Equatorial Ionization Anomaly: The Role of ThermosphericWinds and the Effects of the Geomagnetic Field SecularVariation
Inez S. Batista, Erica M. Diogo, Jonas R. Souza, Mangalathayil Ali Abdu, and GrahamJ. Bailey
Abstract The vertical plasma drift is the well knowndriver of the equatorial ionization anomaly (EIA). Thelatitudinal distribution of ionization in the EIA is deter-mined also by thermospheric meridional wind whoseprecise role can only be evaluated through the use oftheoretical models because it depends not only uponthe local configuration of the wind, but is a complexfunction of its distribution along the entire magneticfield line. Besides, in the Brazilian region, the magneticfield secular variations are fast enough for their effectson the ionosphere to be observed in the time span of theorder of a solar cycle. In this work we use the SheffieldUniversity Plasmasphere-Ionosphere Model (SUPIM)to investigate the role of the vertical plasma drift, ther-mospheric meridional wind and of the magnetic fieldsecular variations in the changing trend of the EIA overthe Brazilian region.
23.1 Introduction
The equatorial ionization anomaly (EIA) is one of theimportant features of the equatorial and low-latitudeionosphere. It is generated by the well known fountaineffect, produced by the upward vertical ExB/B2 plasmadrift that elevates the F-region ionosphere plasma tohigher altitudes over the magnetic equator, followed bydiffusion along the geomagnetic field lines, that movesthe plasma down and away from the equator, forming
I.S. Batista (�)National Institute for Space Research, São José dos Campos,SP, Brazile-mail: [email protected]
ionization peaks/crests in the subtropics on both sidesof the magnetic equator and an ionization trough overthe dip equator. This configuration can be significantlymodified by the thermospheric meridional wind. Thelatitudinal plasma distribution that characterizes theEIA (a through at the magnetic equator and two crestsat approximately ±15◦ latitude) is well reproduced bymany theoretical models (Hanson and Moffett, 1966;Anderson, 1973a, b; Balan and Bailey, 1996; Baileyand Balan, 1996; Bittencourt et al., 2007).
In this work we use the Sheffield UniversityPlasmasphere-Ionosphere Model (SUPIM) (Bailey andSelek, 1990; Bailey et al., 1997) to investigate the EIAresponse to changes in zonal electric fields and ther-mospheric winds, and also to investigate the long termtrends in the EIA over the Brazilian region in termsof how it is connected to the rather rapid secular vari-ation of the geomagnetic field in the Brazilian sectoras manifested in the peculiar northwestward move-ment of the geomagnetic equator over northeast Brazil(Rangarajan and Muniz Barreto, 2000).
The Sheffield University Plasmasphere IonosphereModel (Bailey et al., 1993; Bailey and Balan, 1996)solves the coupled time-dependent equations of thecontinuity, momentum, and energy balance along thegeomagnetic field lines and calculates the density,momentum flux and temperature for electrons andfor the ions O+, He+, N2
+, O2+, NO+ and N+ (see
also Souza et al., 2000a, b) for applications of themodel to low latitudes over Brazil). In the presentversion of the SUPIM, the geomagnetic field is rep-resented by a tilted centered dipole with the angleof tilt and the magnetic declination angle given bythe International Geomagnetic Reference Field (IGRF)model (Maus et al., 2005). The solar flux values usedas input to the SUPIM are derived from the Extreme
Ultra Violet flux model for Aeronomic Calculations(EUVAC) (Richards et al., 1994) that gives the EUVflux at 37 wavelength groups between 50 and 1050 Å.The flux for the X-rays and Lyman-α (1216 Å) weretaken from the model SOLAR2000 (Tobiska et al.,2000). The neutral atmosphere parameters were takenfrom the model NRLMSISE-00 (Picone et al., 2002)that gives density and temperature for the neutral con-stituents He, O, N2, O2, Ar, H, and N. The numberdensity for the neutral constituent NO was calculatedusing the empirical expression given by Mitra (1968)
n(NO) = 4 × 10−1 exp
(−3700
Tn
)n(O2)
+5 × 10−7n(O).
The neutral wind used as input in the SUPIM modelis given by the empirical model HWM93 (HorizontalWind Model) (Hedin et al., 1995) and the vertical driftat the equatorial F-region is given by the empiricalmodel from Scherliess and Fejer (1999).
23.2 Effect of the Vertical Drift on theEIA Development
The vertical ExB plasma drift over the geomagneticequator is the main driver of the EIA. The drift isupward during the day and downward during the night.Before its evening reversal to downward the driftundergoes an enhancement, the pre-reversal enhance-ment (PRE) (see for example Fejer et al., 1989, 1991)that is responsible for the intensification of the EIAafter sunset and during pre-midnight hours. The degreeof development of the post sunset EIA is directlyrelated to the amplitude of the PRE. Depending uponthe amplitude of the PRE the effect on the EIAmay last for extended durations, however. In orderto quantify the effect of the PRE amplitude in theEIA development we have used two drift models,shown in Fig. 23.1, as inputs to the SUPIM model.The main difference between the two drift models isobserved around sunset and at nighttime. The pre-reversal enhancement amplitude in drift model D1 ismuch higher than in model D2. Also the enhance-ment starts much earlier and the reversal occurs laterin model D1 than in model D2. Those differences in
Fig. 23.1 Vertical drift models used as input in the simulations.Model D1 is representative of high solar activity and Model D2is representative of low solar activity
the PRE behavior are reflected in the EIA developmentas shown in Fig. 23.2.
Figure 23.2 shows the latitude versus height dis-tribution of the F layer plasma frequencies at 2000LT (local time) for the September equinox of highsolar activity (left panels) and low solar activity(right panels). All the results were obtained using theSUPIM model run for the Brazilian region (45◦W).Figure 23.2a, c were obtained using the drift modelD1 as input to the SUPIM and Fig. 23.2b, d wereobtained using drift model D2 (note that the plots are ingeographic latitude). At the 45◦W longitude the mag-netic equator is now located at ∼3◦S. The results forboth high and low solar activity (HSA and LSA) peri-ods using the drift model D1 (Fig. 23.2a, c) show awell developed EIA with clear and pronounced den-sity peaks to the north and south of the equator andthe trough close to the magnetic equator. The peaksare formed at similar latitudes during HSA and LSA.The lower intensity of the EIA crest/peak densitiesduring the LSA period is due to the low backgroundionization density that results from lower solar radi-ation intensity. Comparing the latitudinal distributionof the F-layer plasma frequencies obtained from theSUPIM using drift model D1 and D2 it can be noticedthat the reduction of the pre-reversal enhancement inmodel D2 by a factor of approximately 4 times (in rela-tion to the PRE amplitude of model D1) resulted in areduction by ∼25 and ∼35% in the average (of the
23 Equatorial Ionization Anomaly 319
Fig. 23.2 Contour maps of the F layer plasma frequency as afunction of geographic latitude and altitude. The results are forthe 45◦W meridian, September equinox, at 2000 LT. Panels (a)and (b) are the results for high solar activity and panels (c) and
(d) are for low solar activity. Note that the drift model D1 wasused as input for results in panels (a) and (c) and that the driftmodel D2 was used for results in panels (b) and (d)
northern and southern crests) peak electron density1
for HSA and LSA, respectively. Correspondingly theposition of the anomaly crest is shifted equatorwardsby around 6◦, the crest-to-trough ratio is significantlyreduced and the vertical extension of the equatorialanomaly is reduced by approximately 200 km.
23.3 Effect of the Meridional Wind in theEIA Development
The thermospheric neutral wind has a very importanteffect in the final configuration of the EIA. The initiallatitudinal distribution is modified by the neutral-ioncollisional drag that transports the ionization along themagnetic field lines at the same velocity as that of the
1 The relation between peak electron density and frequency ingiven by
N(cm−3) = 1.24 × 104(f (MHz))2
wind component along it. This plasma movement willresult in inter-hemispheric transport of ionization. Alsothe ionization moves upwards in the upwind hemi-sphere and downwards in the downwind hemisphereof the magnetic field line. This upward (downward)movement will bring ionization to regions of lower(higher) atmospheric density and recombination rates.As a result the EIA will present asymmetric crests withrespect to the magnetic dip equator with unequal val-ues of electron densities and heights for the F-regionpeak. In order to evaluate the effect of the meridionalwind in the equatorial anomaly development we haverun the SUPIM model for September equinox condi-tions of HSA and LSA. The vertical drift models D1and D2 (Fig. 23.1) were used as input for the SUPIMruns for HSA and LSA respectively. The results of theplasma frequencies outputs from the SUPIM runs at45◦W longitude for the input winds from HWM areshown for different local times in Figs. 23.4 and 23.6.It is important to stress that, in a region with highmagnetic declination as in the present case (magneticdeclination ∼20◦W), the effective meridional wind,
320 I.S. Batista et al.
which is the same as the magnetic meridional windalong the magnetic field line, has contributions of boththe meridional and the zonal components of the wind.The effective winds, presented in Figs. 23.3 and 23.5were calculated using the expression
Ueff = (Uφ cos D + Uθ sin D) cos I
where Uφ is the meridional component of the wind(positive northward), Uθ is the zonal component of thewind (positive eastward), D is the magnetic declinationangle and I is the dip angle. According to the aboveexpression, it can be seen that Ueff is the projection ofthe magnetic meridional wind along the magnetic fieldline.
Fig. 23.3 Contour maps of effective meridional wind as a function of geographic latitude and altitude at different local times. Theresults are for the 45◦W meridian, September equinox, and high solar activity. Positive winds are northward
23 Equatorial Ionization Anomaly 321
For the HSA period (Fig. 23.4) we can observethat the EIA is well developed during all day hoursand that it is intensified after sunset due to the ver-tical drift intensification (pre-reversal enhancement).For the LSA period (Fig. 23.6) no post-sunset enhance-ment in EIA is observed due to the much less sig-nificant prereversal enhancement in the vertical drift.In fact, during the LSA the anomaly intensity steadilydecreases after 1500 LT.
The 1200 LT panel on Fig. 23.4 shows an almostsymmetrical development of the EIA. At the samelocal time the effective meridional wind from Fig. 23.3is northward in the northern hemisphere and southward(but weak) in the southern hemisphere (diverging windat the equator) which results in a downward move-ment of the ionization (along the magnetic field line)in both hemispheres. The much higher wind inten-sity at the northern hemisphere will imply in a more
Fig. 23.4 Contour maps of F layer plasma frequency as a function of geographic latitude and altitude at different local times. Theresults are for the 45◦W meridian, September equinox, and high solar activity. Drift model D1 was used as input
322 I.S. Batista et al.
effective downward movement of ionization in thathemisphere, and a consequent asymmetry in the twocrests of the EIA. This effect will only be seen approx-imately 3 h later (as seen in the 1500 LT plot) whenthe southern crest is more intense (higher density) andalso at slightly higher altitude than the northern crest.At 1700 LT the wind pattern is again divergent atthe equator, but now the southern hemisphere compo-nent is a bit more intense than the component at thenorthern hemisphere. Approximately 2 h later (as seenin the 1900 LT plot) the relative behavior observedbefore for the crests will reverse, with the northerncrest more intense and at higher altitudes than thesouthern crest. The persistent southward wind that isobserved later is the responsible for the strengtheningof the asymmetry observed at later times (probablydue to the recombination effect under the relativelymore persistent southward wind, and hence the down-ward plasma transport, in the southern hemisphere).The EIA response to the wind during LSA (Figs. 23.5and 23.6) is quite similar to that during the HSA.The asymmetrically divergent wind (stronger at onehemisphere) will affect the latitudinal distribution ofionization approximately 2–3 h later.
In order to evaluate the effect of the wind on theionization distribution and in producing asymmetricEIA crests it is necessary to investigate its behav-ior along the entire field line. Figure 23.7 shows thenet wind integrated between 33◦N and 33◦S latitude(in arbitrary units) as a function of local time. Inthe same figure we show the asymmetry of the EIAmeasured as the relative difference in peak electrondensity and the layer peak height at the anomaly crests.The density asymmetry is calculated as the differ-ence between the peak electron density in the northernhemisphere crest (NmF2NH) and the peak electrondensity in the southern hemisphere crest (NmF2SH),divided by (NmF2NH) that is, Density Asymmetry =(NmF2NH - NmF2SH)/NmF2NH. The height asymme-try is the difference between the peak electron densityheight in the northern hemisphere crest (hmF2NH) andthat at the southern hemisphere crest (hmF2SH), thatis, Height Asymmetry = (hmF2NH - hmF2SH). As dis-cussed before, a northward wind will push the F layerdown in the northern hemisphere and up in the south-ern hemisphere causing negative height asymmetry.This is clearly seen in Fig. 23.7 which shows that theheight asymmetry parameter has opposite phase to thenet integrated wind. The response of the F layer heightto the meridional wind is very prompt as can be seen
from Fig. 23.7, in which the zeros at the two param-eters are very close in time. On the other hand, thedensity changes in response to the meridional windare seen after 2–3 h, as already pointed out in theprevious discussion. From Fig. 23.7 we can see thatthe afternoon reversal of the net integrated wind frompositive (northward) to negative (southward) occurs at1600 LT, but reversal on the peak density asymmetry isobserved only 2 h later (1800 LT). The maximum neg-ative (southward) integrated wind occurs at 2100 LTwhile the maximum density asymmetry occurs aroundmidnight. Although most of the time a net transequa-torial wind (net positive or negative integrated wind)is present, its effect on the electron density latitudi-nal distribution is not straightforward as the heightresponse. The density asymmetry response presents atime delay which could be caused by the plasma diffu-sion along the field line that plays an important role inestablishing a new peak density (foF2) in response toa change in the wind. On the other hand the plasmatransport by meridional/trans-equatorial wind occursthrough (height dependent) collisional effect and there-fore the F layer height responds promptly to a changein the wind intensity. Thus the asymmetry in foF2responds to a change in the meridional/trans-equatorialwind with a time delay of the order of 2–3 h as seenin Fig. 23.7 which is comparable with the observa-tional results over Fortaleza for F layer peak densityresponse to changes in zonal electric field as reportedearlier by Abdu et al. (1990). On the other hand thehmF2 presents a prompt/fast response to a change inthe meridional/trans-equatorial wind clearly seen inthe results of Fig. 23.7. Despite of all the aspects dis-cussed above, the height asymmetry appears to havea significant effect in the development of the den-sity asymmetry (with some time delay that dependson the recombination time constant), at least under theequinox wind configuration used in the present simula-tion, in which a net positive (negative) integrated winddoes not necessarily means that a transequatorial windis present at the magnetic equator.
23.4 Secular Variation of theGeomagnetic Field and Its Effecton the EIA
Over the Brazilian region magnetic field secular vari-ations are fast enough so that their ionospheric sig-natures can be observed in the time interval of one
23 Equatorial Ionization Anomaly 323
Fig. 23.5 Contour maps of effective meridional wind as a function of geographic latitude and altitude. The results are for the 45◦Wmeridian, September equinox, and low solar activity. Positive winds are northward
or more solar cycles (Abdu et al., 1996). Accordingto the International Geomagnetic Reference Field(IGRF) Model (Maus et al., 2005) (the online versioncan be found at http://ngdc.noaa.gov/geomagmodels/struts/calcGridIGRF) the magnetic inclination overnortheast Brazil varies at a rate of 20’ per year, corre-sponding to an apparent northwestward movement ofthe magnetic equator. This peculiarity of the magneticequator over Brazil allowed studying long term trendsof the sporadic E (Es) layer behavior (Abdu et al.,
1996) and of the F3 layer occurrence (Batista et al.,2002) over the Brazilian station Fortaleza (2.8 to 38).In this work we have used the SUPIM model in orderto investigate to what extent this movement of the geo-magnetic equator affects the EIA position over theBrazilian region.
Figures. 23.8 and 23.9 show the distribution of Fregion plasma frequency as a function of geographiclatitude and altitude at 1800 LT for the Septemberequinox of high and low solar activity, respectively,
324 I.S. Batista et al.
Fig. 23.6 Contour maps of F layer plasma frequency as a function of geographic latitude and altitude. The results are for the 45◦Wmeridian, September equinox, and low solar activity. Drift model D2 was used as input
at 45◦W longitude. Results in Fig. 23.8 (HSA) wereobtained using the drift model D1 as input for theSUPIM and those in Fig. 23.9 (LSA) were obtainedusing drift model D2.
Results in Fig. 23.8a–c were obtained running theSUPIM with IGRF inputs for the high solar activ-ity years 1978, 1989 and 2002, respectively. In orderto ensure that the changes observed from 1 year tothe other were due to the secular variation in the
geomagnetic field alone, all the other input parameters(such as vertical drift, solar flux, neutral wind, mag-netic activity, etc.) were kept unchanged from one runto the other. Figure 23.9a–c are similar to Fig. 23.8a–cbut for the low solar activity years of 1986, 1996 and2006, respectively.
As we can see from Fig. 23.8, at the 45◦W longi-tude the crests of the EIA are moving northward (forreference vertical lines are drawn at the latitudes
23 Equatorial Ionization Anomaly 325
Fig. 23.7 Net wind integrated along the magnetic meridian(upper panel). EIA crests density asymmetry calculated as(NmF2NH – NmF2SH)/NmF2NH (middle) and height asymmetrycalculated as (hmF2NH – hmF2SH) (lower panel)
22.5◦S and 8◦N). In the southern hemisphere the posi-tion of the EIA crest has moved from approximately25◦S to ∼21◦S between 1978 and 2002 (Fig. 23.8a,c). Similar variation is also observed in the positionof the northern hemisphere crest, which moved from∼5.5◦N to ∼10◦N. Figure 23.9 shows the same steadymovement of the EIA crest at both hemispheres asobserved in Fig. 23.8, but for low solar activity. Fromthe results presented in Figs. 23.8 and 23.9 it was pos-sible to calculate the rate of change of the geographiclatitude of the EIA crest at the 45◦W longitude merid-ian. These results are shown in Fig. 23.10 as a linearfit to the data points corresponding to the EIA crestposition at the southern hemisphere, for high and lowsolar activity. The geographic latitude of the EIA crestvaries at a rate of 10’/year and 9.5’/year for the highand low solar activity periods, respectively. These ratesare very close to the rate of change of the dip equa-tor at the same meridian that is equal to 11.6’/year,according to IGRF results. The linear fit to the datapoints corresponding to the dip equator position (geo-graphic latitude) at the 45◦W longitude meridian, foryears 1978, 1984, 1990, 1996, 2002 and 2006, is alsoplotted in Fig. 23.10.
23.5 Summary
The effect of the vertical plasma drift and thermo-spheric wind over the equatorial ionization anomaly
was investigated using the Sheffield UniversityPlasmasphere-Ionosphere Model. Furthermore, theeffect of the magnetic field secular variations on theEIA location over the Brazilian region was also inves-tigated using the SUPIM.
The simulation results show that reducing the PREamplitude by a factor of 4 implies in a decrease by ∼25and ∼35% in the F layer peak density at the EIA crestduring high and low solar activity, respectively. Theposition of the anomaly crest is shifted equatorwardsby around 6◦, the crest-to-trough ratio is significantlyreduced and the vertical extension of the equatorialanomaly is reduced by approximately 200 km.
Thermospheric meridional wind can introduceasymmetries in height and density of the EIA crests.The response of the F layer height to the meridionalwind is very fast. A net wind along the magnetic fieldline produces an asymmetry in the EIA crest height.If the net wind is northward (southward), the south-ern hemisphere crest will be observed at higher (lower)altitude than the crest at the opposite hemisphere. Onthe other hand, the density changes in response to themeridional wind are not straightforward. They resultof a more complicated process that involves diffusionand recombination. They seem to occur with a timedelay of 2–3 h (which is similar to the response delayto electric field changes shown from ionosonde mea-surements by Abdu et al., 1990). When the F layerpeak heights at EIA crests are at different heights in theopposite hemispheres, the height dependent collisionfactors (together with the diffusion and recombina-tion processes) could cause density asymmetries at thecrests. This could be the main cause of the delayedresponse of the density asymmetry at the EIA crests.At least under the equinox wind configuration used inthe present simulation, in which a net positive (nega-tive) integrated wind does not necessarily means thata transequatorial wind is present at the magnetic equa-tor, the height asymmetry appears to have a significanteffect in the development of the density asymmetry.
There are some regions of the globe where geo-magnetic field secular variations are fast enough sothat their ionospheric signatures can be observed in thetime interval of a solar cycle. This is the case with thenortheast region over Brazil where the rate of secularvariation is the largest over the low latitude regions ofthe globe. Over the 45◦W meridian (Brazilian region)the EIA crest position undergoes a steady northwardexcursion, closely following the magnetic equator
326 I.S. Batista et al.
Fig. 23.8 Contour maps of the F layer plasma frequency as afunction of geographic latitude and altitude. The results are forthe 45◦W meridian, September equinox, high solar activity, at
1800 LT. Panels (a), (b) and (c) were obtained using IGRF for theyears 1978, 1989 and 2002, respectively. For reference verticallines are drawn at 22.5◦S and 8◦N
23 Equatorial Ionization Anomaly 327
Fig. 23.9 Contour maps of the F layer plasma frequency as afunction of geographic latitude and altitude. The results are forthe 45◦W meridian, September equinox, low solar activity, at
1800 LT. Panels (a), (b) and (c) were obtained using IGRF for theyears 1976, 1986 and 2006, respectively. For reference verticallines were drawn at 22.5◦S and 8◦N
328 I.S. Batista et al.
Fig. 23.10 Time variation of the geographic latitude of the EIAcrest at the southern hemisphere, over the 45◦W meridian highsolar activity (triangles) and low solar activity (circles). Thetime variation of the geomagnetic equator position at the samelongitude is also shown (stars)
displacement. This should be taken into account whenlong term trend studies are undertaken using data fromthis region.
Acknowledgments This work was partially supported by CNPqunder grant 301643/2009-1.
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