SA42A-01
Electric fields in the equatorial ionosphere inferred from magnetic measurements of CHAMP, Oersted and SAC-C
The Equatorial Electrojet (EEJ) and its return currents create a strong signature in magnetic measurements of
low-orbiting satellites. Seven years of data from CHAMP, Oersted and SAC-C with almost 100,000 equator
crossings allow for a detailed characterization of the spatial and temporal structure of the EEJ. In addition to
scalar measurements used in previous studies, we have now also inverted vector magnetic field measurements
from CHAMP and Oersted. In contrast to scalar data, which only provide the magnitude of the EEJ current system,
the vector data can be inverted for absolute current estimates along meridional profiles. These profiles confirm
the presence of strong westward return currents on both sides of the eastward EEJ. In the climatological average,
the peak eastward current strength correlates well with radar measurements of upward drift. As expected, the
early morning hours are dominated by a reversed, westward electric field. A climatological model of the EEJ
strength and variability, parametrized by longitude, local time, season and solar flux is now available online.
http:geomag.colorado.edu/EEJmodel.html
SA42A-02
TIMED/SABER Observations of 4.3 um Emission during Solar-Geomagnetic Storms: Analysis of Ionospheric E-region Chemistry, Kinetics, and Radiation Transfer
Nighttime thermospheric infrared emission at 4.3 um was enhanced by several orders of magnitude during recent solar-geomagnetic storms, as observed by the TIMED/SABER instrument. Auroral electron dosing followed by ion-neutral chemical reactions leads to vibrationally excited NO+ and emission at 4.3 um in the ionospheric E- region. Consequently, nighttime measurements from the SABER 4.3 um radiometer channel provide an excellent dataset to: (1) monitor the global E-region response to solar-geomagnetic disturbances, and (2) conduct a detailed study of E-region electron dosing, ion-neutral chemistry, and energy transfer processes. Specifically, we derive NO+ 4.3 um volume emission rates (VER) from SABER 4.3um limb emission measurements during the April 2002 and October-November 2003 solar storms. The SABER-derived NO+(v) VERs are an observation- based proxy to study storm-induced E-region electron density enhancements and assess current understanding of E-region chemistry and kinetics. NO+(v) VER is derived by removing the contribution of CO2(nu3) from the SABER 4.3 um channel, followed by a standard Abel inversion on the residual radiance. The CO2(nu3) contribution to the SABER 4.3 um channel is modeled using temperature, pressure and CO2 abundance Level 2 data products retrieved from SABER, non-LTE CO2 and infrared radiation transfer models. We have shown in previous studies that the CO2(nu3) contribution can be adequately modeled and removed during magnetically disturbed conditions, which leads to the following objectives of this study. Thus, the first objective of this paper is to study the global morphology of the SABER-derived NO+(v) during the April 2002 and Halloween 2003 solar- geomagnetic storms. The second objective is to asses current understanding of E-region chemistry and kinetics by modeling the SABER-derived NO+(v) during the magnetically disturbed periods using the field-line interhemispheric plasma (FLIP) model, dynamically driven by NOAA/POES measurements of auroral electron energy characteristics, and an NO+(v) chemical-kinetics model.
SA42A-03
DAY-TO-DAY VARIABILITY OF THE IONOSPHERE: MODEL-DATA COMPARISON EXPERIMENTS USING TIME-GCM
We report on a new series of TIME-GCM model runs for the year 2002 that use updated aeronomic parameters and processes that lead to a better portrayal of long-term variability patters (seasonal and annual). Within this context, we then explore the sources of day-to-day ionospheric variability. Separate model runs employ coupling from above (daily solar and geomagnetic forcings) in conjunction with (and without) day-to-day coupling from below via the NCEP formulation. We compare model output for the F2-layer with data from several ionosonde stations to assess how simulations capture true daily variability at locations ranging from sub-auroral to lower mid-latitudes. Of particular interest are the days each month when geomagnetic activity is lowest, and thus when coupling from below might be most visible.
SA42A-04
Retrieval of dynamical ionospheric parameters through high-latitude and geosynchronous FUV imaging.
Emissions of atomic oxygen are a major feature of Earth's Far-Ultraviolet signature. Recombination of ionospheric O+ is the dominant source of FUV light on the nightside away from the aurora, revealing details pertaining to ionospheric morphology and dynamics. The light originating from the bands of the equatorial ionospheric anomaly can reveal the rapid changes that occur just after sunset, such as the steep reductions of plasma density occuring with the development of ionospheric spread-F. Initial studies have used the subsequent drift of these plasma bubble signatures observed by NASA-IMAGE as a method for retrieving the zonal velocity of the ambient ionospheric plasma. The results of these studies are revisited using a newly developed technique for retrieving bubble drift velocities, that automatically identifies and tracks bubbles in the nighttime ionosphere. The technique retrieves velocities as a function of latitude, local time and longitude, allowing for multi-parameter studies of low-latitude ionospheric plasma drift. The threshold of brightness required for bubble identification and tracking using these techniques is studied in support of future NASA missions carrying an FUV imager to geostationary orbit.
SA42A-05 INVITED
The Thermospheric Response to Solar Irradiance Variation
The basic structure and variation of density and composition in the thermosphere has been well-characterized since the beginning of the space age, due to the need for quantifying the effect of atmospheric drag on satellites in low-Earth orbit, and the data obtained from measuring the changes in those orbits. Empirical models constructed using satellite drag, mass-spectrometer, radar, occultation, and other techniques, have carried this forward to a highly developed state, but are still dependent on the indices used to drive them, and are known to be less reliable during periods of significant geomagnetic disturbance. In the case of solar ultraviolet irradiance variation on solar-cycle, solar-rotational, and shorter time scales, the effect on thermospheric density and composition is in principle better understood, but the proxy index approach still has limitations, including systematic solar-cycle non-linearities at lower solar activity, and poorer short-term correlations at high solar activity. Recent work has shown that using measured solar irradiances in empirical and theoretical models can improve the validity of these models in comparison with observed density fluctuations. An additional challenge is to bring theoretical models to a degree of fidelity that could make them competitive with empirical models for near-real-time description or even short-term forecasting of thermospheric density, composition, and temperature, including geomagnetic as well as solar irradiance effects. In this presentation, we briefly review the history and state of the field, and demonstrate use of the NCAR Thermosphere-Ionosphere General Circulation Model, using measured solar ultraviolet irradiance as an input, to simulate the thermospheric density distribution. These simulations are compared with empirical models and with density measurements obtained from satellite drag analyses. We show that when seasonal effects are fully accounted for, this theoretical modeling approach can be an improvement over the empirical approach for describing the density of the thermosphere.
SA42A-06 INVITED
Why Fly ITSP and GEC or Don't We Understand the Ionosphere and Thermosphere?
The ionosphere/thermosphere (I/T) community faces some significant challenges in the next few years. Principal among these challenges is that of conveying to the broader space science community the need for additional, focused space-based research missions that address the major problems of I/T physics. What do we say when we hear that 1) the I/T is basically understood, 2) I/T science is about improving the specification of the I/T rather than answering basic questions and 3) the only reason we study the I/T is for its practical applications to communications, navigation and orbit-dynamics? The ability of first principles models to produce a reasonable fit to observations seems to provide prima facie evidence that we do understand the physics, chemistry and dynamics of the I/T. However, we have so few systematic, well calibrated, unambiguous, global measurements of the I/T and there are so many poorly characterized inputs to the models that there is a great range in the ability of the model to be "tuned" to reproduce a particular set of measurements. The ability of the models to reproduce the general behavior should enable us to determine what our "known unknowns" are and provide valuable insight into those processes or quantities that we must measure in order to make further progress in our understanding. Future missions, especially those like GEC or ITSP as well as potential Explorer-class missions, that look at the I/T in a new way, will tell us if there are "unknown unknowns" that await our investigation.. There are still new and exciting questions at all spatial and temporal scales in the ionosphere and thermosphere. The pending missions - Ionosphere Thermosphere Storm Probes (ITSP) and Geospace Electrodynamics Connections (GEC) - are vital to testing our understanding of the physics of the storm-time response of the I/T and the electrodynamic connection of the ionosphere with geospace, respectively. With these missions we seek to characterize the spatial and temporal variability of the I/T and to understand the root cause of that variability on a global scale and in a global context.. Coupled with the rich variety and history of distributed ground-based measurements, we can address these issues that are at the heart of our need to understand the physical processes that are parameterized as sub-gridscale phenomena on the first principles models.
SA42A-07 INVITED
Recent in-situ measurements of thermospheric density: Results and implications for future experiments
Results from recent in-situ measurements of thermospheric density are presented. The measurements were made by an ionization-gauge based pressure sensor hosted on a space vehicle orbiting in the lower thermosphere. Approximately nine months of nearly continuous operation have produced new insights into the structure of the thermosphere and small scale perturbations, including waves and other disturbances. These insights are presented and compared to those gained from earlier work. The new measurements are also used to highlight the need for new experiments and suggest the measurements that new experiments should make