Frequency domain interferometry mode observations of PMSE using the EISCAT VHF radar

I. Häggström,S. Kirkwood,P. B. Chilson

doi:10.1007/s005850000309

Abstract

During the summer of 1997 investigations into the nature of polar mesosphere summer echoes (PMSE) were conducted using the European incoherent scatter (EISCAT) VHF radar in Norway. The radar was operated in a frequency domain interferometry (FDI) mode over a period of two weeks to study the frequency coherence of the returned radar signals. The operating frequencies of the radar were 224.0 and 224.6 MHz. We present the first results from the experiment by discussing two 4-h intervals of data collected over two consecutive nights. During the first of the two days an enhancement of the FDI coherence, which indicates the presence of distinct scattering layers, was found to follow the lower boundary of the PMSE. Indeed, it is not unusual to observe that the coherence values are peaked around the heights corresponding to both the lower- and upper-most boundaries of the PMSE layer and sublayers. A Kelvin-Helmholtz mechanism is offered as one possible explanation for the layering structure. Additionally, our analysis using range-time-pseudocolor plots of signal-to-noise ratios, spectrograms of Doppler velocity, and estimates of the positions of individual scattering layers is shown to be consistent with the proposition that upwardly propagating gravity waves can become steepened near the mesopause.Key words: Ionosphere (polar ionosphere) Â· Meteorology and Atmospheric Dynamics (middle atmosphere dynamics) Â· Radio Science (Interferometry)

Highlights

The mesosphere-stratosphere-troposphere (MST) class of Doppler VHF radars has been used for remoteCorrespondence to: P
To facilitate an investigation into the small-scale vertical structures the radar was operated in a frequency domain interferometry (FDI) mode
Existing rocket and radar observations suggest that non-turbulent mechanisms must play a role for the generation and/or preservation of polar mesosphere summer echoes (PMSE) (LuÈbken et al, 1993, 1998)

Summary

Introduction

The mesosphere-stratosphere-troposphere (MST) class of Doppler VHF radars has been used for remote. For large Schmidt numbers Sc m=D, where m is the neutral atmospheric molecular diusion rate and D is the electron diusion rate), the inertial subrange of the plasma spectrum can be extended to suciently small scales to allow the Bragg condition to be ful®lled This can be attributed to, e.g., low mobilities of charged aerosols or dust particles. If the aerosols are charged, this in turn leads to electron and ion density gradients If such vortices exist it is expected that their structure could be probed by the interferometric techniques but that they could not be resolved by the conventional radar methods. To facilitate an investigation into the small-scale vertical structures the radar was operated in a frequency domain interferometry (FDI) mode This allows detection of the presence of thin scattering layers within the radar volume. 1990; Chilson et al, 1997b; Muschinski et al., 1999); with the exception of one 37-min data set of observations (Franke et al, 1992), this technique has not been utilized to date for the study of PMSE

Experiment

Application of FDI to PMSE data

Data analysis and overview

Vertical structure within the PMSE

Indications of steepened gravity waves

Conclusions