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GOES 1–15 Space Weather Instruments

The Geostationary Operational Environmental Satellites (GOES) are a series of key satellites NOAA uses to monitor weather and space weather since 1975. GOES superseded the Synchronous Meteorological Satellite (SMS) series that was in operation from 1974–1978. GOES program operated as a two-satellite constellation in GEO above the equator, and between 1975 and 2020, relied on observations from its first 15 launched satellites. GOES 1-15 satellites carried a Space Environment Monitor (SEM) with multiple series of space-weather instruments for measuring particles, the magnetic field, solar irradiance, and for imaging the Sun. GOES-16 (the 1st satellite of the current GOES-R series) was launched in 2016 carrying new space weather instruments, enabling continuity of  GOES 1-15 observations of the Sun’s activity and consequent space weather phenomena covering more than 4 solar cycles.

For questions about the data, use the contact info in the relevant readme files, or email NCEI.info@noaa.gov.

Geosynchronous satellites have an unobstructed view of the Sun for all but a few dozen hours per year when Earth Eclipses the Sun. These intervals appear as gaps of up to 80 minutes in the irradiance and imaging data for about six weeks around the time of the equinoxes.  Please read the available documentation. Be alert for 'anomalies' in the data, and attempt to corroborate them with other sources before assuming they accurately represent the space environment. 

Space Environment Monitor (SEM)

The Space Environment Monitor (SEM) instrument suite was a set of space weather instruments on the SMS and GOES 1–15 satellites. At various times, the instruments aboard these satellites included:  

  • Electron, Proton, and Alpha Detector (EPEAD)
  • Energetic Particle Sensor (EPS)
  • Extreme UltraViolet Sensor (EUVS)
  • High Energy Proton and Alpha Detector (HEPAD)
  • MAGnetometer (MAG)
  • MAGnetospheric Electron Detector (MAGED)
  • MAGnetospheric Proton Detector (MAGPD)
  • Solar X-ray Imager (SXI) 
  • X-ray Sensors (XRS)

GOES 1-15 (and SMS-1 and -2) satellites have provided magnetometer, energetic particles, and soft X-ray data continuously since July 1974. The Solar X-ray Imager (SXI) was introduced on GOES-12 (2001) and provided, for the first time, operational Sun’s full disk images (available until the end of GOES-15 operations). Extreme Ultraviolet Sensors (EUVS) were introduced on GOES-13 (2006), providing complementary solar irradiance measurements to the traditional GOES X-ray sensors (XRS). Energetic particles data from the High Energy Proton and Alpha Detector (HEPAD) is available starting with GOES-6 (1983). The Magnetospheric Electron Detectors (MAGED) and Proton Detectors (MAGPD) were introduced on GOES-13, and the Energetic Particle Sensors (EPS) were renamed Energetic Proton, Electron and Alpha Detectors (EPEAD)

SEM ephemeris and data availability
Satellite Overview Launch Date First Data Last Data Available Instruments Ephemeris
SMS-1 1974-05-17 1974-07 1975-10 EPS, MAG, XRS  
SMS-2 1975-02-06 1975-02 1978-03 EPS, MAG, XRS  
GOES-1 1975-10-16 1976-01 1978-05 EPS, MAG, XRS  
GOES-2 1977-06-16 1977-08 1983-05 EPS, MAG, XRS  
GOES-3 1978-06-16 1978-07 1980-08 EPS, MAG, XRS  
GOES-4 1980-09-09 System Failure n/a none  
GOES-5 1981-05-22 1983-01 1987-02 EPS, MAG, XRS  
GOES-6 1983-04-22 1983-05 1994-11 EPS, MAG, HEPAD, XRS 1-min ephemeris
GOES-7 1987-02-26 1987-03 1996-08 EPS, MAG, XRS 1-min ephemeris
GOES-8 1994-04-13 1995-01 2003-06 EPS, MAG, HEPAD, XRS 1-min ephemeris
GOES-9 1995-05-23 1996-04 1998-07 EPS, MAG, HEPAD, XRS 1-min ephemeris
GOES-10 1997-04-25 1998-07 2009-12 EPS, MAG, HEPAD, XRS 1-min ephemeris
GOES-11 2000-05-03 2000-07 2011-02 EPS, MAG, HEPAD, XRS 1-min ephemeris
GOES-12 2001-07-23 2003-01 2010-08 EPS, MAG, HEPAD, XRS 1-min ephemeris
GOES-13 2006-05-24 2010-05 2017-12 EUVS, MAG, XRS, EPEAD, HEPAD, MAGED, MAGPD 1-min ephemeris
GOES-14 2009-06-27 2010-01 2020-03 EUVS, MAG, XRS, EPEAD, HEPAD, MAGED, MAGPD 1-min ephemeris

GOES-15   

 

2010-03-04 2010-09 2020-03 EUVS, MAG, XRS, EPEAD, HEPAD, MAGED, MAGPD 1-min ephemeris

NOAA geostationary satellites have made operational measurements of X-ray solar irradiance from since 1974 (starting with SMS-1 satellite) and Extreme Ultra Violet (EUV) irradiance since 2006 (starting with GOES-13).

X-Ray Sensor (XRS)

XRS provides solar X-ray fluxes for the wavelength bands of 0.5 to 4 Å and 1 to 8 Å. Science-quality data should be used where available. Operational data should be rescaled as described in the Operational Data Readme (see Documentation tab).

Extreme Ultraviolet Sensors (EUVS)

The GOES-13, 14, and 15 satellites used EUVS sensors to measure extreme ultraviolet irradiance. Transmission grating spectrographs measured five spectral bands between 5 and 127 nm. Primary channels are: A (5-15 nm), B (25-35 nm), and E (115-130 nm). Science-quality data should be used where available; this data is better calibrated and includes solar line data for Lyman alpha (121.6 nm) and He II (30.4 nm).

When available, we strongly recommend using science quality data in your study. These data have been reprocessed with updated calibrations.

Note: Reprocessed science-quality XRS data will be available for GOES 1-7 in 2024.
 

Science Quality XRS Data

XRS Science Quality Data Table
Product GOES Satellite Data Description Plots
3-second Fluxes 8 | 10 | 11 | 12 High cadence XRS measurements
2-second Fluxes 13 | 14 | 15 High cadence XRS measurements
1-min Averages

8 | 9 | 10 | 11 | 12 | 13 | 14 |15

1-minute averages of XRS measurements 1-min. average plots
Flare Summary 13 | 14 | 15 List of solar flares
Daily Background 8| 9 |10 | 11 | 12 | 13 | 14 | 15 Daily X-ray averages and backgrounds

 

Operational XRS Data

Data that were processed for operational use when they were collected.

XRS Operational Data Table
Product Description
High Cadence Full resolution (2 or 3 s cadence) data in daily files in NetCDF and CSV formats
Averages Time-averaged data in monthly files in NetCDF and CSV formats
Plots
Housekeeping Temperatures and other housekeeping data for XRS and EUVS

Data that were processed for operational use when they were collected.

Operational XRS Data

XRS Operational Data Table
Product Description
High Cadence Full resolution (2 or 3 s cadence) data in daily files in NetCDF and CSV formats
Averages  Time-averaged data in monthly files in NetCDF and CSV formats
Plots  
Housekeeping Temperatures and other housekeeping data for XRS and EUVS

Operational EUVS Data

EUVS Operational Data Table
Product Description
Counts and flags for all 5 EUVS channels

Energetic Particle Sensor (EPS)

Solid-state detectors with pulse-height discrimination measure proton, alpha-particle, and electron fluxes. The EPS telescope (channels P1-P3) and the dome D5 detector designs (channels P6 and P7) were the same from GOES-4 through GOES-15. The dome D3 (channels P4, E1, and E2) and dome D4 detector designs (channels P5 and E3) were the same from GOES-8 through GOES-15. 

The E1, E2, E3, and P1 channels respond to trapped outer radiation belt particles. The P2 channel may occasionally respond to trapped particles during magnetically disturbed conditions. P1–P7 measure fluxes that originate outside the magnetosphere–from the sun or the heart of the galaxy. Similarly, the alpha particle channels A1–A6 measure fluxes of solar- and galactic-origin helium-4 nuclei.

The particle data also include significant secondary responses from higher energy particles, other species (especially proton contamination of electron channels E2 and E3), and directions outside the nominal detector entrance aperture. The data also include backgrounds that are caused by instrument noise, not by penetrating radiation. The integral proton fluxes are derived from the instrument channels and have been partially corrected for these effects. E2 (>2 MeV) and E3 (>4 MeV) electron data become unreliable during most solar energetic particle events (solar radiation storms). The alpha particle (helium-4 nuclei) channels are all differential channels. 

On GOES 13–15, the EPS was renamed the Energetic Proton, Electron and Alpha Detector (EPEAD), and two new instruments (Magnetospheric Electron Detector (MAGED), and Magnetospheric Proton Detector (MAGPD)) were added to the suite. MAGED (30-600 keV electrons) and MAGPD (80-800 keV protons) are based on the POES SEM-2 Medium Energy Proton and Electron Detector (MEPED) electron and proton telescope designs, but with different energy channels.  MAGED and MAGPD are each composed of nine telescopes of identical design in a cruciform arrangement, with five telescopes forming a fan in the north-south plane and four additional telescopes forming an east-west fan with the central telescope.

High Energy Proton and Alpha Detector (HEPAD)

The HEPAD telescope consists of two silicon detectors and a photomultiplier tube (PMT) illuminated by a Cherenkov radiator. Triple coincidences between the two silicon detectors and the PMT are used to produce four proton channels and two alpha particle channels. There are three proton differential channels between 330 and 700 MeV and an integral channel above 700 MeV. The two alpha particle channels rarely report fluxes above backgrounds. HEPAD data are available from GOES-6 and GOES 8-15 (The GOES-7 instrument failed). The GOES-6 HEPAD first observed a SEP event with a ground level enhancement (GLE) in February 1984 (GLE 39).  The final GLE observed by HEPAD (on GOES 13–15) was GLE 72 (September 2017).

When available, we strongly recommend using the science quality data in your study. These data have been reprocessed to account for new calibrations, to account for dead time, or to flag backgrounds and contamination. NCEI has also calculated pitch angles for MAGED and MAGPD.

Science Quality Particle Data

1-Minute Averages

The 1-minute averages of the GOES 13–15 EPEAD >0.8 and >2 MeV electron channels (E1 and E2) were reprocessed by NCEI with a science-quality algorithm. Key aspects of this algorithm that distinguished this product from the real-time SWPC product included (1) application of a dead-time correction and (2) replacing near-background values with fill values. 

Data and Documentation 

The 1-minute average particle data can be accessed from the GOES-SEM averages directory. They are contained in the NetCDF folders for GOES 13–15, and use the following file naming convention: g1s_epead_e13ew_1m_20yymm01_20yymmdd_science_v1.0.0.nc.

The GOES EPEAD Science-Quality Electron Fluxes Algorithm Theoretical Basis Document provides a detailed description of the science-quality algorithm and product. 

Pitch Angles

Pitch angles for the GOES 13–15 MAGED and MAGPD instruments were calculated at 1-minute cadence by NCEI using magnetic field measurements from the GOES 13–15 magnetometers. 

Data and Documentation 

The pitch angles for particle data can be accessed from the GOES-SEM averages directory. They are contained in the NetCDF folders for GOES 13–15, and use the following file naming convention: g1s_epead_e13ew_1m_20yymm01_20yymmdd_science_v1.0.0.nc.

The GOES 13–15 MAGE/PD Pitch Angles Algorithm Theoretical Basis Document provides a detailed description of the science-quality algorithm and product.

EPS Operational Data Table
Product Description
Full resolution data in daily files in NetCDF and CSV formats
Time-averaged data in monthly files in NetCDF and CSV formats
 

Magnetometers

Three orthogonal fluxgate magnetometer elements (only two spinning fluxgate elements prior to GOES-8 - 1994) provide geomagnetic field measurements at geosynchronous orbit (L = 6.6) with high resolution since 1974. 

More Information About Magnetometers

Science Quality Data

When available, we strongly recommend using the science quality data in your study. These data have been reprocessed to account for new calibration. 

Operational Data

Magnetometer Operational Data Table
Product Description
High Cadence Full resolution data in daily files in NetCDF and CSV formats
Averages Time-averaged data in monthly files in NetCDF and CSV formats
Plots  
Ephemeris Satellite locations for GOES 8-15

Solar X-ray Imager (SXI)

The GOES 12 through 15 spacecraft each carried an X-ray telescope called the Solar X-ray Imager (SXI) to monitor the solar corona. SXI used a 512 x 512 intensified CCD and broadband filters to obtain images at several wavelength bands between about 6 and 60 Å. The geosynchronous orbit of GOES allows it to have direct line-of-sight observations of the Sun, 24 hours/day, 7 days/week. The only exception to this, is near the equinox, when GOES enters Earth's shadow for up to one hour each day. SXI will collect an image once per minute and the exposure settings follow a sequence that is optimized to observe three primary phenomena as they are reflected in the Solar atmosphere: coronal structures, active regions, and solar flares.

GOES Science-Quality Data

GOES Operational (Real-Time) Data Readmes

GOES Data Books: Overview of the Satellite Platform and Instruments

GOES NOP (13–15) Algorithm Theoretical Basis Documents

GOES NOP (13–15) Engineering Documents

GOES I-M (8–12) Engineering Documents

GOES DEF (4–6) Engineering Documents

GOES 1–3 and SMS Engineering Documents

  • ERL-SEL-42: Space Environment Subsystem, 1975, R.N. Grubb
  • ERL-SEL-48: Solar X-ray Measurements from SMS-1, SMS-1 and GOES-1, Information for Data Users, R.F. Donnelly, et al
  • ERL-SEL-56: SMS-GOES Soft X-ray Measurements, July 1, 1974 - December 31, 1976, R.F. Donnelly

Important GOES Data Announcements

Other Documents

File Naming

D075YYMM.TXT

D -> Data version:
'G' GOES X-ray, Mag., Electrons & Uncorrected Proton Channels
'Z' GOES X-ray, Mag., Electrons & Corrected Proton Channels
'I' GOES X-ray, Mag., Electrons & Corrected Integral Protons
'H' GOES X-ray, Mag., Electrons & HEPAD
'A' GOES X-ray, Mag., Electrons & Uncorrected Alpha-Particles
07 -> GOES-7, etc.
5 -> 5-minute averages, 1 -> 1-minute averages
YY -> year
MM -> month

Data Channels

X-Rays
XL 1 - 8 Å X-rays
XS 0.5 - 4 Å X-rays

Magnetic Field
HP Parallel to satellite spin axis
HE Earthward
HN Normal to HP and HE, points West for GOES 1-4, East for GOES 5+
Htot Magnitude of total magnetic field vector

Energetic Particles from SMS-1 -> GOES7

Electrons
E1 > 2 MeV (Electrons/cm2 sec sr)
 Protons
Corrected and uncorrected
P1 .6 - 4.2 MeV (Protons/cm2 sec sr MeV)
P2 4.2 - 8.7 MeV (Protons/cm2 sec sr MeV)
P3 8.7 - 14.5 MeV (Protons/cm2 sec sr MeV)
P4 15.0 - 44.0 MeV (Protons/cm2 sec sr MeV)
P5 39.0 - 82.0 MeV (Protons/cm2 sec sr MeV)
P6 84.0 - 200 MeV (Protons/cm2 sec sr MeV)
P7 110.0 - 500.0 MeV (Protons/cm2 sec sr MeV)

'I' designates Integrated protons, corrected
I1 > 1 MeV (Protons/cm2 sec sr)
I2 > 5 MeV (Protons/cm2 sec sr)
I3 > 10 MeV (Protons/cm2 sec sr)
I4 > 30 MeV (Protons/cm2 sec sr)
I5 > 50 MeV (Protons/cm2 sec sr)
I6 > 60 MeV (Protons/cm2 sec sr)
I7 > 100 MeV (Protons/cm2 sec sr)

Alpha Particles
'A' designates Alpha-particles, not corrected
A1 3.8 - 9.9 MeV (Alpha-particles/cm2 sec sr MeV)
A2 9.9 - 21.3 MeV (Alpha-particles/cm2 sec sr MeV)
A3 21.3 - 61. MeV (Alpha-particles/cm2 sec sr MeV)
A4 60.0 - 180 MeV (Alpha-particles/cm2 sec sr MeV)
A5 160.0 - 260 MeV (Alpha-particles/cm2 sec sr MeV)
A6 330.0 - 500 MeV (Alpha-particles/cm2 sec sr MeV)

HEPAD
P8 370 - 480 MeV (Protons/cm2 sec sr MeV)
p9 480 - 640 MeV (Protons/cm2 sec sr MeV)
P10 640 - 850 MeV (Protons/cm2 sec sr MeV)
P11 > 850 MeV (Protons/cm2 sec sr)
A7 630 - 850 MeV (Alpha-particles/cm2 sec sr MeV)
A8 > 850 MeV (Alpha-particles/cm2 sec sr)

Energetic Particles from GOES-8 -> GOES-12

Electrons
E0 > 0.6 MeV (Electrons/cm2 sec sr)
E1 > 2.0 MeV (Electrons/cm2 sec sr)
E2 > 4.0 MeV (Electrons/cm2 sec sr)

Protons
P1 <= 0.8 to 4 (Protons/cm2 sec sr MeV)
P2 4 to 9 (Protons/cm2 sec sr MeV)
P3 9 to 15 (Protons/cm2 sec sr MeV)
P4 15 to 40 (Protons/cm2 sec sr MeV)
P5 40 to 80 (Protons/cm2 sec sr MeV)
P6 80 to 165 (Protons/cm2 sec sr MeV)
P7 165 to 500 (Protons/cm2 sec sr MeV)
P8 350 to 420 HEPAD (Protons/cm2 sec sr MeV)
P9 420 to 510 HEPAD (Protons/cm2 sec sr MeV)
P10 510 to 700 HEPAD (Protons/cm2 sec sr MeV)
P11 > 700 HEPAD (Protons/cm2 sec sr)

'I' designates Integrated protons, corrected
I1 > 1 MeV (Protons/cm2 sec sr)
I2 > 5 MeV (Protons/cm2 sec sr)
I3 > 10 MeV (Protons/cm2 sec sr)
I4 > 30 MeV (Protons/cm2 sec sr)
I5 > 50 MeV (Protons/cm2 sec sr)
I6 > 60 MeV (Protons/cm2 sec sr)
I7 > 100 MeV (Protons/cm2 sec sr)

Alpha Particles
A1 4 to 10 (Alpha-particles/cm2 sec sr MeV)
A2 10 to 21 (Alpha-particles/cm2 sec sr MeV)
A3 21 to 60 (Alpha-particles/cm2 sec sr MeV)
A4 60 to 150 (Alpha-particles/cm2 sec sr MeV)
A5 150 to 250 (Alpha-particles/cm2 sec sr MeV)
A6 300 to 500(Alpha-particles/cm2 sec sr MeV)
A7 2560 to 3400 HEPAD (Alpha-particles/cm2 sec sr MeV)
A8 >3400 HEPAD (Alpha-particles/cm2 sec sr)
 

Data Cautions

The volume of these data makes it impossible to issue a guarantee as to the quality of each data point. A quality pass has been made though each file to identify values that make wild excursions from the norm, and instances of such have been looked at on a case by case basis and compared with concurrent data from other satellites. Data identified as bad have been replaced with the bad data flag. Users should be suspicious of 'spikes' in the data and attempt to correlate them with other sources before assuming that they represent the space environment.

The time of these observations has not been corrected for the down-link and preprocessing delays. The Space Weather Prediction Center estimates that delay to be 5-6 seconds.

X-ray Data Quality

The X-ray sensors may experience significant bremsstrahlung contamination. This contamination is caused by energetic particles in the outer radiation belts and depends on satellite local time, time of year, and the local particle pitch- angle distribution. The X-ray sensors are also sensitive to background contamination due to energetic electrons that either deposit their energy directly in the telescope or strike the external structure and produce bremsstrahlung X- rays inside the ion chamber. Comparison of X-ray measurements from two concurrently operating GOES satellites reveals a systematic difference signal that shows both diurnal and seasonal variations. These variations are most noticeable when solar activity is low to moderate. Beginning with the GOES-8 detector the dynamic range of the instrument was shifted upwards to allow the highest flux events to be recorded. As a consequence of this, the lowest flux recordings are clipped.

Ion Data Quality

Users of GOES particle data should be aware that significant secondary responses may exist in the particle data, i.e. responses from other particles and energies and from directions outside the nominal detector entrance aperture. A description of the algorithm that partially corrects for these effects is described below.

Electron Data Quality

The Electron detector responds significantly to protons above 32 MeV; therefore, electron data are contaminated when a proton event is in progress. Beginning with GOES-8 the electron data have had a preliminary correction applied, however, even these data are not to be considered research quality at this time.

The GOES-5 electron channel is noisy from 1986 onwards and readings are a possible factor of 2 high. One component of the GOES-6 particle detector system has had radiation damage since 1986 that reduced its counting efficiency progressively. At present the E1 and P4 channels derived from this component record at only a few percent of their proper rates. In 1991 the telescope component of the GOES-7 energetic particle detector system experienced episodes of malfunction (noise). The first period began at 0330 UT, October 18, 1991 and extended to November 5, 1991. The detector was commanded off for 12 hours. At turn-on the detector appeared to have recovered, but failed again on November 11, with a rerecovery on November 12 after a second turn-off of three hours. The detector has since operated normally. The noise periods may be identified by unusually high rates being shown by the P1 channel and the derived > 1 MeV integral channel. Currently, the GOES-7 Energetic Particle Sensor is left turned off for 4 hours after eclipse to minimize bad data.

More on GOES-8 through GOES-10 Electrons from Terry Onsager:

1. The GOES 11 satellite is in storage mode and spinning. The electron fluxes vary with the spin of the spacecraft, and therefore the flux levels can easily be misinterpreted. It is safest not to use these data.

2. There are questions with the geometric factor used for processing the 0.6 MeV electron channel (GOES-8 thru GOES-12). The relative variations of the 0.6 MeV electrons are useful for scientific studies, but spectral indices inferred from the 0.6 MeV and 2.0 MeV channels may not be accurate.

3. The minimum value allowed in our processing is 1.33E-01. Our processing takes the accumulated electron counts in a short interval, converts to counts/second, and then subtracts off an estimated contamination from protons. When the electron count level is near the background level, the correction we do for proton contamination can take the count rate below zero. To avoid this we impose a floor on the count rate. I forget what this floor is, but when it's converted to flux, you get 1.33E-01.

4. You should not trust any data where the flux is below about 10 (cm2 s sr)^-1. Once you get near the background level of the instrument, the effect of the proton correction can be significant, even when the proton levels are near their background.

Onsager, T. G., A. A. Chan, Y. Fei, S. R. Elkington, J. C. Green, and H. J. Singer, The radial gradient of relativistic electrons at geosynchronous orbit, J. Geophys. Res., 109, A05221, doi:10.1029/2003JA010368, 2004.

Onsager, T. G., G. Rostoker, H.-J. Kim, G. D. Reeves, T. Obara, H. J. Singer, and C. Smithtro, Radiation belt electron flux dropouts: Local time, radial, and particle-energy dependence, J. Geophys. Res., 107(A11), 1382, doi:10.1029/2001JA000187, 2002.

Onsager, T. G., R. Grubb, J. Kunches, L. Matheson, D. Speich, R. Zwickl, and H. Sauer, Operational uses of the GOES energetic particle detectors, SPIE Conference Proceedings, Vol. 2812, p. 281-290, GOES-8 and Beyond, Edward R. Washwell, ed., 1996.

Magnetometer Data Quality

The GOES-5 magnetometer HP component had an artificial offset from January 2, 1986 to March 13, 1986. The data are left as is. The GOES-6 magnetometer experienced irregularities in the magnetometer on September 9, 1991. The transverse component, which is deconvoluted into the HE and HN components (orthogonal to spin axis), began to yield bad values due most likely to an error in locating Earth's limb. The problem persists to this time. Although the possibility exists that a proper deconvolution may be arrived at, the data for these values have been replaced with the bad data flag and will not be plotted.

In summary, the HE and HN components of the GOES-6 magnetometer have been filled with the bad data flag from September 9, 1991 onwards. The HP component is left intact. The GOES-7 magnetometer experienced instrument failure of its transverse component in May 1993. Only the HP component is available from May 1993 onwards. The HN and HE components are filled with the bad data flag. The absolute accuracy of HP (spin axis component) on all GOES can be uncertain because of difficulties in calibration.

Data Gaps

GOES-6 1-minute data from June 5, 1988 to August 14, 1988 are missing particle and magnetometer components. GOES-6 5-minute data from June 5, 1988 to July 31, 1988 are missing particle and magnetometer components.

Due to the failure of the P6 and P7 channels on GOES-12, the "Z" and "I" files will not be generated.


 

GOES Energetic Particle Correction Algorithm

R. D. Zwickl
NOAA Space Weather Prediction Center

In January 1990, an upgraded algorithm for calculating the energetic-particle differential and integral proton flux from measurements made by the energetic particle monitors onboard the GOES-6 and -7 satellites became operational in NOAA's Space Weather Prediction Center (SEC). The following is a brief description of the rationale for the new algorithm and its basic features.

Why Did We Need a New Algorithm?

The energetic particle monitors are simple solid-state sensors, designed to handle large count rates without overwhelming the electronics. Since their launch these instruments have met their design goals and have never saturated, even during the largest events. However, because they were required to measure high rates, the detectors were built with passive shielding (no anti- coincidence). This has allowed particles to pass through the shielding from any direction and be counted as though they had entered through the front collimator.

During solar energetic-particle events the low-energy passbands would detect particles at exactly the same time as the high-energy passbands did, even though it was impossible for the lower-energy particles to be present at such early times. During quiet times, cosmic rays and their secondary particles produce a very high background in the GOES sensors, in contrast to their effect on more advanced sensors that use active shielding (>100 times the "nominal" background).

The initial algorithm, used until January 1990, did not take either of those effects into account. (NGDC has since applied the correction algorithm to the earlier data from 1986 to 1990.)

The Upgraded Algorithm

The count rate as measured by any one of the seven energetic particle proton channels on GOES-6 or -7 (identical systems) can be given by

CMeas = CTrue + S + BG

where CMeas is the actual measured count rate, CTrue is the true count rate, S is the count rate generated by particles entering through secondary energy passbands (i.e., those particles not passing through the collimator), and BG is the background count rate (produced primarily by cosmic rays). Simply stated, the new algorithm solves for CTrue as follows:

CTrue = CMeas - S - BG

The first step in the algorithm is to determine the background count rate for each of the seven channels. Since the background varies with time, a filter technique is used to find a new minimum value within the previous 10 days or use the previous value. This background value is then subtracted from CMeas. It is then assumed that the energy spectrum of the energetic particles, from one energy channel to the next, can be represented by a simple power law in energy ( ), and that the secondary energy passbands that were determined during calibration are responsible for all of the secondary count rate. The resulting set of equations can then be solved, starting with the highest energy channel and working toward lower energies. All seven energy channels must contain data or no values are calculated.

Finally, each set of 5-minute-averaged values is calculated independently of every other set of values. This allows the corrected values to be calculated continuously in an operational environment.