| DATA_SET_DESCRIPTION |
Data Set Overview
=================
Instrument P.I. : Rochus E. Vogt
Data Supplier : National Space Science Data Center
Data sampling rate : variable (1 hr for FPHA data, 15 min.
for all others)
Data Set Start Time : 1979-07-03T00:00:00.000Z
Data Set Stop Time : 1979-08-03T23:45:00.000Z
(The following description has been excerpted from
[NSSDCCRS1979])
As its name implies, the Cosmic Ray Subsystem (CRS) was
designed for cosmic ray studies [STONEETAL1977B]. 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 cm^2 ster)
and long electronic time constants (~ 24 [micro]sec) for low
power consumption and good stability. Normally, the data are
primarily derived from comprehensive ([Delta]E[1], [Delta]E[2]
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. In inter-
planetary 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&STONE1972]. 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 [micro]sec). The high
counting rates, however, caused some baseline shift which may
have raised proton thresholds significantly. In the inner
magnetosphere, the L[2] counting rate was still useful because
it never rolled over. This rate is due to 1.8- to 13-MeV
protons penetrating L[1] (0.43 cm^2 ster) and > 9-MeV protons
penetrating the shield (8.4 cm^2 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
[MCDONALDETAL1979], the contribution from protons penetrating
the shield would be only 3 to 14%.
The LET L[1]L[2]L[4] and L[1]L[2]L[3] 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
L[1], accidental L[1]L[2] coincidences and anticoincidence
losses from L[4]. 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 L[1], L[2]
and L[3] 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 [Delta]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 L[1] only and L[1]L[2]
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 ([VOGTETAL1979A], [VOGTETAL1979B];
[SCHARDTETAL1981]; [GEHRELS1981]). 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.
Description of the Data
-----------------------
(1) LD1 RATE gives the nominal > 0.43-MeV proton flux cm^-2
s^-1 sr^-1. This rate includes all particles which pass
through a 0.8 mg/cm^2 aluminum foil and deposits more than
220 keV in a 34.6 [micron] Si detector on Voyager 1 (209
keV, 33.9 [microns] on Voyager 2) Therefore, heavy ions,
such as oxygen and sulfur are also detected; however, their
contribution 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:
F
F[t] = ----------------
1 - 1.26x10^-4 F
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/cm^2 Al
plus 8.0 mg/cm^2 Si and lose at least 200 keV in a 35
[micron] Si detector (1.8 to 13 MeV) or (b) pass through
> 140 mg/cm^2 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
R
R[t] = ----------------
1 - 2.55x10^-5 R
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 L[1].L[2]. L[4]. SL COINCIDENCE RATE gives the total
proton flux (cm^-2 s^-1 sr^-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^-2 s^-1 sr^-1) between
3.0 and 8.0 MeV with a small alpha particle contribution
(L[1]L[2]L[3] coincidences are required).
(5) IBS4E RATE gives the electron flux (cm^-2 s^-1 sr^-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 D[4] of TET 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
D[4]L 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 LD[2] rate ([tau]
~ 2.55x10^-5 s).
(9) Pulse-height Analyzed Proton Flux (FPHA) is derived from a
[Delta]E vs. E analysis of pulses from L[1], L[2] and L[3]
of LET and gives the average proton flux (cm^-2 s^-1 sr^-1
MeV^-1) in six energy channels. Where required, a
correction should be applied for the dead time in LD1 as
follows:
FPHA
FPHA[t] = -------------------
1 - 1.26x10^-4 FLD1
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.
ENERGY CHANNELS (MEV) OF FPHA
(absolute accuracy ~ 10%)
VOYAGER 1 VOYAGER 2
1 1.829 - 2.045 1.807 - 2.001
2 2.045 - 3.104 2.001 - 3.309
3 3.104 - 3.753 3.309 - 3.984
4 3.753 - 4.530 3.984 - 4.761
5 4.530 - 6.284 4.761 - 6.041
6 6.284 - 8.091 6.041 - 8.043
Data Coverage
================
Filename Recs Start Stop
-----------------------------------------------------------------------
BS2E_RATE.TAB 2424 1979-07-03T00:00:00.000Z 1979-08-03T23:45:00.000Z
BS3E_RATE.TAB 2424 1979-07-03T00:00:00.000Z 1979-08-03T23:45:00.000Z
BS4E_RATE.TAB 2424 1979-07-03T00:00:00.000Z 1979-08-03T23:45:00.000Z
D4L_RATE.TAB 2744 1979-07-03T00:00:00.000Z 1979-08-03T23:45:00.000Z
FPHA_RATE.TAB 632 1979-07-03T00:00:00.000Z 1979-08-03T23:00:00.000Z
LD1_RATE.TAB 2744 1979-07-03T00:00:00.000Z 1979-08-03T23:45:00.000Z
LD2_RATE.TAB 1056 1979-07-03T00:00:00.000Z 1979-07-13T23:45:00.000Z
LD_RATE.TAB 2526 1979-07-03T00:00:00.000Z 1979-08-03T23:45:00.000Z
LDTRP_RATE.TAB 2520 1979-07-03T00:00:00.000Z 1979-08-03T23:45:00.000Z
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