GOES EHIS Heavy Ions

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: