A Study of the Decay Times and Autocorrelation Function Decorrelation Times of Artificial Field-Aligned Irregularities

R.S. DILLON, D.M. WRIGHT and T.R. ROBINSON

University of Leicester, U.K.

The HF CUTLASS radar system is a bistatic component of the SuperDARN array with sites in Finland and Iceland. CUTLASS usually transmits a pulse-sequence consisting of 7 pulses, from which an 18-lag autocorrelation function (ACF) may be calculated. The Doppler velocity and spectral width may then be obtained by fitting a suitable function to the ACF. Following on from the work by Villain et al (JATP, 1996), a diffusion coefficient and correlation parameters may be computed. The CUTLASS Finland radar has been used to collect coherent backscatter return from artificial field-aligned irregularities (FAIs) generated by the EISCAT Heating Facility near Tromso, Norway. This return differs markedly from that obtained for naturally occurring FAIs. The main differences are the extremely low spectral widths and long irregularity decay times. The spectral width has a low value because the associated ACF does not decorrelate for higher lags, i.e. the value remains more or less constant. This approximately constant value corresponds to a fairly static irregularity distribution during the sampling period. The most recent experimental campaign (October 1999) was the first where CUTLASS ran a special long-lag mode. This long-lag mode utilised lag separations of 5 and 10 times the normal lag separation (2400 microseconds). It was found that the ACFs decorrelated for these longer lag times with characteristic times of the order of a second. This decorrelation time was compared to the actual irregularity decay time, i.e. the time taken for the irregularity signature to decay away after switch-off of the heater. It was found that the decay time was approximately 5 times as large as the decorrelation time of the ACF.

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