== Observable ==

It is in the description of the independent variables, what the data
in fact measure, that there is the greatest variation in terminology
among data archives.  Most solar observational data consist of direct
measurements of the intensity of radiation as a function of time,
direction (location), wavelength, and polarization, or combinations
of intensities associated with different independent variables (''e.g.''
line shifts and splittings, Stokes parameters).  These data may be
interpreted as measurements of certain physical observables, such as
temperature, velocity, emission measure, etc. via models.  There are
of course some important exceptions: some solar data archives include
''in situ'' measurements of such observables as particle fluxes and
compositions and magnetic field strengths; some solar data sets represent
not direct observation but the results of complex inversions or modeling,
such as the frequencies of acoustic modes, or the interior structure;
and there are catalogs, histories, and descriptions of features and events.
As long as the various observable classes are orthogonal, however, these
additional cases should present no problem.


The model of describing observables in terms of particular combinations
of intensity measurements or the associated physical parameters to be derived
from them is a natural one for data deriving from imaging spectrographs,
such as magnetographs and helioseismic instruments.  For cameras or
radiometers measuring only intensity or flux at selected wavelengths,
it is not so natural.  People dealing with data from such instruments
tend to think of the observables as being associated with the spectral
wavelength or band selected, or for monochromatic instruments, even the
spatial-temporal target of the observations.  It is important to
understand that the meaning of the term "observable" in the VSO Search
Parameter model may not at all agree with the meaning of the term as
used by the data providers.




==== Physical_Observable ====


  type: ''menu''[[BR]]
  FITS keyword: ''PHYS_OBS''[[BR]]
  The following values are currently recognized:

  
  ''' intensity  '''


    the direct intensity, either integrated over the spectral observing range
    or as a function of wavelength (spectral density)

  
  ''' equivalent_width '''


    differences between intensities measured at nerbay wavelengths,
    typically in line cores, wings, and nearby continuum, whether measured
    as an intensity difference or an equivalent width

  ''' polarization_vector '''


    the net linear polarization

  ''' LOS_magnetic_field  '''


    the frequency/wavelength Zeeman splitting between opposite circular
    polarizations of a magnetically-sensitive line

  ''' vector_magnetic_field '''


    field strengths and directions inferred from Stokes polarimetry

  ''' LOS_velocity '''


    the displacement of line center from rest wavelength/frequency in
    an arbitrary polarization state
  
  ''' vector_velocity '''


    two- or three-dimesnional velocities, typically inferred from
    helioseismic inversion or from directly measured velocities transverse
    to the line of sight, possibly combined with Doppler velocities
  
  ''' wave_power '''


  
  ''' wave_phase '''

 
  
  ''' oscillation_mode_parameters '''


    These all refer to solar internal or atmospheric acoustic-gravity wave
    measurements.  The mode parameters could include frequencies, splittings,
    amplitudes, widths, ''etc.''

  ''' number_density '''

  ''' particle_flux '''

  ''' composition '''

  ''' particle_velocity '''

  ''' thermal_velocity '''

    ''in-situ'' observations


In addition to the above, the following classes have been suggested:


  * Electric Field Strength - the Stark effect splitting
  * Transverse Magnetic Field Strength - Hanle effect measurements
  * Stokes Parameters ('''I, Q, U, V''' - equivalent to observables of net circular, linear and total polarization, and polarization angle
  * ''in situ'' Magnetic Field
  * Differential Emission Measure
  * Model Parameters - Interior, Atmosphere, Solar Wind