SA31B-01 INVITED 08:00h
On Modeling the Coupled Thermosphere/Ionosphere at low Latitudes
The low latitude thermosphere/ionosphere is a particularly challenging region for theoretical modeling and understanding since the charged and neutral components can be strongly coupled. Models must therefore be able to simulate realistically both components as well as their mutual interaction. And they must do this over a large altitude range, since waves propagating from the mesosphere and lower thermosphere can exert tremendous influence on the dynamics, composition, and structure of the upper atmosphere. Tidal, gravity wave, and planetary wave influences on the ionosphere are becoming well known, although the processes are not always well understood. Future progress in theoretical understanding of the low latitude region would be greatly facilitated by simultaneous measurements of neutral and ion densities and velocities at both low and high altitudes.
SA31B-02 08:15h
Assimilated Low Latitude Ionosphere Variability During the First CAWSES Space Wether Campaign
The Utah State University (SUS) Global Assimilation of Ionospheric Measurements (GAIM) has been used to study the first CAWSES Space Weather Campaign period (March 25 through April 5). This period is noteworthy in two distinct ways. First, there is an extended three-day very quiet geomagnetic period, which is followed almost immediately by a marked change in the ionosphere associated with severe geomagnetic activity. GAIM, for this study, assimilated over 500 groundbased GPS TEC and 18 digisonde measurements. The background ionosphere was driven by climatology models that responded to the geomagnetic K$_{p}$ and solar F$_{10.7}$ indices. The GAIM assimilation results show that the ionosphere at low latitudes is highly responsive to (a) quiet time variability from "drivers" that are relatively unknown, and (b) during the storm from drivers assumed to be associated with high latitude auroral inputs, but again relatively unspecific. However, the strength of the assimilation approach is that the observed electron density and total electron density, when assimilated, redistributes the background model ionosphere to reveal both the quiet-time and disturbed ionospheric variability. These GAIM results will be presented with comparison to the purely climatology background.
SA31B-03 INVITED 08:30h
The climatology of low-latitude ionospheric densities and zonal drifts from IMAGE-FUV.
The IMAGE satellite was the first dedicated to magnetospheric imaging, but has also provided numerous images of the nightside ionosphere with its Far-Ultraviolet (FUV) spectrographic imager. Nightside emissions of O I at 135.6-nm originating away from the aurora are due to recombination of ionospheric O$^{+}$, and vary in intensity with (O$^{+}$)$^{2}$. IMAGE-FUV, operating in a highly elliptical orbit with apogee at middle latitudes and $>$7 Re altitude, measures this emission globally with 100-km resolution. During each 14.5 hour orbit, IMAGE-FUV is able to monitor nightside ionospheric densities for up to 6-7 hours. Hundreds of low-latitude ionospheric bubbles, their development and drift speed, and a variety of other dynamical variations in brightness and morphology of the equatorial anomalies have been observed during this mission. Furthermore, the average global distribution of low-latitude ionospheric plasma densities can be determined in 3 days. Imaging data collected from February through June of 2002 are used to compile a dataset containing a variety of parameters (e.g., latitude and brightness of peak plasma density, zonal bubble drift speed) which can be drawn from for climatological studies. Recent results indicate that the average ground speed of low-latitude zonal plasma drifts vary with longitude by up to 50%, and that a periodic variation in ionospheric densities with longitude suggests the influence of a lower-thermospheric non-migrating tide with wave number = 4 on ionospheric densities. An excellent correlation between zonal drift speed and the magnetic storm index Dst is also found.
SA31B-04 08:45h
A climatology of the nighttime ionosphere acquired by the TIMED/GUVI instrument and the changes associated with storm time
We present here a global climatology of the quiet time F region derived from measurements of the TIMED/GUVI instrument. The climatology has been assembled from a large number of electron density profiles inferred from limb measurements of 135.6 nm radiance. Our technique yields maps every 60 days of electron density spanning the entire night side between 45 degrees latitude at all longitudes. This allows us to study seasonal variations of the quiet time ionosphere; in particular, we have constructed maps for the periods within thirty days of the equinoxes and solstices. Signatures of the quiet time winds and electric fields are apparent in the maps, along with a very clear longitudinal dependence due to the variation in the geographic position of the magnetic equator. Along with the maps, we present comparisons with other measurements and climatologies, including IRI, the global ionosonde network and Total Electron Content (TEC) measurements. We show that these climatologies are useful points of departure for studies of the response of the low-latitude ionosphere to geomagnetic storms.
SA31B-05 INVITED 09:00h
The Response of the Low and Middle Latitude Topside Ionosphere to the Superstorm Events of October and November 2003
Superstorms are identified by a large negative excursion in the Dst index, but are also characterized by periods in which the interplanetary magnetic field is also large and negative and the AE index is significantly enhanced. A large increase in high latitude ionospheric electric field is accompanied by an expansion of the latitude range of influence and a penetration of the field beyond the auroral zone. In addition, enhanced Joule and particle heating in the auroral zones create vertical and equatorward winds that affect the ionization distribution. During the superstorms of October and November 2003 the DMSP and ROCSAT-1 satellites observe the changing ion concentration in the topside ionosphere and the associated electrodynamic drift of the ionization. By examining the evolution of these parameters at fixed longitudes and different local times at middle and low latitudes it is possible to describe the effects of these different drivers originating at high latitudes. Here we describe the effects of enhanced upward ExB drifts and equatorward neutral winds in redistributing the ionization in the topside at low and middle latitudes.
SA31B-06 INVITED 09:15h
Low-to-Middle Latitude Plasma Transport During Geomagnetic Storms Observed From Ground and Space-Borne GPS Receivers
Electric fields imposed on the ionosphere during geomagnetic storms have a significant impact on plasma transport. We use total electron content measurements from ground and space-based GPS receivers to infer how low latitude plasma is transported during periods of intense geomagnetic disturbance. Recent studies have identified vertical plasma transport due to zonal electric fields at the magnetic equator as an important contributor to dramatic changes in low and mid-latitude ionospheric structure. We will present results from a few events (October 2003, November 2003 and April 2002) and study the time series of TEC measurements obtained from ground-based GPS receiver networks covering a wide range of local times to infer plasma transport characteristics, focusing on how daytime plasma at low latitudes becomes transported to middle latitudes. We will use observations of TEC from space-borne GPS receivers to study vertical distribution of the plasma associated with TEC increases and decreases observed from the ground. The correlation between TEC and upstream solar-wind conditions is part of the analysis.
SA31B-07 09:30h
Seasonal Dependence of the Low-Latitude Ionosphere Response to Magnetic Storms
We use two chains of GPS receivers in the Asian (120\deg E) and American (70\deg W) sectors to simultaneously observe the variation of the low latitude ionosphere during the disturbance periods. 10 storm events with Dst drops to less than -200 nT during 2000 $\sim$ 2003 have been monitored by the two GPS receiver chains. Large expansion of the equatorial ionization anomaly (EIA) due to the penetrating electric field is seen during the initial phase in every storm event followed by the suppression of the EIA after the storm main phase. In some events, enhanced electron densities in the EIA peaks are seen during the recovery phase. Observations also show seasonal asymmetry of the EIA densities and locations during both expanding and suppression of the EIA. We compare the GPS TEC with simulation results from the Sheffield University Plasmasphere Ionosphere Model (SUPIM) to explain the change of the low-latitude plasma transport due to the storm time electric field, neutral wind, as well as effects of storm-time changes in neutral composition.
SA31B-08 09:45h
Modeling the Thermosphere/Ionosphere During Storms
Important progress has been made recently in developing an understanding of the effects of geomagnetic storms in the thermosphere and ionosphere. The theory developed based on observations and numerical model simulations can explain most of the apparent coherence of local-time and seasonal dependencies, and the apparent randomness of the longitudinal response of the global ionosphere. A true test of the models and the theory is their ability to predict the large scale distribution of storm effects for specific storms. In this paper CTIPe simulation results for specific storm periods are presented. We compare model results with TIMED GUVI measurements. The circumstances leading to the penetration of composition features to very low latitudes and their longitude structure will be explored.