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GOES-R Series Solar X-ray and Ultraviolet Irradiance
C H A P T E R
19 GOES-R Series Solar X-ray and Ultraviolet Irradiance Janet L. Machol*,†, Francis G. Eparvier‡, Rodney A. Viereck*,§, Donald L. Woodraska‡, Martin Snow‡, Ed Thiemann‡, Thomas N. Woods‡, William E. McClintock‡, Steven Mueller‡, Thomas D. Eden, Jr.‡, Randle Meisner‡, Stefan Codrescu*,†, S. Dave Bouwer¶, Alysha A. Reinard*,§ Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado-Boulder, Boulder, CO, United States, †NOAA National Centers for Environmental Information (NCEI), Boulder, CO, United States, ‡ Laboratory for Atmospheric and Space Physics (LASP), University of Colorado-Boulder, Boulder, CO, United States, § NOAA Space Weather Prediction Center (SWPC), Boulder, CO, United States, ¶Space Environment Technologies, Thornton, CO, United States *
19.1 INTRODUCTION The Geostationary Operational Environmental Satellites (GOES)-R Series Extreme Ultraviolet and X-ray Irradiance Sensors (EXIS) is a key operational space weather instrument that continuously measures solar irradiance at short wavelengths where the Sun’s output varies the most (Ermolli et al., 2013). Solar irradiance is electromagnetic radiation from the Sun and has units of power per unit area. Short wavelength radiation is absorbed in Earth’s upper atmosphere and so must be measured from space. The short wavelengths can be defined as hard X-ray (10−3–0.1 nm), soft X-ray (0.1–10 nm), extreme ultraviolet (EUV; 10–121 nm), and far ultraviolet (FUV; 122–200 nm) wavelength ranges (ISO 21348). EXIS has two primary components, the X-ray Sensor (XRS) and the Extreme Ultraviolet Sensor (EUVS). EXIS sits on the same Sun Pointing Platform (SPP) on GOES-R as the Solar Ultraviolet Imager (SUVI) and has a quadrant diode white light detector called the Sun Pointing Sensor (SPS) to monitor its pointing relative to the Sun. XRS instruments have been on GOES since 1974 and have measured the same two soft X-ray bandpasses. XRS measurements are used by space weather forecast agencies such as the National Oceanic and Atmospheric Administration (NOAA) Space Weather Prediction Center (SWPC) to detect solar flares (https://www.swpc.noaa. gov/). When X-rays from solar flares reach Earth, the resulting ionospheric enhancement can block high-frequency (HF) radio communication (e.g., Redmon et al., 2018), which is the primary communication path for commercial airlines. Solar X-ray and ultraviolet radiation affect other radio frequencies such as those used for satellite communication and navigation (GPS). During solar flares, forecast agencies produce warnings of radio blackouts and monitor for subsequent events. Following a large solar flare, there are likely to be two possible space weather events. A coronal mass ejection (CME) can blast away from the Sun, traveling millions of miles an hour, and arrive at Earth in as little as 20 h to create a geomagnetic storm. The solar flare and the CME can also accelerate protons to create a solar particle event (SPE). These protons and heavier ions can travel at 1/4 the speed of light and arrive at Earth in as little as 30 min. XRS measurements are also used to generate temperature and emission measures of hot coronal plasma (Garcia, 1994; White et al., 2005), which have applications in basic solar physics (e.g., Aschwanden and Alexander, 2001) as well as spectral irradiance modeling (e.g., Woods et al., 2008).
The GOES-R Series. https://doi.org/10.1016/B978-0-12-814327-8.00019-6
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Copyright © 2020 Elsevier Inc. All rights reserved.
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FIG. 19.1 Energy deposition (colors) as a function of wavelength and altitude. Solar spectrum (white) is overlaid. The impact on the upper atmosphere of the increased energy absorption is shown in the temperature profiles of solar maximum (orange) and minimum (yellow). This figure was created using temperature profiles from the Naval Research Laboratory (NRL) Mass Spectrometer Incoherent Scatter (MSIS) model (Hedin, 1991), absorption cross sections from Fennelly and Torr (1992), and spectral data from the NRL solar spectral model (courtesy of J. Lean; Warren et al., 2001; Huba et al., 2005).
The atmospheric energy deposition of electromagnetic radiation from the Sun as a function of wavelength is shown in Fig. 19.1. The absorption of solar X-ray and ultraviolet radiation in the upper atmosphere heats the thermosphere and creates the ionosphere, with impacts on radio transmissions and satellite orbits. Precise knowledge of the Sun’s output, such as that from EXIS measurements, is required to predict these impacts. For example, satellite drag, which is the friction between a satellite and the upper atmosphere, increases with solar activity; more solar irradiance expands the upper atmosphere thereby increasing its density as a function of altitude. Although 99% of the Sun’s light is produced at visible wavelengths, there is very little variability at visible wavelengths. The variability of the total solar irradiance is about 0.1% and occurs on time scales of days to years (Kopp, 2016). At shorter wavelengths, the solar irradiance variability is larger and varies on multiple time scales (Fig. 19.2). At EUV wavelengths, the variability over the 11-year solar cycle ranges from a factor of about 2–10 times the irradiance and is about 10 times larger than the variability in one 28-day solar rotation (Chamberlin et al., 2007), and during a flare, the irradiance can further vary by 0.1–10 times the nonflare signal (Thiemann et al., 2017). The first EUVS instruments on GOES are on GOES 13–15 and make five broadband measurements in the EUV (Viereck et al., 2007; Evans et al., 2010). GOES-R EUVS (Eparvier et al., 2009) is quite different from the first-generation instruments; it makes EUV and FUV high-spectral-resolution measurements of distinct solar emission lines representative of different layers of the solar atmosphere. The combined XRS and EUVS measurements are used to reconstruct a EUV spectrum from 5 to 127 nm using an empirical proxy model. Measurements of solar spectral irradiance (SSI), that is, the solar irradiance as a function of wavelength, are used in conjunction with absorption cross sections that are a function of wavelength and altitude as inputs to atmospheric models (Solomon and Qian, 2005).
19.2 X-RAY MEASUREMENTS AND PRODUCTS XRS measures solar irradiance, also called solar flux, in two bandpasses at soft X-ray wavelengths: XRS-A (0.05– 0.4 nm) and XRS-B (0.1–0.8 nm). Improvements of GOES-R XRS (Chamberlin et al., 2009) over previous generations of XRS include dual channels to improve the dynamic range, Si photodiodes instead of ionization cells, quadrant diodes to provide real-time flare locations, and radiometric calibrations at the National Institute of Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF) in Gaithersburg, Maryland (Arp et al., 2002).
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FIG. 19.2 Variability in solar emissions at X-ray, EUV, and FUV wavelengths as seen by a comparison of typical solar irradiance spectra during solar minimum, solar maximum, and during a solar flare. Spectral data are from the solar spectral model of the Naval Research Laboratory (courtesy of J. Lean; Warren et al., 2001; Huba et al., 2005).
As with XRS on earlier GOES (Garcia, 1994), the responsivity Ri for detector channel i, is defined by the assumption of a flat solar spectrum: ∞
A Ai ∫ 0 ε i ( λ ) dλ Ri = 2 ( λ2 − λ1 ) W/m
(19.1)
where Ai is the aperture area (in m2), εi is the channel spectral response (in A/W), and λ1 and λ2 are the lower and upper limits of the channel’s nominal bandpass, respectively. XRS measurements are made at a 1-s cadence, and Level 2 (L2) irradiances are provided at 1-s and 1-min cadences. GOES-R XRS measures the same bandpasses as earlier instruments but provides a dynamic range of over six orders of magnitude, a factor of 10 more than on previous GOES. It achieves this via two channels for each band, which measure overlapping ranges of lower and higher irradiances. The low irradiance channels, called A1 and B1, have larger area photodiodes for detecting weak signals. The high irradiance channels, called A2 and B2, are smaller quadrant photodiodes for measuring large solar flares. The quadrant diodes are also used to estimate flare locations. There are also two covered (“dark”) photodiodes that measure energetic particle impacts. The operational (i.e., real-time) XRS L2 products discussed in this section include high resolution and averaged irradiances, a daily background, an events list, and flare locations.
19.2.1 Irradiances X-ray irradiances are provided in units of W/m2 based on a flat spectrum as described by Eq. (19.1). At each instance in time, a primary channel is defined for XRS-A (either XRS-A1 or -A2) and for XRS-B (either XRS-B1 or -B2). Corrections applied to the data include temperature corrections, dark count subtraction, spike removal, electron contamination removal, and radiation corrections. Fig. 19.3 is a time series of XRS measurements and shows the tremendous variability of the solar irradiance for 41 days in 2017. Spikes in the XRS flux measurements are due to energetic particles, primarily galactic cosmic rays. A flux measurement, x, is flagged as a spike if: x − xmedian > f noise ⋅ xnoise + f flux ⋅ x
(19.2)
where xmedian is the median of the five closest flux values, xnoise is the maximum negative deviation of x − xmedian over 1 h, and fnoise ≈ 1.05 and fflux ≈ 0.01 are adjustable parameters. The noise term has the most impact at low fluxes and
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FIG. 19.3 Time series of 1-min-averaged GOES-16 XRS-B fluxes from August 24 through October 2, 2017. X-class flares occurred on September 6, 7 and 10, 2017.
during solar energetic particle (SEP) events (when the noise increases) while the flux term has impact at high fluxes and ensures that only significant spikes are flagged at high fluxes. During periods of very low X-ray fluxes and high electron fluxes, XRS measurements are dominated by signals that correlate with the local electron flux (instead of the solar X-ray photon flux). The impact is largest in the XRS channel B. This electron contamination will be largely removed from the X-ray data with an algorithm that uses differential electron flux measurements from the GOES-R SEISS MPS-HI (Space Environment In Situ Suite Magnetospheric Particle Sensor — High Energy Range) (Chapter 20). SEP events, which sometimes follow large solar flares (Gopalswamy et al., 2003), produce high particle fluxes, which are mostly protons and which affect the XRS detectors. These result in higher noise on the XRS detectors, increased spikes (which are removed by the spike removal algorithm), and artificial offsets in the detectors. The SEP impacts are corrected by subtracting the radiation impacts as determined by the dark detector measurements. XRS irradiances are used to monitor solar flares. During a flare, the ratio of XRS-A to XRS-B reaches a maximum before the individual XRS channels reach their maxima and is used operationally to aid space weather forecasters in the prediction of the timing of the flare maximum. The traditional solar flare classifications of A, B, C, M, and X are logarithmic and are based on the peak 1-min-averaged XRS-B irradiances of solar flares. As some examples, an A1 flare corresponds to a peak irradiance of 10−8 W/m2. Similarly, B1 corresponds to a peak irradiance of 10−7 W/m2, M5.2 corresponds to 5.2 × 10−5 W/m2, X9.2 corresponds to 9.2 × 10−4 W/m2, and X12 corresponds to 12 × 10−4 W/m2.
19.2.2 Daily Background The daily background describes the mid-to-long-term coronal variations on time scales of weeks to years. The background, provided for both XRS-A and -B, serves as an approximation of coronal background variations for statistical time series comparisons to other solar indices and is useful to describe solar active region evolution and the solar cycle. Because most major X-ray flares last for less than about 8 h, the algorithm is based on the minimum of hourly averages within 8-h bins.
19.2.3 Event Detection Operationally, the event detection algorithm must detect X-ray flares in real time, as soon as possible, while also minimizing ‘false alarms.’ To reduce the noise in the signal, the event detection algorithm is based on XRS-B 1-min-averaged data. Outputs of the algorithm are the times of the event start, event peak, event end, postevent, the integrated flux, the peak flare classification, and the background flux. XRS event detections serve as triggers for the EXIS flare location algorithm and the SUVI bright region and flare location algorithms. The key characteristic of
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X-ray flare events is a sudden, exponential rise above the preflare background (“impulsive phase”), followed by a slower decay (“gradual phase”). The majority of flares have a very rapid rise, reaching the maximum within minutes after the initiation, but because of the variations between flares, a simple percentage rise above the background will not work as a threshold to define flare initiation. A flare start is declared based on either of the following criteria: (1) the flare irradiance suddenly increases to above 5 × 10−5 W/m2 (an M5 flare magnitude) or (2) recent irradiance values show an exponential rise above two standard deviations of the signal and have reached an inflection point as determined from the maximum of the second derivative of the flux. The latency of determining the flare start can be as large as 7 min due to the data frame length needed for the exponential fit. The event end is defined as the time when the irradiance has decayed by onehalf of peak irradiance relative to the preevent background, while the postevent is defined as the time when the flare has decayed to the preflare background.
19.2.4 Flare Location Flare location influences the spectrum of the EUV irradiance that reaches Earth (Chamberlin et al., 2008; Thiemann et al., 2018), and as a result, the impact a flare has on Earth’s upper atmosphere (Qian et al., 2010). Flare locations are estimated using the signals from the four quadrants of the XRS-B2 quadrant diodes and provide rapid situational awareness for the SWPC forecasters especially in the case where real-time solar images are not available. The flare location is determined once per minute during the rising phase of flares. The flare location relative to the field of view (FOV) is Xrelative = ( Q2 + Q4 ) − ( Q1 + Q3 ) / ( Q1 + Q2 + Q3 + Q4 ) Yrelative = ( Q1 + Q2 ) − ( Q3 + Q4 ) / ( Q1 + Q2 + Q3 + Q4 )
(19.3)
where Qi is the background-subtracted corrected current for quadrant diode i. The background current is an average of the current prior to the onset of the flare. The Xrelative and Yrelative of the detected flare are translated, rotated, and scaled into a helioprojective Cartesian coordinate system in units of arcseconds. First, the location is translated to compensate for offsets in the instrument pointing relative to the center of the Sun. Next, it is rotated to set solar north at 0° based on the roll angle between EXIS and solar north. Finally, the translated and rotated position is scaled by the instrument FOV. The final location is reported in helioprojective Cartesian, heliographic Carrington, and heliographic Stonyhurst coordinates. Prelaunch estimates of the error in flare location determination were 3.6 ± 3 arcmin for X-class flares, 4.4 ± 3 arcmin for M-class flares, and 6.3 ± 5 arcmin for C-class flares. The background definition is more challenging when there are multiple flares within a few hours, resulting in somewhat larger location errors.
19.3 EUV MEASUREMENTS AND PRODUCTS The three EUVS channels are grating spectrographs. The EUVS-A channel measures solar lines in the wavelength range from 25 to 31 nm, while the EUVS-B measures lines between 117 and 141 nm. Both of these channels have 0.6-nm resolution and are based on irregularly spaced diode arrays where the diodes are positioned in groups to intercept the solar lines of interest, with 3–6 diodes used to produce the irradiance for each line. EUVS-C observes the Sun between 275 and 285 nm with a 512-element diode array and a net 0.1-nm resolution (for spectral sampling of five pixels per resolution element). Most of the data are provided at high resolution and/or 1-min cadences. The Level 1b (L1b) and L2 EUVS operational products discussed in this section include solar line irradiances at high resolution and 1-min averages, solar flare event detection, the Magnesium (Mg) ii index, and the EUVS proxy spectrum. The cadence of the high resolution products depends on the wavelength; it is currently 30 s for EUVS lines, 3 s for Mg ii, and 30 s for the spectrum. The primary solar lines measured by the three EUVS channels are shown in Table 19.1. The specific wavelengths were chosen to monitor the different layers of the Sun’s outer atmosphere so that they can be combined to create a full EUV spectrum of the Sun every 30 s.
19.3.1 EUVS Calibrations and Degradation Tracking Because some optical materials are modified when they absorb light at highly energetic EUV wavelengths, instruments that measure EUV are prone to degradation or changes in calibration with exposure. Most aspects of EUVS,
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TABLE 19.1 Main Solar Lines Measured by EUVS Wavelength (nm)
Line(s)
Source region
25.632, 25.724
He ii, Fe x
Transition region, Corona
28.415
Fe xv
Corona
30.378
He ii
Transition region
117.5
C iii
Chromosphere
121.567
Hi
Transition region
133.57
C ii
Chromosphere
140.5
Si iv, O iv
Transition region
279.5528, 280.2704
Mg ii h, k
Chromosphere
including the instrument, its preflight calibrations, its in-flight degradation tracking, and its ground processing algorithms were designed with this in mind. The EXIS ground processing algorithms to convert raw signals to irradiances have adjustable parameters to incorporate changes as tracked in-flight calibrations. Both XRS and EUVS underwent extensive preflight calibrations at the NIST SURF-III, which is the standard source for ultraviolet light in North America. The facility allows for direct access to the synchrotron radiation without any intervening optics, thus allowing for a primary calibration with the minimal uncertainty possible. Calibrations performed included quantum throughput (signal measured for a known input photon flux) and response variations as functions of pointing, signal level, and temperature. In addition, dark signals, detector gains, and detector flat-fields were also characterized preflight. On orbit, several different methods are used to track changes in EXIS. The XRS and EUVS-A and -B detectors have built-in gain calibration circuitry to measure changes in the gain. Gain and dark signal calibrations are performed weekly for EUVS and quarterly for XRS. Weekly calibrations of the flatfield (i.e., the pixel-to-pixel response of the detector arrays) for all three EUVS channels are done with visible LED stimulus lamps. On a quarterly basis, the SPP performs a cruciform scan whereby it slews the instrument boresight ±4° in the north-south and east-west directions. An FOV map is also performed quarterly, consisting of a 7 × 7 grid of off-points spanning ±15 arcmin from nominal Sun pointing. These maneuvers are used to track changes in the FOV response due to degradation of the nominal Sun-pointing optical paths in the instrument. The EUVS-C readout noise is also measured quarterly. Filter tests are performed frequently on the EUVS-A filter wheel, which has 24 redundant bandpass filters. One filter is designated as the primary filter to be used for normal operations. Once daily, the filter wheel is rotated to make a solar measurement with a designated secondary filter. Once weekly, a tertiary filter is used for a solar measurement. Comparisons of these measurements are used to track the degradation of the filter transmission. If the primary filter degrades too much to make an accurate solar measurement, one of the redundant filters can be designated as the new primary. For EUVS-C, the degradation trend is monitored by tracking trends in the signal levels. As discussed below, EUVS-C degradation has little impact on the Mg ii index. In the unlikely case that the EUVS-C channel becomes unusable, there are actually two completely redundant EUVS-C optical systems, and operation could switch to the second EUVS-C. Both the EUVS-A and -B channels are also corrected via a “bootstrap” degradation tracking methodology built into the design (Eparvier et al., 2009) that is based on Mg ii index measurements.
19.3.2 EUVS Event Detection The onset of flares at EUV wavelengths is of interest because they may precede by several minutes (and so forecast) the onset of X-ray flares. However, at EUV wavelengths, the onsets of flares are harder to detect in real time because EUV flares, with increases in irradiance on the order of 10%, are much smaller than the X-ray flares, which can increase by multiple orders of magnitude. The L2 EUVS event detection algorithm operates at a 1-min cadence and does not use a shape-based trigger like the XRS event detection but instead uses increases in the signals to independently trigger events for each wavelength. Outputs of the algorithm are event start, event peak, event end, postevent, integrated flux, and background flux.
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19.3.3 Magnesium II Index Another new product from GOES-R EUVS is the Mg ii index (Heath and Schlesinger, 1986; Snow et al., 2009), which is an extremely reliable measure of solar activity at ultraviolet wavelengths and is used to monitor long-term and day-to-day solar variability and to model the resulting upper atmospheric changes at Earth. EUVS measures the Mg ii index with better precision and a much higher cadence (3 s) than for any previous satellite. The Mg ii index is calculated from measurements of solar emissions near 280 nm. The Mg ii h and k doublet is formed in the chromosphere and varies considerably with solar activity, while the emissions in the wings are from the photosphere and have little variation. The Mg ii index is the ratio of the irradiance from the line cores to that of the wings. As a ratio instead of an irradiance measurement, the index is more stable relative to degradation or other instrumental artifacts. The Mg ii index is calculated as
(
Mg II index = ( Dh + Dk ) / DBlueWing + DRedWing
)
(19.4)
where Dh and Dk are the signals for the h and k lines as defined by 9-pixel-wide weighted masks while the signals for the wings are defined by trapezoidal 148-pixel-wide masks (Fig. 19.4A). One year of EXIS Mg ii index measurements are shown in Fig. 19.4B. The EXIS Mg ii index is a major input to the proxy model spectrum and is also used to track instrument degradation on EUVS-A and -B. A second data product is the Mg ii index scaled to be the index that would be produced by a 1.1-nm spectral resolution instrument such as described in Viereck et al. (2004). The L2 Mg ii index is provided at cadences of 3 s and 1 min.
19.3.4 EUV Proxy Spectrum The solar EUV spectrum is composed of emission lines and continuua that are formed in different layers of the solar atmosphere, namely, the chromosphere, the transition region, and the corona. See Chapter 18 for a description of the Sun’s atmosphere. Long-term observations of the solar EUV spectrum from the Solar EUV Experiment (SEE) on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite (Woods et al., 2005) show that spectral features that have the same source regions in the solar atmosphere vary similarly. Based on this information, if measurements are made of each type of region, then a full spectrum of the EUV region can be generated from these measurements with an empirical model. This concept of using representative (proxy) wavelengths to determine a full spectrum is the basis for the EUVS L1b Irradiance Model and has been used by a variety of SSI models including (Hinteregger et al., 1981), the Solar2000 model (Tobiska et al., 2000), the Flare Irradiance Spectral Model (FISM, Chamberlin et al., 2007, 2008), a similar model for measurements at Mars (Thiemann et al., 2017), and a model for GOES 13–15 EUVS (Suess et al., 2016).
FIG. 19.4 (A) GOES-16 EUVS-C spectrum (black) and weighting functions for two core lines and two wings in arbitrary units (shaded). (B) Time series of GOES-16 EXIS Mg ii index for 2017.
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The inputs for the EUVS L1b Irradiance Model are the 10 independent EUVS and XRS measurements (2 XRS bands, 7 EUV lines and the Mg ii index), which are produced by specific solar emission features that represent the different types of variability. The model produces solar EUV spectral irradiance continuously at 30-s cadence in 22 5-nm-wide bins from 5 to 115 nm and a single 10-nm-wide bin from 117 to 127 nm. The irradiance Eλ for bin λ at time t is 10
10
i =1
i =1
Eλ ( t ) = Eλ ,0 + ∑ ji ,λ Pi ( t ) + ∑ ki ,λ Qi ( t )
(19.5)
where Eλ,0 is an offset, the sums are over the ten measurements, ji,λ and ki,λ are model coefficients, and Pi(t) and Qi(t) are long- and short-term measurement components. The time-dependent terms represent a daily average and the contribution at a 30-s cadence. The components are defined Pi ( t ) = Qi ( t ) =
Xi ( t )day − Xi , 0 Xi , 0 Xi ( t ) − Xi ( t )day Xi ( t )day
(19.6)
where Xi(t) is the ith EXIS measurement, Xi(t)day is the daily average, and Xi,0 is the reference offset. The EUVS model parameters of Eλ,0, ji,λ and ki,λ were determined using historical data sets for GOES/XRS, Solar Dynamics Observatory/ (SDO) EUV Variability Experiment (EVE) (Woods et al., 2012), Solar Radiation and Climate Experiment (SORCE)/ Solar Stellar Irradiance Comparison Experiment (SOLSTICE; McClintock et al., 2005), and TIMED/SEE as well as the Mg ii composite time series provided by the University of Bremen based on measurements from Global Ozone Monitoring Experiment (GOME; Weber et al., 1998) and Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY (SCHIAMACHY; Bovensmann et al., 1999). The modeling was done for 2002–2013 and gaps were filled with values from FISM. Modeled spectra for the quietest and most active solar irradiance days measured by EXIS at the time of writing are shown in Fig. 19.5. The absorption cross sections of the major species in the thermosphere are also shown for reference. The model uncertainty for daily average irradiances ranges from 1.6% to 5.4% for the model bins. At a 30-s cadence, model predictions of M-class or greater flares are less than 20% for all bins except for the 10–15-nm and 95–100-nm bins, which have uncertainties of 68% and 31.8%, respectively. The data cadence for the proxy spectrum is 30 s in the L1b data and 1 min in the L2 data. In the future, proxy spectra with other bin spacings will be provided. Of special interest to atmospheric modelers are the spectral bins based on the regions of near-constant atmospheric absorption cross sections as defined by Solomon and Qian (2005).
FIG. 19.5 GOES-16 daily average spectra corresponding to days with the highest and lowest 30.4-nm measurements currently available. Superimposed are the absorption cross sections for the major atmospheric species of O, O2, and N2 as provided by Huebner and Mukherjee (2015).
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19.4 SUMMARY EXIS provides greatly improved continuous measurements of X-ray and EUV irradiance for better space weather prediction and atmospheric modeling. Relative to its operational predecessors, GOES-R XRS has better dynamic range while GOES-R EUVS has completely new measurements of spectral lines and the Mg ii index data. These improved data allow for continued flare detection, the generation of an EUV proxy spectrum at a 1-min cadence, and a variety of other useful space weather products for operational forecasting and scientific uses. EXIS data and information are available at the NOAA NCEI website for GOES-R space weather data: https:// www.ngdc.noaa.gov/stp/satellite/goes-r.html. Additional documents and user resources can be found at the GOES-R Series website: https://www.goes-r.gov/.
Acknowledgments We thank all members of the LASP EXIS team for their careful design, construction, calibration, and validation of EXIS, especially Andrew Jones. We also thank William Rowland, Margaret Tilton, Courtney Peck, and Vicki Hsu for work on the NCEI processing and archiving. The work at CIRES was supported by the GOES-R Program and NCEI through NOAA Cooperative Agreements NA15OAR4320137 and NA17OAR4320101. The EXIS work at LASP was supported under NASA Contract #NNG07HW00C. GOES-R risk reduction work done by SET and LASP was under the GSA Schedule 871 contract GS23F0195N and NOAA order number RA133W-09-NC-2692.
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Snow, M., McClintock, W.E., Crotser, D., Eparvier, F.G., 2009. EUVS-C: the measurement of the magnesium II index for GOES-R EXIS. In: Proc. SPIE 7438, Solar Physics and Space Weather Instrumentation III, 743803 (August 26, 2009). https://doi.org/10.1117/12.828566. Solomon, S.C., Qian, L., 2005. Solar extreme-ultraviolet irradiance for general circulation models. J. Geophys. Res. 110. https://doi. org/10.1029/2005JA011160. Suess, K., Snow, M., Viereck, R., Machol, J., 2016. Solar spectral proxy irradiance from GOES (SSPRING): a model for solar EUV irradiance. J. Space Weather Space Clim. 6, A10. https://doi.org/10.1051/swsc/2016003. Thiemann, E.M., Chamberlin, P.C., Eparvier, F.G., Templeman, T., Woods, T.N., Bougher, S.W., Jakosky, B.M., 2017. The MAVEN EUVM model of solar spectral irradiance variability at Mars: Algorithms and results. J. Geophys. Res. 122, 2748–2767. https://doi.org/10.1002/2016JA023512. Thiemann, E.M.B., Chamberlin, P.C., Eparvier, F.G., Epp, L., 2018. Center-to-limb variability of hot coronal EUV emissions during solar flares. Solar Phys. 293, 19. https://doi.org/10.1007/s11207-018-1244-2. Tobiska, W.K., Woods, T., Eparvier, F., Viereck, R., Floyd, L., Bouwer, D., Rottman, G., White, O.R., 2000. The SOLAR2000 empirical solar irradiance model and forecast tool. J. Atmos. Solar-Terrestrial Phys. 62, 1233–1250. https://doi.org/10.1016/S1364-6826(00)00070-5. Viereck, R.A., Floyd, L.E., Crane, P.C., Woods, T.N., Knapp, B.G., Rothman, G., Weber, M., Puga, L.C., DeLand, M.T., 2004. A composite Mg II index spanning from 1978 to 2003. Space Weather 2, https://doi.org/10.1029/2004SW000084. S10005. Viereck, R., Hanser, F., Wise, J., Guha, S., Jones, A., McMullin, D., Plunket, S., Strickland, D., Evans, S., 2007. Solar extreme ultraviolet irradiance observations from GOES: Design characteristics and initial performance. In: Proc. SPIE 6689, Solar Physics and Space Weather Instrumentation II, 66890K. https://doi.org/10.1117/12.734886. Warren, H.P., Mariska, J.T., Lean, J., 2001. A new model of solar EUV irradiance variability: 1. Model formulation. J. Geophys. Res. 106, 15745– 15758. https://doi.org/10.1029/2000JA000282. Weber, M., Burrows, J.P., Cebula, R.P., 1998. GOME solar UV/VIS irradiance measurements between 1995 and 1997—first results on proxy solar activity studies. Sol. Phys. 177, 63–77. White, S.M., Thomas, R.J., Schwartz, R.A., 2005. Updated expressions for determining temperatures and emission measures from Goes soft X-ray measurements. Sol. Phys. 227, 231. https://doi.org/10.1007/s11207-005-2445-z. Woods, T.N., Eparvier, F.G., Bailey, S.M., Chamberlin, P.C., Lean, J., Rottman, G.J., Soloman, S.C., Tobiska, W.K., Woodraska, D.L., 2005. Solar EUV experiment (SEE): Mission overview and first results. J. Geophys. Res. 110, A01312. https://doi.org/10.1029/2004JA010765. Woods, T.N., Chamberlin, P.C., Peterson, W.K., Meier, R.R., Richards, P.G., Strickland, D.J., Mannucci, A.J., 2008. XUV photometer system (XPS): Improved solar irradiance algorithm using CHIANTI spectral models. Sol. Phys. 250, 235–267. https://doi.org/10.1007/s11207-008-9196-6. Woods, T.N., Eparvier, F.G., Hock, R., Jones, A.R., Woodraska, D., Judge, D., Didkovsky, L., Lean, J., Mariska, J., Warren, H., McMullin, D., Chamberlin, P., Berthiaume, G., Bailey, S., Fuller-Rowell, T., Sojka, J., Tobiska, W.K., Viereck, R., 2012. Extreme ultraviolet variability experiment (EVE) on the solar dynamics observatory (SDO): Overview of science objectives, instrument design, data products, and model developments. Solar Phys 275, 115–143. https://doi.org/10.1007/s11207-009-9487-6.
19 GOES-R Series Solar X-ray and Ultraviolet Irradiance Janet L. Machol*,†, Francis G. Eparvier‡, Rodney A. Viereck*,§, Donald L. Woodraska‡, Martin Snow‡, Ed Thiemann‡, Thomas N. Woods‡, William E. McClintock‡, Steven Mueller‡, Thomas D. Eden, Jr.‡, Randle Meisner‡, Stefan Codrescu*,†, S. Dave Bouwer¶, Alysha A. Reinard*,§ Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado-Boulder, Boulder, CO, United States, †NOAA National Centers for Environmental Information (NCEI), Boulder, CO, United States, ‡ Laboratory for Atmospheric and Space Physics (LASP), University of Colorado-Boulder, Boulder, CO, United States, § NOAA Space Weather Prediction Center (SWPC), Boulder, CO, United States, ¶Space Environment Technologies, Thornton, CO, United States *
19.1 INTRODUCTION The Geostationary Operational Environmental Satellites (GOES)-R Series Extreme Ultraviolet and X-ray Irradiance Sensors (EXIS) is a key operational space weather instrument that continuously measures solar irradiance at short wavelengths where the Sun’s output varies the most (Ermolli et al., 2013). Solar irradiance is electromagnetic radiation from the Sun and has units of power per unit area. Short wavelength radiation is absorbed in Earth’s upper atmosphere and so must be measured from space. The short wavelengths can be defined as hard X-ray (10−3–0.1 nm), soft X-ray (0.1–10 nm), extreme ultraviolet (EUV; 10–121 nm), and far ultraviolet (FUV; 122–200 nm) wavelength ranges (ISO 21348). EXIS has two primary components, the X-ray Sensor (XRS) and the Extreme Ultraviolet Sensor (EUVS). EXIS sits on the same Sun Pointing Platform (SPP) on GOES-R as the Solar Ultraviolet Imager (SUVI) and has a quadrant diode white light detector called the Sun Pointing Sensor (SPS) to monitor its pointing relative to the Sun. XRS instruments have been on GOES since 1974 and have measured the same two soft X-ray bandpasses. XRS measurements are used by space weather forecast agencies such as the National Oceanic and Atmospheric Administration (NOAA) Space Weather Prediction Center (SWPC) to detect solar flares (https://www.swpc.noaa. gov/). When X-rays from solar flares reach Earth, the resulting ionospheric enhancement can block high-frequency (HF) radio communication (e.g., Redmon et al., 2018), which is the primary communication path for commercial airlines. Solar X-ray and ultraviolet radiation affect other radio frequencies such as those used for satellite communication and navigation (GPS). During solar flares, forecast agencies produce warnings of radio blackouts and monitor for subsequent events. Following a large solar flare, there are likely to be two possible space weather events. A coronal mass ejection (CME) can blast away from the Sun, traveling millions of miles an hour, and arrive at Earth in as little as 20 h to create a geomagnetic storm. The solar flare and the CME can also accelerate protons to create a solar particle event (SPE). These protons and heavier ions can travel at 1/4 the speed of light and arrive at Earth in as little as 30 min. XRS measurements are also used to generate temperature and emission measures of hot coronal plasma (Garcia, 1994; White et al., 2005), which have applications in basic solar physics (e.g., Aschwanden and Alexander, 2001) as well as spectral irradiance modeling (e.g., Woods et al., 2008).
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FIG. 19.1 Energy deposition (colors) as a function of wavelength and altitude. Solar spectrum (white) is overlaid. The impact on the upper atmosphere of the increased energy absorption is shown in the temperature profiles of solar maximum (orange) and minimum (yellow). This figure was created using temperature profiles from the Naval Research Laboratory (NRL) Mass Spectrometer Incoherent Scatter (MSIS) model (Hedin, 1991), absorption cross sections from Fennelly and Torr (1992), and spectral data from the NRL solar spectral model (courtesy of J. Lean; Warren et al., 2001; Huba et al., 2005).
The atmospheric energy deposition of electromagnetic radiation from the Sun as a function of wavelength is shown in Fig. 19.1. The absorption of solar X-ray and ultraviolet radiation in the upper atmosphere heats the thermosphere and creates the ionosphere, with impacts on radio transmissions and satellite orbits. Precise knowledge of the Sun’s output, such as that from EXIS measurements, is required to predict these impacts. For example, satellite drag, which is the friction between a satellite and the upper atmosphere, increases with solar activity; more solar irradiance expands the upper atmosphere thereby increasing its density as a function of altitude. Although 99% of the Sun’s light is produced at visible wavelengths, there is very little variability at visible wavelengths. The variability of the total solar irradiance is about 0.1% and occurs on time scales of days to years (Kopp, 2016). At shorter wavelengths, the solar irradiance variability is larger and varies on multiple time scales (Fig. 19.2). At EUV wavelengths, the variability over the 11-year solar cycle ranges from a factor of about 2–10 times the irradiance and is about 10 times larger than the variability in one 28-day solar rotation (Chamberlin et al., 2007), and during a flare, the irradiance can further vary by 0.1–10 times the nonflare signal (Thiemann et al., 2017). The first EUVS instruments on GOES are on GOES 13–15 and make five broadband measurements in the EUV (Viereck et al., 2007; Evans et al., 2010). GOES-R EUVS (Eparvier et al., 2009) is quite different from the first-generation instruments; it makes EUV and FUV high-spectral-resolution measurements of distinct solar emission lines representative of different layers of the solar atmosphere. The combined XRS and EUVS measurements are used to reconstruct a EUV spectrum from 5 to 127 nm using an empirical proxy model. Measurements of solar spectral irradiance (SSI), that is, the solar irradiance as a function of wavelength, are used in conjunction with absorption cross sections that are a function of wavelength and altitude as inputs to atmospheric models (Solomon and Qian, 2005).
19.2 X-RAY MEASUREMENTS AND PRODUCTS XRS measures solar irradiance, also called solar flux, in two bandpasses at soft X-ray wavelengths: XRS-A (0.05– 0.4 nm) and XRS-B (0.1–0.8 nm). Improvements of GOES-R XRS (Chamberlin et al., 2009) over previous generations of XRS include dual channels to improve the dynamic range, Si photodiodes instead of ionization cells, quadrant diodes to provide real-time flare locations, and radiometric calibrations at the National Institute of Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF) in Gaithersburg, Maryland (Arp et al., 2002).
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FIG. 19.2 Variability in solar emissions at X-ray, EUV, and FUV wavelengths as seen by a comparison of typical solar irradiance spectra during solar minimum, solar maximum, and during a solar flare. Spectral data are from the solar spectral model of the Naval Research Laboratory (courtesy of J. Lean; Warren et al., 2001; Huba et al., 2005).
As with XRS on earlier GOES (Garcia, 1994), the responsivity Ri for detector channel i, is defined by the assumption of a flat solar spectrum: ∞
A Ai ∫ 0 ε i ( λ ) dλ Ri = 2 ( λ2 − λ1 ) W/m
(19.1)
where Ai is the aperture area (in m2), εi is the channel spectral response (in A/W), and λ1 and λ2 are the lower and upper limits of the channel’s nominal bandpass, respectively. XRS measurements are made at a 1-s cadence, and Level 2 (L2) irradiances are provided at 1-s and 1-min cadences. GOES-R XRS measures the same bandpasses as earlier instruments but provides a dynamic range of over six orders of magnitude, a factor of 10 more than on previous GOES. It achieves this via two channels for each band, which measure overlapping ranges of lower and higher irradiances. The low irradiance channels, called A1 and B1, have larger area photodiodes for detecting weak signals. The high irradiance channels, called A2 and B2, are smaller quadrant photodiodes for measuring large solar flares. The quadrant diodes are also used to estimate flare locations. There are also two covered (“dark”) photodiodes that measure energetic particle impacts. The operational (i.e., real-time) XRS L2 products discussed in this section include high resolution and averaged irradiances, a daily background, an events list, and flare locations.
19.2.1 Irradiances X-ray irradiances are provided in units of W/m2 based on a flat spectrum as described by Eq. (19.1). At each instance in time, a primary channel is defined for XRS-A (either XRS-A1 or -A2) and for XRS-B (either XRS-B1 or -B2). Corrections applied to the data include temperature corrections, dark count subtraction, spike removal, electron contamination removal, and radiation corrections. Fig. 19.3 is a time series of XRS measurements and shows the tremendous variability of the solar irradiance for 41 days in 2017. Spikes in the XRS flux measurements are due to energetic particles, primarily galactic cosmic rays. A flux measurement, x, is flagged as a spike if: x − xmedian > f noise ⋅ xnoise + f flux ⋅ x
(19.2)
where xmedian is the median of the five closest flux values, xnoise is the maximum negative deviation of x − xmedian over 1 h, and fnoise ≈ 1.05 and fflux ≈ 0.01 are adjustable parameters. The noise term has the most impact at low fluxes and
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19. GOES-R Series Solar X-ray and Ultraviolet Irradiance
FIG. 19.3 Time series of 1-min-averaged GOES-16 XRS-B fluxes from August 24 through October 2, 2017. X-class flares occurred on September 6, 7 and 10, 2017.
during solar energetic particle (SEP) events (when the noise increases) while the flux term has impact at high fluxes and ensures that only significant spikes are flagged at high fluxes. During periods of very low X-ray fluxes and high electron fluxes, XRS measurements are dominated by signals that correlate with the local electron flux (instead of the solar X-ray photon flux). The impact is largest in the XRS channel B. This electron contamination will be largely removed from the X-ray data with an algorithm that uses differential electron flux measurements from the GOES-R SEISS MPS-HI (Space Environment In Situ Suite Magnetospheric Particle Sensor — High Energy Range) (Chapter 20). SEP events, which sometimes follow large solar flares (Gopalswamy et al., 2003), produce high particle fluxes, which are mostly protons and which affect the XRS detectors. These result in higher noise on the XRS detectors, increased spikes (which are removed by the spike removal algorithm), and artificial offsets in the detectors. The SEP impacts are corrected by subtracting the radiation impacts as determined by the dark detector measurements. XRS irradiances are used to monitor solar flares. During a flare, the ratio of XRS-A to XRS-B reaches a maximum before the individual XRS channels reach their maxima and is used operationally to aid space weather forecasters in the prediction of the timing of the flare maximum. The traditional solar flare classifications of A, B, C, M, and X are logarithmic and are based on the peak 1-min-averaged XRS-B irradiances of solar flares. As some examples, an A1 flare corresponds to a peak irradiance of 10−8 W/m2. Similarly, B1 corresponds to a peak irradiance of 10−7 W/m2, M5.2 corresponds to 5.2 × 10−5 W/m2, X9.2 corresponds to 9.2 × 10−4 W/m2, and X12 corresponds to 12 × 10−4 W/m2.
19.2.2 Daily Background The daily background describes the mid-to-long-term coronal variations on time scales of weeks to years. The background, provided for both XRS-A and -B, serves as an approximation of coronal background variations for statistical time series comparisons to other solar indices and is useful to describe solar active region evolution and the solar cycle. Because most major X-ray flares last for less than about 8 h, the algorithm is based on the minimum of hourly averages within 8-h bins.
19.2.3 Event Detection Operationally, the event detection algorithm must detect X-ray flares in real time, as soon as possible, while also minimizing ‘false alarms.’ To reduce the noise in the signal, the event detection algorithm is based on XRS-B 1-min-averaged data. Outputs of the algorithm are the times of the event start, event peak, event end, postevent, the integrated flux, the peak flare classification, and the background flux. XRS event detections serve as triggers for the EXIS flare location algorithm and the SUVI bright region and flare location algorithms. The key characteristic of
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237
X-ray flare events is a sudden, exponential rise above the preflare background (“impulsive phase”), followed by a slower decay (“gradual phase”). The majority of flares have a very rapid rise, reaching the maximum within minutes after the initiation, but because of the variations between flares, a simple percentage rise above the background will not work as a threshold to define flare initiation. A flare start is declared based on either of the following criteria: (1) the flare irradiance suddenly increases to above 5 × 10−5 W/m2 (an M5 flare magnitude) or (2) recent irradiance values show an exponential rise above two standard deviations of the signal and have reached an inflection point as determined from the maximum of the second derivative of the flux. The latency of determining the flare start can be as large as 7 min due to the data frame length needed for the exponential fit. The event end is defined as the time when the irradiance has decayed by onehalf of peak irradiance relative to the preevent background, while the postevent is defined as the time when the flare has decayed to the preflare background.
19.2.4 Flare Location Flare location influences the spectrum of the EUV irradiance that reaches Earth (Chamberlin et al., 2008; Thiemann et al., 2018), and as a result, the impact a flare has on Earth’s upper atmosphere (Qian et al., 2010). Flare locations are estimated using the signals from the four quadrants of the XRS-B2 quadrant diodes and provide rapid situational awareness for the SWPC forecasters especially in the case where real-time solar images are not available. The flare location is determined once per minute during the rising phase of flares. The flare location relative to the field of view (FOV) is Xrelative = ( Q2 + Q4 ) − ( Q1 + Q3 ) / ( Q1 + Q2 + Q3 + Q4 ) Yrelative = ( Q1 + Q2 ) − ( Q3 + Q4 ) / ( Q1 + Q2 + Q3 + Q4 )
(19.3)
where Qi is the background-subtracted corrected current for quadrant diode i. The background current is an average of the current prior to the onset of the flare. The Xrelative and Yrelative of the detected flare are translated, rotated, and scaled into a helioprojective Cartesian coordinate system in units of arcseconds. First, the location is translated to compensate for offsets in the instrument pointing relative to the center of the Sun. Next, it is rotated to set solar north at 0° based on the roll angle between EXIS and solar north. Finally, the translated and rotated position is scaled by the instrument FOV. The final location is reported in helioprojective Cartesian, heliographic Carrington, and heliographic Stonyhurst coordinates. Prelaunch estimates of the error in flare location determination were 3.6 ± 3 arcmin for X-class flares, 4.4 ± 3 arcmin for M-class flares, and 6.3 ± 5 arcmin for C-class flares. The background definition is more challenging when there are multiple flares within a few hours, resulting in somewhat larger location errors.
19.3 EUV MEASUREMENTS AND PRODUCTS The three EUVS channels are grating spectrographs. The EUVS-A channel measures solar lines in the wavelength range from 25 to 31 nm, while the EUVS-B measures lines between 117 and 141 nm. Both of these channels have 0.6-nm resolution and are based on irregularly spaced diode arrays where the diodes are positioned in groups to intercept the solar lines of interest, with 3–6 diodes used to produce the irradiance for each line. EUVS-C observes the Sun between 275 and 285 nm with a 512-element diode array and a net 0.1-nm resolution (for spectral sampling of five pixels per resolution element). Most of the data are provided at high resolution and/or 1-min cadences. The Level 1b (L1b) and L2 EUVS operational products discussed in this section include solar line irradiances at high resolution and 1-min averages, solar flare event detection, the Magnesium (Mg) ii index, and the EUVS proxy spectrum. The cadence of the high resolution products depends on the wavelength; it is currently 30 s for EUVS lines, 3 s for Mg ii, and 30 s for the spectrum. The primary solar lines measured by the three EUVS channels are shown in Table 19.1. The specific wavelengths were chosen to monitor the different layers of the Sun’s outer atmosphere so that they can be combined to create a full EUV spectrum of the Sun every 30 s.
19.3.1 EUVS Calibrations and Degradation Tracking Because some optical materials are modified when they absorb light at highly energetic EUV wavelengths, instruments that measure EUV are prone to degradation or changes in calibration with exposure. Most aspects of EUVS,
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TABLE 19.1 Main Solar Lines Measured by EUVS Wavelength (nm)
Line(s)
Source region
25.632, 25.724
He ii, Fe x
Transition region, Corona
28.415
Fe xv
Corona
30.378
He ii
Transition region
117.5
C iii
Chromosphere
121.567
Hi
Transition region
133.57
C ii
Chromosphere
140.5
Si iv, O iv
Transition region
279.5528, 280.2704
Mg ii h, k
Chromosphere
including the instrument, its preflight calibrations, its in-flight degradation tracking, and its ground processing algorithms were designed with this in mind. The EXIS ground processing algorithms to convert raw signals to irradiances have adjustable parameters to incorporate changes as tracked in-flight calibrations. Both XRS and EUVS underwent extensive preflight calibrations at the NIST SURF-III, which is the standard source for ultraviolet light in North America. The facility allows for direct access to the synchrotron radiation without any intervening optics, thus allowing for a primary calibration with the minimal uncertainty possible. Calibrations performed included quantum throughput (signal measured for a known input photon flux) and response variations as functions of pointing, signal level, and temperature. In addition, dark signals, detector gains, and detector flat-fields were also characterized preflight. On orbit, several different methods are used to track changes in EXIS. The XRS and EUVS-A and -B detectors have built-in gain calibration circuitry to measure changes in the gain. Gain and dark signal calibrations are performed weekly for EUVS and quarterly for XRS. Weekly calibrations of the flatfield (i.e., the pixel-to-pixel response of the detector arrays) for all three EUVS channels are done with visible LED stimulus lamps. On a quarterly basis, the SPP performs a cruciform scan whereby it slews the instrument boresight ±4° in the north-south and east-west directions. An FOV map is also performed quarterly, consisting of a 7 × 7 grid of off-points spanning ±15 arcmin from nominal Sun pointing. These maneuvers are used to track changes in the FOV response due to degradation of the nominal Sun-pointing optical paths in the instrument. The EUVS-C readout noise is also measured quarterly. Filter tests are performed frequently on the EUVS-A filter wheel, which has 24 redundant bandpass filters. One filter is designated as the primary filter to be used for normal operations. Once daily, the filter wheel is rotated to make a solar measurement with a designated secondary filter. Once weekly, a tertiary filter is used for a solar measurement. Comparisons of these measurements are used to track the degradation of the filter transmission. If the primary filter degrades too much to make an accurate solar measurement, one of the redundant filters can be designated as the new primary. For EUVS-C, the degradation trend is monitored by tracking trends in the signal levels. As discussed below, EUVS-C degradation has little impact on the Mg ii index. In the unlikely case that the EUVS-C channel becomes unusable, there are actually two completely redundant EUVS-C optical systems, and operation could switch to the second EUVS-C. Both the EUVS-A and -B channels are also corrected via a “bootstrap” degradation tracking methodology built into the design (Eparvier et al., 2009) that is based on Mg ii index measurements.
19.3.2 EUVS Event Detection The onset of flares at EUV wavelengths is of interest because they may precede by several minutes (and so forecast) the onset of X-ray flares. However, at EUV wavelengths, the onsets of flares are harder to detect in real time because EUV flares, with increases in irradiance on the order of 10%, are much smaller than the X-ray flares, which can increase by multiple orders of magnitude. The L2 EUVS event detection algorithm operates at a 1-min cadence and does not use a shape-based trigger like the XRS event detection but instead uses increases in the signals to independently trigger events for each wavelength. Outputs of the algorithm are event start, event peak, event end, postevent, integrated flux, and background flux.
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19.3 EUV Measurements and Products
19.3.3 Magnesium II Index Another new product from GOES-R EUVS is the Mg ii index (Heath and Schlesinger, 1986; Snow et al., 2009), which is an extremely reliable measure of solar activity at ultraviolet wavelengths and is used to monitor long-term and day-to-day solar variability and to model the resulting upper atmospheric changes at Earth. EUVS measures the Mg ii index with better precision and a much higher cadence (3 s) than for any previous satellite. The Mg ii index is calculated from measurements of solar emissions near 280 nm. The Mg ii h and k doublet is formed in the chromosphere and varies considerably with solar activity, while the emissions in the wings are from the photosphere and have little variation. The Mg ii index is the ratio of the irradiance from the line cores to that of the wings. As a ratio instead of an irradiance measurement, the index is more stable relative to degradation or other instrumental artifacts. The Mg ii index is calculated as
(
Mg II index = ( Dh + Dk ) / DBlueWing + DRedWing
)
(19.4)
where Dh and Dk are the signals for the h and k lines as defined by 9-pixel-wide weighted masks while the signals for the wings are defined by trapezoidal 148-pixel-wide masks (Fig. 19.4A). One year of EXIS Mg ii index measurements are shown in Fig. 19.4B. The EXIS Mg ii index is a major input to the proxy model spectrum and is also used to track instrument degradation on EUVS-A and -B. A second data product is the Mg ii index scaled to be the index that would be produced by a 1.1-nm spectral resolution instrument such as described in Viereck et al. (2004). The L2 Mg ii index is provided at cadences of 3 s and 1 min.
19.3.4 EUV Proxy Spectrum The solar EUV spectrum is composed of emission lines and continuua that are formed in different layers of the solar atmosphere, namely, the chromosphere, the transition region, and the corona. See Chapter 18 for a description of the Sun’s atmosphere. Long-term observations of the solar EUV spectrum from the Solar EUV Experiment (SEE) on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite (Woods et al., 2005) show that spectral features that have the same source regions in the solar atmosphere vary similarly. Based on this information, if measurements are made of each type of region, then a full spectrum of the EUV region can be generated from these measurements with an empirical model. This concept of using representative (proxy) wavelengths to determine a full spectrum is the basis for the EUVS L1b Irradiance Model and has been used by a variety of SSI models including (Hinteregger et al., 1981), the Solar2000 model (Tobiska et al., 2000), the Flare Irradiance Spectral Model (FISM, Chamberlin et al., 2007, 2008), a similar model for measurements at Mars (Thiemann et al., 2017), and a model for GOES 13–15 EUVS (Suess et al., 2016).
FIG. 19.4 (A) GOES-16 EUVS-C spectrum (black) and weighting functions for two core lines and two wings in arbitrary units (shaded). (B) Time series of GOES-16 EXIS Mg ii index for 2017.
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19. GOES-R Series Solar X-ray and Ultraviolet Irradiance
The inputs for the EUVS L1b Irradiance Model are the 10 independent EUVS and XRS measurements (2 XRS bands, 7 EUV lines and the Mg ii index), which are produced by specific solar emission features that represent the different types of variability. The model produces solar EUV spectral irradiance continuously at 30-s cadence in 22 5-nm-wide bins from 5 to 115 nm and a single 10-nm-wide bin from 117 to 127 nm. The irradiance Eλ for bin λ at time t is 10
10
i =1
i =1
Eλ ( t ) = Eλ ,0 + ∑ ji ,λ Pi ( t ) + ∑ ki ,λ Qi ( t )
(19.5)
where Eλ,0 is an offset, the sums are over the ten measurements, ji,λ and ki,λ are model coefficients, and Pi(t) and Qi(t) are long- and short-term measurement components. The time-dependent terms represent a daily average and the contribution at a 30-s cadence. The components are defined Pi ( t ) = Qi ( t ) =
Xi ( t )day − Xi , 0 Xi , 0 Xi ( t ) − Xi ( t )day Xi ( t )day
(19.6)
where Xi(t) is the ith EXIS measurement, Xi(t)day is the daily average, and Xi,0 is the reference offset. The EUVS model parameters of Eλ,0, ji,λ and ki,λ were determined using historical data sets for GOES/XRS, Solar Dynamics Observatory/ (SDO) EUV Variability Experiment (EVE) (Woods et al., 2012), Solar Radiation and Climate Experiment (SORCE)/ Solar Stellar Irradiance Comparison Experiment (SOLSTICE; McClintock et al., 2005), and TIMED/SEE as well as the Mg ii composite time series provided by the University of Bremen based on measurements from Global Ozone Monitoring Experiment (GOME; Weber et al., 1998) and Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY (SCHIAMACHY; Bovensmann et al., 1999). The modeling was done for 2002–2013 and gaps were filled with values from FISM. Modeled spectra for the quietest and most active solar irradiance days measured by EXIS at the time of writing are shown in Fig. 19.5. The absorption cross sections of the major species in the thermosphere are also shown for reference. The model uncertainty for daily average irradiances ranges from 1.6% to 5.4% for the model bins. At a 30-s cadence, model predictions of M-class or greater flares are less than 20% for all bins except for the 10–15-nm and 95–100-nm bins, which have uncertainties of 68% and 31.8%, respectively. The data cadence for the proxy spectrum is 30 s in the L1b data and 1 min in the L2 data. In the future, proxy spectra with other bin spacings will be provided. Of special interest to atmospheric modelers are the spectral bins based on the regions of near-constant atmospheric absorption cross sections as defined by Solomon and Qian (2005).
FIG. 19.5 GOES-16 daily average spectra corresponding to days with the highest and lowest 30.4-nm measurements currently available. Superimposed are the absorption cross sections for the major atmospheric species of O, O2, and N2 as provided by Huebner and Mukherjee (2015).
REFERENCES 241
19.4 SUMMARY EXIS provides greatly improved continuous measurements of X-ray and EUV irradiance for better space weather prediction and atmospheric modeling. Relative to its operational predecessors, GOES-R XRS has better dynamic range while GOES-R EUVS has completely new measurements of spectral lines and the Mg ii index data. These improved data allow for continued flare detection, the generation of an EUV proxy spectrum at a 1-min cadence, and a variety of other useful space weather products for operational forecasting and scientific uses. EXIS data and information are available at the NOAA NCEI website for GOES-R space weather data: https:// www.ngdc.noaa.gov/stp/satellite/goes-r.html. Additional documents and user resources can be found at the GOES-R Series website: https://www.goes-r.gov/.
Acknowledgments We thank all members of the LASP EXIS team for their careful design, construction, calibration, and validation of EXIS, especially Andrew Jones. We also thank William Rowland, Margaret Tilton, Courtney Peck, and Vicki Hsu for work on the NCEI processing and archiving. The work at CIRES was supported by the GOES-R Program and NCEI through NOAA Cooperative Agreements NA15OAR4320137 and NA17OAR4320101. The EXIS work at LASP was supported under NASA Contract #NNG07HW00C. GOES-R risk reduction work done by SET and LASP was under the GSA Schedule 871 contract GS23F0195N and NOAA order number RA133W-09-NC-2692.
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