Description of the Experiments
Ion and Electron Esa Spectrometers (IES and EES)
The IES and EES measure 32 pitch angle directions simultaneously (11.25 degree
resolution), and sweep over 48 energy steps in 1/64 of a spacecraft spin. The energy range
is nominally 3 eV to 25 keV and 4 eV to 30 keV for ions and electrons, respectively. Each
spectrometer consists of two half sensor heads with180x10 degree field-of-views (FOVs) that
are mounted on opposite sides of the spacecraft to form a 360 degree planar FOV. The
out-of-plane response is roughly gaussian with a FWHM of about 5 degrees. The planar FOV is
within the spacecraft spin plane which is oriented with the spin vector normal to the
nominal magnetic field direction. This orientation facilitates continuous pitch angle
measurements. Deflectors are used to steer the sensor FOV out of this plane by up to 10
degrees to account for variations of the magnetic field direction during the orbit. In
general, the pitch angle distribution always includes both the parallel and anti-parallel
magnetic field directions.
Burst data consists of a 32 angle by 48 energy bin array of counts. Counts are
pseudo-log compressed from 16 to 8 bits onboard. The first energy step is a high voltage
retrace and should be ignored. Detector anodes rotate by 1/2 an anode (5.625 degrees) each
sweep.
Survey data is averaged burst data which has been despun onboard. Slow survey is
averaged into a 32 angle by 48 energy array each 1/2 spin (~2.5 s). Some blurring of the
angular resolution results from the half anode rotation on alternate sweeps. Fast survey is
averaged into a 64 angle by 48 energy array that maintains the angular resolution. Fast
survey time resolution varies from 1/16 to 1/4 of a spin.
Stepped Electron Spectrograph (SES)
The SES consists of six pairs of ESA half analyzers to form six full pitch-angle
(360 degree) sensors. Each sensor has 22.5 degree resolution and twice the geometric factor
of the electron spectrometer, giving an overall sensitivity 12 times the electron
spectrometer. As with the spectrometers, orientation of the sensor FOVs is the spin plane
to give complete angular coverage for nominal magnetic field orientations. These sensors do
not have deflectors, so that out of plane variations in the magnetic field can result in
the detectors missing narrow field aligned beams. Several different energy level schemes
are used for the SES to optimize different science goals. The sensors can be operated at 6
fixed energy levels to optimize time resolution, or can toggle over 12, 24 or 48 energy
steps. The sensors can also track the energy flux peak with 6 or 12 energy steps. Finally
the sensors can be swept over the entire energy range, with sensors shifted by 1/6 of a
sweep so the entire energy range is covered 6 times faster than the spectrometer.
Burst data consists of six count arrays (SES1, SES2,...SES6) of 16 angles by N
energies, where N=6, 12, 24, or 48. Counts are pseudo-logged. The first energy step is a
high voltage retrace. Detectors rotate a fraction of an anode in a sweep.
Survey data is averaged burst data which has been despun onboard. Slow survey is
averaged into 16 angles by N energies, where N=6, 12, 24, or 48 and where N for burst may
differ from N for survey data. Some blurring of the angular resolution results from the
anode rotation during the average. Fast survey is generally identical to slow survey but
with much faster time resolution.
Known Problems with FAST ESA data
For a discussion: McFadden et al., Ion and Electron Characteristics in Auroral Density
Cavities Associated with Ion Beams: No Evidence for Cold Ionospheric Plasma, JGR, p.14671,
1999.
Sunlight contamination
Sunlight background counts are observed in all sensors when the spacecraft is near a
noon-midnight orbit. Rates the order of ~100/s-anode are observed in the worst look
directions.
Scattered electrons.
Scattered electrons can produce a low level of background counts in the all the sensors.
These are most noticeable in the electron sensors as pitch-angle wings on narrow field
aligned beams, or as background counts above and below an intense mono-energetic peak. The
background is greater at energies near the incident electron energy. In the IES sensor, the
incident electrons must have >2 keV energy to reach the front of the negatively biased MCP
detector to be observed.
Spacecraft photo-electrons and secondary electrons
Spacecraft produced photo and secondary electrons are observed in the EES and SES sensors
at 90 degrees pitch angle at energies up to ~100 eV. Photo-electrons dominate accept in
darkness. Secondary electrons produced in the electron sensor aperture can also generate a
low energy background.
Missing field-aligned beams
On occasion, the onboard IES and EES deflector settings may be wrong so
the sensors can miss narrow field aligned beams. This is especially
true in the southern hemisphere, where the magnetic field is sometimes beyond
the deflection range. The SES sensors have no deflectors so these sensors
will miss field-aligned beams when the magnetic field direction is out
of the sensor field of view (+/- 5 degrees).
ExB effects on ion beams
S/C motion through the plasma normally affects only low energy oxygen conics. However,
within electrostatic shocks a significant effect on ion beams is observed. The ion
spectrometer FOV is nominally aligned along the magnetic field using deflectors. Although
most ion beams are observed since the beam widths are large (10-30 degrees), beam
intensities may be reduced and moment calculations may be in error when ExB is large.
Noise counts
On occasion there are noise counts at the highest energy steps of the sensor probably due
to weak high voltage discharges. If the MCPs get blasted with a high count rate, for
example during spacecraft charging at low altitudes, false counts will appear on following
energy samples. This background rate decays over several tens of milliseconds and is
normally noticed at the highest energy steps of the following sweep.
Spacecraft charging.
Negative spacecraft charging from a few volts to several tens of volts is common and is
obvious in the low energy ion spectra. Current moment calculations do not account for the
spacecraft potential.
Incorrect pitch angle due to magnetic field phase errors
When the magnetometer is off, the magnetic field phase (magphase) in the
ESA packet data header is incorrect. This often happens during low latitude
(ILAT less than 60 deg) data collection beginning around June 2000. The
magphase is used to sort the ESA measurements by pitch angle, thus the pitch
angles calculated by the software are incorrect when magphase is wrong. This
problem can be easily identified in the data by incorrect pitch angles for
the loss cone. An initial correction has been implemented for sdt. By setting
the environment variable with the command
"setenv FAST_COMPUTE_MAG_PHASE 1",
sdt will be forced to calculate "magphase" on the ground based upon
a magnetic field model and the sun pulse timing. This fix should work
whenever the spacecraft is in sunlight. This fix will also work when
the spacecraft is in darkness provided that the onboard "sun-nadir"
table is accurate. The sun-nadir table allows FAST to calculate a
pseudo-sun pulse based upon horizon sensor data. Early in the mission,
FAST had some problems with the sun-nadir table updates. Thus the use
of ground calculated magphase may occasionally fail. For pre-June 2000
data we suggest using the onboard magphase by setting
"setenv FAST_COMPUTE_MAG_PHASE 0".
Software will eventually be updated to allow the user to toggle between
these choices without exiting the sdt program.
Finally, the ground computed magphase will also be incorrect for those few
orbits where the attitude data has not been calculated. A warning message
should be implemented in the near future to warn the user of this problem.
The onboard magphase and computed magphase can be plotted in sdt by selecting
from the menu: " Add Plot -> FAST -> Esa_survey -> EsaSrvSpinMagAndMagPhase"
then adding the appropriate choice.
