ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-27-967-2009 Zonal asymmetry of daytime 150-km echoes observed by Equatorial Atmosphere Radar in Indonesia Yokoyama T. 1 Hysell D. L. 1 Patra A. K. 2 Otsuka Y. 3 Yamamoto M. 4 Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA National Atmospheric Research Laboratory, Gadanki, India Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan 02 03 2009 27 3 967 974 Copyright: © 2009 T. Yokoyama et al. 2009 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/27/967/2009/angeo-27-967-2009.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/27/967/2009/angeo-27-967-2009.pdf

Multi-beam observations of the daytime ionospheric E-region irregularities and the so-called 150-km echoes with the 47-MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (0.20° S, 100.32° E, 10.36° S dip latitude) are presented. 150-km echoes have been frequently observed by the EAR, and their characteristics are basically the same as the equatorial ones, except for an intriguing zonal asymmetry; stronger echoes in lower altitudes in the east directions, and weaker echoes in higher altitudes in the west. The highest occurrence is seen at 5.7° east with respect to the magnetic meridian, and the altitude gradually increases as viewing from the east to west. Arc structures which return backscatter echoes are proposed to explain the asymmetry. While the strength of radar echoes below 105 km is uniform within the wide coverage of azimuthal directions, the upper E-region (105–120 km) echoes also show a different type of zonal asymmetry, which should be generated by an essentially different mechanism from the lower E-region and 150-km echoes.

 References Balsley, B. B.: Evidence of stratified echoing region at 150 km in the vicinity of magnetic equator during daylight hours, J. Geophys. Res., 69, 1925–1930, 1964. Chau, J. L.: Unexpected spectral characteristics of VHF radar signals from 150-km region over Jicamarca, Geophys. Res. Lett., 31, L23803, https://doi.org/10.1029/2004GL021620, 2004. Chau, J. L. and Woodman, R. F.: Daytime vertical and zonal velocities from 150-km echoes: Their relevance to F-region dynamics, Geophys. Res. Lett., 31, L17801, https://doi.org/10.1029/2004GL020800, 2004. Choudhary, R. K., St.-Maurice, J.-P., and Mahajan, K. K.: Observation of coherent echoes with narrow spectra near 150 km altitude during daytime away from the dip equator, Geophys. Res. Lett., 31, L19801, https://doi.org/10.1029/2004GL020299, 2004. Cohen, R. and Bowles, K. L.: Secondary irregularities in the equatorial electrojet, J. Geophys. Res., 72, 885–894, 1967. Fukao, S., Hashiguchi, H., Yamamoto, M., Tsuda, T., Nakamura, T., Yamamoto, M K., Sato, T., Hagio, M., and Yabugaki, Y.: Equatorial Atmospheric Radar (EAR): System description and first results, Radio Sci., 38(3), 1053, https://doi.org/10.1029/2002RS002767, 2003. Kudeki, E. and Fawcett, C. D.: High resolution observations of 150 km echoes at Jicamarca, Geophys. Res. Lett., 20, 1987–1990, 1993. Kudeki, E., Fawcett, C. D., Ecklund, W. L., Johnston, P. E., and Franke, S. J.: Equatorial 150-km irregularities observed at Pohnpei, Geophys. Res. Lett., 25, 4079–4082, 1998. Patra, A. K. and Rao, N. V.: Radar observations of daytime 150-km echoes from outside the equatorial electrojet belt over Gadanki, Geophys. Res. Lett., 33, L03104, https://doi.org/10.1029/2005GL024564, 2006. Patra, A. K., Tiwari, D., Devasia, C. V., Pant, T. K., and Sridharan, R: East-west asymmetries of the equatorial electrojet 8.3 m type-2 echoes observed over Trivandrum and a possible explanation, J. Geophys. Res., 110, A11305, https://doi.org/10.1029/2005JA011124, 2005. Patra, A. K., Yokoyama, T., Yamamoto, M., Nakamura, T., Tsuda, T., and Fukao, S.: Lower E region field-aligned irregularities studied using the Equatorial Atmosphere Radar and meteor radar in Indonesia, J. Geophys. Res., 112, A01301, https://doi.org/10.1029/2006JA011825, 2007. Patra, A. K., Yokoyama, T., Otsuka, Y., and Yamamoto, M.: Daytime 150-km echoes observed with the Equatorial Atmosphere Radar in Indonesia: First results, Geophys. Res. Lett., 35, L06101, https://doi.org/10.1029/2007GL033130, 2008. Tsunoda, R. T., and Ecklund, W. L.: On the nature of 150-km radar echoes over the magnetic dip equator, Geophys. Res. Lett., 27, 657–660, 2000. Tsunoda, R. T. and Ecklund, W. L.: On the visibility and zenithal confinement of 150 km (\it F$_1$) radar echoes, Geophys. Res. Lett., 34, L21102, https://doi.org/10.1029/2007GL031276, 2007. Tsunoda, R. T. and Ecklund, W. L.: On the sheet-like nature of 150 km (\it F$_1$) radar echoes, Geophys. Res. Lett., 35, L05102, https://doi.org/10.1029/2007GL032512, 2008. Yokoyama, T., Patra, A. K., Fukao, S., and Yamamoto, M.: Ionospheric irregularities in the low-latitude valley region observed with the Equatorial Atmosphere Radar, J. Geophys. Res., 110, A10304, https://doi.org/10.1029/2005JA011208, 2005.