D. C. Thompson

Global Assimilation of Ionospheric Measurements (GAIM)

Abstract : Our primary goal is to construct a real-time data assimilation model for the ionosphere-plasmasphere system that will provide reliable specifications and forecasts. A secondary goal is to validate the model for a wide range of geophysical conditions, including different solar cycle, seasonal, storm, and substorm conditions.

Ionospheric Weather Forecasting on the Horizon

In an effort to mitigate the adverse effects of the ionosphere on military and civilian operations, specification and forecast models are being developed that employ state-of-the-art data assimilation techniques. Utah State University has recently developed two data assimilation models for the ionosphere as part of the USU Global Assimilation of Ionospheric Measurements (USU GAIM) program. One of these models is currently being implemented at the Air Force Weather Agency for operational use. The USU-GAIM models are also being used for scientific studies, and this should lead to a dramatic advance in our understanding of ionospheric physics similar to what occurred in meteorology and oceanography after the introduction of data assimilation models in those fields.

Invited Article: Data analysis of the Floating Potential Measurement Unit aboard the International Space Station

We present data from the Floating Potential Measurement Unit (FPMU) that is deployed on the starboard truss of the International Space Station. The FPMU is a suite of instruments capable of redundant measurements of various plasma parameters. The instrument suite consists of a floating potential probe, a wide-sweeping spherical Langmuir probe, a narrow-sweeping cylindrical Langmuir probe, and a plasma impedance probe. This paper gives a brief overview of the instrumentation and the received data quality, and then presents the algorithm used to reduce I-V curves to plasma parameters. Several hours of data are presented from August 5, 2006 and March 3, 2007. The FPMU derived plasma density and temperatures are compared with the International Reference Ionosphere (IRI) and Utah State University-Global Assimilation of Ionospheric Measurement (USU-GAIM) models. Our results show that the derived in situ density matches the USU-GAIM model better than the IRI, and the derived in situ temperatures are comparable to the average temperatures given by the IRI.

Correlator measurements of megahertz wave‐particle interactions during electron beam operations on STS

We report on the analysis of megahertz modulation of electrons as measured by the Shuttle Potential and Return Electron Experiment (SPREE) during dc firing of the shuttle electrodynamic tether system (SETS) fast pulsed electron generator (FPEG). The SPREE and FPEG were flown aboard the space shuttle Atlantis flight STS 46 as part of the Tethered Satellite System (TSS 1) mission. The principal data reported here are from the SPREE multiangular electrostatic analyzers (ESAs) and Space Particle Correlator Experiment (SPACE). The ESAs, mounted on rotary tables, measured electrons and ions in the energy range from 10 eV to 10 keV over a solid angle of 2π sr. The SPACE is a signal processing system that analyzes the pulse stream from the SPREE ESAs to identify bunching of the electrons and ions produced by coherent wave-particle interactions (WPIs). The SPACE detects modulations in the electron fluxes in frequency range 0- to 10-MHz. This paper concerns 2- to 4-MHz modulations of the electron flux detected by the SPACE when the FPEG was firing in a dc mode at pitch angles close to 90°. During such operations, FPEG emitted a current of 100 mA at an energy of 1 keV. For these times, electrons with energies from 10 to 1850 eV were measured by the SPREE. For energies between ∼10 and 100 eV the electron flux is basically isotropic. At higher energies the flux increases for pitch angles near 90°. The electron distribution functions generally decrease monotonically with increasing energy up to 100 eV. At energies >100 eV the distributions either monotonically decrease or exhibit a peak or plateau at energies near the beam emission energy. Megahertz modulations were observed for electrons with energies from 10 to 1180 eV, on both positive and negative slopes in the distribution function and throughout the 2π sr sampled by the ESAs. The occurrence and strength of the modulations exhibit no clear dependence on the pitch angle at which the electrons are measured. However, they appear to be limited to low parallel velocities (<3×10 6 m s −1 ) where beam-generated waves are in resonance with suprathermal electrons.

Charging of the International Space Station as Observed by the Floating Potential Measurement Unit: Initial Results

The floating potential measurement unit (FPMU) is a multiprobe package designed to measure the floating potential of the International Space Station (ISS) as well as the density and temperature of the local ionospheric plasma environment. The purpose of the FPMU is to provide direct measurements of ISS spacecraft charging as continuing construction leads to dramatic changes in ISS size and configuration. FPMU data are used for refinement and validation of the ISS spacecraft charging models used to evaluate the severity and frequency of occurrence of ISS charging hazards. The FPMU data and the models are also used to evaluate the effectiveness of proposed hazard controls. The FPMU consists of four probes: a floating potential probe, two Langmuir probes, and a plasma impedance probe. These probes measure the floating potential of the ISS, plasma density, and electron temperature. Redundant measurements using different probes support data validation by interprobe comparisons. The FPMU was installed by ISS crew members during an extra-vehicular activity on the starboard (S1) truss of the ISS in early August 2006 when the ISS configuration included only one 160-V U.S. photovoltaic (PV) array module. The first data campaign began a few hours after installation and continued for over five days. Additional data campaigns were completed in 2007 after a second 160-V U.S. PV array module was added to the ISS. This paper discusses the general operational characteristics of the FPMU as integrated on ISS, the functional performance of each probe, the charging behavior of the ISS before and after the addition of a second 160-V U.S. PV array module, and initial results from model comparisons.

USU Global Ionospheric Data Assimilation Models

The USU global ionospheric data assimilation model is part of the Global Assimilation of Ionospheric Measurements (GAIM). This model uses a physics-based ionosphere-plasmasphere-polar wind model and a Kalman filter as a basis for assimilating a diverse set of real-time (or near real-time) measurements. Some of the data that are assimilated include in situ electron density measurements from the DMSP satellites, bottomside electron density profiles from the Air Force network of digisondes, GPS-TEC data from a network of more than 900 stations, and occultation data. GAIM provides specifications and forecasts on a spatial grid that can be global, regional, or local. The primary GAIM output is in the form of 3-dimensional electron density distributions from 90 km to the geosynchronous altitude. GAIM also provides auxiliary parameters (NmF2, hmF2, NmE, hmE, slant and vertical TEC) and global distributions of the self-consistent ionospheric drivers (neutral winds and densities, electric fields, and particle precipitation). In its specification mode, GAIM provides quantitative estimates for the accuracy of the reconstructed ionospheric densities. In addition to the physics-based, Kalman filter model, we have also developed a Gauss-Markov Kalman filter model. The status of the models and the relevant applications are discussed.

Data Assimilation Models: A ‘New’ Tool for Ionospheric Science and Applications

The Earth’s space environment is a complex and dynamic system that exhibits weather features at all times. As shown by meteorologists and oceanographers, a powerful way of modeling dynamic systems is with the use of data assimilation models. Recently, two data assimilation models for the ionosphere have been developed at Utah State University that provide global and regional specifications of the 3-dimensional (3-D) ionospheric plasma densities. The two models are based on approximations to the full Kalman filter in order to reduce the enormous computational requirements associated with it. The first model uses a physics-based ionosphere model to provide the background plasma density field but uses a simpler statistical Gauss-Markov process to replace the physical model in the Kalman filter. The second model is an ensemble Kalman filter model, which uses a physics-based model for the ionosphere-plasmasphere system. The latter model covers the ionosphere-plasmasphere system from 90 to 30,000 km altitude and includes 6 ion species. An important strength of this model is that in addition to the 3-D plasma density distribution it also self-consistently determines the corresponding ionospheric drivers, including the thermospheric neutral winds and the low-latitude electric fields. Both models can assimilate a variety of space- and ground-based data types. Some of the data that can be assimilated include total electron content (TEC) from hundreds of ground-based GPS receivers, in situ electron densities (N e) and ultraviolet (UV) emissions from several DMSP satellites, bottomside N e profiles from tens of ionosondes, and limb TEC data from occultation satellites. The applied data assimilation techniques, although here used to specify ionospheric parameters, should also be beneficial to the study of other regions of the space environment.

Observations of electron beam propagation perpendicular to the Earth's magnetic field during the TSS 1 mission

We report on measurements by the Shuttle Potential and Return Electron Experiment (SPREE), acquired during a period of the Electrodynamic Tethered Satellite mission when the fast pulsed electron generator (FPEG) injected a 1-keV electron beam nearly perpendicular to the Earths magnetic field. Using multiangular electrostatic analyzers mounted on rotary tables, SPREE was capable of determining the flux of electrons and ions in the energy range from 10 eV to 10 keV and over a solid angle of 2π sr. SPREE was located in the shuttle bay where it could observe beam electrons after they had completed ∼1 gyrocycle when fired nearly perpendicular to the local magnetic field. For the case presented here, the beams intensity decreased from ∼100 mA cm−2 at FPEGs aperture to ∼0.18 nA cm−2 at the location of SPREE. The spectrum of the return electrons displays a sharp peak at the beam energy with an intensity at the peak of approximately 2×1010 electrons cm−2 s−1 sr−1. The distribution of the electrons around the peak has a half width of several hundred eV, with observed energies as high as 1850 eV. For energies between 10 and a few hundred eV, intense fluxes of electrons are seen at all look angles. For angles where the beam is observed the spectrum in this energy range has a power law shape. At angles away from the direction of beam return, the spectrum in this energy range can display a more thermal shape with a peak at energies up to 50 eV. In general, the flux intensity in the lower-energy portion of the spectrum is isotropic with an average integral flux of 0.5 to 2×1012 electrons cm−2 s−1 sr−1. Integrating over energy and pitch angle gives number densities of ∼ 5×104 electrons cm−3. The return current density of 0.5 to 2µA cm−2 s−1 sr−1 carried by this isotropic component is sufficient to balance that emitted by FPEG and keep the shuttle at a low potential. We find that both scattering and spreading of the beam near FPEG are necessary for primary electrons to reach the locations of the SPREE detectors.

Ionosphere Data Assimilation: Problems Associated with Missing Physics

Physics-based data assimilation models can be used for a wide range of applications in space physics, but as with all models the data assimilation models have limitations. The limitations can be associated with the data, the assimilation technique, or the background physics-based model. Here, we focused on the ionosphere and on elucidating the problems associated with missing physics in the background ionosphere model. The study was conducted with the Global Assimilation of Ionospheric Measurements-Gauss-Markov (GAIM-GM) physics-based data assimilation model. Simulations relevant to the low and middle latitude ionosphere were conducted in order to show how missing physics in the background ionosphere model affects the reconstructions. The low-latitude simulation involved the presence of equatorial plasma bubbles and a background physics-based ionosphere model that does not self-consistently describe bubbles. This problem, coupled with insufficient data, led to a Gauss-Markov reconstruction that contained a broad region of relatively low nighttime Total Electron Density (TEC) values instead of the individual plasma bubbles. The implications of plasma bubbles for reconstructions with the GAIM-FP (Full Physics) data assimilation model, where the electric fields and neutral winds are determined self-consistently, are noted. The mid-latitude simulation involved a Gauss-Markov ionosphere reconstruction for a geomagnetic storm where a Storm Enhanced Density (SED) appeared across the United States. Again, the background physics-based model (Ionosphere Forecast Model) could not produce the SED feature because this model does not take account of penetrating electric fields. Nevertheless, in this case there were sufficient data to overcome the deficiency in the background ionosphere model and the Gauss-Markov data assimilation reconstruction successfully described the SED feature and surrounding ionosphere.

TSS‐1R vertical electric fields: Long baseline measurements using an electrodynamic tether as a double probe

This paper presents measurements of vertical electric fields obtained using the Tethered Satellite System (TSS) as an electrodynamic double probe. During five hours of TSS-1R deployment from the space shuttle Columbia in February 1996, observations of induced tether potential were gathered over a baseline from 2 to 20 km in the mid and low latitude F-region ionosphere. The corresponding electric fields derived from the potential measurements are found to be consistent with previous satellite and ground-based measurements for similar ionospheric conditions. The data are unique and demonstrate a new capability for measuring the vertical component of the ionospheric electric field. Comparison of the measured electric field at different tether deployment lengths for corresponding local times reveal that the vertical component of the ambient electric field exists on scales of at least 20 km.