The equatorial rotation rate of the photosphere was measured at effect data. It was found that scattered light has a large influence and must be taken into account properly. When this was done it was found that the rotation rate from Doppler shifts agreed very well with the rate found for sunspots. Short-term fluctuations in rotation rate (i.e. from day to day) were less than plus or minus 15 m/s and were thus within observational errors

Scherrer, P. H.

Svalgaard, L.

Wilcox, J. M.

English

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https://ntrs.nasa.gov/search.jsp?R=19790003425 2020-03-22T02:17:27+00:00Z
SUIPR Report No. 766
October 1978
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THE EQUATORIAL ROTATION VELOCITY OF THE
PHOTOSPHERE IS MEASURED TO BE THE
SAME AS SUNSPOTS wl
f byL. Svalgaard, P.H. Scherrer, J.M. Wilcox
(NASA-CF- 15792 _S) 	THE %UATORIAL ForATICN	 N79-11596
VELOCITY OF THE PHOTOSPHERE IS MEASUPED TO
BE THE SAME AS SUNSPOTS (Stanford Univ.)
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Office of Naval Research
Contract N00014-76-C-0201
National Aeronautics and Space Administration
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	 Grant NGR 05-020-559
Contract NAS5-24420
and
National Science Foundation
Grant ATM77-20580
Reproduction in whole or in part
is permitted for any purpose of
the United States Government. t
II	 INSTITUTE FOR PLASMA RESEARCHSTANFORD UNIVERSITY, STANFORD, CALIFORNIA
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SUIPR Report No. 766
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The Equatorial Rotation Velocity of thr
Photosphere is Measured to be the Same as
Sunspots
AUINtt111d
L. Svalgaard, P.H. Scherrer, J.M. Wilcox
Itt .11) INSIt1UCrIONS
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Solar Rotation
Scattered Llgllt
Photosphere
-.	 FACT Icontrmre on re,
The
,.Jr If r1rCNS4aly erld Idrntrly by block numbed -
 equatorial rotation rate or the photosphere wa% measured at the Stanford
Solar Observatory during 19707-1978 using spectroscopic Bata. The following points
are made: 1) Scattered liq_ht has a large influence and must he taken into ICCOLIllt
properly. 2) When this is done we find that the rotation rate from Doppler shifts
agrees vrr y well with the rate found for sunspots. 	 3) Short-term fluCtuat ions in
rotation rate (i.e. from day to day) are less than 1 15 m/s and are thus within
observational errors.
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THE EQUATORIAL ROTATION VLLOCITY OF THE
PHOTOSPHERE iS MEASURED TO BE THE
•	 SAME AS SUNSPOTS
L. Svalgaard • P.H. Scherrer, J.M. Wilcox
Institute for Plasma Research
Stanford University
Stanford, California U.S.A.
Abstract
The equatorial rotation rate of the photosphere was measured at the
Stanford Solar Observatory during 1976-1978 using spectroscopic data. The
following points are made: 1) Scattered light has a large influence and
musr be taken i r.to account properly. 2) When this is done we find that the
rotation rate from Fbprd ershifts agrees very well with the rate found for
sunspots. 3) Short-term fluctuations in rotation rate (i.e. from day to
day) are less than t 15 m/s and are thus within observational errors.
The solar rotation has been observed at the Stanford Solar Observ-
atory during the interval 1976-1978. 	 It is found that the equatorial
rotation rate obtained from spectroscopic observations is the same as the
equatorial rate found by Newton and Nunn (1951) for sunspots.
The Stanford Solar Observatory consists of a coelor}at approximately
8 m above the ground that forms all
	
approximately 6 cm in diameter
that is fed into a Babcock solar magnetograph. Th- magnetograph is con-
nedted to a 22.9 m vertical Littrow spectrograph. The spectrograph optics
are ,imilar to those at the Mt. Wilson Observatory. The observations to
be reported were obtained using the Doppler compensator of the magneto-
_	 graph with the line reI
	 X 52508 with a square aperture of size 3'. i.e.
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1/10 of the solar disk diameter. The Stanford Solar Observatory has been
41	 described by Scherrer et al. (1977).
For each day we computed the average rotation velocity in the
•	 center half of the area of the solar disk, i.e. within a circle of radius
0.7 Ro . This average velocity was converted to an equatorial rotation
velocity assuming the form of the differential rotation given by Newtcn
and Nunn (1951). The computed equatorial velocity is rather insensitive
to the precise form of differential rotation assumed.
Figure 1 shows daily values of the equatorial rotation velocity as
measured, without the correction for scattered light that will be
described below. Each vertical shift in Figure 1 corresponds to the time
at which mirrors and/or the objective lens were cleaned or a new mirror
was installed. The abrupt increase in measured rotation velocity at the
time of cleaning of the optics suggests that scattered light has an
important influence on the measured velocities, as was first pointed out
by DeLury (1916).
How can one correct the observed rotation velocity for the effect of
scattered light, WE measured the intensity of scattered light 2' off the
solar disk. We started the investigation on a fine, clear day with clean
mirrors and lenses. Figure 2 shows the measured equatorial velocity and
the measured intensity at the center of the solar disk as a function of
the scattered light 2' off the limb. The points at the left side of
Figure 2 were obtained with clean optics. We then put some fine chalk
dust on the objective lens, and in this situation of increased scattered
light repeated the above measurements to obtain another point in Figure 2.
We then put more fine dust on the objective lens and again measured
rotation velocity and intensity as a function of scattered light. Repeat-
ing this process gave the several points in Figure 2 which were fitted
with a straight line as shown. This straight line can be described with
the equation V.
 = 2025 - 24S 2 m/s, where Vo is the equatorial rotation
velocity and S 2 is the scattered light measured 2' off the limb in
2
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percent of central intensity of the disk. If the straight line is
extrapolated to zero scattered light an equatorial rotation velocity
of 2025 m/s is found. Fo- comparison the equatorial velocity of sun-
spots found by Newton and Nunn (1951) was 2035 m/s.
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The straight line in Figure 2 indicates that 1' , scattered light 2'
off the limb reduces the measured rotational velocity by about 1 1A. On
the disk scattered light is typicall y
 four times larger. The dashed curve
in Figure 2 shows the measured intensity at the center of the disk as a
function of scattered light 2' off the limb. This curve is described by
I o = 4250 e -0 ' `SZ , where 
1  
is the measured intensity at the center of the
disk. The measured intensity - corrected for varying airmass and solar
distance - for each observation can thus be used to correct for the
influence of scattered light. We conclude that it is essential that
each such observation be so corrected.
Figure 3 shows the daily values of equatorial rotation velocity
after the correction for scattered light has been made. During the
interval of our observations 1976-78 the correctec, equatorial rotation
was constant within less than 1''.. and indistinguishable from the
rotation velocity of spots obtained by Newton and Nunn (1951).
A detailed comprehensive analysis of the effects of scattered light
on solar observations is in preparation. and a collaborative investigation
of relation of tho present results to previous observations is in progress.
tkcknowledgements
This work was supported in part by the Office of Naval Research under
Contract N00014-76-C-0201, by the National Aeronautics and Space Admin-
istration under Grant NGR 05- 020-559 and Contract NAS5-24420, , nd b y the
National Science Foundation under Grant ATM17-20680.
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keferences
DeLury, R.E.: 1916, Astrophys ; J. 44, 177.
Newton, H.W. and Nunn, M.L.: 1951. Mor..Not.R.Astron.Soc. 111, 413.
Scherrer, P.H.. Wilcox. J.M. Svalgaard, L., and Gustafson. E.K.: 1977,
Solar Physics 54. 353.
Ap
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7


The equatorial rotation velocity of the photosphere is measured to be the same as sunspots

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