Variables

We have identified three types of variables to be included in ISTP/IACG CDF files: data variables of primary importance (e.g., density, magnetic_field, particle_flux), support_data variables of secondary importance (e.g., time, energy_bands associated with particle_flux) and metadata variables (e.g., a variable holding "Bx,By,Bz" to label magnetic field). Variables are defined with CDF specifications and required attributes. Data variables also have attached variables for time and dependencies (support_data) and labels (metadata). The support_data variables can be attached to data variables via DEPEND_i variable attributes. Metadata variables can be attached to data variables via LABL_PTR_i variable attributes (see below).

Examples of data and support_data variables commonly found in ISTP/IACG investigations are shown below. They are mapped to their corresponding dimensions and sizes in CDF.

  • Density and Temperature (data) in this example are scalars; in CDF they are associated with zero dimension and no size.
  • Plasma velocity, electric and magnetic fields (data) are vectors, i.e., three orthogonal components in some coordinate system, stored in one-dimension of size 3.
  • Particle flux, for this example, has values at eight energy channels. Flux (data) and Energy (support_data) are stored as one-dimensional variables of size 8.
  • The image array (data) maps into a two-dimensional variable with sizes 256 and 256. Latitude and longitude (support_data) are one-dimensional variables of size 256, providing the necessary coordinate indices for the image array.
  • Data

    These are variables of primary importance (e.g., density, magnetic_field, particle_flux). Data variables are completely defined with the combination of CDF specifications, variable attributes, and attached variables such as time and dependencies (support_data) and labels (metadata).

    Data is always either Real or Integer type. Data is always time (record) varying, but can be of any dimensionality. Real or Integer data are always defined as having one element.

    The following variable attributes are required.
     

  • CATDESC
  • DEPEND_0 = Epoch
  • DEPEND_i
  • DISPLAY_TYPE (time_series, spectrogram, stack_plot,image)
  • FIELDNAM
  • FILLVAL
  • FORMAT/FORM_PTR
  • LABLAXIS/LABL_PTR_i
  • UNITS/UNIT_PTR
  • VALIDMIN
  • VALIDMAX
  • VAR_TYPE = data

  • The need for DEPEND_i (other than DEPEND_0) and either LABLAXIS or LABL_PTR_i depends on the data itself and how it will be displayed.

    Support_Data

    These are variables of secondary importance (e.g., time, energy_bands associated with particle_flux) .

    Support_data is always either Real or Integer type. Support_data is usually time invariant, but can be time varying.

    If a support_data variable is attached to a data variable via DEPEND_i, then it must be of the same size as the dimension i. See example below. Real or Integer data are always defined as having one element.

    The following variable attributes are required.
     

    Metadata

    These are variables of secondary importance e.g., a variable holding "Bx,By,Bz" to label magnetic field).

    Metadata is always character type. Metadata is always time invariant if it is used to label a data variable. Metadata can be time varying if it is NOT used as a label.

    If a metadata variable is attached to a data variable via LABL_PTR_i, then it must be of the same size as the dimension i.

    Character metadata must define the number of elements to be the same as the number of characters used in its value.

    The following variable attributes are required.

    See also: