No abstract

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.

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.

[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).