Magnetic activity linked generation of nighttime equatorial spread F irregularities

摘要

Results derived from a statistical study of the generation of equatorial spread F (ESF) irregularities as a result of magnetic activity based on spaced receiver ionospheric scintillation data recorded at a dip equatorial station is reported here. For a study of this nature it is essential to establish whether the observed scintillations are caused by freshly generated irregularities or by irregularities generated earlier, which later drift onto the signal path. It has been observed in the past that the maximum cross-correlation between the spaced receiver signals is significantly less than 1 during the initial phase of development of ESF irregularities due to the presence of perturbation electric fields associated with the Rayleigh-Taylor (R-T) instability that produces equatorial plasma bubbles (EPBs), whereas in the later phase, when these perturbation electric fields die down, the correlation between the two signals increases rapidly. This feature is used in the present study to identify freshly generated ESF irregularities associated with EPBs using spaced receiver scintillation data. Magnetically disturbed days are chosen by using three hourly geomagnetic activity index ap, daily index Ap, and also AE index to study the cases of prompt penetration of high-latitude electric field to the equatorial ionosphere. Disturbed time statistical occurrence pattern of freshly generated irregularities shows seasonal variation for all three types of magnetic disturbances: disturbance dynamo, prompt penetration, combination of disturbance dynamo and prompt penetration. However, it is found that fresh generation of the irregularities due to magnetic activity is most likely to occur around midnight hours in all seasons. Suppression of generation of irregularities immediately after sunset due to inhibition of the growth of the R-T instability on the bottomside of the equatorial F region is clearly seen in vernal equinox (March and April) and solstice months but is not observed for autumnal equinox (September and October).