Abstract
Measurements of midlatitude E region coherent backscatter obtained during four summers with SESCAT, a 50 MHz Doppler system operating in Crete, Greece, and concurrent ionosonde recordings from the same ionospheric volume obtained with a CADI for one of these summers, are used to analyse the long-term variability in echo and Es occurrence. Echo and Es layer occurrences, computed in percent of time over a 12-h nighttime interval, take the form of time sequences. Linear power spectrum analysis shows that there are dominant spectral peaks in the range of 2â9 days, the most commonly observed periods appearing in two preferential bands, of 2â3 days and 4â7 days. No connection with geomagnetic activity was found. The characteristics of these periodicities compare well with similar properties of planetary waves, which suggests the possibility that planetary waves are responsible for the observed long-term periodicities. These findings indicate also a likely close relation between planetary wave (PW) activity and the well known but not well understood seasonal Es dependence. To test the PW postulation, we used simultaneous neutral wind data from the mesopause region around 95 km, measured from Collm, Germany. Direct comparison of the long-term periodicities in echo and Es layer occurrence with those in the neutral wind show some reasonable agreement. This new evidence, although not fully conclusive, is the first direct indication in favour of a planetary wave role on the unstable midlatitude E region ionosphere. Our results suggest that planetary waves observation is a viable option and a new element into the physics of midlatitude Es layers that needs to be considered and investigated.Key words: Ionosphere (ionosphere irregularities; mid-latitude ionosphere) â Meteorology and atmospheric dynamics (waves and tides)
Highlights
Recent studies of the unstable midlatitude E region ionosphere suggested that planetary waves may play a role in coherent backscatter occurrence (Tsunoda et al, 1998; Voiculescu et al, 1999) and the formation of strong sporadic E layers (Voiculescu et al, 1999; Shalimov et al, 1999)
The most fundamental aspect, relates to the role played by planetary wave (PW) in the formation of strong Es layers, since midlatitude coherent echoes are observed in close connection with strong sporadic E layers
We have analysed a large database of midlatitude 50 MHz backscatter, observed with the Sporadic E Scatter Experiment (SESCAT) from Crete, Greece over four summers of continuous operation, to study the long-term variability in echo occurrence
Summary
Recent studies of the unstable midlatitude E region ionosphere suggested that planetary waves may play a role in coherent backscatter occurrence (Tsunoda et al, 1998; Voiculescu et al, 1999) and the formation of strong sporadic E layers (Voiculescu et al, 1999; Shalimov et al, 1999). These studies are the latest to report large-period wavelike eects on midlatitude ionospheric plasma processes most likely caused by neutral atmospheric oscillations of global scale, which are known to exist in the mesosphere and lower thermosphere (MLT). We provide for ®rst time some direct evidence in favour of a PW role by comparing the long-term periodicities in radar backscatter and ionosonde sporadic E layer observations with those in concurrent neutral wind measurements at the lower E region (near 95 km) made in the European sector
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