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SAMI2 model results for the quiet time low latitude ionosphere over India
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ScienceDirect Advances in Space Research xxx (2018) xxx–xxx www.elsevier.com/locate/asr
SAMI2 model results for the quite time low latitude ionosphere over India S.S. Rao a, Shweta Sharma b, R. Pandey a,⇑ a
Department of Physics, MLS University, Udaipur 313001, India 8G Pocket Ki Sheikh Sarai Phase 2, New Delhi 110017, India
b
Received 4 August 2017; received in revised form 5 January 2018; accepted 9 January 2018
Abstract Efficacy of SAMI2 model for the Indian low latitude region around 75°E longitudes has been tested for different levels of solar flux. With a slight modification of the plasma drift velocity the SAMI2 model has been successful in reproducing quiet time ionospheric low latitude features like Equatorial Ionization Anomaly. We have also showed the formation of electron hole in the topside equatorial ionosphere in the Indian sector. Simulation results show the formation of electron hole in the altitude range 800–2500 km over the magnetic equator. Indian zone results reveal marked differences with regard to the time of occurrence, seasonal appearances and strength of the electron hole vis-a-vis those reported for the American equatorial region. Ó 2018 Published by Elsevier Ltd on behalf of COSPAR.
Keywords: SAMI2 model; Electron hole; Low latitude ionosphere; Equatorial ionization anomaly
1. Introduction A new low latitude ionospheric model has been developed at the Naval Research Laboratory, USA by Huba et al. (2000b) that takes into account detailed chemistry, electrodynamics and geomagnetic field configuration. The model has been termed SAMI2 (SAMI2 is Another Model of the Ionosphere). This model is shown to explain quiet time ionospheric features at American low latitudes. A number of workers have used this model to explain quiet time as well as disturbed time ionospheric features. (e.g., Huba et al., 2000a; McDonald et al., 2008; Wang et al., 2010; Klenzing et al., 2013; Joshi et al., 2016). Using the SAMI2 model Huba et al. (2000a) reported a remarkable phenomenon, namely, an ‘electron hole’ wherein a reduction in electron density in the topside ionosphere in the ⇑ Corresponding author.
E-mail addresses: [email protected] (S.S. Rao), shweta.phy@ gmail.com (S. Sharma), [email protected] (R. Pandey).
height range of about 1500–2500 km near the magnetic equator has been reported. This reduction in electron density has been attributed to the transhemispheric O+ flows that collisionally couple to H+ and transport it to lower altitudes, and thereby reduce the electron density at high altitudes. The transhemispheric O+ flows are caused by an interhemispheric pressure anisotropy that can be generated by the neutral wind, primarily during solstice conditions. They further reported that the formation of the electron depletion has a seasonal and longitudinal dependence. The first empirical support of an electron hole in the topside equatorial ionosphere is recently given by Gallagher (2016) who computed ratios of Retarding Ion Mass Spectrometer-RIMS (Chappell et al., 1981) derived H+, He+ and O+ ion density and IRI (International Reference Ionosphere) electron density. The RIMS measurements were made onboard the Dynamic Explorer 1 satellite. Thus, the work of Gallagher (2016) is an empirical suggestion that the SAMI2 model finding of an electron
https://doi.org/10.1016/j.asr.2018.01.016 0273-1177/Ó 2018 Published by Elsevier Ltd on behalf of COSPAR.
Please cite this article in press as: Rao, S.S., et al. SAMI2 model results for the quite time low latitude ionosphere over India. Adv. Space Res. (2018), https://doi.org/10.1016/j.asr.2018.01.016
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hole represents the physical behavior of the topside equatorial ionosphere. In order to verify seasonal and longitudinal dependence of the topside electron hole we have used SAMI2 model for the Indian low latitudes around 75°E longitude. Therefore, the present work aims at simulating ionospheric variability in the Indian low latitudes during quiet solar conditions using the SAMI2 model. 2. Results and discussions As a first step of simulation we used the same model of ionospheric drift velocity and HWM93 empirical wind model that were used by Huba et al. (2000a). We used a 201 100 mesh that corresponds to 201 grid points along every field line, for each of the 100 field lines from an altitude of 90 km to 6000 km. The simulations were run for 48 h, out of which initial 24 h period take care of the transients, and the results for the rest 24 h have been used to obtain diurnally variability. The SAMI2 ion drifts are such that the ionospheric flux tubes rise during the day, from 0700 LT to 1900 LT and fall during the night from 1900 LT to 0700 LT. The daytime equatorial and low latitude ionosphere is characterized by an F region electron density trough at the geomagnetic equator and two crest within ±20° magnetic latitudes. This phenomenon has been well recognized as ‘‘Appleton anomaly” (Appleton, 1946) or equatorial ionization anomaly (EIA). This anomaly is formed due to the so called ‘‘fountain effect” and is first envisaged by Martyn (1947). In the daytime the ambient eastward electric field at the geomagnetic equator gives rise to an upward drift of the F region plasma at the magnetic equator due to the crossed electric and magnetic field (Martyn, 1953). This rising plasma reaches very high altitudes until it loses momentum whereafter the plasma diffuses downward along the geomagnetic field lines, as was first suggested by Mitra (1946). This process makes an ionization trough at the equator and crests around ±15° magnetic latitudes on both sides of the equator. (Duncan, 1960). Moffet and Hanson, 1965 have presented results of modeling of formation of the EIA due to the fountain effect. The EIA feature has repeatedly been reviewed in the literature (e.g., Anderson, 1981; Stenning, 1992; Walker et al., 1994). Thus; location and strength of the EIA crest and the extent of the EIA are intimately related to the vertical plasma drift velocity at the equator. As measurements of the F-region vertical drifts at the equator in the Indian sector are not available, to estimate vertical plasma drifts, we have appropriately scaled vertical drift velocity, as modeled by Fejer (1997), and neutral wind speed data used in SAMI2 model. If E stands for the zonal electric field, then E = 1 would signify that vertical drift as provided by Fejer (1997) are used. If E = 0, no plasma drifts are used. So, the scaling factor could vary from 0 to 1. Similarly, W = 0 indicates no neutral winds are used. When W = 1, wind model (HWM93) used by Huba et al. (2000a) has been used. The day of low solar activity is
chosen as solar flux provided for the low solar activity year 2008 and the day of high solar activity is chosen as solar flux provided for the high solar activity year 2000 for the solar cycle 23 (http://www.spaceweather.gc.ca). 2.1. EIA formation We have run SAMI2 model for the following set of inputs – day no: 173, year 2000 (high solar activity case), longitude 75°E, geographic latitude 24.6°N (locationUdaipur), Ap: 6 and F10.7A: 180. In the first case, we set zero vertical drift velocity so that uplifting of plasma and its subsequent dumping is not expected. In this case, whether we set W = 0, or 1, formation of EIA at local noon was not observed in simulation results (These results are not given here). Also, when we set E = 1 but W = 0, results showed the formation of the EIA. Thus, simulation calculations confirmed the results obtained earlier by other researchers (e.g., Moffett and Hanson, 1965; Fejer et al., 2008; Stolle et al., 2008) that the zonal electric field is the primary factor for the formation of EIA. For the next case vertical drift velocity and neutral wind speed as employed by Huba et al. (2000a) have been used. So, the scale factor is 1 for both E and W. Fig. 1 (left panel) shows results of simulation at 1339 h when a well developed EIA with trough at around 10°N (corresponding to the geomagnetic equator in the region) and crests around 25°N and 5°S are seen to be formed. However, simulation results reveal that the EIA formation is sustained almost till mid night. As a representative case we show model results at around mid night in Fig. 1 (right panel). In order to compare our model results with observations, authors are hamstrung by the paucity of observational data in the Indian region. While Sripathi (2012) have reported electron density distribution using cosmic observations in the Indian zone, there are very limited use of present case. However, for very low solar activity year 2008, results of Sripathi (2012) revealed that the crest of ionization to be 10°S and 25°N. Results of present work are consistent with these observations. For the Indian region, the SROSS C2 satellite data are also of limited use as satellite head an circular orbit at an altitude of 500 km. For these, we have to take reports to the data of total electron content (TEC) in the region that have been obtained using the GPS receivers (e.g., Rama Rao et al., 2006; Kumar and Singh, 2009; Bagiya et al., 2009; Galav et al., 2010; Venkatesh et al., 2015). Galav et al. (2010) found that (for the years 2005–2008), the largest crest of TEC was formed during 1400–1600 h and it shifted equatorward as the solar activity lowered. Venkatesh et al. (2015) have provided detailed studies of diurnal variation of TEC in the Indian low latitudes at various stations, from equator to the anomaly crest and beyond. They have shown that the EIA existed between 1000 LT–1800 LT and it is maximized 1400 h. Therefore, all observational studies of TEC in the Indian low latitudes have revealed that the EIA is the strongest in the afternoon hours, with its crests located
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Fig. 1. A color contour plot of the electron density as a function of geographic latitude and altitude is shown using SAMI2 model. The simulation was run using vertical drift velocity and neutral wind speed as employed by Huba et al. (2000a). The left panel shows the well developed EIA at mid day (1340 LT) and right panel shows the existence of EIA during midnight (2400 h) along 75°E longitude. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
around 10–15° (magnetic latitude), depending on whether the solar activity is low or high. Studies of local time variation of EIA from longitude sectors other than India (Yeh et al., 2001; Liu et al., 2007; Xiong et al., 2013, 2016), based on the observations from ground-based instruments as well as in situ measurements, have revealed that the EIA typically first appear at around 0900 LT, and peaked around 1400 LT. These are in consonance with the findings reported in the Indian sector. These facts have been the motivating factors to scale the drift velocity data as provided by Huba et al. (2000a). Thus, drift velocities employed in SAMI2 model cannot be used as such in the Indian low latitudes. The result also suggests that either a different drift velocity model should be incorporated, or the drift velocity of Fejer (1997) should be suitably scaled so that anomaly formation is consistent in the India region for all phases of solar activity. After a number of runs employing different scale factors for the drift velocity and wind, we have found that a scale factor of 0.65 is appropriate for scaling the drift velocity. Since scaling of wind did not have a significant effect on gross features of the anomaly, we have used the HWM93 wind model of SAMI2 model as such. Here-in-below we give results for a high solar activity day (day number 173) of the year 2000, with Ap: 6, solar flux at 10.7 cm, i.e., F10.7 = 180 sfu and its three months average, F10.7A = 180. Consistent with TEC observations in the Indian region, simulation results reveal that no EIA formation takes place in the morning hours (till about 1100 LT). The EIA crests were seen to be formed after 1100 LT which intensified during the afternoon until around 1600 LT. Fig. 2 shows the results at around 1430 h when the anomaly was found
to be strongest along 75°E longitude. The strength of the EIA decreased thereafter and EIA totally disappeared after 2030 LT. Existence of EIA during the post sunset hours depends not only on the intensity of the daytime EIA but also on the pre-reversal enhancement of the ambient electric field. The detail could be found elsewhere (e.g., Hanumath, 1998; Sastri, 1998; Prakash et al., 2009). Variability in the post-sunset anomaly is one of the largest sources of error in ionospheric specification models (Coker et al., 2012). We have also simulated latitudinal electron density profile for very low solar activity for which the year 2008 has been chosen. Simulation uses following parameters-day 173 (summer solstice), Geog. Longitude; 75°E, Geog. Lat; 24.6°N, AP: 4, F10.7: 60. F10.7A: 60. Simulation results reveal that the southern crest of EIA starts to develop 0939 LT and northern EIA crest 1040 LT, and the crest amplitude is largest 1400 LT. The results further reveal that the EIA crest first appears near equator and then shifts towards higher latitude with increasing strength. Results also reveal that the southern crest disappears earlier, by about an hour, at 1730 LT, than the northern crest. A representative case in Fig. 3 gives results 1400 LT when the EIA formation was found to be strongest with crests around 20°N and 5°S (geographic) in the northern and southern low latitude, respectively. Thereafter, the EIA crests receded towards the equator with decreasing strength. It can be also seen from Fig. 3 that compared to the high solar activity period (Fig. 2), the electron density is significantly decreased during low solar activity. The model results revel that the EIA starts to develop after the sunrise and decays after the sunset during the low solar activity year, but persist late into the night during
Please cite this article in press as: Rao, S.S., et al. SAMI2 model results for the quite time low latitude ionosphere over India. Adv. Space Res. (2018), https://doi.org/10.1016/j.asr.2018.01.016
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Fig. 2. A color contour plot of the electron density as a function of geographic latitude and altitude is shown employing scale factor 0.65 for the drift velocity for the Indian zone in SAMI2 model. Figure shows the results at around 1430 LT on a day of high solar activity case when the EIA was found to be strongest along 75°E longitude. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. A color contour plot of the electron density as a function of geographic latitude and altitude is shown using SAMI2 model employing scale factor 0.65 for the Indian zone in SAMI2 model. Figure shows the results 1400 LT on a day of low solar activity case when the EIA formation was found to be strongest with crests around 20°N and 5°S (geographic) in the northern and southern low latitude, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
high solar activity. Results regarding asymmetry of northern and southern crests of EIA are in agreement with the ones reported earlier by Lin et al. (2007) for the low solar
activity year 2006 and by Luan et al. (2015) for the low to moderately high solar activity years 2007–2012 using measurements from COSMIC satellites. They suggested that
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In Fig. 4, the hole appears as a closed, dark blue contour centered around 1500 km with a latitudinal extent in excess of 10° centered on the magnetic equator. The time of occurrence and duration of sustenance of the hole are quite different from those reported by Huba et al. (2000a). Plasma density of ionosphere varies with time and space, and its rate of change is governed by the continuity equation. We have used (SAMI2) model to provide vertical electron density profile over an equator at geographic latitude of 5°N for the case of high solar activity year 2000 as shown in Fig. 5 for different hours, starting from evening hours until post mid night hours. It can be seen from Fig. 5 that the electron density increases from a height of around 50 km and maximizes around 500 km. Thereafter, it decreases exponentially and becomes asymptotic beyond 1800 km. It can be seen that no electron hole formation till 2100 LT (blue curve). The green curve at 0123 LT and red curve at 0353 LT shows well developed electron hole at an altitude 1500 km by the post mid-night hours. While the simulation results produce gross features of the electron density profile, the magnitude of peak electron density is slightly higher, of the order of 106 electrons per cc. In summary, the SAMI2 model reproduces gross features of the EIA and the vertical electron density profiles in the Indian low latitudes when the vertical plasma drift is suitably scaled. To see the effect of solar activity variation over formation of electron hole, we have also run SAMI2 model for low solar activity year 2008 with day number 173 (summer solstice), for 75°E longitude. Results of this case are shown
the stronger southern EIA crest may result from both the summer-to-winter hemisphere plasma transport and the asymmetric neutral composition distribution. Simulation result regarding the stronger southern EIA crest is also consistent with the results of Croom et al. (1959) from topside ionograms. 2.2. Electron hole formation As noted earlier, occurrence of an electron hole has been reported by Huba et al. (2000a) for the American equatorial region wherein they found reduction in electron density in the altitude range 1000–2500 km. They also reported seasonal variability of the electron hole formation. In this section, we report results of simulation runs for the formation of electron hole and its seasonal variability over the geomagnetic equator in the Indian zone in low as well as high solar activity. We have run SAMI2 model for high solar activity year 2000 for the day number 173, summer solstice, for 75°E longitude. Our simulation results reveal that a shallow electron hole starts developing in the local evening hours at a height of about 2000 km and becomes prominent during the mid-night. It descends in height with the passage of time. The hole may reach a low altitude of about 800 km past the mid night. It is localized and more intense about the geomagnetic equator. In the early morning hours the hole disappears altogether. As a representative result, Fig. 4 shows a contour plot of electron density as a function of geographic latitude versus altitude for 0353 LT. 4500
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Fig. 4. An electron hole formation is shown using simulation run by SAMI2 model for high solar activity year 2000, summer solstice, along 75°E longitude. The Electron hole appears as a closed, dark blue contour centered around 1500 km with a latitudinal extent in excess of 10° centered on the magnetic equator at 0353 LT. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. A vertical electron density profile is shown over an equator at geographic latitude of 5°N for different hours. The green curve at 0123 LT and red curve at 0353 LT shows well developed electron hole at an altitude 1500 km by the post mid-night hours. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in Fig. 6. Our simulation result shows that an electron hole first appears at a height of around 700 km at 2200 LT centered over 8°N geographic latitude. In contrast to the high solar activity case, it ascended in height up to about 3000 km with passage of night hours and became prominent at
0345 LT. After that, it started to shrink and totally diminished after 0515 LT. To check the seasonal effects on presence or absence of electron hole in the ionosphere we have run simulation for the equinoctial day of high as well as low solar activity years. The high solar activity case is again the year 2000 with day number 265 whereas for low solar activity case the year is 2008 with day number 265. Huba et al. (2000a) reported that the electron hole is not formed during equinoxes due to the absence of strong pressure anisotropy between the geomagnetic hemispheres which weaken O+ transhemispheric flows. They have also stated that the ionosphere in equinoxes is reasonably symmetric about the geomagnetic equator at 5°, and the O+ velocities are quite small compared to those in solstices. Our results for 75°E longitude reveal that while the EIA is symmetric during equinoxes (Fig. 2), contrary to Huba et al. (2000a), formation of electron hole occurs during equinoxes also. There are, however, some differences with regard to the time of occurrence of the hole and its movement with time compared to those observed during solstices. For the high solar activity case, we have observed that the electron hole starts to form by 1950 LT in the evening at around 1600 km centered over 10°N geographic latitude. Thereafter, it symmetrically widened latitudinally in shape and became intense at about 2120 LT (Fig. 7, left panel). The most interesting result is that the hole remained at a height of 1600 km during its growth period. It started to decay after 2230 LT and descended in height down to about 1300 km. It totally disappeared at 0500 LT. For the low solar activity case (year 2008, day number 265), the hole started to develop around mid night at an altitude
Fig. 6. An electron hole formation is shown for low solar activity year 2008 (day of summer solstice) along 75°E longitude. The electron hole intensified at 0345 LT and centered over 8°N geographic latitude.
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Fig. 7. An electron hole formation along 75°E longitude is shown for the equinox condition choosing day of high solar activity year 2000 (left panel) and low solar activity year 2008 (right panel).
of 1800 km and descended to about 800 km by 0500 LT when it was maximum (Fig. 7, right panel). With the day break it disappeared totally around 0600 LT. Incidentally Klimenko and Klimenko (2011, 2012) have shown mechanism for reduction in electron density at the topside ionosphere in the height range of 1500–2500 km near the magnetic equator from slightly different point of view. In those papers Klimenko and Klimenko (2011, 2012) showed the formation of an additional layer namely a G layer comprising of the H+ ions at night in the equinoctial conditions when solar condition was minimum. They attributed the formation of a G layer to meridional component of the thermospheric wind and also association of the G layer with the mid-latitude maximum in the F region. Formation of the electron hole in equinox for 75°E longitude needs further analysis. Huba et al. (2000a) have reported that (i) neutral winds alone are sufficient for the formation of the hole. In order to ascertain the role of the neutral winds, simulation runs were made without the neutral winds but electric field (plasma drift) included. Though the results for this case are not given here, this run did not produce any electron hole. Hence the trans-hemispheric neutral winds are necessary for the formation of the electron hole. (ii) ion-ion coupling of the H+ and O+ was critical to the formation of the hole. The reduction in electron density has been attributed to the trans-hemispheric O+ flows that
collisionally couple to H+ and transport it to lower altitudes, and thereby reduce the electron density at high altitudes. To test this hypothesis, simulation runs were made without the H+ ions. No electron hole formation was found (Figure not shown here). This implies that ion-ion coupling of the H+ and O+ is critical to the formation of the hole in the equinoxes along 75°E longitude. In solstices, when we set neutral wind to zero and the zonal electric field is allowed, simulation run does not produce any electron hole. In the case when neutral winds were allowed and drifts were set to zero (i.e., W = 1 and E = 0), electron hole does form. The electron hole first appeared around 2100 LT and sustained till early morning hours (figures are not shown here). Hence the trans-hemispheric neutral winds are necessary and are present over the 75°E longitude in the Indian sector, both during the solstices and equinox. As the electron hole during the equinoxes is smaller in size and shallower in terms of density than that during the solstices, it implies that the trans-hemispheric winds, though present during equinox, are smaller in magnitude than those during the solstices. 3. Summary This work presents results obtained from simulation runs based on the SAMI2 model by adopting it for the Indian low latitudes along 75°E longitude. In agreement
Please cite this article in press as: Rao, S.S., et al. SAMI2 model results for the quite time low latitude ionosphere over India. Adv. Space Res. (2018), https://doi.org/10.1016/j.asr.2018.01.016
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with the TEC variations in the region the anomaly is seen to be strongest in the afternoon hours. Our results also showed hemispheric asymmetry of the EIA crests with regard to strength and time of occurrence/disappearance which is consistent with the results reported by Lin et al. (2007) and Luan et al. (2015). Our results also show seasonal and solar activity dependences of the EIA. The strength of EIA is found to decrease in low solar activity. Barring some differences, results using SAMI2 model gave gross feature of vertical electron density profile for the Indian low latitude. The main thrust of this work is the study of the ‘‘Electron hole” in the Indian EIA region along 75°E longitude. Simulation results show that the reduction in electron density occurs in the altitude range 800–2500 km over the magnetic equator. Compared to the results of Huba et al. (2000a), significant differences with regard to the strength and time of occurrence of the electron hole have been found in the Indian region along 75°E longitude. Contrary to the result of Huba et al. (2000a), our results for the Indian region showed the occurrence of an electron hole during equinoxes also. Acknowledgements This work uses the SAMI2 ionosphere model written and development by the Naval Research Laboratory, USA. This work is supported from research grant received from Council of Scientific and Industrial Research (CSIR)New Delhi, India under Emeritus Scientist scheme vide sanction no. 21(954)/13/EMR-II. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j. asr.2018.01.016. References Anderson, D.N., 1981. Modeling the ambient, low latitude F-region ionosphere – a review. J. Atmos. Terr. Phys. 43, 753–762. Appleton, E.V., 1946. Two anomalies in ionosphere. Nature 157, 691–693. Bagiya, M.S., Joshi, H.P., Iyer, K.N., Aggrawal, M., Ravindran, S., Pathan, B.M., 2009. TEC variations during low solar activity period (2005–2007) near the equatorial anomaly crest region in India. Ann. Geophys. 27, 1047–1057. Chappell, C.R., Fields, S.A., Baugher, C.R., Hoffman, J.H., Hanson, W. B., Wright, W.W., Hammack, H.D., Carigen, G.R., Nagy, A.F., 1981. The retarding ion mass spectrometer on Dynamics Explorer. Space Sci. Instrum. 5 (4), 477–491. Coker, C., Dandenault, P.B., Dymond, K., Budzien, S.A., Nicholas, A.C., 2012. A report on global specification of the post sunset equatorial ionization anomaly presented at NASA Astrophysics Data System (ADS). Science. Gov. on 2012-12-01. Croom, S., Robbins, A., Thomas, J.O., 1959. Two anomalies in the behavior of the F2 layer of the ionosphere. Nature 184, 2003–2004. Duncan, R.A., 1960. The equatorial F-region of the ionosphere. J. Atmos. Terr. Phys. 18, 89–100.
Fejer, B.G., 1997. Electrodynamics of the low latitude ionosphere: recent results and future challenges. J. Atmos. Sol. Terr. Phys. 59, 1456–1482. Fejer, B.G., Jensen, J.W., Su, S.-Y., 2008. Quiet time equatorial F region vertical plasma drift model derived from ROCSAT-1 observations. J. Geophys. Res. 113, A05304. Galav, P., Dashora, N., Sharma, S., Pandey, R., 2010. Characterization of low latitudes GPS-TEC during very low solar activity phase. J. Atmos. Terr. Phys. 72, 1309–1317. Gallagher, D., 2016. Is there a hole in the topside, equatorial ionosphere? Results Phys. 6, 1181–1182. Hanumath, S.J., 1998. On the development of abnormally large post sunset upward drift of equatorial F region under quiet geomagnetic conditions. J. Geophys. Res. 103 (A3), 3983–3991. Huba, J., Joyce, G., Fedder, J.A., 2000a. The formation of electron hole in the topside equatorial ionosphere. Geophy. Res. Lett. 27 (2), 181–184. Huba, J., Joyce, G., Fedder, J.A., 2000b. Sami2 is Another Model of the Ionosphere (SAMI2): a new low-latitude ionosphere model. J. Geophys. Res. 105 (A10), 23035–23053. Joshi, L.M., Sripathi, S., Singh, R., 2016. Simulation of low-latitude ionospheric response to 2015 St. Patrick’s Day super geomagnetic storm using ionosonde-derived PRE vertical drifts over Indian region. J. Geophys. Res. Space Phys. 121 (3), 2489–2502. Klenzing, j., Burrell, A.G., Heelis, R.A., Huba, J.D., Pfaff, R., Simo˜es, F., 2013. Exploring the role of ionospheric drivers during the extreme solar minimum of 2008. Ann. Geophys. 31, 2147–2156. Klimenko, M.V., Klimenko, V.V., 2011. Numerical simulation of the F2 layer stratification and appearance of the F3 and G layers in the equatorial ionosphere: the morphology of the phenomena. Geomag. Aeron. 51 (5), 646–655. Klimenko, M.V., Klimenko, V.V., 2012. Mechanism of stratification of the F2 layer and formation of the F3 layer and G layers in the equatorial ionosphere. Geomag. Aeron. 52 (3), 321–334. Kumar, S., Singh, A.K., 2009. Variation of ionospheric total electron content in Indian low latitude region of the equatorial ionization anomaly during May 2007-April 2008. Adv. Space Res. 43, 1555–1562. Lin, C.H., Liu, J.Y., Fang, T.W., Chang, P.Y., Tsai, H.F., Chen, C.H., Hsiao, C.C., 2007. Motions of the equatorial ionization anomaly crests imaged by FORMOSAT-3/COSMIC. Geophys. Res. Lett. 34, L19101. Liu, H., Stolle, C., Fo¨rster, M., Watanabe, S., 2007. Solar activity dependence of the electron density in the equatorial anomaly regions observed by CHAMP. J. Geophys. Res. 112, A11311. Luan, X., Wang, P., Dou, X., Liu, Y.C.-M., 2015. Interhemispheric asymmetry of the equatorial ionization anomaly in solstices observed by COSMIC during 2007–2012. J. Geophys. Res. Space Phys. 120, 3059–3073. Martyn, D.F., 1947. Atmospheric tides in the ionosphere. I. Solar tides in the F2 region. Proc. R. Soc. Lond. Ser. A 189, 241–260. Martyn, D.F., 1953. Electric currents in the ionosphere. III. Ionization drifts due to winds and electric fields. Phil. Trans. R. Soc. Lond. 246 (A), 306–320. McDonald, S.E., Dymond, K.F., Summers, M.E., 2008. Hemispheric asymmetries in the longitudinal structure of the low-latitude nighttime ionosphere. J. Geophys. Res. Space Phys. 113, A08308. Mitra, S.K., 1946. Geomagnetic control of region F2 of the ionosphere. Nature 158, 608–669. Moffett, R.J., Hanson, W.B., 1965. Effect of ionization transport on the equatorial F-region. Nature 206, 705–706. Prakash, S., Pallamraju, D., Sinha, H.S.S., 2009. Role of the equatorial ionization anomaly in the development of the evening pre reversal enhancement of the equatorial zonal electric field. J. Geophys. Res. 114, A02301. https://doi.org/10.1029/2007JA012808. Rama Rao, P.V.S., GopiKrishna, S., Niranjan, K., Prasad, D.S.V.V.D., 2006. Temporal and spatial variations in TEC using simultaneous measurements from the Indian GPS network of receivers during the low solar activity period of 2004–2005. Ann. Geophys. 24, 3279–3292. Sastri, J.H., 1998. On the development of abnormally large post sunset upward drift of equatorial region under quiet geomagnetic conditions. J. Geophys. Res. 103 (A3), 3983–3991.
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S.S. Rao et al. / Advances in Space Research xxx (2018) xxx–xxx Sripathi, S., 2012. Cosmic observations of ionospheric density profiles over Indian region: Ionospheric condition during extremely low solar activity period. Indian J. Radio Appl. Phys. 41, 98–109. Stenning, R.J., 1992. Modelling the low latitude F-region. J. Atmos. Terr. Phys. 54 (11/12), 1387–1412. Stolle, C., Manoj, C., Luhr, H., Maus, S., Alken, P., 2008. Estimating the daytime Equatorial Ionization Anomaly strength from electric field proxies. J. Geophys. Res. 113, A09310. Venkatesh, K., Fagundes, P.R., Prasad, D.S.V.V.D., Denardini, C.M., de Abreu, A.J., Jesus, R., Gende, M., 2015. Day-to-day variability of equatorial electrojet and its role on the day-to-day characteristics of the equatorial ionization anomaly over the Indian and Brazilian sectors. J. Geophys. Res. Space 120, 9117–9131. Walker, G.O., Ma, J.H.K., Golton, E., 1994. The equatorial ionospheric anomaly in electron content from solar minimum to solar maximum for South East Asia. Ann. Geophys. 12, 195–209.
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Wang, X., Berthelier, J.J., Lebreton, J.P., 2010. Ionosphere variations at 700 km altitude observed by the DEMETER satellite during the 29 March 2006 solar eclipse. J. Geophys. Res. Space Phys. 115, A11. Xiong, C., Luhr, H., Ma, S.-Y., 2013. The magnitude and interhemispheric asymmetry of equatorial ionization anomaly-based on CHAMP and GRACE observations. J. Atmos. Solar-Terr. Phys. 105– 106, 160–169. Xiong, C., Zhou, Y.-L., Lu¨hr, H.M., Ma, S.-Y., 2016. Diurnal evolution of the F region electron density local time gradient at low and middle latitudes resolved by the Swarm constellation. J. Geophys. Res. Space Phys. 121, 9075–9089. Yeh, K.C., Franke, S.J., Andreeva, E.S., Kunitsyn, V.E., 2001. An investigation of motions of the equatorial anomaly crest. Geophys. Res. Lett. 28 (24), 4517–4520.
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ScienceDirect Advances in Space Research xxx (2018) xxx–xxx www.elsevier.com/locate/asr
SAMI2 model results for the quite time low latitude ionosphere over India S.S. Rao a, Shweta Sharma b, R. Pandey a,⇑ a
Department of Physics, MLS University, Udaipur 313001, India 8G Pocket Ki Sheikh Sarai Phase 2, New Delhi 110017, India
b
Received 4 August 2017; received in revised form 5 January 2018; accepted 9 January 2018
Abstract Efficacy of SAMI2 model for the Indian low latitude region around 75°E longitudes has been tested for different levels of solar flux. With a slight modification of the plasma drift velocity the SAMI2 model has been successful in reproducing quiet time ionospheric low latitude features like Equatorial Ionization Anomaly. We have also showed the formation of electron hole in the topside equatorial ionosphere in the Indian sector. Simulation results show the formation of electron hole in the altitude range 800–2500 km over the magnetic equator. Indian zone results reveal marked differences with regard to the time of occurrence, seasonal appearances and strength of the electron hole vis-a-vis those reported for the American equatorial region. Ó 2018 Published by Elsevier Ltd on behalf of COSPAR.
Keywords: SAMI2 model; Electron hole; Low latitude ionosphere; Equatorial ionization anomaly
1. Introduction A new low latitude ionospheric model has been developed at the Naval Research Laboratory, USA by Huba et al. (2000b) that takes into account detailed chemistry, electrodynamics and geomagnetic field configuration. The model has been termed SAMI2 (SAMI2 is Another Model of the Ionosphere). This model is shown to explain quiet time ionospheric features at American low latitudes. A number of workers have used this model to explain quiet time as well as disturbed time ionospheric features. (e.g., Huba et al., 2000a; McDonald et al., 2008; Wang et al., 2010; Klenzing et al., 2013; Joshi et al., 2016). Using the SAMI2 model Huba et al. (2000a) reported a remarkable phenomenon, namely, an ‘electron hole’ wherein a reduction in electron density in the topside ionosphere in the ⇑ Corresponding author.
E-mail addresses: [email protected] (S.S. Rao), shweta.phy@ gmail.com (S. Sharma), [email protected] (R. Pandey).
height range of about 1500–2500 km near the magnetic equator has been reported. This reduction in electron density has been attributed to the transhemispheric O+ flows that collisionally couple to H+ and transport it to lower altitudes, and thereby reduce the electron density at high altitudes. The transhemispheric O+ flows are caused by an interhemispheric pressure anisotropy that can be generated by the neutral wind, primarily during solstice conditions. They further reported that the formation of the electron depletion has a seasonal and longitudinal dependence. The first empirical support of an electron hole in the topside equatorial ionosphere is recently given by Gallagher (2016) who computed ratios of Retarding Ion Mass Spectrometer-RIMS (Chappell et al., 1981) derived H+, He+ and O+ ion density and IRI (International Reference Ionosphere) electron density. The RIMS measurements were made onboard the Dynamic Explorer 1 satellite. Thus, the work of Gallagher (2016) is an empirical suggestion that the SAMI2 model finding of an electron
https://doi.org/10.1016/j.asr.2018.01.016 0273-1177/Ó 2018 Published by Elsevier Ltd on behalf of COSPAR.
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hole represents the physical behavior of the topside equatorial ionosphere. In order to verify seasonal and longitudinal dependence of the topside electron hole we have used SAMI2 model for the Indian low latitudes around 75°E longitude. Therefore, the present work aims at simulating ionospheric variability in the Indian low latitudes during quiet solar conditions using the SAMI2 model. 2. Results and discussions As a first step of simulation we used the same model of ionospheric drift velocity and HWM93 empirical wind model that were used by Huba et al. (2000a). We used a 201 100 mesh that corresponds to 201 grid points along every field line, for each of the 100 field lines from an altitude of 90 km to 6000 km. The simulations were run for 48 h, out of which initial 24 h period take care of the transients, and the results for the rest 24 h have been used to obtain diurnally variability. The SAMI2 ion drifts are such that the ionospheric flux tubes rise during the day, from 0700 LT to 1900 LT and fall during the night from 1900 LT to 0700 LT. The daytime equatorial and low latitude ionosphere is characterized by an F region electron density trough at the geomagnetic equator and two crest within ±20° magnetic latitudes. This phenomenon has been well recognized as ‘‘Appleton anomaly” (Appleton, 1946) or equatorial ionization anomaly (EIA). This anomaly is formed due to the so called ‘‘fountain effect” and is first envisaged by Martyn (1947). In the daytime the ambient eastward electric field at the geomagnetic equator gives rise to an upward drift of the F region plasma at the magnetic equator due to the crossed electric and magnetic field (Martyn, 1953). This rising plasma reaches very high altitudes until it loses momentum whereafter the plasma diffuses downward along the geomagnetic field lines, as was first suggested by Mitra (1946). This process makes an ionization trough at the equator and crests around ±15° magnetic latitudes on both sides of the equator. (Duncan, 1960). Moffet and Hanson, 1965 have presented results of modeling of formation of the EIA due to the fountain effect. The EIA feature has repeatedly been reviewed in the literature (e.g., Anderson, 1981; Stenning, 1992; Walker et al., 1994). Thus; location and strength of the EIA crest and the extent of the EIA are intimately related to the vertical plasma drift velocity at the equator. As measurements of the F-region vertical drifts at the equator in the Indian sector are not available, to estimate vertical plasma drifts, we have appropriately scaled vertical drift velocity, as modeled by Fejer (1997), and neutral wind speed data used in SAMI2 model. If E stands for the zonal electric field, then E = 1 would signify that vertical drift as provided by Fejer (1997) are used. If E = 0, no plasma drifts are used. So, the scaling factor could vary from 0 to 1. Similarly, W = 0 indicates no neutral winds are used. When W = 1, wind model (HWM93) used by Huba et al. (2000a) has been used. The day of low solar activity is
chosen as solar flux provided for the low solar activity year 2008 and the day of high solar activity is chosen as solar flux provided for the high solar activity year 2000 for the solar cycle 23 (http://www.spaceweather.gc.ca). 2.1. EIA formation We have run SAMI2 model for the following set of inputs – day no: 173, year 2000 (high solar activity case), longitude 75°E, geographic latitude 24.6°N (locationUdaipur), Ap: 6 and F10.7A: 180. In the first case, we set zero vertical drift velocity so that uplifting of plasma and its subsequent dumping is not expected. In this case, whether we set W = 0, or 1, formation of EIA at local noon was not observed in simulation results (These results are not given here). Also, when we set E = 1 but W = 0, results showed the formation of the EIA. Thus, simulation calculations confirmed the results obtained earlier by other researchers (e.g., Moffett and Hanson, 1965; Fejer et al., 2008; Stolle et al., 2008) that the zonal electric field is the primary factor for the formation of EIA. For the next case vertical drift velocity and neutral wind speed as employed by Huba et al. (2000a) have been used. So, the scale factor is 1 for both E and W. Fig. 1 (left panel) shows results of simulation at 1339 h when a well developed EIA with trough at around 10°N (corresponding to the geomagnetic equator in the region) and crests around 25°N and 5°S are seen to be formed. However, simulation results reveal that the EIA formation is sustained almost till mid night. As a representative case we show model results at around mid night in Fig. 1 (right panel). In order to compare our model results with observations, authors are hamstrung by the paucity of observational data in the Indian region. While Sripathi (2012) have reported electron density distribution using cosmic observations in the Indian zone, there are very limited use of present case. However, for very low solar activity year 2008, results of Sripathi (2012) revealed that the crest of ionization to be 10°S and 25°N. Results of present work are consistent with these observations. For the Indian region, the SROSS C2 satellite data are also of limited use as satellite head an circular orbit at an altitude of 500 km. For these, we have to take reports to the data of total electron content (TEC) in the region that have been obtained using the GPS receivers (e.g., Rama Rao et al., 2006; Kumar and Singh, 2009; Bagiya et al., 2009; Galav et al., 2010; Venkatesh et al., 2015). Galav et al. (2010) found that (for the years 2005–2008), the largest crest of TEC was formed during 1400–1600 h and it shifted equatorward as the solar activity lowered. Venkatesh et al. (2015) have provided detailed studies of diurnal variation of TEC in the Indian low latitudes at various stations, from equator to the anomaly crest and beyond. They have shown that the EIA existed between 1000 LT–1800 LT and it is maximized 1400 h. Therefore, all observational studies of TEC in the Indian low latitudes have revealed that the EIA is the strongest in the afternoon hours, with its crests located
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Fig. 1. A color contour plot of the electron density as a function of geographic latitude and altitude is shown using SAMI2 model. The simulation was run using vertical drift velocity and neutral wind speed as employed by Huba et al. (2000a). The left panel shows the well developed EIA at mid day (1340 LT) and right panel shows the existence of EIA during midnight (2400 h) along 75°E longitude. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
around 10–15° (magnetic latitude), depending on whether the solar activity is low or high. Studies of local time variation of EIA from longitude sectors other than India (Yeh et al., 2001; Liu et al., 2007; Xiong et al., 2013, 2016), based on the observations from ground-based instruments as well as in situ measurements, have revealed that the EIA typically first appear at around 0900 LT, and peaked around 1400 LT. These are in consonance with the findings reported in the Indian sector. These facts have been the motivating factors to scale the drift velocity data as provided by Huba et al. (2000a). Thus, drift velocities employed in SAMI2 model cannot be used as such in the Indian low latitudes. The result also suggests that either a different drift velocity model should be incorporated, or the drift velocity of Fejer (1997) should be suitably scaled so that anomaly formation is consistent in the India region for all phases of solar activity. After a number of runs employing different scale factors for the drift velocity and wind, we have found that a scale factor of 0.65 is appropriate for scaling the drift velocity. Since scaling of wind did not have a significant effect on gross features of the anomaly, we have used the HWM93 wind model of SAMI2 model as such. Here-in-below we give results for a high solar activity day (day number 173) of the year 2000, with Ap: 6, solar flux at 10.7 cm, i.e., F10.7 = 180 sfu and its three months average, F10.7A = 180. Consistent with TEC observations in the Indian region, simulation results reveal that no EIA formation takes place in the morning hours (till about 1100 LT). The EIA crests were seen to be formed after 1100 LT which intensified during the afternoon until around 1600 LT. Fig. 2 shows the results at around 1430 h when the anomaly was found
to be strongest along 75°E longitude. The strength of the EIA decreased thereafter and EIA totally disappeared after 2030 LT. Existence of EIA during the post sunset hours depends not only on the intensity of the daytime EIA but also on the pre-reversal enhancement of the ambient electric field. The detail could be found elsewhere (e.g., Hanumath, 1998; Sastri, 1998; Prakash et al., 2009). Variability in the post-sunset anomaly is one of the largest sources of error in ionospheric specification models (Coker et al., 2012). We have also simulated latitudinal electron density profile for very low solar activity for which the year 2008 has been chosen. Simulation uses following parameters-day 173 (summer solstice), Geog. Longitude; 75°E, Geog. Lat; 24.6°N, AP: 4, F10.7: 60. F10.7A: 60. Simulation results reveal that the southern crest of EIA starts to develop 0939 LT and northern EIA crest 1040 LT, and the crest amplitude is largest 1400 LT. The results further reveal that the EIA crest first appears near equator and then shifts towards higher latitude with increasing strength. Results also reveal that the southern crest disappears earlier, by about an hour, at 1730 LT, than the northern crest. A representative case in Fig. 3 gives results 1400 LT when the EIA formation was found to be strongest with crests around 20°N and 5°S (geographic) in the northern and southern low latitude, respectively. Thereafter, the EIA crests receded towards the equator with decreasing strength. It can be also seen from Fig. 3 that compared to the high solar activity period (Fig. 2), the electron density is significantly decreased during low solar activity. The model results revel that the EIA starts to develop after the sunrise and decays after the sunset during the low solar activity year, but persist late into the night during
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Fig. 2. A color contour plot of the electron density as a function of geographic latitude and altitude is shown employing scale factor 0.65 for the drift velocity for the Indian zone in SAMI2 model. Figure shows the results at around 1430 LT on a day of high solar activity case when the EIA was found to be strongest along 75°E longitude. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. A color contour plot of the electron density as a function of geographic latitude and altitude is shown using SAMI2 model employing scale factor 0.65 for the Indian zone in SAMI2 model. Figure shows the results 1400 LT on a day of low solar activity case when the EIA formation was found to be strongest with crests around 20°N and 5°S (geographic) in the northern and southern low latitude, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
high solar activity. Results regarding asymmetry of northern and southern crests of EIA are in agreement with the ones reported earlier by Lin et al. (2007) for the low solar
activity year 2006 and by Luan et al. (2015) for the low to moderately high solar activity years 2007–2012 using measurements from COSMIC satellites. They suggested that
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In Fig. 4, the hole appears as a closed, dark blue contour centered around 1500 km with a latitudinal extent in excess of 10° centered on the magnetic equator. The time of occurrence and duration of sustenance of the hole are quite different from those reported by Huba et al. (2000a). Plasma density of ionosphere varies with time and space, and its rate of change is governed by the continuity equation. We have used (SAMI2) model to provide vertical electron density profile over an equator at geographic latitude of 5°N for the case of high solar activity year 2000 as shown in Fig. 5 for different hours, starting from evening hours until post mid night hours. It can be seen from Fig. 5 that the electron density increases from a height of around 50 km and maximizes around 500 km. Thereafter, it decreases exponentially and becomes asymptotic beyond 1800 km. It can be seen that no electron hole formation till 2100 LT (blue curve). The green curve at 0123 LT and red curve at 0353 LT shows well developed electron hole at an altitude 1500 km by the post mid-night hours. While the simulation results produce gross features of the electron density profile, the magnitude of peak electron density is slightly higher, of the order of 106 electrons per cc. In summary, the SAMI2 model reproduces gross features of the EIA and the vertical electron density profiles in the Indian low latitudes when the vertical plasma drift is suitably scaled. To see the effect of solar activity variation over formation of electron hole, we have also run SAMI2 model for low solar activity year 2008 with day number 173 (summer solstice), for 75°E longitude. Results of this case are shown
the stronger southern EIA crest may result from both the summer-to-winter hemisphere plasma transport and the asymmetric neutral composition distribution. Simulation result regarding the stronger southern EIA crest is also consistent with the results of Croom et al. (1959) from topside ionograms. 2.2. Electron hole formation As noted earlier, occurrence of an electron hole has been reported by Huba et al. (2000a) for the American equatorial region wherein they found reduction in electron density in the altitude range 1000–2500 km. They also reported seasonal variability of the electron hole formation. In this section, we report results of simulation runs for the formation of electron hole and its seasonal variability over the geomagnetic equator in the Indian zone in low as well as high solar activity. We have run SAMI2 model for high solar activity year 2000 for the day number 173, summer solstice, for 75°E longitude. Our simulation results reveal that a shallow electron hole starts developing in the local evening hours at a height of about 2000 km and becomes prominent during the mid-night. It descends in height with the passage of time. The hole may reach a low altitude of about 800 km past the mid night. It is localized and more intense about the geomagnetic equator. In the early morning hours the hole disappears altogether. As a representative result, Fig. 4 shows a contour plot of electron density as a function of geographic latitude versus altitude for 0353 LT. 4500
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Fig. 4. An electron hole formation is shown using simulation run by SAMI2 model for high solar activity year 2000, summer solstice, along 75°E longitude. The Electron hole appears as a closed, dark blue contour centered around 1500 km with a latitudinal extent in excess of 10° centered on the magnetic equator at 0353 LT. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. A vertical electron density profile is shown over an equator at geographic latitude of 5°N for different hours. The green curve at 0123 LT and red curve at 0353 LT shows well developed electron hole at an altitude 1500 km by the post mid-night hours. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in Fig. 6. Our simulation result shows that an electron hole first appears at a height of around 700 km at 2200 LT centered over 8°N geographic latitude. In contrast to the high solar activity case, it ascended in height up to about 3000 km with passage of night hours and became prominent at
0345 LT. After that, it started to shrink and totally diminished after 0515 LT. To check the seasonal effects on presence or absence of electron hole in the ionosphere we have run simulation for the equinoctial day of high as well as low solar activity years. The high solar activity case is again the year 2000 with day number 265 whereas for low solar activity case the year is 2008 with day number 265. Huba et al. (2000a) reported that the electron hole is not formed during equinoxes due to the absence of strong pressure anisotropy between the geomagnetic hemispheres which weaken O+ transhemispheric flows. They have also stated that the ionosphere in equinoxes is reasonably symmetric about the geomagnetic equator at 5°, and the O+ velocities are quite small compared to those in solstices. Our results for 75°E longitude reveal that while the EIA is symmetric during equinoxes (Fig. 2), contrary to Huba et al. (2000a), formation of electron hole occurs during equinoxes also. There are, however, some differences with regard to the time of occurrence of the hole and its movement with time compared to those observed during solstices. For the high solar activity case, we have observed that the electron hole starts to form by 1950 LT in the evening at around 1600 km centered over 10°N geographic latitude. Thereafter, it symmetrically widened latitudinally in shape and became intense at about 2120 LT (Fig. 7, left panel). The most interesting result is that the hole remained at a height of 1600 km during its growth period. It started to decay after 2230 LT and descended in height down to about 1300 km. It totally disappeared at 0500 LT. For the low solar activity case (year 2008, day number 265), the hole started to develop around mid night at an altitude
Fig. 6. An electron hole formation is shown for low solar activity year 2008 (day of summer solstice) along 75°E longitude. The electron hole intensified at 0345 LT and centered over 8°N geographic latitude.
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Fig. 7. An electron hole formation along 75°E longitude is shown for the equinox condition choosing day of high solar activity year 2000 (left panel) and low solar activity year 2008 (right panel).
of 1800 km and descended to about 800 km by 0500 LT when it was maximum (Fig. 7, right panel). With the day break it disappeared totally around 0600 LT. Incidentally Klimenko and Klimenko (2011, 2012) have shown mechanism for reduction in electron density at the topside ionosphere in the height range of 1500–2500 km near the magnetic equator from slightly different point of view. In those papers Klimenko and Klimenko (2011, 2012) showed the formation of an additional layer namely a G layer comprising of the H+ ions at night in the equinoctial conditions when solar condition was minimum. They attributed the formation of a G layer to meridional component of the thermospheric wind and also association of the G layer with the mid-latitude maximum in the F region. Formation of the electron hole in equinox for 75°E longitude needs further analysis. Huba et al. (2000a) have reported that (i) neutral winds alone are sufficient for the formation of the hole. In order to ascertain the role of the neutral winds, simulation runs were made without the neutral winds but electric field (plasma drift) included. Though the results for this case are not given here, this run did not produce any electron hole. Hence the trans-hemispheric neutral winds are necessary for the formation of the electron hole. (ii) ion-ion coupling of the H+ and O+ was critical to the formation of the hole. The reduction in electron density has been attributed to the trans-hemispheric O+ flows that
collisionally couple to H+ and transport it to lower altitudes, and thereby reduce the electron density at high altitudes. To test this hypothesis, simulation runs were made without the H+ ions. No electron hole formation was found (Figure not shown here). This implies that ion-ion coupling of the H+ and O+ is critical to the formation of the hole in the equinoxes along 75°E longitude. In solstices, when we set neutral wind to zero and the zonal electric field is allowed, simulation run does not produce any electron hole. In the case when neutral winds were allowed and drifts were set to zero (i.e., W = 1 and E = 0), electron hole does form. The electron hole first appeared around 2100 LT and sustained till early morning hours (figures are not shown here). Hence the trans-hemispheric neutral winds are necessary and are present over the 75°E longitude in the Indian sector, both during the solstices and equinox. As the electron hole during the equinoxes is smaller in size and shallower in terms of density than that during the solstices, it implies that the trans-hemispheric winds, though present during equinox, are smaller in magnitude than those during the solstices. 3. Summary This work presents results obtained from simulation runs based on the SAMI2 model by adopting it for the Indian low latitudes along 75°E longitude. In agreement
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with the TEC variations in the region the anomaly is seen to be strongest in the afternoon hours. Our results also showed hemispheric asymmetry of the EIA crests with regard to strength and time of occurrence/disappearance which is consistent with the results reported by Lin et al. (2007) and Luan et al. (2015). Our results also show seasonal and solar activity dependences of the EIA. The strength of EIA is found to decrease in low solar activity. Barring some differences, results using SAMI2 model gave gross feature of vertical electron density profile for the Indian low latitude. The main thrust of this work is the study of the ‘‘Electron hole” in the Indian EIA region along 75°E longitude. Simulation results show that the reduction in electron density occurs in the altitude range 800–2500 km over the magnetic equator. Compared to the results of Huba et al. (2000a), significant differences with regard to the strength and time of occurrence of the electron hole have been found in the Indian region along 75°E longitude. Contrary to the result of Huba et al. (2000a), our results for the Indian region showed the occurrence of an electron hole during equinoxes also. Acknowledgements This work uses the SAMI2 ionosphere model written and development by the Naval Research Laboratory, USA. This work is supported from research grant received from Council of Scientific and Industrial Research (CSIR)New Delhi, India under Emeritus Scientist scheme vide sanction no. 21(954)/13/EMR-II. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j. asr.2018.01.016. References Anderson, D.N., 1981. Modeling the ambient, low latitude F-region ionosphere – a review. J. Atmos. Terr. Phys. 43, 753–762. Appleton, E.V., 1946. Two anomalies in ionosphere. Nature 157, 691–693. Bagiya, M.S., Joshi, H.P., Iyer, K.N., Aggrawal, M., Ravindran, S., Pathan, B.M., 2009. TEC variations during low solar activity period (2005–2007) near the equatorial anomaly crest region in India. Ann. Geophys. 27, 1047–1057. Chappell, C.R., Fields, S.A., Baugher, C.R., Hoffman, J.H., Hanson, W. B., Wright, W.W., Hammack, H.D., Carigen, G.R., Nagy, A.F., 1981. The retarding ion mass spectrometer on Dynamics Explorer. Space Sci. Instrum. 5 (4), 477–491. Coker, C., Dandenault, P.B., Dymond, K., Budzien, S.A., Nicholas, A.C., 2012. A report on global specification of the post sunset equatorial ionization anomaly presented at NASA Astrophysics Data System (ADS). Science. Gov. on 2012-12-01. Croom, S., Robbins, A., Thomas, J.O., 1959. Two anomalies in the behavior of the F2 layer of the ionosphere. Nature 184, 2003–2004. Duncan, R.A., 1960. The equatorial F-region of the ionosphere. J. Atmos. Terr. Phys. 18, 89–100.
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