Ionospheric Imaging of E-Region Densities

1. Ionospheric Imaging of E-Region Densities Gary S. Bust and Fabiano Rodrigues Atmospheric Space Technology & Research Associates (ASTRA) www.astraspace.net Mike Nicolls SRI International

2. Outline • Introduction • Description of IDA4D • Concept for improved E-region imaging • Data sources, new measurements • Current NSF project to image E-region densities • Description • Initial results • Using bi-static oblique HF to image E-region • Concept • Example low-vhf transionospheric results with Forte • Summary

3. Introduction • Many science topics leverage requirements to obtain improved 3D imaging of ionospheric E-region • Equatorial spread-F • High latitude conductances – AMIE • Global current system • Lightning-ionosphere coupling • Traditional data assimilation sources are not sensitive to E-region • GPS TEC • In-situ Satellite measurements • Beacon TEC • To accurately estimate E-region densities we need: • New data sources • Customized data assimilation algorithms for E-region imaging

4. Ionospheric Data Assimilation Four-Dimensional: IDA4D • 4D global ionospheric electron density imaging algorithm • Based on more than 15 years experience in data analysis, and ionospheric tomographic research and development • Mathematical formulation follows closely the meteorological 3DVAR methodology found in Daley and Barker • Can be considered a maximum likelihood solution, or Gauss Markov Kalman Filter • At each time step, previous best estimate for electron density is used as initial guess and then data are used to improve global solution • Typically 5-15 minute temporal cadence • Non-linear estimator of log density • Solving for the log of the density rather than density guarantees positivity • However to do so requires a non-linear estimation process • IDA4D methodology: • non-linear methodology in Daley and Barker • Combined with a Levenberg-Marquardt type iteration process.

5. IDA4D as Data Assimilative Test Bed • Simple forward model being added • Electron density continuity equation • Use data as much as can – AMIE ExB, ExB from Anderson magnetometers • NSF Reverse Engineering project • Estimate drivers directly from IDA4D density outputs • New Data Sources • Easily tested • HF • 6300 Optical • Others

6. Standard: IDA4D Data Sources • Ground Based • GPS TEC (~1400 sites) • DORIS TEC (~57 sites, 4 LEO satellites) • Digisonde virtual height vs frequency (~40) • Space Based • GPS Occultations • CHAMP, SACC, GRACE, COSMIC (9 satellites) • GPS SST topside TEC • Same 9 satellites • Insitu Electron Denisities • DMSP, CHAMP

7. IDA4D: Data Sources sensitive to Bottom-side F- and E-region • Space-based GPS Occultations • 9 satellites -> ~ 30-40 occultations over New Mexico / day • New Data Sources • Bistatic oblique HF time-delay versus frequency • N transceivers -> N(N-1)/2 link paths • Range over frequency – height information as well as horizontal – lower frequencies sensitive to E region • Use 3D ray-tracer Tracker inside IDA4D non-linear iteration to get densities along path • 6300 ground optical imaging data • Measurements ~ integral Ne*F(neutrals) • Estimate F(neutrals) from ASPEN/TIMEGCM • Perhaps scalar correction • Ingest in IDA4D – imaging of bottom-side altitudes • IDA4D can calibrate counts to Raylieghs

8. IDA4D: New Methods • Model and Data Error Covariance Matrix • Size of estimated variance error on model predictions (TIMEGCM, SAMI2 etc), compared to data error, dictates how strongly solution is weighted by data • Over estimation of data errors reduces influence on solution • Very important in E-region • Vertical error correlations dictate over what altitudes measurements can influence profile • Horizontal error correlations dictate over what horizontal ranges data can influence densities • Mis-specification of any of these parameters can lead to large errors – particularly in E-region • Grid • High resolution 25-50 km horizontal resolution • High resolution ~ 1 km vertical resolution in E-region • Run IDA4D in Global Background Mode (w/o occultations) • Use results as background model input to high resolution regional IDA4D run

9. Lightning E Region Imaging • GPS Occultations • Currently 9 satellites • Occultations extend down to bottom of E-region • Issue is errors due to mis-specification of F-region bleeding into E-region • Deploy 6 HF transceivers around New Mexico • Links optimized for E-region measurements • 30 independent horizontal links • Time-delay versus frequency measurements (~ few micro-second accuracy) • Deploy 6300 optical imager (2??) • Ingest all data into IDA4D along with normal data sources • Use methods being developed in other NSF grant to optimize E-region imaging

10. Synergy NSF: Investigation of Global E-region Conductivities Relevant to the Seeding and Variability of Equatorial Spread F Using Measurements from COSMIC • 3 year project to developed optimized algorithms to accurately estimate E-region densities at off equatorial latitudes • All methods being investigated • IDA4D “assistance” to Abel or Maximum Entropy inversions (M. Nicolls) • Full IDA4D estimation of E-region • First year complete • Initial comparison with daytime Jicamarca E-region densities very promising

11. Direct Comparisons to Jicamarca E-region Ne ASPEN - Solid IDA 4D - + Jicamarca - *

12. F-region Gradients from IDA-4D - Run global IDA runs, incorporating datasets from 4/5/2007: no occultations - Evaluate IDA TEC along occultation geometry above 150 km - Subtract from measurement to estimate E-region TEC - apply inverse transform - Including F region gradients produces more reasonable low altitude profiles (go to 0 at low alts) - Quite good agreement with Jicamarca - Substantially different from direct Abel results

13. Using Bi-static HF Links to Imge E-Region • Bistatic HF links over ~ 300 – 1000 km or so • Range over frequency (~1-2 MHz – 15 MHz) • Data is time-delay versus frequency along oblique paths • Thus for a given link, we get a range of measurements that probe different altitudes between the E- and F-region peak • The propagation path through the ionosphere can be modeled by a full 3D ray-trace algorithm that uses the Appleton-Hartree (AH) equation for the complex refraction. • AH depends on the 3D electron density and magnetic field. • Therefore, HF data can be used with 3D ray-trace model to non-linearly iteratively adjust electron densities along the path (and adjust the path!!!!) until the model matches the data • We have implemented the full 3D ray-tracer “tracker” in IDA4D along with a non-linear estimation method to be able to use HF oblique data to improve estimation of densities in the bottom-side F and E region. • Improved bottom-side imaging due to inclusion of HF links can provide accurate ionospheric specification to HF applications

14. Possible Configuration of 6 HF Transceivers in New Mexico

15. Second Possible Configuration

16. IDA4D Results using Broadband Low VHF Forte Data • As an example of how Oblique HF would be ingested into IDA4D – use Actual low VHF data from Forte Satellite • 25-90 MHz • 800 Km altitude near-polar orbit • LANL Pulsar provides broadband transmission • April 10, 2001 (storm) 18 UT • 6 separate sets of data at 6 different Forte satellite positions (28 – 42 degrees latitude) • Each set of data ~ 20 frequencies versus (relative) time delays • Ingest in IDA4D use Tracker, and non-linear iteration method

17. IDA4D Forte: Post-IDA4D fit to Forte Data Fit to time delay versus frequency Blue is ASPEN model Red IDA4D post-fit to data 38.2 Satellite Latitude 28.4 Satellite Latitude

18. IDA4D Forte: Model versus IDA4D April 10, 2001 (storm day) 18 UT 6 Forte paths over New Mexico All other data available also ASPEN IDA4D

19. HF/Forte • Example using actual low VHF broadband data demonstrates cabability of IDA4D to fit the data and to adjust densities, providing an improved estimation of density • Same thing can be done for range of frequencies for ground-based HF links • Multiple HF links will provide crossing paths in the bottom-side, and therefore provides the cabability of actually imaging the 3D bottom-side F and E region ionosphere • Example configuration above provides 15 independent links.

20. AM/HF Absorption Measurements • Low cost MF/HF receivers designed to measure changes in received signal strength • Use transmitters of opportunity – frequencies that monitor E/D region of ionosphere • Monitor changes in signal strength versus time • Sensitive to solar flares etc • Sensitive to lightning strikes??? • Deploy on same links as HF transceivers • Get electron density from HF/ IDA4D • Use absorption to then estimate electron collision frequencies • Temperature changes • Neutral composition changes

21. Low-cost monitoring of the lower ionosphere Uses a commercial shortwave radio receiver (NRD-535)‏ PC-controlled / unattended operation Initial observations made to detect particle precipitation in the SAMA region Used AM broadcast stations in Brazil Effects of solar flares detected Rodrigues et al., 2004; Contreira et al., 2005

22. Low-cost monitoring of the lower ionosphere Examples of observations: Daily variation of signal strength (daytime absorption)‏ Rodrigues et al., 2004

23. Low-cost monitoring of the lower ionosphere Examples of observations: Effects of solar flares Contreira et al., 2005

24. Summary • Imaging E-region and bottom-side F-region is difficult since easily available data sets consist of integrated TEC which is not very sensitive to E-region • Data sources that are sensitive to bottomside F and E-region include • GPS Occultations • Ground based bi-static HF links • Ground based optical data • ASTRA and SRI have a joint NSF project to improve imaging of E-region densities at low latitudes that is synergistic with this project • IDA4D can ingest all the above data sets – particularly ground based HF data that can be used to provide improved 3D imaging of regional E-region densities