Seasonal variation of twilight sodium emission at Victoria, B.C., 1967-68
Date
1969
Authors
Roberts, Michael George
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Abstract
Twilight sodium emission from the upper atmosphere has been studied at Victoria, B.C. for a period of more than one year from February 4, 1967 to February 29, 1968 using a birefringent photometer directed towards the zenith sky.
The observations have shown that the emission intensity increased to a maximum in March 1967, decreased rapidly during this month to a summer time low in June and then increased to a broad maximum in October, November and December. After December, the intensity appeared to decrease to a minimum during the early part of February and then to rise to a secondary maximum in early March. Thus the pattern of the seasonal intensity variation obtained during the first part of 1967 was repeated in 1968.
In addition to the winter-summer seasonal intensity variation, more rapid fluctuations were observed over periods of several weeks during the autumn and winter months. These fluctuations may be due to local conditions, the global effect being simply a high intensity during the winter and a low intensity during the summer. Since the secondary intensity maximum observed in March could be due to similar local conditions which occurred at the same time of year, before it can be concluded that the typical seasonal variation exhibits two maxima and two minima, more observations should be made, especially during the autumn and winter months.
Seasonal variations of both the morning and evening heights of maximum density were also studied. The morning heights appeared to exhibit maxima in April and November and minima in September and January whereas the evening heights appeared to exhibit maxima in March and October and minima in June and December. The apparent phase difference of approximately six to eight weeks between the two height variations is unexplained.
Although the mean monthly evening-morning intensity ratios did not differ from unity by more than ± 20%, and no seasonal trend could be detected, an interesting evening-morning variation of the height of maximum density was observed, the evening heights being generally greater than the morning ones. This evening-morning height difference variation appears to show a symmetry about the solstices, the difference being least at these times, but greatest near the 90 km. equinoxes.
Although the evening-morning height effect indicates apparent vertical motions it is difficult to explain the effect on the basis of subsidence due to overnight cooling, in view of the seasonal height variation. An alternative explanation might be found in a change in the value of the screening height between evening and morning. This would be reflected by a change in the abundance or distribution of atmospheric ozone during the night. If the latter explanation is correct the height of the sodium layer may not vary appreciably from evening to morning.
In this thesis comparisons with temperature variations at the 90 km. level were made and a weak correlation was found with the seasonal evening height variations; however, the sparseness of the temperature data at high levels, makes the correlation somewhat uncertain.
Comparisons have been made in the past, by other workers, between stratospheric temperature variations and the seasonal variations of the intensity of the sodium twilight emissions, but again, the correlation has been very uncertain.
If the intensity and the height variations of the sodium layer are reflected by temperature changes or vertical motions of the upper atmosphere, or by changes in the vertical distribution of atmospheric ozone, such variations would be of considerable interest to meteorologists who may be led to a better understanding of atmospheric dynamics in the 90 km. region and thence to a more complete comprehension of the general circulation of the earth's atmosphere.