Threshold height (h′F)_c for the meridional wind to play a deterministic role in the bottom side equatorial spread F and its dependence on solar activity

Abstract

Detailed investigations have been carried out on the occurrence of bottom side Equatorial Spread F (ESF) and the thermospheric meridional wind characteristics just before the former's initiation using ground based ionospheric data corresponding to the equinoctial periods of 1993–1998, from Trivandrum (8.5°N, 76.5°E, dip = 0.5°N) and Sriharikota (13.7°N, 80.2°E, dip ∼ 10°N) in the Indian longitudes. Critical analysis of the base height of the F‐region h′F at the time of triggering of ESF and the polarity of the meridional winds revealed that, if the h′F is above a certain level ESF occurred under both equatorward and poleward wind conditions. Below that level, ESF occurred only when equatorward winds were present implying that the equatorward winds must somehow be able to offset the reduced growth rate of the plasma instability responsible for ESF. A plausible explanation linking Equatorial Ionization Anomaly (EIA) and the consequent Equatorial Temperature and Wind Anomaly (ETWA) and the consequent neutral dynamics effectively enabling the instability even at lower height has been offered. The threshold height (h′F)c gleaned out on the basis of the polarity of the meridional winds has been shown to bear a linear relation to the solar activity and sheds light on the enigmatic short and long term variability of ESF.

On the seasonal variations of the threshold height for the occurrence of equatorial spread F during solar minimum and maximum years

“…They suggested that the meridional winds were the outcome of the pressure bulges associated with ETWA (Raghavarao et al, 1993). Jyoti et al (2004) brought out the dependence of the threshold height h F c , on solar activity for the equinox. In the present study, the seasonal variation in the threshold height for the triggering of ESF has been examined, corresponding to the solar minimum and maximum phases.…”

“…Only those events for which ESF is triggered before 20:00 IST are considered for this study. As mentioned earlier, the criterion of ESF occurrence irrespective of the polarity of meridional winds is applied to h F to determine the threshold height h F c below which equatorward wind is essential for the triggering of ESF (Devasia et al, 2002;Jyoti et al, 2004). Figure 1a shows the scatter plot of the meridional wind velocities against h F values of each of the ESF events during the summer solstice of solar minimum (left panel) and solar maximum (right panel) periods.…”

“…But the ETWA-related meridional winds, which follow the day-to-day variability of the EIA, modulate the inter-hemispheric winds and thereby result in higher or lower values of meridional winds either poleward or equatorward (depending on the magnitude and direction of the ETWA related winds). For the winter solstice, the upper bound (Jyoti et al, 2004) of the threshold height could only be delineated and it was found to increase from 240 km at solar minimum to 264 km at solar maximum. It is seen that the threshold height shows an increase with solar activity for winter solstice similar to the summer solstice, but to a significantly lesser extent.…”

“…Above the critical height the polarity did not matter. This threshold height of h F (defined by them) has been found to be ∼300 km for the occurrence of ESF during equinoctial periods and showed a linear solar activity dependence (Devasia et al, 2002;Jyoti et al, 2004). Raghavarao et al (1993) conjectured that such equatorward winds could be produced as a result of the pressure bulges associated with Equatorial Temperature and Wind Anomaly (ETWA).…”

The seasonal and solar cycle variations of electron density gradient scale length, vertical drift and layer height during magnetically quiet days: Implications for Spread F over Trivandrum, India

“…These conditions include the movement of the F-layer to very high altitudes in the postsunset hours, and/or formation of steep bottom side F-layer electron density gradient. Aspects related to the effect of meridional winds, on ESF occurrence, have been examined by some workers (Maruyama and Matuura, 1984;Devasia et al, 2002;Jyoti et al, 2004;Manju et al, 2007). Studies on the role of seed perturbations, in the generation of F region irregularities have been reported by Whitehead (1971) and Fejer et al (1999).…”