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The GOES-R Space Environment In Situ Suite (SEISS): Measurement of Energetic Particles in Geospace
C H A P T E R
20 The GOES-R Space Environment In Situ Suite (SEISS): Measurement of Energetic Particles in Geospace Brian T. Kress*,†, Juan V. Rodriguez*,†, Terrance G. Onsager‡ NOAA National Centers for Environmental Information, Boulder, CO, United States, †Cooperative Institute for Research in Environmental Sciences (CIRES) at CU, Boulder, CO, United States, ‡NOAA Space Weather Prediction Center (SWPC), Boulder, CO, United States *
20.1 INTRODUCTION The solar wind is a plasma of ions and electrons emitted by the solar corona at speeds of about 400 km/s. The solar wind is deflected by Earth’s magnetic field creating a cavity called the magnetosphere (see Fig. 21.1 in Chapter 21). The plasma properties in the magnetosphere are distinct from those in the solar wind with, in general, lower densities and higher temperatures in the outer magnetosphere than in the solar wind. The sunward boundary of the magnetosphere, called the magnetopause, is typically ~10 Earth radii (RE) sunward of Earth’s dayside, where the solar wind dynamic pressure is balanced by magnetic pressure of the geomagnetic field. During solar active periods when there is high solar wind dynamic pressure, the magnetopause is sometimes compressed inside of geosynchronous orbit. In the nightside portion of the magnetosphere, the geomagnetic field is stretched 100s of RE in the anti-sunward direction. This region is called the magnetotail. Geostationary Operational Environmental Satellites (GOES) orbit within the magnetosphere at geosynchronous altitude at ~6.6 RE. The Space Environment In Situ Suite (SEISS) (Dichter et al., 2015) is a suite of particle detectors flown on the GOES-R Series that measure plasma properties and energetic particle populations at geosynchronous orbit. In contrast to remote sensing performed by the Solar Ultraviolet Imager (SUVI, Chapter 18), and Extreme Ultraviolet and X-ray Irradiance Sensors (EXIS, Chapter 19), SEISS measures the local spacecraft charged-particle environment. Charged particle populations in the magnetosphere are a hazard to spacecraft operations and humans in space. Electrons can cause surface and internal charging to spacecraft components causing electrostatic discharges (ESDs) that damage electronics and interfere with electronic operations. Ions with energies in the 10s of MeV and greater expose humans to harmful levels of radiation, can damage electronics, and cause single event upsets in electronic operations. SEISS data are used by the National Oceanic and Atmospheric Administration (NOAA) National Weather Service (NWS) Space Weather Prediction Center (SWPC) for monitoring the radiation environment in near-Earth geospace (https://www.swpc.noaa.gov/). The National Aeronautics and Space Administration (NASA) Applications Technology Satellite (ATS) series (1966–1974), provided the first opportunities to make in situ energetic particle measurements at geosynchronous altitude (Paulikas et al., 1968; DeForest, 1972). The ATS spacecraft were followed by Synchronous Meteorological Satellites SMS-1 and -2 (1974–1976) and the GOES system (1975–present). The original Space Environment Monitor (SEM) (Grubb, 1975) was flown on SMS-1 and -2 and the first GOES. SEM has undergone a series of enhancements of its capabilities since. The SEM flown aboard GOES-13, -14, and -15 measures electrons from 30 keV to several MeV and protons from 80 keV to 100s of MeV. SEM also measures MeV to GeV alpha particles (helium-4 nuclei). The NOAA workshop on energetic particle measurements for the GOES-R Series, held in October 2002, led to the
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FIG. 20.1 The Space Environment In Situ Suite (SEISS) is composed of five particle sensor units: Low- and High-Energy Magnetospheric Particle Sensors, MPS-LO and MPS-HI; two (east and west facing) Solar and Galactic Proton Sensor (SGPS) units; and an Energetic Heavy Ion Sensor (EHIS). The four distinct sensor units are shown. The red telescope aperture covers seen on MPS-HI and SGPS are removed before flight. Images from GOES-R website https://www.goes-r.gov/spacesegment/seiss.html.
s pecification of a new suite of particle detectors. These specifications included the capability to measure populations that contribute to differential charging of satellite surfaces, surface dose and damage (100 eV–30 keV electrons and ions), and to measure the heavy ion component of solar and galactic cosmic rays (GCRs). SEISS is composed of five particle sensor units shown in Fig. 20.1: an electrostatic analyzer (ESA) for measuring low-energy magnetospheric ions and electrons in 12 angular zones (MPS-LO), a high-energy magnetospheric particle sensor that includes fans of five electron and five proton telescopes (MPS-HI), two Solar and Galactic Proton Sensor (SGPS) units (east and west facing), and an Energetic Heavy Ion Sensor (EHIS). Collectively the SEISS sensors measure electrons with energies from 30 eV to >4 MeV in 26 differential energy channels and one integral channel, protons from 30 eV to >500 MeV in 40 channels, and 28 species of heavy ions (He-Cu) in five energy bands in the 10–200 MeV/nucleon range. Measurement of 30 eV–30 keV particles by MPS-LO and heavy ions by EHIS are new capabilities not previously flown on GOES. SEISS also has increased energy resolution of MeV to 100s of MeV protons. SEISS will fly on four GOES-R Series satellites for a combined mission duration of approximately 20 years. The following sections provide an overview of the individual SEISS instruments, the particle populations they measure, and operational use of the data. This is followed by brief descriptions of the Level 1b (L1b) and Level 2 (L2) processing and data. For more details on the design and calibration of SEISS, please refer to Dichter et al. (2015). Brief summaries of the L1b and L2 data products are provided here. Complete descriptions are given in the L1b Product Definition and Users’ Guide (PUG) and L2 Algorithm Theoretical Basis Documents (ATBDs). The L1b PUG gives an overview of the L1b ground processing system and the complete descriptions of Level 0 (L0) and L1b data. The L2 ATBDs describe the theoretical basis of the algorithms, assumptions and limitations, exception handling, inputs, outputs, and data flow.
20.2 MAGNETOSPHERIC PARTICLE SENSOR—LOW ENERGY (MPS-LO) MPS-LO measures electrons and ions in 15 energy channels between 0.03 and 30 keV. MPS-LO consists of two ESAs based on the Special Sensor J (SSJ/5) flown on the Defense Meteorological Satellite Program. ESAs apply an electric field between two curved surfaces, allowing charged particles in a narrow energy range to reach the detector; the electric field is stepped rapidly (15 energy steps plus a return step in 1 s) to fixed values to provide the energy coverage. During each 1-s sweep, counts are accumulated at each energy for 0.0615 s, and the complete energy-angle distribution of counts is reported for both species every second. MPS-LO has 12 unique zones (viewing directions), each ~15°, spanning 180° in the north–south plane. The outputs of all zones are registered simultaneously on a set of microchannel plates (MCPs). The MCPs also include dark zones for measuring counts from penetrating radiation that are used by the ground processing algorithm to remove backgrounds from the illuminated zones. The pitch angle (angle between particle velocity and local magnetic field direction) associated with each zone is calculated using the magnetic field vector measured by the GOES-R Magnetometer (MAG) described in Chapter 21. MPS-LO measures ions and electrons in the magnetosphere at thermal energies (~1 keV at geosynchronous). Surface charging on spacecraft is associated with <50 keV electrons (NASA, 2011, p. 27). ESD due to spacecraft charging causes most of the environmentally related anomalies on spacecraft, and surface charging has caused the most serious anomalies, that is, those that have resulted in the loss of the mission (Koons et al., 1999). At a given plasma temperature, electron speeds are much greater than ion speeds; thus, in a neutral plasma the electrons impinging on a spacecraft surface far outnumber the ions resulting in a negative current onto the
20.3 Magnetospheric Particle Sensor—High Energy (MPS-HI)
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FIG. 20.2 GOES-16 MPS-LO spectrogram of dead-time-corrected count rates from zone 4, with ion line signatures of spacecraft charging. The horizontal axis units are UTC hours on March 22, 2017. The first ion line enhancement appears near 1 keV at ~05:30–06:30 UTC during an eclipse period. The subsequent ion lines are after the eclipse period at ~09:00–10:30 UTC and ~11:00–12:00 UTC. The afternoon is dominated by contamination from 100s of keV electrons enhanced during the latter part of the day.
spacecraft surface. Alternatively, when the spacecraft is sunlit, negative charge can be removed via the photoelectric effect, whereby electrons are ejected from the spacecraft surface by incident photons of sufficiently high energy (e.g., photon frequencies greater than near-ultraviolet for metals). Additional currents include those due to secondary electrons ejected when ambient particles strike the spacecraft surface and ion and/or electron currents away from the spacecraft during thruster firing. The spacecraft attains a potential that balances these various currents. When GOES-16 is in eclipse and/or during periods of high ambient electron flux, it typically charges to a negative potential. When the spacecraft is negatively charged, ions entering the MPS-LO detector have their energies increased by the magnitude of the spacecraft potential (Φ). When there is a sufficient population of low-energy ambient ions, this produces an “ion line” signature in the MPS-LO data. All ambient ions with energies below Φ are accelerated to energies at and greater than Φ. A typical ion distribution, with greater numbers of ions at lower energies, produces a pronounced peak in flux vs. energy at energies near Φ, and background levels at energies below this. Ion line signatures of negative spacecraft charging are seen in the GOES-16 MPS-LO spectrogram from March 22, 2017 shown in Fig. 20.2. When present, the ion line is used by the SEISS Level 2 density and temperature moments and level of spacecraft charging (henceforth SEISS L2 moments) algorithm to determine the spacecraft potential. Densities and temperatures from the SEISS L2 moments algorithm are input to the magnetometer magnetopause crossing detection algorithm (described in Chapter 21), which uses the electron density-temperature ratio as an indicator of magnetopause crossings at geosynchronous.
20.3 MAGNETOSPHERIC PARTICLE SENSOR—HIGH ENERGY (MPS-HI) MPS-HI consists of five solid-state electron telescopes and five solid-state proton telescopes with 30-deg full angle cylindrical fields of view arranged in a fan in the north–south plane and centered at 0°, ±35°, and ±70°. Each electron telescope reports rates that are converted by the ground processing algorithm into fluxes in 10 differential channels spanning 50–4000 keV plus a >2 MeV integral channel. Each proton telescope reports on-orbit coincidence rates forming 11 differential channels spanning 80–12,000 keV. MPS-HI also includes two dosimeters under hemispherical shields of 100 mils (0.254 cm) and 250 mils (0.635 cm) aluminum. MPS-HI measures electrons at radiation belt energies. Radiation belts are toroidal regions of intense fluxes of energetic ions and electrons trapped in Earth’s magnetic field, surrounding Earth between ~1–7 RE near the geomagnetic equatorial plane, and extending poleward along geomagnetic field lines to the upper atmosphere near ±60° latitude. The radiation belts are separated into two distinct zones, an inner zone (~1–3 RE) consisting mainly of 10–100s of MeV ions, and an outer zone (~3–7 RE) with 100s of keV to several MeV electrons. Radiation belt intensities can vary over several orders of magnitude on timescales of minutes to months. GOES orbit near the outer boundary of the outer electron belts. Radiation belt electrons are a source of charging of internal spacecraft components, which can cause ESDs that degrade dielectric materials in microelectronic devices and interfere with electronic operations, for example, by producing electromagnetic pulses causing spurious commands (Vampola, 1987). ESDs due to internal charging have been found to cause the majority of environmentally related anomalies on spacecraft (Koons et al., 1999). The primary operational use for MPS-HI is to monitor Earth’s outer radiation belts. Five-minute averaged fluxes from the MPS-HI >2 MeV electron channel will be used by NOAA SWPC to issue its real-time radiation belt alerts when the >2 MeV flux exceeds 1000 (electrons/cm2-sr-s).
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FIG. 20.3 Signatures of injections of energetic protons and electrons into the inner magnetosphere on January 19, 2017. Injections at geostationary orbit from Earth’s magnetotail appear as sudden increases in the flux level. Higher energies arrive at the spacecraft’s location first, followed by subsequent pulses called ‘drift echoes,’ as the injected protons and electrons drift around Earth and repeatedly reappear at the spacecraft. The time series shown are from MPS-HI proton P1–P7 channels and electron E1–E7 channels. SGPS+X P1 is also shown in the top panel. Telescopes 2 and 4 are the central proton and electron telescopes, looking radially outward in the orbital (geographic equatorial) plane, nominally directed perpendicular to the magnetic field direction.
Protons with energies in the 100s of keV play an important role in the magnetospheric current systems that drive geomagnetic storms. Perturbation of the geomagnetic field by an enhanced ring current (at ~3–7 RE) is the primary source of geomagnetic storms. Geomagnetic storms induce voltages/currents in power systems that can degrade power grid operations and damage transformers. Geomagnetic storms can also degrade the accuracy of radio navigation systems (e.g., GPS and GNSS). GOES are ideally stationed for making in situ observations of source populations for Earth’s ring current and outer radiation belts. Fig. 20.3 shows signatures of injections of energetic protons and electrons from the magnetotail into the inner magnetosphere that are frequently observed near geosynchronous orbit.
20.4 SOLAR AND GALACTIC PROTON SENSOR (SGPS) There are two SGPS sensor units mounted on each GOES-R Series spacecraft, facing in the spacecraft −X and +X directions. When the spacecraft is not in the yaw-flipped configuration, SGPS-X faces west and SGPS+X faces east. Each SGPS unit has three solid-state (silicon detector) telescopes T1, T2, and T3 for measuring 1–25, 25–80, and 80–500 MeV protons, respectively. All three telescopes have the same look direction (i.e., +X or −X). T1 and T2 have 60° (full cone angle) fields of view, and T3 has a 90° field-of-view. Each unit measures 1–500 MeV proton fluxes in 13 logarithmically spaced differential channels (P1-P10) and >500 proton flux in a single integral channel (P11). Two sources of radiation exposure in space are galactic cosmic rays (GCRs) and solar cosmic rays, also called solar energetic particles (SEPs). Ever-present GCRs come from outside the solar system and are thought to originate from supernovae and galactic nuclei. GCRs consist mainly of ions with energies between ~100 MeV and 10 GeV. SEPs are ions in the 0.1–1 GeV range produced by the Sun during solar active periods, usually in association with a solar flare
20.5 Energetic Heavy Ion Sensor
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FIG. 20.4 Five-minute averaged differential fluxes from GOES-16 SGPS+X on September 10–14, 2017. The time of the X8.2 class solar flare associated with this solar particle event is shown by the vertical dashed line at 16:06 UTC on September 10, 2017.
or coronal mass ejection (CME) and lasting several days. Approximately 50 solar particle events occur per solar cycle with greatly varying degrees of intensity. During a solar particle event, SEP fluxes are typically several orders of magnitude greater than GCR fluxes but have a softer spectrum, with fluxes dropping rapidly to background levels at energies >500 MeV. Both solar and galactic cosmic rays are composed primarily of high-energy protons. While GCR and inner zone radiation belt intensities are relatively steady and predictable, solar particle events are transient and difficult to predict. An example of SEP fluxes observed by GOES-16 SGPS+X during the September 2017 solar particle event is shown in Fig. 20.4. The primary operational use of SGPS is to support real-time alerts of solar energetic proton events, which can cause single-event effects in space electronics (Tylka et al., 1996) and excessive radiation dose in humans in space and at commercial aviation altitudes over the poles (Dyer et al., 2003). During a solar particle event >1 MeV protons penetrate below ∼100 km altitude producing ionization that disrupts high frequency (HF) communication and navigation in the polar regions. Solar particle events also cause solar array degradation. The SWPC Solar Radiation Storm (S) scale according to which these events are classified is based on integral fluxes above 10 MeV derived from the channel fluxes by the SEISS L2 differential-to-integral flux algorithm. The SEISS L2 solar proton event (SPE) detection algorithm is an automated event detection algorithm that operates on integral fluxes.
20.5 ENERGETIC HEAVY ION SENSOR EHIS data are processed in real time as differential ion fluxes for hydrogen (H), helium (He), carbon‑nitrogen‑ oxygen (CNO), neon‑sulfur (Ne-S), and chlorine‑nickel (Cl-Ni) mass groups, and 26 individual species from beryllium (Be) to copper (Cu). The fluxes are produced in five energy bands and reported every 5 min. The five energy bands are spaced logarithmically, spanning 10–200 MeV/nucleon for H and He. The energy range for ions heavier than He is species-dependent, corresponding approximately to the same stopping range as He. The energy coverage ranges from 10 to 192 MeV/nucleon for hydrogen to 39–1067 MeV/nucleon for copper. EHIS has a single 60° (full cone angle) field-of-view directed radially outward from Earth (toward the zenith). Outside of SEP events, EHIS observes GCR fluxes. EHIS is an entirely new GOES operational capability, namely the observation of heavy ions emitted by the Sun during SEP events. Heavy ions in space cause single event upsets in satellite electronics and contribute to ionizing radiation dose. Although solar heavy ions are much less abundant than hydrogen (protons) or helium ions (which EHIS also observes), they are much more likely to cause single-event effects due to their greater linear energy transfer (LET) in matter. EHIS uses an angle detecting inclined sensor (ADIS) detector geometry to determine the angle of incidence and thereby distinguish species by atomic number (Z) (Connell et al., 2001). During a moderate solar particle event, the heavy ion count rates are sufficiently low that the 5-min cadence heavy ion fluxes should not be used if the maximum likelihood fit (to the histogram of counts in fractional Z bins calculated
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FIG. 20.5 EHIS histogram of E1 (lowest energy) channel counts accumulated during the September 10–14, 2017 SEP events. Histogram bins 0–29 correspond to hydrogen ions, bins 30–59 correspond to helium ions, and there are 20 bins per element for Z = 3–29 (Li to Cu). The first five prominent peaks from the right correspond to H, He, C, N, and O. The prominent peaks correspond to the most abundant elements ejected by the Sun during the solar particle event.
on-orbit) reports only an upper limit (best-fit plus one sigma) to the fluxes. In this case, it is necessary to reprocess EHIS L0 data to obtain histograms of sums of counts over the total event period, which can be used to obtain event fluences. In addition to the histograms, EHIS reports a subset of pulse heights, prioritized for heavy ions that can also be analyzed. An event-summed histogram of EHIS protons, helium ions, and heavy ions from the September 2017 solar particle events is shown in Fig. 20.5.
20.6 LEVEL 1B (L1B) PROCESSING AND DATA PRODUCTS SEISS L1b processing includes unpacking L0 data, dead-time correction of raw count rates, out-of-band contamination and/or background removal, and conversion from count rates to differential directional number fluxes. Deadtime parameters, out-of-band removal coefficients, geometric factors, and additional L1b processing parameters are provided by the instrument vendor in a contract data requirements list (CDRL) document and maintained in ground system lookup tables (LUTs) read by the L1b ground processing code. The dead time is the time after each recorded count during which the electronics are not able to process another count. The dead-time correction is applied to correct for in-band counts missed during the dead time. A dead-time correction is applied to all SEISS channels reported in L1b output. Out-of-band counts generally refer to counts from particles not in the intended species, field of view, or energy- range of the channel. Contamination from out-of-band counts and implementation of the out-of-band removal step varies among the SEISS instruments and channels. Some channels have individually tailored out-of-band removal, while others have no out-of-band removal step. At the time of writing, work is ongoing to determine sources of backgrounds and correct values for out-of-band removal coefficients. Count rates are converted to directional-differential number flux using a geometric factor unique to each channel. This step is common to all SEISS channels. L1b data consists mainly of time series of fluxes (#/cm2-sr-s-keV), associated errors, and data quality flags. MPS-LO, MPS-HI, and SGPS L1b fluxes are reported on a 1-s cadence. SEISS L1b output contains time series from a grand total of 804 energy-direction-species channels (including 31 heavy ion elements and mass groups in five energy bands on a 5-min cadence). As with MPS-HI and SGPS, the EHIS H and He fluxes are derived directly from coincidence rates (by EHIS on a 3-s cadence in the raw Level 0 data). The heavy ion fluxes, however, are derived using a maximum likelihood fit to a histogram of counts binned in Z values determined on-orbit (on 1-min cadence) using the ADIS system incorporated into the EHIS telescope (Connell et al., 2001).
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20.8 GOES Data in Support of Space Science Research
MPS-LO L1B
MPS-LO one- and fiveminute averages
1-min averages
Density and temperature moments, pitch angles and level of spacecraft charging
MPS-HI L1B
MPS-HI one- and fiveminute averages
1-min averages
Density and temperature moments and pitch angles
SGPS L1B
SGPS one- and fiveminute averages
1-min averages
1 sec cadence L1B
EHIS L1B
Level 2
5-min averages
Differential to integral flux
Rate of rise
Differential to integral flux
Solar proton event detection
Solar energetic particle event linear energy transfer
FIG. 20.6 SEISS data flow diagram. The moments calculation also ingests magnetometer data, to calculate pitch angles.
20.7 LEVEL 2 (L2) ALGORITHMS AND DATA PRODUCTS A SEISS data flow diagram is shown in Fig. 20.6. The Level 2 algorithms are shown to the right of the vertical dashed line. SEISS L2 one- and five-minute averages are computed in L2 processing. A boxcar averaging of all valid data points within sequential one- or five-minute intervals is performed. Errors associated with the averaged fluxes are computed using RMS sums of L1b 1-s cadence errors. The SEISS L2 density and temperature moments and level of spacecraft charging algorithm calculates the partial densities and temperatures of electrons and ions/protons from 1-min averages of MPS-LO and MPS-HI fluxes. The algorithm uses measurements from the GOES-R Magnetometer to calculate the pitch angles of the particles measured by each zone/telescope. The algorithm also identifies electron and ion signatures of spacecraft surface (frame) charging in MPS-LO fluxes. From these observations, or, in the absence of clear signatures, from the electron temperature, it estimates the spacecraft charging voltage and uses this to correct the measured MPS-LO spectra for the effective change in particle energy prior to calculating the moments. The SEISS L2 differential-to-integral flux algorithm was developed to calculate proton integral flux values using the 5-min differential proton flux averages. The differential-to-integral flux algorithm uses the fundamental physical assumption of a piecewise power law spectrum. Performance comparisons with the GOES I-P legacy algorithm, using theoretical spectra derived from measurements, show that the combination of the new instrument and new algorithm improves performance substantially over the legacy system. The SEISS L2 SPE detection algorithm operates on 5-min-cadence L2 solar proton integral fluxes. It has legacy and new components. The legacy components (currently produced by SWPC) include the S-Scale detection, peak event fluxes, and daily fluences. The new components include event onset and end (at thresholds below the S1 level), which in turn support the calculation of event fluences. Some SPEs rise too rapidly for the event detection algorithm to provide sufficient advance warning. The SEISS L2 rate-of-rise algorithm addresses this capability gap by operating on 1-min-cadence >10 and >100 MeV integral fluxes. It estimates the rate-of-rise using an exponential fit to the 1-min fluxes and uses this rate-of-rise to predict when the event will reach different levels of the S-Scale. The purpose of the SEISS L2 SEP event LET algorithm is to transform energy spectra measured by EHIS into LET (MeV cm2/mg) spectra as a real-time indicator of potential radiation hazards for the satellite community. The L2 SEP event LET algorithm estimates the LET spectrum in silicon after degrading the observed EHIS spectra due to one- dimensional transport through selected spacecraft shielding thicknesses.
20.8 GOES DATA IN SUPPORT OF SPACE SCIENCE RESEARCH As with its predecessors, postoperational data from space weather instruments on the GOES-R Series will be made publicly available at NOAA NCEI and other data services. GOES data provides an important contribution to space science research. A search for 2017 abstracts containing reference to GOES in the Smithsonian Astrophysical
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Observatory/NASA Astrophysics Data System (ADS) yields 172 abstracts: 76 peer-reviewed publications (e.g., 27 in The Astrophysical Journal, 11 in Solar Physics, 5 in Journal of Geophysical Research—Space Physics, etc.), 93 meeting presentations, two ArXiv e-prints, and one PhD thesis. In addition to providing operational support for space weather monitoring and prediction at SWPC, the space weather instruments on the GOES-R Series will continue to provide valuable data sets to the space science community, advancing basic space science research, first-principles space weather modeling and prediction, and understanding of space weather effects on satellite systems. GOES-R space weather product user guides and additional information are at the NOAA National Centers for Environmental Information website 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 The authors thank the SEISS team at Assurance Technology Corporation (ATC) for their work on design, assembly, testing, calibration, and operations support of the SEISS instruments. The Authors also thank the NOAA GOES-R Program. The work at CIRES was supported by the GOES-R Program and the National Centers for Environmental Information (NCEI) through NOAA Cooperative Agreements NA15OAR4320137 and NA17OAR4320101. The views, opinions, and findings contained in this report are those of the authors and should not be construed as an official National Oceanic and Atmospheric Administration or US Government position, policy, or decision.
References Connell, J.J., Lopate, C., McKibben, R.B., 2001. The angle detecting inclined sensors (ADIS) system: measuring particle angles of incidence without position sensing detectors. Nucl. Instrum. Methods 457 (1–2), 220–229. DeForest, S.E., 1972. Spacecraft charging at synchronous orbit. J. Geophys. Res. 27, 651–659, 1972. Dichter, B.K., Galica, G.E., McGarity, J.O., Tsui, S., Golightly, M.J., Lopate, C., Connell, J.J., 2015. Specification design and calibration of the space weather suite of instruments on the NOAA GOES-R program spacecraft. IEEE Trans. Nuclear Science 62 (6), 2776–2783. https://doi. org/10.1109/TNS.2015.2477997. Dyer, C.S., Lei, F., Clucas, S.N., Smart, D.F., Shea, M.A., 2003. Calculations and observations of solar particle enhancements to the radiation environment at aircraft altitudes. Adv. Space Res. 32 (1), 81–93. Grubb, R.N., 1975. The SMS/GOES Space Environment Monitor Subsystem. NOAA Technical Memorandum ERL SEL-42, Space Environment Laboratory, Boulder, Colorado. Koons, H.C., Mazur, J.E., Selesnick, R.S., Blake, J.B., Fennell, J.F., Roeder, J.L., Anderson, P.C., 1999. The Impact of Space Environment on Space Systems. The Aerospace Corporation, El Segundo, CA. TR-99(1670)-1. NASA, 2011. Mitigating in-space charging effects—a guideline. In: NASA Technical Handbook NASA-HDBK-4002A, March 3. Paulikas, G.A., Blake, J.B., Freden, S.C., Imanoto, S.S., 1968. Low-energy cosmic-ray cutoffs: Diurnal variation and pitch-angle distributions. J. Geophys. Res. 73 (87), 1968. Tylka, A.J., William, W.F., Dietrich, P.R., Boberg, E.C., Edward, C., Smith, J.H., Adams, J.H., 1996. Single event upsets caused by solar energetic heavy ions. IEEE Trans. Nucl. Sci. 43 (6), 2758–2766. Vampola, A.L., 1987. Thick dielectric charging on high-altitude spacecraft. J. Electrost. 20, 21–30.
20 The GOES-R Space Environment In Situ Suite (SEISS): Measurement of Energetic Particles in Geospace Brian T. Kress*,†, Juan V. Rodriguez*,†, Terrance G. Onsager‡ NOAA National Centers for Environmental Information, Boulder, CO, United States, †Cooperative Institute for Research in Environmental Sciences (CIRES) at CU, Boulder, CO, United States, ‡NOAA Space Weather Prediction Center (SWPC), Boulder, CO, United States *
20.1 INTRODUCTION The solar wind is a plasma of ions and electrons emitted by the solar corona at speeds of about 400 km/s. The solar wind is deflected by Earth’s magnetic field creating a cavity called the magnetosphere (see Fig. 21.1 in Chapter 21). The plasma properties in the magnetosphere are distinct from those in the solar wind with, in general, lower densities and higher temperatures in the outer magnetosphere than in the solar wind. The sunward boundary of the magnetosphere, called the magnetopause, is typically ~10 Earth radii (RE) sunward of Earth’s dayside, where the solar wind dynamic pressure is balanced by magnetic pressure of the geomagnetic field. During solar active periods when there is high solar wind dynamic pressure, the magnetopause is sometimes compressed inside of geosynchronous orbit. In the nightside portion of the magnetosphere, the geomagnetic field is stretched 100s of RE in the anti-sunward direction. This region is called the magnetotail. Geostationary Operational Environmental Satellites (GOES) orbit within the magnetosphere at geosynchronous altitude at ~6.6 RE. The Space Environment In Situ Suite (SEISS) (Dichter et al., 2015) is a suite of particle detectors flown on the GOES-R Series that measure plasma properties and energetic particle populations at geosynchronous orbit. In contrast to remote sensing performed by the Solar Ultraviolet Imager (SUVI, Chapter 18), and Extreme Ultraviolet and X-ray Irradiance Sensors (EXIS, Chapter 19), SEISS measures the local spacecraft charged-particle environment. Charged particle populations in the magnetosphere are a hazard to spacecraft operations and humans in space. Electrons can cause surface and internal charging to spacecraft components causing electrostatic discharges (ESDs) that damage electronics and interfere with electronic operations. Ions with energies in the 10s of MeV and greater expose humans to harmful levels of radiation, can damage electronics, and cause single event upsets in electronic operations. SEISS data are used by the National Oceanic and Atmospheric Administration (NOAA) National Weather Service (NWS) Space Weather Prediction Center (SWPC) for monitoring the radiation environment in near-Earth geospace (https://www.swpc.noaa.gov/). The National Aeronautics and Space Administration (NASA) Applications Technology Satellite (ATS) series (1966–1974), provided the first opportunities to make in situ energetic particle measurements at geosynchronous altitude (Paulikas et al., 1968; DeForest, 1972). The ATS spacecraft were followed by Synchronous Meteorological Satellites SMS-1 and -2 (1974–1976) and the GOES system (1975–present). The original Space Environment Monitor (SEM) (Grubb, 1975) was flown on SMS-1 and -2 and the first GOES. SEM has undergone a series of enhancements of its capabilities since. The SEM flown aboard GOES-13, -14, and -15 measures electrons from 30 keV to several MeV and protons from 80 keV to 100s of MeV. SEM also measures MeV to GeV alpha particles (helium-4 nuclei). The NOAA workshop on energetic particle measurements for the GOES-R Series, held in October 2002, led to the
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20. The GOES-R Space Environment In Situ Suite (SEISS): Measurement of Energetic Particles in Geospace
FIG. 20.1 The Space Environment In Situ Suite (SEISS) is composed of five particle sensor units: Low- and High-Energy Magnetospheric Particle Sensors, MPS-LO and MPS-HI; two (east and west facing) Solar and Galactic Proton Sensor (SGPS) units; and an Energetic Heavy Ion Sensor (EHIS). The four distinct sensor units are shown. The red telescope aperture covers seen on MPS-HI and SGPS are removed before flight. Images from GOES-R website https://www.goes-r.gov/spacesegment/seiss.html.
s pecification of a new suite of particle detectors. These specifications included the capability to measure populations that contribute to differential charging of satellite surfaces, surface dose and damage (100 eV–30 keV electrons and ions), and to measure the heavy ion component of solar and galactic cosmic rays (GCRs). SEISS is composed of five particle sensor units shown in Fig. 20.1: an electrostatic analyzer (ESA) for measuring low-energy magnetospheric ions and electrons in 12 angular zones (MPS-LO), a high-energy magnetospheric particle sensor that includes fans of five electron and five proton telescopes (MPS-HI), two Solar and Galactic Proton Sensor (SGPS) units (east and west facing), and an Energetic Heavy Ion Sensor (EHIS). Collectively the SEISS sensors measure electrons with energies from 30 eV to >4 MeV in 26 differential energy channels and one integral channel, protons from 30 eV to >500 MeV in 40 channels, and 28 species of heavy ions (He-Cu) in five energy bands in the 10–200 MeV/nucleon range. Measurement of 30 eV–30 keV particles by MPS-LO and heavy ions by EHIS are new capabilities not previously flown on GOES. SEISS also has increased energy resolution of MeV to 100s of MeV protons. SEISS will fly on four GOES-R Series satellites for a combined mission duration of approximately 20 years. The following sections provide an overview of the individual SEISS instruments, the particle populations they measure, and operational use of the data. This is followed by brief descriptions of the Level 1b (L1b) and Level 2 (L2) processing and data. For more details on the design and calibration of SEISS, please refer to Dichter et al. (2015). Brief summaries of the L1b and L2 data products are provided here. Complete descriptions are given in the L1b Product Definition and Users’ Guide (PUG) and L2 Algorithm Theoretical Basis Documents (ATBDs). The L1b PUG gives an overview of the L1b ground processing system and the complete descriptions of Level 0 (L0) and L1b data. The L2 ATBDs describe the theoretical basis of the algorithms, assumptions and limitations, exception handling, inputs, outputs, and data flow.
20.2 MAGNETOSPHERIC PARTICLE SENSOR—LOW ENERGY (MPS-LO) MPS-LO measures electrons and ions in 15 energy channels between 0.03 and 30 keV. MPS-LO consists of two ESAs based on the Special Sensor J (SSJ/5) flown on the Defense Meteorological Satellite Program. ESAs apply an electric field between two curved surfaces, allowing charged particles in a narrow energy range to reach the detector; the electric field is stepped rapidly (15 energy steps plus a return step in 1 s) to fixed values to provide the energy coverage. During each 1-s sweep, counts are accumulated at each energy for 0.0615 s, and the complete energy-angle distribution of counts is reported for both species every second. MPS-LO has 12 unique zones (viewing directions), each ~15°, spanning 180° in the north–south plane. The outputs of all zones are registered simultaneously on a set of microchannel plates (MCPs). The MCPs also include dark zones for measuring counts from penetrating radiation that are used by the ground processing algorithm to remove backgrounds from the illuminated zones. The pitch angle (angle between particle velocity and local magnetic field direction) associated with each zone is calculated using the magnetic field vector measured by the GOES-R Magnetometer (MAG) described in Chapter 21. MPS-LO measures ions and electrons in the magnetosphere at thermal energies (~1 keV at geosynchronous). Surface charging on spacecraft is associated with <50 keV electrons (NASA, 2011, p. 27). ESD due to spacecraft charging causes most of the environmentally related anomalies on spacecraft, and surface charging has caused the most serious anomalies, that is, those that have resulted in the loss of the mission (Koons et al., 1999). At a given plasma temperature, electron speeds are much greater than ion speeds; thus, in a neutral plasma the electrons impinging on a spacecraft surface far outnumber the ions resulting in a negative current onto the
20.3 Magnetospheric Particle Sensor—High Energy (MPS-HI)
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FIG. 20.2 GOES-16 MPS-LO spectrogram of dead-time-corrected count rates from zone 4, with ion line signatures of spacecraft charging. The horizontal axis units are UTC hours on March 22, 2017. The first ion line enhancement appears near 1 keV at ~05:30–06:30 UTC during an eclipse period. The subsequent ion lines are after the eclipse period at ~09:00–10:30 UTC and ~11:00–12:00 UTC. The afternoon is dominated by contamination from 100s of keV electrons enhanced during the latter part of the day.
spacecraft surface. Alternatively, when the spacecraft is sunlit, negative charge can be removed via the photoelectric effect, whereby electrons are ejected from the spacecraft surface by incident photons of sufficiently high energy (e.g., photon frequencies greater than near-ultraviolet for metals). Additional currents include those due to secondary electrons ejected when ambient particles strike the spacecraft surface and ion and/or electron currents away from the spacecraft during thruster firing. The spacecraft attains a potential that balances these various currents. When GOES-16 is in eclipse and/or during periods of high ambient electron flux, it typically charges to a negative potential. When the spacecraft is negatively charged, ions entering the MPS-LO detector have their energies increased by the magnitude of the spacecraft potential (Φ). When there is a sufficient population of low-energy ambient ions, this produces an “ion line” signature in the MPS-LO data. All ambient ions with energies below Φ are accelerated to energies at and greater than Φ. A typical ion distribution, with greater numbers of ions at lower energies, produces a pronounced peak in flux vs. energy at energies near Φ, and background levels at energies below this. Ion line signatures of negative spacecraft charging are seen in the GOES-16 MPS-LO spectrogram from March 22, 2017 shown in Fig. 20.2. When present, the ion line is used by the SEISS Level 2 density and temperature moments and level of spacecraft charging (henceforth SEISS L2 moments) algorithm to determine the spacecraft potential. Densities and temperatures from the SEISS L2 moments algorithm are input to the magnetometer magnetopause crossing detection algorithm (described in Chapter 21), which uses the electron density-temperature ratio as an indicator of magnetopause crossings at geosynchronous.
20.3 MAGNETOSPHERIC PARTICLE SENSOR—HIGH ENERGY (MPS-HI) MPS-HI consists of five solid-state electron telescopes and five solid-state proton telescopes with 30-deg full angle cylindrical fields of view arranged in a fan in the north–south plane and centered at 0°, ±35°, and ±70°. Each electron telescope reports rates that are converted by the ground processing algorithm into fluxes in 10 differential channels spanning 50–4000 keV plus a >2 MeV integral channel. Each proton telescope reports on-orbit coincidence rates forming 11 differential channels spanning 80–12,000 keV. MPS-HI also includes two dosimeters under hemispherical shields of 100 mils (0.254 cm) and 250 mils (0.635 cm) aluminum. MPS-HI measures electrons at radiation belt energies. Radiation belts are toroidal regions of intense fluxes of energetic ions and electrons trapped in Earth’s magnetic field, surrounding Earth between ~1–7 RE near the geomagnetic equatorial plane, and extending poleward along geomagnetic field lines to the upper atmosphere near ±60° latitude. The radiation belts are separated into two distinct zones, an inner zone (~1–3 RE) consisting mainly of 10–100s of MeV ions, and an outer zone (~3–7 RE) with 100s of keV to several MeV electrons. Radiation belt intensities can vary over several orders of magnitude on timescales of minutes to months. GOES orbit near the outer boundary of the outer electron belts. Radiation belt electrons are a source of charging of internal spacecraft components, which can cause ESDs that degrade dielectric materials in microelectronic devices and interfere with electronic operations, for example, by producing electromagnetic pulses causing spurious commands (Vampola, 1987). ESDs due to internal charging have been found to cause the majority of environmentally related anomalies on spacecraft (Koons et al., 1999). The primary operational use for MPS-HI is to monitor Earth’s outer radiation belts. Five-minute averaged fluxes from the MPS-HI >2 MeV electron channel will be used by NOAA SWPC to issue its real-time radiation belt alerts when the >2 MeV flux exceeds 1000 (electrons/cm2-sr-s).
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20. The GOES-R Space Environment In Situ Suite (SEISS): Measurement of Energetic Particles in Geospace
FIG. 20.3 Signatures of injections of energetic protons and electrons into the inner magnetosphere on January 19, 2017. Injections at geostationary orbit from Earth’s magnetotail appear as sudden increases in the flux level. Higher energies arrive at the spacecraft’s location first, followed by subsequent pulses called ‘drift echoes,’ as the injected protons and electrons drift around Earth and repeatedly reappear at the spacecraft. The time series shown are from MPS-HI proton P1–P7 channels and electron E1–E7 channels. SGPS+X P1 is also shown in the top panel. Telescopes 2 and 4 are the central proton and electron telescopes, looking radially outward in the orbital (geographic equatorial) plane, nominally directed perpendicular to the magnetic field direction.
Protons with energies in the 100s of keV play an important role in the magnetospheric current systems that drive geomagnetic storms. Perturbation of the geomagnetic field by an enhanced ring current (at ~3–7 RE) is the primary source of geomagnetic storms. Geomagnetic storms induce voltages/currents in power systems that can degrade power grid operations and damage transformers. Geomagnetic storms can also degrade the accuracy of radio navigation systems (e.g., GPS and GNSS). GOES are ideally stationed for making in situ observations of source populations for Earth’s ring current and outer radiation belts. Fig. 20.3 shows signatures of injections of energetic protons and electrons from the magnetotail into the inner magnetosphere that are frequently observed near geosynchronous orbit.
20.4 SOLAR AND GALACTIC PROTON SENSOR (SGPS) There are two SGPS sensor units mounted on each GOES-R Series spacecraft, facing in the spacecraft −X and +X directions. When the spacecraft is not in the yaw-flipped configuration, SGPS-X faces west and SGPS+X faces east. Each SGPS unit has three solid-state (silicon detector) telescopes T1, T2, and T3 for measuring 1–25, 25–80, and 80–500 MeV protons, respectively. All three telescopes have the same look direction (i.e., +X or −X). T1 and T2 have 60° (full cone angle) fields of view, and T3 has a 90° field-of-view. Each unit measures 1–500 MeV proton fluxes in 13 logarithmically spaced differential channels (P1-P10) and >500 proton flux in a single integral channel (P11). Two sources of radiation exposure in space are galactic cosmic rays (GCRs) and solar cosmic rays, also called solar energetic particles (SEPs). Ever-present GCRs come from outside the solar system and are thought to originate from supernovae and galactic nuclei. GCRs consist mainly of ions with energies between ~100 MeV and 10 GeV. SEPs are ions in the 0.1–1 GeV range produced by the Sun during solar active periods, usually in association with a solar flare
20.5 Energetic Heavy Ion Sensor
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FIG. 20.4 Five-minute averaged differential fluxes from GOES-16 SGPS+X on September 10–14, 2017. The time of the X8.2 class solar flare associated with this solar particle event is shown by the vertical dashed line at 16:06 UTC on September 10, 2017.
or coronal mass ejection (CME) and lasting several days. Approximately 50 solar particle events occur per solar cycle with greatly varying degrees of intensity. During a solar particle event, SEP fluxes are typically several orders of magnitude greater than GCR fluxes but have a softer spectrum, with fluxes dropping rapidly to background levels at energies >500 MeV. Both solar and galactic cosmic rays are composed primarily of high-energy protons. While GCR and inner zone radiation belt intensities are relatively steady and predictable, solar particle events are transient and difficult to predict. An example of SEP fluxes observed by GOES-16 SGPS+X during the September 2017 solar particle event is shown in Fig. 20.4. The primary operational use of SGPS is to support real-time alerts of solar energetic proton events, which can cause single-event effects in space electronics (Tylka et al., 1996) and excessive radiation dose in humans in space and at commercial aviation altitudes over the poles (Dyer et al., 2003). During a solar particle event >1 MeV protons penetrate below ∼100 km altitude producing ionization that disrupts high frequency (HF) communication and navigation in the polar regions. Solar particle events also cause solar array degradation. The SWPC Solar Radiation Storm (S) scale according to which these events are classified is based on integral fluxes above 10 MeV derived from the channel fluxes by the SEISS L2 differential-to-integral flux algorithm. The SEISS L2 solar proton event (SPE) detection algorithm is an automated event detection algorithm that operates on integral fluxes.
20.5 ENERGETIC HEAVY ION SENSOR EHIS data are processed in real time as differential ion fluxes for hydrogen (H), helium (He), carbon‑nitrogen‑ oxygen (CNO), neon‑sulfur (Ne-S), and chlorine‑nickel (Cl-Ni) mass groups, and 26 individual species from beryllium (Be) to copper (Cu). The fluxes are produced in five energy bands and reported every 5 min. The five energy bands are spaced logarithmically, spanning 10–200 MeV/nucleon for H and He. The energy range for ions heavier than He is species-dependent, corresponding approximately to the same stopping range as He. The energy coverage ranges from 10 to 192 MeV/nucleon for hydrogen to 39–1067 MeV/nucleon for copper. EHIS has a single 60° (full cone angle) field-of-view directed radially outward from Earth (toward the zenith). Outside of SEP events, EHIS observes GCR fluxes. EHIS is an entirely new GOES operational capability, namely the observation of heavy ions emitted by the Sun during SEP events. Heavy ions in space cause single event upsets in satellite electronics and contribute to ionizing radiation dose. Although solar heavy ions are much less abundant than hydrogen (protons) or helium ions (which EHIS also observes), they are much more likely to cause single-event effects due to their greater linear energy transfer (LET) in matter. EHIS uses an angle detecting inclined sensor (ADIS) detector geometry to determine the angle of incidence and thereby distinguish species by atomic number (Z) (Connell et al., 2001). During a moderate solar particle event, the heavy ion count rates are sufficiently low that the 5-min cadence heavy ion fluxes should not be used if the maximum likelihood fit (to the histogram of counts in fractional Z bins calculated
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20. The GOES-R Space Environment In Situ Suite (SEISS): Measurement of Energetic Particles in Geospace
FIG. 20.5 EHIS histogram of E1 (lowest energy) channel counts accumulated during the September 10–14, 2017 SEP events. Histogram bins 0–29 correspond to hydrogen ions, bins 30–59 correspond to helium ions, and there are 20 bins per element for Z = 3–29 (Li to Cu). The first five prominent peaks from the right correspond to H, He, C, N, and O. The prominent peaks correspond to the most abundant elements ejected by the Sun during the solar particle event.
on-orbit) reports only an upper limit (best-fit plus one sigma) to the fluxes. In this case, it is necessary to reprocess EHIS L0 data to obtain histograms of sums of counts over the total event period, which can be used to obtain event fluences. In addition to the histograms, EHIS reports a subset of pulse heights, prioritized for heavy ions that can also be analyzed. An event-summed histogram of EHIS protons, helium ions, and heavy ions from the September 2017 solar particle events is shown in Fig. 20.5.
20.6 LEVEL 1B (L1B) PROCESSING AND DATA PRODUCTS SEISS L1b processing includes unpacking L0 data, dead-time correction of raw count rates, out-of-band contamination and/or background removal, and conversion from count rates to differential directional number fluxes. Deadtime parameters, out-of-band removal coefficients, geometric factors, and additional L1b processing parameters are provided by the instrument vendor in a contract data requirements list (CDRL) document and maintained in ground system lookup tables (LUTs) read by the L1b ground processing code. The dead time is the time after each recorded count during which the electronics are not able to process another count. The dead-time correction is applied to correct for in-band counts missed during the dead time. A dead-time correction is applied to all SEISS channels reported in L1b output. Out-of-band counts generally refer to counts from particles not in the intended species, field of view, or energy- range of the channel. Contamination from out-of-band counts and implementation of the out-of-band removal step varies among the SEISS instruments and channels. Some channels have individually tailored out-of-band removal, while others have no out-of-band removal step. At the time of writing, work is ongoing to determine sources of backgrounds and correct values for out-of-band removal coefficients. Count rates are converted to directional-differential number flux using a geometric factor unique to each channel. This step is common to all SEISS channels. L1b data consists mainly of time series of fluxes (#/cm2-sr-s-keV), associated errors, and data quality flags. MPS-LO, MPS-HI, and SGPS L1b fluxes are reported on a 1-s cadence. SEISS L1b output contains time series from a grand total of 804 energy-direction-species channels (including 31 heavy ion elements and mass groups in five energy bands on a 5-min cadence). As with MPS-HI and SGPS, the EHIS H and He fluxes are derived directly from coincidence rates (by EHIS on a 3-s cadence in the raw Level 0 data). The heavy ion fluxes, however, are derived using a maximum likelihood fit to a histogram of counts binned in Z values determined on-orbit (on 1-min cadence) using the ADIS system incorporated into the EHIS telescope (Connell et al., 2001).
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20.8 GOES Data in Support of Space Science Research
MPS-LO L1B
MPS-LO one- and fiveminute averages
1-min averages
Density and temperature moments, pitch angles and level of spacecraft charging
MPS-HI L1B
MPS-HI one- and fiveminute averages
1-min averages
Density and temperature moments and pitch angles
SGPS L1B
SGPS one- and fiveminute averages
1-min averages
1 sec cadence L1B
EHIS L1B
Level 2
5-min averages
Differential to integral flux
Rate of rise
Differential to integral flux
Solar proton event detection
Solar energetic particle event linear energy transfer
FIG. 20.6 SEISS data flow diagram. The moments calculation also ingests magnetometer data, to calculate pitch angles.
20.7 LEVEL 2 (L2) ALGORITHMS AND DATA PRODUCTS A SEISS data flow diagram is shown in Fig. 20.6. The Level 2 algorithms are shown to the right of the vertical dashed line. SEISS L2 one- and five-minute averages are computed in L2 processing. A boxcar averaging of all valid data points within sequential one- or five-minute intervals is performed. Errors associated with the averaged fluxes are computed using RMS sums of L1b 1-s cadence errors. The SEISS L2 density and temperature moments and level of spacecraft charging algorithm calculates the partial densities and temperatures of electrons and ions/protons from 1-min averages of MPS-LO and MPS-HI fluxes. The algorithm uses measurements from the GOES-R Magnetometer to calculate the pitch angles of the particles measured by each zone/telescope. The algorithm also identifies electron and ion signatures of spacecraft surface (frame) charging in MPS-LO fluxes. From these observations, or, in the absence of clear signatures, from the electron temperature, it estimates the spacecraft charging voltage and uses this to correct the measured MPS-LO spectra for the effective change in particle energy prior to calculating the moments. The SEISS L2 differential-to-integral flux algorithm was developed to calculate proton integral flux values using the 5-min differential proton flux averages. The differential-to-integral flux algorithm uses the fundamental physical assumption of a piecewise power law spectrum. Performance comparisons with the GOES I-P legacy algorithm, using theoretical spectra derived from measurements, show that the combination of the new instrument and new algorithm improves performance substantially over the legacy system. The SEISS L2 SPE detection algorithm operates on 5-min-cadence L2 solar proton integral fluxes. It has legacy and new components. The legacy components (currently produced by SWPC) include the S-Scale detection, peak event fluxes, and daily fluences. The new components include event onset and end (at thresholds below the S1 level), which in turn support the calculation of event fluences. Some SPEs rise too rapidly for the event detection algorithm to provide sufficient advance warning. The SEISS L2 rate-of-rise algorithm addresses this capability gap by operating on 1-min-cadence >10 and >100 MeV integral fluxes. It estimates the rate-of-rise using an exponential fit to the 1-min fluxes and uses this rate-of-rise to predict when the event will reach different levels of the S-Scale. The purpose of the SEISS L2 SEP event LET algorithm is to transform energy spectra measured by EHIS into LET (MeV cm2/mg) spectra as a real-time indicator of potential radiation hazards for the satellite community. The L2 SEP event LET algorithm estimates the LET spectrum in silicon after degrading the observed EHIS spectra due to one- dimensional transport through selected spacecraft shielding thicknesses.
20.8 GOES DATA IN SUPPORT OF SPACE SCIENCE RESEARCH As with its predecessors, postoperational data from space weather instruments on the GOES-R Series will be made publicly available at NOAA NCEI and other data services. GOES data provides an important contribution to space science research. A search for 2017 abstracts containing reference to GOES in the Smithsonian Astrophysical
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Observatory/NASA Astrophysics Data System (ADS) yields 172 abstracts: 76 peer-reviewed publications (e.g., 27 in The Astrophysical Journal, 11 in Solar Physics, 5 in Journal of Geophysical Research—Space Physics, etc.), 93 meeting presentations, two ArXiv e-prints, and one PhD thesis. In addition to providing operational support for space weather monitoring and prediction at SWPC, the space weather instruments on the GOES-R Series will continue to provide valuable data sets to the space science community, advancing basic space science research, first-principles space weather modeling and prediction, and understanding of space weather effects on satellite systems. GOES-R space weather product user guides and additional information are at the NOAA National Centers for Environmental Information website 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 The authors thank the SEISS team at Assurance Technology Corporation (ATC) for their work on design, assembly, testing, calibration, and operations support of the SEISS instruments. The Authors also thank the NOAA GOES-R Program. The work at CIRES was supported by the GOES-R Program and the National Centers for Environmental Information (NCEI) through NOAA Cooperative Agreements NA15OAR4320137 and NA17OAR4320101. The views, opinions, and findings contained in this report are those of the authors and should not be construed as an official National Oceanic and Atmospheric Administration or US Government position, policy, or decision.
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