1Instituto Nacional de Pesquisas Espaciais, São José dos Campos, SP, Brazil ... el campo eléctrico y los vientos no son representativos de las latitudes bajas ...
Geofísica Internacional (2000), Vol., 39, Num. 1, pp. 57-64
F3 layer observations at low and equatorial latitudes in Brazil I. S. Batista,1 N. Balan,1,2 M. A. Abdu,1 J. MacDougall,3 and P. F. Barbosa Neto1 1
Instituto Nacional de Pesquisas Espaciais, São José dos Campos, SP, Brazil On leave from Department of Physics, University of Kerala, Trivandrum, India 3 University of Western Ontario, London, Canada 2
Received: November 6, 1998; accepted: January 10, 1999. RESUMEN Estudios recientes usando modelos y observaciones ionosféricas han revelado la existencia de una capa adicional en el tope de la ionosfera ecuatorial, la capa F3 . En el presente trabajo analizamos los datos de ionosondas de dos estaciones: una de baja latitud y otra ecuatorial en Brasil, durante el año 1995, un periodo de baja actividad solar, con el objeto de estudiar la capa F3 en estas latitudes. Se observa que en la estación magnética de São Luís (2.3° S, 44° W, dip angle –0.5°), la ocurrencia de la capa F3 es mucho menor que en la estación de baja latitud de Fortaleza (4° S, 38° W, dip angle –9°). Los ionogramas de Fortaleza en 1995 muestran la existencia de la capa en el 50% de los días. Dicha ocurrencia es frecuente y distintiva (75%) en verano (mes de diciembre). Los ionogramas también muestran la existencia de la capa en el 66% de los días en invierno (mes de junio) y en el 28% en los equinoccios. La capa empieza a aparecer alrededor de las 0930 hora local y dura desde unos minutos hasta varias horas. Esto significa que la altura de la capa varía desde 570 km en verano hasta 440 km en invierno, aunque día a día la altura puede variar desde 375 km hasta 775 km. El modelo ionosférico SUPIM (Sheffield University Plasmasphere-Ionosphere Model) se usó para simular las características de la capa sobre Fortaleza. El modelo explica bien la ocurrencia de la capa en verano, pero no en invierno, a menos que se modifiquen algunos parámetros clave tales como el campo eléctrico y los vientos. Esto indica que los modelos existentes para el campo eléctrico y los vientos no son representativos de las latitudes bajas brasileñas en el invierno. PALABRAS CLAVE: Ionosfera, capa F3, baja latitud.
ABSTRACT Recent studies using model calculation and ionospheric observations have revealed the existence of an additional layer in the topside equatorial ionosphere, the F3 layer. In this work we analyze ionosonde data from two low/equatorial latitude stations in Brazil, during the year 1995, a low solar activity period, in order to study the F3 layer at those locations. At the magnetic station São Luís (2.3° S, 44° W, dip angle –0.5°) the occurrence of the F3 layer is much less evident than at the low latitude station Fortaleza (4° S, 38° W, dip angle –9°). The ionograms recorded at Fortaleza in 1995 show the existence of the layer on 50% of the days, with the occurrence being more frequent (75%) and distinct in summer. The layer appears on 66% of the days in winter and 28% in equinoxes. The layer starts to appear around 0930 LT and lasts from a few minutes to several hours. Its mean height varies from 570 km in summer to 440 km in winter, although on a day to day basis it can vary from as low as 375 km to 775 km. The ionospheric model SUPIM (Sheffield University Plasmasphere-Ionosphere Model) explains well the occurrence of the layer in summer, but not in winter, unless some key input parameters, such as electric field and winds are modified. This suggests that the existing models for electric fields and winds are not representative for the Brazilian low latitude region during winter. KEY WORDS: Ionosphere, F3 Layer, low latitudes.
1. INTRODUCTION
Some recent theoretical studies concerning the equatorial plasma fountain and its effects revealed the possibility of an additional layer, above the F2 peak. It was first called G layer [Balan and Bailey, 1995], and later renamed as the F3 layer [Balan et al., 1997; Jenkins et al., 1997]. Since the layer arises from the dynamics of the F layer at low latitudes, it was found that F3 was the most appropriate nomenclature to denote the layer.
The low latitude ionosphere has been known to exhibit unique features in plasma density, plasma temperature, and plasma velocity, which arise basically from the horizontal orientation of the geomagnetic field at the geomagnetic equator. In plasma density, the ionosphere exhibits the equatorial ionization anomaly (EIA), characterized by an ionization trough at the equator and crests on either side, at around ±16° magnetic latitude [Appleton, 1946; Anderson,1981; Walker, 1981; Stening, 1992; Abdu, 1997; and Bailey et al., 1997]. The cause of the EIA is the plasma fountain [Hanson and Moffett, 1966], that is a unique feature caused by plasma velocity.
2. OBSERVATIONS OF THE F3 LAYER In September 1994 a Canadian Advanced Digital Ionosonde (CADI) was installed in Fortaleza, Brazil (4° S, 38° W, dip angle –9°). Ionograms are taken at 5-min inter57
I. S. Batista et al. vals. A DGS256 was already installed in São Luís, Brazil (2.3° S, 44° W, dip angle –0.5°), since the end of August 1994. It takes ionograms at 15-min intervals, except during campaign periods when it is programmed to operate at 7.5min intervals. In this work we analyzed one year of Fortaleza’s data, from January to December 1995, in order to study the F3 layer occurrence and characteristics over that station. São Luís data for the same period is also analyzed in this study. This is a low solar activity period, characterized by solar radio flux in the 10.7 cm (F10.7) range of 66 to 93 (in units of 10-22 watts/m2/Hz). The F 3 layer appears in the ionogram as a cusp near the high frequency end. Figure 1 shows an example obtained at Fortaleza, on January 8, 1995. The top panel shows an ionogram where no F3 layer is present. The middle panel shows the formation of the layer, where a stratification appears at the high frequency end of the ionogram. In this case top frequency of F 3 layer (f0F3) is almost equal to f0F2. In the bottom panel the layer is fully developed (f0F 3 > f0F2). The F3 layer can be detected by ground based ionosondes only when this last conditions is satisfied. Figure 2 shows some characteristics of the of the F3 layer occurrence and parameters on Fortaleza ionograms, during the year of 1995, grouped in 4 periods. In the top panel the layer occurs more than 75% of the time during the local summer period. It is also frequent (more than 65%) in the local winter period, and is less frequent in the equinoxes (less than 35% on March-May and around 20% on September-November). The layer occurs at higher altitudes in the December solstice. The mean virtual height of the F3 layer (h’F3) varies from 567 km in the December solstice, to 440 km in the June solstice, passing by an intermediate value (450 km) on the equinoxes. The altitude difference between the F3 and F2 layers (h’F3 – h’F2) varies from 172 km in December to 86 km in June, with an intermediate value (120 km) on the equinoxes. However, the critical frequency of the F3 layer (foF3) exceeds that of the F2 layer (foF2) by a maximum amount in the equinoxes and in winter. This may be related to the winter anomaly and semiannual variation of the ionosphere, which provides more ionization in winter and the equinoxes than in summer. The value of foF3 exceeds that of foF2 by an yearly average of about 1.3 MHz. Figure 2 (bottom panel) shows the times of occurrence and duration of the F3 layer over Fortaleza. The lower curve indicates the mean time of appearance, the upper curve indicates the mean time where F3 was last observed, and the middle curve (dotted) indicates the mean time of strongest F3 occurrence. The layer appears earlier in the December solstice (around 0930 LT) and it lasts longer (around 3 hours). It starts around 1100 LT in the June solstice and it 58
Fig. 1. Sample of F3 layer formation over Fortaleza, Brazil.
has a shorter duration on the equinoxes (around 1 hour and 15 minutes). Figure 3 shows the day-to-day variability of the F3 layer characteristics over Fortaleza for January 1995. The daily magnetic activity index (∑ Kp) is shown as histograms,
Observations of F3 layer in Brazil Note that the F3 layer occurs preferentially during the quiet-time periods, and that its occurrence is inhibited during disturbed periods. When F3 occurs on disturbed days, such as on January 2, 3, 5, 17, and 18, the time of its beginning is delayed in relation to its occurrence on quiet days. In São Luís the occurrence of the F3 layer is much lower than over Fortaleza. On January 1995, the F3 layer is observed over São Luís on days 7, 12, 17, 20, 21, and 29. It generally occurs later than over Fortaleza, with beginning time around 1400 UT (1100 LT) and its duration rarely exceeds one hour. In March 1995 the F3 layer was observed over São Luís on the 10th (not simultaneous with Fortaleza) and on the 26th (simultaneous with Fortaleza). Both days were disturbed, with the sum of Kp equal to 25o and 30+, respectively. On June 1995 we observed an extra layer over São Luís only on the 19th, a disturbed day (sum of Kp = 32) at 1215 and at 1515 UT, but this layer is very different from that in summer, resembling a stratification of the normal F layer and not an extra high-altitude layer, as in the previous cases. 3. MODEL RESULTS REVIEW AND COMPARISON WITH OBSERVATIONS
Fig. 2. Frequency of occurrence, critical frequency, minimum virtual height, and time of occurrence of F3 layer over Fortaleza, Brazil, in the year 1995.
shaded histograms correspond to F3 layer days. The critical frequency foF3 can exceed foF2 by 0.2 MHz to 1.2 MHz. The layer virtual height can vary from 500 km to 800 km, and the height difference between h’F3 and h’F2 can vary from 70 km to 350 km. The vertical lines in Figure 3 indicate the presence of the F3 layer, and its duration, as a function of local time (LT). The bottom of each line indicates the time in LT at which the F3 layer is first seen in the ionograms, and the top of the line indicates the time of the last ionogram on which the layer is observed. The first appearance of the layer can be as early as 0930 (Jan 16) and as late as 1215 LT (Jan 17) and it can lasts from a few minutes (Jan 3 and 5) to more than 7 hours (Jan 24).
The physical mechanism, location, and latitude extent of the F3 layer were calculated by Balan et al. (1998), based on model calculations carried out for the longitude of Fortaleza (38°W) using the Sheffield University plasmasphere-ionosphere model (SUPIM) [Bailey and Balan, 1996; Bailey et al., 1997]. Coupled time-dependent equations of continuity, momentum, and energy balance for the O+, H+, He+, N2+, O2+, and NO + ions, and the electrons, were solved along closed dipole magnetic field lines between a base altitude of about 130 km in conjugate hemispheres to obtain values for the concentrations, field-aligned fluxes, and temperatures of the ions and the electrons at a discrete set of points along field lines. The important parameters for the formation of the F3 layer are the vertical E x B plasma drift velocity and the neutral wind velocity in the magnetic meridian. The drift velocity is obtained from measurements made at Jicamarca [Fejer et al., 1991], for magnetic field lines with apex altitude less than 450 km, and at Arecibo (77° W) [Fejer, 1993], scaled to the magnetic equator, for field lines with apex altitudes greater than 2000 km. The vertical E x B drift velocity models available for Fortaleza [Batista et al., 1996] are not used because they are valid for high solar activity. The neutral wind velocity in the magnetic meridian is calculated from the HWM90 model [Hedin et al., 1991]. The concentrations, and temperatures of neutral gases, solar EUV fluxes, photoelectron heating rates, and other in59
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Fig. 3. Critical frequency, virtual height, and local time duration (vertical lines) of F3 layer recorded at Fortaleza in January 1995. The histograms at the bottom give the daily magnetic activity ÂKp with the shaded histograms corresponding to the F3 layer days. The dots in the duration lines represent the local time of strongest F3 layers. The virtual height (h’F, h’F2, h’F 3) and critical frequencies (foF2, f oF3) correspond to this time.
puts used in the model calculations are described in Balan et al. (1997). 3.1 The formation of the F3 layer The F3 layer arises from daytime chemical and dynamical processes in the equatorial region. The model electron 60
density (N e) profiles, calculated using SUPIM (Balan et al., 1998), and shown in Figure 4, illustrate the physical mechanisms that lead to the formation and maintenance of the F3 layer. The layer forms during the morning-noon period when ionization dominates over loss, and there is a large upward flow of ionization due to the combined effect of the E x B drift and the neutral wind. Starting in the morning (0800 LT)
Observations of F3 layer in Brazil
Fig. 4. Model electron density profiles for the longitude of Fortaleza under conditions of strongest F3 layer.
the usual F2 layer becomes much broader than at other latitudes, due to the large production and unique dynamical effects the peak of the layer rises in altitude, faster than at other latitudes, due to the E x B drift. Until about 0930 LT there is only a single peak. By that time the peak has risen to the altitude range where both chemical and dynamical effects are important in maintaining a single peak layer structure; above this altitude range, the dynamical effects dominate over the chemical effects. Thus, while the original F2 peak rises further in altitude due to dynamical effects, another peak forms below it due to both chemical and dynamical effects. The new peak develops into the usual F2 layer and the earlier peak drifts upward and forms the F3 layer. The two layers become distinct around 1030 LT. After the two layers become distinct the peak electron density of the F3 layer (Nm F3) decreases in time mainly due to chemical loss and diffusion. Production at F 3 layer altitudes is not important. On the other hand Nm F2 increases due to production and dynamical effects. There is a period of time between 1030 and 1230 LT when NmF 3 remains larger than Nm F2. During that period both F2 and F3 can be detected by ground based ionosondes. Although N mF2 decreases after that time, the layer may exist until after sunset, as has been observed in the model and observed results.
3.2 Location of the F3 layer The F3 layer forms near the equatorial region and is centered where there is a vertically upward plasma velocity near and above the F2 peak during the morning-noon period. Otherwise the plasma will diffuse downward along geomagnetic field lines to other latitudes. The vertical velocity should be sufficiently large for Nm F3 to exceed NmF 2. The plasma velocity is determined by the resultant of the upward E x B drift velocity and the neutral wind velocity at the magnetic meridian. However, since the drift velocity during the morning-noon period is more or less the same for all seasons and is supposed to be symmetric in relation to the magnetic equator, the neutral wind should be the main factor in deciding the location and latitudinal extent of the F3 layer. Following model calculations of Balan et al. [1998], at Fortaleza (38° W), where the geomagnetic equator is located south of the geographic equator, the F3 layer is expected to be centered south of the magnetic equator at the December (summer) solstice and north at the June (winter) solstice; at the equinoxes the center is expected to be located at an intermediate latitude. This is illustrated in Figure 5, which shows 61
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Fig. 5. Latitude extent (shaded lozenges) of the modeled F3 layer at 1100 LT at the longitude of Fortaleza for December solstice, June solstice and equinox during periods of low solar activity; the thick portion of the shaded areas correspond to the latitude centers of the layer. The electron density profiles shown correspond to the latitude centers; latitude is magnetic. The mark F denotes the location of Fortaleza.
the location and latitude extent (shaded lozenges) of the F3 layer at 1100 LT at 38° W, during the solstices and equinoxes at low solar activity. The corresponding N e profiles are also shown in Figure 5. 3.3 Comparison between observations and model results The results of Jenkins et al. [1997] and Balan et al. [1998] for winter solar minimum do not predict the formation of an F3 layer over Fortaleza or São Luís. The reason is that during the winter, although the vertical E x B drift is upward during the morning and is larger than during the summer, the neutral wind blows southwards, acting to move plasma down the field line, in the opposite sense of the upward drift. Yet our observations for Fortaleza do show that the F3 layer is a very common feature at the June solstice (Figure 2), although some of the cases have characteristics very different from those observed during the summer, and could 62
certainly be considered to be a stratification of the F layer. The simulations by Jenkins et al. [1997] and Balan et al. [1998] used some inputs to the model, such as the neutral constituents concentration, the vertical plasma drift and the winds. The wind model HWM90 [Hedin et al., 1991] is used in the simulations. Model results for low/equatorial Brazilian latitudes conducted by Batista et al. [1996] and Souza [1997] showed that the HWM90 is not appropriate for this region. Souza [1997] used SUPIM in order to model the ionosphere over Fortaleza and São José dos Campos (23° S, 45° W), two low-latitude stations in Brazil, and compared the modeled results with ionosonde data. HWM90 shows a strong southward daytime wind, which inhibits the formation of the F3 layer. In Souza’s results the deduced wind for winter low solar activity is enough to generate the F3 layer. This confirms that the HWM90 is not appropriate for the region, at least during winter low solar activity. The results of Balan et al. [1998] for the equinox of low solar activity (Figure 5) do not predict the formation of
Observations of F3 layer in Brazil an F3 layer over Fortaleza in agreement with the observations.
APPLETON, E. V., 1946. Two anomalies in the ionosphere, Nature, 157, 691-693.
The occurrence of the F3 layer at Fortaleza during summer (December solstice) and equinoxes are in general agreement with what is expected from the E x B drifts and neutral winds used in the model predictions. However in winter (June solstice) when the a F 3 layer is not expected to occur, it is observed almost as frequently as in summer (Figure 2), although for shorter duration. Balan et al. [1998] showed that a combination of strong upward drift and weak poleward wind can cause the occurrence of the F3 layer during the winter at Fortaleza.
BAILEY, G. J. and N. BALAN, 1996. A low-latitude ionosphere-plasmasphere model. In: STEP Handbook edited by R. W. Schunk, pp. 173-206, Utah State University, Logan.
4. SUMMARY The occurrence of F3 layer was studied over two equatorial/low latitude locations in Brazil, for a low solar activity period. The layer was observed much more frequently over Fortaleza than over the equatorial station of São Luís. The occurrence of the layer over Fortaleza is higher on the December solstice (local summer) and on the June solstice (local winter), and is very low in the equinox. Model simulations using HWM90 do not predict the F 3 layer formation in winter months. The simulation can explain our observations by a combination of strong upward drift and weak poleward wind. Our results confirm that the HWM90 wind model is not appropriate at Brazilian equatorial/low latitudes, at least during a winter of solar minimum, in agreement with previous results from Batista et al. [1996] and Souza [1997]. Over Fortaleza the F3 layer occurs most frequently between 0830 and 1030 LT, in the rising part of vertical E x B drift, and its peak density is higher than foF2 for approximately 2 hours. Over São Luís the layer generally occurs later than over Fortaleza, starting around 1400 UT (1100 LT) and its duration is generally not greater than one hour. The F3 layer is a very common feature of the near equatorial ionosphere during summer solar minimum, and it must be taken in to account in low-latitude ionospheric models. BIBLIOGRAPHY ABDU, M. A., 1997. Major phenomena of the equatorial ionosphere-thermosphere system under disturbed conditions. J. Atmos. Sol. Terr. Phys., 59, 1505-1519. ANDERSON, D. N., 1981. Modeling the ambient low latitude F region ionosphere – A review, J. Atmos. Terr. Phys., 43, 753-762.
BAILEY, G. J., N. BALAN and Y. Z. SU, 1997. The Sheffield University plasmasphere-ionosphere model – A review, J. Atmos. Sol. Terr. Phys., 59, 1541-1552. BALAN, N. and G. J. BAILEY, 1995. Equatorial plasma fountain and its effects: Possibility of an additional layer, J. Geophys. Res., 100, 21241-21432. BALAN, N., G. J. BAILEY, M. A. ABDU, K. I. OYAMA, P. G. RICHARDS, J. MACDOUGALL and I. S. BATISTA, 1997. Equatorial plasma fountain and its effects over three locations: Evidence for an additional layer, the F3 layer, J. Geophys. Res., 102, 2047-2056. BALAN, N., I. S. BATISTA, M. A. ABDU, J. MACDOUGALL and G. J. BAILEY, 1998. Physical mechanism and statistics of occurrence of an additional layer in the equatorial ionosphere, J. Geophys. Res., 103, 29169-29181. BATISTA, I. S., R. T. de MEDEIROS, M. A. ABDU, J. R. DE SOUZA, G. J. BAILEY and E. R. DE PAULA, 1996. Equatorial ionospheric vertical plasma drift model over the Brazilian region, J. Geophys. Res., 101, 1088710892. BLANC, M. and A. D. RICHMOND, 1980. The ionospheric disturbance dynamo, J. Geophys. Res., 85, 1669-1686. FEJER, B.G., 1993. F region plasma drifts over Arecibo: Solar cycle, seasonal and magnetic activity effects, J. Geophys. Res., 98, 13645-13652. FEJER, B. G., E. R. de PAULA, S. A. GONZALEZ and R. F. WOODMAN, 1991. Average vertical and zonal F region plasma drifts over Jicamarca, J. Geophys. Res., 96, 13901-13906. HANSON, W. B. and R. J. MOFFET, 1966. Ionization transport effects in the equatorial F region, J. Geophys. Res., 71, 5559-5572. HEDIN, A.E., M.A. BIONDI, R.G. BURNISIDE, G. HERNANDEZ, R. M. JONHNSON, T. L. KILLEEN, 63
I. S. Batista et al. C. MAZAUDIER, J. W. MERIWETHER, J. E. SALAH, R.J. SICA, R.W. SMITH, N.W. SPENCER, V. W. SPENCER, V. B. WICKWAR and T. S. VIRDI, 1991. Revised global model of thermosphere winds using satellite and ground-based observations, J. Geophys. Res., 96, 76577688. JENKINS, B., G. J. BAILEY, M. A. ABDU, I. S. BATISTA and N. BALAN, 1997. Observations and model calculations of an additional layer in the topside ionosphere above Fortaleza, Brazil, Ann. Geophys., 15, 753-759. SOUZA, J. R., 1997. Modelagem ionosférica em baixas latitudes no Brazil, Instituto Nacional de Pesquisas Espaciais (INPE), São José dos Campos, Brazil (Tese de Doutorado) STENING, R. J., 1992. Modeling the low latitude F region, J. Atmos. Terr. Phys., 54, 1387-1412. WALKER, G. O., 1981. Longitudinal structure of the F region equatorial anomaly – A review, J. Atmos. Terr. Phys., 43, 763-774. __________
I. S. Batista, 1 N. Balan, 1,2 M. A. Abdu, 1 J. MacDougall,3 and P. F. Barbosa Neto1 1
Instituto Nacional de Pesquisas Espaciais, São José dos Campos, SP, Brazil. 2 On leave from Department of Physics, University of Kerala, Trivandrum, India. 3 University of Western Ontario, London, Canada.
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