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The altitude of the maximum luminosity of single, discrete auroral arcs wasmeasured by photometric triangulation from two stations (College and FortYukon, Alaska) located 226km apart on nearly the same magnetic meridian. Theaverage height of the evening aurora decreases smoothly with increasing solardepression angle (sda) from 160km near 12° sda to 100km after18° sda. The average height remains constant until around 12° sdain the morning. This diurnal variation is similar to that of the electrondensity in the F region of the ionosphere. Thus, the behavior is consistentwith the concept that the mean auroral electron energy increases as theionospheric conductivity decreases due to ionospheric recombination in theevening twilight. However, the mean electron energy decreases in magnitude atdawn when the solar ionizing radiation returns and the electron density inthe F region increases. The magnetospheric acceleration mechanism associatedwith discrete auroral arcs thus appears to be inversely proportional to theionospheric conductivity, because the time variation of the accelerationmechanism coincides with the local F region electron density and not with anyobvious magnetospheric process. Previous auroral altitude observations, usingsimilar triangulation methods, showed that the altitude of discrete auroralarcs increases as a function of latitude. When these data are corrected forthe twilight effect, the dependence of altitude on latitude disappears. Thus,the average altitude of discrete auroral arcs and, by inference themagnetospheric auroral electron acceleration mechanism, is significantlyinfluenced by the initial ionospheric conductance

Influence of the ionosphere on the altitude of discrete auroral arcs