During September, at the mean nighttime reflection height the summer zonal westerlies decrease significantly. This is well expressed in the upper panel of Figure 1; during the first half of the month the zonal winds are still strong, but after that the mean winds decrease to near zero. The zonal westerlies in summer are the reaction of the lower thermosphere on the stratospheric and mesospheric easterlies: the middle atmosphere jet passes only eastward travelling gavity waves and planetary waves, which lead to an acceleration of the mean flow towards the east. The decrease of the westerlies thus is an indicator for decreasing easteliers below. The meridional prevailing wind vom still is mainly negative, i.e. southward directed. This southward wind belongs to the mean meridional circulation which is steered by the upward circulation at summer polar latitudes. The semidurnal amplitudes and phases are rather stable, this is a regular feature in September. The semidiurnal tide reaches its highest values in the course of the year now. However, the September 2000 amplitudes are relatively small compared to those in other years (up to more than 25 m/s on a monthyl average). Generally the day-to-day variability in September is rather small.
As shown by the predictions of the SIDC RWC Brussels World Data Center for the Sunspot Index we probably now are actually very near to solar maximum (see from SIDC, referred to on 4.10.2000). It is know since a long time that radio reflection heights decrease with solar activity owing to the increased ionisation during solar maximum (Entzian, 1967). This is well visible in Figure 2, showing the monthly mean nighttime reflections heights (Kürschner et al, 1987) at Collm vs. sunspot number. Although differences of about 5 km in different years are found, the mean reflection height difference between solar minimum and solar maximum is 1 - 2 km only. The variability is statstically significant, although some years (e.g. 1995) do not fit into this picture.
To give an impression of the total of measurements, in Figure 3 we compare the histogram of all measured half-hourly means in September 2000 with those of September 1996. The latter month was well within solar minimum (monthly mean sunspot number R = 1.6) while in September 2000 R = 109.9 (provisional, taken from National Geophysical Data Center (NGDC) on ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SUNSPOT_NUMBERS/MONTHLY on 4.10.2000. It can be seen that the entire histogram is shifted, and both the high (nightime) and low (daytime) values are higher during solar maximum.
The same conclusion can be drawn from the monthly median reflection height daily cycle, presented in Figure 4. During daylight hours at times no measurements are possibly, but throughout the rest of the day in 2000 the reflection heights are lower by 1 - 2 km.
Figure 4: Monthly median half-hourly virtual reflection heights in September 2000 and 1996.
Entzian, G., 1967: Der Sonnenfleckenzyklus in der Elektronenkonzentration der D-Region. Kleinheubacher Berichte 12, 309-313.
Kürschner, D., Schminder, R., Singer, W. and Bremer, J., 1987: Ein neues Verfahren zur Realisierung absoluter Reflexionshöhenmessungen an Raumwellen amplitudenmodulierter Rundfunksender bei Schrägeinfall im Langwellenbereich als Hilfsmittel zur Ableitung von Windprofilen in der oberen Mesopausenregion. Zeitschrift für Meteorologie 37, 322 - 332.
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4.10.2000, last modification: 3.11.2000