SA24A-01 INVITED
Initial Results of the Spread F Experiment (SpreadFEx): Overview and Evidence of Possible Gravity Wave Excitation of Equatorial Plasma Bubbles
The Spread F Experiment (SpreadFEx) was performed in Brazil by Brazilian and U.S. researchers during two ~20- day periods extending from September to November 2005. We employed extensive ground-based and space- based observations of gravity waves, plasma structures, electron densities, and mean atmospheric and ionospheric conditions using airglow, digisonde, VHF and meteor radar, balloon, GPS and satellite instrumentation at multiple sites in Brazil and with GUVI aboard the TIMED satellite. These measurements focused on deep convection, gravity waves, and plasma bubble structures. This comprehensive data set has provided the first promising indications of the specific roles of gravity waves arising from deep convection and other sources in contributing to the seeding of equatorial spread F and plasma bubbles extending to high altitudes. This talk will summarize the campaign results related to possible neutral atmosphere seeding of spread F and plasma bubbles during these observations. Specifically, our measurements have revealed significant neutral density (and related wind and temperature) perturbations extending from ~80 km well into the thermosphere and ionosphere. Many of these appear to arise from deep convection over the Amazon basin. Others occurring at larger scales under magnetically-disturbed conditions may have auroral or other higher-latitude sources. Both appear to lead, on occasion, to sufficiently large perturbations of the bottomside F layer to trigger plasma bubbles extending to much higher altitudes thereafter. Upon completion of our analyses, we believe that these observations will yield the first persuasive evidence of the role of neutral atmosphere gravity waves in the seeding of equatorial plasma bubbles.
SA24A-02
A Two-Year Study of 50-MHz Mesospheric Echoes and Winds at Jicamarca, Peru
The Jicamarca 50-MHz radar has been operated since December 2004 for over 30 days in a MST-ISR mode receiving coherent scatter between 11 to 180 km with nominal 150 m resolution and incoherent scatter between 200 and 900 km. This paper will focus on the highly structured turbulent layers from the daytime mesosphere (D region) between 55 and 85 km. Non-linear fitting of a generalized Gaussian yields power, Doppler velocities and spectral width for four fixed beams, from which horizontal and vertical winds, momentum fluxes and turbulent velocities are calculated. The ISR data are modeled to determine electron density profiles (which are compared to ionosonde data) and also calibration factors for all beams to derive absolute reflectivities for the mesospheric turbulent scatter. Earlier experiments have shown evidence for annual and semiannual variability in the winds, signatures of the diurnal tide, inertia-gravity waves and instabilities. The more comprehensive data set covering over two years is employed to study the variability of small-scale dynamics and its correlation with the seasonal change of the prevailing winds.
SA24A-03 INVITED
Multi-Instrument Studies of the Magnetic Conjugate Nature of F-region Structure
By combining data from multiple types of instrumentation collected at different locations, we can come to a better understanding of the physics and properties of structures in the low-latitude ionosphere and their effects on trans-ionospheric radio wave propagation. In this study, we present examples of spread-F events collected from both the Pacific (Hawaii/Christmas Island) and South American (Chile/Peru) sectors using optical imaging systems and coherent backscatter radar in addition to both ground- and space-based GPS receivers. The optical data provide background information on the location and spatial/temporal dynamics of irregularity-containing depletions over a wide spatial region. In order to put the observations made with the other instrumentation at different locations into a broader context, we map the structure observed with the optical instrumentation along magnetic field lines. In the case of coincident imaging/radar data, we show that the images aid in interpretation of the spatial characteristics of the structure observed by the narrow radar beam. We also compare velocities measured by the radar and imaging systems to study the conjugate nature of these irregularities in more detail. In the case of coincident imaging/occultation data, we highlight how the imaging data can be used to determine when the spherical symmetry conditions assumed in the occultation inversion breaks down. We will also discuss future experiments that will employ additional instruments to further study the conjugate nature of these irregularity regions.
SA24A-04
Observations of Electric Fields Associated With Internal Gravity Waves
Measurements of the ion drift perpendicular to the magnetic field allow an unambiguous determination of the electric field in that plane. At the Jicamarca Radio Observatory the vertical drift component yields a very accurate measure of the eastward electric field since the spectrum of the ISR signal is extremely narrow in the plane perpendicular to B. Occasionally this drift component displays a downward-phase progression, which is evidence for a relationship to a gravity wave. The idea that gravity waves can create electric fields has been around for awhile but there are only two cases reported in the literature, one being from the same data set discussed here. We examined the Jicamarca database for events of this type and made an attempt to determine the properties of the associated waves. The only measureables we have are the frequency in the earth-fixed frame and the vertical wavelength. We extend the information as follows. In order to avoid shorting by the current along magnetic field lines, we argue that the propagation must be close to pure zonal. We then use measurements or models of the zonal plasma drift and argue that the zonal wind should be in the same direction and about 15% higher. Using this estimate, along with temperature and density estimates from the MSIS computer model, we then solve the dispersion relation for gravity waves and the Doppler-shift equation simultaneously. This allows us to determine the frequency in the wind frame. A typical value for the horizontal wavelength, vertical wavelength, and period in the wind frame is 600 km, 350 km, and 25 minutes, respectively. All but one event found thus far occurred at night but the daytime case is fascinating since the E region is expected to short out such fields. The typical gravity wave-induced vertical drift perpendicular to B in these events is a few m/s. This is sufficient to seed the Rayleigh-Taylor instability.
SA24A-05
ESF-related airglow depletions at Arecibo and conjugate observations
Data from the Boston University all-sky imager located at Arecibo, Puerto Rico (18.3° N, 66.7° W, 28° N mag lat), have been used to identify several nights with 630.0 nm airglow patterns that are typical signatures of equatorial Spread-F (ESF) and distinctly different from the more common airglow bands frequently observed there. Two case studies show the occurrence of simultaneous airglow depletions observed with another all-sky imager located at El Leoncito, Argentina (31.8° S, 69.3° W, 18° S mag lat), relatively close to the Arecibo conjugate point. Supporting information is obtained from DMSP, ROCSAT-1 and GPS data, all of them showing the presence of strong ionospheric irregularities collocated with the airglow depletions. Mapping the circular field of view from Arecibo into the southern hemisphere reveals a distorted pattern due to the differences in the magnetic field characteristics in both hemispheres. Arecibo's location can play a pivotal role in attempts to understand how low to mid-latitude coupling occurs. The region poleward of ~18° magnetic latitude and equatorward of ~25° magnetic latitude can be considered a good example of a transition region in which low latitude processes can be seen extending to midlatitudes.
SA24A-06
New observations of upper mesospheric OH Prompt emission from two satellites: Direct measurements of water vapor
Satellite measurements of mesospheric water vapor provide important constraints to global-scale dynamical models of the Earth's upper atmosphere. One measurement approach is to observe hydroxyl (OH) prompt emission between 300-330 nm, which can be produced directly from the photolysis of water vapor by solar Lyman-α. Identification of this relatively weak non-thermal emission requires a rigorous understanding of the Earth's airglow between 300-330 nm, including the spectrally complex Rayleigh scattered background and OH(0,0) solar resonance fluorescence. Here we present the first satellite observations of mesospheric OH prompt emission from two limb sounding experiments: the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) and the Optical Spectrograph and Infra-Red Imaging System (OSIRIS). MAHRSI flew twice on a satellite deployed and retrieved by the space shuttle in 1994 and 1997, measuring OH(0,0) solar resonance fluorescence near 309 nm at a spectral resolution of 0.02 nm. OSIRIS was launched on the Odin satellite in 2001 and the airglow measurements between 280-810 nm include both OH(0,0) and (1,1) solar fluorescence at a lower spectral resolution of 1 nm. We will compare the two sets of OH prompt observations under similar geophysical conditions, retrieve vertical profiles of water vapor and compare them to other available water vapor measurements. Emphasis will be placed on the potential of this technique compared with other more traditional approaches to measuring mesospheric water vapor.
SA24A-07
Comparison Of The Mesospheric Bore Characteristics Imaged At Equatorial And Mid- Latitudes
Mesospheric bores are remarkable gravity wave events that are characterized by a sharp leading front usually followed by a coherent train of waves that grow in number with time. To date, there have been only a few reports of such events in the literature. However, recent analysis of mesospheric airglow image data from at São João do Cariri, Brazil, and from Bear Lake Observatory, Utah, USA have revealed a relatively large number of bore-like (typically 25-50 events per year). This paper focuses on a detailed comparison of these two data sets which were obtained from two well-separated sites at equatorial and mid-latitudes under differing mesospheric conditions. At São João do Cariri (7°S, 36°W) all-sky CCD image measurements of gravity waves in the near infrared OH Meinel, O2 (0,1) bands, and the OI (557.7 nm) line nightglow emissions were made over a two- year period from September 2000 to September 2002. At Bear Lake Observatory (41.6°N, 111.6°W) all-sky measurements of mesospheric gravity waves in the same three nightglow emissions were obtained over a 2-year period from January 2002 to December 2003. These two data sets have been investigated to determine the occurrence and typical properties of undular mesospheric bore events as observed at equatorial and mid-latitudes. In this paper we will describe their main characteristics and contrast their differing properties.
SA24A-08
Joint Radar Backscatter Observations from Pohnpei and Christmas Island
The space-time characteristics of radar backscatter associated with the development of plasma bubbles in the nighttime equatorial ionosphere were investigated using two approaches. One was to operate two radars, separated by 45 degrees in longitude and 3 hours in local time, simultaneously and continuously, in order to assess whether the spatial correlation of conditions favorable for bubble development remains high over such a distance, and whether such conditions can persist for that length of time. The other approach was to operate the radars using multiple beams, in order to separate spatial from temporal effects, such as produced by large-scale wave structure during the post-sunset rise of the F layer. Multiple beams were also used to determine the temporal evolution of backscatter plumes (bubbles) as they drifted eastward, successively through the radar beams. These results are interpreted in terms of possible processes that lead to bubble development, and its day-to-day variability.