Long‐term behavior of the OI 558 nm emission in the night sky and its aeronomical implications

Study of the long‐term behavior of the OI 558 nm emission in the night‐sky, using data encompassing observations at low geomagnetic activity from 1923 to present, gives the result that these long‐term, noisy, geophysical time series need to include nearly 20 cycles of an oscillation before a stable statistically meaningful answer is achieved. Periodogram analysis reveals that the strongest and predominant statistically significant oscillation in the OI 558 nm emission is a yearly variation. When this yearly oscillation is the main oscillation present in the spectrum, it has a fractional amplitude of value near 0.4. A semiannual oscillation is often also found in addition to the annual oscillation and, when significant annual and semiannual components are present, the fractional amplitudes are typically near 0.3 and 0.2 for the annual and semiannual oscillations, respectively. However, the annual fractional amplitude increases with latitude, while the semiannual fractional amplitude decreases with latitude. These results provide an insight on the nature of the seasonal variation of the OI 558 nm emission. Our longest time series are about 12 years long and thus set the lowest frequency to be studied. It is found that the OI 558 nm emission rate increases with solar activity, but in general, the emission is higher during the decreasing phase of solar activity; the apparent relationship changes with different solar cycles. OI 558 nm emission observations have been used to provide observational constraints to global dynamical atmospheric models. However, those constraining observational series have been of limited length and location and are possibly atypical. In the context of a model constrained by such observations, one can infer differences in the dynamical behavior of the atmosphere with latitude in order to account for the observed climatological emission behavior.