Postsunset rise of F layer height in the equatorial region and its relation to the F layer dynamo polarization fields

Abstract

The ionosonde data are studied for equatorial station, Thumba, to delineate various features of the evening height rise of F layer. Sharp increase of h'F and h_F2 is obß 13 served •n the postsunset period for high solar activity. Seasonal variation is observed in this increase of h'F and it is maximum for equinox months. For summer months, there is a delay of about an hour in the time of occurrence of h'F (peak) as compared to winter and equinox months. This delay is shown to be associated with the delay in sunset times in the conjugate E regions. As for magnetic activity dependence, it is found that this height increase is less pronounced for disturbed days for winter and equinox whereas for summer it is marginally higher over the quiet day values. Further, it is observed that the value of h'F (peak) during disturbed periods is almost at the same value of 350 km for all the three seasons. Thus the seasonal variation of magnetic activity effects appears to be mainly governed by the average seasonal variation for quiet times. The increase in F layer height is due to zonal eastward electric fields developed after sunset which is believed to be due to F region dynamo fields. While the main driving force for these fields is the zonal neutral winds, the development of these fields depend on the ratio of the F region to E region conductivity and the longitudinal gradient in the E region conductivity. Experimental observations of both the neutral winds and ionospheric conductivities are examined for their variation with solar activity, season and magnetic activity as both these factors will contribute for the various observed features of the height rise. Through model calculations it is shown that the E region density (conductivity) gradient in the postsunset period is higher by a factor of 2 for high solar activity compared to low solar activity and hence it is partly responsible for the observed solar activity variation of the postsunset height rise of the F layer. The importance of the studies using available ionosonde data for understanding the F region dynamo electric fields is emphasized.

Faraday polarization fluctuations and their dependence on post sunset secondary maximum and amplitude scintillations at Delhi

“…The driving force for the polarization field, which generates the vertical drift or zonal electric field, is the zonal neutral wind. Goel et al (1990) have shown that the polarization field is dependent upon the Eregion density, so its effect will be maximum when both the conjugate points in the E-region correspond to the F-region at the equator sunset. Thus, during equinoxes, with the sunset in the E-region being simultaneous, polarization fields that developed in the equatorial F-region are maximum, showing a prereversal enhancement to be maximum and consequently, the post sunset IEC value.…”

Study of disturbance dynamo effects at nighttime equatorial F region in Indian longitude

“…In many investigations the rate of change of h ′ F is used to get the vertical drift of base height of F layer [ Goel et al , 1990; Subbarao and Krishna Murthy , 1994; Oyekola , 2009]. However the vertical apparent drift obtained from h ′ F measurements deviate considerably from true vertical E × B drift during early and late evening hours due to effects of chemical recombination.…”

Role of the equatorial ionization anomaly in the development of the evening prereversal enhancement of the equatorial zonal electric field

“…It has been suggested [e.g., Heelis et al , 1974; Tsunoda , 1985; Batista et al , 1986; Goel et al , 1990] that the longitudinal gradient of integrated Pedersen conductivity (LGIPC) in the E region at sunset time can play a positive role in strengthening the EPRE magnitudes. Tsunoda [1985] has shown that the occurrence maxima in scintillation activity at a given longitude are coincident with time of the year when the sunset is simultaneous at the conjugate E layer.…”

“…The study of the seasonal variation of V p is therefore not suitable for the determination of the exclusive influence of LGIPC in the E region on V p . In connection with the variation of V p with solar flux, Goel et al [1990] using E region model by Muggleton [1975] found that because of the variation in electron density from low to high solar flux period, the LGIPC in the E region increases by a factor of two. They suggested that theoretical calculations of V p incorporating the above variation in the LGIPC in the E region be made to assess the role of these gradients in explaining observed variations in V p with increase of solar flux.…”

“…Therefore, we have examined whether the expected variation in the FTIC of the F region with the solar flux can account for the observed increase in the strength of the EPRE from low to high solar flux conditions. We also examined whether the increase in LGIPC in the E region due to the change of solar flux, as suggested by Goel et al [1990], can account for the increase of the EPRE from the low to high solar flux period. For developing a mechanism to explain the TF 3 we limit our discussion exclusively to equinoxes for an equatorial station with zero declination angle.…”