LOW ENERGY ELECTRON EXPERIMENT ATMOSPHERE EXPLORER C AND D SPACECRAFT Data Description June 1977 R. A. Hoffman R. W . Janetzke Code 625 Goddard Space Flight Center Greenbelt, MD 20771 EXPERIMENT OVERVIEW The Low Energy Electron (LEE) experiment* provides direct measure- ments of the energy input into the upper atmosphere due to electrons and ions in the energy range 0.2 to 25 keV. This is done by measuring the fluxes of electrons and ions through the use of cylindrical electro-static analyzers and Spiraltron electron multipliers. The AE-C instrument contains three such detectors, one measuring electrons and one ions over the total energy range in 16 logarithmically-spaced steps each second, and one measuring 5 keV electrons continuously. Angular distributions are acquired during the spinning mode of the spacecraft. The D instrument contained 19 detectors, one ion stepped-energy analyzer, and two electron stepped analyzers at two different angles. In addition it contained 16 fixed-energy detectors which obtained high time-resolution angular distributions in the spacecraft one revolution-per-orbit mode at five energies between 0.72 and 18 keV. * Radio Science, 8, 393-400, April 1973. DATA DESCRIPTION The LEE data returned via telemetry are 8 bit logarithmically com- pressed words which represent the number of particles accumulated by each detector over 55.7 millisecond. The stepped detectors have the voltage on the analyzer plates stepped synchronously once each tele- metry (TM) minor frame to the next lowest energy in the 16 step sequence, and then recycled to the highest energy step. The accumulation of counts from all detectors in each instrument occurs over precisely the same time period, and the data are read out from each accumulator during the fol- lowing minor frame. The actual particle energy measured during a minor frame can be determined from the telemetry of the analyzer plate voltage (TM word 128) of the preceding minor frame. The LEE instrument also uses 10 words in the 8 second subcom sequence to monitor other voltages and modes of the instrument. Table 1 gives a summary of LEE data and their position in the major frame for the C instrument and Table 2 for the D instrument. Table 3 provides the relation between the voltage on the analyzer plates and the step number. DETECTOR PROPERTIES 1. Geometric factor: G0 = 1.291 x 10**-4 x EC cm2 - ster - keV where EC is the center energy of the passband (keV). The values of G0 for each energy step are listed in Tables 4 and 5. 2. Passband An electrostatic analyzer has a relative passband independent of the center energy, or voltage on the analyzer plates: dE/E = 31% or +17%, -14%. The specific numbers for each energy step are listed in Tables 4 and 5 3. Efficiencies The efficiency of a Spiraltron detector is a function of the energy of the particle being measured, and the species of the particle. The efficiencies of electrons adopted for processing appear in Tables 4 and 5. Post launch analysis in isotropic flux regions has shown that the relative efficiencies of the AE-D detectors vary 20% from the values listed. The relative efficiencies can be obtained from the Investigator. The efficiency of ions adopted for processing was 1.00. However, the members of the LEE analysis team have adopted the following relationship between energy and efficiency: Ion Energy (keV) Efficiency (%) >=10 100 5 98 3 93 2 88 1 67 0.5 40 0.3 24 0.2 14 TABLE 1 AE-C INSTRUMENT TELEMETRY DESCRIPTION TM(43)* Logarithmically compressed data word for the stepped electron detector. TM(44)* Logarithmically compressed data word for the stepped ion detector. TM(45)* Logarithmically compressed data word for the fixed electron detector. TM(128) Analyzer plate voltage. TM(68,28)1,2 High voltage level. TM(68,28)3 Spare. TM(68,28)4 Calibrate mode flag. TM(68,28)5,6,7 Spare. TM(68,28:68,29)9,8,14 Threshold for the stepped electron de- tector. TM(68,28:68,29)2,8,15 Threshold for the stepped ion detector. TM(68,28:68,29)9,8,16 Threshold for the fixed energy electron detector. TM(68,29)2,3 Calibrate amplitude. TM(68,29)4,5 Calibrate frequency. TM(68,30:68,31 ) Minor mode command (zeros). TM(68,57) Low voltage monitor. TM(68,58) High voltage monitor. TM(68,59) Fixed energy detector high voltage. TM(68,60) Detector continuity. TM(68,61 ) Spare. TM(68,106)7 High voltage flag. TM(68,106)8 Sun sensor override. * The formula for converting the compressed datum word to counts/readout (Q) is, Q=(M+16)*2**(10-C) where M is the unsigned integer value of the last 4 bits and C is the unsigned integer value of the first 4 bits. TABLE 2 AE-D INSTRUMENT ANGLE TO TELEMETRY ENERGY (keV) PARTICLE +Y AXIS TM(10) 0.2 to 25 keV electron -7 (16 steps) TM(16) 6.88 electron +7 TM(23) 0.2 to 25 keV ion +60 (16 steps) TM(36) 0.2 to 25 keV electron +60 (16 steps) TM(43) 2.62 electron -7 TM(44) 2.62 electron +7 TM(45) 1.38 electron +7 TM(49) 0.72 electron +7 TM(56) 0.72 electron -7 TM(62) 1.38 electron -7 TM(75) 6.88 electron +35 TM(80) 2.62 electron +35 TM(87) 1.38 electron +35 TM(94) 0.72 electron +35 TM(100) 2.62 electron +60 TM(113) 1.38 electron +60 TM(126) 0.72 electron +60 TM(127) 18.1 electron +35/+7* TM(128) Analyzer plate - - voltage TM(68,59) Fixed energy ana- - - lyzer plate volt- age. *Toggled between main frames. TABLE 3 V (PLATES) STEP # 1249 16 907 15 657 14 476 13 345 12 250 11 182 10 132 9 95 8 69 7 50 6 36 5 26 4 19 3 14 2 10 1 TABLE 4 Properties of the Stepped Energy Detectors, AE-C and D STEP EC EMIN EMAX dE EFF* G0 16 24.90 20.978 28.635 7.657 .64 3.21E-03 15 18.08 15.21 20.978 5.762 .70 2.33 14 13.10 11.03 15.216 4.186 .72 1.69 13 9.50 7.993 11.03 3.037 .76 1.23 12 6.88 5.787 7.993 2.206 .80 8.88E-04 11 4.98 4.198 5.787 1.589 .82 6.43 10 3.62 3.045 4.198 1.163 .86 4.67 9 2.62 2.206 3.045 .839 .87 3.38 8 1.90 1.601 2.206 .605 .90 2.45 7 1.38 1.159 1.601 .442 .92 1.78 6 .996 .838 1.159 .321 .93 1.29 5 .722 .609 .838 .229 .94 9.32E-05 4 .524 .440 .609 .169 .95 6.76 3 .379 .318 .440 .122 .96 4.89 2 .273 .231 .318 .087 .95 3.52 1 .200 .170 .231 .061 .94 2.58 * Efficiency for electrons. TABLE 5 Properties of the Fixed Energy Detectors, AE-D DET # EC EMIN EMAX dE EFF G0 2 6.88 5.787 7.993 2.206 .80 8.88E-04 5 2.62 2.206 3.045 .839 .87 3.38E 6 2.62 2.206 3.045 .839 .87 3.38E 7 1.38 1.159 1.601 .442 .92 1.78E 8 .722 .609 .838 .229 .94 1.55E-05 9 .722 .609 .838 .229 .94 9.32E-05 10 1.38 1.159 1.601 .442 .92 1.78E-04 11 6.88 5.787 7.993 2.206 .80 8.88 12 2.62 2.206 3.045 .839 .87 3.38 13 1.38 1.159 1.601 .442 .92 1.78 14 .722 .609 .838 .229 .94 9.32 15 2.62 2.206 3.045 .839 .87 3.38 16 1.38 1.159 1.601 .442 .92 1.78 17 .722 .609 .838 .229 .94 9.32E-05 18 18.08 15.216 20.978 5.762 .70 2.33E-03 19 18.08 15.216 20.978 5.762 .70 2.33E-03 UNITS OF OUTPUT PARMETERS 1. Differential Particle Flux: J=C/( EFF*dT*G0) particle /cm2-sec-ster-keV where C = counts/accumulation period T = accumulation period = 5.566 x 10**-2 sec. 2. Differential Energy Flux: psi = J*EC*S ergs/cm2-sec-ster-keV where S = 1.61 x 10**-9 ergs/keV 3. Total Energy Flux: psi(T)= SUM[i,i=1,16](psi(i)*dE(i)) ergs/cm**2-sec-ster where psi(i) = differential energy flux for the i-th step. The second sum is required for AE-C because the passbands of the detectors do not provide a continuous monitor of all energies. GEOPHYSICAL UNIT FILE The Geophysical Unit File represents a condensation of the telemetry words and a transformation to more scientifically meaningful units. The file will have one physical record of fixed length (540 words) for every minute of data. Each physical record consists of 60 logical records, of 9 words each, containing values for one second of data. The date and time by which the physical record is accessed corresponds to the first logical record. The format of a logical record for data from AE-C is: Word Contents 1 Total energy flux for electrons 2 N0 for electrons 3 E0 for electrons 4 Correlation coefficient for electrons 5 Total energy flux for ions 6 E0 for ions 7 N0 for ions 8 Correlation coefficient for ions 9 RMS for the fixed energy electron detector PARAMETER DESCRIPTION: total energy = (ergs/cm2-ster-sec) the total energy flux for a given particle determined from one second of data. E0 = (keV) the characteristic energy of the particles as deter- mined by fitting one second of data to a Maxwellian velocity distribution function. N0 = (particles/cm3) the characteristic density of the particles determined in the same manner as E0. Correlation Coefficient: ( N* SUM(X*Y) - SUM X * SUM Y)/ /(((N*SUM X**2 - SUM X*SUM X) * (N*SUM Y**2 - SUM Y * SUM Y))**1/2) RMS = (percent) the rms of the measured values as compared to the averaged value, over one second of data, divided by the average. It was found through experience that a single Maxwellian distribu- tion did not usually represent the data. Thus, for AE-D the format for the logical record was changed to: Word Contents 1 Total flux for electrons in the energy range 0.2 to 0.8 keV. 2 Same for 0.8 to 2 keV. 3 Same for 2 to 7 keV; 4 Same for 7 to 25 keV. 5 Average energy for electrons. 6 Total electron flux. 7 Total energy flux for ions. 8 Average energy for ions. 9 Total ion flux. UNIFIED ABSTRACT FILE The Unified Abstract File is similar to the Geophyical Unit File except it provides 15 second summaries of the data. The data are condensed using data from 7.5 seconds before and 7.5 seconds after the 15 second time mark, making available 15 stepping sequences for processing. However, when a spacecraft is in the 15 second spin mode, only data from the upper hemisphere are accepted, so that 7.5 sequences of the 16 steps from each detector are used. Each record of the AE-C UA file contains 80 items for a 15 second period. The LEE items are: ITEM NUMBER DESCRIPTION #15 Total energy flux for electrons. #16 N0 for electrons. #17 E0 for electrons. #18 Correlation coefficient for electrons. #19 Total energy flux for ions. #20 N0 for ions. #21 E0 for ions.- #22 Correlation coefficient for ions. PARAMETER DESCRIPTION: total energy = (ergs/cm2-sec-ster) the average total energy flux from the upper hemisphere for a fifteen second period. E0 = (keV) the characteristic energy of the particles as deter- mined by fitting fifteen seconds of upper hemisphere data to a Maxwellian velocity distribution function. N0 = (particles/cm3) the characteristic density of the particles determined in the same manner as Eo Correlation Coefficient: as described early Each record of the AE-D UA file contains 84 items. The LEE items ITEM NUMBER DESCRIPTION 15 Total energy flux for electrons, 0.2 to 0.8 keV. 16 Same, 0.8 to 2 keV. 17 Same, 2 to 8 keV. 18 Same, 7 to 30 keV. 19 Average energy for electrons. 20 Total electron flux. 21 Total energy flux for ions. 22 Average energy for ions. 23 Total ion flux.