Contrasting features of the F 3 layer during high and low solar activity conditions observed from Indian low latitude

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1 Indian Journal of Radio & Space Physics Vol 41, April 2012, pp Contrasting features of the F 3 layer during high and low solar activity conditions observed from Indian low latitude P Pavan Chaitanya 1,$,*, A K Patra 2,# & S V B Rao 1 1 Department of Physics, S V University, Tirupati , India 2 National Atmospheric Research Laboratory, Gadanki , India $ pavanpeddapati@gmail.com, # akpatra@narl.gov.in Received October 2011; accepted 22 March 2012 Observations of the F 3 layer made during the extremely low solar activity (LSA) condition during and high solar activity (HSA) condition during from 6.5 o N magnetic latitude in the Indian sector are presented. Structure of the ionogram, displaying the F 3 layer, is found to be very distinct and clear in the LSA as compared to that in the HSA. The occurrence rate and the duration of occurrence of the F 3 layer is maximum during the summer solstice, moderate during the equinox, and minimum during the winter solstice. While there is no significant difference in the occurrence of the F 3 layer in the equinox and the summer solstice during the HSA and LSA periods; in the winter solstice, it is extremely low in the LSA when compared to those observed in the HSA. Mean f o F 3 is as high as 13 MHz in the HSA and 7 MHz in the LSA. In general, mean f o F 3 is found to be 4-6 MHz higher in the HSA than that of the LSA. Notably, f o F 3 values observed during are found to be 2-3 MHz lower than those reported for the previous LSA periods. While in the summer and winter solstices, mean h m F 3 is higher in the HSA than in the LSA; in the equinox, it is higher in the LSA than in the HSA. Also there exists large variability of h m F 3 irrespective of local time, season and solar activity levels, indicating the roles of E B drift and meridional neutral wind on the height of the F 3 layer. In an overall sense, while the observations agree well with the model prediction, they are found to agree on some counts and differ on some other counts with those reported earlier. The similarity and dissimilarity of the observations from different low-latitude locations are discussed in the light of current understanding on the F 3 layer. Keywords: F 3 layer occurrence, F 3 layer critical plasma frequency, Low solar activity, High solar activity PACS Nos: dj; Q- 1 Introduction The existence of an additional stratification of the equatorial F-layer, now known in the scientific community as F 3 layer 1, was first detected in 1940s using ground-based ionosonde 2,3 and subsequently by satellite-based Langmuir probe 4 and topside sounder 5,6. Much emphasis in uncovering the F 3 layer properties and the governing processes, however, was given when the Sheffield University Plasmasphere Ionosphere Model (SUPIM) showed the possibility of an additional layer in the equatorial F-region 7. Following the prediction of the additional layer 7, the layer was observationally studied using ionograms recorded at an equatorial station Fortaleza (3 o S, 38 o W, dip 2 o S) in Brazil 1,8. Balan et al. 9 proposed a physical mechanism for the formation of the F 3 layer. According to him, an F 3 layer is formed in the equatorial region when the combined effect of an upward E B drift and neutral wind provides a vertical upward plasma drift velocity at altitudes near and above the F 2 peak. This velocity causes the F 2 peak to drift upward and form the F 3 layer, while the normal F 2 layer develops at lower altitudes through the usual photochemical and dynamical effects of the equatorial region. Once the F 3 layer is formed, its peak density remains greater than that of the F 2 layer for a period of time when both the F 2 and F 3 layers can be recorded by groundbased ionosondes. Subsequently, if the F 3 peak density decreases due to chemical loss and diffusion and becomes less than that of the F 2 layer, the F 3 layer would not be seen by ground-based ionosondes. The fact that the F 3 layer is often detected during the morning-noon period supports the above viewpoint. Balan et al. 8 also predicted that the F 3 layer would be seen frequently and distinctly on the summer side of the equator during the low solar activity and less distinct as solar activity increases. They also suggested that the latitudinal extent of the F 3 layer can extend to the winter side of

2 122 INDIAN J RADIO & SPACE PHYS, APRIL 2012 the geomagnetic equator if a strong upward E B drift, or a weak poleward wind or a combination of the two, exists. Subsequently, extensive studies on the F 3 layer have been made, covering its diurnal and seasonal variations from different low-latitude sites for the solar quiet condition and also on its storm time behaviour Observations also showed that the F 3 layer occurrence is maximum at 7-8 o magnetic latitude 13,22. On the other hand, modeling studies 9 predicted that they would occur frequently in summer and in low solar activity. Thus, it would be important to study the F 3 layer properties at the long deep solar minimum and previous high solar activity years and compare the F 3 layer features at the deep solar minimum with those at normal solar minima. In this paper, the features of the F 3 layer observed from 6.5 o N magnetic latitude in the Indian sector during the extremely low solar activity (LSA) period of and the high solar activity (HSA) period of have been studied. These results are compared with those reported earlier from the Indian sector as well as those from elsewhere and discussed in the light of current understanding on the F 3 layer. 2 Observations The observational results on the F 3 layer presented here were made using a digital ionosonde (model IPS-42). This ionosonde was situated at Sriharikota (13.6 o N, 80.2 o E, magnetic latitude 6.5 o N) until 2006, which was later moved to Gadanki (13.5 o N, 79.2 o E, magnetic latitude 6.5 o N), another low latitude location about 100 km west of Sriharikota. Thus, the observations made during the HSA period of are from Sriharikota and those of LSA period of are from Gadanki. For the present study, quarter hourly ionograms, recorded during hrs IST during the year and , have been used. 2.1 Solar activity conditions Figure 1 shows daily values of 10.7 cm solar flux (F 10.7 ), as a measure of solar activity, for the period The hatched regions represent the period for which the ionosonde data analysis has been performed to study the F 3 layer characteristics. It may be noted that during , the F 10.7 happened to be the lowest with average value of 63 and during , it varied from 200 to General features of F 3 layer during high and low solar activity conditions Figures 2(a and b) show sample ionograms, displaying the clear presence of the F 3 layer, observed on 1 July 2002 corresponding to the HSA and 15 July 2008 corresponding to the LSA, respectively. In this study, such ionograms have been used to study the F 3 layer characteristics and their occurrence statistics. While the F 3 layer can be easily noticed in both the ionograms, the structures of the ionograms themselves are remarkably different in the two cases. The distinct features of the F 3 layer observed during the LSA than those observed during the HSA have been noted. The virtual peak height of the F 3 layer, h m F 3 and the critical plasma frequency of the F 3 layer, f o F 3 are 720 km and 9.16 MHz for the observations made in the HSA, while they are 591 km and 6.58 MHz corresponding to the LSA. Fig. 1 Daily variation of 10.7 cm solar flux during the period (hatched portions represent the periods of observations used in the present study)

3 CHAITANYA et al.: CONTRASTING FEATURES OF F 3 LAYER DURING HSA AND LSA 123 Fig. 2 Sample ionograms displaying the F 3 layer observed on: (a) 01 July 2002 at 10:00 hrs IST corresponding to the high solar activity (HSA); and (b) 15 July 2008 at 10:00 hrs IST corresponding to the low solar activity (LSA) period [ionograms display both ordinary (O) and extraordinary (X) modes; F 1, F 2, and F 3 layers are also marked in both the ionograms] 2.3 Occurrence statistics of the F 3 layer Figure 3 shows statistics of the F 3 layer occurrence. Figure 3(a) shows number of observational days (dark black vertical bar) and number of days on which the F 3 layer was observed (grey vertical bar) in each month corresponding to the HSA period of Figure 3(b) shows the same but for the LSA period of Figure 3(c) shows the occurrence percentage per month for the HSA (continuous line) and LSA (dashed line) conditions. Occurrence percentage, S, is calculated as follows: S = (N F3 /N o ) x 100 (1) where, N F3 and N o, represent the number of days on which the F 3 layer was observed and the total number of days of observations in every month, respectively. It is interesting to note from Fig. 3(c) that the occurrence percentages of the F 3 layer during the summer solstice (June-August) are nearly 100% both in the LSA and HSA conditions and the occurrence is slightly lower in the HSA than in the LSA. In other seasons, occurrence percentages, however, are found to be somewhat lower in the LSA period than those in the HSA period. Especially, the occurrence percentages during November and December are found to be remarkably lower in the LSA period than those in the HSA period. These statistics suggest that the F 3 layer occurs on more than 75% days during January-September and on 50-75% days during October-December, except during November- December of the LSA condition. The above statistics, however, does not include the time duration of the daytime hours during which the F 3 layer is observed. In order to reveal such statistics, the duration (in hours) of the F 3 layer occurrence per day has been examined. Figures 4(a and b) show the occurrence statistics of the F 3 layer in terms of hours per month. Percentage occurrence is calculated in the same way as that shown in Fig. 3(c), but in this case the number of days in Eq. (1) is replaced by the number of hours. As is evident, the F 3 layer occurrence is maximum in May-September (40-50% of the daytime hours) and minimum in October- December (only 10-20% of the daytime hours) both during the HSA and LSA conditions. During January- April, the occurrence is moderate (25% of the daytime hours) both during the HSA and LSA conditions. 2.4 Local time and day-to-day variation in the F 3 occurrence Figures 5(a-c) show the local time and day-to-day variations in the F 3 layer occurrence for different seasons observed during the HSA and LSA conditions. Observations corresponding to the HSA period are presented in the left panels and those corresponding to the LSA period are in the right panels to make a one-to-one comparison. The dotted lines represent no-observation on those days. As is evident, during the summer solstice (July-August), the occurrence as well as the duration of the F 3 layer occurrence is maximum both in the HSA and LSA conditions. Although the F 3 layer is mostly seen during hrs IST, on some occasions it is seen to form as early as 0800 hrs IST and last until 1800 hrs IST. Further, there is no remarkable difference in both local time and day-to-day variation in the F 3 layer occurrence corresponding to the HSA and LSA conditions. During the equinox, the occurrence is more regular during the LSA than in the HSA. Further, while the F 3 layer is found to commence at around 0900 hrs IST and observed

4 124 INDIAN J RADIO & SPACE PHYS, APRIL 2012 Fig. 3 Number of observational days (dark black vertical bar) and number of days on which the F 3 layer was observed (grey vertical bar) in each month corresponding to: (a) HSA period of , and (b) LSA period of ; (c) F 3 layer occurrence percentage per month for the HSA (continuous line) and the LSA (dashed line) conditions mostly until 1400 hrs IST during the LSA, they are found to commence around 1000 hrs IST and observed mostly until 1500 hrs IST in the HSA. This implies that during the LSA condition, they occur and vanish earlier (about an hour) than those in the HSA condition. During the winter solstice, the occurrence rate is very low as compared to those of summer solstice and equinox. Again, it is much lower during the LSA condition than during the HSA condition. Further, the F 3 layer is seen to commence around 1015 hrs IST and last until 1400 hrs IST. In all the seasons and both in the HSA and LSA, there exists large day-to-day variability in the occurrence of the F 3 layer. Fig. 4 Occurrence percentage per month based on duration of F 3 layer occurrence in hours during: (a) HSA of and (b) LSA of Critical plasma frequency and height of the F 3 layer Figures 6(a-c) show mean and standard deviation of the F 3 layer critical plasma frequency, f o F 3 (in the left panels) and virtual peak height, h m F 3 (in the right

5 CHAITANYA et al.: CONTRASTING FEATURES OF F 3 LAYER DURING HSA AND LSA 125 panels) as a function of local time corresponding to the equinox, summer solstice and winter solstice, respectively. Observations made during the HSA are presented in red color and those corresponding to the LSA are in black color. During the HSA period, while the mean value of f o F 3 is found to be as high as 13 MHz; the maximum value of f o F 3 during the LSA is only 7 MHz. The f o F 3 values lie in the ranges of 8-13 MHz in the HSA and 5-7 MHz in the LSA. Note that the difference is found to be more in the equinox and winter solstice than in the summer solstice. Further, f o F 3 is found to be the highest in the equinox, followed by the winter solstice and the summer solstice. In the summer solstice, the mean value of f o F 3 is found to have an increasing trend with time in the HSA as against that in the LSA when the mean f o F 3 does not change with time. Seasonal variation in h m F 3, however, show very different picture. In the summer and winter solstices, while h m F 3 is found to be higher in the HSA than in the LSA, it is found to be just reverse in the equinox, i.e. h m F 3 is higher in the LSA than in the HSA. Further, h m F 3 is found to be the highest in the HSA summer solstice (the mean value is as high as 700 km) and continues to be the same for most part of the day. In the LSA equinox, while the mean value of h m F 3 attains an altitude of 700 km, it is observed only for a short time (about an hour) during the morning hours ( hrs IST) and decreases to an altitude of ~600 km. On an average, the mean values of h m F 3 for the HSA are found to be 500, 700 and 600 km in the equinox, summer solstice and winter solstice, respectively; while those for the LSA are 600, 600, and 500 km, respectively. Further, note that there exists large variation in the h m F 3 irrespective of local time, season and solar activity levels unlike those of f o F 3. Fig. 5 Local time and day-to-day variations in the F 3 layer occurrence for: (a) equinox, (b) summer solstice, and (c) winter solstice, observed during the HSA (left panels) and LSA (right panels) conditions (dotted lines represent no-observation on those days)

6 126 INDIAN J RADIO & SPACE PHYS, APRIL 2012 Fig. 6 Mean and standard deviation of the F 3 layer critical plasma frequency, f o F 3 (left panels) and virtual height, h m F 3 (right panels) as a function of local time corresponding to: (a) equinox, (b) summer solstice, and (c) winter solstice (observations made during HSA in red color and LSA in black color) 3 Results and Discussions Important results on the characteristics and occurrence statistics of the F 3 layer for the HSA and LSA periods observed from 6.5 o N magnetic latitude in the Indian sector can be summarized as follows. 1. Structure of the ionogram, displaying the F 3 layer, is very distinct and clear in the LSA as compared to that observed in the HSA. 2. The occurrence rate and the duration of occurrence of the F 3 layer are maximum during the summer solstice, moderate during the equinox, and minimum during the winter solstice. 3. While there is no significant difference in the occurrence of the F 3 layer in the equinox and the summer solstice during the HSA and LSA periods, in the winter solstice it is extremely low in the LSA when compared to that of the HSA. 4. Mean f o F 3 is as high as 13 MHz in the HSA and 7 MHz in the LSA. In general, mean f o F 3 is found to be 4-6 MHz higher in the HSA than in the LSA. 5. Mean h m F 3 in the summer and winter solstices are higher in the HSA than in the LSA. In the equinox, however, it is higher in the LSA than in the HSA. As far as the morphology of the ionograms observed during the HSA and LSA is concerned, it is found that the layers are much clearer in the LSA than in the HSA. This morphological difference clearly suggests the role of the solar activity on the layer properties, including the F 3 layer. The morphological difference reported here is consistent with the model prediction of Balan et al. 9 13, 21 and those reported earlier based on ionosonde observations. The occurrence statistics, however, is found to be similar on some counts and drastically different on some other counts when compared with those reported earlier from low latitudes including those from the Indian sector. Ramarao et al. 13, based on a detailed study using ionosonde observations from Waltair (17.7 o N, 83.3 o E, magnetic latitude 8.2 o N)

7 CHAITANYA et al.: CONTRASTING FEATURES OF F 3 LAYER DURING HSA AND LSA 127 reported that the F 3 layer occurrence rate including the duration of occurrence is maximum during the summer of the LSA and the occurrence rate decreases as the solar activity increases. They also showed that the occurrence rate of the F 3 layer was maximum in the summer solstice and minimum in the winter solstice. Further, based on case studies using ionosonde observations from four different magnetic latitudes in the Indian sector, namely Trivandrum (0.5 o N), Sriharikota (6.5 o N), Waltair (8.2 o N) and Ahmedabad (14.4 o N), Ramarao et al. 13 conjectured that the latitude of Waltair is the most favourable location for the observation of the F 3 layer. Sreeja et al. 18, based on ionosonde observations from Trivandrum (8.5 o N, 77 o E, magnetic latitude 0.5 o N), found that while the occurrence of the F 3 layer is low (< 30%) when compared to that of Waltair, the occurrence rate is higher in the LSA than in the HAS, which is in agreement with those reported by Ramarao et al. 13. The seasonal variation, however, was shown to be different for Trivandrum. In the LSA, the occurrence was shown to be maximum in the summer solstice while in the HSA it was maximum in the equinox. The low occurrence rate over Trivandrum was found not to be consistent with that predicted for the equatorial latitude by Balan et al. 1 It was, however, found that while the summer time occurrence is indeed the most, the occurrence in terms of total number of days are higher in the LSA than in the HSA [Fig. 3(c)], but in terms of total number of hours they are identical both for the HSA and LSA (Fig. 4), which is different from those reported from Waltair 13 and Trivandrum 18. However, seasonal variations of the F 3 layer occurrence shown by Ramarao et al. 13 and those shown here are very consistent with each other irrespective of solar activity. From the perspective of seasonal variation of the F 3 layer occurrence, however, these results are different from those of Trivandrum 18, where the maximum occurrence was shown to be in the summer solstice for the LSA and in the equinox for the HSA. The above results apparently suggest that the occurrence of the F 3 layer both in terms of solar activity and season is different from one location to another. In addition to the underlying governing processes at the three magnetic latitudes, there are two possibilities that would have resulted in this difference: (1) different periods of data used for the analysis; and (2) the critical frequency of the F 3 layer, f o F 3 (i.e. f o F 3 > f o F 2 ) that decides the visibility of the layer by a ground based ionosonde. It is believed that the discrepancy in the seasonal occurrence of the F 3 layer at the three Indian latitudes to some extent might have come from the uncommon datasets used in these studies. Coming to the discrepancy of the results shown by Sreeja et al. 18 that the occurrence rate of the F 3 layer at the equatorial location Trivandrum was not consistent with that predicted by Balan et al. 1, it is believed that it may be linked with magnetically equatorial F 3 layer having relatively low value of f o F 3 as compared to f o F 2. Topside sounder observations would be much useful in resolving this. Coming to the studies made from other longitudes, observations from Fortaleza (4 o S, 38 o W, magnetic latitude 5.5 S) in the Brazilian sector, reported by Batista et al. 12 showed that the F 3 layer occurs maximum in the summer solstice (75%) followed by the winter solstice (65%). They found that the F 3 layer occurrence decreased as the solar activity increased. For Fortaleza, however, the dip angle was also varying at the rate of ~ 0.5 o per year, which also played a role in the occurrence pattern as a function of solar activity. On the other hand, observations from Sao Jose dos Campos (23.2 o S, 45 o W, magnetic latitude 17.6 o S), another location in Brazil, reported by Fagundes et al. 21 showed that the F 3 layer occurrence was much higher in the HSA than in the LSA; and while during the HSA, the occurrence was maximum in the summer solstice and minimum in the winter solstice; during the LSA, there was no seasonal variation in the F 3 layer occurrence. Thus the large occurrence rate (65%) observed in the winter solstice at Fortaleza is apparently different from that observed over Sao Jose dos Campos and also from those reported here as well as those reported earlier from the Indian longitude. Zain et al. 17, based on ionosonde observations from Parit Raja (1.86 o N, o E) an equatorial location in Malaysia, reported that during the LSA year of 2005, the F 3 layer occurred more in the winter solstice and in the equinoxes. This finding apparently is at variance with those reported earlier from the Indian sector and those reported here. In a more recent study based on GPS-RO observations, Zhao et al. 22 showed that the occurrence usually maximizes in the summer months and at the ±7-8 o magnetic latitude belt. Further, they found that the occurrence has a clear longitudinal dependence during the summer, with relatively higher occurrence at -80 to -100 o, -20 to 20 o, 80 to 120 o and -160

8 128 INDIAN J RADIO & SPACE PHYS, APRIL 2012 to -170 o longitudes, suggesting the association of wavenumber-3 diurnal tide (DE3). Thampi et al. 16, using TEC observations, also found that the position of occurrence of the F 3 layer was 7-8 o magnetic latitude. Thus, while the observations reported here and those reported from Waltair 13, both from low-latitude (6.5 o N and 8.2 o N, respectively), are in agreement with each other and agree with those reported by Zhao et al. 22 in terms of occurrence rate and seasonal variation, they differ from those reported by Fortaleza 12 and Parit Raja 17 in terms of seasonal pattern. Balan et al. 9 predicted that high occurrence rate of the F 3 layer could extend to the winter side of the geomagnetic equator if a strong upward E B drift, or a weak poleward wind, or a combination of the two, exists. Thus, it is quite likely that such favourable conditions might be prevalent in the observations reported from Fortaleza 12 and Parit Raja 17. This implies that in the absence of E B drift and meridional neutral wind measurements, it would be difficult to uncover the truth of the observed seasonal variations of F 3 layer occurrence reported from different longitude/latitude sectors. In this context, it is important to mention that at Gadanki, the authors have started measuring the E B drift using the daytime 150 km echoes 27 and it would be possible to examine the effect of E B drift predicted by Balan et al. 9, a task to be taken up in a future study. Coming to f o F 3, it has been found that it was the highest in the equinox and the lowest in the summer solstice. Further, the f o F 3 values were 8-13 MHz in the HSA as compared to 5-7 MHz in the LSA. These results are consistent with those expected from solar activity dependence of the F-layer electron density. Based on observations from Waltair, Ramarao et al. 13 found that the average values of f o F 3 were 8 MHz during 1997 and 9 MHz during The average F 10.7 was 73 in 1997 and 106 in The f o F 3 values for the deep solar minimum period of reported here are found to be 5-7 MHz, which are lower than those of reported from Waltair 13. The average F 10.7 for the period was 63, which was lower than that of Thus, the f o F 3 values presented for the deep solar minimum period of are 2-3 MHz lower than those of Further, it has been found that our results on f o F 3 for the long deep solar minimum are very similar to those reported earlier from Fortaleza by Balan et al. 9 and Parit Raja by Zain et al. 17 Thus, the observed f o F 3 values are quite consistent with those expected from solar activity dependence of the F-layer electron density. As far as h m F 3 is concerned, while it is higher in the summer and winter solstices of the HSA than those in the LSA, interestingly in the equinox, it is found to be higher in the LSA than in the HSA. Also, it has been found that there exists large variability of h m F 3, as evident from large standard deviation, irrespective of local time, season and solar activity levels. In general, the observed values of h m F 3 are larger than those reported from Waltair 13 and of the same order, except for the LSA equinox, as those reported from Fortaleza 9. Observations of h m F 3 reported by Balan et al. 9 and Ramarao et al. 13 also showed large day-today variability when compared to those of f o F 3 and are quite similar to those reported here. It implies that h m F 3 is highly dynamic and controlled by dynamical factors such as E B drift and meridional neutral wind 9. The observed difference in h m F 3 during the HSA and LSA equinox does not appear to be consistent with those expected from solar cycle dependence of E B drift 28 alone. This aspect needs a detailed modeling study incorporating the measured E B drift and meridional neutral wind. In conclusion, while much of the features of the F 3 layer, including their seasonal and solar activity dependence behaviour, are found to be consistent with those reported from other low-latitude locations, a few differences also exist, which need further investigation. Notably, the results presented here are in good agreement with those expected from theoretical/modeling point of view 1,9. As far as the day-to-day and seasonal variabilities are concerned, it is possible to evaluate these by incorporating the E B drifts 27 in a suitable model, which will be carried out in the future study. Acknowledgements The work was carried out at National Atmospheric Research Laboratory (NARL) and one of the authors (PPC) gratefully acknowledges the support provided by NARL for carrying out this research work. References 1 Balan N, Bailey G J, Abdu M A, Oyama K I, Richards P G, MacDougall J & Batista I S, Equatorial plasma fountain and its effects over three locations: Evidence for an additional layer, the F 3 layer, J Geophys Res (USA), 102 (1997) 2047, doi: /95JA Sen H Y, Stratification of the F2-layer of the ionosphere over Singapore, J Geophys Res (USA), 54 (1949) Ratcliffe J A, Some regularities in the F2 region of the ionosphere, J Geophys Res (USA), 56 (1951) 487.

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