Venus Express ASPERA-5 ELS PAD Field Definitions NASA ROSES Solar System Workings (SSW) Grant 80NSSC21K0151 Author: R. A. Frahm Southwest Research Institute 6220 Culebra Road San Antonio, TX 78238 PI: Shaousi Xu University of California at Berkeley Berkeley, CA 94710 Date: 28 September 2021 Version 1.0 Table of Contents 1.0 Introduction 2.0 File Names 3.0 The Data File Definition 3.1 The Data File Time Definition 3.2 The Data File Scan Definition 3.3 The Data File PAD Definition 4.0 The Mode File Format 4.1 The Mode File Time Format 4.2 The Mode File Pitch Angle Range Format 4.3 The Mode File Sweep Type Format 4.4 The Mode File Sector Pitch Angle Definition 4.5 The Mode File Number Sectors Used Definition 4.6 The Mode File Sector Background Definition 4.7 The Mode File Used Magnetic Field Definition 4.8 The Mode File Software Version Definition 1.0 Introduction There are two types of data files which describe the Venus Express (VEx) Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) Electron Spectrometer (ELS) Pitch Angle Distribution (PAD) data. These types are the Data and Mode. Files are written in ASCII where the Data file describes the pitch angle sorted data in Comma Separated Variables (CSV) format and the Mode file describes how the Data was generated in txt format. For each Data file, there is one corresponding Mode file. Each file contains 1 day of data. The Mode fields are separated by a blank character while the Data fields are separated by a comma (,) character. 2.0 File Names Files are created for each day of the year. They are either named VExELSPADRG_yyyyddd_Data.csv or VExELSPADRG_yyyyddd_Mode.txt depending on whether that contain Data or Mode information where: VEx means the Venus Express spacecraft, ELS means the electron spectrometer of the Analyzer of Space Plasmas and Energetic Atoms experiment, PAD means the sorted pitch angle distribution, RG means only regular unblocked data is included, yyyy means the four digit year, ddd means the day of year (with day 1 as January 1), Data or Mode means the type of data stored in the data file, csv means that the file is an ASCII csv file with comma separators between fields. txt means that the file is an ASCII text file with blank separators between fields. 3.0 The Data File Definition The Data file contains three header lines followed by the data values The header lines give a brief description of the data fields, of which there are 23. Fields 1 and 2 contain the start time and end time of the data spectrum. Fields 3, 4, and 5 describe the index, energy, and velocity of the scan. Fields 6-23 contain the pitch angle sorted data. 3.1 The Data File Time Definition The start time and end time (fields 1 and 2, respectively) describe the time of the spectrum. The format is YYYY-DDDTHH:MM:SS.SSS which means: YYYY is the year, DDD is the Julian day of year (with day 1 as January 1), HH is the hour of the day, MM is the minute of the hour, SS.SSS is the second of the minute (resolution to the millisec). Times are given in universal time (UT). The start time field is given as field 1 and the end time as field 2. Together these fields describe the time range of the spectrum. Below is the header lines for the first two fields: Start Time , End Time , YYYY-DDDTHH:MM:SS.SSS,YYYY-DDDTHH:MM:SS.SSS, ---------------------,---------------------, 3.2 The Data File Scan Definition The scan index, energy, and velocity (fields 3, 4, and 5, respectively) describe the energy scan of the data. The scan index is a pure number which ranges from 0 to 30 when the electron instrument is in the 1 sec/sweep mode and from 0 to 126 when the electron instrument is in the 4 sec/sweep mode (note that there is 1 additional flyback step which is not included in this analysis). The scan is a decay sweep, which means that low scan index represents large energy. The energy for each scan index is given in units of eV and depends on the instrument configuration. The same scan index does now always correspond to the same energy value. The energy is also expressed in terms of velocity in the units of m/sec. Electron energy (eV) is converted into velocity using the formula: Velocity (m/sec) = SQRT [2 * Q * Energy (eV) / M] where Q converts the energy from eV into Joule (Q = 1.602e-19 Joule/eV), M is the mass of the electron (9.11e-31 kg), and SQRT [] is the math function. The scan index, energy, and velocity header lines are as follows: SCN, Energy , Velocity , INX, eV , m/sec , ---,----------,----------, 3.3 The Data File PAD Definition The pitch angle array covers the range from 0 deg to 180 deg in 10 deg linearly spaced angular bins, each having a width of 10 deg. Pitch angle bins are marked with the pitch angle at the center of each bin. All pitch angle bins are filled with values, where -3.400e+38 is used as a fill value. Values less than zero result from background subtraction and are retained for further processing by the user. Each value represents the electron Pitch Angle Distribution in units of sec**3/(m**6 sr) [labeled as PADs3/m6sr in the Data file]. The Table 1 gives the amount of sr in a pitch angle measurement. Header lines for the pitch angle sorted values are shown below: PADs3/m6sr,PADs3/M6sr,PADs3/m6sr, ... ,PADs3/m6sr,PADs3/m6sr,PADs3/m6sr 5 deg PA, 15 deg PA, 25 deg PA, ... ,155 deg PA,165 deg PA,175 deg PA ----------,----------,----------, ... ,----------,----------,---------- Table 1. The amount of volume (sr) in each pitch angle bin assuming a gyrotropic electron distribution. Pitch Angle Bin (deg) Gyrotropic Data File Start Center Stop Weight (sr) Position 0 5 10 0.095456 6 10 15 20 0.283467 7 20 25 30 0.462865 8 30 35 40 0.628199 9 40 45 50 0.774446 10 50 55 60 0.897161 11 60 65 70 0.992617 12 70 75 80 1.057912 13 80 85 90 1.091064 14 90 95 100 1.091064 15 100 105 110 1.057912 16 110 115 120 0.992617 17 120 125 130 0.897161 18 130 135 140 0.774446 19 140 145 150 0.628199 20 150 155 160 0.462865 21 160 165 170 0.283467 22 170 175 180 0.095456 23 4.0 The Mode File Definition The Mode file contains three header lines followed by the mode values. the mode values describe how the spectral data was processed to create the PAD. The header lines give a brief description of the mode fields, of which there are 40. Fields 1 and 2 contain the start time and end time of the data spectrum. Fields 3 and 4 describe approximate pitch angle range. Field 5 describes the sweep type. Fields 6-21 gives a rough value of the pitch angle of each ELS sector. Field 22 gives the number of sectors used in the sort. Fields 23-38 describes the resolution of the background which was subtracted. Field 39 describes the time resolution of the magnetometer data used while field 40 describes the version of the processing code used. 4.1 The Mode File Time Definition The start time and end time (fields 1 and 2, respectively) describe the time of the spectrum. The format is YYYY-DDDTHH:MM:SS.SSS which means: YYYY is the year, DDD is the day of year (with day 1 as January 1), HH is the hour of the day, MM is the minute of the hour, SS.SSS is the second of the minute (resolution to the millisec). Times are given in universal time (UT). The start time field is given as field 1 and the end time as field 2. Together these fields describe the time range of the spectrum. Below is the header lines for the first two fields: Start Time End Time YYYY-DDDTHH:MM:SS.SSS YYYY-DDDTHH:MM:SS.SSS --------------------- --------------------- 4.2 The Mode File Pitch Angle Range Definition The approximate pitch angle range is given by the "Minimum Pitch Angle" (field 3) and "Maximum Pitch Angle" (field 4) indices, which are in terms of pitch angle bin. Each pitch angle bin is 10 deg wide. The value of the "Minimum Pitch Angle" is the index representing the angular bin in which data with the minimum pitch angle was detected. The "Maximum Pitch Angle" is the index representing the angular bin in which data with the maximum pitch angle was detected. Table 2 shows the translation for each index into the pitch angle of coverage (in deg). A value of 255 is used to indicate the field is filled. Below is the three header lines for the second two fields: PA Range MIN MAX --- --- Table 2. The Pitch Angle Range of the Pitch Angle Index. Mode File Pitch Angle Location Index Minimum Center Maximum --------- ------- -------- ------- 0 0 deg 5 deg 10 deg 1 10 deg 15 deg 20 deg 2 20 deg 25 deg 30 deg 3 30 deg 35 deg 40 deg 4 40 deg 45 deg 50 deg 5 50 deg 55 deg 60 deg 6 60 deg 65 deg 70 deg 7 70 deg 75 deg 80 deg 8 80 deg 85 deg 90 deg 9 90 deg 95 deg 100 deg 10 100 deg 105 deg 110 deg 11 110 deg 115 deg 120 deg 12 120 deg 125 deg 130 deg 13 130 deg 135 deg 140 deg 14 140 deg 145 deg 150 deg 15 150 deg 155 deg 160 deg 16 160 deg 165 deg 170 deg 17 170 deg 175 deg 180 deg 4.3 The Mode File Sweep Type Definition The sweep type (field 5) is meant to indicate the sweep length of the electron instrument. Possible values for the ELS sweep type are: 0 = 127 steps, 1 = 31 steps, 2 = 1 step. The 1 step mode was not used often and indicates that the electron instrument measured a single energy at 32 times per sec. The 31 step mode was used to measure the electron spectrum at the 1 sec rate and covered a limited energy range. The 127 step mode was the normal scan rate of the electron instrument and measured the electron spectrum in 4 sec. Below is the header line for the fifth field: ST TYP --- 4.4 The Mode File Sector Pitch Angle Definition The approximate pitch angle for each ELS Sector which was included in the PAD is shown in fields 6-21. Values are given in degrees from 0 to 180 and the number of 255 is used as a fill. A sector may be active, but excluded in the PAD determination. These excluded sectors are also considered filled. Below is the header line for the fields 6-21: Individual Pitch Angle for Anode 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 -- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- 4.5 The Mode File Number Sectors Used Definition The number of ELS sectors used in the PAD calculation is given in field 22. This number ranges from 1 to 16 indicating that a PAD was computed. A value of 0 indicates that no ELS sector could be used to generate a PAD. A fill value of 255 means the number was filled. Below is the header line for the field 22: USD SEC --- 4.6 The Mode File Sector Background Definition Fields 23-38 indicate the type of background which was removed from the spectral data for each of the ELS anodes. Possible values are: 0 = No background subtraction, 1 = background determined from the spectrum was removed, 2 = background determined from a 1 minute average was removed, 3 = background determined from a 5 minute average was removed, and 4 = background determined from a 25 minute average was removed, Values are filled with the number 255 indicating that the ELS sector was not used in PAD determination. Below is the header line for the fields 23-38: Background Type Used for Anode 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- 4.7 The Mode File Used Magnetic Field Definition The magnetic field is used when generating a PAD for orientation. There are two types of magnetometer resolution data and field 39 indicates which magnetic field time resolution was used. Possible values are: 0 = 4-sec magnetic field, 1 = 1-sec magnetic field, 3 = high res magnetic field (128 samples/sec). The fill value for this field is 255 and no PAD data product can be produced with out magnetic field orientation. Below is the header lines for the field 39: MAG TYP --- 4.8 The Mode File Software Version Definition The last Mode field (40) is the software version number which generated the data file. This value should be 1 or larger. Below is the header lines for the field 40: SW VER ---