NSSDCA ID: PSFP-00682
Availability: Archived at NSSDC, accessible from elsewhere
This description was generated automatically using input from the Planetary Data System.
This data set consists of all of the spacecraft potential data generated from the Cassini Plasma Spectrometer (CAPS) electron spectrometer uncalibrated data. The data set currently contains the spacecraft potential data from the Electron Spectrometer (ELS_SCPOT). The data is also contained in the electron moments file. Data was generated using uncalibrated data from the electron spectrometer on Cassini CAPS. The uncalibrated data were acquired in a mix of CAPS operating modes beginning with the first instrument checkout in January 1999 and containing throughout the Cassini Tour and through the end of prime mission. The data set covers the time period from 1999-004T03:07:47 UT until end of prime mission (July 2008). In addition, it will cover data received during extended missions. Sampling rates were variable and depended upon the downlink capabilities and other activities on-board. For times when CAPS is not producing data due to being turned off or due to communication issues, the data set will not contain data. Additionally, there will be no data during times when the calculations are not possible. Electron Moment File Discussion ========== First, some notes on the code itself - it assumes a zero bulk flow velocity, due to the limited view of the ELS. Zero velocity in assumed in the spacecraft frame. In plasmas, the zeroth moment gives the density, the first moment gives the bulk flow velocity vector, and the second moment gives a pressure tensor or a temperature tensor. Each moment in turn requires the previous moments, i.e. the velocity moment requires density and the pressure moment requires density and velocity. But as we don't have a 3d sensor (like Cluster), we have to assume the plasma is isotropic, i.e. what you see in one direction you also see in the opposite direction. So, any velocity vector that you measure in one direction will be exactly cancelled by the velocity vector in the opposite direction. So in our case the bulk flow velocity will be zero. In this case the pressure/temperature tensor will become a scalar. Secondly, the calibrated numbers. We have to convert the data into phase space density so we can subtract off the photoelectrons properly. This can be summarized as follows: The ELS accumulation time is 0.0234375s. So: data in counts/sec = data recorded / accumulation time after taking into account telemetry periods where several accumulation periods were summed or averaged. To get the higher calibrated units, which are anode and energy dependent: data in DEF = data in counts/sec / geometric factor data in DNF = data in DEF / (energy * electron charge) data in PSD = (data in DEF * electron mass squared) / (2 * (energy * electron charge) squared) The geometric factor (with efficiency rolled in) can be found in the CALIB directory. The units on geometric factor are str m^2 eV/eV per anode. The updated file is always on the ELS managed web site: http://www.mssl.ucl.ac.uk/~lkg/ELS_calibrations/geometric_factor.html For more information, the ELS calibration paper talks in depth about the geometric factor. [LEWISETAL2010] Background subtraction comes from analysis of data from 15th Sep 06, 14th Jan 07 and 17th Jun 07, which assigns background levels per anode based on actuator position. The analysis was published in 2009 in a paper by Arridge et al [ARRIDGEETAL2009]. Here's a summary of what the code does. Read in data, average by A-cycle (and A-cycle is a 32 second instrument cycle). Per A-cycle: Subtract background, taking the actuator position from the mid-point of the A-cycle. Convert data to PSD (phase space density), remove photoelectrons. Remove spacecraft potential (also taken from mid-point of the A-cycle) from instrument energy array Get the radial cell limits for each energy bin in m/s from the spacecraft-potential-subtracted energy array For each bin, density = 4pi/3 * data * (top limit velocity ^3 bottom limit velocity ^3) Sum all bin densities to get total density. Calculate temperature constant t_const = (4 * mass_electron * pi) / (15 * density * Boltzmann's constant) For each bin, temperature = t_const * data * (top limit velocity ^5 bottom limit velocity ^5) Sum all bin temperatures to get total temperature. Convert temperature from Kelvin to eV with temperature = temperature * 8.61752d-5 Then get a quality factor by: Get peak energy of a Maxwellian from the actual density and temperature from the moments calculated. Calculate the theoretical peak energy that a Maxwellian would have with the temperature from the calculated moments (identically equal to TWICE the temperature in energy units): kbt = temperature * charge_e And work out the mid-energy of the bin that comes in (energy), and the geometric factor at that energy (geom) Get peak phase space density of same: peak_psd = density*SQRT((mass_e/(2*!PI*kbt))^3)*EXP(-1) Get the peak count rate from that lot: peak_cr=peak_psd*2(energy*charge_e)^2*geom/(mass_e^2) Get the Poissonian error on the peak count rate: std_dev_cr=SQRT(peak_cr/accutime) Roll them into a quality factor: quality_factor=(peak_cr-42.7)/std_dev_cr As a note, 42.7 Hz is the one count level. Data ==== The data are stored in multiple data files and have been organized in folders, first by higher order data type and then by year. Each file contains a maximum of 24 hours of data. Note that data is included in the file based on the start time, and not the end time of the data. Format of the data files can be found in the CAPS instrument archive specification [FURMANETAL2013]. The format can also be found in the .LBL files in the FM/HIGHERORDER/SCPOT/YYYY specific directory, co-located with the data. Ancillary Data ============== Ancillary data can be found in the ANC data file provided with the CAPS UNCALIBRATED data set. References ======== [FURMANETAL2005] CAPS standard data products and archive volume software interface specification, Version 1.9, JPL SIS ID: IO-AR-017, Southwest Research Institute, San Antonio, TX 78250, 2005. [ARRIDGEETAL2009] The effect of spacecraft radiation sources on electron moments from the Cassini CAPS electron spectrometer, Planetary and Space Science, 57, 854-869, doi:10.1016/j.pss.2009.02.011, 2009. [GURNETTETAL2004] The Cassini Radio and Plasma Wave Investigation, Space Sci. Rev. 114, 395-463, 2004. [LEWISETAL2008] Derivation of density and temperature from the Cassini Huygens CAPS electron spectrometer, Planetary and Space Science, 56, 901-912, doi: 10.1016/j.pss.2007.12.017, 2008. [LEWISETAL2010] The calibration of the Cassini-Huygens CAPS Electron Spectrometer, Plan. and Space Sci., 58, 427?436, doi:10.1016/j.pss.2009.11.008, 2010. [THOMSENETAL2005] Numerical moments computation for CAPS/IMS, Los Alamos National Laboratory Report LA-UR-05-1542, 2005. [THOMSENETAL2010] Survey of ion plasma parameters in Saturn's magnetosphere, J. Geophys. Res., 115, A10220, doi:10.1029/2010JA015267, 2010. [WILSONETAL2012] PDS User's Guide for Cassini Plasma Spectrometer (CAPS), 2012. [YOUNGETAL2004] Cassini Plasma Spectrometer Investigation, Space Sci. Rev. 114, 1-112, 2004.
These data are available on-line from the Planetary Data System (PDS) at:
https://pds-ppi.igpp.ucla.edu/data/CO-E_J_S_SW-CAPS-5-DDR-SC-POTENTIAL-V1.0/
Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II
Name | Role | Original Affiliation | |
---|---|---|---|
Dr. David T. Young | Data Provider | Southwest Research Institute | dyoung@srwi.edu |
Dr. Judith D. Furman | General Contact | Southwest Research Institute | jfurman@swri.edu |