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 Moon has been shown to be an extremely stable radiometric reference for calibration and long-term stability
measurements of on-orbit sensors. The majority of previous work has been in the visible part of the spectrum,
using ground-based lunar images. The SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) on the
SOlar Radiation and Climate Experiment (SORCE) can be used to extend the lunar spectral irradiance dataset
to include the 115-300 nm range. SOLSTICE can directly measure both the solar and lunar spectra from orbit,
using the same optics and detectors. An observing campaign to map out the dependence on phase angle began
in mid 2006, and continues through the present. The geometry of SORCE's orbit is very favorable for lunar
observations, and we have measurements of almost the entire 0-180 degree range of phases. In addition to
Earth Observing Systems using the Moon for calibration, recent planetary missions have also made ultraviolet
observations of the Moon during Earth flyby, and these SOLSTICE measurements can be useful in calibrating
their absolute responsivity.
Variability in the solar UV and EUV irradiance is an important driver of the temperature of the Earth's upper atmosphere. The variation of the Magnesium II emission at 280 nm is a useful proxy for the variation at shorter wavelengths. Currently, the emission by the Mg II lines is measured only a few times per day at best, and often only a single measurement per day is available.
An instrument dedicated to measuring only the Magnesium II solar output on a high time cadence will greatly improve our knowledge of the energy input to the atmosphere. Such an instrument would have very modest power, size, and weight requirements, and could easily be one component of a suite of solar instruments on a future space mission. We present some design options for a lightweight solar Magnesium II monitor that would provide measurements suitable for space weather studies.