The following document is an NSSDC instrument and data set description for NSSDC data set 77-048A-08A. The data described have been reformatted at the PDS/PPI node. Consequently, portions of this document describing the previous format have been omitted. No data or data information was lost in the data reformat.

VOYAGER 1 AND 2

COSMIC RAY SUBSYSTEM

Description of Jupiter Encounter Data

Instrumentation

As its name implies, the Cosmic Ray Subsystem (CRS) was designed for cosmic ray studies (Stone et al., 1977). It consists of two high Energy Telescopes (HET), four Low Energy Telescopes (LET) and The Electron Telescope (TET). The detectors have large geometric factors (~ 0.48 to 8 cm² ster) and long electronic time constants (~ 24 µsec) for low power consumption and good stability. Normally, the data are primarily derived from comprehensive (E₁, E₂ and E) pulse-height information about individual events. Because of the high particle fluxes encountered at Jupiter and Saturn, greater reliance had to be placed on counting rates in single detectors and various coincidence rates. The detectors used for most of our work are listed in Table 1 and illustrated in Figure 1. In interplanetary space, guard counters are placed in anticoincidence with the primary detectors to reduce the background from high-energy particles penetrating through the sides of the telescopes. These guard counters were turned off in the Jovian magnetosphere when the accidental anticoincidence rate became high enough to block a substantial fraction of the desired counts. Fortunately, under these conditions the spectra were sufficiently soft that the background, due to penetrating particles, was small.

The data on proton and ion fluxes at Jupiter were obtained with the LET. The thicknesses of individual solid-state detectors in the LET and their trigger thresholds were chosen such that, even in the Jovian magnetosphere, electrons made, at most, a very minor contribution to the proton counting rates (Lupton and Stone, 1972). Dead time corrections and accidental coincidences were small (< 20%) throughout most of the magnetotail, but were substantial (> 50%) at flux maxima within 40 R_J Of Jupiter. Data have been included in this package for those periods when the corrections are less than ~ 50% and can be corrected by the user with the dead time appropriate to the detector (2 to 25 µsec). The high counting rates, however, caused some baseline shift which may have raised proton thresholds significantly. In the inner magnetosphere, the L₂ counting rate was still useful because it never rolled over. This rate is due to 1.8- to 13-MeV protons penetrating L₁ (0.43 cm² ster) and > 9-MeV protons penetrating the shield (8.4 cm² ster). For an E^{- 2} spectrum, the two groups would make comparable contributions; but in the magnetosphere, for the E^-3 to E^-4 spectrum above 2.5 MeV (McDonald et al., 1979), the contribution from protons penetrating the shield would be only 3 to 14%.

The LET L₁L₂L₄ and L₁L₂L₃ coincidence-anticoincidence rates give the proton flux between 1.8 and 8 MeV and 3 to 8 MeV with a small alpha particle contribution (~ 10^-3). Corrections are required for dead time losses in L₁, accidental L₁L₂ coincidences and anticoincidence losses from L₄. Data are given only for periods when these corrections are relatively small. In addition to the rates listed in the table, the energy lost in detectors L₁, L₂ and L₃ was measured for individual particles. For protons, this covered the energy range from 0.42 to 8.3 MeV. Protons can be identified positively by the E vs. E technique, their spectra obtained and accidental coincidences greatly reduced. Because of telemetry limitations, however, only a small fraction of the events could be transmitted, and statistics become poor unless pulse-height data are averaged over a period of one hour.

HET and LET detectors share the same data lines and pulse-height analyzers; thus, the telescopes can interfere with one another during periods of high counting rates. To prevent such an interference and explore different coincidence conditions, the experiment was cycled through four operating modes, each 192 seconds long. Either the HETs or the LETs were turned on at a time. LET-D was cycled through L₁ only and L₁L₂ coincidence requirements. The TET was cycled through various coincidence conditions, including singles from the front detectors. At the expense of some time resolution, this procedure permitted us to obtain significant data in the outer magnetosphere and excellent data during the long passage through the magnetotail region.

Some of the published results from this experiment required extensive corrections for dead time, accidental coincidences and anticoincidences (Vogt et al., 1979a, 1979b; Schardt et al., 1981; Gehrels et al., 1981). These corrections can be applied only on a case-by-case basis after a careful study of the environment and many self-consistency checks. They cannot be applied on a systematic basis and we have no computer programs to do so; therefore, data from such periods are not included in the Data Center submission. The scientists on the CRS team will, however, be glad to consider special requests if the desired information can be extracted from the data.

In order to acquaint the potential user of these data with the type of information that can be extracted from the CRS data, we are showing typical rates and fluxes in Figures 2 through 7.

Description of the Data

(1) LD1 RATE gives the nominal > 0.43-MeV proton flux cm^-2s^-1sr^-1. This rate includes all particles which pass through a 0.8 mg/cm² aluminum foil and deposits more than 220 keV in a 34.6 µ Si detector on Voyager 1 (209 keV, 33.9 µ on Voyager 2) Therefore, heavy ions, such as oxygen and sulfur are also detected; however, their contributin is believed to be relatively small. Only a small percentage of the pulses in this detector are larger than the maximum energy that can be deposited by a proton. Heavy ions would produce such large pulses, unless their energy spectra were much steeper than the proton spectrum. The true flux, F_t, can be calculated from the data:

and corrections are small for F < 1000 cm^-2 s^-1.

(2) LD2 RATE is not suitable for an absolute flux determination and is given in counters per s. The detector responds to protons and ions that penetrate either (a) 0.8 mg/cm² Al plus 8.0 mg/cm² Si and lose at least 200 keV in a 35 µ Si detector (1.8 to 13 MeV) or (b) pass through > 140 mg/cm² Al. For an E^-2 proton spectrum, the contributions from (a) and (b) would be about equal; however, the proton spectrum is substantially softer throughout most of the magnetosphere and the detector should respond primarily to (a). Dead time corrections are given by

where R is the count rate in counts/s. Thus, correction to the supplied data are small for R < 4000 c/sec, but become 80 large in the middle magnetosphere that the magnitude of even relative intensity changes becomes uncertain.

(3) LD L₁.L₂. L₄. SL COINCIDENCE RATE gives the total proton flux (cm^-2s^-1sr^-1) between ~ 1.8 and ~ 8.1 MeV with a small admixture of alpha particles. Accidental coincidences become substantial at higher rates and the flux derived from pulse-height analysis should be used if accuracy is desired.

(4) LDTRP RATE gives proton flux (cm^-2s^{- 1}sr^-1) between 3.0 and 8.0 MeV with a small alpha particle contribution (L₁L₂L₃ coincidences are required).

(5) IBS4E RATE gives the electron flux (cm^{- 2}s^-1sr^-1) for electrons with a range between 4 and 10 mm in Si; this corresponds approximately to the energy range of 2.6-5.1 MeV. Accidental coincidence and dead time corrections are generally small in the magnetotail and have not been applied to these data. Because of differences between Voyager 1 and 2, we give the average rate for HET I and II for Voyager 1 and the HET I rate for Voyager 2.

(6) IBS3E RATE is the same as (5); but the electron range falls between 10 and 16 mm of Si, or approximately 5.1-8 MeV.

(7) IBS2E RATE is the same as (5); but the electron range falls between 16 and 22 mm of Si, or approximately 8-12 MeV.

(8) D4L RATE is not suitable for an absolute electron flux determination. This counting rate includes all pulses from detector D₄ of TET (Fig. 1) which exceed 0.5 MeV. The shielding varies with direction of incidence but is at least 1.2 cm of Si. In the Jovian environment, the detector responds primarily to electrons with energies above ~ 6 MeV. The D₄L rate is useful primarily for determining relative changes in the high-energy electron flux. This rate has a high background from the RTG. Where needed, the dead time corrections should be applied as to the LD₂ rate ( ~ 2.55x10^-5 s).

(9) Pulse-height Analyzed Proton Flux (FPHA) is derived from a E vs. E analysis of pulses from L₁, L₂ and L₃ of LET (Fig 1) and gives the average proton flux (cm^-2s^-1sr^-1MeV^-1) in six energy channels. Where required, a correction should be applied for the dead time in LD1 as follows:

where FPHA is the listed flux of this rate (9) and FLD1 is the flux given in rate 1. FPHA gives the most accurate value of the proton flux available from this experiment; however, the counting statistics are poorer than for the other rates because of limited sampling. Fluxes derived from rate 3 (LD) which cover the same energy range as FPHA will be higher because of poorer definition of the energy threshold, accidental coincidences and a variable, but small, background contribution.

Table 1
CRS DETECTORS USED DURING JUPITER ENCOUNTER

Fig. 1. Schematic diagram of the High Energy Telescope (HET), Low Energy Telescope (LET) and the Electron Telescope (TET) systems.

Fig. 2. Proton and > 5 MeV electron flux observed during the inbound pass of Voyager 1. Bow shock and magnetopause crossings are indicated by S and M, respectively. Jovicentric longitudes ( _III1965) of flux maxima near magnetic equatorial crossings are indicated.

Fig. 3. Proton and electron intensities observed by Voyager 2

Fig. 4. Relative proton intensities in the middle magnetosphere observed with Voyager 1. Due to the extreme fluxes, the detector threshold shifted with counting rate.

Fig. 5. Approximate intensity of protons with energies above 2.5 MeV observed with Voyager 2 near the orbit if Ganymede. Note the large intensity fluctuations which fall within ±4 hours of the closest approach to Ganymede.

Fig. 6. Electron (2.6-5.1 MeV) and proton fluxes observed during the outbound passes of Voyagers 1 and 2. Electron and > 1.8 MeV proton fluxes above 10³ cm^-2 s^-1 sr^-1 are uncertain because of large corrections and show only relative trends.

Fig. 7. The fluxes shown in Fig. 6 are extended from 130 to 250 Jovian radii. The shading near the distance scale indicates when the spacecraft were in the magnetotail.

REFERENCES

Gehrels, N., E.C. Stone and J.H. Trainor, "Energetic Oxygen and Sulful in the Jovian Magnetosphere," submitted to J. Geophys. Res., 1981.

Lupton, J.E., and E.C. Stone, "Measurement of Electron Detection Efficiencies in Solid-state Detectors," Nucl. Instr. and Meth. 98, 189, 1972.

McDonald, F.B., A.W. Schardt and J.H. Trainor, "Energetic Protons in the Jovian Magnetosphere," J. Geophys. Res. 84, 2579, 1979.

Schardt, A.W., F.B. McDonald and J.H. Trainor, "Energetic Particles in the Pre-dawn Magnetotail of Jupiter," J. Geophys. Res., special Voyager issue, 1981.

Stone, E.C., R.E. Vogt, F.B. McDonald, B.J. Teegarden, J.H. Trainor, J.R. Jokipii and W.R. Webber, "Cosmic Ray Investigation for the Voyager Mission: Energetic Particle Studies in the outer Heliosphere--and Beyond," Space Sci. Rev. 21, 355, 1977.

Vogt, R.E., W.R. Cook, A.C. Cummings, T.L. Garrard, N. Gehrels, E.C. Stone, J.H. Trainor, A.W. Schardt, T. Conlon, N. Lal and F.B. McDonald, "Voyager 1: Energetic Ions and Electrons in the Jovian Manetosphere," Science 204, 1003, 1979a.

Vogt, R.E., A.C. Cummings, N. Gehrels, E.C. Stone, J.H. Trainor, A.W. Schardt, T.F. Conlon and F.B. McDonald, "Voyager 2: Energetic Ions and Electrons in the Jovian Magnetosphere," Science 206, 984, 1979b.