SA22A-01
Longitudinal Variability of the Low Latitude Ionosphere and Scintillation Activity
Longitudinal variability of the post-sunset low latitude ionosphere is provided by Tiny Ionospheric Photometers (TIP) on the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites. The TIP sensor is a compact, nadir directed, narrow-band, ultraviolet photometer operating at the 135.6 nm wavelength. This emission is produced by radiative recombination of O+ ions and electrons. At night, the strength of the emission is proportional to the square of the peak electron density, Nmax. TIP measures the horizontal structure of the ionosphere with 15-30 km resolution and high sensitivity, providing detailed observations of the post-sunset equatorial anomaly even during solar minimum conditions. It has previously been demonstrated by Whalen, that the maximum value of the pre-reversal enhancement in vertical ExB drift is linearly related to the value of Nmax at the crest of the equatorial anomaly at 2000 LT. A linear relationship has also been established between the TIP 135.6 nm radiances at the crest of the equatorial anomaly at 2000 LT and the pre-reversal enhancement in vertical ExB drift velocities at 1900 LT in the Peruvian longitude sector. This relationship is independent of the magnitude of the daytime vertical ExB drift velocities. Based on this relationship and TIP 135.6 nm observations, a longitude variation in the pre-reversal enhancement in ExB drift reveals a 4-cell pattern that is attributed to non-migrating tides which arise from tropospheric weather patterns in the tropics. The longitudinal pattern of the pre-reversal enhancement suggests that certain longitudes are more favored than others for the subsequent development of scintillation activity. The strength of this relationship is investigated using scintillation observations from the SCINDA network of ground-based VHF and UHF receivers. A strong relationship implies a connection between tropospheric weather and the occurrence of scintillation activity.
SA22A-02 INVITED
Equatorial Scintillation Initiated From Low Altitude Forcing
Equatorial scintillation is often associated with equatorial plasma bubbles (ESF). Thus, understanding the forcing mechanisms behind bubble formation is necessary in order to predict the occurrence of most equatorial scintillation. Gravity waves are a strong driver for initiating spread-F. However identifying the source of the gravity waves remains a difficult task. Since the 1950's tropospheric/ionospheric coupling by gravity waves has been presented as a viable coupling mechanism in the literature. Recent GPS, ionosonde, and ISR studies have linked mid- and low latitude scintillation to the passage of large convective tropospheric storms. In this talk, an overview of recent observational and modeling work is presented showing the coupling between the two regions. Next, using GPS occultation data, evidence of equatorial scintillation due to nearby tropical storms is shown. The source of gravity waves within the tropical storms is then described and the type of gravity waves capable of reaching the bottom-side F-region is characterized. Finally, specific storm examples occurring at low magnetic latitudes are shown and their affects on the local ionosphere are discussed.
SA22A-03
Longitudinal Variability of Low-Latitude Total Electron Content: Tidal Influences
Recently, nighttime ultraviolet (UV) observations obtained by IMAGE FUV and TIMED GUVI instruments have revealed a longitudinal wavenumber-four pattern in the nighttime airglow intensity and in the position of the equatorial anomalies during equinox and high solar flux conditions. In the present study, we have extended this work and determined the longitudinal variability of the low-latitude total electron content (TEC) climatology during different geophysical conditions with a special emphasis on the longitudinal wavenumber-four structure in the low-latitude ionosphere. We have used more than 5 million low-latitude TOPEX TEC observations covering the entire 13 years of TOPEX TEC data from August 1992 until October 2005. This data set was used to determine the local time, seasonal, solar cycle, and geomagnetic activity dependence of the longitudinal variability of TEC at equatorial and low latitudes, and in particular, to address the existence and evolution of the wavenumber-four longitudinal pattern under these conditions. Our study shows that the wavenumber-four pattern is created during the daytime hours at equinox and June solstice but is absent, or washed out by other processes, during December solstice. During equinox the wavenumber-four pattern is created around noon with well-defined longitudinal enhancements in the low-latitude TEC. These enhancements, which are symmetric about the geomagnetic equator during this season, last for many hours and can be clearly seen past midnight. The longitudinal patterns are found to be nearly identical between the vernal (March/April) and autumnal (September/October) equinoxes and largely independent of the solar cycle conditions. The wavenumber-four pattern is also observed during geomagnetically active conditions, indicating that the processes that create this pattern are also present during active times. The variations between the well-defined longitudinal maxima and minima are of the order of 20%. During June solstice, the wavenumber-four pattern is also observed in the afternoon hours but, in contrast to the equinox cases, it exhibits a strong hemispheric asymmetry and is not observed during the night. The low-latitude TEC exhibits clear longitudinal variations during December solstice, with large daytime enhancements over the East-Asian and Pacific regions and a third enhancement emerging in the afternoon over the Atlantic ocean, but a clear wavenumber-four pattern is not observed during this season. Although the equatorial and low-latitude TEC values exhibit clear longitudinal patterns during all seasons, a significant amount of scatter remains in the TEC data that is not accounted for by changes in the solar cycle, the season, the local time or by the longitudinal variability. This remaining scatter is largest near the poleward edges of the anomalies and is of the order of 40%
SA22A-04
Longitudinal Variability in the Geomagnetically Quiescent Ionosphere
We report on the effects of nonmigrating tides on the Earth's upper atmosphere based upon calculations made with the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere- mesosphere-electrodynamics general circulation model (TIME-GCM) and global-scale wave model (GSWM). We extend the results reported by Hagan et al. (2007) who demonstrated that longitude variations in IMAGE satellite airglow brightness measurements associated with equatorial ionization anomaly peak densities can be attributed to longitudinally variable nonmigrating zonal wind tides which modulate the E-region dynamo and produce electric field effects that map into the F-region aloft. We explore the local time, altitude and seasonal variability of these nonmigrating tidal components in the TIME-GCM along with viable sources of excitation, including latent heat release associated with raindrop formation in deep convective towers in the tropical troposphere as parameterized in the GSWM. We also assess the reliability of our TIME-GCM predictions via comparison with accessible observations of the quiescent ionosphere. Hagan, M.E., A. Maute, R. G. Roble, A. D. Richmond, T. J. Immel, and S. L. England, (2007), Connections between deep tropical clouds and the Earths ionosphere, Geophys. Res. Lett., in press.
SA22A-05
Electric fields and zonal winds in the equatorial ionosphere inferred from 30,000 individual CHAMP orbits
The Equatorial Electrojet (EEJ) is an eastward current in the ionospheric E-region, flowing along the dip equator on the day-side. This current system produces a strong signal in the magnetic measurements of the CHAMP satellite. The resulting latitudinal current profiles can be used to place constraints on the electric fields and zonal winds in the equatorial ionosphere. More than six years of CHAMP observations are now available, comprising more than 30,000 equator crossings. In an earlier study we had shown that the eastward current density at the magnetic equator is a good proxy for the electric field. To further improve the method, we now use the MSIS and IRI models to compute the conductivity in a meridional cross-section of the ionosphere and solve the governing differential equation directly for the eastward electric field and zonal wind. This leads to more accurate electric field estimates and places useful constraints on the ambient zonal wind.
SA22A-06
Hemispheric asymmetries in the longitudinal structure of the low-latitude nighttime ionosphere
Several recent studies suggest that non-migrating diurnal tides generated by tropical weather in the troposphere influence the longitudinal morphology of the low latitude F region ionosphere; in particular, a wave number four pattern is observed in the peak densities and magnetic latitudes of the equatorial anomaly. It is suspected that this variability is driven by E region processes on the dayside. Observations made with the Low Resolution Airglow and Aurora Spectrograph (LORAAS), which flew aboard ARGOS from May 1999 to April 2002, show additional evidence of periodic variations in the densities and latitudes of the equatorial anomaly crests. In this investigation, electron density profiles are reconstructed from limb scans of OI 135.6 nm emissions to obtain maps of the NmF2 at 0230 LT. Our findings show that the longitudinal variability of the NmF2 of the northern anomaly crest matches that of other global observations, but there is a pronounced hemispheric asymmetry in the longitudinal variations observed in the southern anomaly crest. We show that this asymmetry is tied to longitudinal variations in the neutral winds at F region altitudes, which act to enhance the observed wave number four pattern. We discuss the role of the neutral winds by comparing the LORAAS results to both empirical (IRI-90) and physics-based models (SAMI3).
SA22A-07 INVITED
Imaging Coherent Backscatter Radar Observations in Brazil
An upgrade of the coherent backscatter radar in Sao Luis, Brazil makes possible the construction of interferometric radar images of equatorial ionospheric scattering structures. We will be presenting in-beam radar images of equatorial Spread-F scattering layers. Among other results, we will be presenting interesting observations of periodically structured bottom-type layers preceding full-blown Spread-F. Details about these layers, their possible origin and relationship with bottomside/topside Spread-F will be discussed during this talk.
SA22A-08
Validating the Communications/Navigation Outage Forecasting System (C/NOFS) algorithms using data from the COSMIC campaigns
The Communication/Navigation Outage Forecasting System (C/NOFS) satellite is scheduled for launch in June 2008 into a low inclination (13°), elliptical (~ 400 x 850 km) orbit. Models have been developed at AFRL that will ingest C/NOFS data in order to forecast ionospheric density and the presence of irregularities. The C/NOFS forecast models can be initialized using an assimilative model (e.g., GAIM), or alternatively, using a physics based model driven by the electric fields and winds that will be measured by C/NOFS (i.e., PBMOD). We present a validation study of the C/NOFS PBMOD and JPL/USC GAIM models using ground and satellite instrument data from a recent COSMIC campaign. Total electron content from ground-based GPS receivers and from the COSMIC GPS receivers are included in the GAIM assimilation. We compare the models with electron density from the Jicamarca incoherent scatter radar as well as with peak density from several equatorial ionosondes using metrics to show their strengths and limitations. We find that the assimilation of GPS TEC improves GAIM's ability to predict vertical TEC. The assimilation of COSMIC GPS TEC in particular improves the topside shape. On the other hand, assimilation tends to degrade GAIM's specification of NmF2 relative to PBMOD or to GAIM's climatology in this study. Recent changes to GAIM and the use of higher resolution are expected to enhance GAIM's specification.