Changes between Initial Version and Version 1 of PhysicalObservable

Timestamp:

06/09/05 12:29:38 (20 years ago)

Author:

rick

Comment:

--

PhysicalObservable

@@ +1 @@
+== 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