8. OCCURRENCE OF A PECULIAR SEQUENTIAL Es AT KARACHI

 

Z. M. Khan, (Mrs) Husan Ara and G. Murtaza

Ionospheric Research Division, SUPARCO, Karachi, PAKISTAN

8.1. Abstract

This study reveals the frequent occurrence, maximising in Winter, of a peculiar sequential Es recorded by the Digisonde DGS-256 at Karachi during 1992-93. The sequential Es is peculiar in the sense that it is formed by a clear detachment of the bottom side of the F1-layer. The detached part of the F1-layer is converted, directly or through E2-layer, into a sequential Es; which descends from around 200km to about 130km at a velocity of about 6m/sec. A study of this sequential Es may enable us to deduce the neutral meridional wind pattern in the middle ionosphere at Karachi.

8.2. Introduction

Sequential Es (Sporadic E) is frequently the last part of a much longer sequence starting high up in the F-region (Robinson, 1960). Sequential Es appears preferentially at low latitudes and quite often it descends at a velocity of about 1m/sec, increasing in electron density and becoming thinner (Whitehead, 1970). Rawer (1962b, 1963) pointed out that the sequential Es did not occur often over the European zone. Bibl (1960) found both decreases and increases in h'Es with foEs varying in anti phase to it. Lanchester et al. (1991) reported that descending Sporadic-E Layers above Tromso are generated by a wind shear associated with the semi-diurnal solar tide. They, using the data of the EISCAT (European Incoherent SCATter) UHF radar, reported that on 3 and 4 August 1988, Sporadic-E Layers, associated with the afternoon wave of the semi-diurnal tide, were formed and descended from the bottom of the F1-layer at Tromso (I=76° N).

The main aim of this short paper is to present examples of a peculiar sequential Es, recorded by Digisonde DGS-256 during 1992-93 and a preliminary study thereof; which occurs frequently at Karachi (24.95° N, 67.14° E,I=36° N). This sequential Es is peculiar in the sense that it is formed by a clear detachment of the bottom side of the F1-layer, which is often converted directly into sequential Es of h-type and goes on descending to join E-layer at foE; thereby forming a c-type sequential Es. At times, the detached part of F1-layer is first converted partly into E2-layer and Es, which soon merge with each other forming a single sequential Es of h-type.

8.3. Examples of peculiar sequential Es at Karachi

Two clear examples of conversion of the bottom side of the F1-layer into 'h' type (high type) sequential Es at Karachi on 26-02-1993 and 01-07-1993 are shown in Figs. 1(a-g) and 2(a-h), respectively. Both sets of ionograms have the same format. The frequency scale starts at 1 MHz and there is a vertical dotted line marking each 1MHz increase in frequency. The frequency scale is linear. The vertical virtual height scale is also linear and starts at 50km with a horizontal line for each step of 50km. Virtual heights of 100km, 200km and 400km are marked on the ionograms, but may not be easily read.

Fig 1(a) shows the normal F1-layer at 1014 hours LT on 26-02-1993, while Fig, 1(b-c) shows the detachment of the bottom side of the F1-layer from the parent layer. In Fig. 1(b) the bottom side (3.6-4.2 MHz) of the F1-layer tends to get detached from the parent layer (indicating the onset of the sequential Es) at 1029 hours LT, while Fig. 1(c) shows the process of detachment is complete at 1044 hours LT. Later, this part of F1-layer is fully converted into h-type sequential Es at 1059 hours LT as is seen in Fig. 1(d). This sequential Es then continues to descend (Fig. 1(e-g)) until it joins E-layer at foE and is converted into a c-type sequential Es at 1144 hours LT.

Another example of conversion of the bottom side of the F1-layer into sequential Es, observed on 01-07-1993, is shown in Fig. 2(a-h). in this example, unlike the previous one, the bottom side (3.9-4.5 MHz) of the F1-layer is first converted partly into E2-layer and Es at 1344 hours LT, which persist up to 1359 hours LT (Fig. 2(b-c)). Later, E2-layer and Es merge together to form a single h-type sequential Es at 1429 hours LT (Fig. 2(e)); which continues to descend until it is ultimately converted into a c-type sequential Es at 1559 hours LT (Fig. 2(g)).

8.4. Results

A preliminary study of the peculiar sequential Es occurring at Karachi during 1992-93 has shown that:

(i) This is a daytime phenomenon which (irrespective of the season) starts at Karachi around noon and continues up to 1800 hours LT at the most. This phenomenon when started before noon is usually found to be stronger and prolonged.

(ii) The occurrence of sequential Es is more frequent in Winter (35%) than in Equinoxes (18%) and Summer (13%). In Winter (1992-93), its occurrence is found to be maximum in the month of November (53%)

(iii) The virtual height of the sequential Es is found to be around 200Kms at the time of its onset.

(iv) As the sequential Es descends, foEs often remains constant or shows a nominal increase. At times, foEs may increase abruptly when sequential Es is converted into a 'c' type Es.

(v) The downward speed, d(h'Es)/dt, of sequential Es is on average about 6m/sec.

(vi) The sequential Es is blanketing in nature.

8.5. Conclusions and Discussion

A study of the Karachi ionograms for the year 1992-93 reveals conversion of the bottom side of the F1-layer, directly or through E2-layer, into sequential Es. This peculiar sequential Es, with its onset around noon, descends from a virtual height of around 200Km to about 130Km at a velocity of about 6m/sec. A study of sequential Es, which normally descends from 200Km to 130Km, may help us to deduce the neutral meridional wind patterns in the middle ionosphere at Karachi. This is because the wind shear theory predicts that, below about 130Km, Es layers are controlled more by the zonal wind that by the meridional wind (Chen and Harris, 1971).

8.6. Acknowledgments

The authors feel highly indebted to Mr Sikandar Zaman, Chairman (SUPARCO) for his constant support and patronage to the cause of ionospheric studies in Pakistan. The authors also feel indebted to Dr M I Mirza, Member (Space Research) for his valuable advice and helpful suggestions during the course of this study. Dr Khatri, SSO and Mr M Wagar Hasan, SSO are acknowledged respectively for reading the proof of this paper and helping in the preparation of the diagrams included in it.

8.7. References

Bibi, K., Ann. Geophys., 16, 148, 1960

Chen, W. M. and Harris, R.D., J. Atmos. Terr. Phys., 33, 1193, 1971.

Lanchester, B. S., Nygren, T., Huuskonen, A., Turunen, T. and Jarvis, M. J., Planet. Space Sci., 39, 1421-1434, 1991.

Rawer, K., Ionospheric Sporadic E., edited by E. K. Smith and S. Matsushita, p. 292, Pergamon Press, New York, 1962.

Rawer, K., Proceedings of the International Conference on the Ionosphere, London, 1962, p. 239, Institute of Physics and Physical Society, 1963.

Robinson, B. J., J. Atmos. Terr. Phys., 19, 160, 1960.

Whitehead, J. D., Rev. of Geophys. and Space Phys., 8, p. 65-127, 1970.