From thompson@orpheus.nascom.nasa.gov Tue Nov 18 16:42:01 2003
Date: Mon, 23 Jun 97 17:11:25 -0400
From: thompson@orpheus.nascom.nasa.gov
Reply-To: sumer-soft@esa.nascom.nasa.gov
To: cds-data@esa.nascom.nasa.gov, sumer-soft@esa.nascom.nasa.gov
Subject: SUMER-SOFT: Solar rotation compensation 

Folks:

One of the things I've recently had occasion to do is to check the algorithm
used for solar differential rotation compensation in the SOHO software.  The
routine which actually calculates the rotation is DIFF_ROT.  Many people use
ROT_XY as a wrapper around DIFF_ROT, because it works in arcseconds.

The default algorithm used in DIFF_ROT is

	rotation = ddays*(14.38-2.7*sin2l)

which is what you get when you call ROT_XY.  The documentation says that this
comes from Allen's Astrophysical Quantities.  I looked it up, and found out
that the parameters come from the second edition of that work, published in
1955, and that the later third edition lists somewhat different quantities,
namely

	rotation = ddays*(14.44-3.0*sin2l)

In addition, DIFF_ROT gives the user the option of using a different algorithm
from Howard, Harvey, and Forgach, Solar Physics, 130, 295, 1990, which has the
evidently more precise algorithm for small magnetic features

      rotation = ddays*(14.434-1.772*sin2l-2.569*sin4l)

which mainly differs at high solar latitudes.

It seems to me that there are a couple of things that should be done.  First of
all, DIFF_ROT should be corrected to use the more recent quantities from Allen
(is there a later edition than the third?).  Second, maybe the Howard et
al. equations should be the default.  There should at least be a way of
selecting between the /HOWARD and /ALLEN options from within ROT_XY.

What is the opinion of others on the best algorithm to use for differential
solar rotation?

Bill Thompson