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Appendix A
A.0 Introduction
Our investigation of electron radiation belt dynamics primarily uses data from the
High Sensitivity Telescope (HIST) onboard the Polar spacecraft [Blake et al., 1995]. This
chapter describes how the Polar spacecraft’s orbit samples the magnetosphere, how the
HIST instrument functions, and gives details of the data. The coverage is spatially
extensive but the instrument and the data acquired from it have limitations. Every
instrument has idiosyncrasies that must be understood to know in what regions and under
what conditions the data are reliable. To understand the limitations of the HIST
instrument we describe how it translates electronic signals to output data and identify
periods when the instrument gives anomalous data based on significant deviations from
expected measurements. The measurements from the anomalous periods are compared to
data from other spacecraft to verify that the observations are indeed instrument artifacts
and not physical phenomena. We conclude the chapter with a probable explanation for
the anomalous instrument response and finally give a method for identifying problem
periods and interpreting the anomalous data.
A.1 Polar and HIST
HIST is part of a suite of instruments that make up the Comprehensive Energetic
Particle and Pitch Angle Distribution (CEPPAD) experiment onboard the Polar
spacecraft. The suite of instruments measures flux of energetic electrons with energies >
25 kev and protons with energies > 10 kev. As part of this instrument set, the objective of
HIST is to measure the flux and pitch angles of 0.35 MeV to 10 MeV electrons as well as
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3.25 MeV to 100 MeV protons. Within this range of energies the instrument measures 16
logarithmically spaced energy bins. The mean energy and width of the 16 bins were
determined using a bow-time method explained in detail by Selesnick and Blake [2000]
and are given in Table 1. The instrument collimator is oriented perpendicular to the spin
axis of the spacecraft. Pitch angle distributions are obtained by making 16 measurements
per spacecraft rotation.
Polar was launched on February 24, 1996 in an elliptical orbit with a 9 Re apogee
at 90 inclination. The initial orbit crossed the equator near L~2.5 approximately 2 Re
earthward of the nominal peak in flux of the outer electron radiation belt. In the initial
orbital configuration, the larger L values were sampled only at high off-equatorial
latitudes. Over the lifetime of the mission the orbital line of apsides moved slowly
towards the equator. By the year 2000 the orbit crossed the equator near L~5-6. The
rotation of the orbit allowed measurements at high L values near equatorial latitudes.
Figure 1 is an illustration of the elliptical orbit and the progressive changes. The figure
shows two sample orbits in GSM coordinates from the early mission (May 16, 1997)
when the orbit was highly inclined as well as a later near equatorial orbit (July 1, 2000).
Because of the changing inclination and elliptical shape of the Polar orbit, HIST
measures particles at a wide range of L values over varying latitudes and local times. The
wide spatial coverage will be made use of throughout this thesis.
A.2 How HIST works
Understanding the limitations of the HIST data requires knowledge of how the
instrument operates. Here we describe how the energy of a particle entering the detector
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is determined and how electrons and protons are separated and counted. The HIST
instrument collimator has a 26o field of view that consists of three detectors labeled A, B
and C in Figure 2. A particle entering the aperture (left hand side of Figure 2) will deposit
energy in all or some of the detectors as it finally comes to a stop. Detectors A and B are
solid state silicon detectors. The amount of energy deposited by a particle entering one of
these detectors is related to the number of electron-hole pairs produced in the detector
during the interaction. The electron-hole pairs create a charge pulse that is converted to a
voltage pulse using a charge sensitive pre-amplifier and pulse shaping circuitry. Detector
C, which is an organic plastic scintillator, works in a slightly different manner. A particle
entering this detector causes photons to be emitted that are collected by a photomultiplier
tube (PMT). The PMT creates a voltage pulse proportional to the detected light which in
turn is related to deposited energy.
From the simple description above, it would seem that summing the voltage
pulses from all three detectors determines the total energy of the particle but this is not
the case. Determining the total energy is complicated because the detected energy
measured by the sensors is not equal to the actual energy deposited by a particle.
Appropriate gains must be applied to equate detected energy to the incident energy of the
particle. Figure 3, taken from Contos [1997], shows how the energy measured by each
sensor relates to the actual incident energy of a beam of particles. The cyan trace shows
the energy measured by sensor A, the blue trace shows the energy measured by sensor B
and the green trace shows the energy measured by sensor C. The red trace shows the total
energy measured by all three sensors. Generally, the detected and incident energy are
similar but significant deviations exist. In particular, at low energies the detected energy
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is nearly four times lower than the incident energy. Therefore, variable gains must be
applied. Multiplying the measured energy by the appropriate gain gives the total incident
energy of a particle.
A multi-step process separates proton and electron counts and determines the
appropriate gain and energy of the particles. First, a particle enters the collimator and
deposits energy in all or some of the detectors as it comes to a stop. Each detector
registers a voltage pulse that is related to deposited energy. Here the voltage signal is split
as shown in Figure 4. One branch carries the signal to a processor that sums the analog
voltage from all three detectors. The other branch carries the signal to a set of voltage
discriminators.
The discriminators measure whether the energy deposited in each detector reaches
fixed threshold levels. The threshold levels are shown for each detector in tables 2-4. If
the threshold is met, the discriminator output is set to a logical 1. For example, if the
energy deposited in detector B is greater than 1.1 MeV, the B3 descriminator signal is set
to a logical 1. Boolean logic is applied to the discriminator outputs as shown in Table 3.
The Boolean logic separates protons and electrons and determines gains. Electrons and
protons are separated using the output of TH= AH + BH + CH. If the output is a logical 1
the particle is identified as a proton. If the output is a logical 0 the particle is identified as
an electron. The Boolean logic used to determine gains depends on the instrument mode
of operation as described below.
The instrument has three modes of operation, ABC, HBC and BC. Here we are
concerned only with the ABC and HBC modes. The BC mode was used only for a short
period at the start of the mission and has not been calibrated. BC data are not used in our
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analysis. In the ABC mode, the A1 discriminator threshold (0.053 MeV) must be met to
indicate a valid count. The instrument sums the energy from all three detectors and
assigns the gain determined using the Boolean logic in Table 3 labeled ‘ABC mode’. In
the HBC mode the B1 discriminator threshold (0.083 MeV) must be met to indicate a
valid count. In this mode the energy deposited in detectors A and B are not used to
calculate the total energy. Instead the appropriate gain is assigned using the Boolean logic
in table 3 labeled, ‘HBC mode . The gain is applied only to energy deposited in detector
C and not all three detectors. The purpose of the HBC mode is to minimize the problem
of coincident counts. A coincident count occurs when two or more particles enter the
detector simultaneously and are treated as a single particle. When this happens the
instrument measures multiple particles as one particle with very high energy. Coincident
counts occur when the total electron flux is high. The HBC mode minimizes this problem
by using the B detector instead of the A detector to trigger a valid event. The number of
particles entering this detector is lower because of the steep fall off of flux with energy.
In addition, only the C detector is used to determine the total energy rather than the
summed energy from all three detectors. Therefore, any energy from a coincident low
energy particle is not included in the energy determination. During times of high counts
the instrument automatically begins switching every 96 seconds between the HBC and
ABC modes. Each particle is assigned an ID tag that represents the appropriate gain. The
ID tag is used in the process of binning counts into various energy bins as described next.
Now that energy is determined the count is assigned to one of 16 logarithmically
spaced energy bins. An onboard look up table (LUT) shown as Table 6 defines the energy
bins for both protons and electrons. The look up table uses the particle ID tag that
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represents the gain as well as the summed voltage to bin each count. For example, the E0
electron channel counts electrons with a summed voltage between 33-40 and a gain of
9.88 keV/bit represented by ID 1. Multiplying the voltage by the gain shows that this
energy channel counts electrons with energy between 0.326 MeV and 0.395 MeV. There
are separate look-up tables for the HBC and ABC modes. Two versions of look up tables
have been used during the course of the mission. The original look-up tables used from
launch to January 24th, 1997 were created based on expected instrument response to
particles of different energies. Extensive simulations of instrument response showed that
the original look up tables did not always accurately bin the data [Contos, 1997]. Again
using simulations, new look-up tables were created that optimized binning. These lookup tables were implemented onboard the spacecraft on January 24th, 1997 and are
currently still in use.
A.3 Observations of Instrument Anomalies
Through much of the mission the HIST instrument accurately measured electron
flux but during periods of high count rates the data show anomalies. The anomalies are
apparent when comparing the electron flux versus L profiles measured by HIST to
expected flux versus L profiles. The AE8MIN and AE8MAX models [Vette, 1991] give
our best expectation of flux versus L profiles. The AE8MIN and AE8MAX models
provide average electron flux based on data from 20 spacecraft from the early sixties to
the mid-seventies. The AE8MIN profile is based on data taken during solar minimum
while the AE8MAX profile is based on data taken during solar maximum. Figure 5
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shows AE8MIN and AE8MAX flux versus L profiles for electrons with energy from .1 to
7 MeV.
During geomagnetically quiet periods when the total particle count rate is low,
the HIST electron measurements qualitatively match the AE8 expected flux versus L
profiles. But as the total particle count rate increases during geomagnetically active
periods the expected and measured data differ. Figure 6 shows the changing flux versus L
profiles before and after a geomagnetic storm which began on 01:00 UT April 24, 1998
(day 114). On day 113, prior to the start of the storm the total count rate is low and the
flux versus L profile is qualitatively consistent with the AE8 models. But as the storm
progresses, the HIST data show features not consistent with the profiles predicted by the
AE8 models. An unusual dip in the flux of electrons with low energy (<1.6 MeV) appears
in the HIST data between L=4-5. The flux minimum of low energy electrons occurs at the
same L value where the flux of high energy electrons (>1.6 MeV) peaks. The low energy
electron flux dip at L~4.5 persists for several days and becomes more pronounced as the
flux of high energy electrons increases.
An additional unusual feature apparent in Figure 6, is the response of the 1.6 MeV
electron flux. As the storm progresses the flux of 1.6 MeV electrons becomes
anomalously high and increases above the flux level of .8 MeV electrons. Figure 7 plots
electron energy spectra at L=4.5 from day 113 to 119. The plot shows a flux peak in the
energy spectra at 1.6 MeV beginning on day 117 and persisting through day 119.
Electron energy spectra with flux peaks at a limited range of energies are not uncommon.
However these types of distributions are unstable and generally do not persist for 2 or
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more days as shown in Figure 7. Whether this unusually high energy channel is related to
the low energy electron flux minimum or simply an additional problem is unclear.
Pitch angle distributions show a noticeable change in the region of the anomalous
low energy flux minimum. As the flux minimum becomes more pronounced, the pitch
angle distributions of low energy electrons become increasingly isotropic. Figure 8
demonstrates the changing pitch angle distributions plotted as a function of both L and
time. Panels A1-A6 show pitch angle distributions as a function of L prior to the start of
the storm. At this time the distributions are peaked at 90o at all energies. Panels B1-B6
and C1-C6 show distributions 1 day and 3 days after the storm has begun. One day after
the start of the storm the flux minimum begins to form. The pitch angle distributions from
the same region (Panels B3 and B4) become slightly isotropic. Three days after the start
of the storm the low energy flux minimum is clearly developed and the pitch angle
distributions from the region are now completely isotropic (Panels C3 and C4).
Unusual flux profiles and pitch angles do not automatically indicate an instrument
anomaly. However, a survey of many storms when count rates reached high levels shows
the same three systematic features in the HIST data: a low energy (<1.6 MeV) electron
flux minimum occurring at the high energy (>1.6 MeV) flux peak, isotropic pitch angles
in the region of the flux minimum, and an anomalously high flux of 1.6 MeV electrons.
The systematic recurring nature of these events suggests the features are instrument
artifacts.
A.4 Comparison of HIST data to HEO and CRRES
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To establish whether the unusual features are instrument artifacts or physical
phenomena we compare measurements of HIST to measurements of the DSU3
instrument onboard the HEO3 spacecraft and the Medium Energy Analyzer (MEA)
[Gussenhoven et al., 1985] onboard the Combined Release and Radiation Effects satellite
(CRRES) [Johnson and Ball, 1992]. Surprisingly, comparisons between the two sets of
instruments yield different results. The CRRES/MEA data are not consistent with
Polar/HIST data. Low energy flux minima are present in the CRRES data but the minima
do not occur systematically during high count periods, they do not occur at the high
energy flux peak, the pitch angle distributions do not become isotropic and there is no
anomalously high flux of 1.6 MeV electrons. The HEO/DSU3 dataset, on the other hand
is similar to the Polar data in that it also shows anomalous low energy electron flux
minima at times of high fluxes. Pitch angle distributions are not obtained by this detector.
Below we describe the comparison between the datasets. We discuss why the
comparisons give different results and explain why we consider the CRRES comparison
to be more reliable.
The CRRES mission was short-lived from July 1990 to October 1991. The orbit
was elliptical with a ~6 Re apogee but, unlike Polar, it remained within 18 degrees of the
equatorial plane. MEA, the high energy particle instrument onboard CRRES, measured
electrons with energies between 0.123 –1.714 MeV. Eleven of the energy channels with
energy from 0.648 to 1.714 MeV overlap with the energy range of electrons measured by
HIST. Direct comparisons between the Polar and CRRES data are not possible because
the missions were flown at different times and latitudes, but qualitative comparisons are.
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A qualitative comparison is made by looking for the same systematic features in the two
datasets.
The CRRES data periodically show dips in the flux of low energy electrons.
However, a presentation of representative events reveals clear differences between these
flux dips and the observations of Polar. For example, the flux dips observed by CRRES
are not just associated with geomagnetically active periods. They also occur during quiet
periods with no total electron count increases as shown in Figure 9. The figure plots flux
versus L profiles of .6 MeV, 1.1 MeV, and 1.7 MeV electrons from 5 consecutive
CRRES orbits. The data shows a dip in the electron flux at L=4-5.5 even though the total
electron counts during these orbits is not high and no geomagnetic storm has occurred.
The event plotted also differs from the Polar observations in that the flux minimum is not
concurrent with a peak in the high energy electron flux. At CRRES all three energy
channels including the high energy 1.7 MeV channel show a flux minimum. At no time
during the CRRES mission do we observe a flux minimum of .6 MeV electrons
concurrent with a peak in flux of 1.7 MeV electrons.
The CRRES data also show low energy electron flux minima occurring during
geomagnetically active periods with high total electron counts similar to the Polar
observations. Figure 10 plots an example of such an event. This event is the most similar
to the observations of Polar yet differences are still apparent. As in the last example, there
is no peak in the high energy channel concurrent with the low energy electron flux dip.
All three energy channels show a flux minimum. In addition, the pitch angle distributions
differ from those seen in the Polar data. Figure 11 plots pitch angle distributions within
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the flux minimum during this same event. No isotropic distributions are observed in the
region of the flux minimum.
Finally, Figure 12 shows an example of CRRES data that bears no resemblance to
the HIST observations. In this figure, the relativistic electron fluxes increase at all
energies with no evidence of a low energy electron flux minimum between L=4-5.
Comparisons between the Polar and HEO data, on the other hand, do show
similarities. Direct comparisons can be made between measurements from the HEO and
Polar spacecrafts because the two orbits overlap in position and time. Both the HEO and
Polar spacecraft have elliptical highly inclined orbits and therefore probe similar regions
of the magnetosphere. Figure 13 plots the HEO and Polar orbits for a geomagnetically
active period from April 23, 1998 to April 30, 1998. The HEO orbit is shown in black
and the Polar orbit is shown in gray. At times the high latitude portion of the Polar orbit
is within a few degrees latitude of the HEO orbit.
Whereas the latitudes of the spacecraft are similar, the instruments measure
different quantities. The DSU3 instrument onboard HEO3 measures omnidirectional
integral flux at > 0.63 MeV, >1.5 MeV and >3.0 MeV. Polar/HIST measures differential
flux between 0.67-7.2 MeV at 16 pitch angles. To make meaningful comparisons we
calculate equivalent quantities from the two datasets. With the HEO dataset we create
pseudo differential flux detectors by subtracting the integral flux measurements. The
subtraction gives two differential flux measurements of electrons with energy range 0.63
MeV- 1.5 and MeV 1.5 – 3.0 MeV to be compared to the HIST data. It would also be
possible to carry out the reverse operation and make integral flux measurements from the
Polar data equivalent to the integral detectors of HEO. However the anomalous minimum
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of low energy electron flux is generally not observed in the integral detectors because the
flux of high energy electrons peaks in the region of the low energy electron flux
minimum. The decrease of low energy electron flux is compensated for by an increase of
high energy electron flux.
To compare the HEO and Polar data, the Polar data must also be processed
further. First we use the Polar measurements at 16 pitch angles to obtain equivalent
pseudo-omnidirectional flux by averaging the measurements over one spacecraft spin. In
addition, the range of the DSU3 pseudo-differential energy channels is still wider than the
Polar differential energy channels. We sum the fluxes measured in the .67-1.6 MeV
energy channels and 1.8-3.2 MeV channels to obtain flux over the range equivalent to
that of HEO. The final comparisons of flux measured by our pseudo-detectors are shown
in Figure 14 for April 28, 1998. The four panels show data taken at four different
latitudes and local times. Panel A shows the comparison between the southern nightside
passes marked with diamonds in the trajectory plot of Figure 13. The latitude of the Polar
spacecraft at L=4.5 is ~50o while the latitude of the HEO spacecraft is ~60o. At these high
latitudes neither spacecraft sees evidence of a flux minimum. The comparison here shows
the peak flux measured by Polar increased above that of HEO by a factor of ~1.5. The
increased flux at Polar is expected because the latitude of Polar is 10o lower than HEO at
this time. The two measurements are both made on the same day but the times may differ
by several hours. The magnetic local time separations of the measurements may also
contribute to the observed flux differences.
Panel B compares the northern nightside passes of HEO and Polar. Both
spacecraft reach L=4.5 at a latitude of ~30o. Here we expect the best correspondence of
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the two instruments because of the comparable latitudes. Both spacecraft show evidence
of a low energy flux minimum concurrent with a high energy flux peak. The flux
measured by Polar is now a factor of ~1.5 less than the fluxes measured by HEO. The
difference in flux levels could be because of the time or local time differences.
Panel C shows the comparison of the southern dayside passes. Here Polar reaches
L=4.5 at latitude of ~45o while HEO is at a latitude of ~ 60o. The Polar data shows a
flattening while the HEO data does not. The differences here are expected because of the
large latitude separation. The last panel compares the measurements of the northern
dayside passes. Here both spacecraft show evidence of a minimum but the minimum at
Polar is more extreme.
The CRRES observations lead us to reject the Polar HIST data as invalid. Why do
the HEO observations not lead to the same conclusion? The differences between the
Polar-HEO and Polar-CRRES comparisons can be explained based on the accuracy of the
calibrations of the instruments. Every instrument must be calibrated in order to transform
measured counts to particle flux. The transformation requires multiplying measured
counts by an efficiency factor. Efficiency factors are known only to within certain error
margins. A large enough error in the efficiency factor may produce a flux minimum in
the difference of flux calculated from two integral channels as was done to create the
pseudo-differential channels. The efficiencies of the HEO instrument are known to within
factors of 2. Figure 15 shows how the HEO pseudo-differential channels are affected by a
factor of 2 change in the efficiency. In this figure, we have multiplied the HEO-DSU3
>1.5 MeV integral channel by a factor of 0.5 before subtracting it from the >0.6 MeV
integral channel. The flux minimum is no longer apparent. Because the results of the
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HEO data are susceptible to calibration errors we believe that the CRRES data are the
more reliable. The differences between the CRRES/MEA and Polar/HIST data suggest
that the anomalous flux minima measured by HIST are indeed an instrumental artifact.
A.5 Possible Explanation of Anomalies
Having concluded that the anomalies are instrumental artifacts, we now discuss a
probable cause. One explanation is that the dip in flux results from missing counts during
instrument dead-time. The dead-time is the time required for the instrument to fully
process one particle count. The maximum counts/sec measured by any instrument equals
1/dead-time. If the dead-time is a constant value, the total measured counts will level off
at a constant value when the incident particle flux exceeds 1/dead-time. This can not
explain the observed dip influx in the HIST data. However, a decrease in counts can
occur if the dead-time is not constant. Such is the case with the HIST instrument. The
time required to process an electron passing through the two solid state detectors
(detectors A and B) is shorter than the time required to process an electron that passes
through the photo-multiplier tube (detector C). As a result, higher energy particles take
longer to process. In this case, the maximum counts/sec the instrument measures
decreases as the number of high energy particles increases. Figure 16 shows total
counts/sec measured by HIST during an anomalous period. The total counts do not level
off to a fixed value. At L~3.5 the total counts still show a small dip that occurs at
approximately the peak of >1.6 MeV particles consistent with the proposed scenario.
A.6 Identifying and Accounting for Problem Data
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We have identified limitations in the HIST data and determined a satisfactory
explanation of the anomalies in the measured flux. We now present a simple criterion for
flagging suspect data and establishing what data are still usable even during these
problem periods.
Our criterion for flagging suspect data is based on the total measured counts per
sample. We find the maximum total counts measured at any time over a period of many
storms. We flag data as suspect when the total counts per sample equals 2*104 which is
roughly half that maximum. Figure 17 shows the same anomalous data plotted previously
in Figure 6 with flagged data shown in red. The criterion clearly identifies those data we
believe to be a problem.
While we’ve flagged large regions of data as being suspect, some of the data in
this region are still usable in limited ways. The HBC mode data under most
circumstances are still usable. The HBC mode is generally unaffected by the dead-time
problem because this mode counts only electrons with energy >~2 MeV therefore the
total counts/sec remain low. In addition, even the flagged data are still of some use. The
flagged data represent a lower bound of the electron flux that is a useful measurement in
some analysis. Throughout the remainder of the thesis, flagged data are either removed or
marked in gray and identified as lower bounds.
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References
Blake, J. B., et al., CEPPAD: Comprehensive energetic particle and pitch angle
distribution experiment on Polar, Space Sci. Rev., 71, 531, 1995.
Contos, A. R., Complete description and characterization of the high sensitivity telescope
(HIST) onboard the polar satellite., M.S. thesis, Boston University, Boston, Mass., 1997.
Gussenhoven, M.S., E.G. Mullen, and R.C. Sagalyn, CRRES/Spacerad experiment
descriptions, Air Force Geophysics Lab, Rept. AFGL-TR85-0017, Hanscom AFB,
MA,1985.
Johnson, M.H., and J.K. Ball, Combined Release and Radiation Effects Satellite
(CRRES) :Spacecraft and mission, J. Spacecraft and Rockets, 29, 556, 1992.
Selesnick, R. S. and J. B. Blake, On the source location of radiation belt electrons, J.
Geophys. Res., 105, 2607, 2000.
Vette, J. I., The AE-8 Trapped Electron Model Environment, NSSDC WDC-A-R&S 9124, 1991.
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Figure Captions Chapter 2
Table1. Energy width and mean energy for the 16 HIST energy channels given in units of
MeV. Values are given for both the ABC and HBC instrument modes. Two sets of values
are given because the instrument thresholds were changed on January 24, 1997.
Table 2. Threshold energy levels used to set discriminator outputs for detector A.
Table 3. Threshold energy levels used to set discriminator outputs for detector B.
Table 4. Threshold energy levels used to set discriminator outputs for detector C.
Table 5. Boolean logic used by the HIST instrument to identify protons and electrons and
set appropriate energy gains. A, B, and C are logical outputs from the discriminators
associated with each sensor. An asterik represents a logical ‘AND’ operator, a ‘+’ symbol
represents a logical ‘OR’ operator, and a bar over a variable represents a logical ‘NOT’
operator. The TH output determines whether a particle is an electron or proton.
Table 6. Onboard look-up table used by the HIST instrument to bin electron and proton
counts into 16 energies during the ABC mode as described in the text.
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Figure 1. Sample Polar orbits from May 16, 1997 and July 01, 2000 in GSM coordinates.
Panels A and B plot the May 16th orbit in the X-Z and Y-Z planes. Panels C and D plot
the July 1st orbit also in the X-Z and Y-Z planes.
Figure 2. Schematic of the HIST instrument showing the two silicon detectors labeled A
and B and the scintillator labeled C. Particles enter the collimator from the left-hand side
of the schematic.
Figure 3. Plot showing the incident energy of electrons entering the HIST instrument
versus the energy detected by the instrument. The cyan trace shows energy measured by
detector A. The blue trace shows energy measured by detector B and the green trace
shows energy measured by detector C. The red trace shows the total energy measured by
all three detectors.
Figure 4. Block diagram outlining the electronic circuitry of the HIST instrument.
Figure 5. Plot showing omnidirectional differential flux (#/cm2-s-str-keV) of electrons
with energy from .1-7 MeV as a function of L from the AP8MAX and AP8MIN models.
Panel A plots data from the AP8MAX model and panel B plots data from the AP8MIN
model.
Figure 6. Plot of spin averaged differential electron flux (#/cm2-s-str-MeV) from the
HIST instrument as a function of time and L during a geomagnetically active period. The
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left panel plots differential electron flux on a linear scale to highlight the flux minimum
in the .7 MeV energy channel. The right panel plots the same differential electron flux
but on a log scale to show the increase of the 7.2 MeV electron flux.
Figure 7. Energy spectra from L=4.5 plotted as differential electron flux (#/cm2-s-strMeV) versus energy (MeV). Panels A-E show energy spectra from day 113 to119. On
day 117 the 1.6 MeV energy channel shown in red becomes anomalously high.
Figure 8. Two plots showing the Polar (gray) and HEO (black) orbit. Panel A shows the
magnetic latitude as a function of L for the two spacecraft. Panel B shows the magnetic
local time as a function of L. The symbols separate the orbits by local time and latitude.
Triangles mark the portions of the orbits occurring in the northern-dusk part of the
magnetosphere. Asterisks mark the portions of the orbit in the northern dawnside
magnetosphere. Diamonds mark the orbit in the southern duskside magnetosphere. And
circles mark the orbit in the southern dawnside magnetosphere.
Figure 9. Plot showing a comparison of HEO/DSU3 and Polar/HIST electron data for all
passes occurring on April 24, 1998. The data has been transformed into similar pseudodetectors as described in the text. All four panels show differential electron flux (#/cm2-sstr-MeV) as a function of L with energy from 0.6-1.5 MeV and 1.5-3.0 MeV. HEO 0.61.5 MeV electron flux is shown as thick black traces while HEO 1.5-3.0 MeV electron
flux is plotted as a thin black line. Thick gray lines and thin gray lines mark the Polar .61.6 MeV electron flux and 1.8-3.2 MeV electron flux respectively. The gaps in the Polar
20
data are a result of mode switching as described in the text. Each panel plots data taken
from different portions of the orbit as marked.
Figure 10. Plot of pitch angle distributions as a function of time and L during the
geomagentically active period from April 23-April 30 1998. Panels A, B and C shows
HIST differential electron flux at four energies from day April 23 (day 113), April 25
(day 115) and April 27 (day 117). Vertical lines labeled A-F mark the L value of the
corresponding pitch angle distribution plots. The pitch angle plots labeled A-F plot
differential electron flux versus equatorial pitch angle.
Figure 11. Plot of differential electron flux measured by the MEA/CRRES instrument.
The plot shows flux as a function of L and time from 5 orbits. The blue, green, and red
traces plot flux of electrons with energy equal to 0.6 Mev, 1.1 MeV and 1.7 MeV
respectively. The bottom panel shows the Dst index in units of nT. The vertical lines
labeled A-E correspond to the panels labeled A-E and mark the time of each orbit.
Figure 12. Plot of differential electron flux measured by the MEA/CRRES instrument.
The plot shows flux as a function of L and time from 5 orbits. The blue, green, and red
traces plot flux of electrons with energy equal to 0.6 Mev, 1.1 MeV and 1.7 MeV
respectively. The bottom panel shows the Dst index in units of nT. The vertical lines
labeled A-E correspond to the panels labeled A-E and mark the time of each orbit.
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Figure 13. Plot of differential electron flux and pitch angle distributions measured by the
CRRES/MEA instrument as a function of L and time. Panel A shows data taken from
orbit 595. The vertical lines labeled A1-A6 mark the L value of the pitch angle
distributions plotted in panels A1-A6. The pitch angle distributions are plotted as flux
versus equatorial pitch angle. The blue, green, and red traces correspond to flux of
electrons with energy equal to 0.6 Mev, 1.1 MeV and 1.7 MeV respectively. Panel B
plots data in the same format from CRRES orbit 596 and panel C plots data from orbit
597. The bottom panel shows the Dst index. The vertical lines mark the time of each
orbit.
Figure 14. Plot of differential electron flux measured by the MEA/CRRES instrument.
The plot shows flux as a function of L and time from 5 orbits. The blue, green, and red
traces plot flux of electrons with energy equal to 0.6 Mev, 1.1 MeV and 1.7 MeV
respectively. The bottom panel shows the Dst index in units of nT. The vertical lines
labeled A-E correspond to the panels labeled A-E and mark the time of each orbit.
Figure 15. Plot showing how errors in the HEO efficiency factors will affect the pseudodetector flux. This plot is similar to Figure 8 but the efficiency factor of the HEO/DSU3
>1.5 MeV channel has been changed as described in the text. All four panels show
differential electron flux as a function of L from pseudo-detectors with energy from 0.61.5 MeV and 1.5-3.0 MeV. HEO 0.6-1.5 MeV electron flux is shown as thick black
traces while HEO 1.5-3.0 MeV electron flux is plotted as a thin black line. Thick gray
lines and thin gray lines mark the Polar 0.6-1.6 MeV electron flux and 1.8-3.2 MeV
22
electron flux respectively. The gaps in the Polar data are a result of mode switching as
described in the text. Each panel plots data taken from different portions of the orbit as
marked.
Figure 16. Plot of HIST total counts/sample during an anomalous period.
Figure 17. Plot showing HIST electron flux (#/cm2-s-str-MeV) as a function of L and
time during an anomalous period. Data shown in red has been flagged as suspect. Data
shown in black is uncorrupted data.
23
Before day 24 hour 20 1997
After day 24 hour 20 1997
Mode-bin
Mean Energy
dE
Mode-bin
Mean Energy
dE
ABC E0
1.2664
.0039
ABCE 0
.7602
.0139
ABC E1
.8356
.0108
ABC E1
.6687
.0328
ABC E2
.6785
.01083
ABC E2
.7264
.0587
ABC E3
.7219
.0603
ABC E3
.8443
.0849
ABC E4
.8618
.1108
ABC E4
1.069
.2198
ABC E5
1.0692
.1712
ABC E5
1.2930
.1745
ABC E6
1.2918
.2381
ABC E6
1.6045
.1036
ABC E7
1.5680
.2550
ABC E7
1.8142
.3200
ABC E8
1.9010
.4398
ABC E8
2.1862
.3255
ABC E9
2.4408
.4895
ABC E9
2.6784
.3796
ABC E10
3.130
.6233
ABC E10
3.2889
.4778
ABC E11
4.030
.7479
ABC E11
3.9626
.6588
ABC E12
5.1279
.9499
ABC E12
4.8720
.7840
ABC E13
6.4304
1.1030
ABC E13
5.9704
.9539
ABC E14
8.0603
1.2868
ABC E14
7.2066
1.070
ABC E15
10.1138
.9418
ABC E15
8.8641
1.0661
HBC E8
2.1818
.0433
HBC E8
2.1818
.0433
HBC E9
2.5812
.0602
HBC E9
2.5812
.0602
HBC E10
2.6822
.3833
HBC E10
2.6822
.3833
HBC E11
3.3621
.6406
HBC E11
3.3621
.6406
HBC E12
4.3526
.8742
HBC E12
4.3526
.8742
HBC E13
5.6226
1.0898
HBC E13
5.6226
1.0898
HBC E14
7.2590
1.2520
HBC E14
7.2590
1.2520
HBC E15
9.26742
1.1597
HBC E15
9.26742
1.1597
Table 1
24
Discriminator Thresholds for Detector A
A1
0.053 MeV
AH
0.980 MeV
Table 2
Discriminator Thresholds for Detector B
B1
0.083 MeV
B3
1.10 MeV
B4
5.56 MeV
BH
3.62 MeV
Table 3
Discriminator Thresholds for Detector C
C1
0.095 MeV
C3
1.10 MeV
CH
10.00 MeV
Table 4
25
ID tag
ABC mode
HBC mode
Gain(kev/bit)
Electrons
ID 0
A1 * B1 * C1 * B3 *TH
B1 * C1 * B3 *TH
9.88 keV/bit
ID 2
A1 * B1 * C1 * B3 * C3 *T H
B1 * C1 * B3 * C3 *T H
9.88 keV/bit
ID 4
A1 * B1 * C1 * B3 *TH
B1 * C1 * B3 *TH
39.2 keV/bit
ID 6
A1 * B1 * C1 * ( B3  C3 ) *TH
B1 * C1 * ( B3  C3 ) *TH
39.2 keV/bit
Protons
ID 1
A1 * B1 * C1 *TH
B1 * C1 *TH
39.2 keV/bit
ID 3
A1 * B1 * C1 * B4 *TH
B1 * C1 * B4 *TH
39.2 keV/bit
ID 4
A1 * B1 * C1 * B4 *TH
B1 * C1 * B4 *TH
313.7 keV/bit
ID 5
A1 * B1 * C1 * TH
B1 * C1 * TH
313.7 keV/bit
Electron/Proton determination
TH  AH  BH  C H
TH  B H  C H
Table 5
26
Table 6
Energy
ID 0
ID 1
ID 2
ID 3
ID 4
ID 5
ID 6
ID 7
bin
(e-)
(p+)
(e-)
(p+)
(e-)
(p+)
(e-)
(p+)
E15
-
-
-
-
-
-
209-246
249-255
E14
-
-
-
-
-
-
169-208
-
E13
-
-
-
-
-
-
137-168
-
E12
-
-
-
-
-
-
111-136
-
E11
-
-
-
-
-
-
89-110
175-249
E10
-
-
-
-
-
-
73-88
121-174
E9
-
-
-
-
59-63
-
59-72
73-120
E8
-
-
189-219
-
49-58
-
47-58
53-72
E7
-
-
153-188
-
39-48
49-65
39-46
-
E6
-
-
119-152
-
35-38
39-48
-
-
E5
107-128
-
-
-
-
31-38
-
-
E4
77-106
-
-
199-212
-
25-30
-
-
E3
63-76
137-160
-
159-198
-
-
--
-
E3
51-62
97-136
-
-
-
-
-
-
E1
41-50
61-96
-
-
-
-
-
-
E0
33-40
31-60
-
-
-
-
-
-
27
28
29