Skip to main content

Statistical studies of fast and slow Z-mode plasma waves in and beyond the equatorial plasmasphere based on long-term Akebono observations


In order to investigate spatial and temporal variations of fast and slow Z-mode waves frequently observed in the equatorial plasmasphere, statistical studies have been performed by using plasma wave observation data obtained by the Akebono satellite within a period from 1989 to 1995. It has been clarified that fast and slow Z-mode waves are intensified within ±5° of geomagnetic latitudes in an altitude range from 6000 km to the apogee (10500 km) of the satellite without obvious local time dependence. Long-term averaged intensity of fast Z-mode waves has almost the same orders of magnitude as that of slow Z-mode waves. These results indicate that significant part of fast Z-mode waves are not produced by the linear mode conversion process from slow Z-mode waves, but excited by more direct process. Furthermore, the region of intensified fast and slow Z-mode waves has been spread in a wider geomagnetic latitude range of ±10° during geomagnetic storms. These evidences suggest that one of the possible free energy sources is ring current particles injected into the equatorial region of the plasmasphere during geomagnetic storms.


  • Anderson, B. J., R. E. Erlandson, and L. J. Zanetti, A statistical study of Pc 1-2 magnetic pulsations in the equatorial magnetosphere. I—Equatorial occurrence distributions. II—Wave properties, J. Geophys. Res., 97, 3075–3101, 1992.

    Article  Google Scholar 

  • Goertz, C. K. and R. J. Strangeway, Plasma waves, in Introduction to Space Physics, edited by M. G. Kivelson and C. T. Russell, Cambridge Univ. Press, 1995.

    Google Scholar 

  • Gurnett, D. A., R. R. Anderson, F. L. Scarf, R. W. Fredricks, and E. J. Smith, Initial results from the ISEE-1 and -2 plasma wave investigation, Space Sci. Rev., 23, 103–122, 1979.

    Article  Google Scholar 

  • Hashimoto, K., W. Calvert, and H. Matsumoto, Kilometric continuum detected by Geotail, J. Geophys. Res., 104, 28,645, 1999.

    Article  Google Scholar 

  • Iyemori, T., Storm-time magnetospheric currents inferred from mid-latitude geomagnetic field variations, J. Geomagn. Geoelectr., 42, 1249–1265, 1990.

    Article  Google Scholar 

  • Olsen, R. C, S. D. Shawhan, D. L. Gallagher, C. R. Chappell, and J. L. Green, Plasma observations at the earth’s magnetic equator, J. Geophys. Res., 92, 2385–2407, 1987.

    Article  Google Scholar 

  • Oya, H., Conversion of electrostatic plasma waves into electromagnetic waves: Numerical calculation of the dispersion relation for all wavelengths, Radio Sci., 1131–1141, 1971.

    Google Scholar 

  • Oya, H., A. Morioka, K. Kobayashi, M. Iizima, T. Ono, H. Miyaoka, T. Okada, and T. Obara, Plasma wave observation and sounder experiments (PWS) using the Akebono (EXOS-D) satellite—Instrumentation and initial results including discovery of the high altitude equatorial plasma turbulence, J. Geomagn. Geoelectr., 42, 411–442, 1990.

    Article  Google Scholar 

  • Oya, H., M. Iizima, and A. Morioka, Plasma turbulence disc circulating the equatorial region of the plasmasphere identified by the plasma wave detector (PWS) onboard the Akebono (EXOS-D) satellite, Geophys. Res. Lett., 18, 329–332, 1991.

    Article  Google Scholar 

  • Roeder, J. L. and H. C. Koons, A survey of electron cyclotron waves in the magnetosphere and the diffuse auroral electron precipitation, J. Geophys. Res., 94, 2529–2541, 1989.

    Article  Google Scholar 

  • Shinbori, A., T. Ono, M. Iizima, A. Kumamoto, S. Shirai, A. Hanaoka, K. Okamoto, M. Ohashi, and H. Oya, Electrostatic electron cyclotron harmonic waves observed by the Akebono Satellite near the equatoral region of the plasmasphere, J. Geophys. Res., 2004 (submitted).

    Google Scholar 

  • Shinbori, A., Y. Nishimura, T. Ono, M. Iizima, A. Kumamoto, and H. Oya, Electrodynamics in the duskside inner magnetosphere and plasmasphere during a super magnetic storm on March 13–15, Earth Planets Space, 57, 643–659, 2005.

    Article  Google Scholar 

  • Tsurutani, B. T. and E. J. Smith, Two types of magnetospheric ELF chorus and their substorm dependences, J. Geophys. Res., 82, 5112–5128, 1977.

    Article  Google Scholar 

  • Walsh, D., F. T. Haddock, and H. F Schulte, Cosmic radio intensities at 1.225 and 2.0 Mc/s measured up to an altitude of 1700 km, in Space Research, edited by P. Muller, vol. 4, pp. 935–959, The Netherlands, 1964.

    Google Scholar 

Download references

Author information

Authors and Affiliations


Corresponding author

Correspondence to Y. Nishimura.

Rights and permissions

Open Access  This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

To view a copy of this licence, visit

Reprints and permissions

About this article

Cite this article

Nishimura, Y., Ono, T., Iizima, M. et al. Statistical studies of fast and slow Z-mode plasma waves in and beyond the equatorial plasmasphere based on long-term Akebono observations. Earth Planet Sp 58, 343–346 (2006).

Download citation

  • Received:

  • Revised:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

Key words