Vertical coupling of atmospheres: dependence on strength of sudden stratospheric warming and solar activity

Fazlul I Laskar,Duggirala Pallamraju,Bhaskara Veenadhari

doi:10.1186/1880-5981-66-94

Abstract

Comprehensive behavior of the low-latitude upper atmosphere during sudden stratospheric warming (SSW) events at varying levels of solar activity has been investigated. The equatorial electrojet (EEJ) strength and the total electron content (TEC) data from low latitudes over Indian longitudes during the mid-winter season in the years 2005 to 2013 are used in this study. Five major and three minor SSW events occurred in the observation duration, wherein the solar activity had varied from minimum (almost no sunspots) to mini-maximum (approximately 50 sunspots of the solar cycle 24). Spectral powers of the large-scale planetary wave (PW) features in the EEJ and the TEC have been found to be varying with solar activity and SSW strengths. Specially, the spectral powers of quasi-16-day wave variations during the three very strong SSW events in the years 2006, 2009, and 2013 were found to be very high in comparison with those of other years. For these major events, the amplitudes of the semi-diurnal tides and quasi-16-day waves were found to be highly correlated and were maximum around the peak of SSW, suggesting a strong interaction between the two waves. However, this correlation was poor and the quasi-16-day spectral power was low for the minor events. A strong coupling of atmospheres was noted during a relatively high solar activity epoch of 2013 SSW, which was, however, explained to be due to the occurrence of a strong SSW event. These results suggest that the vertical coupling of atmospheres is stronger during strong major SSW events and these events play an important role in enabling the coupling even during high solar activity.

Highlights

The low-latitude upper atmosphere of the Earth is coupled vertically to the lower atmospheric and horizontally to the high-latitude dynamical processes
This plasma diffuses along the magnetic field lines and accumulates at around ±15° latitudes off the magnetic equator, building up regions of enhanced densities in plasma, and this phenomenon is known as the equatorial ionization anomaly (EIA) (Raghavarao et al 1988)
Waves propagating from the lower atmosphere perturb the E-region dynamo electric field which is mapped to the F-region and thereby contribute to the redistribution of plasma

Summary

Introduction

The low-latitude upper atmosphere of the Earth is coupled vertically to the lower atmospheric and horizontally to the high-latitude dynamical processes. While the neutral motions are affected via waves in the atmosphere and incoming solar radiation, the plasma motions are driven by the electrodynamical processes. Since both plasma and neutral species share the same space, their motions are coupled to each other in the upper atmosphere. It is believed that they can influence the upper atmosphere by modulating the shorter period gravity waves and tides (Lastovicka 2006). The gravity waves and tides with greater horizontal wavelengths are allowed to propagate to the upper atmosphere under suitable conditions, thereby enabling communication of the largescale lower atmospheric waves into the upper atmosphere. During sudden stratospheric warming (SSW) events, the PW amplitudes get amplified and the stratospheric and mesospheric zonal winds are altered drastically rendering the conditions conducive for the propagation of the small-scale waves which are already modulated by the PWs (Liu et al 2010; Yiğit and Medvedev 2012)

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Conclusion