Lighting-Ionosphere Coupling Workshop, LANL, 20 Aug 2008 Optical imaging of the mesosphere and...
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Transcript of Lighting-Ionosphere Coupling Workshop, LANL, 20 Aug 2008 Optical imaging of the mesosphere and...
Lighting-Ionosphere Coupling Workshop, LANL, 20 Aug 2008
Optical imaging of the mesosphere and ionosphereJonathan J. Makela (University of Illinois)
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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)
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
• 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
Prominent Airglows in Mesosphere
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
Fenergy =−ρ0ω i
2g2
kz2N2
ρ 'ρ
⎛
⎝⎜⎞
⎠⎟
2
ω z =kh
2 N 2 −ω i2( )
ω i2 − fc
2−
1
4H s2
• Pertinent parameters to know include:– Wave number (kh and kz)– Intrinsic wave frequency (ωi)– Amplitude or perturbation (A or
ρ’/ ρ)
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
Airglow Layer
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
Airglow Layers
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
Single-Layer/Multi-Site Observation
• Provides multiple-angle observations of the same perturbation– Obtain z through standard tomography, tomography of
Fourier Descriptors, or Parameter EstimationAirglow Layer
• Ph.D. work of D. Scott Anderson– Examined these
techniques and their suitability to retrieving estimates of z
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
Data Model
• Gi is 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
Gi x, y( ) =A x,y( )e−σ 2
2wi (x,y)+ωz( )
exp j β x,y( ) + zc −H( )wi x,y( )( )⎡⎣ ⎤⎦
wi x, y( ) =kxx−x0H −z0
+ kyy−y0H −z0
β x,y( ) =kxx+ kyy+ kzz+ω tt+φ x,y( )
x=x'z'
H −z0( ) + x0
y=y'z'
H −z0( ) + y0
where
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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, ωt, is obtained from time sequence of images
∠Gi x, y( ) = kxx + kyy + kzz +ω tt + zc − H( )wi x, y( ) +φ(x, y)
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
Amplitude Analysis (PE-mag)
• Amplitude of wave perturbation, A(x,y), obtained if imaging systems are well calibrated
• Layer thickness, σ, and vertical wave number, kz, obtainable
• Requires multi-site observations of a single (or multiple) layer(s)
Gi x, y( ) =A x,y( )e−σ 2
2wi (x,y)+kz( )
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
Example Experimental Campaign
OH and OI imagerNa lidarOH and OI imagerNa lidar
OH and OI imagerOH and OI imager
OH and OI imagerOH and OI imager
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
Campaign Results• Several wave packets observed in the
different imagers
• Basic parameters obtained from the raw images alone
Wave 1 Wave 2
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
Campaign Results• Collocated Na lidar measurements at UAO
site confirm downward phase propagation
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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)
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
• 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 ne
2
– Prompt emission (no blurring)– Very dim emission
Prominent Airglows in Thermosphere/Ionosphere
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
nm = I 7774 ×3.06 ×105 cm-3⎡⎣ ⎤⎦
Hm =e0.171×ln I7774 / I6300( )+6.43 km[ ]
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
Example from 15-16 Sept 1999
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
Example from 15-16 Sept 1999
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
Pedersen Airglow Technique• Technique allows estimation of F-region Pedersen
airglow over a large area (10001000 km)– Based on modeling study, RMS difference of 0.271 mhos
is expected (0.172 mhos if layer altitude is known)
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
Comparison to ISR-derived P
F
• Technique validated against estimates of PF
derived from the Arecibo ISR• Estimates were very good, especially given
knowledge of the F-layer altitude
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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 NWSE
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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
20 Aug 2008Lightning-Ionosphere Coupling Workshop, LANL
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