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Solar Occultation For Ice Experiment Science Overview. Larry Gordley & Mark Hervig. AIM Is a Mission to Study Noctilucent Clouds (NLCs). 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.

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Solar occultation for ice experiment science overview
Solar Occultation For Ice ExperimentScience Overview

Larry Gordley


Mark Hervig

Aim is a mission to study noctilucent clouds nlcs
AIM Is a Mission to Study Noctilucent Clouds (NLCs)

  • 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

Why do they form?

Why do they vary?

  • What is the role of

  • Temperature

  • H2O

  • Dynamics

  • Chemistry?

  • NLC are changing:

  • Increasing numbers

  • Moving equatorward

  • Increasing brightness

Is there a relationship to global change?

SOFIE will help AIM provide answers

Sofie measurement overview
SOFIE Measurement Overview

  • 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

Differential absorption measurements
Differential Absorption Measurements

  • Each SOFIE channel uses two detectors to make three measurements:

    • Strong band absorption

    • Weak band absorption

    • 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

Progress since pdr from a scientific view point
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

Silicon carbide detectors bands 1 2
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

Sofie spectral response
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 13

Band 13

Sofie performance requirements
SOFIE Performance Requirements

  • Performance Drivers

  • Precision & altitude range: signal-to-noise (S/N), spectral response, pointing

  • Vertical Resolution: FOV, S/N, pointing

  • Horizontal & Temporal Resolution: inherent to SOFIE

  • Sofie signal to noise s n exceeds requirements
    SOFIE Signal-to-Noise (S/N) Exceeds Requirements

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

    Retrieval simulations
    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,…)

    Simulated sofie water vapor retrievals
    Simulated SOFIE Water Vapor Retrievals

    Retrievals in the presence of clouds

    Simulated sofie temperature retrievals
    Simulated SOFIE Temperature Retrievals

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

    Retrievals in the presence of clouds

    Simultaneous CO2 retrievals

    Predicted performance exceeds requirements

    Horizontal and temporal resolution requirements are met

    Predicted Performance Exceeds Requirements

    Sofie particle measurements
    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

    Simulated pmc size distribution retrievals
    Simulated PMC Size Distribution Retrievals

    SOFIE will measure PMC extinction at 10 wavelengths

    These measurements will allow accurate PMC size distribution retrievals

    Cosmic dust smoke measurements
    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


    • SOFIE will provide critical contributions to AIM science

    • CBE performance meets or exceeds requirements:

      • Signal to noise

      • Pointing

      • Spectral response

      • Retrieval precision, resolution, etc…

    Band 1 spectral response sic detector
    Band 1 Spectral Response, SiC Detector

    Predictions indicate excellent performance using silicon carbide detector

    Solar out-of-band = 0.02%

    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.

    Autonomous Start and Operation





    SR Orbital Period – ΔtP

    SS Orbital Period – ΔtP

    Simulated sofie co 2 retrievals
    Simulated SOFIE CO2 Retrievals

    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 methane retrievals
    Simulated SOFIE Methane Retrievals

    Current best estimate performance

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

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

    Altitude range: 15 – 90 km (30 – 90 km required)

    Simulated sofie nitric oxide retrievals
    Simulated SOFIE Nitric Oxide Retrievals

    Current best estimate performance

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

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

    Altitude range: 80 – 120 km (80 – 95 km required)

    Simulated sofie ozone retrievals
    Simulated SOFIE Ozone Retrievals

    Current best estimate performance

    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

    Sofie refraction angle temperature retrievals
    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

    Cosmic dust smoke measurements1
    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