ANGEO Annales Geophysicae ANGEO 1432-0576 Copernicus GmbH Göttingen, Germany 10.5194/angeo-27-3781-2009 Intriguing aspects of F-region plasma irregularities revealed by the Gadanki radar observations during the SAFAR campaign Patra A. K. 1 Phanikumar D. V. 1 2 National Atmospheric Research Laboratory, Gadanki, India now at: Korea Astronomy and Space Science Institute, Daejeon, Republic of Korea 06 10 2009 27 10 3781 3790 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.ann-geophys.net/27/3781/2009/angeo-27-3781-2009.html The full text article is available as a PDF file from http://www.ann-geophys.net/27/3781/2009/angeo-27-3781-2009.pdf

Intriguing new results of F-region irregularities observed using the Gadanki MST radar during the SAFAR campaigns, which were conducted during the equinox and summer of 2008 that corresponds to low solar activity condition, are presented. The summer observations are first of its kind from Gadanki. Observations revealed remarkably different morphology of the F-region irregularities in summer when compared to that in equinox. In summer, the F-region irregularities were observed as horizontally stratified structures, while in equinox they were observed as plume structures. Further, the irregularities in summer commenced during the post-midnight hours in contrast to their commencement in the post-sunset hours and occurrence extending to post-midnight hours in equinox. In addition, an intriguing observation of the summer time irregularities is that they occurred when the background electron density was remarkably low as characterized by the disappearance of the F layer trace in the ionograms. An interesting event of equinox that was observed for 10 h and extended beyond the sunrise time displayed multiple plume structures having periods similar to those of the E-region velocity variations. These observations are discussed with due focus on the genesis of post-midnight F-region irregularities and their possible linkage to the E-region dynamics.

 References Burke, W. J.: Plasma bubbles near the dawn terminator in the topside ionosphere, Planet. Space. Sci., 27, 1187–1193, 1979. Chandra, H. and Rastogi, R. G.: General features of the ionosphere at Thumba, J. Inst . Telecom. Engrs. 17, 207–216, 1971. Chandra, H. and Rastogi, R. G.: Spread-F at magnetic equatorial station Thumba, Ann Geophys., 28, 37–44, 1972. Chandra, H., Vyas, G. D., Sinha, H. S. S., Misra, R. N., and Prakash, S.: Ionospheric scintillation observations from SHAR, J. Atmos. Terr. Phys., 54, 167–172, 1992. de La Beaujardiere, Retterer, J. M., Pfaff, R. F., Roddy, P. A., Roth, C., Burke, W. J., Su, Y. J., Kelley, M. C., Ilma, R. R., Wilson, G. R., Gentile, L. C., Hunton, D. E., and Cooke, D. L.: C/NOFS observations of deep plasma depletions at dawn, Geophys. Res. Lett. 36, L00C06, doi:10.1029/2009GL038884, 2009. Fejer, B. G., Scherliess, L., and De Paula, E. R.: Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F, J. Geophys. Res., 104, 19859–19869, 1999. Fritts, D. C., Vadas, S. L., Riggin, D. M., Abdu, M. A., Batista, I. S., Takahashi, H., Medeiros, A., Kamalabadi, F., Liu, H.-L., Fejer, B. G., and Taylor, M. J.: Gravity wave and tidal influences on equatorial spread F based on observations during the Spread F Experiment (SpreadFEx), Ann. Geophys., 26, 3235–3252, 2008. Haldoupis, C., Kelley, M. C., Hussey, G. C., and Shalimov, S.: Role of unstable sporadic-E layers in the generation of midlatitude spread F, J. Geophys. Res., 108, 1446, doi:10.1029/2003JA009956, 2003. Hari, S. S. and Krishna Murthy, B. V.: Equatorial night-time F-region zonal electric fields, Ann. Geophys., 13, 871–878, 1995. Huang, C.-S. and Kelley, M. C.: Nonlinear evolution of equatorial spread F 2. Gravity wave seeding of the Rayleigh-Taylor instability, J. Geophys. Res., 101, 293–302, 1996. Hysell, D. L., Kelley, M. C., Swartz, W. E., Farley, D. T., and Woodman, R. F.: Seeding and layering of equatorial spread F by gravity waves, J. Geophys. Res., 95, 17253–17260, 1990. Hysell, D. L. and Burcham, J. D.: Long term studies of equatorial spread F using the JULIA radar at Jicamarca. J. Atmos. Solar Terr. Phys., 64, 1531–1543, 2002. Hysell, D. L. and Kudeki, E.: Collisional shear instability in the equatorial F region ionosphere, J. Geophys. Res., 109, A11301, doi:10.1029/2004JA010636, 2004. Immel, T. J., Sagawa, E., England, S. L., Henderson, S. B., Hagan, M. E., Mende, S. B., Frey, H. U., Swenson, C. M., and Paxton, L. J.: Control of equatorial ionospheric morphology by atmospheric tides, Geophys. Res. Lett., 33, L15108, doi:10.1029/2006GL026161, 2006. Jayaraman, A., Venkatratnam, M., Patra, A. K., Rao, T. N., Sridharan, S., Rajeevan, M., Gadhavi, H., Kesarkar, A. P., Srinivasulu, P., and Raghunath, K.: Study of Atmospheric Forcing and Responses (SAFAR) campaign: Overview, Ann. Geophys., in review, 2009. Kelley, M. C., Larsen, M. F., LaHoz, C., and McClure, J. P.: Gravity wave initiation of equatorial spread F: A case study, J. Geophys. Res., 86, 9087–9100, 1981. Kelley, M. C., Rodrigues, F. S., Makela, J. J., Tsunoda, R. T., Roddy, P. A., Hunton, D. E., Retterer, J. M., de La Beaujardiere, O., de Paula, E. R., and Ilma, R. R.: C/NOFS and radar observations during a convective ionospheric storm event over South America, Geophys. Res. Lett., 36, L00C07, doi:10.1029/2009GL039378, 2009. Krishnamurthy, B. V., Ravindran, S., Viswanathan, K. S., Subbarao, K. S. V., Patra, A. K., and Rao, P. B.: Small-scale ($\sim $3 m) E region irregularities at and off the magnetic equator, J. Geophys. Res., 103, 20761–20772, 1998. Kudeki, E. and Bhattacharyya, S.: Postsunset vortex in equatorial F-region plasma drifts and implications for bottomside spread-F, J. Geophys. Res., 104(A12), 28163–28170, 1999. Kudeki, E., Akgiray, A., Milla, M., Chau, J. L., and Hysell, D. L.: Equatorial spread-F initiation: Post-sunset vortex, thermospheric winds, gravity waves, J. Atmos. Solar-Terr. Phys., 69, 2416–2427, 2007. Lastovicka, J.: Forcing of the ionosphere by waves from below, J. Atmos. Sol.-Terr. Phys., 68, 479–497, 2006. MacDougall, J. W., Abdu, M. A., Jayachandran, P. T., Cecile, J.-F., and Batista, I. S.: Presunrise spread F at Fortaleza, J. Geophys. Res., 103, 23415–23425, 1998. Niranjan, K., Brahmanandam, P. S., Ramakrishna Rao, P., Uma, G., Prasad, D. S. V. V. D., and Rama Rao, P. V. S.: Post midnight spread-F occurrence over Waltair (17.7° N, 83.3° E) during low and ascending phases of solar activity, Ann. Geophys., 21, 745–750, 2003. Patra, A. K., Sripathi, S., and Tiwari, D.: Coupling effect of the equatorial F region irregularities on the low latitude E region instability processes, Geophys. Res. Lett., 31, L17803, doi:10.1029/2004GL020486, 2004. Patra, A. K., Tiwari, D., Sripathi, S., Rao, P. B., Sridharan, R., Devasia, C. V., Viswanathan, K. S., Subbarao, K. S. V., Sekar, R., and Kherani, E. A.: Simultaneous radar observations of meter-scale F region irregularities at and off the magnetic equator over India, J. Geophys. Res., 110, A02307, doi:10.1029/2004JA010565, 2005. Prakash, S.: Production of electric field perturbations by gravity wave winds in the E region suitable for initiating equatorial spread F, J. Geophys. Res., 104, 10051–10069, 1999. Rao, P. B., Patra, A. K., Sarma, T. V. C., Krishnamurthy, B. V., Subbarao, K. S. V., and Hari, S. S.: Radar observations of updrafting and downdrafting plasma depletions associated with the equatorial spread F, Radio Sci., 32, 1215–1227, 1997. Roettger, J.: Wavelike structures of large scale equatorial spread F irregularities, J. Atmos. Terr. Phys., 35, 1195–1206, 1973. Saito, S., Fukao, S., Yamamoto, M., Otsuka, Y., and Maruyama, T.: Decay of 3-m scale ionospheric irregularities associated with a plasma bubble with the Equatorial Atmosphere Radar, J. Geophys. Res., 113, A11318, doi:1029/2008JA013118, 2008. Sastri, J. H.: Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum, Ann. Geophys., 17, 1111–1115, 1999. Sekar, R., Chakrabarty, D., Sarkhel, S., Patra, A. K., Devasia, C. V., and Kelley, M. C.: Identification of active fossil bubbles based on coordinated VHF radar and airglow measurements, Ann. Geophys., 25, 2099–2102, 2007. Su, S.-Y., Yeh, H. C., Chao, C. K., and Heelis, R. A.: Supercooled ion temperatures observed in the topside ionosphere at dawn median during storm periods, J. Geophys. Res., 109, A06307, doi:10.1029/2003JA010139, 2004. Subbarao, K. S. V. and Krishna Murthy, B. V.: Seasonal variations of equatorial spread-$F$, Ann. Geophys., 12, 33–39, 1994. Tsunoda, R. T. and White, B. R.: On the generation and growth of equatorial backscatter plumes – 1. Wave structure in the bottomside F layer, J. Geophys. Res., 86, 3610–3616, 1981. Venkateswara Rao, N., Patra, A. K., Pant, T. K., and Rao, S. V. B.: Morphology and seasonal characteristics of low latitude E region quasi-periodic echoes studied using large database of Gadanki radar observations, J. Geophys. Res., 113, A07312, doi:10.1029/2007JA012830, 2009. Woodman, R. F. and LaHoz, C.: Radar observations of F region equatorial irregularities, J. Geophys. Res., 81, 5447–5466, 1976.