Changes between Initial Version and Version 1 of PhysicalObservable


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Timestamp:
06/09/05 12:29:38 (17 years ago)
Author:
rick
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  • PhysicalObservable

    v1 v1  
     1== Observable == 
     2 
     3It is in the description of the independent variables, what the data 
     4in fact measure, that there is the greatest variation in terminology 
     5among data archives.  Most solar observational data consist of direct 
     6measurements of the intensity of radiation as a function of time, 
     7direction (location), wavelength, and polarization, or combinations 
     8of intensities associated with different independent variables (''e.g.'' 
     9line shifts and splittings, Stokes parameters).  These data may be 
     10interpreted as measurements of certain physical observables, such as 
     11temperature, velocity, emission measure, etc. via models.  There are 
     12of course some important exceptions: some solar data archives include 
     13''in situ'' measurements of such observables as particle fluxes and 
     14compositions and magnetic field strengths; some solar data sets represent 
     15not direct observation but the results of complex inversions or modeling, 
     16such as the frequencies of acoustic modes, or the interior structure; 
     17and there are catalogs, histories, and descriptions of features and events. 
     18As long as the various observable classes are orthogonal, however, these 
     19additional cases should present no problem. 
     20 
     21 
     22The model of describing observables in terms of particular combinations 
     23of intensity measurements or the associated physical parameters to be derived 
     24from them is a natural one for data deriving from imaging spectrographs, 
     25such as magnetographs and helioseismic instruments.  For cameras or 
     26radiometers measuring only intensity or flux at selected wavelengths, 
     27it is not so natural.  People dealing with data from such instruments 
     28tend to think of the observables as being associated with the spectral 
     29wavelength or band selected, or for monochromatic instruments, even the 
     30spatial-temporal target of the observations.  It is important to 
     31understand that the meaning of the term "observable" in the VSO Search 
     32Parameter model may not at all agree with the meaning of the term as 
     33used by the data providers. 
     34 
     35 
     36 
     37 
     38==== Physical_Observable ==== 
     39 
     40 
     41  type: ''menu''[[BR]] 
     42  FITS keyword: ''PHYS_OBS''[[BR]] 
     43  The following values are currently recognized: 
     44 
     45   
     46  ''' intensity  ''' 
     47 
     48 
     49    the direct intensity, either integrated over the spectral observing range 
     50    or as a function of wavelength (spectral density) 
     51 
     52   
     53  ''' equivalent_width ''' 
     54 
     55 
     56    differences between intensities measured at nerbay wavelengths, 
     57    typically in line cores, wings, and nearby continuum, whether measured 
     58    as an intensity difference or an equivalent width 
     59 
     60  ''' polarization_vector ''' 
     61 
     62 
     63    the net linear polarization 
     64 
     65  ''' LOS_magnetic_field  ''' 
     66 
     67 
     68    the frequency/wavelength Zeeman splitting between opposite circular 
     69    polarizations of a magnetically-sensitive line 
     70 
     71  ''' vector_magnetic_field ''' 
     72 
     73 
     74    field strengths and directions inferred from Stokes polarimetry 
     75 
     76  ''' LOS_velocity ''' 
     77 
     78 
     79    the displacement of line center from rest wavelength/frequency in 
     80    an arbitrary polarization state 
     81   
     82  ''' vector_velocity ''' 
     83 
     84 
     85    two- or three-dimesnional velocities, typically inferred from 
     86    helioseismic inversion or from directly measured velocities transverse 
     87    to the line of sight, possibly combined with Doppler velocities 
     88   
     89  ''' wave_power ''' 
     90 
     91 
     92   
     93  ''' wave_phase ''' 
     94 
     95  
     96   
     97  ''' oscillation_mode_parameters ''' 
     98 
     99 
     100    These all refer to solar internal or atmospheric acoustic-gravity wave 
     101    measurements.  The mode parameters could include frequencies, splittings, 
     102    amplitudes, widths, ''etc.'' 
     103 
     104  ''' number_density ''' 
     105 
     106  ''' particle_flux ''' 
     107 
     108  ''' composition ''' 
     109 
     110  ''' particle_velocity ''' 
     111 
     112  ''' thermal_velocity ''' 
     113 
     114 
     115  '''    ''in-situ'' observations ''' 
     116 
     117 
     118In addition to the above, the following classes have been suggested: 
     119 
     120 
     121  * Electric Field Strength - the Stark effect splitting 
     122  * Transverse Magnetic Field Strength - Hanle effect measurements 
     123  * Stokes Parameters ('''I, Q, U, V''' - equivalent to observables of net circular, linear and total polarization, and polarization angle 
     124  * ''in situ'' Magnetic Field 
     125  * Differential Emission Measure 
     126  * Model Parameters - Interior, Atmosphere, Solar Wind