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Ann. Geophys., 23, 759-766, 2005
www.ann-geophys.net/23/759/2005/
© European Geosciences Union 2005
Influence of the ionosphere on the altitude of discrete auroral arcs
C. S. Deehr, M. H. Rees, A. E. H. Belon, G. J. Romick, and D. Lummerzheim
The Geophysical Institute University of Alaska Fairbanks 903 Koyukuk Ave Fairbanks, Alaska 99775-7320, USA
Abstract. The altitude of the maximum luminosity of single, discrete auroral arcs was
measured by photometric triangulation from two stations (College and Fort
Yukon, Alaska) located 226km apart on nearly the same magnetic meridian. The
average height of the evening aurora decreases smoothly with increasing solar
depression angle (sda) from 160km near 12° sda to 100km after
18° sda. The average height remains constant until around 12° sda
in the morning. This diurnal variation is similar to that of the electron
density in the F region of the ionosphere. Thus, the behavior is consistent
with the concept that the mean auroral electron energy increases as the
ionospheric conductivity decreases due to ionospheric recombination in the
evening twilight. However, the mean electron energy decreases in magnitude at
dawn when the solar ionizing radiation returns and the electron density in
the F region increases. The magnetospheric acceleration mechanism associated
with discrete auroral arcs thus appears to be inversely proportional to the
ionospheric conductivity, because the time variation of the acceleration
mechanism coincides with the local F region electron density and not with any
obvious magnetospheric process. Previous auroral altitude observations, using
similar triangulation methods, showed that the altitude of discrete auroral
arcs increases as a function of latitude. When these data are corrected for
the twilight effect, the dependence of altitude on latitude disappears. Thus,
the average altitude of discrete auroral arcs and, by inference the
magnetospheric auroral electron acceleration mechanism, is significantly
influenced by the initial ionospheric conductance.
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