SIGNIFICANCE OF FIELD-ALIGNED CURRENTS FOR F-REGION PERTURBATIONS

Diurnal variations of decay time of heater-induced small-scale irregul arities in the mid-latitude ionospheric F-layer were measured by means of diagnostic stimulated electromagnetic emissions (DSEE). The abrupt (15-20 min) and very strong (10-fold or more) increase in DSEE decay times was observed simultaneously over a wide height range around a tu rbulence location. This increase was assumed to be dictated by a natur al mechanism, supporting artificial irregularities by utilization of t he diagnostic wave energy. Analysis of the experimental data, concerni ng features of both heater-induced and natural irregularities, shows t hat such a natural mechanism was initiated by the S-q current system. To account for small-scale irregularity growth, the thermomagnetic ins tability realized for a downward directed field-aligned current was co nsidered. This instability allows us to explain the natural generation of irregularities with scale lengths of 25 m or longer. (C) 1996 Else vier Science Ltd