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This work presents the first simultaneous observations of Na and K between 120 and 150 km altitudes along with ionospheric tidal ion layers (TILs) obtained on 30 January 2006 data from Arecibo. The latter displays an average downward phase velocity of ~14.7 ms−1. However, the neutral layers descend together at a much slower velocity of about ~0.69 ms−1. This indicates that thermospheric atomic metal layers are not necessarily associated with TILs. The ratio of the average Na/K abundances in thermosphere is ~35.5 as compared to 150 in the main layer (80–105 km). The long lifetimes of ions at ~140 km implies that neutral layers cannot result from direct neutralization of metal ions in the TILs. We investigate different mechanisms that can deposit neutral atoms at thermospheric altitudes.

[1] We present incoherent scatter radar measurements of electron density, electron and ion temperatures, and ion composition made at Arecibo Observatory (18.35°N, 66.75°W), which is at a geomagnetic latitude of 30°N (or 46.7°dip latitude), during the recent extreme solar minimum of [2007][2008][2009] and find agreement between our data and recent reports of corresponding satellite observations. Both the in situ spacecraft measurements and our ground-based radar profiles exhibit unusually low electron densities and cold temperatures. These two factors result in an extraordinarily contracted ionosphere and thermosphere. This contraction in the ionosphere in turn causes the O + /H + transition height to descend; thus, the base of the low-latitude plasmasphere, or protonosphere, is found at extraordinary low altitudes. We show that during the geomagnetically quiet period of October 2009, the transition height h t , where, was observed at altitudes as low as 800-820 km during daytime and descended as low as 450 km during the night. At night, when T e = T i = T n , temperatures below 675 K were measured at 03:00 Atlantic Standard Time. These values are about 100 K lower than corresponding temperatures observed by the Arecibo incoherent scatter radar during the previous solar minimum period (1995)(1996)(1997).

Dynamical, electro-dynamical and electrical coupling processes originating from upward propagation of atmospheric waves, and magnetosphere-ionosphere interaction are responsible for the large degree of variabilities observed in the low latitude ionosphere. One of the most outstanding aspects of its phenomenology is related to the sunset electrodynamical processes responsible for the evening enhancements in zonal and vertical electric fields and the associated spread of F/plasma bubble irregularity development. Recent observational results have provided evidence of significant contribution to their quiet time variability arising from thermospheric wind patterns, upward propagating planetary waves and possibly sporadic E layers. This paper provides an overview and some new results on planetary wave coupling with the equatorial F region, the E layer conductivity as key connecting mechanism, a possibly interactive role by sporadic E layers, and the resulting day-today variability in the evening prereversal electric field enhancements with consequences on spread F development.

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.

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Since 1980, we have observed the thermospheric neutral wind at the Arecibo Observatory using a Fabry‐Perot interferometer to measure the O(1D) 630 nm emission. Burnside and Tepley (1989) examined the first 8 years of this extended data set and found that there were no significant or systematic solar cycle influences on the magnitude or direction of the neutral wind field, nor on its horizontal gradients. Such affects have been observed previously at other locations around the globe, and their absence at Arecibo may have been due to the limited data set. Thus, we have extended the period of acquisition and analysis of our neutral wind measurements to include nearly three complete solar cycles (or approximately 30 years) and will present our results within the framework of the earlier work. While the earlier conclusion that no major systematic solar cycle influence on the neutral winds at Arecibo generally remains intact, we did find a slight increase in wind magnitude and a gradual, yet consistent rotation of the thermospheric neutral wind vector from a general southeast to a more eastward flow during 30 years of observation. We explain the magnitude and directional variations in terms of long‐term changes in the density and temperature of the upper atmosphere and their possible dissimilar influences on each wind component that appear as a rotation of the neutral wind vector.

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