Optical imaging of the mesosphere and ionosphere

1. Optical imaging of the mesosphere and ionosphere Jonathan J. Makela (University of Illinois)

2. Overview • Imaging as a remote sensing tool • Estimating GW parameters in the mesosphere • Observing structure and inferring pertinent parameters in the thermosphere/ionosphere • Airglow emissions of interest • Parameter estimation techniques • Deployment considerations Lightning-Ionosphere Coupling Workshop, LANL

3. Why Imaging? • Many methods exist to probe the upper atmosphere. Imaging provides several advantages over other techniques: • Large coverage area from a single site (650  650 km in mesosphere; 1750  1750 in thermosphere) • High spatial resolution (< km in mesosphere; ~km in thermosphere) • Good temporal resolution (~90 s  # filters used) Lightning-Ionosphere Coupling Workshop, LANL

4. Lightning-Ionosphere Coupling Workshop, LANL

5. Why Not Imaging? • As with any observing method, there are also disadvantages, including: • Passive technique (rely on what Mother Nature gives us) • Measuring (height) integrated quantities • Difficulty in obtaining absolute quantities • Requirement of dark/clear skies There are many applications where the pros outweigh the cons and imaging is an appropriate technique to probe the upper atmosphere Lightning-Ionosphere Coupling Workshop, LANL

6. Airglow • Chemilluminescent processes • Chemistry determines altitude a given emission occurs at • Perturbations to the medium (AGWs, TIDs, etc) can modify the chemistry and are therefore observed as changes in emission intensity • Visible from the ground with sensitive CCD cameras Lightning-Ionosphere Coupling Workshop, LANL

7. Prominent Airglows in Mesosphere • OH (Hydroxyl) • Peak altitude ~88 km • Broad band emission (770 – 2000 nm) • Bright! • O2 (Molecular Oxygen) • Peak altitude ~94 km • Narrow band emission (860 – 870 nm) • Na (Sodium) • Peak altitude ~94 Km • Metal caused by meteor ablation • Used for resonance lidar • OI (Atomic oxygen: Green Line) • Peak altitude ~98 km • Atomic line emission (557.7 nm) • Weakest of the three but most visible to human eye Lightning-Ionosphere Coupling Workshop, LANL

8. Atmospheric Gravity Waves • Transverse buoyancy waves • Transport energy across different regions of the atmosphere (one of the largest sources through mesospshere) • Perturbations modify mesospheric airglow emission intensities and can thus be imaged • Pertinent parameters to know include: • Wave number (kh and kz) • Intrinsic wave frequency (i) • Amplitude or perturbation (A or ’/ ) Lightning-Ionosphere Coupling Workshop, LANL

9. Airglow Layer Single-Layer/Single-Site Observation • Provides horizontal wave numbers and “true” frequency • No vertical wavelength • No intrinsic frequency • Need wind or vertical wavelength • No amplitude • Requires vertical wavelength Lightning-Ionosphere Coupling Workshop, LANL

10. Airglow Layers Multi-Layer/Single Site Observation • Vertical wavelength can be estimated by comparing phases in two different layers • Assumes known heights of layers • Problems: • Wave does not always show up in two layers • Potential for 2 ambiguity Lightning-Ionosphere Coupling Workshop, LANL

11. Airglow Layer Single-Layer/Multi-Site Observation • Provides multiple-angle observations of the same perturbation • Obtain z through standard tomography, tomography of Fourier Descriptors, or Parameter Estimation • Ph.D. work of D. Scott Anderson • Examined these techniques and their suitability to retrieving estimates of z Lightning-Ionosphere Coupling Workshop, LANL

12. Parameter Estimation • If the goal of measurements is to infer a few parameters of AGWs, full-blown tomography is unnecessary • Parameter estimation can be performed using multi-site observations and an appropriate forward model without requiring the complexity of tomographic inversion • Significantly reduces computational requirements and improves end results Lightning-Ionosphere Coupling Workshop, LANL

13. Data Model • Giis the result of a Gabor filter (a complex band pass filter) on the mapped pixel intensity data which selects the horizontal wavelength to be modeled where Lightning-Ionosphere Coupling Workshop, LANL

14. Phase Analysis (PE-Phase) • If the layer centroid, zc, is assumed to be known this simplifies to a two-unknown problem • Vertical wave number, kz, obtained from single-site observations of multiple layers • Layer centroid obtainable from multi-site observations of a single layer • Observed frequency, t, is obtained from time sequence of images Lightning-Ionosphere Coupling Workshop, LANL

15. Amplitude Analysis (PE-mag) • Amplitude of wave perturbation, A(x,y), obtained if imaging systems are well calibrated • Layer thickness, , and vertical wave number, kz, obtainable • Requires multi-site observations of a single (or multiple) layer(s) Lightning-Ionosphere Coupling Workshop, LANL

16. OH and OI imager OH and OI imager Na lidar OH and OI imager Example Experimental Campaign Lightning-Ionosphere Coupling Workshop, LANL

17. Campaign Results • Several wave packets observed in the different imagers • Basic parameters obtained from the raw images alone Wave 1 Wave 2 Lightning-Ionosphere Coupling Workshop, LANL

18. Campaign Results • Using the PE-phase technique, parameters are estimated • 2 phase ambiguity leads to two solutions • Calculating winds from the dispersion relation tells us which direction is correct Lightning-Ionosphere Coupling Workshop, LANL

19. Campaign Results • Collocated Na lidar measurements at UAO site confirm downward phase propagation Lightning-Ionosphere Coupling Workshop, LANL

20. Considerations • PE-phase contains a 2 ambiguity • Can be mitigated by using the dispersion relation • Observing additional emission layers would also help on this front • PE-mag (not shown) is heavily dependent on proper absolute calibration of each imager • Difficult to do as unknown atmospheric extinction is non-negligible and non-uniform • Can partially be mitigated by fitting PE-mag results to PE-phase results (for kz) Lightning-Ionosphere Coupling Workshop, LANL

21. Prominent Airglows in Thermosphere/Ionosphere • Dissociative Recombination of O2+ • Peak emission below the F peak • Narrow band emission (630.0 nm) • Chemistry depends on both electron and neutral densities • Long lifetime (~110 s) can cause blurring of features • Radiative Recombination of O+ • Peak emission at the F peak • Narrow band emission (777.4 nm) • Assuming an O+ plasma, intensity is proportional to ne2 • Prompt emission (no blurring) • Very dim emission Lightning-Ionosphere Coupling Workshop, LANL

22. Ionospheric “Topography” • Using the combination of the height-dependent 630.0-nm emission and density-dependent 777.4-nm emission can give estimates of F-layer altitude and density Lightning-Ionosphere Coupling Workshop, LANL

23. Example from 15-16 Sept 1999 Lightning-Ionosphere Coupling Workshop, LANL

24. Example from 15-16 Sept 1999 Lightning-Ionosphere Coupling Workshop, LANL

25. Example from 15-16 Sept 1999 • “Bands” in radar data caused by gradients in electron density; higher densities to the south • Increase in density slightly before local midnight • F layer is observed to decrease in altitude over time Lightning-Ionosphere Coupling Workshop, LANL

26. F-Region Pedersen Conductivity • Important parameter for understanding: • E- and F-region coupling • Instability processes (e.g., Perkins instability at mid-latitudes) • 630.0-nm volume emission rate is similar to the equation for Pedersen conductivity • Both can be shown to have dependence on ne and O2 • 630.0-nm intensity is proportional to PF Lightning-Ionosphere Coupling Workshop, LANL

27. Pedersen Airglow Technique • Technique allows estimation of F-region Pedersen airglow over a large area (10001000 km) • Based on modeling study, RMS difference of 0.271 mhos is expected (0.172 mhos if layer altitude is known) Lightning-Ionosphere Coupling Workshop, LANL

28. Comparison to ISR-derived PF • Technique validated against estimates of PF derived from the Arecibo ISR • Estimates were very good, especially given knowledge of the F-layer altitude Lightning-Ionosphere Coupling Workshop, LANL

29. Example During Mid-Latitude Event • Evolution of structure at mid-latitudes typically understood as Perkins’ instabilities • Depends on variations in conductivities associated with altitude variations of the F layer that align from NWSE Lightning-Ionosphere Coupling Workshop, LANL

30. Possibilities for Technique Improvements • Uncertainties in techniques from: • Reliance on (climatological) background models • Imperfectly known absolute calibration and systematic factors (e.g., flat-fielding) • Unknown atmospheric extinction • Improvements can be gained by either using better models (e.g., assimilative models) or actually integrating images as an assimilated data source • Initial work being performed to integrate into IDA4D Lightning-Ionosphere Coupling Workshop, LANL

31. Deployment Requirements • Dark skies that are typically clear from cloud cover • Availability of • power • facility for housing instrument • Internet connectivity • For PE technique, need multiple sites separated by 100-120 km viewing a common volume Lightning-Ionosphere Coupling Workshop, LANL

32. Summary • Imaging of the mesosphere and thermosphere/ionosphere can lead to estimates of parameters important for understanding coupling processes • Provide observations of spatiotemporal dynamics over a large area • Integrating images into assimilative models may resolve some of the short comings of current parameter estimation techniques Lightning-Ionosphere Coupling Workshop, LANL