Steve Allen wrote:
> On Fri 2008-03-14T02:39:27 +0000, Paddy Leahy hath
writ:
>> Example: From (IQUV) absorption spectra, calculate
the optical depth
>> separately for polarized and unpolarized emission.
The ratio (fractional
>> polarization of optical depth) tells you something
interesting about the
>> absorbing material. The polarized absorption may
also be anisotropic,
>> hence there may be an angle for the polarized
optical depth. This can
>> happen in theory for absorption by Zeeman-split
lines, although I have to
>> admit that I've not heard about a measurement in an
astronomical context.
>
> Isn't this pretty much the data used by a solar
magnetograph?
> If so, we need input from practicing solar astronomers
before we're done.
I can give you some information from my own experience.
There are three cases
that I'm aware of where polarization is important:
1. Line-of-sight or vector magnetographs
2. Coronagraphs
3. Solar radio astronomy
My own experience is with coronagraph data. There are two
quantities that are
important in coronagraphic polarimetry, the total brightness
(denoted B), and
the linearly polarized brightness (pB). Since the scattered
light from the
corona is always polarized tangential to the limb, the
actual direction of
polarization (mu) is not generally of interest, except as a
check on the
validity of the data. I will point out, however, that the
definition of the
angle of polarization as we use it differs by a factor of
two from that
described in Paddy Leahy's original post.
The coronagraphs on STEREO take measurements at 3
polarization angles separated
by 120 degrees. These are stored separately as individual
2D FITS files, with
the keyword POLAR in the header giving the angle of the
linear polarizer for
each exposure. From image triplets the total brightness and
polarized
brightness images (or the ratio) are formed and stored in
separate 2D FITS
files. I doubt it would really be of interest to define a
"polarization axis"
where both B and pB are combined into a single array. In
any case, one couldn't
include the fractional polarization or angle in this scheme
because of the
different units.
I've talked to some of my colleagues about magnetometer
data, which I'm not as
familiar with. Usually, magnetometer data is distributed
already combined into
physical quantities, such as magnetic field strength, or
azimuth and elevation
angles. When the actual polarization state of the data are
distributed, they're
distributed as I,Q,U,V images, which are already well
covered by the current
standard. (However, I'm not aware of anybody who actually
*uses* that standard
for magnetograph data--probably, they don't even know it
exists.) The actual
filtergrams are not routinely distributed.
Finally, I have very little familiarity with solar radio
data. I'm given to
understand that radio data are usually distributed as either
separate left and
right polarization data, or as Stokes I and V, both of which
are covered by the
current standard. Some of my colleagues who are more
familiar with radio
observations are gone for the day, so I may have more to say
on the subject
later. In any case, there's a lot of overlap between solar
and celestial radio
astronomy, e.g. both use the VSO, so I would expect that
community practices
would be very similar.
Personally, I think that one has to make a distinction
between data axes and
data types, and they should not be confused together. The
current Stokes
standard in the WCS makes a certain amount of sense, because
a "Stokes vector"
is a well defined quantity. These other parameters, such as
B and pB, are
quantities, and should be described as such, e.g. through a
keyword such as
BYTPE. One should definitely *not* use degenerate axes for
this kind of
information.
Bill Thompson
--
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD 20771
USA
301-286-2040
William.T.Thompson nasa.gov
(Note changed email address)
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