Indian Journal of Radio & Space Physics Vol 41, April 2012, pp 208-219

Equatorial F-region irregularities during low and high solar activity conditions

L M Joshi1,$,*, A K Patra1,# & S V B Rao2

1National Atmospheric Research Laboratory, Gadanki (AP) 517 112 2Department of Physics, S V University, Tirupati (AP) 517 502

E-mail: $lalit@narl.gov.in, #akpatra@narl.gov.in, patra_nmrf@rediffmail.com

Received October 2011; accepted 26 March 2012

In this paper a comparative study on equatorial F-region irregularities observed during the extremely low solar activity period of 2007-2009 and high solar activity period of 2002-2003 based on the Gadanki radar and ionosonde observations has been made. Observations show that the occurrence probability of the F-region field-aligned irregularities (FAI) on night-to-night basis was 65% in the high solar activity and 30-40% in the low solar activity. During the high solar activity condition, F-region FAI extended to higher altitude (600 km) than those in the low solar activity condition (400-500 km). Intriguingly, the altitude extent of the FAI during 2008 was 100 km higher than that of 2009. The lowest altitude at which the F-region FAI were observed during the high solar activity period was 250 km while it was 200 km during the low solar activity period of 2008-2009. During the high solar activity condition, the high altitude plume structures were observed earlier than those observed in the low solar activity period. Also, in the high solar activity period, the FAI were mostly confined to pre-midnight hours while during the low solar activity condition, the FAI extended well beyond post-midnight and to dawn hours. While the duration of the ionosonde observed range-spread echoes was generally consistent with that of the radar observed F-region FAI echoes, on a few occasions, range-spread echoes were observed during the post-midnight hours when radar echoes were completely absent. Concurrent observations of Es layer displayed descending behaviour reminiscent of tidal/gravity wave associated winds. Importantly, the descending property led to disruption of Es during the evening hours when equatorial plasma bubbles are formed. Interestingly, some of the high altitude plumes appear to be linked with such Es disruption. Examination of the growth perspective of the Rayleigh-Taylor instability suggests that even though the height of F-layer in the post sunset hour was lower during the low solar activity period than that of the high solar activity period. The chance of bubble formation is near equal owing to the advantage of lower ion-neutral collision frequency in the low solar activity than in the high solar activity. Detailed observational and theoretical results, including the E-F region coupling, are presented and discussed in the light of the current understanding of the equatorial plasma bubble phenomenon.

Keywords: Equatorial F-region irregularities, Field-aligned irregularities (FAI), High altitude plume structures, Equatorial plasma bubbles, E-F region coupling

PACS Nos: 94.20.dj; 94.20.dt; 94.20.Vv; 96.60.Q-

1 Introduction Solar minimum between the solar cycles 23 and 24

was unique amongst last several solar minima for two reasons. Firstly, the solar activity during this period was the quietest in last 60 years. Secondly, it prolonged for much more time than that of other solar minimum period. Impact of this extreme low solar period on terrestrial environment is visible in the form of extreme low level of thermospheric density1,2. Recent findings suggest that in addition to the low solar activity, increased greenhouse gas emissions due to anthropogenic activities have contributed significantly to the recent extreme low level of thermospheric density through radiative or IR cooling3,4. Over the last few years, two different

aspects of climatological research related to ionosphere-thermosphere system have emerged. First one is related to the role of natural and anthropogenic activities on thermospheric density and/or temperature and its relevance to global change research. Second one is related to the role of thermospheric density and/or temperature changes on the plasma instability processes in the ionosphere.

As far as F-region plasma irregularities are concerned, ionosonde observations showed that with declining solar activity, the occurrence of post-sunset spread-F decreases while the occurrence of the post-midnight spread-F increases5. On the other hand, radar observations made using the JULIA radar from Jicamarca showed that during the high solar activity,

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the irregularities occur earlier, extend to higher altitude, and vanish earlier than those of low solar activity6. JULIA observations also revealed that the post-midnight F-region irregularities occur most frequently during solar minimum in consistence with those of ionosonde observations. Morphologically, however, no significant variation was found between high and low solar activity. Gadanki radar observations made during the low solar activity of 2008, however, exhibited remarkable difference in the morphology of F-region irregularities when compared to those observed during the high solar activity7,8. During the low solar activity period of 2008, the post-sunset F-region irregularities were shown to extend to the post-midnight hours7 and the post-midnight F-region irregularities observed during the June solstice showed the occurrence probability to be nearly 100 percent8. Based on observations made using a 30.8 MHz radar from Kototabang, Indonesia, Otsuka et al.9 also observed frequent occurrence of post-midnight irregularities during the June solstice and found that they either drift westward or have no drift. Interestingly, Otsuka et al.10 found that such irregularities were related to meso-scale traveling ionospheric disturbance (MSTID) and proposed the E-F region coupling effect as the cause of the instability growth.

In this paper, Gadanki MST radar observations of F-region field-aligned irregularities (FAI) and ionosonde observations made, during the March equinox of 2002-2003 corresponding to high solar activity, and 2008-2009 corresponding to extreme low solar activity condition, have been analysed. While the observations made during the high and low solar activity conditions were found to be different, interestingly significant difference has been found between the observations made during 2008 and 2009. These results are presented and discussed in the light of current understanding of F-region irregularities in terms of solar activity dependence. The E-F region coupling aspects linked with the post-sunset ionosphere applicable to the contrasting solar activity conditions have also been discussed. 2 Experiments and Data

Observations, used in the present study, were made using the MST radar located at Gadanki (13.5°N, 79.2°E, 6.5°N magnetic latitude) and an ionosonde operated from either Gadanki or Shriharikota, a location ~ 100 km east of Gadanki.

Gadanki MST radar is a high power pulsed Doppler radar, which is operated at 53 MHz with a peak power of 2.5 MW. The antenna array, consisting of 1024 Yagi-Uda antenna, provides a radiation pattern having half power full width of 2.8° and the antenna beam can be positioned at any off zenith angle within ±20° in the east-west or north-south plane. More details on the radar system are given by Rao et al.11. For studying the ionospheric field-aligned irregularities (FAI), the radar beam was positioned at a zenith angle of 15° towards north, which made it perpendicular to earth’s magnetic field12. Radar observations were made with range and time resolutions of 2.4 km and 30 s, respectively. Only the March equinoxial observations, made during 2002-2003 and 2008-2009 when the F-region FAI are known to be initiated soon after the sunset through the Rayleigh-Taylor (RT) instability, have been used. Observations were made in the form of Doppler power spectrum, which were processed offline to obtain signal-to-noise ratio (SNR), mean Doppler velocity, and spectral width (2σ).

Ionosonde observations used here were made quarter hourly using an IPS-42 digital ionosonde. This ionosonde works in the frequency range of 1-22.6 MHz and provides ionogram with a range resolution of about 3 km. Ionograms were scaled off-line to obtain virtual heights and frequencies of reflected/ scattered signals from the E and F-layers. It may be noted that the ionosonde observations prior to August 2008 were made from Sriharikota and those of latter period were made from Gadanki. 3 Solar activity conditions and magnetic field geometry relevant to E-F region coupling

Figure 1 shows daily values of 10.7 cm solar flux (F10.7), as a measure of solar activity, for the period 2000-2011. Note that F10.7 was ~63 during 2008-2009 and 100-200 during 2002-2003 for which observations are presented and discussed.

Gadanki is located at 6.5oN magnetic latitude and hence, observations made from Gadanki offer unique opportunity not only to investigate the equatorial F-region plasma irregularities but also to study the electrodynamical coupling between the low-latitude E region and the equatorial F-region since the E-region over Gadanki connects to the bottom part of the F-region over the magnetic equator. Figure 2 shows the magnetic field geometry illustrating the magnetic field line connection between the ionospheric regions

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over Gadanki and those over the equator. The shaded region represents the E-region. Note that the E-region over Gadanki (6.5°N dip latitude) is connected to the F-region over the geomagnetic equator at an altitude of about 220 km via the magnetic field lines. Also, it may be noted that 350 km at the F-region over the magnetic equator connects the low-latitude E-region at ~10° magnetic latitude. 4 Observational results 4.1 F-region FAI occurrence

F-region FAI were observed on 21 nights out of 32 nights in 2002-2003, 15 nights out of 37 nights in 2008, and 12 nights out of 38 nights in 2009. This means that the occurrence probability of F-region FAI

on a given night varied from 65% in 2002-2003 to 40% in 2008 and 31% in 2009, implying that the occurrence rate was lower during the low solar activity than those in the high solar activity. Further, the FAI occurrence in 2009 is lower than that of 2008.

In Figs 3(a-c), the height-time occurrence of F-region FAI observed by the Gadanki radar during 2002-2003, 2008, and 2009, respectively are shown. The occurrence statistics are derived based on 15 nights of F-region FAI observations each for 2002-2003 and 2008, and 12 nights for 2009. Height and time resolutions of these maps are 4.8 km and 15 minutes, respectively. White portions represent data gaps. In the high solar activity condition of 2002-2003, the F-region FAI is extended to altitudes as high as 600 km while those during the low solar activity condition of 2008 and 2009, these are confined to only 500 km. Further, the FAI are mostly confined to pre-midnight during the high solar activity condition while they extended to post-midnight hours during low solar activity condition. It is interesting to notice couple of differences in the height-time occurrence of FAI in 2008 and 2009 despite the fact that both in 2008 and 2009, the average F10.7 was 63. FAI confined to 400 km in 2009 while they extended to 500 km in 2008. FAI were observed for longer period in the post-midnight hours in 2008 than in 2009. Also, FAI were observed during the dawn time in 2008 but not in 2009. 4.2 Case studies

In an effort to understand the contrasting behaviour of the F-region irregularities during the high and low solar activity conditions, a few case studies are presented here.

Fig. 1 — Daily values of 10.7 cm solar flux (F10.7) as a measure of solar activity for the period 2000-2011

Fig. 2 — Low-latitude magnetic field geometry and the different regions of the ionosphere depicting how different regions are connected to each other through the conducting magnetic field lines (E-region over Gadanki connects 220 km altitude over the magnetic equator)

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4.2.1 Observations during the high solar activity of 2003 Figures 4(a-b) and 4(c-d) illustrate the observations

made on 25 March 2003 and 28 March 2003, respectively. Radar observed FAI are presented in panels (a) and (c) in the form of height-time SNR map and ionosonde observed F-region echoes are presented in panels (b) and (d) in the form of height variation of the F-region echoes, including the range spread. In panels (b) and (d), while the height of the box (parallel to the y-axis) indicates the range spread of the F-region trace seen in ionograms, the base level of the box represents the virtual height of the base of the F-layer echoes (h’F).

In the radar observations made on 25 March 2003, shown in panel (a), well separated periodic plumes, representing plasma bubbles, are clearly seen along with bottom-type and bottom-side irregularity bands.

Bottom-type irregularities started appearing one hour prior to the occurrence of the first plume. It is clearly seen that plasma bubbles penetrated to the top side with the presence of irregularities seen up to 600 km over Gadanki at 23:00 hrs IST. The apex altitude of these structures at the magnetic equator is estimated to be 750 km. One can also notice the weakening of the E-region FAI echoes during the appearance of the first F-region plume in the radar map. During the later phase, however, the E-region FAI echoes were found to be strong as indicated by SNR values.

On the same night, ionosonde observations, shown in panel (b), suggest that h’F ascended to ~350 km and subsequently the equatorial spread-F commenced. A sinusoidal pattern in the range spread-F can be seen, which lasted for a longer period than those of the radar observed FAI. It can also be seen that

Fig. 3 — Height-time occurrence (in percentage) of F-region field-aligned irregularities (FAI) as observed by the MST radar during: (a) 2002-2003, (b) 2008, and (c) 2009

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spread-F was not observed at ~22:00 hrs IST, which is very much consistent with non-occurrence of FAI at that time (as can be seen in the radar observed SNR map). This seems to indicate that ionosonde was capable of resolving the large scale irregularities in the F-region despite having wide field of view possibly due to large separation between the plumes observed on this night. Also, note that the ionosonde detected range spread-F at ~19:45 hrs IST, whereas the radar map shows only bottom-type irregularities at that time. It is quite likely that the ionosonde observations of spread-F during that period are either due to the bottom-type irregularities that were seen by the radar or due to plasma bubble that occurred slightly east of the radar field of view, which subsequently drifted eastward and thus was not seen by the radar.

In Fig. 4, panels (c and d) illustrate the results obtained on 28 March 2003. Panel (c) illustrates the occurrence of bottom-type irregularities starting at

19:30 hrs IST and followed by plume structures at 20:00 hrs IST. After the occurrence of these plumes, the bottom-side irregularities were seen to descend and disappear at 20:30 hrs IST. After a brief period, the valley region irregularities also occurred. A few patches of F-region FAI also can be seen at ~300 km during 22:00-23:30 hrs IST. It is important to mention that the sequence of events involving the occurrence of bottom-type irregularities, plume structures, subsequent descent of the bottom-side echoing layer and finally after a brief gap the occurrence of valley region irregularities, were very similar to those reported earlier by Patra et al.13 based on the observations made using the same radar.

As can be seen from panel (b), h’F was uplifted to ~350 km prior to the occurrence of spread-F echoes as well as the plume structures. Spread-F echoes were observed only during 19:45-20:45 hrs IST in consistence with the radar observations. It may be noted that

Fig. 4 — (a) Height-time-SNR map of F-region FAI observed by the MST radar on 25 March 2003, (b) ionosonde observations displaying the range spread echoes (vertically elongated box) on 25 March 2003, (c) Height-time-SNR map of F-region FAI observed by the MST radar on 28 March 2003, and (d) ionosonde observations displaying the range spread echoes (vertically elongated box) on 28 March 2003

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corresponding to the patchy type structures in the radar observations during 22:00-23:30 hrs IST, there was no spread-F echoes in the ionosonde observations. 4.2.2 Observations during the low solar activity of 2008

Figure 5 shows the results obtained on 5 February and 8 March 2008 in the same way as those shown in Fig. 4. Panel (a) shows a positively tilted radar plume to commence at 21:00 hrs IST and extend to 500 km in altitude. Subsequently, plumes were also observed, but they were confined to 400 km in altitude. It may be noticed that in the post-midnight period, the plumes were seen to stretch in time, indicating longer dwell time with no height migration of the FAI in the radar field of view than those of pre-midnight period. It may be noted that during pre-midnight hours, the

E-region FAI were weak and occurred sporadically. In the post-midnight hours, they got intensified.

Ionosonde observations [panel (b)] show that spread-F was observed concurrently, but there was no significant height rise of h’F unlike those observed in 2003. It may also be noticed that the base height of the F-region FAI in the radar observations is also lower than those of 2003. It is interesting to note that although radar observations disappeared or weakened after 01:30 hrs IST, spread-F echoes were noted in the ionosonde observations until morning. In the post-midnight period, however, the range spread was somewhat smaller than those observed in the pre-midnight period. Moreover, the post-midnight spread-F was accompanied by frequency spread-F during 03:00-06:00 hrs IST when foF2 was ≤ 4 MHz.

Fig. 5 — (a) Height-time-SNR map of F-region FAI observed by the MST radar on 5 February 2008, (b) ionosonde observations displaying the range spread echoes (vertically elongated box) on 5 February 2008, (c) Height-time-SNR map of F-region FAI observed by the MST radar on 8 March 2008, and (d) ionosonde observations displaying the range spread echoes (vertically elongated box) on 8 March 2008

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The results of 8 March 2008, illustrated in panels (c and d), also show plume activity, but in this case the plume was observed at an earlier time than that of 5 February. In this case, the F-region FAI are characterized with periodic plume structures. The temporal separations of the plumes in this case, however, are smaller than those observed on 25 March 2003 [Fig. 4(a)]. In this case, also the E-region FAI clearly disappeared during the early phase of the plume events (prior to 22:30 hrs IST). Ionosonde observations [panel (d)] clearly display range spread to occur right at the onset time of the radar plume and continued with the same range spread until 23:00 hrs IST. After 23:00 hrs IST, although the range spread decreased in consistence with the decrease in radar echo SNR, range spread was observed after the disappearance of the radar echoes also. It may, however, be noted that the morphology of the plumes observed after 23:00 hrs

IST was different than those observed prior to 23:00 hrs IST. The plumes seem to be stretched in time without vertical development indicating the decaying signature of the F-region FAI. 4.2.3 Observations during the low solar activity of 2009

Figure 6 shows the observations made on 23 March and 21 April 2009. Radar observations made on 23 March 2009, illustrated in panel (a), are found to occur in the altitude range of 200-400 km. Note that the upper limit of the radar echoing height is only 400 km unlike those observed during the high solar activity period. Assuming that the plume have mapped from the magnetic equator, the apex height of these plumes over the magnetic equator is estimated to be 550 km. During the post-midnight hours, a few weak patches of irregularities centering on 200 km altitude were also observed. Ionosonde observations (panel b) clearly show occurrence of range spread-F

Fig. 6 — (a) Height-time-SNR map of F-region FAI observed by the MST radar on 23 March 2009, (b) ionosonde observations displaying the range spread echoes (vertically elongated box) on 23 March 2009, (c) Height-time-SNR map of F region FAI observed by the MST radar on 21 April 2009, and (d) ionosonde observations displaying the range spread echoes (vertically elongated box) on 21 April 2009

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during 21:00-02:00 hrs IST. It may be noted that despite the radar observations of FAI were insignificant after ~23:00 hrs IST, ionosonde observations clearly reveal the occurrence of range spread-F. The range spread in this case, however, is smaller than those observed in 2008. The base height of the F-layer trace (which includes h’F as well as the base height of the range spread echoes) shows quasi-sinusoidal height migration as a function of time. The plume structures are found to be associated with the upliftment of the F-layer height. During the second upliftment of the F-layer, although the F-region FAI was observed by the radar, they were very weak in SNR and also in height extent, possibly indicating the presence of already decayed irregularities.

Coming to the E-region observations, E-region FAI echoes were seen to be absent during the occurrence of F-region plume structures. During this period, however, valley region FAI structures can be observed. It is interesting to note that after ~ 23:00 hrs IST, E-region FAI were present despite the occurrence of spread-F in the ionosonde observations and weak radar echoes from the F-region.

Panels (c and d) show the observations made on 21 April 2009. Radar observations of the F-region FAI were observed during 20:30 - 00:30 hrs IST and were confined to the altitude region of 200-350 km. Although plume type structures can be noted in the SNR map, they did not extend to the topside on this day. Ionosonde observations suggest that the F-layer descended to 200 km at 19:30 hrs IST, which subsequently ascended to 300 km at 20:30 hrs IST and then spread-F was observed. Spread-F continued until 01:30 hrs IST. The range spread is similar to that of 23 March 2009 and hence, is lower than those observed in 2008. In this case, however, h’F variations do not show quasi-sinusoidal variations unlike that of 23 March 2009. Interestingly, in this case, range spread was observed with the maximum range of the echoes reaching 400 km, which is more than that observed as 350 km in the radar observations. These two datasets suggest that the ionosonde echoes with larger range extent than those of radar may be due to the detection of oblique echoes (which resulted in large range spread-F in the ionogram).

Coming to the E-region observations, several interesting aspects are found in this case compared to those on 23 March 2009. As far as the E-region FAI echoes were concerned, they were observed until the

commencement of the first plume and the FAI echoes disappeared during 21:30-00:30 hrs IST, when the plumes were observed. Once the plume occurrence was ceased, the E-region FAI occurrence was reinitiated and continued during the rest of the observational period. During the later part, E-region FAI also show quasi-periodic (QP) echo characteristics14. During the disruption of the E-region FAI echoes, however, valley region echoes were observed for some time. 4.3 Es characteristics

In an effort to evaluate the possible linkage of the low-latitude nighttime Es, which varies due to the variabilities in the neutral dynamical and metallic ions, with the equatorial plasma bubble phenomenon, three case studies have been picked up, one each from 2003, 2008 and 2009. The Es characteristics observed on the three nights are presented in Fig. 7. Figures 7 (a and b) show ftEs/fbEs and height of the Es layer, respectively, observed on 28 March 2003. Note the descending property with decreasing ftEs of the Es layer before its disruption during 20:00-21:00 hrs IST. After 21:00 hrs IST, it recurred at 160 km with ftEs of 2.5 MHz and descended to 125 km where ftEs reached a value of 6 MHz and the Es clearly displayed blanketing property with fbEs of 5 MHz. Es having descending properties observed soon after 21:00 hrs IST can be related to intermediate layer. No Es was observed during 22:30-04:15 hrs IST, indicating that E-region plasma density was well below the detectability limit of the ionosonde. Es observations made on 8 March 2008, which are shown in Figs 7 (c and d), also show disruption of Es after 19:00 hrs IST and their reappearance during 00:45-02:30 hrs IST. In this case also, prior to the disruption, Es layers showed descending properties. Es observations made on 21 April 2009, which are shown in Figs 7(e and f), however, show very different properties than those observed on 28 March 2003 and 8 March 2008. Note that Es was observed throughout the night except for a very brief period during the midnight. Further, the Es was blanketing type with fbEs of about 3 MHz. Note that on this night also, Es observations show descending property. It is interesting to recall that on 28 March 2003 and 8 March 2009, the F-region plumes extended to higher altitude than those observed on 21 April 2009. Also the high altitude plumes observed on 28 March 2003 and 8 March 2008 occurred when the Es layer was seen to be disrupted.

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5 Summary and Discussion The results of the study can be summarized as:

1. Occurrence probability of the F-region FAI on night-to-night basis was 65% in the high solar activity of 2002-2003 and 30-40% in the low solar activity of 2008-2009.

2. During the high solar activity condition, F-region FAI extended to higher altitude (600 km) than those

in the low solar activity condition (400-500 km). Intriguingly, the altitude extent of the FAI during 2008 was 100 km higher than that of 2009.

3. The lowest altitude at which the F-region FAI were observed during the high solar activity period of 2002-2003 was 250 km while it was 200 km during the low solar activity period of 2008-2009.

Fig. 7 — Es parameters observed on 28 March 2003 (a and b), 8 March 2008 (c and d), and 21 April 2009 (e and f) [ftEs/fbEs shown in panels (a, c, and e); and the height of the Es and their range spread shown in panels (b, d, and f)

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4. During the high solar activity condition, the high altitude plume structures were observed at an earlier time than those observed in the low solar activity period. Also, in the high solar activity period, the FAI were mostly confined to pre-midnight hours while during the low solar activity condition, the FAI extended well beyond post-midnight and to dawn hours.

5. While the duration of the ionosonde observed range-spread echoes was generally consistent with that of the radar observed F-region FAI echoes, on a few occasions, range-spread echoes were observed during the post-midnight when radar echoes were completely absent.

6. Concurrent observations of Es layer displayed descending behaviour reminiscent of tidal/ gravity wave associated winds. Importantly, the descending property led to disruption of Es during the evening hours when equatorial plasma bubbles are formed. Interestingly, some of the high altitude plumes appear to be linked with such Es disruption.

As far as the F-region FAI occurrences and their height extents in the high and low solar activity are concerned, they are found to be broadly consistent with those reported by Hysell & Burcham6 based on Jicamarca radar observations. Especially, the FAI occurring earlier, extending to higher altitude, and decaying earlier during high solar activity than those during low solar activity are very similar to those reported by Hysell & Burcham6. The height extent of the FAI, however, is lower in the current observations than those reported by Hysell & Burcham6. Radar observations made from Trivandrum15 during high solar activity period of 1998-2000 showed that the plumes used to extend to 800 km and on occasions as high as 1200 km. Thus the height extent of the plumes observed at Gadanki is quite expected considering the fact that Jicamarca/Trivandrum is located at the magnetic equator whereas Gadanki is located at 6.5°N magnetic latitude. Further, the radar observations of FAI occurrence in the high and low solar activity are also consistent with those reported based on ionosonde observations5,16. Also the duration of spread-F observations longer than that of VHF radar observations of FAI reported here is in agreement with those reported earlier from Trivandrum15. This fact is also consistent with the understanding considering that small scale irregularities would disappear faster than larger scale irregularities

and ionosonde detects larger scale irregularities than that of VHF radar.

A finding that the lowest altitude of the FAI is only 200 km during the low solar activity period when compared to 250 km in the high solar activity period, however, needs evaluation in terms of the generation mechanism of the FAI themselves. At the magnetic equator, the F-region FAI which occur at the bottommost height of the F-region are usually characterized as bottom-type or bottom-side irregularities. When it comes to the low-latitude like Gadanki, most of the time, the low altitude irregularities do not look like the way the equatorial bottom-type irregularities do. Also, the equatorial bottom-type irregularities may not map since the scale sizes associated with such layer are generally small and their mapping efficiencies are low. This argument, however, does not preclude the locally generated bottom-type echoing layer if the bottom side of the F-layer is locally unstable through a shear driven instability. In fact, bottom-type layer has been observed on occasions such as that observed on 25 March 2003 [Fig. 4(a)], implying that such irregularity layers might have formed due to shear flow. On the other hand, observations suggest that the structures at altitudes as low as 200 km (in low solar activity) are reminiscence of plume structures, indicating the manifestation of plasma bubbles at such altitudes. This being the case, it is quite likely that such bubbles are of equatorial origin and they may be linked with equatorial plasma bubbles having base altitude of ~300 km. In the high solar activity, the observed bottom altitude is 250 km and that maps to ~350 km over the equator. Considering that they are directly linked with the equatorial plasma bubble, one can evaluate their origin in terms of the known premise of the RT instability.

Another finding is the occurrence of the high altitude plumes coinciding with the disruption of Es or alternately, the height extent of the radar plumes were confined to 400 km in the presence of strong Es activity. From the observations presented above, it may be recalled that while Es layer generally disappears after the sunset, it re-appears subsequently displaying descending pattern reminiscent of tidal/gravity wave winds. Such layers are composed of metallic ions, due to their long lifetime, and are formed by wind shear of tidal/gravity wave wind origin. If such metallic ion layer is located at the height of maximum Pedersen conductivity, it could

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contribute significantly to the field-line integrated Pedersen conductivity and could affect the growth rate of the RT instability.

Following Sultan17, a simplified growth rate expression for the RT instability, incorporating the field-line integrated conductivity, can be written as:

γ =in

gLFPEP

FPν

1)()(

)(×

+∑ ∑∑ … (1)

where, ∑P(E), is the field line integrated Pedersen conductivity in the E-region; ∑P(F), the field line integrated Pederson conductivity in the F-region; L, the ambient plasma density scale length; and νin, the ion neutral collision frequency. While ∑P(E) could vary drastically from one night to another due to the Es activity, the variation in g/νin is rather smooth.

In an effort to examine the role of solar activity on the growth of plasma bubble, g/νin has been estimated for the maximum and minimum solar activity conditions. Figure 8 shows the estimated values of g/νin as a function of height (bottom height of the F-layer) corresponding to the maximum and minimum solar activity conditions. Note that g/νin is nearly equal corresponding to the base height of 265 km in the solar minimum and 365 km in the solar maximum. This implies that the threshold base height of the F-layer decreases by about 100 km from the high solar activity to the low solar activity.

Jyoti et al.18, based on ionosonde observations from Trivandrum, found that prior to the onset of spread-F the threshold height of the F-layer increased from 370 km to 220 km with the decrease in the solar

activity. Their study, however, was linked with the bottom-side spread-F and hence does not relate directly with the occurrence of plasma bubble. In fact, Tiwari et al.15, based on simultaneous observations made using a 18 MHz radar and ionosonde from Trivandrum, clearly showed that in general bottom- side spread-F occurs prior to the onset of radar plume. Their observations also showed that prior to the occurrence of radar plumes, the base of the F-layer rose further while displaying bottom-side spread-F echoes, implying that the actual height of the F-layer was somewhat higher than those reported by Jyoti et al.18.

Coming back to the role of Es, it has been clearly found that their occurrence while follow a pattern mainly controlled by tidal winds, at times the Es could be much irregular and strong. One such case reported here was observed on 21 April 2009 and on this night, the plumes were confined to 350 km. Although these results provide adequate experimental evidence on role of the E-region Pedersen conductivity in the RT growth rate, a more rigorous examination, however, is required to establish the true role of Es in the equatorial plasma bubble phenomenon. It is proposed to examine this aspect in detail in future by incorporating required observations and model computation. Acknowledgments

The authors are grateful to the technical staff of National Atmospheric Research Laboratory (NARL) for carrying out the MST radar observations. One of the authors (LMJ) gratefully acknowledges NARL for providing research fellowship for carrying out this research work. References 1 Solomon S C, Woods T N, Didkovsky L V, Emmert J T

& Qian L, Anomalously low solar extreme ultraviolet irradiance and thermospheric density during solar minimum, Geophys Res Lett (USA), 37 (2010) L16103, doi: 10.1029/2010GL044468.

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Fig. 8—Height profiles of gνin

-1 corresponding to high and low solar activity conditions

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