References

ANGEO

Annales Geophysicae

ANGEO

Ann. Geophys.

1432-0576

Copernicus Publications

Göttingen, Germany

10.5194/angeo-24-2931-2006

Solar cycle dependent characteristics of the equatorial blanketing Es layers and associated irregularities

Devasia

C. V.

¹ Sreeja

¹ Ravindran

Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, 695022, India

21 11 2006

24 11 2931 2947

2006

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The occurrence of blanketing type Es (Esb) layers and associated E-region irregularities over the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip ~0.5°) during the summer solstitial months of May, June, July and August has been investigated in detail for the period 1986–2000 to bring out the variabilities in their characteristics with the solar cycle changes. The study has been made using the ionosonde and magnetometer data of Trivandrum from 1986–2000 along with the available data from the 54.95 MHz VHF backscatter radar at Trivandrum for the period 1995–2000. The appearance of blanketing Es layers during these months is observed to be mostly in association with the occurrence of afternoon Counter Electrojet (CEJ) events. The physical process leading to the occurrence of a CEJ event is mainly controlled by the nature of the prevailing electro dynamical/neutral dynamical conditions before the event. Hence it is natural that the Esb layer characteristics like the frequency of occurrence, onset time, intensity, nature of gradients in its top and bottom sides etc are also affected by the nature of the background electro dynamical /neutral dynamical processes which in turn are strongly controlled by the solar activity changes. The occurrence of Esb layers during the solstitial months is found to show very strong solar activity dependence with the occurrence frequency being very large during the solar minimum years and very low during solar maximum years. The intensity of the VHF radar backscattered signals from the Esb irregularities is observed to be controlled by the relative roles of the direction and magnitude of the prevailing vertical polarization electric field and the vertical electron density gradient of the prevailing Esb layer depending on the phase of the solar cycle. The gradient of the Esb layer shows a more dominant role in the generation of gradient instabilities during solar minimum periods while it is the electric field that has a more dominant role during solar maximum periods.

References 1

Abdu, M. A., Batista, I. S., Muralikrishna, P., and Sobral, J. H. A.: Long term changes in the sporadic E-layer and electric fields over Fortaleza, Brazil, Geophys. Res. Lett., 23, 757–760, 1996.

Ananda Rao, B. G. and Raghava Rao, R.: Structural changes in the current fields of the equatorial electrojet due to zonal and meridional winds, J. Geophys. Res., 92, 2514–2526, 1987.

Anderson, D., Anghel, A., Yumoto, K., Ishitsuka, M., and Kudeki, E.: Estimating daytime vertical ExB drift velocities in the equatorial F-region using ground-based magnetometer observations, Geophys. Res. Lett., 29, 1596, https://doi.org/10.1029/2001GL014562, 2002.

Balsley, B. B.: Electric fields in the equatorial ionosphere: A review of techniques and measurements, J. Atmos. Terr. Phys., 35, 1035–1044,1973.

Balsley, B. B. and Farley, D. T.: Radar observations of two-dimensional turbulence in the equatorial electrojet, J. Geophys. Res., 78, 7471–7479, 1973.

Buneman, O.: Excitation of field aligned sound waves by electron streams, Phys. Rev. Lett., 10, 285–287, 1963.

Crochet, M., Hanuise, C., and Broche, P.: HF radar studies of two-stream instability during an equatorial counter electrojet, J. Geophys. Res., 84, 5223–5233, 1979.

Devasia, C. V., Jyoti, N., Subbarao, K. S. V., Tiwari, D., Raghava Reddi, C., and Sridharan, R.: On the role of vertical electron density gradients in the generation of type II irregularities associated with blanketing Es(Esb) during counter equatorial electrojet events: A case study, RadioScience, 39, RS3007, https://doi.org/10.1029/2002RS002725, 2004.

Devasia, C. V.: Blanketing sporadic-E characteristics at the equatorial stations of Trivandrum and Kodaikanal, Indian J. Radio Space Phys., 5, 217–220, 1976.

Farley, D. T.: Theory of equatorial electrojet plasma waves: New developments and current status, J. Atmos. Terr. Phys., 47, 729–744, 1985.

Farley, D. T., Fejer, B. G., and Balsley, B. B.: Radar observations of two-dimensional turbulence in the equatorial electrojet, 3, Nighttime observations of type I waves, J. Geophys. Res., 83, 5625–5632, 1978.

Farley, D. T. and Balsley, B. B.: Instabilities in the equatorial electrojet, J. Geophys. Res., 78, 227–239, 1973.

Farley, D. T.: A plasma instability resulting in field-aligned irregularities in the ionosphere, J. Geophys. Res., 68, 6083–6097, 1963.

Fejer, B. G. and Kelley, M. C.: Ionospheric irregularities, Rev. Geophys., 18, 401–454, 1980.

Fejer, B. G., Farley, D. T., Balsley, B. B., and Woodman, R. F.: Vertical structure of the VHF backscattering regions in the equatorial electrojet and the gradient drift instability, J. Geophys. Res., 80, 1313–1324, 1975.

Forbes, J. M. and Lindzen, R. S.: Atmospheric solar tides and their electrodynamic effects-I The global Sq current system, J. Atmos. Terr. Phys., 38, 897–910, 1976a.

Forbes, J. M. and Lindzen, R. S.: Atmospheric solar tides and their electrodynamic effects- II The equatorial electrojet, J. Atmos. Terr. Phys., 38, 911–920, 1976b.

Hanuise, C., Mazaudier, C., Vila, P., Blanc, M., and Crochet, M.: Global dynamo simulation of ionospheric currents and their connection with the equatorial electrojet and counter electrojet: A case study, J. Geophys. Res., 88, 253–270, 1983.

Krishna Murty, B. V. and Sen Gupta, K.: Disappearance of equatorial Es associated with magnetic field depressions, Planet. Space Sci., 20, 371–378, 1972.

Kudeki, E. and Sürücü, F.: Radar interferometric imaging of field-aligned plasma irregularities in the equatorial electrojet, Geophys. Res. Lett., 18, 41–44, 1991.

Kudeki, E. and Farley, D. T.: Aspect sensitivity of equatorial electrojet irregularities and theoretical implications, J. Geophys. Res., 94, 426–434, 1989.

Kudeki, E., Fejer, B. G., Farley, D. T., and Hanuise, C.: The condor equatorial electrojet campaign: Radar results, J. Geophys. Res., 92, 13 561–13 577, 1987.

Kudeki, E., Farley, D. T., and Fejer, B. G.: Theory of spectral asymmetries and nonlinear currents in the equatorial electrojet, J. Geophys. Res., 90, 429–436, 1985.

Kudeki, E., Farley, D. T., and Fejer, B. G: Long wavelength irregularities in the equatorial eletrojet, Geophys. Res. Lett., 9, 684–687, 1982.

Marriott, R. T., Richmond, A. D., and Venkateswaran, S. V.: On quiet-time equatorial electrojet and counter electrojet, J. Geomagn. Geoelectr., 31, 311–340, 1979.

Mayaud, P. N.: The equatorial counter electrojet-A review of its geomagnetic aspects, J. Atmos. Terr. Phys., 39, 1055–1070, 1977.

Rogister, A. and D'Angelo, N.: Type II irregularities in the equatorial electrojet, J. Geophys. Res., 75, 3879–3887, 1970.

Rastogi, R. G.: The equatorial electrojet: Magnetic and ionospheric effects, Geomagnetism, 3, 461–525, Academic Press, Sandigo, Calif, 1989.

Rastogi, R. G., Chandra, H., and Chakravarty, S. C.: The disappearance of equatorial Es and the reversal of electrojet current, Proc. Indian Acad. Sci., 74, 62–67, 1971.

Reddy, C. A.: The equatorial electrojet, Pure Appl. Geophys., 131, 485–508, 1989.

Reddy, C. A. and Devasia, C. V.: Height and latitude structure of electric fields and currents due to local east-west winds in the equatorial electrojet, J. Geophys. Res., 86, 5751–5767, 1981.

Reddy, C. A. and Devasia, C. V.: VHF radar observation of gradient instabilities associated with blanketing Es layers in the equatorial electrojet, J. Geophys. Res., 82, 125–128, 1977.

Reddy, C. A. and Devasia, C. V.: Formation of blanketing sporadic E layers at the magnetic equator due to horizontal wind shears, Planet. Space Sci., 21, 811–817, 1973.

Richmond, A. D.: Equatorial electrojet-I. Development of a model including winds and instabilities, J. Atmos. Terr. Phys., 35, 1083–1103, 1973.

Rishbeth, H. and Garriott, O. K.: Introduction to ionospheric Physics, p. 199, Academic New York, 1969.

Smith, L. G.: Rocket observations of sporadic E and related features of the E region, Radio Sci., 1, 178–186, 1966.

Somayajulu, V. V., Selvamurugan, R., Devasia, C. V., and Cherian, L.: VHF backscatter radar observations of type-I waves during a counter electrojet event, Geophys. Res. Lett., 21, 2047–2050, 1994.

Somayajulu, V. V. and Viswanathan, K. S.: VHF radar observations during equatorial counter electrojet events, Indian J. Radio Space Phys., 16, 380–383, 1987.

Stening, R. J., Meek, C. E., and Manson, A. H.: Upper atmosphere wind systems during reverse equatorial electrojet events, Geophys. Res. Lett., 23, 3243–3246, 1996.

Stening, R. J.: What drives the equatorial electrojet, J. Atmos. Terr. Phys., 57, 1117–1128, 1995.

Stening, R. J.: Modelling the Equatorial Electrojet, J. Geophys. Res., 90, 1705–1719, 1985.

Woodman, R. F. and Chau, J. L.: First Jicamarca radar observations of two-stream E-region irregularities under daytime counter electrojet conditions, J. Geophys. Res., 107(A12), 1482, https://doi.org/10.1029/2002JA009362, 2002.