SA52A-01 INVITED
Ducted Gravity Waves and their Effects in the Upper Atmosphere
Ducted gravity waves are trapped internal gravity waves that can be sustained with minimal diminishment in the absence of sustained forcing. They are near-resonant waves trapped by evanescence, or rapid vertical variations in the background state. There is accumulating evidence of a large population of ducted gravity waves in the upper mesosphere and thermosphere. Ducted waves may account for much, if not most, of the coherent wave forms seen in the upper atmosphere. Persistence and long distance propagation is favored by some degree of ducting. Wave amplification in the upper atmosphere related to ducting provides a means for waves to propagate to great heights with significant amplitude where they may interact with the ionosphere. Ducts may be caused by thermal stratification (peaks in the Brunt-Väisälä frequency) or by variations in the intrinsic frequency due to wind shear. A thermal duct is always present in the lower thermosphere and another is often present in the upper mesosphere. The latter is due to an inversion below the mesopause and collocated or nearly collocated with the OH emission layer. When the winds are favorable these ducts can support very strong ducting events. I present the theory of wave ducting, show calculations of favored regions in phase-speed, wavenumber space for wave ducting as a function of the background wind and thermal structure and discuss observational evidence for ducting including recent observations of strong bore-like ducting events and standing waves in ionospheric ion-density perturbations.
SA52A-02 INVITED
Mid-Latitude Spread-F Variability Observed by the Wallops Island Digisonde
During the 2006 and 2007 launch windows for the Storms sounding rocket experiment, the Wallops Island digisonde made nightly observations at a 5-minute cadence. Six out of the 30 nights of observations contained spread-F traces that lasted for more than 30 minutes. Each of these events varied in duration, range spread, and onset time. Additionally, the events occurred during different levels of geomagnetic activity and tropospheric conditions. The more intense range spreading and longer duration spreading events tend to correspond to higher geomagnetic activity conditions. This talk will present a brief introduction to mid-latitude spread-F, its formation theories, and possible sources of forcing. The remainder of the talk will focus on several of the cases that represent Spread-F resulting from geomagnetic activity induced TIDs and tropospheric weather induced gravity waves.
SA52A-03 INVITED
New Ionosonde Observations of Mid Latitude Spread-F at Wallops Island
A new generation digital ionosonde, a Vertical Incidence Pulsed Ionospheric Radar (VIPIR) was installed at the NASA Wallops Island Flight Facility in Virginia and saw first operation in 2007. The VIPIR features dramatic improvements in ionosonde and radar system design, including multiple phase-coherent, fully digital receivers, improved antenna designs, and higher data resolution. This produces markedly improved ionograms. From the mid 1980's through today, NASA Wallops hosts a first generation digital ionosonde, a US Air Force Digital Ionospheric Sounding System or Digisonde 256. Phase coherent digital sampling of analog receiver output, digital data storage and automated real time ionogram scaling allowed spread-F quantification and use of the data for launch decisions. In the 1970's and early 1980's an analog ionosonde, at Wallops recorded data on photographic film. Hourly characteristics were later scaled by hand, with spread-F being qualitatively identified. Recently, select periods of film ionograms have been digitized by NOAA/NGDC and are available in electronic format. This presentation compares spread-F observations made with three generations of ionospheric sounding equipment operated at the NASA Wallops Island Flight Facility. Comparison of ionogram data from both VIPIR and Digisonde ionosondes during several different events reveals newly observed mid-latitude spread- F features.
SA52A-04
Results from a Comprehensive Rocket Investigation of Mid-Latitude Spread F
In late October of 2007 an instrumented sounding rocket was launched into a mid-latitude spread F (MSF) condition over Wallops Island. The MSF event occurred several hours after an abrupt increase in the Kp magnetic activity index. The rocket experiment measured the plasma density, electric field, and horizontal neutral wind as it traversed a region of the ionosphere that simultaneously produced spread-F signatures on the Wallops Island digisonde system. Prior to and during the rocket flight the ionospheric medium was continually under observation by a second vertical incidence radar system, a TID-detection radar, scintillation receivers, and the east coast GPS receiver network. This presentation will provide an overview of the complete experiment, followed by detailed rocket results. Several competing theories of mid-latitude spread F development will be quantitatively compared to the rocket measurements to determine which are consistent with the observations.
SA52A-05
TIDDBIT HF Doppler Sounder Measurements of TIDs During the Wallops Island Rocket Launch of October 2007
The TID Detector Built In Texas (TIDDBIT) sounder was deployed on the East Coast near Wallops Island to support a rocket launch in October 2007. The purpose of the rocket experiment was to study mid-latitude spread-F (MSF), and TIDDBIT provided information on the TID characteristics during the launch and for several days surrounding the launch. The sounder data confirm that waves were present during the rocket launch. This presentation reviews the TIDDBIT results from the experiment, contrasting data collected on different days, and from the same dates a year earlier. HF Doppler sounders represent a low-cost and low- maintenance solution for monitoring acoustic and gravity wave activity in the F-region ionosphere. HF Doppler sounders together with modern data analysis techniques provide both horizontal and vertical phase trace velocities across the entire TID spectrum from periods of 30-s to several hours. ASTRA has extensive experience with HF systems, and is currently building TIDDBIT sounders in New Mexico, and Peru.
SA52A-06
Observations of Equatorward Propagating Backscatter Enhancements on the Nightside with the Wallops SuperDARN Radar
The first mid-latitude SuperDARN radar was built at NASA Wallops Flight Facility in 2005. It has observed the expansion of storm-time ionospheric electric fields, backscatter from F region plasmapause irregularities, sporadic E layers, and a variety of effects that are not yet understood. One such effect is the equatorward propagation of trains of backscatter power enhancements on the nightside, especially in the premidnight sector. This is a wave-like phenomena that bears similarities to observations with the high-latitude SuperDARN radars of TIDs associated with atmospheric gravity waves. However, the characteristics of the activity at Wallops are quite distinctive. The night of the October rocket launch was very active with this type of backscattering, which was clearly seen to reach to the immediate vicinity of Wallops; the associated perturbation in the ionosphere may have conditioned the rocket measurements. In this talk we describe the unusual properties of this type of backscatter activity, characterize the wave-like propagation, and interpret the activity in terms of the comprehensive set of measurements collected before and during the launch.
SA52A-07
Daytime low- and middle-latitude spread-F driven by Rayleigh-Taylor instability during the PPEF events
The possibility of Rayleigh-Taylor (RT) instability of the dayside ionosphere is examined under the condition of a strong prompt penetrating electric field (PPEF). Strong PPEF events have been shown to occur during superstorms with Dst less than -250 nT. These PPEFs have been noted to cause significant electron density redistribution in the ionosphere (the dayside ionospheric superfountain, or DIS). We focus on dynamics of the daytime and postdusk ionosphere at low- and middle latitudes. We present first estimates of the RT instability growth rates based on a modified SAMI-2 model. An electric field value of ~ 4 mV/m is assumed. We find that there are two regions in the daytime ionosphere where the RT instability develops. Bottom-side F-layer instability occurs at the lower portion of the uplifted equatorial ionospheric anomaly (EIA). The corresponding altitude range of instability is between ~500 and 700 km. Another region of instability is below ~150 km altitude. With the addition of the PPEF in the post-dusk sector, the RT growth rates are considerable higher at the bottom-side of the EIAs. Corresponding linear growth time for the RT instability vary from 0.8 minutes to 4 minutes, depending on local time and altitude.
SA52A-08 INVITED
Occurrence of Elevated Total Electron Content in the Dusk Sector During Associated with F-Region Structuring at Tropical Latitudes
We examine the total electron content (TEC) derived from dual-frequency Global Positioning System (GPS) receivers during geomagnetically active events where airglow images of tropical F-region emissions show anomalous structuring. The airglow data are typified by structures that are reminiscent of quiet-time medium- scale traveling ionospheric disturbances (MSTIDs) in terms of their orientation, propagation velocity and wavelength. However, during storm-time events, there is significantly more small-scale structuring seen in the airglow data and larger variations in TEC than during quiet-time MSTID events. These structures may be associated with mid-latitude spread-F. We suggest that the development of these structures is due to the presence of an increased background total electron content, at times factors of 5 times higher than quiet- time values, caused by the redistribution of equatorial plasma by storm-time penetration electric fields in the dusk sector as suggested by Basu et al. (2001). We present data collected during several storm-time events in the Pacific and American sectors during which simultaneous and near-collocated imaging and GPS data are available.
SA52A-09
Mid-latitude Spread-F and Large Magnetic Storm-Driven Nighttime Flows
Plasma structuring at mid-latitudes causing spread-F on ionograms and VHF/UHF scintillations can have unexpected impacts on GPS signals at L-band if these events occur in a background of large magnetic storm-driven nighttime flows. Two such storm results are presented. The common distinguishing feature of both storms has been the absence of appreciable storm-enhanced density or total electron content (TEC) gradients, which are generally observed in the afternoon sector; rather, the mid-latitude region was enveloped by the auroral oval and the ionospheric trough, within which the sub auroral polarization stream (SAPS) was confined during the local dusk to nighttime hours. The intense flow channel in the ionosphere was revealed by both space and ground-based measurements, through DMSP over-flights and Dynasonde drifts. The competing effects of irregularity amplitude ÄN/N, the background F-region density and the magnitude of SAPS or auroral convection are discussed in establishing the extent of the region which can impact GPS-based navigation systems such as the Wide Area Augmentation System (WAAS) that operates in the North American sector.
SA52A-10
Three-Dimensional Simulation of the Coupled Perkins and Es-Layer Instabilities in the Nighttime Midlatitude Ionosphere
Plasma density structures and associated irregularities in the nighttime midlatitude ionospheric E and F regions are frequently observed as frontal structures elongated from northwest to southeast (NW-SE) in the northern hemisphere. The frontal structures and the coupling process between the E and F regions are studied with a three-dimensional numerical model which can simulate two instability mechanisms: Perkins instability in the F region and sporadic-E (Es)-layer instability in the E region, both of which have directional preference which can account for the NW-SE structure. Applying sinusoidal perturbation on an Es layer, it is confirmed that the fastest growth of the coupled instability occurs when the unstable conditions on NW-SE perturbation are satisfied in both regions. The perturbation of F-region integrated conductivity grows much faster than the isolated Perkins instability. The predominant process in the coupled system depends on the ratio of integrated Hall to Pedersen conductivity. A larger Hall conductivity given by an Es layer is important for growth of the coupled instability as well as the Es-layer instability itself. NW-SE structure in the E region can be formed from random perturbation regardless of the F-region condition. When the F region is unstable on NW-SE perturbation, however, the Es-layer instability is reinforced through the coupling process, and the NW-SE structure is formed in both regions with a common scale length which is attributed to the Perkins instability. We conclude that (1) the Es-layer instability plays a major role in seeding NW-SE structure in the F region, and the Perkins instability is required to amplify its perturbation, and (2) the coupling process has a significant effect on the scale of the Es-layer perturbation rather than the growth speed of the Es-layer instability.
SA52A-11
New Metrics for Mid-Latitude Spread F
A new methodology has been developed for detection and quantification of mid-latitude spread F (MSF) and applied over an eleven year period from 1996-2006. The data for this time period were obtained from the digisonde at Wallops Island (37.95° N, 284.53° E, 67.5° dip angle). Raw binary format data were processed and filtered to obtain ionograms. A pattern recognition algorithm analyzes these ionograms to determine the start time, duration and intensity of mid-latitude spread F. Range and frequency spreading on the ionograms are differentiated with this algorithm, and the results can be useful for a statistical understanding of MSF. This presentation will focus on comparing, under various geophysical conditions, this new MSF detection algorithm with the automated metrics produced by the ionosonde in real time and currently contained in the ionosonde data archive.