Significance of field-aligned currents for F-region perturbations

Diurnal variations of decay time of heater-induced small-scale irregularities 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 turbulence location. This increase was assumed to be dictated by a natural mechanism, supporting artificial irregularities by utilization of the diagnostic wave energy. Analysis of the experimental data, concerning features of both heater-induced and natural irregu larities, shows that such a natural mechanism was initiated by the S_q current system. To account for small-scale irregularity growth, the thermomagnetic instability realized for a downward directed field-aligned current was considered. This instability allows us to explain the natural generation of irregularities with scale lengths of 25 m or longer.