Some optical observations of auroral forms have indicated anomalous enhancement of emission within a very limited altitude range of about 2 km (Oguti, 1975; Stenbaek-Nielsen and Hallinan, 1979; Stenbaek-Nielsen, 1980a, b), and have been called as enhanced aurora. More than one layers can sometimes be seen simultaneously in the typical altitude range 100-120 km. These layers can occur with any type of auroras (Hallinan et al., 1985), although pulsating aurora is most often mentioned. Similar thin ionization layers have also been seen relating to pulsating aurora by incoherent scatter radar (Wahlund et al., 1989) and rocket (MacDonald et al., AGU Fall meeting 1998) measurement.
It has been suggested that a large-amplitude, turbulent ac electric field in the frequency range of electron plasma waves is produced by a beam-plasma instability, energizing ambient ionospheric electrons above the ionization threshold and thereby increasing electron density (and auroral emissions) by secondary ionization. See also the laboratory experiments by Bernstein et al. (1978) and Hallinan et al. (1984).
- Bernstein, W., H. Leinback, C. Kellogg, S. Monson, T. Hallinan, O. K. Garriot, A. Konradi, J. McCoy, C. Daly, B. Baker, and H. R. Anderson, Electron beam-injection experiments: The beam-plasma discharge at low pressures and magnetic field strengths, Geophys. Res. Lett., 5, 127, 1978.
- Hallinen, T. J., H. Leinback, G. Mantjoukis, and W. Bernstein, Measurements of the optical emission produced during the laboratory beamplasma discharge, J. Geophys. Res., 89, 2335, 1984.
- Hallinan, T. J., H. C. Stenbaek-Nielsen, and C. S. Deehr, Enhanced aurora, J. Geophys. Res., 90, 8461, 1985.
- Oguti, T., Two-tiered auroral band, J. Atmos. Terr. Phys., 37, 1501, 1975.
- Stenbaek-Nielsen, H. C., Pulsating aurora: The importance of the ionosphere, Geophys. Res. Lett., 7, 353, 1980a.
- Stenbaek-Nielsen, H. C., On the comparison between inferred and observed electron flux in pulsating auroras, J. Geophys. Res., 85, 4299, 1980b.
- Stenbaek-Nielsen, H. C., and T. N. Hallinan, Pulsating auroras: Evidence for noncollisional thermalization of precipitating electrons, J. Geophys. Res., 84, 3257, 1979.
- Wahlund, J.-E., H. J. Opgenoorth, and P. Rothwell, Observations of thin auroral ionization layers by EISCAT in connection with pulsating aurora, J. Geophys. Res., 94, 17223-17233, 1989.