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NAME:twavpol
PURPOSE:To perform polarisation analysis of three orthogonal component time
series data, using tplot variables.
EXAMPLE: twavpol,'in_data',prefix='in_data',freqline=fl
INPUTS: tvarname: the name of the tplot variable upon which it will
operate
prefix(optional): the prefix to be assigned to the tplot variables that will be
output, defaults to tvarname
Subroutine assumes data are in righthanded fieldaligned
coordinate system with Z pointing the direction
of the ambient magnetic field.
Keywords:
nopfft(optional) = Number of points in FFT
steplength(optional) = The amount of overlap between successive FFT intervals
OUTPUTS:
error(optional): named variable in which to return the
error state of this procedure call. 1 = success, 0 = failure
freqline(optional): assign a named variable to this keyword
to store the frequencies of each y-index
timeline(optional): assign a named variable to this keyword
to store the times of each x-index
The program outputs five spectral results derived from the
fourier transform of the covariance matrix (spectral matrix)
This version stores these outputs as tplot variables with the
specified prefix
These are follows:
Wave power: On a linear scale, at this stage no units
Degree of Polarisation:
This is similar to a measure of coherency between the input
signals, however unlike coherency it is invariant under
coordinate transformation and can detect pure state waves
which may exist in one channel only.100% indicates a pure
state wave. Less than 70% indicates noise. For more
information see J. C. Samson and J. V. Olson 'Some comments
on the description of the polarization states
of waves' Geophys. J. R. Astr. Soc. (1980) v61 115-130
Wavenormal Angle:
the angle between the direction of minimum
variance calculated from the complex off diagonal
elements of the spectral matrix and the Z direction
of the input
ac field data. For magnetic field data in
field aligned coordinates this is the
wavenormal angle assuming a plane wave.
Ellipticity:The ratio (minor axis)/(major axis) of the
ellipse transcribed by the field variations of the
components transverse to the Z direction. The sign
indicates the direction of rotation of the field vector in
the plane. Negative signs refer to left-handed
rotation about the Z direction. In the field
aligned coordinate system these signs refer to
plasma waves of left and right handed
polarisation.
Helicity:Similar to Ellipticity except defined in terms of the
direction of minimum variance instead of Z. Stricltly the Helicity
is defined in terms of the wavenormal direction or k.
However since from single point observations the
sense of k cannot be determined, helicity here is
simply the ratio of the minor to major axis transverse to the
minimum variance direction without sign.
NOTES:
1. Although the input is in the form of a tplot variable, the
output is currently in the form of arrays
2. -If one component is an order of magnitude or more greater than
the other two then the polarisation results saturate and erroneously
indicate high degrees of polarisation at all times and
frequencies.
3. Time series should be eyeballed before running the program.
For time series containing very rapid changes or spikes
the usual problems with Fourier analysis arise.
Care should be taken in evaluating degree of polarisation results.
4. For meaningful results there should be significant wave power at the
frequency where the polarisation approaches
100%. Remembercomparing two straight lines yields 100% polarisation.
$LastChangedBy: aaflores $
$LastChangedDate: 2012-01-26 17:01:34 -0800 (Thu, 26 Jan 2012) $
$LastChangedRevision: 9630 $
$URL: svn+ssh://thmsvn@ambrosia.ssl.berkeley.edu/repos/ssl_general/tags/tdas_7_00/science/wavpol/twavpol.pro $
(See ssl_general/science/wavpol/twavpol.pro)
NAME: twavpol_scm_crib Purpose: Demonstrate the usage of the wave polarization routines. NOTES: Shortened version of Olivier Le Contel's <olivier.lecontel@lpp.polytechnique.fr> wave polarization crib(scm_mfa_wpol_ole_fc_crib.pro) $LastChangedBy: aaflores $ $LastChangedDate: 2012-01-10 17:02:16 -0800 (Tue, 10 Jan 2012) $ $LastChangedRevision: 9534 $ $URL: svn+ssh://thmsvn@ambrosia.ssl.berkeley.edu/repos/ssl_general/tags/tdas_7_00/science/wavpol/twavpol_scm_crib.pro $
(See ssl_general/science/wavpol/twavpol_scm_crib.pro)
NAME:wavpol
MODIFICATION HISTORY:Written By Chris Chaston, 30-10-96
:Modified by Vassilis, 2001-07-11
:Modified by Olivier Le Contel, 2008-07
to be able to change nopfft and steplength
PURPOSE:To perform polarisation analysis of three orthogonal component time
series data.
EXAMPLE: wavpol,ct,Bx,By,Bz,timeline,freqline,powspec,degpol,waveangle,elliptict,helict
CALLING SEQUENCE: wavpol,ct,Bx,By,Bz,timeline,freqline,powspec,degpol,waveangle,elliptict,helict
INPUTS:ct,Bx,By,Bz, are IDL arrays of the time series data; ct is cline time
Subroutine assumes data are in righthanded fieldaligned
coordinate system with Z pointing the direction
of the ambient magnetic field.
threshold:-if this keyword is set then results for ellipticity,
helicity and wavenormal are set to Nan if below 0.6 deg pol
;Keywords:
nopfft(optional) = Number of points in FFT
steplength(optional) = The amount of overlap between successive FFT intervals
OUTPUTS: The program outputs five spectral results derived from the
fourier transform of the covariance matrix (spectral matrix)
These are follows:
Wave power: On a linear scale, at this stage no units
Degree of Polarisation:
This is similar to a measure of coherency between the input
signals, however unlike coherency it is invariant under
coordinate transformation and can detect pure state waves
which may exist in one channel only.100% indicates a pure
state wave. Less than 70% indicates noise. For more
information see J. C. Samson and J. V. Olson 'Some comments
on the description of the polarization states
of waves' Geophys. J. R. Astr. Soc. (1980) v61 115-130
Wavenormal Angle:
the angle between the direction of minimum
variance calculated from the complex off diagonal
elements of the spectral matrix and the Z direction
of the input
ac field data. For magnetic field data in
field aligned coordinates this is the
wavenormal angle assuming a plane wave.
Ellipticity:The ratio (minor axis)/(major axis) of the
ellipse transcribed by the field variations of the
components transverse to the Z direction. The sign
indicates the direction of rotation of the field vector in
the plane. Negative signs refer to left-handed
rotation about the Z direction. In the field
aligned coordinate system these signs refer to
plasma waves of left and right handed
polarisation.
Helicity:Similar to Ellipticity except defined in terms of the
direction of minimum variance instead of Z. Stricltly the Helicity
is defined in terms of the wavenormal direction or k.
However since from single point observations the
sense of k cannot be determined, helicity here is
simply the ratio of the minor to major axis transverse to the
minimum variance direction without sign.
RESTRICTIONS:-If one component is an order of magnitude or more greater than
the other two then the polarisation results saturate and erroneously
indicate high degrees of polarisation at all times and
frequencies. Time series should be eyeballed before running the program.
For time series containing very rapid changes or spikes
the usual problems with Fourier analysis arise.
Care should be taken in evaluating degree of polarisation results.
For meaningful results there should be significant wave power at the
frequency where the polarisation approaches
100%. Remembercomparing two straight lines yields 100% polarisation.
$LastChangedBy: aaflores $
$LastChangedDate: 2012-01-10 17:02:16 -0800 (Tue, 10 Jan 2012) $
$LastChangedRevision: 9534 $
$URL: svn+ssh://thmsvn@ambrosia.ssl.berkeley.edu/repos/ssl_general/tags/tdas_7_00/science/wavpol/wavpol.pro $
(See ssl_general/science/wavpol/wavpol.pro)