5. PAS L2 Data Products

We produce 3 PAS Level-2 (L2; scientific data level) products in CDF (NASA's Common Data Format) as follows.

  1. 3D velocity distribution function
    e.g., solo_L2_swa-pas-vdf_YYYYMMDD_Vxx.cdf
  2. 1D energy differential flux
    e.g., solo_L2_swa-pas-eflux_YYYYMMDD_Vxx.cdf
  3. Ground moments
    e.g., solo_L2_swa-pas-grnd-mom_YYYYMMDD_Vxx.cdf

where YYYYMMDD is the year, month, day of the data (e.g., 20220403) and Vxx is the version of the file (e.g., V02).

5.1 3D velocity distribution function data

In this section the CDF variables are shown in bold.

The 3D velocity distribution function CDF file consists of 3D arrays (variable “vdf”) of [11 azimuth, 9 elevations, 96 energies] of the ion number density in the phase space expressed in s^3 m^{-6}. Since the instant sampling may cover just a part of the full angular-energy range, the data record contains also the start and number of the elevation bins (“start_elevation”, “nb_elevation”), the start and number of the azimuth bins (“start_CEM”,”nb_CEM”), and the start and number of the energy bins (“start_energy”,”nb_energy”). User shall use only records of the categories (variable “Info”) as follows:

  1. Normal sampling, one “vdf” each 4 sec
  2. Snapshot sampling, “nb_K” VDFs per second
  3. Burst sampling, “nb_K” VDFs per second

As of the version 2 (V02) of VDF data products, we have the following parameters

Table 5.1 Brief description of parameters in the PAS 3D VDF file

Parameters Description Data type Dimension
Azimuth Azimuth bins CDF_REAL4 [11]
Elevation Elevation bins CDF_REAL4 [9]
Elevation_correction Correction of the elevation CDF_REAL4 [96]
Energy Center of energy bins CDF_REAL4 [96]
Epoch Timestamps CDF_TIME_TT2000 [21314]
Full_azimuth Full definition azimuth table CDF_REAL4 [11,9,3]
Full_elevation Full definition elevation table CDF_REAL4 [11,9,3]
Half_interval Acquisition half interval CDF_REAL4 [11,9,3]
Info Information related to PAS operation CDF_UINT1 [21314]
K Number of samplings per second CDF_INT2 [21314]
PAS_to_RTN Coordinate transformation from PAS to RTN CDF_REAL8 [21314,3,3]
SCET Spacecraft local time CDF_REAL8 [21314]
delta_Azimuth Width of azimuth bins CDF_REAL4 [11]
delta_Elevation Width of elevation bins CDF_REAL4 [9]
delta_m_Energy Delta minus energy bins CDF_REAL4 [96]
delta_p_Energy Delta plus energy bins CDF_REAL4 [96]
nb_CEM Number of CEM (channeltron) CDF_INT2 [21314]
nb_elevation Number of elevation CDF_INT2 [21314]
nb_energy Number of energy CDF_INT2 [21314]
quality_factor Data quality factor CDF_REAL4 [21314]
start_CEM Starting number of CEM CDF_INT2 [21314]
start_elevation Starting number of elevation bin CDF_INT2 [21314]
start_energy Starting number of energy bin CDF_INT2 [21314]
total_count Total counts CDF_REAL4 [21314]
unrecovered_count Unrecovered counts CDF_REAL4 [21314]
vdf Velocity Distribution Function CDF_REAL4 [21314,11,9]

Examples of how to read in a CDF file using Python

SpacePy provides a module called pycdf that facilitates reading in a CDF file. Once SpacePy module and CDF library have been installed properly, one can use

from spacepy import pycdf

cdf = pycdf.CDF('solo_L2_swa-pas-vdf_20220403_V02.cdf')
print(cdf)
cdf.close()

This will show you the parameters and their dimensions as shown in Table 5.1.

Then, to read in the parameters, e.g., epoch

time = cdf['Epoch'][...]
print('Time: ',time)

this gives a list of timestamps

Time:  [datetime.datetime(2022, 4, 3, 0, 0, 2, 398445)
 datetime.datetime(2022, 4, 3, 0, 0, 6, 398452)
 datetime.datetime(2022, 4, 3, 0, 0, 10, 398459) ...
 datetime.datetime(2022, 4, 3, 23, 49, 58, 549214)
 datetime.datetime(2022, 4, 3, 23, 50, 2, 549221)
 datetime.datetime(2022, 4, 3, 23, 50, 6, 549228)]

5.2 1D Differential Energy Flux

1D Differential Energy flux CDF file contain original velocity distribution function integrated over elevation and azimuth and converted to the differential energy flux. The value is expressed in cm^{-2} s^{-1} eV / eV. Since the instant sampling may cover just a part of the full energy range, the data record contains also the start and number of the energy bins (“start_energy”,”nb_energy”). User shall use only records of the categories (variable “Info”) as follows.

  1. Normal sampling, one “eflux” each 4 sec
  2. Snapshot sampling, “nb_K” efluxs per second
  3. Burst sampling, “nb_K” efluxs per second

The full description is in Table 5.2 below.

5.3 PAS Ground Moments

Ground moments contain the number density, the velocity vector, the pressure tensor and the temperature of the proton peak, extracted from the 3D VDF. User shall use only records of the categories (variable “Info”) as follows:

  1. Normal sampling, one measurement each 4 sec
  2. Snapshot sampling, “nb_K” measurements per second
  3. Burst sampling, “nb_K” measurements per second

The “validity” flag shows the data quality. Do not use the data labeled as “1” since the number density and the pressure could be too low. Use the data labeled “2” with attention because the data could be noisy.

The full description is in the Table 5.3 below.