Summer 1999 quasi two-day wave at Collm

The measurements at Collm (52°N, 15°E, 80 - 110 km altitude) are carried out continuously. During summer 1998, a quasi 2-day wave modulated by long-period oscillations was measured at Collm.

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This figure shows time series of prevailing winds and 48 h oscillation amplitudes and phases at 95 km. The rsults are calculated using a regression analysis with linearly height-dependent coefficients and a data base of 5 days each. Note the change in 48-h phase during specific periods. This indicates a period different from 48 h. The amplitude itself is strongly modulated by a 10 - 15 day oscillation.

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The graphic shows the QTDW period, with teh colour code indicating the amplitude. Mean winds are shown in the lower panel. As can be seen from this graphic, the QTDW period is strongly variable, and at times split into several secondary peaks, which indicates interaction with other oscillations. However, the strongest amplitudes are found at a period of nearly exactly 48 h. This was also the case for the 1998 QTDW event.

The summer 99 quasi 2-day wave event was, on the whole, of moderate intensity. It fits into the long-term variation of 2-day wave amplitude.

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These figures show the mean July/August quasi 2-day wave zonal amplitude and the 13-monthly smoothed sunspot number. A dependence of wave amplitude on the solar variation is indicated.

It has been assumed that the solar cycle dependece of the QTDW is due to a solar control of the zonal prevailing wind gradient and thus in connection with the zonal prevailing wind TTO that has been monitored in the Collm long-term measurements (Jacobi, 1998). However, the summer 1999 zonal prevailing winds do not fit into the long-term picture, being extraordinarily strong for the moderate solar activity in this summer.

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References

Jacobi, Ch., 1998: On the solar cycle dependence of winds and planetary waves as seen from midlatitude D1 LF mesopause region wind measurements. Ann. Geophysicae 16, 1534 - 1543. (Abstract)

last modification: 21.9.1999