[1] Incoherent scatter radar (ISR) power profile observations at Arecibo Observatory (AO) have revealed long vertical wavelength (>100 km) quasi-coherent waves with periods of $1 hour that are observed to be nearly continuously present over two $35-hour geomagnetically quiet observation periods. When properly filtered, results from both 22-23 March 2004 and 5-6 June 2005 provide unambiguous views of these waves. The waves are strong throughout the F region, often spanning 160 to above 500 km in altitude and are present day and night in the F2 layer. Filtering techniques that were used to better reveal the waves are extensively discussed. Barometric pressure and imager data, both of which were taken on site at Arecibo, are used to provide further insight into the nature of these waves. These waves may be linked to high-latitude medium-scale traveling ionospheric disturbances (MSTIDs) observed with the superDARN radar network and associated with E region auroral electrojet source(s) that are quasi-coherent under geomagnetic quiet conditions. As auroral-zone MSTIDs are believed to dissipate before reaching Arecibo latitudes (Vadas, 2007), various alternative sources including weather patterns are explored.

[1] Wave-like disturbances in electron density n e have been observed in the thermosphere above Arecibo Observatory, Puerto Rico throughout its 45 year history. However, only recently has it become evident that these waves are continuously present in the Arecibo thermosphere. The wave characteristics are fairly constant between day and night and from season to season. High-resolution electron density measurements obtained by applying the coded long-pulse radar technique to photoelectron-enhanced Langmuir waves are presented. These new observations strongly suggest that the perturbations in electron density are the result of internal acoustic-gravity waves (AGWs) propagating through the Arecibo thermosphere. The AGWs appear to be broadbanded in wave number space. The downward phase trajectories of Dn e /n e between 400 and 120 km combined with the low horizontal phase velocities obtained from airglow measurements support the idea that the AGWs are not ducted but rather are locally produced. In addition, the altitudes at which major peaks in Dn e /n e are observed follow theoretical estimates for nonducted waves. The nominal period of the AGWs is ∼60 min at 250 km altitude, but periods of ∼20 min are also evident at lower attitudes. Classic sources of AGWs do not appear to be consistent with the Arecibo observations of a continuous flux of background AGWs. Ray tracing of the AGWs combined with 630.0 nm airglow observations point to a source location in the Atlantic Ocean that is roughly 2100 km east northeast of Arecibo. Internal ocean waves generated in response to the internal tide at the mid-Atlantic Ridge are the most likely source of Arecibo's thermospheric AGWs.

F-region plasma dynamics has been one of the main foci of the ionospheric research community for decades. The mid-latitude F-region has been considered to be relatively calm; however recent observations using highly sensitive CCD imaging systems and Global Positioning System (GPS) receivers have revealed that mid-latitude F-region mesoscale electrodynamics are more complex and this region is more active than usually assumed. Here we report combined incoherent scatter radar (ISR), imager, and GPS observations of F-region MediumScale Traveling Ionospheric Disturbance (MSTID) structures over the Arecibo Observatory in Puerto Rico. In particular, the plasma structures seen in the narrow-beam ISR cannot be understood fully without the all-sky images, which provide the context for the radar results-specifically, the spatial and temporal properties of the mesoscale structure. The GPS-derived total electron content (TEC) data provide additional information on the intensity of the MSTIDs. Here we present analysis of two specific plasma depletion events, which we prefer to call "MSTID bands". Important results on the 3D geometry of these structures were found using a newly developed observation technique. For the first time, it is shown that the southern part of MSTID bands reaches higher altitudes than the northern part (vertically tilted by 12• towards magnetic south). These results give a much broader perspective on nighttime, mid-latitude F-region structure and point to new ways of interpreting these structures and how they appear in ISR results.

Incoherent Scatter Radar power profile observations at Arecibo, Millstone Hill, and the Poker Flat AMISR have revealed the continuous presence of Coherent Omnipresent Fluctuations in the Ionosphere (COFIs) with periods ranging from roughly 30 to 60 minutes and apparent vertical wavelengths increasing with altitude from tens to hundreds of kilometers. Upon high‐pass filtering of the Incoherent Scatter Radar power profile and electron concentration data, the COFIs are seen unambiguously and ubiquitously in Arecibo results from 22–23 March 2004, 5–6 June, 21–25 September, and 17–20 November 2005, as well as Millstone Hill results from 4 October to 4 November 2002. The COFIs are strong throughout the F region, often spanning altitudes of 160 km to above 500 km, and are detected day and night in the F2 layer. In fact, the COFIs are seen at every time and altitude that there is sufficient plasma to detect them. The COFIs are also observed at Poker Flat, although the poor signal‐to‐noise ratio over segments of the data makes it difficult to determine whether or not they are always present. The consistent detection of the COFIs, along with the longitudinal alignment and large latitudinal spread of the observation sites, suggests that these waves are always present over at least North America. This phenomenon appears to have been reported in Total Electron Concentration (TEC) maps of the ionosphere over much of North America Tsugawa et al. (2007b) as well as in airglow images from Arecibo and many other midlatitude sites around the world. These observations give us insight into the horizontal properties of the waves. While Medium‐Scale Ionospheric Disturbances (MSTIDs) are generally associated with aurorally generated acoustic gravity waves, the properties of the COFIs may suggest otherwise. We present other possible source mechanisms, notably a possible link to oscillations in the solar wind and magnetosphere. We have observed consistent fluctuations with periods of about an hour observed in magnetic field measurements taken at geosynchronous altitudes by the Geostationary Operational Environmental Satellites (GOES)‐10 and ‐12 satellites, which may be linked to the COFIs. We give corresponding solar wind results from ACE and discuss possible coupling mechanisms.

[1] Perhaps the most persistent and prominent of the midlatitude nighttime F region phenomena observed with all-sky imagers is the Medium-Scale Traveling Ionospheric Disturbances (MSTID) that occur when the geomagnetic activity is low. We introduce a simple empirical 3-D model of these bands, which was developed on the basis of an extension of results given by Seker et al. (2008). Seker et al. (2008) reported observational results on the 3-D geometry of MSTID structures found using combined incoherent scatter radar (ISR) and all-sky imager observations over the Arecibo Observatory in Puerto Rico. To confirm and further investigate these findings, a simple empirical 3-D model of nighttime MSTID bands is constructed. We show ISR and all-sky imager results derived from the model, compare them with the actual observations, and highlight the important findings. This model is intended to replicate both the azimuth-scanning ISR and the all-sky imager results and is especially useful in explaining how these complex structures appear in azimuth-scanning ISR results. In addition, confidence in the empirical model allows us to know what to expect from the theory (of MSTID) and to assess how well geomagnetically quiet, nighttime, midlatitude, F region electrodynamics models succeed. Furthermore, the technique described in the paper is not limited to F region or midlatitudes only; it could be applied to any other phenomenon which can be observed by both radar and imager.