S2. EUV Irradiance

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S2. EUV Irradiance Calibration
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EUV Observations

• Most of the new missions that make the next 5
  years of solar observations look so exciting
  carry EUV/SXR instruments
• Solar-B EIS, XRT
• STEREO SECCHI EUVI
• GOES SXI, XRS
• Two of the three SDO instruments are strongly
  focused on the EUV
• Calibration of these EUV instruments is essential
  for a number of reasons
• EVE calibration is important for understanding
  the effects of irradiance variability on the
  atmosphere
• AIA calibration is important for understanding
  the thermal structure of the corona
• Even scientific investigations that dont
  explicitly rely on calibrated EUV observations
  will benefit from cross-calibration of EUV
  instruments

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Agenda

• (slightly modified since the announcement was
  posted)
• Overview of EVE calibration
• Overview of AIA-EVE cross-calibration
• Discussion
• cross-calibration with other instruments
• problems
• priorities
• procedures
• Wanted practical ideas and questions, not
  necessarily solutions (yet)

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EVE and those other instruments on SDO

• Frank Eparvier
• LASP / University of Colorado
• eparvier_at_lasp.colorado.edu

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Reminder EVE Instrument Overview

• Key Components
• EVE Optical Package (EOP)
• MEGS
• MEGS A SAM
• MEGS B P
• ESP
• EVE Electrical Box (EEB)
• Processor Memory
• Interfaces (1553 HSB)
• Power / Heaters / Control
• CCD power converters
• ESP power converters

EVE
AIA
HMI
EVE Resources EVE Resources
Power (orbit average) 43.9 Watts
Mass 54.2 kg
Data Rate 2 kbps (engineering) 7 Mbps (science)
Dimensions (L x W x H) 99 cm x 61 cm x 36 cm
SDO Spacecraft
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How does EVE measure the EUV?

• Multiple EUV Grating Spectrograph (MEGS)
• At 0.1 nm resolution
• MEGS-A 5-37 nm
• MEGS-B 35-105 nm
• At 1 nm resolution
• MEGS-SAM 0-7 nm
• At 10 nm resolution
• MEGS-Photometers _at_ 122 nm
• Ly-a Proxy for other H I emissions at 80-102 nm
  and He I emissions at 45-58 nm
• EUV Spectrophotometer (ESP)
• At 4 nm resolution
• 17.5, 25.6, 30.4, 36 nm
• At 7 nm resolution
• 0-7 nm (zeroth order)
• In-flight calibrations from ESP and MEGS-P on
  daily basis and also annual calibration rocket
  flights

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EVE Science Requirements
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EVE Data Products
Level Algorithm purpose Scientifically Useful Description File Duration Daily Volume (MB)
0A Fast validity check No TLM consistency/quality checking 1 minute 76000
0B Assemble images, detailed data verification No Data checks for CRC and pixel parity, parse data packets, merge image data, separate by science channel and filter wheel position 1 minute 76000
0C Space Weather Yes Quick-look indices, MEGS-A, MEGS-B, SAM, MEGS-P ESP 1 minute 36
1 Apply calibration Yes (SAM, ESP, MEGS-P) Use measurement equations to produce irradiance units 1 hour 1095
2 Re-grid, extract lines Yes Bin data to fixed wavelength scale, integrate over emission features with background removal 1 hour 1160
3 Daily average Yes Merge all component data into daily averages, bin to 0.1 and 1 nm 24 hours 0.026
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Calibration is a Lifetime Commitment

• The Calibration Essentials
• Understand the Measurement Equation
• Know all the parameters that go into the
  measurement to irradiance conversion and assess
  how to best quantify each
• Do a thorough error analysis and uncertainty
  budget
• Calibrate pre-flight
• Use a standard radiometric EUV source
• Primary standards, such as NIST SURF-III source,
  are preferred (note SURF beam flux known to lt1
  for EUV ranges)
• Track in-flight
• Any instrument changes that will affect results
• E.g. detector flat fields, gain changes,
  temperature effects, background signals,
• Re-Calibrate in-flight
• As close after launch as possible (changes since
  pre-flight calib.)
• On a regular basis thereafter in order to track
  absolute changes
• E.g. redundant channels, on-board sources, rocket
  underflights, proxy models
• Validate
• With measurements made with other instrumentation
  
• Comparisons with models

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MEGS A B Measurement Equations
Where
E Solar spectral irradiance
(x,y) Detector pixel location
S Raw signal from detector
?t Integration time
G Detector gain
fFF Flatfield correction
fLin Linearity correction
CBkg Background signal
CSL Scattered light signal
fImage Pixel contribution weighting to slit image
Good(x,y) Good pixels in slit image
ASlit Slit area
?? Dispersion (bandpass of single detector element)
Rc Responsivity at center of FOV
fFOV Pointing within FOV correction
fDegrad Degradation correction
f1AU Normalization to 1-AU
? Wavelength
EOS Higher order correction
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MEGS-A B Error Analysis

• The uncertainties of the various correction
  factors must be propagated through to determine
  the accuracy of the measured irradiance (note ?
  denotes uncertainty in the units of the variable)

• For bright solar emission features the primary
  contributors to accuracy are the uncertainties in
  RC (the responsivity of the instrument) and the
  fDegrad (degradation correction)
• For dim solar emissions, other uncertainties
  dominate, such as the precision of the
  measurement and the various corrections to the
  signal

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EVE Uncertainty Budget and Verification Matrix
Symbol Parameter Description Error Budget Component Level Instrument Level Spacecraft Level On-Orbit Level
S Signal 34 X X X X
?t Integration Time 0.02 X
G Gain 1 X X
fFF Flatfield 2 X X X X
fLin Detector Linearity 0.2 X X
CBkg Background 20 X X X X
CSL Scattered Light 20 X X X
fImage Slit Image Weight 2 X X
ASlit Slit Area 8 X
?? Dispersion 6 X X
RC Responsivity at Center 12 X X
fFOV FOV Correction 10 X X
fDegrad Degradation Correction 18 X
f1AU 1-AU Correction 0.02 X
? Wavelength 0.2 X X
EOS Order Sorting 2 X X
EMeasured Irradiance Product 25
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EVE In-Flight Calibration Activities

• Continuous Internal Cross-Calibrations
• Overlapping Channels within EVE
• Daily
• Filter wheel movements (dark, alternate filters)
• Flat field lamps for MEGS CCDs (LEDs)
• Quarterly Maneuvers
• Cruciform Scans 150 arcmin in 3 arcmin steps
• Gives gross FOV changes and locates edges of FOV
  for relative boresight calibrations to SAM and
  AIA guide telescope
• FOV Maps 10 arcmin in 5 arcmin steps (5x5 map)
• Gives finer FOV changes over nominal FOV pointing
  area (with margin)
• Also get bonus mapping when AIA and HMI require
  maneuvers (though their mappings are different
  and not optimized for EVE needs).
• Annual Rocket Underflights
• Fly prototype instruments on sounding rocket
  periodically.
• Calibrate rocket instruments at NIST before and
  after flight to transfer best calibration to EVE.

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Cross-calibrationAIA-EVE,SDO-everybody else
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Overview AIA/EVE cross-calibration

• Spectral response ?(?) (effective area) of AIA
  channels determined by component-level
  calibration measurements
• Mirrors (primary determination of bandpass)
• Filters
• CCDs
• System-level effects
• Estimated BOL relative calibration accuracy for
  AIA is 15
• Absolute calibration is more difficult
• Calibration will change due to contamination,
  degradation, etc.
• Therefore, cross-calibration with EVE is highly
  desireable
• First-order cross-calibration procedure
• Use EVE MEGS-A measurements of full-disk solar
  spectral irradiance to predict a full-disk count
  rate in each AIA channel
• Compare EVE-predicted count rate with AIAs
  measured full-disk count rate, and produce a
  scaling factor for each channel

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Refining the Cross-calibration

• First-order calibration should be easy to
  implement, but a few questions remain
• What cadence? (yearly? monthly? daily? 10
  seconds?)
• How do we interpret the resulting scale factors?
• Contamination?
• Something else?
• or is it just an empirical correction, and we
  dont worry about it?
• There are some potential pitfalls to the
  first-order AIA-EVE cross-calibration
• Field of view
• Spectral resolution
• Bandpass uncertainty

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Field of View

• AIA field of view is 41 arc-minutes (to edge of
  CCD) / 46 arc-minutes (vignetting circle)
• 1.3-2.0 pressure scale heights (at T 3.0 MK)
• Based on Yohkoh observations, we estimate that
  AIA will observe 96 of the total coronal
  radiance
• Higher fraction for lower-temperature lines
• Depends on size and location of particular
  structures

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Spectral Resolution

• Spectral resolution of 1 Å results in
  calibration errors
• Less than 1 for longer-wavelength (broad)
  channels
• Up to 25 for 171 and 94 Å
• Can be corrected by modeling higher-resolution
  spectrum

Simulated full-disk spectrum (10 AR, 90 QS)
shown in blue. Blurred with 1 Å FWHM gaussian and
binned at 6 pixels/Å in black. Response of AIA
194 channel shown in red. Folding the black
spectrum through the red instrument response
results in errors of 1-25 compared to using the
blue spectrum.
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Bandpass Uncertainty

• First-order cross-calibration only allows us to
  correct the overall scale of the AIA response
  functions
• Uncertainties in the bandpass shape are more
  important can we use EVE to correct those?

Measurements of the MSSTA multilayers. This is
not data from an AIA telescope, but the
illustration of bandpass variations over the
mirror surface is relevant. See the poster by R.
Soufli et al.
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Questions (1 of 3)

• For AIA-EVE cross-calibration
• How often should we perform "first-order"
  calibration?
• What data products are necessary for this
  cross-calibration?
• What sort of operational coordination is
  necessary? Coordination with rocket underflights?
• How do we interpret the resulting scaling
  factors?
• contamination?
• something else?
• not at all?
• How do we deal with the field-of-view
  discrepancy?
• How do we use EVE to correct the bandpass shape
  of the AIA?
• To what extent will cross-calibrations rely on
  spectral modeling?
• What improvements in spectral modeling can be
  made to enhance calibration accuracy?
• Can AIA-EVE cross-calibration be used to
  constrain Fe abundance?
• What role can DEM extraction from AIA play in
  cross-calibration, and extending the spectral
  range of EVE?

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Questions (2 of 3)

• For EIS-AIA-EVE inter-calibration
• How does EIS-AIA cross-calibration feed back into
  AIA-EVE cross-calibration?
• Is it possible to get full-disk spectra with EIS?
• If not, how do we cross-calibrate with EVE?
• If so, how can we coordinate this
  cross-calibration?
• Will it be possible to cross-calirbate EIS with
  the LASP rocket this year?
• For XRT-EVE cross-calibration
• Can the EVE SAM and ESP be used to
  cross-calibrate with XRT?
• Would this be useful?
• What sort of coordination is necessary? How often
  should this be done? etc.
• Can XRT and AIA be cross-calibrated? How? (Using
  DEM extraction?)

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Questions (3 of 3)

• Are TRACE and EIT going to be observing during
  SDO?
• If so, how do we cross-calibrate with AIA?
• If not, how do we establish continuity between
  the AIA dataset and the EIT/TRACE datasets?
• How important is this cross-calibration?
• For AIA, how important is it to have accurate
• Absolute calibration?
• Relative calibration (channel-to-channel)?
• Bandpass shape calibration?

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Backup Slides
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EVE and AIA Inter-Calibrations

• EVE spectra can be convolved with AIA bandpasses
  and compared with integrated images to transfer
  an absolute irradiance calibration from EVE to
  AIA.
• Whats needed for this transfer?
• AIA bandpasses (!)
• AIA image conversion to irradiance
• EVE irradiances
• Can EVE be used to track changing AIA bandpasses?
• Probably, but how the bleep do we do that?
• Logistical Questions
• Do AIA and EVE integrations need to be
  coincident?
• Do special data products need to be made for
  inter-calibrations?
• How frequently should comparisons be done?
• Are there special calibration activities on-orbit
  that should be planned? In conjunction with
  rocket underflights?

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Action Items from EVE Science Workshop (Nov, 2005)
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Comments from EVE Science Workshop (Nov, 2005)