Alpha particles (helium-4 nuclei) are significant contributors to dose rates during solar energetic particle (SEP) events. Real-time measurements of alpha particle fluxes are required for inputs to models serving the aviation, satellite, and human space exploration industries. The GOES 16-19 alpha particle fluxes are measured by the two Solar and Galactic Proton Sensors (SGPS) on each satellite, the same instrument that measures the solar proton fluxes used by SWPC for solar radiation storm alerts. One SGPS looks eastward, and one looks westward in the orbit. The ion fluxes that are incident on the westward-looking telescopes are least attenuated by the shielding effects of the geomagnetic field. The capability of the spacecraft to perform a yaw flip, which results in a switch of the eastward- and westward-looking directions, drives the requirement for two SGPS units on each spacecraft. The proton fluxes from the westward-looking SGPS are used for SWPC’s solar radiation storm alerts.

From each SGPS, eleven alpha particle energy channels are processed (Table 1). The JSON files contain 5-minute averages of the SGPS alpha particle fluxes at all eleven energies from the SGPS unit that looks westward. The files also contain the lower and upper channel energies. An ‘effective’ center energy can be approximated as the geometric mean of the lower and upper energies, though, strictly speaking, this is only correct for a power law exponent of -2. Background levels from galactic cosmic rays have not been removed from these fluxes.

Note: The energy units in the SGPS files are keV, not MeV, because of the way the GOES SGPS requirements were written.

• To convert channel energies from keV to MeV, divide by 1000.
• To convert alpha particle fluxes from α/cm²-s-sr-keV to α/cm²-s-sr-MeV, multiply by 1000.

To convert alpha particle fluxes from α/cm²-s-sr-MeV to α/cm²-s-sr-(MeV/nucleon), multiply by 4.

differential-alphas-6-hour.json

differential-alphas-1-day.json

differential-alphas-3-day.json

differential-alphas-7-day.json

The GOES 16-19 heavy ion fluxes are measured by an Energetic Heavy Ion Sensor (EHIS) on each satellite. EHIS looks radially outward, thus midway between the look directions (eastward and westward) of the two Solar and Galactic Proton Sensors (SGPS) on each satellite. SGPS is the primary GOES 16-19 source for solar energetic proton and alpha particle (helium-4 nucleus) fluxes.

EHIS Level 1b (L1b)

The EHIS Level 1b (L1b) product contains energy spectra of differential number fluxes (particles/cm²-sr-s-(MeV/nucleon)) of hydrogen, helium and heavy ion (carbon through copper) nuclei, consisting of five energy bands. 

Data files also include data quality information that provides an assessment of the differential flux values, including whether they are invalid, and an indication of good or degraded quality. The rationale for the quality assessment is also given. 

For each mass group, heavy ion flux is reported in five energy bands for one angular zone. Similarly, for each element in the elemental group, heavy ion flux is reported in five energy bands for one angular zone. The five energy bands are evenly spaced logarithmically spanning from 10 to 200 MeV per nucleon for the H and He mass groups. For all the mass groups and the elemental group, the energy band bounds are dynamic and included in the product. They are the weighted averages of the Prime and Non-Prime band energies. The Non-Prime and Prime states correspond respectively to when the external scintillator is included and is not included in the logic. Owing to MeV radiation belt electrons in GEO, the scintillator count rates are nearly always high, driving the EHIS units into the Prime state. The energy bands vary by atomic number (Z).

The one angular zone has a central look-angle that is anti-earthward and has a 60 degree conical field of view. The units of measure for the directional differential flux values are particles per second per square centimeter per steradian per (megaelectron volt per nucleon). The Level 1b product has a 5-minute cadence, corresponding to an accumulation of counts over a 5-minute period.

EHIS Level 2 (L2)

The EHIS Level 2 (L2) files contain linear-energy-transfer (LET) spectra of differential number fluxes (particles/cm²-sr-s-(MeV/mg-cm²)) and integral number fluxes (particles/cm²-sr-s) for the nine most abundant species above hydrogen (He, C, N, O, Ne, Mg, Al, Si, and Fe). These spectra are calculated under three thicknesses of Aluminum shielding (50, 100 and 500 mils). The input helium energy spectra are read from SGPS Level-2 5-minute averages because SGPS provides better energy resolution. Also, based on cross-calibrations performed by NCEI, the SGPS alpha particle number flux levels are closer to those reported by the predecessor GOES instruments (Energetic Particle Sensors on GOES-15 and prior satellites) and by instruments on ACE (SIS) and SOHO (ERNE).

EHIS is a small instrument compared to the ACE/Solar Isotope Spectrometer (SIS). (The SIS geometric factor is ~40 cm² sr, while the largest EHIS geometric factor is 2 cm² sr.)  It is designed to measure the largest SEP events observed by GOES without saturation.  The September 2017 and June 2024 SEP events demonstrated that EHIS could measure fluxes of the most abundant heavy ions (including iron) in the lowest-energy channel at 5-minute cadence during an S3 solar radiation storm.  Fluxes are determined from a non-linear fit to a histogram of counts vs. an approximation to atomic number (ZCAL). The effective lower limits of the flux levels are summarized in Table 2 for the June 2024 SEP event.  

Additional Documentation:

GOES R Series Product Definition and Users’ Guide (PUG) Volume 3: Level 1b Products, March 23, 2021, Revision 2.3 - see section 5.4.1 for Energetic Heavy Ions data format

GOES-R SEISS.20 Solar Energetic Particle Event Linear Energy Transfer Algorithm Theoretical Basis Document Version 1.2: