| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The Cassini Magnetospheric Imaging Instrument (MIMI) Low Energy
Magnetospheric Measurements (LEMMS) sensor uses two oppositely
directed telescopes to measure ions and electrons in the energy range
0.03 to 18 MeV for ions and 0.015 to 0.884 MeV for electrons. LEMMS
measures electrons in two ways: the first method of detection uses
magnets to bend low-energy electrons into detectors and the second uses
a stack of detectors capable of separating electrons by energy
discrimination.
The low energy end of LEMMS (LET) has 15 degree aperature and is
designed to measure ions with E >= 30 keV and electrons in the range 15
keV - 1 MeV. The high energy end of LEMMS (HET) consists of a stack of
five detectors D1, D2, D3a, D3b and D4 to measure high-energy ions (1.5
- 150 MeV/N) and electrons (0.1 - 5MeV). The opening angle of the HET
is 30 degrees.
The LEMMS detector assembly is mounted onto a turntable capable of
continous rotation. When the spacecraft itself is also rotating, the
platform allows LEMMS to detect particles covering essentially the
entire unit sphere. The turntable rotation can be either clockwise or
counter clockwise but in practice after the arrival at Saturn, the
rotation is always clockwise. One LEMMS turntable rotation is divided
into 16 divisions called subsector. Subsectors are divided into 16
microsectors, but the priority data are taken every other microsector,
so in practice there are a maximum of 8 priority data values taken
within a subsector.
The turntable became stuck in early 2005, after which it has been in a
fixed position.
Launch (1997) - 2000:154 - turntable was rotating, orientation was
not well known
2000:154 - 2005:032 - turntable was rotating, orientation was well
known.
2005:032 - 2005:109 - sporadic rotation with intermittent sticking.
2005:032 and beyond - turntable stopped rotating
All the orientation data for LEMMS after 2000:154 including the stuck
period has been captured in SPICE C-kernels, which are included in the
MIMI analysis kernel set available on the MIMI web site.
This LEMMS data set is provided in a calibrated form in conjunction
with a PDS MIMI calibration volume COMIMI_0000 which provides specific
algorithms for the derivation of calibrated data. Calibration of LEMMS
data is described in the section on LEMMS in the MIMI User Guide
[VANDEGRIFFETAL2012].
Parameters
==========
Each standard product ID in the dataset is described as follows:
LPHASPGM0 Particle intensity plots in energy-time spectrograms
for the high energy resolution PHA channels.
LPHAAVG0_1MIN One minute average particle intensity data for PHA
ion and electron channels in daily files.
LPHAAVG0_1HR One hour average particle intensity data for PHA
ion and electron channels in daily files.
LACCAVG0_1MIN One minute average particle intensity data for
accumulator rate ion and electron channels in daily
files.
LACCAVG0_1HR One minute average particle intensity data for
accumulator rate ion and electron channels in daily
files.
LCPRESS0 Daily plots of particle pressure for LEMMS with CHEMS
pressures for comparison. Also, a spectrogram of LEMMS
data from the accumulator rates provides intensity
information up to 60 MeV.
Processing
==========
Data in this data set were processed by the use of a number of software
programs which calibrate, perform the 1 minute and 1 hour averages on
the data, and create the spectrograms and particle pressure plots.
For many LEMMS channels, it is possible to do a background subtraction.
Background levels are variable over time, and the MIMI calibration file
contains background levels for many LEMMS channels obtained as a
function of time throughout the mission. Background levels are recorded
as counts per second, and should be subtracted from the raw count rate
before that rate is converted to intensity.
Background subtraction has been applied to the LEMMS calibrated data in
this data set.
Converting uncalibrated counts into a background subtracted
differential intensity in units of counts/(sec ster cm^2 keV) is done
in the following way. Counts are divided by the accumulation time, and
then background is subtracted. Then the intensity is computed from the
rate with this formula:
intensity = (bg. subtracted rate) / (dt * gf * eff * dE)
Where dt = accumulation time in seconds
gf = geometry factor in steradians
eff = efficiency
dE = energy width in keV
The procedures for calibrating LEMMS data are described in the
section on LEMMS in the MIMI User Guide [VANDEGRIFFETAL2012].
To calculate the particle pressure over a given number of energy
channels is:
P = 4 * pi * SQRT((1.673E-24 * 0.0000000016) / 2) *
SUM{Ji * SQRT(Ei) * BWi}
Where Ji = differential flux for the ith energy channel
[1/(cm^2-s-sr-keV)]
Ei = nominal energy for the ith energy channel (in keV)
BWi = bandwidth for the ith energy channel (in keV)
and SUM means the sum of all the energy channels specified for that
sensor.
Data
====
The LEMMS calibrated data set includes the LEMMS normal rate channel
and PHA ion and electron particle spectrograms and PHA ion particle
pressures plots as well as the 1 minute and 60 minute averaged
particle intensities.
LEMMS PHA and Accumulator Rates Averages - 1 Hour and 1 minute
---------------------------------------
The 1 hour and 1 minute files are very similar except for the averaging
interval. In each file, the data columns contain 1 hour or 1 minute
averages of LEMMS accumulator rates data. The data are averaged by
first defining fixed one hour intervals throughout the day and then
averaging any data points that fall in the intervals. If an interval
contains no data, it is still listed in the file, but all data
intensity values will be fill. The fill value for data in this file is
-1.0E38.
These data are background subtracted, and thus occasionally there will
be negative values when the count rate falls below the background
level. Also, the uncertainties are based on the counting statistics,
and thus because of the background subtraction, the uncertainties can
be larger than the data value.
In the accumulator rates data, there are four types of channels
included in these averages.
The channels A0 through A8 are measured by the low energy end of
LEMMS (the Low Energy Telescope, or LET), and for protons these
channels have an energy range of 27 to 4000 keV.
The P1 through P9 channels are measured by the LEMMS High Energy
Telescope (HET) and have an energy range of 1 to 60 MeV.
Channels C0 through C7 measure electrons, and have an energy of 20 to
900 keV.
The E0 through E4 channels are also electrons covering the 150 to
3000 keV range.
The MIMI instrument paper describes these channels, as does the MIMI
Data User Guide.
In the PHA data, there are three types of channels included in these
averages: the A channels, the E channels and the F1 channels.
The A channels measure ions, mostly protons.
The E and F1 channels measure electrons.
The MIMI instrument paper describes these channels, as does the MIMI
Data User Guide.
It is important to note that a subset of the PHA channels are
typically used for science analysis, and only these channels
are provided in calibrated PDS data. For the A channels, channel
indices 8 through 62 are present (energies of 26-758 keV).
For the E channels, indices 15 to 62 (20-410 keV), and for F1,
26 through 59 (206-1717 keV). For all PHA sensor elements
(A, E, and F1), the highest energy channel (index 63, with 0
being the first channel) contains all the counts that did not
fit into any other channel, and so it is often very noisy. Some
of the lowest energy A and E channels are also not used because
the detection efficiency drops off rapidly as energy decreases,
and also the efficiency is less well characterized at those lower
energies. Thus the counts in these low energy A and E channels
are very uncertain. For PHA electrons, high energy E channels
overlap in energy with low energy F1 channels. Efficiencies in
the transition energies between E and F1 are difficult to
characterize, since they depend on spectral shape and incident
angle. Thus the constant values used for these efficiencies (in
the energy overlap region) are only an approximation. The choice
of typical channels for E and F1 was made to provide some overlap
in energy but still provide good agreement in intensities in most
situations.
The file header and leading columns of the file are present to make the
format consistent with other MIMI data in the PDS. Note that much of
the values for the leading columns are -1, indicating that they do not
have useful values, but their presence allows this file to be parsed by
the same code that reads other MIMI PDS data. The first column is
always the string 'sci', the next column is a UTC string, and then
these prefix columns have no data in them but are just there to
preserve the format:
UTC, SCLOCK, Start_Ephemeris_s End_Ephemeris_s, Spin_Counter,
Sector, Start_Sector_Sclock_major Subsector, Microsectors_Covered,
Spin_Period_s Staring Sensor_Bitrate
Just before the data columns are three columns giving the position of
the Cassini spacecraft relavtive to Saturn in the SZS frame. The
position is given for the middle of the time interval. The units are
Saturn radii (60268 km) and the SZS frame is defined as: +Z is Saturn
spin axis, +Y is +Z cross the Saturn-Sun line, and +X is +Y cross +Z.
LEMMS PHA Spectrograms
---------------------------------------
This data set consists of daily energy-time spectrograms for the LEMMS
PHA electron data (top panel) and LEMMS PHA ion data (second panel).
The ions are almost always protons, and these come from channels 8
through 62 in the LEMMS PHA data.
For protons, these channels have an energy range of 25 to 780 keV. The
electrons (the lower of the two data panels) come from the LEMMS PHA E
channels 15 through 62 and the LEMMS PHA F1 channels (26 through 59)
and have energy ranges of 20 to 410 (for E channels) and 205 to 1700
keV (for F1 channels). Note that there is some overlap. Also note that
in the data, it is usually possible to see the transition region from
the lower energy E channels to the higher energy F1 channels. This is
because the efficiencies adn geometric factors used work for most
situations, but in some environments, the detectors have different
responses and this causes a visible seam in the data.
The thin plot between the two data panels is a status panel indicating
when sunlight contamination may be present in the data. This status
panel is color coded such that green indicates the data is likely free
of sunlight problems. Red indicates a likely contamination problem.
Black and gray both indicate that attitude data could not be obtained
to make a determination. The Cassini MIMI Data User Guide discusses
LEMMS light contamination in more detail.
There is also a panel showing angles between the LEMMS Low Energy
Telescope (LET) and various other vectors. The green line is the SZS
longitude of the LET boresight. The black line is the latitude of the
LET boresight in the SZS frame. The blue line is the Sun angle - the
angle between the LET boresight and the Cassini-Sun line. This is the
angle used to determine possible sunlight contamination.
The X axis is labeled with the time of day and the local time (in SZS),
and also the radial distance of Cassini to the center of Saturn.
The bottom three panels show the position of Cassini relative to Saturn
in the KSM frame. The first plot is a top view, with the Sun to the
left. The blue trace is the projection of the bow shock into the X-Y
plane, and the brown line is the magnetopause projection. The equation
for the bow shock was obtained from Went, et al, 2011, A new
semiempirical model of Saturn's bow shock based on propagated solar
wind parameters, DOI: 10.1029/2010JA016349.
The magnetopause used is from Kanani et al, 2009, A new form of
Saturn's magnetopause using a dynamic pressure balance model, based on
in situ multi-instrument Cassini measurements DOI:
10.1029/2009JA014262.
In each of the trajectory plots, Saturn is in the middle, and the
actual orbit of Titan is shown (centered on the time of the data, but
extendeing several days before and after so that the whole orbit can be
seen). The location of Titan at the middle of the day of the data plots
is indicated with a small red circle.
The trajectory of Cassini is shown in black, and the part of the
trajectory covered by the data is shown with a thicker blue line.
The position of Cassini at the start of the data plot is indicated by a
red X.
In the KSM YZ projection, the magnetopause and bow shock are shown
projected into the plane containing the spacecraft. Note that if the
spacecraft is far enough inside or outside, these boundaries may not
appear.
The KSM frame is defined as follows:
KSM, +X points from Saturn to the Sun.
+Y is the Saturn dipole axis crossed into the +X axis. (In practice,
the spin axis of Saturn is used in place of the dipole axis.)
The +Z axis is then +X cross +Y.
LEMMS Particle Pressures
---------------------------------------------
These are daily plots of particle pressure spectrograms for LEMMS and
CHEMS. It also shows total pressure for LEMMS and CHEMS, and for LEMMS,
a comparable energy-time spectrogram. The location of the Cassini
spacecraft is also shown.
The top panel shows a spectrogram of CHEMS particle pressure. The
source for this data are the 32 energy steps (3 to 230 keV) for the BR0
and BR3 summed channels of just Telescope 2. This telescope was chosen
because it points most closely to the now-fixed look direction of
LEMMS. The intensities are averaged into 5 minute bins before
converting to pressure.
The units are dyne/cm^2. Note that in the magnetosphere of Saturn, the
CHEMS pressures can only be trusted outside of L values around 4.
The next panel is particle pressure based on LEMMS PHA ions, presumed
to be protons. Channels A08 through A62 are used and cover an energy
range of 25 to 780 keV. These data are averaged into 120 second bins
before calculating the pressure. The units are the same as for the
CHEMS data in the top panel. Note that in the magnetosphere of Saturn,
the LEMMS pressures can only be trusted outside of L values around 6.
Inside this distance, background levels become significant and cause
the reported pressure to be too high. There is essentially very little
particle pressure in this region anyway.
The next panel shows the total pressure, as computed by summing the
LEMMS and CHEMS pressure spectrograms in the above panels. The units
for the pressure are still the same.
The next panel is a LEMMS energy-time spectrogram based on these LEMMS
A and P channels: A0 through A7 and P2 through P8 excluding P4. These
channels cover an energy range of 27 keV to 59 MeV and are averaged
into 120 second bins. The units are particles/sec/cm^2/ster/keV.
Directly below this LEMMS spectrogram is a thin panel showing times of
potential light contamination for LEMMS. Red indicates times when the
angle between the LET boresight and the Cassini-Sun vector is less than
60 degrees. Black and gray both indicate that attitude data could not
be obtained to make a determination. The Cassini MIMI Data User Guide
discusses LEMMS light contamination in more detail.
The bottom three panels provide the Cassini position relative to Saturn
as well as the magnetopause and bow shock. These panels are the same as
those in the PHA Spectorgram plots and are described above.
Coordinate System
=================
The averaged data products in this dataset also include spacecraft
position information in the Saturn Equatorial System frame (also called
the SZS frame) in units of Saturn radii, Rs, where 1 Rs = 60268 km. The
SZS frame has the following definition:
+Z points along the dipole axis of Saturn. For practical purposes,
the spin axis of the IAU_Saturn frame is used for this direction.
+Y is in the direction of the cross product of +Z with the
Saturhn-to-Sun line.
+X is then +Y cross +Z.
Software
========
There is access on the MIMI web site, to an analysis package currently
being used by the MIMI team to browse, plot and analyze MIMI data that
reads data directly from the PDS and makes available to anyone the same
analysis and visualization capabilities currently used by the MIMI
team. The main benifit of the tools is that they provide an intuitive
way to browse and analyze the data without having to worry about
calibration details or file access and file reading details.
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