solar occultation for ice experiment science overview

SOFIE CDR, October 7 & 8, 2004

SOFIE Science Overview1

GATS

Proprietary

Solar Occultation For Ice Experiment

Science Overview

Larry Gordley&

Mark Hervig



GATS

Proprietary

Why do they form? Why do they vary?

Is there a relationship to global change?

SOFIE will help AIM provide answers

What is the role of•Temperature•H2O•Dynamics•Chemistry?

•NLCs are the highest (83 km) clouds in our atmosphere

•NLCs occur pole-ward of 50° latitude in both hemispheres

•NLCs occur only during summer

NLC are changing:

•Increasing numbers

•Moving equatorward

•Increasing brightness

AIM Is a Mission to Study Noctilucent Clouds (NLCs)



GATS

Proprietary

SOFIE Measurement Overview

SOFIE Channel Summary

channel 1 2 3 4 5 6 7 8

Target O3 particles H2O CO2 particles CH4 CO2 NO

Center (m)

Strong band

Weak band

0.290

0.328

0.862

1.031

2.60

2.45

2.77

2.94

3.06

3.19

3.37

3.51

4.25

4.63

5.32

4.98

Differential absorption measurements to determine:

•Gas abundance: H2O, O3, CH4, NO, CO2

•Particle extinction: 10 wavelengths from 0.29 to 5.3 m

•Temperature

High signal-to-noise: 106 to 109

Precise solar tracking: 1 arcsec knowledge



GATS

Proprietary

Differential Absorption Measurements

Each SOFIE channel uses two detectors to make three measurements:

1. Strong band absorption

2. Weak band absorption

3. Difference signal (weak band – strong band)

Difference signal measurements

• remove atmospheric interference

• Nearly eliminate common-mode noise (e.g. tracking jitter, chopper noise, sun spots, etc..)

Simultaneous NLCs, temperature, and gas measurements

Targets the mesosphere and above,

but easily obtains stratospheric measurements



GATS

Proprietary

Progress since PDR from a Scientific View Point

• Sun Sensor: Verification of 1 arcsec tracking performance

• Simplified instrument operation: A single data collection mode, command tables provide flexibility for different scenarios.

• Analysis of Signal Drifts: Beam steering, filter thermal effects, etc…, signal drifts are low risk

• Verification of Beam Defocus: Suppression of false signals due to signal gradient on non-uniform detectors.

• Spectral Performance: Predictions for all bands exceed requirements, Delivered flight filters (bands 11 – 16) exceed requirements.

• Band 1 & 2 detectors: Changed from UV-enhanced silicon to silicon carbide: relieves extraordinary out of band rejection requirements



GATS

Proprietary

Silicon Carbide Detectors, Bands 1 & 2

Ozone measurements originally used UV-enhanced silicon detectors

The spectral response of UV-Si detectors is less than ideal:

•UV-Si response combined with solar spectrum = amplified out-of-band energy

•Can be addressed with stacked filters, difficult to verify

Our New Approach: Silicon Carbide (SiC) detectors

Ideal spectral response

twice as rad-hard as Si

Eliminates 2nd filter

solar source detector response



GATS

Proprietary

SOFIE Spectral Response

•Drivers:

Filter Response

Preceding Optics

Detector Response

•Spectral response evaluation must consider solar source

•Out-of-band performance is critical

(OOB < 1% required)

•Predictions meet requirements

•Filters 11 - 16 are now built

they meet requirements

•Example: Band 13Band 13



GATS

Proprietary

SOFIE Performance Requirements

SOFIE Measurement Requirements from the AIM SRD (short list)

Geophysical Parameter

Precision

(near 83 km)

Altitude Range (km)

Vertical Resolution

(km)

Horizontal Resolution

Temporal Resolution

Cloud extinction

NIR: 5x10-6 km-1

IR: 5x10-5 km-1

78 – 85 3 at com. vol. 1 min @ com. vol.

temperature 5 K 70 – 90 3 5lat x 24lon 1 min @ com. vol.

H2O mixing ratio

0.6 ppmv 78 – 90 3 5lat x 24lon 1 day

Performance Drivers Precision & altitude range: signal-to-noise (S/N), spectral response, pointingVertical Resolution: FOV, S/N, pointingHorizontal & Temporal Resolution: inherent to SOFIE



GATS

Proprietary

SOFIE Signal-to-Noise (S/N) Exceeds Requirements

Current best estimates indicate signal to noise margin from 3.8 to 2000



GATS

Proprietary

Retrieval Simulations

Retrieval simulations are used to:

•Relate measurement requirements to instrument requirements

•Track instrument performance

Retrieval simulations are based on an advanced model:

•Rigorous line-by-line calculations

•Detailed atmospheric geometry and optics

•20 year heritage (LIMS, HALOE, CLAES, CRISTA, SABER,…)



GATS

Proprietary

Retrievals in the presence of clouds

Simulated SOFIE Water Vapor Retrievals

SOFIE H2O Measurement performance

Required Current Best Estimate (Margin)

Signal to Noise 2.5 104 3.1 106 (124)

Retrieval Precision 0.6 ppmv 0.02 ppmv (30)

Retrieval Altitude Range 79 – 90 km 15 – 110 km



GATS

Proprietary

Based on differential CO2 absorption from 2 channels (2.8 and 4.3 m)

Simulated SOFIE Temperature Retrievals

SOFIE Temperature Measurement performance

Required Current Best Estimate (Margin)

Signal to Noise 3.0 105 2.5 106 (8.3)

Retrieval Precision 5 K 0.6 K (8.3)

Retrieval Altitude Range 70 - 90 15 – 100 km

Retrievals in the presence of clouds

Simultaneous CO2 retrievals



GATS

Proprietary

Geophysical Parameter

Precision

Required / Current Margin

Altitude Range, km

Required / Current

Vertical Resolution, km

Required / Current

NIR cloud extinction 5x10-6 km-1 / >10 78 – 85 / 75 – 90 3 / 1.5

IR cloud extinction 5x10-5 km-1 / 10 78 – 85 / 75 – 90 3 / 1.5

temperature 5 K / 8.3 70 – 90 / 15 - 100 3 / 1.5

O3 mixing ratio 0.1 ppmv / 50 78 – 90 / 15 - 110 3 / 1.5

H2O mixing ratio 0.6 ppmv / 30 78 – 90 / 15 - 110 3 / 1.5

CO2 mixing ratio 10 ppmv / 6.3 80 – 100 / 15 - 105 3 / 1.5

CH4 mixing ratio 0.05 ppmv / 2.5 30 – 90 / 15 - 90 3 / 1.5

NO mixing ratio 107 cm-3 / 10 80 – 95 / 80 - 120 5 / 3

Horizontal and temporal resolution requirements are met

Predicted Performance Exceeds Requirements



GATS

Proprietary

SOFIE Particle Measurements

Particle extinctions at 10 wavelengths (290 nm – 5.3 m):

•Two dedicated particle channels (4 wavelengths)

•Gas channel weak bands (6 wavelengths)

Measurements from the tropopause to the mesopause:

•Primary: NLCs

•Secondary: cosmic dust, PSCs, cirrus, SSA

Unique combination of UV thru IR wavelengths allows:

•Particle size distribution retrievals

•Inference of particle composition



GATS

Proprietary

Simulated PMC Size Distribution Retrievals

SOFIE will measure PMC extinction at 10 wavelengths

These measurements will allow accurate PMC size distribution retrievals



GATS

Proprietary

Cosmic Dust (Smoke) Measurements

Model predictions suggest that SOFIE may be sensitive enough to measure the smoke layer

Accounting for molecular (Rayleigh) extinction will be important



GATS

Proprietary

Summary

•SOFIE will provide critical contributions to AIM science

•CBE performance meets or exceeds requirements:

Signal to noise

Pointing

Spectral response

Retrieval precision, resolution, etc…



GATS

Proprietary

Supplemental Material



GATS

Proprietary

Band 1 Spectral Response, SiC Detector

Predictions indicate excellent performance using silicon carbide detector

Solar out-of-band = 0.02%



GATS

Proprietary

Autonomous Start and Operation

By observing a CO2 V signal for 3 consecutive events, future event initiation and termination times can be determined, along with all other event command times. This allows autonomous start-up and operations.

Use t0.5 (when CO2 V is 50% of maximum) as a reference time.

Z(t0.5) is known to ± 3km (± 1.0 seconds)

Therefore, t0.5 values observed for the first and third events allows SOFIE to determineΔtP – Orbital Period, which in combination with the second event allows times to be predicted for all future events:tI – Event Initialization TimestB – Event Balance TimestT – Event Termination TimestC’s – Other Command Times

These times are refined continuously as more events are observed.

SS Orbital Period – ΔtP

SR Orbital Period – ΔtP

tISS tB

SS

t0.5SS

tTSS tI

SR

t0.5SR

tBSR tT

SR



GATS

Proprietary

Current best estimate performance

S/N: 3.0E6 (3.0E5 required, margin = 10)

Precision: 1.6 ppmv (10 ppmv required, margin = 6)

Altitude range: 15 – 105 km (80 – 100 km required)

Simulated SOFIE CO2 Retrievals



GATS

Proprietary


S/N: 2.9E6 (4.0E5 required, margin = 7.2)

Precision: 0.02 ppmv (0.05 ppmv required, margin = 2.5)


Simulated SOFIE Methane Retrievals



GATS

Proprietary


S/N: 1.5E6 (3.0E5 required, margin = 4.1)

Precision: 9.8 x 105 cm-3 (107 cm-3 required, margin = 10)


Simulated SOFIE Nitric Oxide Retrievals



GATS

Proprietary


S/N: 1.9E10 (1.0E4 required, margin > 1000)

Precision: 0.002 ppmv (0.1 ppmv required, margin = 50)

Altitude range: 15 - 110 km (78 – 90 km required)

Note: weak band signal will be used to retrieve O3 below 50 km

Simulated SOFIE Ozone Retrievals



GATS

Proprietary

SOFIE Refraction Angle Temperature Retrievals

Measurements of solar refraction angle vs. height can be used to retrieve temperature

Refraction angle known to 0.25 arcsec from the SOFIE sun sensor

2 K precision below 35 km



GATS

Proprietary

Cosmic Dust (Smoke) Measurements?

Can SOFIE measure cosmic smoke?

Model predictions of cosmic smoke properties show an extremely tenuous layer

These predictions were used to simulate SOFIE signals

solar occultation for ice experiment science overview

Documents