Fast and ultrafast Kelvin wave modulations of the equatorial evening F region vertical drift and spread F development

Mangalathayil A Abdu,Inez S Batista,Subramanian Gurubaran,Hisao Takahashi,Christiano Gm Brum,Paulo P Batista,Jose V Bageston,Dora Pancheva

doi:10.1186/s40623-014-0143-5

Abstract

In this paper, we investigate the role of eastward and upward propagating fast (FK) and ultrafast Kelvin (UFK) waves in the day-to-day variability of equatorial evening prereversal vertical drift and post sunset generation of spread F/plasma bubble irregularities. Meteor wind data from Cariri and Cachoeira Paulista (Brazil) and medium frequency (MF) radar wind data from Tirunelveli (India) are analyzed together with Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) temperature in the 40- to 100-km region to characterize the zonal and vertical propagations of these waves. Also analyzed are the F region evening vertical drift and spread F (ESF) development features as diagnosed by Digisonde (Lowell Digisonde International, LLC, Lowell, MA, USA) operated at Fortaleza and Sao Luis in Brazil. The SABER temperature data permitted determination of the upward propagation characteristics of the FK (E1) waves with propagation speed in the range of 4 km/day. The radar mesosphere and lower thermosphere (MLT) winds in the widely separated longitude sectors have yielded the eastward phase velocity of both the FK and UFK waves. The vertical propagation of these waves cause strong oscillation in the F region evening prereversal vertical drift, observed for the first time at both FK and UFK periodicities. A delay of a few (approximately 10) days is observed in the F region vertical drift perturbation with respect to the corresponding FK/UFK zonal wind oscillations, or temperature oscillations in the MLT region, which has permitted a direct identification of the sunset electrodynamic coupling process as being responsible for the generation of the FK/UFK-induced vertical drift oscillation. The vertical drift oscillations are found to cause significant modulation in the spread F/plasma bubble irregularity development. The overall results highlight the role of FK/UFK waves in the day-to-day variability of the ESF in its occurrence season.

Highlights

Plasma structuring of the equatorial nighttime ionosphere, a.k.a, equatorial spread F (ESF) irregularities, is known to suffer large degree of spatial and temporal variabilities of wide-ranging scales due to external forcing
In this work, we undertook to characterize the global nature of the eastward propagating Kelvin waves in the mesosphere and lower thermosphere (MLT) region and to understand the upward propagation characteristics of the waves involving vertical coupling through neutral dynamics extending from the stratosphere to the MLT region and further to the ionospheric F region involving electrodynamic coupling processes
Extensive focus in this work was on the upward propagation characteristics of the waves into the MLT region followed by the electrodynamic coupling to F region heights resulting in strong modulation of the prereversal vertical plasma drift and consequent effects in the post sunset equatorial spread F bubble irregularity development

Summary

Introduction

Plasma structuring of the equatorial nighttime ionosphere, a.k.a, equatorial spread F (ESF) irregularities, is known to suffer large degree of spatial and temporal variabilities of wide-ranging scales due to external forcing. From meteor radar wind measurement over Ascension Island, Pancheva et al (2004) found the overall wind characteristics to be in agreement with those of other equatorial longitudes, and the zonal wind was dominated by 3- to 7-day waves while the meridional wind component by quasi 2-day waves around southern summer months The manifestations of these waves at ionospheric heights are well established in the form of the associated oscillations in the F layer critical frequencies (Chen 1992; Forbes and Leveroni 1992), sporadic E layer top frequencies (Pancheva et al 2003), nighttime F layer heights, and evening prereversal vertical drift and ESF (Abdu et al 2006a, b; Takahashi et al 2005, 2006; Fagundes et al 2009). Since the PW's penetration all the way to ionospheric F region heights is uncertain, their observed signatures at these heights are widely believed to arise from their modulation of the tidal wind modes (that are active in the dynamo region) by nonlinear interaction (Pancheva et al 2003; Haldoupis et al 2004)

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Discussion

Conclusion