Personnel Involved with the SOPA and CPA Detectors:

Principal Investigator

Richard D. Belian

Co-Investigators

Geoffrey D. Reeves - (Primary Data Analyst)

Thomas Cayton

Reiner Friedel

Michael G. Henderson

John C. Ingraham

Robert C. Reedy

Michel Tuszewski

Scientific Programmers

Roderick A. Christensen - (Primary)

Peter S. McLachlan

The LANL satellites:

Orbit: geo-synchronous (6.6 Re, equatorial)

Spin period: ~10 seconds

Spacing: roughly 120 degrees apart

Spin Axis: earth pointing

* Currently supplying data

The Charged Particle Analyzer (CPA) experiment consists of four detectors including the LoE, HiE, LoP and HiP. Each will be described in some detail. The reader is referred to Higbie, et al., 1978, or Baker, et al., 1985, for further instrument information.

CPA experiments were flown on a series of LANL satellites beginning in 1976. In all, 8 satellites were flown, the last was launched in 1987. After 1979 a constellation of at least three satellites with CPA detectors were continuously maintained and the data from some subset were processed up until 1995. Currently two satellites remain operational, however, the CPA data from these is not processed due to processor limitations and their function has been replaced with other LANL satellites with SOPA detectors.

The LoE (low energy electron) subsystem is a set of five, similar, solid state sensors (700 microns thick) in a fan arrangement at angles of +/- 60, +/- 45 and 0 degrees to the normal to the satellite spin axis (directed toward the Earth). Each telescope has a collimating aperture with a half angle of ~2.6 degrees, which provides a geometrical factor of 3.096E-03 cm-sq sr. As the satellite spins, (period of ~10 seconds), the telescopes sweep out parallel bands. During the course of a single rotation, the five LoE detectors record 200 samples of the unit sphere at each energy. There are six nested energy channels with approximate lower thresholds of 30, 45, 65, 95, 140, and 200 keV. Each channel has an upper threshold of 300 keV. Each of the five telescopes is fronted by an aluminized Mylar sunscreen. The LoE telescopes do not discriminate between particle species, but rely instead on the normal preponderance of electrons at GEO. Therefore, there is always a small contamination of each electron channel by protons and other ions (usually less than 1%).

The HiE (high energy electron) subsystem is a single element, collimated, solid state sensor (3000 microns thick) mounted perpendicular to the spacecraft spin axis with a field-of-view half angle of 3.7 degrees and a geometrical factor of 1.407 E-02 cm-sq sr. It is designed to monitor high-energy electrons. There are six nested energy channels with approximate lower thresholds of 200, 290, 430, 630, 930, and 1350 keV. Each channel has an upper threshold of 2000 keV. The HiE detector records 40 samples of the particle distribution each rotation. It is fronted by an aluminized Mylar sunscreen. As with the LoE, the HiE depends on the preponderance of electrons at GEO for species discrimination.

The LoP (low energy proton) subsystem is a single element, collimated, solid state sensor (85 microns thick) mounted perpendicular to the spacecraft spin axis with a field-of-view half angle of 2.8 degrees and a geometrical factor of 2.948E-03 cm-sq sr. It is designed to monitor protons, and has 10 nested energy channels with lower thresholds that vary from satellite to satellite, but which are about 80, 90, 110, 135, 175, 200, 240, 300, 365, 455 keV and an upper threshold of ~580 keV. The LoP detector records 40 samples of the particle distribution each rotation. Electron contamination to the proton channels is minimized by the action of a sweeping magnet. An anti-coincidence scintillation discriminator vetoes particles penetrating the sensor. The LoP is fronted by a 90 micro-inch, nickel sunscreen.

The HiP (high energy proton) subsystem is a single, tri-element, collimated, solid state telescope (45 microns, 3300 microns, and 500 microns thick) mounted perpendicular to the spacecraft spin axis. Except for the collimated aperture, the stack of three solid state sensors is completely surrounded by a plastic anti-coincidence shield. The HiP detector has two slightly different fields-of-view and geometrical factors, depending on which sensor element the particle is analyzed by field-of-view half angles of 6.5 and 6.7 degrees and geometrical factors of 4.685 E-02 and 4.813 E-02 cm-sq sr. The HiP is designed to monitor high-energy protons with 16 differential energy channels. The channels differ slightly from satellite to satellite due to slight variations in sensor dead-layers and in the thickness of the front elements. However, the nominal energy thresholds for the 16 channels are 0.4 MeV, 0.5, 0.6, 0.8, 1.0, 1.3, 1.7, 2.8, 4.3, 8.0 14, 23, 33, 48, 71, and 100, with an upper threshold of ~160 MeV. Like the LoP, the HiP records 40 samples of the particle distribution each rotation. Again, electron contamination to the proton channels is minimized by the action of a one kilogauss sweeping magnet. The HiP collimator is fronted by an aluminized Mylar sunscreen.

Table of Characteristics of the CPA instrument

Rate Channels

Full spatial distribution time - one spin period (~10 s)

References:

Higbie, P. R., R. D. Belian, and D. N. Baker, "High-Resolution Energetic Particle Measurements at 6.6 Re: 1 Electron Micropulsations," J. Geophys. Res., 88, 4851, 1978.

Baker, D. N., W. Aiello, J. R. Asbridge, R. D. Belian, P. R. Higbie, R. W. Klebesadel, J. G. Laros and E. R. Tech, "Los Alamos Energetic Particle Sensor Systems at Geostationary Orbit," AIAA 85-0243. 1985.

The SOPA instrument is designed to provide high spatial, high-resolution energetic particle measurements at geo-synchronous orbit on spinning satellites. As such it monitors electrons, protons, helium, carbon, nitrogen, and oxygen ions individually and all particles with Z > sulfur and all particles with Z > strontium above certain energies. Finally, the SOPA E by dE/dx capability is exploited to provide dual parameter pulse height analysis (PHA). This feature extends the ion coverage, in principal, to all ions. The SOPA instrument, because it collects many samples of the surrounding particle populations (64 per spin period), can be used to determine the symmetry axis of the distribution thus providing the magnetic field orientation. The reader is referred to Belian, et al., 1992, for further information.

SOPA experiments have been flown on a series of LANL satellites beginning in 1989. In all, five satellites have been flown, the last in 1997. These satellites were flown to replace the earlier LANL satellites that had on-board the CPA experiment. Since 1991 a constellation of at least three satellites with SOPA detectors have been continuously maintained and the data from some subset of the five are processed. Usually, three satellites are providing SOPA data, but on occasion, four or even five will be monitored during the course of a day.

The instrument consists of three solid state detector telescopes (T1, T2 and T3) that accept particles from three different directions relative to the spacecraft spin axis. Each telescope consists of a thin, 4 µm, 10 mm² front detector followed by a thick, 3000 µm, 25 mm² back detector. A collimator, with 11° (full width) field of view fronts the detector stack. High and low Z passive shielding completely surround the active solid state detectors providing protection from side penetrating particles. The shielding is sized to stop 50% of the 6 MeV electrons and 65 MeV protons incident normal to the shielding surface.

Satellites, currently On-Orbit with SOPA Instruments:

1989-046, 1990-080, 1991-095, 1994-084 and LANL-97A

Satellites, providing data as of May 19, 1999:

1989-046, 1990-080, 1991-095, 1994-084 and LANL-97A

Sampling sequence: T1, T2, T3, T2, T1, T2, T3 etc.

Duty cycles: T1 - 25%, T2 - 50%, T3 - 25%

Telescope fields-of-view: ~11° full angle

Geometrical factors: Ions - 8.5 X 10-4 cm2-sr

Spatial coverage: 11° bands around the unit sphere, 30°, 90° and 120° to spin axis

Full spatial distribution time - one spin period (~10 s)

Passive Shielding - Ions ~65 MeV, Electrons ~6 MeV

Rate Channels

Accumulation

In addition there are 2 integral "Heavies" channels that are designed to count any particles of sufficient energy whose mass exceeds sulfur in the one case and strontium in the other.

* See the notes associated with the table of nominal channel energies below.

PHA capabilities: 1024 X 1024 channels covering ~0.1 to ~60 MeV on each axis; 8 pulse pairs can be processed each 160 ms. Can monitor all ions except protons.

Nominal Channel Energies – all in keV

In addition there are 2 integral "Heavies" channels that are designed to count any particles of sufficient energy whose mass exceeds sulfur in the one case and strontium in the other.

* See the notes associated with the table of nominal channel energies below.

PHA capabilities: 1024 X 1024 channels covering ~0.1 to ~60 MeV on each axis; 8 pulse pairs can be processed each 160 ms. Can monitor all i

* 1989-046, 1990-080, 1991-095 satellites only

** 1994-084, LANL-97A, and all subsequent LANL series satellites

*** 1994-084, LANL-97A, and certain subsequent LANL series satellites

References:

Belian, R. D., G. R. Gisler, T. Cayton, and R. Christensen, "High-Z Energetic Particles at Geostationary Orbit During the Great Solar Proton Event Series of October 1989", J. Geophys. Res., 97, 16,897, 1992.