Magnetic activity linked generation of nighttime equatorial spread F irregularities

Abstract

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).

Evolution of Freshly Generated Equatorial Spread F (F‐ESF) Irregularities on Quiet and Disturbed Days

“…If we compare Q‐ and D‐days, we find that the postmidnight occurrence of F‐ESF is more likely on D‐days as compared to Q‐days. This tendency is in general agreement with the earlier studies (Abdu et al, ; Kakad et al, ; Li et al, ) and attributed to the westward to eastward reversal of ambient electric field around midnight due to DD electric fields (Fejer & Scherliess, ; C.‐M. Huang et al, ; Richmond et al, ).…”

“…Studies have demonstrated that on disturbed (D) days, a disturbance dynamo (DD) electric field (Blanc & Richmond, ), promptly penetrated (PP) electric field (Kikuchi et al, , ), or combination of DD and PP electric fields affects the dynamics of F‐region (Kakad et al, ; N. Maruyama et al, ). Apart from usual postsunset generation, fresh development of EPBs is often encountered in the postmidnight hours on the D‐days (Kakad et al, ). The seasonal and solar flux dependence of these magnetic activity linked F‐EPBs is not studied so far.…”

Study of equinoctial asymmetry in the Equatorial Spread F (ESF) irregularities over Indian region using multi-instrument observations in the descending phase of solar cycle 23

“…C I is often found to be <0.5 in the initial phase of EPB development due to presence of perturbation electric fields associated with the R‐T instability but remains close to 1 in the later phase under magnetically quiet periods. Using this technique, Kakad et al [2007] have studied the fresh generation of irregularities around midnight hours and suppression of scintillations during pre‐midnight hours during magnetically active periods. From Tiwari et al [2006], one can notice that smaller the C I , higher the maximum height reached by the irregularity layer.…”

Super‐medium‐scale traveling ionospheric disturbance observed at midlatitude during the geomagnetic storm on 10 November 2004

“…During geomagnetically active periods, plasma bubbles can be generated after midnight which are probably affected by electric perturbations associated with the magnetic activity [ Bhattacharyya et al , 2002; Kakad et al , 2007; Tulasi Ram et al , 2008]. Kakad et al [2007] studied local time when plasma bubbles were generated with ground‐based scintillation measurement. According to their study, plasma bubbles are mostly generated around midnight hours during disturbed days.…”