Recognizing that the solar extreme ultraviolet (EUV) irradiance is an important driver of space weather, the National
Oceanic and Atmospheric Administration (NOAA) has added an Extreme Ultraviolet Sensor (EUVS) to its
Geostationary Operational Environmental Satellite (GOES) program, starting with the recently launched GOES-N, now
designated GOES-13. For the GOES-R series (slated for launch starting in 2015) , the EUVS measurement concept has
been redesigned. Instead of measuring broad bands spanning the EUV, the GOES-R EUVS will measure specific solar
emissions representative of coronal, transition region, and chromospheric variability. From these measurements, the
geo-effective EUV wavelength range from 5 to 127 nm can be reconstructed using models based on spectrally resolved
measurements gathered over the full range of solar variability. An overview of the GOES-R EUVS design is presented.
A description of the in-flight degradation tracking utilizing similar measurement and modeling techniques used to
generate the EUV irradiance is also provided.
The EUV and X-ray Irradiance Sensors (EXIS) on the upcoming GOES-R mission will include a spectrograph
to measure the Magnesium II doublet at 280 nm (channel C). The ratio of the core of this spectral feature to
the line wings is the well-known Mg II index. This ratio is often used as a proxy for chromospheric activity,
since changes in the index are highly correlated with changes in other chromospheric emission lines. As a ratio
measurement, the Mg II index is relatively insensitive to instrumental effects. The A and B channels of the
Extreme UltraViolet Sensor (EUVS) will make use of this fact and use the Mg II index measured by channel C
in their degradation correction. EUVS C channel has sufficient spectral resolution and sampling to measure the
Mg II index with high precision and will make this measurement at better than 30 s time cadence. This paper
describes the design and measurement requirements of the C channel.
The NASA Solar Dynamics Observatory (SDO), scheduled for launch in 2009, incorporates a suite of instruments
including the EUV Variability Experiment (EVE). The EVE instrument package contains grating spectrographs that will
measure the solar extreme ultraviolet (EUV) irradiance from 0.1 to 105 nm. The Multiple EUV Grating Spectrograph
(MEGS) channels use concave reflection gratings to image solar spectra onto CCDs. MEGS will provide 0.1nm
spectral resolution between 5-105nm every 10 seconds with an absolute accuracy of better than 25% over the SDO 5-
year mission. MEGS-A utilizes a unique grazing-incidence, off-Rowland circle (RC) design to minimize angle of
incidence at the detector while providing ≥ 0.1nm resolution between 5-37 nm. MEGS-B utilizes a double-pass, cross-dispersed
double-Rowland circle design while providing ≥ 0.1nm resolution between 35-105 nm. We present the as-built
performance of the MEGS optical design, including spectral resolution, wavelength shift, focus and alignment.
The NASA Solar Dynamics Observatory (SDO), scheduled for launch in early 2009, incorporates a suite of instruments
including the EUV Variability Experiment (EVE). Two channels of EVE, the Multiple EUV Grating Spectrograph
(MEGS) A and B channels use concave reflection gratings to image solar spectra onto CCDs to measure the solar
extreme ultraviolet (EUV) irradiance from 5 to 105 nm. MEGS provides these spectra at 0.1nm spectral resolution every
10 seconds with an absolute accuracy of better than 25% over the SDO 5-year mission. The calibration of the MEGS
channels in order to convert the instrument counts in to physical units of W/m2/nm was performed at the National
Institute for Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility III (SURF III) located in
Gaithersburg, Maryland. Although the final post-environmental calibrations have yet to be performed, preliminary
results from the pre-environmental calibrations show very good agreement with the theoretical optical design given by
Crotser et al. Further analysis is still needed in regards to the higher order contributions to determine the final first
order QT for all channels, but two techniques are currently being analyzed and show promising results.
The NASA Solar Dynamics Observatory (SDO), scheduled for launch in 2009, incorporates a suite of instruments
including the EUV Variability Experiment (EVE). The Multiple EUV Grating Spectrograph (MEGS) channels use
concave reflection gratings to image solar spectra onto CCDs that are operated at -100°C. MEGS provides 0.1nm
spectral resolution between 5-105nm every 10 seconds with an absolute accuracy of better than 25% over the SDO 5-
year mission. Characterizations and selection testing of the CCDs and the thin foil filters for SDO EVE have been
performed with both in-band and visible illumination. CCD selection was based on results from testing in LASP facility
Calibration and Test Equipment (CTE3) as well as results from at testingMIT. All CCDs meet the requirements for
electronics gain, flat field, Quantum Efficiency (QE), dark current, reverse clock, CTE, bad pixels and the -120°C
survival test. The thin foil filters selection was based on tests performed at LASP facilities and NIST. All filters provide
>106 attenuation of visible light with the proper EUV transmission needed for order sorting capabilities and are free of
critical pinholes.
The highly variable solar extreme ultraviolet (EUV) radiation is the major energy input into the Earth’s upper atmosphere and thus impacts the geospace environment that affects satellite operations and communications. The Extreme ultraviolet Variability Experiment (EVE) aboard the NASA Solar Dynamics Observatory (SDO, to be launched in 2008) will measure the solar EUV spectral irradiance from 0.1 to 105 nm with unprecedented spectral resolution (0.1 nm), temporal cadence (10-sec), and accuracy (10%). The EVE program will provide solar EUV irradiance data for the Living With the Star (LWS) program, including near real-time data products to be used in operational atmospheric models that specify the space environment and to assist in forecasting for space weather operations. The EVE includes several instruments to cover the full EUV range. The Multiple EUV Grating Spectrographs (MEGS) has two grating spectrographs. The MEGS-A is a grazing-incidence spectrograph to measure the solar EUV irradiance in the 5 to 37 nm range with 0.1 nm resolution, and the MEGS-B is a normal-incidence, dual-pass spectrograph to measure the solar EUV irradiance in the 35 to 105 nm range with 0.1 nm resolution. The MEGS channels have filter wheel mechanisms, holographic gratings, and cooled CCD detectors. For in-flight calibration of the MEGS, the EUV SpectroPhotometer (ESP) measures the solar EUV irradiance in broad bands between 0.1 and 39 nm, and a MEGS-Photometer to measure the bright hydrogen emission at 121.5 nm. In addition, underflight rocket experiments are planned on about an annual basis to assure that the EVE measurements have an absolute accuracy of better than 25% over the five-year SDO mission. This paper will describe the optical design of the EVE instrumentation and the plans for pre-flight and in-flight calibrations.
The NASA Solar Dynamics Observatory (SDO), scheduled for launch in 2008, incorporates a suite of instruments including the EUV Variability Experiment (EVE). The EVE instrument package contains grating spectrographs used to measure the solar extreme ultraviolet (EUV) irradiance from 0.1 to 105 nm. The Multiple EUV Grating Spectrograph (MEGS) channels use concave reflection gratings to image solar spectra onto CCDs that are operated at -100°C. MEGS provides 0.1nm spectral resolution between 5-105nm every 10 seconds with an absolute accuracy of better than 25% over the SDO 5-year mission. MEGS-A utilizes a unique grazing-incidence, off-Rowland circle (RC) design to minimize angle of incidence at the detector while meeting high resolution requirements. MEGS-B utilizes a double-pass, cross-dispersed double-Rowland circle design. MEGS-P, a Ly-α monitor, will provide a proxy model calibration in the 60-105 nm range. Finally, the Solar Aspect Monitor (SAM) channel will provide continual pointing information for EVE as well as low-resolution X-ray images of the sun. In-flight calibrations for MEGS will be provided by the on-board EUV Spectrophotometer (ESP) in the 0.1-7nm and 17-37nm ranges, as well as from annual under-flight rocket experiments. We present the methodology used to develop the MEGS optical design.
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