GOES SEM Data Notes
Important Information for Data Users
D -> Data version:
Energetic Particles from SMS-1 -> GOES7
'I' designates Integrated protons, corrected
Energetic Particles from GOES-8 -> GOES-12
'I' designates Integrated protons, corrected
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.
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.
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
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.