Proceedings with Extended Abstracts (single PDF file) - Radio ...

SPORADIC E LAYER DEPENDENCE ON PLANETARY WAVES.AN EVENT STUDY SHOWING AN INDIRECT RELATIONSHIPTROUGH MODULATED ATMOSPHERIC TIDESChristos Haldoupis 1 and Dora Pancheva 21: Physics Department, University of Crete, Iraklion, Crete, Greece2: Department of Electronic and Electrical Engineering, University of Bath, Bath, UK1. Introduction.The sporadic E layers (E s ), which are thin layers of dense plasma forming in the midlatitudeE region ionosphere, have been the subject of numerous investigations over many years (e.g.,see review by Mathews, 1998). Their formation is governed by the complexity of neutralwind dynamics in the mesosphere and lower thermosphere (MLT). In particular, the diurnaland semidiurnal tides, and the vertical wind shears carried by them, are of fundamentalimportance to E s , which sometimes are referred to as "tidal ion layers". In addition to theundisputed role of atmospheric tides, recent results suggested that planetary waves (PW) playa role on E s formation as well, a fact that went unnoticed in the long-going research ofsporadic E. The first indirect evidence for a link between E s and PW was provided by fieldalignedbackscatter observations (obtained with the MU radar in Japan and the SESCATsystem in Crete, Greece) that showed PW-period like modulations in the occurrence ofmagnetic aspect-sensitive E region radar echoes which relate with strong sporadic E layers.Here we combine and summarize the results of two recent studies on the PW - E s relationshippublished in JGR by Haldoupis and Pancheva [2002] (hereafter called Paper A), andPancheva et al. [2003] (Paper B). Paper A provided the first direct evidence in favor of aPW role on E s generation, whereas Paper B showed that planetary waves can affect E sformation indirectly through nonlinear interaction and modulation of the semidiurnal anddiurnal atmospheric tides. Both of these studies relied on a conspicuous PW event thatoccurred in August-September 1993 and detected in MLT ground radar and satellite windmeasurements (e.g., Clark et al., 2002 and references therein). This large scale atmosphericwave was a westward propagating 7-day PW of zonal number S = 1 that lasted for about 25days, seen mostly in the meridional wind with peak amplitudes in excess of 15 m/s.2. Direct Evidence for PW Modulation of Sporadic E LayersGiven the dominant 7-day PW event in August-September 1993 and the need to prove (ordisprove) the postulated PW- E s relationship, we obtained from the Colorado World DataCenter mean hourly E s critical frequency (foEs) time series for all the northern hemispheremidlatitude stations. In this respect, we found usable data from 8 stations spanning from 58 o Eto 157 o W, that is, covering about 215 o in longitude around the globe. A first look inspectionof the hourly mean foEs times series has been surprisingly reassuring, as all stations appearedto have recorded a long period modulation in foEs during the 7-day PW occurrence.164To investigate the spectral dynamics of all eight time series we applied wavelet transformtechniques. The wavelet spectrograms showed a dominant 7-day periodicity to be present,from about middle of August and well into September, in all eight ionosonde stations fromthe eastmost Ashkhabad to the westmost Maui. This is illustrated in Figure 1 which showswavelet spectrograms for all eight stations. The foEs wavelike modulation maximizeseverywhere around end of August, very much in agreement with the maximum of the 7-dayPW activity observed in the meridional component of the mesospheric neutral wind, asdiscussed by Clark et al. [2002]. We stress that this 7-day periodicity did not relate to aglobal geomagnetic activity, as confirmed from similar 3-hourly Ap wavelet analysis.