Data Set Information
DATA_SET_TERSE_DESCRIPTION This data set contains the calibrated Saturn tour data collected by the Low Energy Magnetospheric Measurements (LEMMS) sensor of the MIMI instrument aboard the Cassini spacecraft for the Cassini mission.
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

   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

     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

     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

     LACCAVG0_1HR     One minute average particle intensity data for
                      accumulator rate ion and electron channels in daily

     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.

   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
           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


   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

   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

   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:

   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

   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.

   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.
DATA_SET_RELEASE_DATE 2018-07-01T00:00:00.000Z
START_TIME 2004-01-01T12:00:00.000Z
STOP_TIME 2017-09-16T12:00:00.000Z
MISSION_START_DATE 1997-10-15T12:00:00.000Z
MISSION_STOP_DATE 2017-09-15T12:00:00.000Z
NODE_NAME Planetary Plasma Interactions
Confidence Level Overview
   This data set contains all calibrated data for the LEMMS Detector as
   part of the Cassini MIMI instrument for the interval described above.
   Every effort has been made to ensure that all data returned to APL from
   the spacecraft is included and that the calibrated browse data products
   are accurate.

   One key issue in dealing with LEMMS data is possible sunlight
   contamination. The plots provided in the dataset have indicators
   flagging times when sunlight may be affecting the detectors. The
   sunlight issue and other caveats to using the LEMMS data are described
   in the Cassini MIMI Data User Guide.

   The MIMI LEMMS data will be reviewed internally by the Cassini MIMI
   team prior to release to the PDS. The data set will also be peer
   reviewed by the PDS.

   Data Coverage and Quality
   All data in the stated interval are included, to the best of our
   knowledge and attempts to determine completeness.

   Large Data Gaps:
     1. Post SOI                   2004-187 - 2004-193 - Spacecraft Safing
                                   shut off

     2. Tour                       2005-006 - 2005-015 -

     3. Tour                       2007-254 - 2007-258 - Spacecraft Safing
                                   shut off

     4. Tour                       2007-279 - 2007-291 - Spacecraft Flight
                                   S/W testing shut off

     5. Extended Mission           2009-071 - 2009-076 - Spacecraft Thruster
                                   branch swap,deactivated S48 background

     6. Extended-Extended Mission  2010-306 - 2010-319 - Spacecraft Safing
                                   shut off

     7. Extended-Extended Mission  2012-293 - 2012-303 - Spacecraft
                                   Propellant Gauging Test shut off.

     On those days for which the instrument did not collect data either due
     to the instrument being off or due to communication issues, the data
     set will not contain any files.
ABSTRACT_TEXT The Cassini Magnetospheric Imaging Instrument (MIMI) Low Energy Magnetospheric Measurements (LEMMS) measures ions and electrons in the energy range 0.03 to 18 MeV for ions and 0.015 to 0.884 MeV for electrons. The LEMMS calibrated data set includes two types of daily plots -- one presenting energy-time spectorgrams in units of intensity, and one with plots of particle pressures. There are also four types of averaged data -- minute and hourly averages for accumulator rate data and PHA data (ions and electrons). These data have been created at the Johns Hopkins University Applied Physics Laboratory by applying the calibrations and averaging. Averaging is done by first defining time periods, and then averaging all data points within those fixed intervals. The time reported is the midpoint time of the interval.
  • Planetary Plasma Interactions Website