
5
19UT (14LT)-IRI
0 -20 -15 -10
-5
0
5
10
15
20
20
foF2 (MHz)
foF2 (MHz)
20
15 10 5 0
Geomagnetic latitude (degrees)
0
12
0
Fig. 3. Variation of foF2 observations and storm-time model predictions with geomagnetic latitude at specified hours during daytime on March 15– 17, 1960.
12
UT (hours)
Huancayo
3. Discussion and conclusion
foF2 (MHz)
20 15 10 5 0 0
0
12
La Paz 20 15 10 5 0 0
This paper presents a comparison between foF2 values from a chain of ionospheric stations located in the region of the equatorial anomaly (longitude range: 278°–295°) and the response of the empirical storm-time correction model (STORM) in IRI-2000 during geomagnetic storm conditions. Although the STORM model is not suitable for low latitudes (e.g., Araujo-Pradere and Fuller-Rowell, 2003) since its development is based on foF2 data from mid-latitude ionosondes, our results show that the model follows to some extent the variation of the experimental values. At equatorial and subequatorial latitudes the greatest disagreement is observed during the main phase and early
12
UT (hours)
foF2 (MHz)
Fig. 6 presents the latitudinal variation of foF2 measurements and outputs of IRI-2000 with STORM model at 12 LT and 14 LT on April 17 and 18. It can be seen that during the first stage of the main phase the equatorial anomaly structure is not affected. It is known that as a rule, the EA is ‘‘smoothed’’ (less contrast between the valley and the crests) during geomagnetic storms. In this study case, it is even ‘‘reversed’’ on April 18: the electron density at the magnetic equator is higher than at the crest. The STORM model cannot represent these stormtime variation patterns because it was build with mid-latitude data.
12
0
12
0
12
UT (hours) Fig. 4. Temporal variation of Dst geomagnetic index (upper panel), foF2 data (solid circles) and output of the IRI-2000 model with the STORM model (thin lines) in lower panels at equatorial and low latitude stations for the April 17–19, 1965 storm period.
stage of the recovery phase. In general, there is an overestimation of the data during the main phase of the storm while during the recovery phase the IRI-2000 outputs can be greater or smaller than experimental values. It is well known that the formation of quiet equatorial F2-layer is strongly controlled by ExB drifts and many morphological features are related to zonal electric field variations. During
672
G.A. Mansilla, M. Mosert / Advances in Space Research 39 (2007) 668–673 April 17 20
50 0 -50 -100 -150 -200
foF2 (MHz)
Dst (nT)
April 17-19, 1965
0
12
0
12
0
12
15
19UT (14LT)
10
17UT (12LT)-IRI 19UT (14LT)-IRI
5 0 -10
UT (hours)
17UT (12LT)
-5
0
5
10
15
20
Geomagnetic latitude (degrees)
April 18
100
20
0 0
12
0
12
0
12
-100
UT (hours) storm ON
foF2 (MHz)
DfoF2 (%)
Bogota
15
DfoF2 (%)
19UT (14LT)-IRI
-5
0
5
10
15
20
Geomagnetic latitude (degrees)
Fig. 6. Variation of foF2 observations and storm-time model predictions with geomagnetic latitude at specified hours during daytime on April 17–18, 1965.
100 0 0
12
0
12
0
12
-100
UT (hours) storm ON
storm OFF
Huancayo DfoF2 (%)
17UT (12LT)-IRI
5
Talara
100 0 0
12
0
12
0
12
-100
UT (hours) storm ON
storm OFF
La Paz DfoF2 (%)
19UT (14 LT)
10
0 -10
storm OFF
17UT (12LT)
100
plasma drift at equatorial latitudes and so to the NmF2 decrease (negative ionospheric storm). This storm time ionospheric modifications are produced without any time delay immediately after SC. Such electric field effects are not currently considered in the model and therefore the predicted values are greater than the measured ones. Moreover, enhancements of NmF2 (positive ionospheric storms) are also known to occur at equatorial latitudes during storm time periods, which are attributed to a decrease in the eastward (during sunlit hours) electric field (e.g., Fejer, 1981, 1991). It is quite possible that the STORM model predictions underestimate the observations in these situations. This brief study shows a marginal improvement when using the STORM model in IRI for a specific low latitude station during a magnetic storm event. References
0 0
12
0
12
0
12
-100
UT (hours) storm ON
storm OFF
Fig. 5. Temporal variation of Dst (upper panel) and relative deviations for both IRI outputs (with and without the STORM model) from foF2 measurements for the April 17–19, 1965 storm period.
the initial phase or initial part of main phase of geomagnetic storms, pronounced perturbations of magnetospheric origin electric field are produced. An increase of the eastward electric field is usually observed during severe magnetic storms, which leads to an increase of the upward
Anderson, D.N. A theoretical study of the ionospheric F-region equatorial anomaly, II, Results in the American and Asian sectors. Planet. Space Sci. 21, 421, 1973. Anderson, D.N., Mendillo, M., Hertniter, B. A semiempirical low latitude ionospheric model. Radio Sci. 21, 421, 1987. Araujo-Pradere, E.A., Fuller-Rowell, T.J., Codrescu, M.V. STORM: an empirical storm-time ionospheric correction model, 1, Model description. Radio Sci. 37, 1070, 2002a. Araujo-Pradere, E.A., Fuller-Rowell, T.J., Codrescu, M.V. STORM: an empirical storm-time ionospheric correction model, 2, validation. Radio Sci. 37, 1071, 2002b. Araujo-Pradere, E.A., Fuller-Rowell, T.J. Validation of the STORM response in IRI2000. J. Geophys. Res. 108, 1120, 2003. Araujo-Pradere, E.A., Fuller-Rowell, T.J., Codrescu, M.V., Anghel, A. Evaluation and prospects for storm-time corrections in the International Reference Ionosphere. Adv. Space Res. 33, 902, 2004.
G.A. Mansilla, M. Mosert / Advances in Space Research 39 (2007) 668–673 Barghausen, A.L., Finney, J.W., Proctor, L.L., Schultz, L.D. Predicting long-term operational parameters of high frequency sky-wave telecommunication systems, ESSA Tech. Rep., ERL 110-ITS 78, 1969. Bent, R.B., Llewellyn, S.K., Nesterczuk, G., Schmid, P.E. The development of highly successful worldwide empirical ionospheric model its use in certain areas of space communications and worldwide total electron content investigations, in: Goodman, J. (Ed.), Effect of the Ionosphere on Space Systems and Communications. Springfield, pp. 13–28, 1976. Bilitza, D. International reference ionosphere: recent developments. Radio Sci. 21, 343, 1986. Bilitza, D., International Reference Ionosphere, Rep. NSSDC/WDC-R&S 90-22, Natl. Space Sci. Data Cent./World Data for Rockets and Satellites, Greenbelt, MD, 1990. Bilitza, D. International reference ionosphere 2000. Radio Sci. 36, 261, 2001.
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Fejer, B.G. The equatorial ionospheric electric fields. A review. J. Atmos. Terr. Phys. 43, 377, 1981. Fejer, B.G. Low latitude electrodynamic plasma drifts: a review. J. Atmos. Terr. Phys. 53, 677, 1991. Fuller-Rowell, T.J., Araujo-Pradere, E.A., Codrescu, M.V. An empirical ionospheric storm-time correction model. Adv. Space Res. 25, 139, 2000. Fuller-Rowell, T.J., Codrescu, M.V., Araujo-Pradere, E.A. Capturing the storm-time ionospheric response in an empirical model. Space Weather AGU Geophys. Monogr. 125, 393–401, 2001. Llewellyn, S.K., Bent, R.B. Documentation and description of the Bent ionospheric model, AFCRL Tech. Rep., AFCRL-TR-73-0657, 1973. Mansilla, G.A., Mosert, M., Ezquer, R., Miro Amarante, G. Variability of ionospheric parameters during geomagnetic storms at a middle latitude station: comparisons with IRI model. Adv. Space Res. 34, 1907, 2004.
Two cases of ionospheric storms at the equatorial anomaly G.A. Mansilla a
a,*
, M. Mosert
b
Laboratorio de Ionosfera, Departamento de Fı´sica, Universidad Nacional de Tucuma´n – CONICET, Av. Independencia 1800, 4000 San Miguel de Tucuma´n, Argentina b CASLEO, Av. Espan˜a 1512 (sur), CP 5400, San Juan, Argentina Received 15 December 2005; received in revised form 26 June 2006; accepted 24 October 2006
Abstract This paper presents an evaluation of the IRI-2000 model, which includes the storm-time correction (STORM model), to predict foF2 during two intense geomagnetic storms (peak Dst < 100 nT) in the equatorial anomaly region. To this end, both IRI model outputs (with the STORM model included and without the STORM model) are compared with ionosonde data from a chain of stations located in the 278°–295° longitudinal range. The results show that the STORM model, even though it is not designed for low latitudes in general captures the variations of the measured values. But there is an overestimation of the foF2 data during the main phase and first stage of the recovery of the storms. Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Geomagnetic storm; STORM model; Equatorial anomaly
1. Introduction The knowledge of the critical frequency of the F2-layer, foF2 (proportional to the square root of the peak electronic density of F2-layer, NmF2) is essential for the prediction of ionospheric behaviour. Several empirical and semi-empirical models (e.g., Barghausen et al., 1969; Anderson, 1973; Llewellyn and Bent, 1973; Bent et al., 1976; Anderson et al., 1987) have been developed to predict ionospheric parameters during quiet conditions. One of the most widely empirical models used to describe the ionosphere during magnetically quiet conditions is the International Reference Ionosphere, IRI (Bilitza, 1986, 1990, 2001) which provides median values of electron density, electron temperature, and ion composition as a function of height for a given location, time and sunspot number. This model is being continuously revised and updated through an international cooperative effort sponsored by the Committee on Space Research and the International Union of Radio Science. *
Corresponding author. E-mail addresses: [email protected] (G.A. Mansilla), [email protected] (M. Mosert).
The latest version of the IRI contains the empirical storm-time ionospheric correction (STORM model) designed by Fuller-Rowell et al. (2000, 2001). It dependents on the intensity of the storm, and is a function of latitude and season (Araujo-Pradere et al., 2002a,b). A number of recent studies have compared the IRISTORM predictions with ionosonde data at mid-latitudes during storm time periods (Araujo-Pradere and FullerRowell, 2003; Araujo-Pradere et al., 2004; Mansilla et al., 2004). In general, the results indicate a significant improvement of IRI-2000 with the STORM model over IRI-2000 without the STORM model. In this paper, the performance of IRI-2000 with the correction of the STORM model included is evaluated in the equatorial anomaly (EA) region during two intense geomagnetic storms (peak Dst < 100 nT) by comparing ionosonde data with predictions obtained from the IRI model. The database used in this analysis, provided by the Space Physics Interactive Data Resource (SPIDR), are hourly values of foF2 from a chain of stations located in the 278°–295° longitudinal sector. The stations and their locations are listed in Table 1.
0273-1177/$30 Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2006.10.022
G.A. Mansilla, M. Mosert / Advances in Space Research 39 (2007) 668–673
669
Table 1 Station list Station
Geographic latitude
Geographic longitude
Geomagnetic latitude
Geomagnetic longitude
Bogota Talara Huancayo La Paz Tucuma´n
4.5 4.6 12.0 16.5 26.9
285.8E 278.6E 284.6E 292.8E 294.6E
15.9 6.6 0.6 5.0 15.5
4.9W 11.9W 5.7W 1.3E 3.8E
The sudden commencements (SC) of the selected geomagnetic storms occurred at 0818 UT on March 15, 1960 (storm 1) and at 1312 UT on April 17, 1965 (storm 2). Storm 1 occurred during descending phase of solar cycle and storm 2 under low solar activity. These events were selected because no recent simultaneous measurements from an American chain located at equatorial and low latitudes are available. Although some stations present data gaps, nevertheless they are useful to investigate how well the IRI-STORM model is able to represent foF2 observations in the EA region. 2. Results The variation of the Dst geomagnetic index during the March 15–17 1960 storm period (storm 1) is shown at the top of Fig. 1. The SC occurred in predawn hours in this longitudinal sector. The main phase continued for about 24 h until around 07–08 UT (02–03 LT) on March 16 followed by a long-duration recovery. The lower part of Fig. 1 shows the diurnal behaviour of foF2 over equatorial and low latitude ionosonde stations during the storm period (solid circles) and superposed the prediction of IRI-2000 with the STORM model included (solid line). There are no data for Bogota´. It can be seen that the STORM model overestimates foF2 a few hours after SC at all the stations in the daytime hours on March 15 (15–20 UT). During the recovery phase predicted values capture the variation of measured values mainly at Talara (geomagnetic latitude = 6.6°) and Huancayo (geomagnetic latitude = 0.6°) while at La Paz (geomagnetic latitude = 5.0°) and Tucuma´n (geomagnetic latitude = 15.5°) the STORM model significantly underestimates the observations. To quantify the improvement of IRI-2000 with the STORM model included over IRI-2000 without the STORM model, the differences for both the IRI predictions from the measured foF2 values, in percentage, were calculated. These differences are given by: DfoF2 ð%Þ ¼ ½ðfoF2 foF2Þ=foF2 100 where foF2* is the modelled critical frequency (with the STORM model ‘‘turn on’’ or ‘‘turn off’’) and foF2 is the corresponding measured hourly critical frequency. Fig. 2 shows these relative differences for the storm on 15–17 March 1960. It can be seen that no appreciable improvement is obtained by using the STORM model in IRI because the STORM model introduces only a very
small change. The initial (09–12 UT on March 15) overestimation is between 60% and 90%. During the recovery phase the relative differences are in general lower than 30%. The strong disagreement among predicted and measured frequencies observed at Tucuma´n during this stage of the storm is possibly because this station is located below the south crest of the equatorial anomaly where local effects are produced. In order to illustrate the actual and modelled equatorial anomaly structure Fig. 3 shows the latitudinal variation of foF2 data and STORM-model predictions at specified times during daytime on March 15, 16 and 17. It can be seen that the equatorial anomaly arch is present during the storm period. The model predictions follow the variation of the foF2 data. As was already mentioned, IRI prediction with the STORM model overestimate foF2 measurements during the main phase, but there is evidently good agreement during the recovery phase of the storm mainly at equatorial latitudes. The upper panel of Fig. 4 presents the variation of Dst for the period April 17–19, 1965 (storm 2). The SC occurred during the sunrise hours in the longitude sector under consideration. The main phase remained until around 09–10 UT on April 18 in the predawn hours followed by a slow recovery. The time variations of foF2 data and IRI predictions with the STORM model at equatorial and low latitudes are also shown. No data were available for Tucuma´n. In general the STORM model predictions follow the variation of the data in the daytime hours during the initial stage of the storm. At Bogota (geomagnetic latitude = 15.9°) and Talara (geomagnetic latitude = 6.6°) there is a small overestimation of the data (the trend of available data at La Paz shows also an overestimation) whereas at Huancayo (geomagnetic latitude = 0.6°) the overestimation is greater. During the recovery phase, at Bogota and Talara the predictions of the model are closer to the observations than at Huancayo and La Paz, where the STORM model significantly underestimates the foF2 data. Fig. 5 presents the relative differences between the IRI2000 outputs (with and without the STORM model) and foF2 measurements. In general, there is not a considerable difference between the IRI-2000 predictions with and without STORM model. An improvement can be seen with the storm-time ionospheric correction model at Huancayo (smaller relative differences) during the end of the main phase and early stage of the recovery. During the main
670
G.A. Mansilla, M. Mosert / Advances in Space Research 39 (2007) 668–673
March 15-17, 1960
March 15-17, 1960 50
0
Dst [nT]
Dst [nT]
50 -50 -100 -150
0 -50 -100 -150 -200
-200 0
12
0
12
0
0
12
12
0
UT [hours]
0
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Talara
Talara 20
DfoF2 (%)
foF2 (MHz)
12
UT [hours]
15 10 5
100 0 0
12
0
12
0
12
-100 -200
0 0
12
0
12
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UT [hours]
12
UT [hours]
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Huancayo
storm OFF
Huancayo DfoF2 (%)
foF2 (MHz)
20 15 10 5 0
100 0 0
12
0
12
0
12
0
12
12
UT [hours]
UT [hours]
storm ON
La Paz
storm OFF
La Paz DfoF2 (%)
20
foF2 (MHz)
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-200 0
15 10 5
100 0 0
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0
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0
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0 0
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UT [hours]
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Tucuman
storm OFF
Tucuman DfoF2 (%)
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foF2 (MHz)
12
-100
15 10 5
100 0 -100
0
12
0
12
0
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-200
0 0
12
0
12
0
12
UT [hours]
UT [hours] storm ON
storm OFF
Fig. 1. Temporal variation of Dst geomagnetic index (upper panel), foF2 data (solid circles) and output of the IRI-2000 model with the STORM model (thin lines) in lower panels at equatorial and low latitude stations for the March 15–17, 1960 storm period.
Fig. 2. Temporal variation of Dst (upper panel) and relative deviations for both IRI outputs (with and without the STORM model) from foF2 measurements for the March 15–17, 1960 storm period.
phase the relative differences are in general lower than 50% at all the stations including Huancayo, which overall shows the greatest disagreement with the model. During recovery
phase the underestimation of the model does not exceed 50% and the overestimation at Bogota´ is also smaller than 50%.
G.A. Mansilla, M. Mosert / Advances in Space Research 39 (2007) 668–673 March 15
April 17-19, 1965 17UT (12LT)
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Bogota 17UT (12LT)
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Talara
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Fig. 3. Variation of foF2 observations and storm-time model predictions with geomagnetic latitude at specified hours during daytime on March 15– 17, 1960.
12
UT (hours)
Huancayo
3. Discussion and conclusion
foF2 (MHz)
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La Paz 20 15 10 5 0 0
This paper presents a comparison between foF2 values from a chain of ionospheric stations located in the region of the equatorial anomaly (longitude range: 278°–295°) and the response of the empirical storm-time correction model (STORM) in IRI-2000 during geomagnetic storm conditions. Although the STORM model is not suitable for low latitudes (e.g., Araujo-Pradere and Fuller-Rowell, 2003) since its development is based on foF2 data from mid-latitude ionosondes, our results show that the model follows to some extent the variation of the experimental values. At equatorial and subequatorial latitudes the greatest disagreement is observed during the main phase and early
12
UT (hours)
foF2 (MHz)
Fig. 6 presents the latitudinal variation of foF2 measurements and outputs of IRI-2000 with STORM model at 12 LT and 14 LT on April 17 and 18. It can be seen that during the first stage of the main phase the equatorial anomaly structure is not affected. It is known that as a rule, the EA is ‘‘smoothed’’ (less contrast between the valley and the crests) during geomagnetic storms. In this study case, it is even ‘‘reversed’’ on April 18: the electron density at the magnetic equator is higher than at the crest. The STORM model cannot represent these stormtime variation patterns because it was build with mid-latitude data.
12
0
12
0
12
UT (hours) Fig. 4. Temporal variation of Dst geomagnetic index (upper panel), foF2 data (solid circles) and output of the IRI-2000 model with the STORM model (thin lines) in lower panels at equatorial and low latitude stations for the April 17–19, 1965 storm period.
stage of the recovery phase. In general, there is an overestimation of the data during the main phase of the storm while during the recovery phase the IRI-2000 outputs can be greater or smaller than experimental values. It is well known that the formation of quiet equatorial F2-layer is strongly controlled by ExB drifts and many morphological features are related to zonal electric field variations. During
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G.A. Mansilla, M. Mosert / Advances in Space Research 39 (2007) 668–673 April 17 20
50 0 -50 -100 -150 -200
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April 18
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Bogota
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DfoF2 (%)
19UT (14LT)-IRI
-5
0
5
10
15
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Fig. 6. Variation of foF2 observations and storm-time model predictions with geomagnetic latitude at specified hours during daytime on April 17–18, 1965.
100 0 0
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plasma drift at equatorial latitudes and so to the NmF2 decrease (negative ionospheric storm). This storm time ionospheric modifications are produced without any time delay immediately after SC. Such electric field effects are not currently considered in the model and therefore the predicted values are greater than the measured ones. Moreover, enhancements of NmF2 (positive ionospheric storms) are also known to occur at equatorial latitudes during storm time periods, which are attributed to a decrease in the eastward (during sunlit hours) electric field (e.g., Fejer, 1981, 1991). It is quite possible that the STORM model predictions underestimate the observations in these situations. This brief study shows a marginal improvement when using the STORM model in IRI for a specific low latitude station during a magnetic storm event. References
0 0
12
0
12
0
12
-100
UT (hours) storm ON
storm OFF
Fig. 5. Temporal variation of Dst (upper panel) and relative deviations for both IRI outputs (with and without the STORM model) from foF2 measurements for the April 17–19, 1965 storm period.
the initial phase or initial part of main phase of geomagnetic storms, pronounced perturbations of magnetospheric origin electric field are produced. An increase of the eastward electric field is usually observed during severe magnetic storms, which leads to an increase of the upward
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