Introduction and Overview of Our Challenge - |LASP|CU …lasp.colorado.edu/media/projects/SORCE/documents... · Introduction and Overview of Our Challenge . Gary Rottman . LASP

Solar Spectral Irradiance (SSI) Variations NIST Workshop — February 28 to March 1, 2012

Introduction and Overview of Our Challenge

Gary Rottman LASP (retired)

[email protected]

2/27/12 1 SSI Variations Workshop

Goals of SSI Workshop(s)

• Examine SSI instruments, their capabilities, their observations, and the uncertainties associated with the measurements.

• Consider how these data were analyzed. How are solar variations separated from instrument effects.

• Establish an understanding of SSI differences. Refine their uncertainties.

• Make plans for the future — studies, calibrations and future meetings.


Why it’s Important to measure the solar irradiance — TSI and SSI


Incoming Solar Radiation


2.0

1.5

1.0

0.5

0.0

I r r a

d i a n

c e ( W

m - 2

n m - 1

)

2000 1500 1000 500

Wavelength (nm)

Top of Atmosphere

At Surface

10 m Below Ocean Surface

The total Solar irradiance (TSI) or radiant flux density is the radiant flux across a surface element, dA :

{W / m2}

Basic Radiometric Quantities- 1


Record of Total Solar Irradiance

1% 1) At most one of these data sets is correct 2) The mean of all four is almost certainly not correct

1) Apply Hooke’s Law: — restoring force is proportional to the displacement from equilibrium

2) Emphasize corrections that move the data in the desired direction


Success Story of the TSI Measurement Program


8

Model of TSI

Solar variability on all temporal and spatial scales is intimately connected with variations of the solar magnetic field

2/27/12 SSI Variations Workshop

Achievable

Required

Model estimates of Solar Variations vs. Wavelength

Solanki and Unruh


The Solar Spectral Irradiance (SSI), Eλ, is the radiant flux density per unit wavelength interval: {W / m3}

Basic Radiometric Quantities- 2

Solar Irradiance Digital Data to the Ground

[Σ=1361 W/m2]

NOTE: the Total Solar Irradiance, TSI, is the integral over all wavelengths of the Solar Spectral Irradiance.


First, do everything you can to reduce optical instability

— maintain cleanliness, select materials, reduce exposure

Goal is to establish (in-flight) change in responsivity and correct solar data accordingly

1. Conduct in-flight calibrations: • Carry an irradiance standard — FEL lamp, D2 lamp, etc. • Use an astronomical standard — stars, the moon, the Sun (!)

2. Use redundant systems — instruments, optical channels, detectors, etc. Employ varying duty cycles to solar exposure (1, 0.1, 0.01, etc.) and then

build an exposure/time dependent model of the responsivity, R(,t)

3. Return instrument from space and repeat calibration. From pre-flight and post-flight calibrations, interpolate responsivity to time of solar observation.

In-flight Change in Instrument Responsivity


Ground Data Processing detector data

Processing algorithm or — data transform or — measurement equation Pre-launch knowledge (σ’s)

and assumptions (σ’s)

• Instrument data • Spacecraft data • Orbit/attitude

E(λ,t) =C(t)

T(t,λ)D(t,λ)As∆(λ)− SL + diff ± ?

Model estimates

Other observations


Example of a time series


1980

1985

1990

1995

2000

2005

2010

Extended Time Series from Multiple Instruments

Lean, J., and M. DeLand, 2012: How Does the Sun’s Spectrum Vary?, J. Climate Doi:10.1175/JCLI-D-11-00571.1


1980

1985

1990

1995

2000

2005

2010

Extended Time Series from Multiple Instruments

σ = 2%


Extended Time Series from Multiple (one) Instrument


Internal Instrument Changes

aging temperature electronic drifts dosage shifts in optics

scattered light overlapping orders contamination solar exposure +++++++


Environment/Operational Changes

— changes in solar pointing — atmospheric absorption (SZA) — airglow (SZA) — off-axis scattered light

a. limb scattering b. f.o.v. intrusions

— energetic particles in space — spacecraft outgassing — plus others


Exposure Changes

UV-A UV-B

UV-C

work functions of most materials

• Outgassing throughout the interior of the instrument is likely ongoing

• Solar exposure of optical surfaces and/or contamination on these surfaces may change transmission

• Most of the “train wreck” happens at the first optical surface

• Some scattering may proceed from there to the walls of the spectrometer

• Fluorescence may occur • Photo-polymerization (or other

chemistry) may occur at the first optic • UV is used to smooth Plexiglas

Pick your poison


What Can this Workshop Accomplish?

• Wait to see what we accomplish in three days. What new insight?

• Networking — Expand the number of experts who understand the SSI instruments and their data processing

• Evaluate assumptions made about instrument performance — postulate alternate assumptions and approaches to reconcile differences

• Evaluate uncertainties presently associated with the SSI data sets

• Advise the instrument teams on methods of reducing uncertainties 2/27/12 20 SSI Variations Workshop

• Model results are based on TSI and UV observations

• The UARS instruments provided the required accuracy for:

λ < 300 nm

• SIM provides the required accuracy for:

200 < λ <2000 nm

Estimates of Solar Cycle Variability (model results of Solanki and Unruh)

Required Capability


TSI Measurements

• TSI varies by ~ 0.1% over the solar cycle • Solar cycles 21, 22 and 23 are roughly the same amplitude • Standard uncertainties have steadily improved from 5000 ppm to about 500 ppm

• TSI measurements require random uncertainty (Type A, or precision) on the order 50 ppm

• A single instrument can measure TSI variability even with large systematic uncertainty (Type B, or bias)

• Individual data sets are limited to about 5 years. — with overlap additional observations can extend the time base. Without overlap observations require combined standard uncertainties of ~ 100 ppm.

21 22 23


Spectral Irradiance Measurements

Note: For measurements of spectral irradiance, all requirements are wavelength dependent. • If a single instrument is used, the systematic uncertainty can be large, as long as the random uncertainty is small (< .1 of variation — 10% in EUV, 1% in the UV, and 0.01% in the visible)

• For multiple data sets (again limited to < 5 years), if the measurement sets overlap the data can be combined. If the sets do not overlap, the measurements must have a combined standard uncertainty of less than 0.1 (ideal) to 0.3 (acceptable) of the solar variation.

(From Solanki et al.)

Solar cycle variation of spectral irradiance — 300 < λ < 2000 nm


10 ppm

.01%

.1%

1%

10%

UARS Capability

SIM Capability

2000 1500 1000 500 Wavelength (nm)

M a x

i m u m

/ M i n

i m u m

- 1

To Control Type A Uncertainty

• Limit detector noise (phase-lock detection)

• Limit electronic noise (phase-lock detection)

• Repeatable and stable mechanisms

• Thermal stability

• Pointing stability and knowledge

• others terms ?


To Control Type B Uncertainty Establish the responsivity of a “flight” instrument relative to International System of

Units (SI)

1) Transfer calibration from a known “standard” instrument ( > 1%) — the resulting uncertainty of the “flight” unit is the combined uncertainty of

the “standard” + uncertainty of the transfer technique + uncertainty from instrument unique parameters and corrections.

2) Measure “flight” instrument response against an “irradiance standard” ( > 1%) — the resulting uncertainty of the “flight” unit is the combined uncertainty of

the “irradiance standard” + uncertainty from instrument unique corrections.

3) Characterize the “flight” instrument as an “absolute sensor” ( < 1%) — characterize each term in the measurement equation. Roll-up the table of

the uncertainties in each term — may be a unit level calibration or a calculation.


Documents

Introduction and Overview of Our Challenge - |LASP|CU …lasp.colorado.edu/media/projects/SORCE/documents... · Introduction and Overview of Our Challenge . Gary Rottman . LASP