This file contains information on the OV3-3 Medium Energy Magnetic Electron Spectrometer data set. The data were originally supplied to the National Space Science Data Center in 1971 on a set of seventeen 7-track tapes and were formatted for use on a 60-bit CDC machine. In 1993, those tapes were copied at NSSDC and supplied to Al Vampola, the original Principal Investigator, for reprocessing. The reprocessing done at that time included translating the modified Control-Data-Corporation format (which consisted of 60-bit words on 7-track tapes, copied to 8 mm tapes with each 6-bit byte padded to 8-bits during the reading of the old 7-track tapes) and converting the 60-bit CDC floating-point values to 32-bit Intel REAL*4 values. In 1999, the data were further processed to eliminate some erronious data and to correct for saturation effects in several detectors. The following information is provided in this information file:

1. OV3-3 (1966-70A) description

2. Description of the OV3-3 Medium Energy Magnetic Electron Spectrometer (MEMES), including a brief discussion of the physics of the instrument and its calibration.

3. Data processing prior to shipment of the data to NSSCD in 1971.

4. Data processing after retrieval of the data from NSSCD in 1993, including reprocessing done in 1999.

Additional files may be accessed which provide the following:

5. Geometric-Energy Factors for the MEMES for power-law spectra

6. The data format of the OV3-3 MES PC-compatible data files.

7. A program listing for OV33PLOT.FOR and its associated subroutines. This program is used to access the data in the OV3-3 MES data base.

8. A list of the data files including date, time, and percent of data coverage in the orbit

9. Survey Plots

An executable module, OV33PLOT.EXE, which is compiled to run on an Intel processor, is provided in the root directory with the data files. The data are now in INTEL PC-compatible format.

Direct access is provided to the following sections:
OV3-3 Medium Energy Magnetic Electron Spectrometer (MEMES)
Physical Theory
Design, Construction
Calibration
Data Processing
Data Reprocessing

Due to poor experience with electron spectrometers which used pulse-height analysis on energy deposits in scintillators or solid state detectors, the personnel in the Space Physics Laboratory at Aerospace Corporation decided in 1964 to fly electron spectrometers which could make absolute energy measurements and which had well-defined efficiencies as a function of energy. The 180-degree type spectrometer had been in use in laboratories for over 50 years and the optics and performance were well known. Power was very limited on the early satellites, so it was not feasible to use an electromagnet for focussing. (The OGO-1 and OGO-3 magnetic-focussing electron spectrometers used electromagnets, but had a very small measurement cycle. A capacitor was slowly charged, the charging rate limited by power availabilty, and then discharged through the magnet solenoid. The measurements were made during the discharge cycle, which was about 1% of the total time.) For the Aerospace spectrometers, it was decided that permanent magnets would be used. The highest energy-product material at the time was Indox V, which was a ceramic material. It was chosen for the Aerospace spectrometers and the instruments were designed around it. Initially, two instruments were designed, one using a gap field of 80 gauss and one using a gap field of 2.1 kilogauss. Six units with the lower field and two with the higher field were eventually built. The lower energy units (LEMS) were flown on the OV2-1 (launch failure), OV2-3 (launch failure), OV2-5 (useless telemetry link from geosynchronous orbit), OV1-14 (1968-26B, data at NSSDC), OV1-19 (1969-25C, data at NSSDC), and S3-2 (1975-114B, data at NSSDC). The higher energy units (MEMS) were flown on OV1-14 and OV3-3. An even higher energy unit (HEMS) was flown on OV1-19 and the OV1-19 backup unit was modified and flown on CRRES in 1990. Other similar units (MES), though using Placovar (a 50-50 mixture of cobalt and platinum) as the permanent magnet material, were flown by the PI on a number of other satellites and rockets.

OV3-3 (1966-70A) Satellite

The electron data on this CD were obtained by a magnetic spectrometer on board satellite 1966-70A, a United States Air Force Office of Aerospace Research vehicle designated OV3-3 (Orbiting Vehicle Series 3, Number 3). The OV3-3 was launched at 10h 45m UT on August 4, 1966. Initial orbital parameters were: perigee, 362 kilometers; apogee, 4488 kilometers; inclination, 91.6º; period, 137 minutes. The spacecraft was spin-stabilized at 8.8 rpm. Directional particle detectors were mounted with their fields of view perpendicular to the spin axis, and the spin provided a scanning function. A 150-minute capacity tape recorder permitted acquisition of data over an entire orbit. Approximately 250 orbits of data were received before failure of the tape recorder fourteen months after launch. Real-time data received subsequently was generally obtained by low-latitude tracking stations and were of short duration. They were not processed due to the high cost of computer processing such data relative to the cost of tape-recorded data (the cost was proportional to the real-time extent of the telemetry transmission, which provided 16 times more data per unit of time from tape recorder dumps than from real-time transmissions). An on-board triaxial fluxgate magnetometer provided local magnetic field data, used primarily for pitch-angle determination in the case of the directional particle detectors. The aperture of the Medium Energy Magnetic Electron Spectrometer is covered by the temporary circular plate in the above picture. The picture can be enlarged by clicking on it.

OV3-3 Medium Energy Magnetic Spectrometer

OV3-3 MEMES

The magnetic spectrometer was of the 180º type with a planar array of nine solid-state detectors at the primary focus. Indox V magnets were used to produce a 1.6 kG magnetic field. Detectors were 1.0 cm x 1.5 cm x 1000-micron thick lithium-drifted diodes with a 0.95-cm x 1.37-cm unshielded sensitive area. A cross-section view of the analyzing chamber is shown below. The instrument weighed 12.4 lbs and drew 800 milliwatts of power. Internal baffling, around the edges of the uniform field volume and on the pole plates themselves, presented a structure that served as a disk-loaded collimator in the geometry of the electron trajectory over the entire path length of the trajectory within the instrument. This reduced electron scattering within the instrument to the point that no scattered electrons were observed at the detectors during calibration runs. However, bremsstrahlung produced on and within the chamber (~1 x 10^-6 count/electron at 1.5 MeV, 3 x 10^-6 count/electron at 3 MeV) may have masked some scattered electrons. Bremsstrahlung sensitivity was sufficiently low that no bremsstrahlung corrections to the data are necessary for spectra harder than about E^-12. The softest spectrum measured was about E^-7; therefore, bremsstrahlung has been ignored in the data analysis.

The instrument was completely insensitive to light and uv. Baffles were a matte-finish black anodize and were grounded to prevent charging and thereby alteration of the electron trajectories in the chamber. Each of the nine detectors had an independent set of electronics consisting of amplifiers, a two-level pulse height analyzer, and two pulse-rate-to-analog converters having a useful range of 1 to 10^3 and 300 to 2 x 10^5 pulses per second, respectively, in a logarithmic response. Each converter output was sampled once per second. All lower thresholds were set at 150 keV, and all but one upper threshold were set at 2.65 MeV to obtain a uniform bremsstrahlung and penetrating proton background in each channel (the Channel 9 upper threshold was set at 2.9 MeV). Energy deposits above 2.65 Mev in Channel 1 were also monitored once per second to determine the energetic proton contributions. The minimum energy for direct proton penetration was 105 MeV.

All internal voltages, individual detector currents, and internal temperatures were sampled once each 8 seconds using internal subcommutators. These diagnostic data were included primarily to determine the state of the detectors, but no changes were observed, apparently due to the high threshold for proton penetration, so the subcom data were not retained in the data base. Throughout its life, the instrument operated as it did in the laboratory during calibration except for Channel 8. This channel became noisy (a few counts per second) after about eighteen months in orbit. The data provided on this CD were obtained during the first fourteen months after launch.

Not shown are an external collimator and internal vertical fins on the pole pieces which were used to control forward scattering. The magnetic field was 1.60 kilogauss. Collimator nominal and actual horizontal angles were 4.25º and 12.38º. The difference between "nominal" and "actual" is due to the finite aperture length. Vertical nominal and actual angles were 32.69º and 40.94º (for the lowest energy channel; higher energy channels are progressively smaller; see documentation for additional details). Count-rate meters were used and were read out once per second. Energies and geometric factors (for a flat spectrum) were as follows:

Nominal Response
Ch1 Ch2 Ch3 Ch4 Ch5 Ch6 Ch7 Ch8 Ch9
Energy 300 446 690 945 1206 1471 1738 2007 2277
GEF 8.29 21.8 17.7 14.7 12.5 10.9 9.59 8.57 7.74
(Energies are in keV, the geometric-energy factor is in units of cm2-sec-ster-keV)

Magnetic Focusing Spectrometers
The magnetic-focusing electron analyzer was the instrument of choice in the laboratory for many decades. It has the advantages that it makes a precise determination of energy separate from the detection process and can be made with a high luminosity, which translates into good statistics with weak sources or low flux intensities. Several types exist, but the one that has been most useful is the 180º transverse type, since it can be made in a compact design with permanent magnets and measure multiple energies simultaneously.

The measurement principle used is momentum analysis in a solenoidal magnetic field (diagram above). In a uniform magnetic field, the motion of a charged particle moving perpendicular to the field is given by Bvq=mv2/r, which just equivalences the electric force on the charge q due to motion across the magnetic field B with the centrifugal force on a particle of mass m and velocity v gyrating in a circle with radius r. Thus, in a magnetic electron spectrometer, momentum (and therefore, energy) analysis is done by geometric means (B and r) and the information derived from the energy deposit in the detector can be used for other purposes.

In a 180° magnetic electron spectrometer, particles entering an aperture encounter a uniform solenoidal magnetic field and travel a circular path in the plane transverse to the field. After being bent through approximately 180°, the particle is detected by a planar array. All momenta are measured simultaneously. First order focusing occurs in the plane. Electrons with the same momentum, though at different angles, are focused on almost the same vertical line on the detection plane. There is no focusing in the vertical direction. The focusing in the plane occurs because the length of a chord subtending angles near 180° in a circle does not change rapidly with a change in the subtended angle. The chord varies as 2r(1-cos h), where r is the radius of curvature and h is the subtended angle. Monoenergetic electrons entering through an aperture width w which has an acceptance half-angle of h will have an image width w' given by: w' = w+2r(1-cos h), where r is the radius of curvature of the electrons in the magnetic field. The factor 2r(1-cos h) is the image broadening (see figure above).

For a typical spectrometer, this amounts to less than one percent. However, the drawback in this type of instrument is also shown by the same equation: w, the aperture width! The geometric factor is linear with the width of the aperture. The momentum resolution is governed by the width of the aperture and so is proportional to the geometric factor. This limits the energy resolution. A further consequence of the cylindrical geometry and the lack of focusing in the vertical direction is that the geometric factor is a function of the momentum of the particle. This must be taken into consideration when analyzing the data from the instrument. The centroid of response of an individual element in the detector array depends on the energy spectrum being measured. This prevents direct comparison of data from a single channel in a magnetic focusing spectrometer with an equivalent energy channel in another type of spectrometer, such as a solid-state detector telescope.

Since the energy of the electron is known from its radius of curvature (by knowing where it impinged on the detector array), the energy information from the detector can be used to increase the efficiency of detection to approximately 100% and to identify penetrating particle backgrounds (energetic protons in the inner zone, cosmic rays, bremsstrahlung).

Since the physics of the instrument is straightforward, the geometric factors, the energy responses, and the efficiencies of the individual channels can be determined with very good accuracy through computational means. The approach is to calculate the energy and angle cutoffs for each channel and then to use electron beam tests to verify the proper operation of the instrument. The mathematical model is then used to calculate the detailed response of the instrument. In the usual case, the magnetic field in the chamber of a magnetic electron spectrometer is quite uniform. The two dimensional angular response can be checked quite accurately and the energy cutoffs of each channel can be determined with a precision that is limited only by the energy spread of the particle beam used in the determination. As a result, data from magnetic electron spectrometers can be used for absolute calibrations.

The time-scale for design and construction of the MEMES was very short--less than a year, and the PI had primary duties that did not involve research, so most of the MEMES activities were done in "spare time" (mostly at home in the evening). The PI designed the chamber, magnets, and all circuitry. While the material for the yoke (an oxygen-free iron to ensure a magnetically "soft" yoke) and pole pieces were being procured, the circuitry was designed and layed out on double-sided copper-clad fiberglass board. The era predated precision etching techniques. A "stripline" technique was used for each channel, in which the signal propagated down the center of a line of components and the outside edges of the line were at signal ground. The copper base was at ground and the back side of the board provided power to the components. The purpose for this technique was to avoid cross-talk between adjacent detector channels. Holes for component-mounting pins were drilled through the board using a photo-deposited mask for guidance. The holes were then spot faced on one or both sides (depending whether B+, ground, or neither were needed on the pin). The drilling and spotfacing were done at the home of the PI in the evening and his wife swaged the pins into place, after which he mounted the components (almost literally a "garage" job). Checkout and setting of amplifier and discriminator levels was done at The Aerospace Corporation in their laboratory. After checkout, the boards were coated with a conformal coating to provide stability during launch and to prevent contamination (by bits of wire or solder, which would short out components).

The chamber, after bonding of the Indox V pole pieces, was placed between the poles of a large bending magnet at the University of Southern California accelerator laboratory. To magnetize the Indox V, a uniform field of >14 kG was required over an area of about one square foot with a 4" gap. The chamber was then disassembled and reassembled to stabilize the field, which was measured to be 2.07 kG and quite uniform within the chamber. During a "shake test" during satellite integration, the shelf upon which the MEMES was mounted experienced a resonance at 80 Hz which exceeded 24 gees. One of the Indox V pole pieces separated cleanly in the plane of the pole piece. It was bonded with an epoxy resin and the magnetic field was remeasured. It was still uniform, but was only 1.60 kG. Since the field was now even more stable and the primary effect was to lower the upper energy range of the instrument from 2.7 MeV to 2.3 MeV, it was decided to fly the unit, even though a backup unit was available. The backup unit had not been as thoroughly tested and calibrated at this unit.

Calibration was performed by using electrons from a 4 MeV Van de Graaff accelerator to check energy levels, angular acceptance, and bremsstrahlung sensitivity. Detector efficiencies were determined using electrons from a Ru-106 --Rh-106 source in a laboratory 180º magnetic spectrometer. Protons up to 150 MeV were used to determine the response of the instrument to penetrating particles. Final calibration of the energy levels and geometric factors was done by trajectory analysis on a computer by using measured field and fringing field geometries and the known physical dimensions of the magnetic chamber and its collimators.

Energy-angle response curves were generated that were much more detailed than could be readily obtained experimentally. These were produced by calculating sample trajectories on a desk calculator, graphing the results, and then using graphical techniques to interpolate and integrate the appropriate functions. Area integrals were obtained using a planimeter. Actual particles were used to check the detailed curves at selected points (typically, eight energies at twenty-one different angles for each of the detectors.) Energy determination in calibration was accurate to about 3%, although much larger errors would not be significant due to the energy spread accepted by each detector.

The actual response function of a given channel was a complicated function of energy and incident angle. To simplify data analysis, the actual response functions were represented by functions in which the geometric factor was independent of energy and the integrated geometric-energy factors in the actual and representative cases are equal. This produced a slight bias toward higher energies, which is insignificant except in very soft spectra. For soft spectra, the geometric-energy factors and energy centroids calculated for the apropriate spectrum should be used.

The magnetic field in the instrument was measured with a Hall-effect probe inserted through the aperture. The flux meter was calibrated with a secondary-standard magnet which had been sent to the National Bureau of Standards for calibration. The NBS calibration accuracy was about 1%. Before and after each measurement of the field within the MEMES chamber, the probe was checked with the secondary-standard magnet. No variations in the chamber in excess of about 3% were seen. In the central horizontal plane of the instrument, the geometry of the collimation external to the analyzing chamber was described by a set of six points corresponding to six collimator edges which controlled entrance angles. The internal collimation (which included fins all around the periphery of the analyzing chamber) was described by an additional 20 points. Finally, the detectors in the array were described by 18 points (two per detector corresponding to the edges nearer to the entrance collimator and further from the collimator). For the vertical response, the same elements in the external and internal collimators were used, plus the height of the sensitive area of the detectors. Calculations of the response of the instrument were made using these elements.

Particle calibrations were made using radioactive sources. Details were given in Vampola, 1969. In use, the electrons emitted by a Sr-Pr source were passed through a collimator, bent in a uniform magnetic field generated by an electromagnet, passed out through another collimator and formed a slightly divergent nearly monoenergetic beam. The MEMES was then placed so that the beam could enter the MEMES aperture. The beam was reasonably uniform over the diameter required to fill the MEMES aperture. Calibrations of the electromagnet were done by using emission lines in the energy spectrum from the radioactive source. Cutoff points in each channel were measured as a function of horizontal and vertical angle and as a function of energy. These cutoff points were always within agreement with the mathematical description of the optics in the instrument (within 1º in angle and within about 3% in energy, which were the limits of accuracy of the laboratory measurements). Geometric factors and energy responses were then generated from the mathematical model.

On-orbit measurements of solar flare electrons over the polar caps result in very well-behaved power-law spectra. This indicates that the relative energy centroids and geometric-energy-efficiency factors are proper. It does not provide information on the absolute energies or geometric factors. The absolute efficiencies were obtained by comparing the response of a test detector/electronics channel to a beam which was also measured with an end-window Anton-203 Geiger-Muller tube. The cutoff for electrons for this tube is about 40 keV (it was the same type of GM tube that many of the early satellites carried). For energies above about 120 keV, count rates were identical (within statistics) for both the GM tube and the test detector (which had an electronic threshold set at 60 keV). Below that, the GM efficiency started to drop off--being about 3% below the test detector channel at 110 keV and about 10% low at 100 keV. The primary reason for this was that the dead layer for the silicon detectors used in the MEMES was about 8 keV, compared to the 40 keV threshold for the GM window. At the lower energy range, many electrons scatter back out of the GM tube window without being counted. Modelling the detector dead layer, noise, and amplifier noise indicated that the efficiency should be approximately 100% throughout the range of energies which could actually reach the various detectors in the MEMES. Some electrons backscatter in this dead layer. The lower level threshold in the MEMES channels was always set low enough that even if a particle scattered out of the detector, it would still be counted if it deposited half of its energy before backscattering out. In an electron spectrometer in which pulse-height analysis of the energy deposit in a silicon detector is used, the efficiency of detection typically varies 20% to 70%, depending on energy. The efficiency is lower both at low energy and at high energy. Higher energy electrons are identified as being lower energy electrons, distorting the spectrum. Such spectrometers do not provide absolute fluxes.

The magnetic field in the instrument was measured with a Hall-effect probe inserted through the aperture. The flux meter was calibrated with a secondary-standard magnet which had been sent to the National Bureau of Standards for calibration. The NBS calibration accuracy was about 1%. Before and after each measurement of the field within the MEMES chamber, the probe was checked with the secondary-standard magnet. No variations in the chamber in excess of about 3% were seen. In the central horizontal plane of the instrument, the geometry of the collimation external to the analyzing chamber was described by a set of six points corresponding to six collimator edges which controlled entrance angles. The internal collimation (which included fins all around the periphery of the analyzing chamber) was described by an additional 20 points. Finally, the detectors in the array were described by 18 points (two per detector corresponding to the edges nearer to the entrance collimator and further from the collimator). For the vertical response, the same elements in the external and internal collimators were used, plus the height of the sensitive area of the detectors. Calculations of the response of the instrument were made using these elements to define path limitations.

Reliability of the Calculated GEFs
Hand calculations of geometric-energy factors were made for the first three magnetic spectrometers flown by the PI (LEMS on the OV2-1 and OV2-3 in 1965 and a MEMES on the OV3-3 in 1966--the OV2-1 and OV2-3 launches were failures although the hardware actually went into orbit in an inert state.) In late 1966, personnel in Aerospace's Information Processing Division developed a program for computing the geometric-energy factors in a magnetic electron spectrometer using the optics in a cyllindrical geometry. The program was validated by comparing it with the hand-calculated GEFs for the OV3-3 instrument. Agreement was satisfactory. In the hand computation, the entrance aperture was divided into 5 vertical slits and the geometric factor extracted graphically for each slit. In the computer program, the aperture was divided into 21 slits and the integrations were done digitally. Because of the finer grid resolution and digital integration, it is assumed that the computer calculation is more accurate than the hand calculation.

In 1970, the program which calculated geometric factors for the magnetic electron spectrometers using the optics of the instrument was converted by the PI from a non-relativistic energy calculation to a relativistic momentum calculation. (Momenta are converted to energy only for printouts.) All LEMS, MEMS, HEMS, and MES geometric-energy factors in use by the PI were calculated with this program. Intercomparisons of OV1-19 LEMS and HEMS fluxes with OGO-5 fluxes during a solar flare electron event show impressive agreement (West and Vampola, 1971). Both instruments had a channel that was nominally 830 keV. These two channels agreed with each other within 5% to 8%, with the OV1-19 being higher for some and lower for others. Geometric-energy factors for the OGO-5 instrument were obtained by calibrating the instrument with an extended radioactive source which was essentially uniform over the entire aperture. The S3-2 LEMS was also calibrated in this manner, using an Hg203 source. Within statistics, knowledge of the absolute intensity of the source, and conversion accuracy of the beta spectrum (after correcting for absorption in a thin window covering the source), the geometric-energy factors obtained in this way agreed with the calculated GEFs.

The CRRES fluxes agree very well with the Lockheed electron spectrometer and the MPAE electron spectrometer when energetic fluxes are low enough that background is not a problem in the Lockheed and MPAE instruments. There is good agreement with the AFGL High Energy Electron Flux meter in the range 1 to 2 MeV, but at higher energies the HEEF has a steep spectrum which does not agree with the other three instruments at lower energy. Since the calculation of the geometric factor for the Lockheed and AFGL instruments is quite straightforward, it can be assumed that the MES GEFs are correct.

The Background Channel
The detector in Ch 1 was intended to be only a background detector, with counts in Ch 1 being mostly bremsstrahlung and counts in "Ch 10" being "thin-down" hits from energetic protons or long path-length protons. During calibration Ch 1 was found to show a direct response to electrons incident at a large angle from one side (the curvature of the path allowed electrons to be focussed on the detector because of the design of the obscuring plate). Since it was too late to redesign the internal collimator and rebuild the instrument with the new design (due to the long time involved in testing and certifying a flight instrument, not to mention the recalibration), the bremsstrahlung channel was accepted as-is as an electron channel. The angular response was integrated manually by measuring the response for a beam entering at 5 vertical angles across the aperture with incident horizontal angles from +15 deg to -25 deg. At the end-points, the response was zero. The response increased linearly from 0.37% (of total response) at +12.5 deg to 13.24% at -22.5 deg (data points were obtained every 2.5 deg). Full energy and angle calibrations were carried out on Ch 1.

References

Vampola, A. L., "Energetic Electrons at Latitudes above the Outer Zone Cutoff," J. Geophys. Res., 74, 1254-1269, 1969.

West, H. I., and A. L. Vampola, "Simultaneous Observations of Solar Flare Electron Spectra in Interplanetary Space and Within the Earth's Magnetosphere," Phys. Rev. Lett., 26, 458-463, 1971.

Data samples from the various channels were obtained once per second. The sampling was done by two commutators, a 60-channel and a 120 channel unit. The MEMES data were on the 120-channel unit while the magnetometer data were on the 60-channel unit. The analog samples were stored on an on-board analog tape recorder and then, during playback, were used to frequency-modulate two FM subcarriers. At the tracking station, the FM subcarriers were stripped off of the signal and put on tapes, along with time codes, which were then shipped to the investigating agency. There the subcarriers were processed through square-law demodulators, digitized, and recorded on computer tapes. The digitization involved setting calibration levels carefully and maintaining them during the duration of the digitization process. The data stream included fixed voltage levels which were intended to be used for calibration of the square-law demodulators and also for final correction during the digital processing. Unfortunately, calibrations were not always maintained during the digitization process. At times, the two count rate meters from each MEMES channel do not smoothly join. Either excessive overlap or a gap showed between the two. In converting the voltage levels to counts using the count-rate-meter calibration curve, inaccuracies rose from this source.

Measurements were limited in accuracy by statistics at low counting rates and by a quantization process at higher rates. The quantization process occured in the computer data processing and was of the order of +/-5% in flux level. Since the telemetry system was FM/FM, noise on weak signals contributed an instantaneous error of up to 15%, although experience indicated that source of error was equivalent in size to the quantization error. Over-all accuracies in flux determination were better than 10% when statistics permitted.

On-Orbit Background Correction
The on-board correction for penetrating particles (the upper threshold on each channel) is not 100% efficient due to the fact that some fraction of the particles (cosmic rays, energetic protons) will have path lengths through the detector which are short ("corner cutters") and deposit an amount of energy equivalent to that which would be deposited by an electron focussed onto that detector. Approximately 5% of the omnidirectional solid angle results in such paths. Thus, about 5% of the penetrating particles should masquerade as real events. The upper threshold on the background detector (Ch 10) counts the penetrating particles. This channel is used for background correction.

First order correction coefficients were obtained by integrating counts in each channel while the instrument aperture was aligned sufficiently close to the local magnetic field line that any particles observed would be in the local loss cone. Only about 1% to 2% of the electrons backscatter from the atmosphere. Therefore, the difference in intensity along the field line compared to the locally mirroring particle population is usually about three to four orders of magnitude. The assumption is made that all of the counts in the channel, under these conditions, is due to background. The ratio between the channel counts and the background counts is used for the first order background correction. Using this correction, data near the equator are corrected and a pitch-angle plot is made. Then, another equatorial pitch-angle distribution is obtained by using data obtained near 90º local pitch angle at various places along the field line. The second approach should be more reliable since the electron count rate is higher (due to being at Jperp) and the background is lower (lower position along the field line and the trapped energetic protons have an equatorial distribution that is of the order of Sin7a). Comparisons of the pitch angle distributions provide the analyst with the information as to how low in local pitch angle the first order background correction is sufficient. For the OV3-3 MEMES, the correction is valid down to 25º degrees, but it has to be a local-angle-dependent correction, as shown previously.

In June, 1971, seventeen 7-track tapes were sent to the National Space Science Data Center for archival. These tapes contained the entire OV33 MEMES (Medium Energy Magnetic Spectrometer) data base that existed at that time. In 1992, copies of these tapes were retrieved. The copies are continuous streams in which the original 6-bit bytes are written out as 8-bit bytes (2 MSB zeros are padded). This is a result of reading a 7-track tape on a 9-track drive.

The archive tapes were reformatted to delete the padded zeros, resulting in a copy of the original 60-bit word CDC tapes. In 1992, the CDC version of the tapes was further processed. All CDC floating point values were changed to Intel floating point. Additionally, the data were "reverse engineered" to get back to the original form--voltage outputs from count rate meters. This involved changing the fluxes back to count-rates using the original conversion factor, then adding back the proton background which had been subtracted, and finally changing the count-rate back to the original voltage outputs from the count-rate meters. In the original data processing, only a single count-rate-meter curve had been used for the conversion. The CRM characteristics of the various channels had been carefully matched, but they weren't identical. Only one conversion curve had been used because of the limitations in computers of that period. In the reprocessing, voltages were changed back to count rates using the original calibration curves for each individual energy channel. In addition, due to a mis-punched card, the geometric factors used in the conversion on the original 17 tapes was in error by a factor of pi. This was also corrected.

When the OV3-3 MEMES data were used at NSSDC in their modelling activities, it was noticed that the lowest energy channel had an apparent "flattening" of the pitch-angle curves at 90º when count rates were very high. The source of this flattening was unknown, since the electronics in the instrument were capable of count rates that were at least an order of magnitude higher than the rate at which the flattening occurred. Two decades later, tests were made of lithium-drifted silicon detectors and it was found that there is a limitation in count rate that is possibly produced by an internal polarization in the device. The level was in agreement with the rate at which the MEMES effect occurred and was undoubtedly the source of the flattening in the MEMES pitch angle. As part of the reprocessing of the data for archival at NSSDC in 1999, cross-plots of the various MEMES channels were made and used to determine at what level saturation started to occur in the MEMES channels. Algorithms were derived for correcting for the saturation in Ch1, Ch2, and Ch3 and have been applied to the data on this CD. The first three channels have been adjusted, but the original uncorrected values are also carried along in the data base.

Significant data gaps occur in the data files. The data were originally telemetered on an FM-FM link. FM-FM links tended to be noisy at times and typical recovery of data from a satellite-to-ground transmission was in the 80%-95% range. The digital processed data were stored at NSSDC for approximately 20 years on the original 7-track tapes and additional errors occurred in copying them in 1992. In the data file log, the value in the column headed "pct" is the percent of data between the start and stop times in the present data file. Differences from 100% are due to these losses in the original processing in the late 60's and recovery from the 7-track tapes in December 1993. MOST of the loss was in the original digitization. Files with low percentages will have a lot of dropout and may not be very useful.

The data files (OV33nnn.MES) contain raw counts. They also contain corrected count rates for those channels which were non-linear at the highest count rates. To correct for the non-linearities, survey plots of all of the data were made in order to select appropriate data segments with which to work while developing the correction algorithms. Those survey plots were saved and are provided in the subdirectory SURVEY. This resource provides hypertext links to the screen-size .JPG files containing the survey plots of the data files on this disk. Clicking on the file name will open the .JPG file. The points plotted are 7-second averages of the data, which corresponds to approximately 1/2 spin period during the earlier portions of the data set. If there are data frames missing from the averaged point, the point may be significantly higher (if only points near Jperp are present) or significantly lower (if only loss-cone samples are present) than surrounding points. These effects make it appear that bad data is being plotted. Actually, only misleading averages are being plotted. Averaging intervals which are not exact multiples of half-spin periods will also produce other effects, such as a scalloped appearance. The data presented in these plots are RAW data which have not been corrected for saturation effects in the lowest energy channels. They are presented for qualitative evaluation, only. The background (proton) monitor is the heavy black line at lowest intensity. It is prominent in the inner zone. The other 9 channels are the electron channels, varying from 2.31 MeV (center energy) -- thin red line to 300 keV --thick red line. The correspondance is:
Energy Color
2.31 MeV thin red
2.01 MeV green
1.77 MeV dark blue
1.47 MeV yellow
1.11 MeV magenta
957 keV blue-green
731 keV brown
475 keV blue
300 keV heavy red
Background black