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ADS astronomy entries on 2022-09-14
author:"Snodgrass, Herschel B."
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Title: Obituary: Peter Robert Wilson, 1929-2007
Authors: Snodgrass, Herschel B.
2009BAAS...41..583S Altcode:
It is with great sadness that I report the passing of Peter Robert
Wilson, a well-known and well-loved figure in the solar physics
community. Peter was on the faculty of the Department of Applied
Mathematics at the University of Sydney for 39 years, and Chair of the
department for 24 of these years. He was the author or co-author of
more than 80 scientific research papers and a book, Solar and Stellar
Activity Cycles (1994), published by Cambridge University Press. He
died suddenly of a heart attack, at his home in Glebe, Australia, in
the early morning of 11 November 2007. <P />Peter was an organizer of,
and participant in, many international conferences and workshops. He
traveled extensively, holding visiting appointments at the University
of Colorado (JILA), at Cambridge University, at the College de France
(Paris), and at the California Institute of Technology [CalTech]. Most
of his work was in the field of solar physics, but he also did some work
on the philosophy of science and on tides. <P />Peter came from a line
of mathematicians. His father, Robert Wilson, immigrated to Australia
from Glasgow in 1911, and became a mathematics teacher at Scotch
College, a private school in Melbourne. There his name was changed
to 'Bill' because 'Bob' was already taken." <P />Peter's enjoyment
of this story as characteristic of Australian academia (as any fan
of Monty Python would understand) is indicative of his infectious
sense of humor. In a similar vein, he claimed ancestry traced back
to the eighteenth-century Scottish mathematician Alexander Wilson,
Professor of Astronomy at the University of Glasgow. That Wilson
is famous in the solar physics community for his discovery, known
as the "Wilson Effect," of the photospheric depressions associated
with sunspots. Peter himself could not resist writing a paper on
this subject, and was delighted when the bait was taken by some
less-informed colleagues who chided him for "naming an effect after
himself." <P />"Bill" Wilson married Naomi Christian, a Melbourne
native, and together they had three children. Peter was the eldest;
he was born on 17 October 1929. He attended Scotch College, where his
father taught, and went on to the University of Melbourne where he
eventually earned an M. Sc. in experimental physics. This was not his
cup of tea, however, and he first endeavored to follow in his father's
footsteps, taking short-term appointments teaching mathematics at the
secondary-school level abroad, in England, and in Scotland. After
a few years Peter returned to Melbourne and took a post at Scotch
College following his father's retirement. He soon decided, however,
that teaching young boys in a private school was not his cup of tea
either, and in 1959 he secured a position in applied mathematics
at the University of Sydney. He had just married his first wife,
Margaret, and they moved north together to start their family. <P
/>Peter flourished at the University of Sydney, but his advancement
in rank was hampered by the lack of a Ph. D. The problem was solved
by Ron Giovanelli, Chief of the Division of Physics at Australia's
Commonwealth Scientific and Industrial Research Organization [CSIRO],
an astrophysicist whose interest lay in the transfer of radiation
through the outer layers in the Sun. Giovanelli took Peter on as a
thesis student. This both earned him the needed Ph. D. and started
him on his research career in solar physics. He now began to move up
the academic ladder at Sydney. <P />To satisfy his love of adventure,
Peter was also able to take a series of visiting positions in the United
States, working with Dick Thomas and others at JILA and Sacramento Peak
Observatory (National Solar Observatory) in New Mexico. During this time
he created a framework for further collaborations that became known
as the Sydney-Boulder Astrophysics Association [SBAA]. <P />In 1971
Peter was appointed Professor and Chair of the Department of Applied
Mathematics at Sydney, and for the next two decades he worked hard to
strengthen this department. He was very successful in this endeavor;
he had a reputation for fairness and honesty and was well liked. Under
his leadership the department grew in both size and quality. Peter
fostered a group of outstanding students, including Chris Cannon,
David Rees, and Lawrence Cram. One of his proudest accomplishments was
to recruit several women onto the faculty and to increase the number
of female students. One of these, Nalini Joshi, is presently Head of
School. After Peter resigned as Chair, he went on to several other
positions associated with the governance of the University, including
the Academic Senate, the Governing Council of the Women's College, and
the Board of Trustees. <P />Peter and his first wife were divorced in
1982, after their two children, Sally and Michael, had grown up and left
home. A few years later he met and married Geraldine Barnes, a Senior
Lecturer in the English Department. This proved to be a fabulous match;
they supported each other's academic pursuits, attended each other's
conferences, enjoyed a rich social life centered around the university,
and traveled extensively together. Their marriage helped both of them
refocus their careers. Geraldine steadily advanced in rank, and is
now Head of the School of Letters, Arts and the Media. Peter became
one of the chief organizers of a series of workshops focused on the
solar activity cycle. <P />The first solar cycle workshop was held
in 1986 at CalTech's Big Bear Solar Observatory [BBSO], and it was
at this meeting that I first met Peter. There were three subsequent
meetings, roughly a year apart, held at the University of Sydney,
at Stanford's Fallen Leaf Lake in the Sierras, and at Sacramento Peak
Observatory, and these were very successful in bringing together the
main players in this research field. My subsequent association with
Peter involved several trips back and forth between Portland (Oregon),
Boulder, and Sydney and collaborations on about a dozen controversial
research papers. Together with Peter Fox and Pat McIntosh, we became the
solar-physics "gang of four." <P />A dinner in Sydney with Geraldine,
Peter, and their friends always meant liberal amounts of fine Australian
wine, lively conversations on every imaginable topic (except physics),
much laughter, and a deliciously endless meal. A weekend at their
beach house in Killcare was even better, featuring long walks on the
golden-sand beach and in the nearby bush. Kookaburras, Currawongs,
and Rainbow Lorikeets frequented the outdoor deck, and the bush
teemed with large and fascinating spiders. Back in Sydney, short-term
visitors enjoyed lodgings and excellent breakfasts at the University
of Sydney's Women's College, with Peter on the Council. <P />Peter
was a man of many interests. He was an expert sailor, a small-plane
pilot who took colleagues and friends on adventurous flights, and a
lover of sports. He was a skier, a hiker, and a good tennis player who
disdained proper form but usually won the point. In 1994, one day after
his 65th birthday, Peter suffered a serious stroke. Recovery from this
was extremely difficult, painful, and slow; he did, however, recover
to a remarkable degree. He had to learn to walk all over again and
his vocal chords were partially paralyzed, but after several years of
determined work, Peter was able to play a little tennis and squash,
and he could bowl and hike. During the last decade of his life he
traveled to Easter Island, to the Galapagos, and to the Ross Ice Shelf
in Antarctica. <P />Peter continued to take pleasure in his research to
the end, in collaboration with close colleagues who were always among
his closest friends. Among these was Chris Durrant, who had been Head of
the School of Mathematics and Statistics from 1994 to 1998. They were
writing a series of papers on the mechanism of the Sun's polar field
reversals. I was looking forward to joining them this coming summer. My
last visit with Peter was in Phoenix, Arizona, where Geraldine was
participating in a conference. We hiked into the Superstition Mountains,
and I remember him walking slowly, being careful of his balance, but
going the whole distance with pride and in good spirits. <P />Peter
was a truly remarkable man with, as Geraldine has put it, "a genuine
gift for leadership and the encouragement of team spirit." He was
a creative and productive scientist with a tremendous life force,
a great sense of adventure, and a warm heart. My own collaborations
with him were a joy. His death is a sad loss to all who knew him, and
he will be sorely missed, but Peter R. Wilson lived life to the fullest
and gave his best to the world. We should be glad for him. At the end
of his (unpublished) autobiography, where he describes his recovery
from the stroke, he writes: <P />"So as I forecast in 1994, I have
continued to 'soldier on', and must admit that a miracle has indeed
occurred, at least 80%; I wouldn't have missed the past ten years for
anything. Who knows what the inevitable advance of old age may hold,
but I cannot complain that I have been 'short changed' in any way."
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Title: Patterns of vorticity on the solar surface
Authors: Brown, Benjamin P.; Snodgrass, Herschel B.
2003ESASP.517..109B Altcode: 2003soho...12..109B
Local Correlation tracking of Hydrogen-alpha images taken at one minute
intervals at Big Bear Solar Observatory is used to make flow maps that
reveal large-scale, high-velocity patterns that appear to be associated
with the Sun's magnetic activity. We discuss the possible connections
of these patterns to the azimuthally averaged meridional flow and
torsional oscillations. We then use the flow maps to compute global
maps of vorticity at the solar surface. The vorticity maps contain
plume-like patterns of alternation resembling the patterns seen in the
maps of the Sun's background magnetic field. The vorticity plumes may
account for the disparity in diffusion constants determined for the
dispersal of the field and the polar field reversals.
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Title: Properties and Motions of Photospheric Magnetic Features away
from Active Regions
Authors: Tucker, J. F.; Snodgrass, H. B.
2001AAS...198.7106T Altcode: 2001BAAS...33..893T
Analysis of the Pearson correlation amplitude as a function of latitude,
lag in longitude and time separation of correlated magnetograms
reveals interesting mean properties of the magnetic field features
seen in the photosphere. Small features decorrelate after a few days,
but correlations persist at all latitudes for lag times spanning
several rotations, revealing the ubiquitous presence of concentrated
aggregates of small features that (1) differentially rotate like the
small features, (2) have very long lifetimes, and (3) have areal sizes
an order of magnitude larger than supergranules. These “meso-scale"
features comprise the unipolar plumes extending poleward from the
active regions, and the more rigid rotation of the plumes stems from the
poleward meridional drift and random walk of these features. The random
walk is consistent with a diffusion constant of 600 Km<SUP>2</SUP>
s<SUP>-1</SUP>, but these features are too large for this to be
propelled by supergranular convection. We discuss the evolution of the
properties and motions of these features, or aggregates, during the
activity cycle. This work is supported through NSF Grant ATM98-14145.
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Title: Torsional Oscillations: Vorticity; Solar Cycle Predictions
Authors: Snodgrass, H. B.
2001AAS...198.7102S Altcode: 2001BAAS...33Q.893S
The azimuthal wind bands known as the torsional oscillations have
been revealed primarily by studying the longitudinally averaged solar
rotation over a period spanning several full solar rotations. This
averaging yields what look like broad but slow, oppositely-moving (
~ 5 m s<SUP>-1</SUP>) bands lying to either side of the centroid of
the sunspot butterfly, making the activity band appear to be a zone
of weakly enhanced shear. In most discussions, the pattern has been
characterized as axially symmetric, but such longitudinal averages could
equally well arise from a system of large-scale vortices associated
with the active regions, if such vortices rotated counterclockwise in
the Northern hemisphere and clockwise in the Southern hemisphere. For
some time Doppler charts made from the Mount Wilson data, though noisy,
have indicated that the torsional pattern is not axially symmetric,
at least during the active phase of the cycle; and recent maps of
local velocities determined from short-term tracer tracking at Big
Bear Solar Observatory suggest that there are large vortical motions
superposed on the mean differential rotation. In any case, it is
evident that the torsional pattern tells us something about the cycle,
and since it precedes the onset of activity, it might be useful as a
predictor of the level of activity to come. For the present cycle 23,
the torsional pattern did not emerge until just before solar minimum,
whereas for cycles 21 and 22 it appeared several years earlier. This
would have suggested by 1996 that that the present cycle would be weaker
than the previous two (as it apparently is), while other predictors
as late as 1998 forecasted a very strong cycle. This work is supported
through NSF Grant ATM98-14145.
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Title: On the Use of Correlations to Determine the Motions and
Properties of Mesoscale Magnetic Features in the Solar Photosphere
Authors: Snodgrass, Herschel B.; Smith, Adam A.
2001ApJ...546..528S Altcode:
The use of correlations to determine both intrinsic properties and
collective motions of patterns is investigated, and the results
are applied to the study of the magnetic features in the solar
photosphere. Simulations with artificial data are used as a bridge
between theory and practical correlation calculations. It is shown
that the correlation amplitude as a function of lag can be used to
determine not only pattern displacement, but also feature sizes and
lifetimes. It is found that reliable results are obtained only when a
normalized correlation function is employed, and then only when the
signal-to-noise level is greater than ~1.5. For weak correlations,
we show that this ratio must be enhanced by averaging the correlation
amplitudes, but when applied to the photospheric magnetic field
patterns, this gives a result different from that obtained by averaging
the individual correlation results. We find this to be the root of the
differences between the magnetic rotation rates that have been reported
and resolve this long-standing puzzle. The correlations indicate the
ubiquitous presence of differentially rotating magnetic features of
two types: small-scale features that have lifetimes of ~1 day, and
“mesoscale” features with lifetimes of many solar rotations. The
latter are estimated to have diameters on the order of 100 Mm, and their
motions relative to the ambient plasma are consistent with a random
walk with diffusion constant D<SUB>m</SUB>=530+/-100 km<SUP>2</SUP>
s<SUP>-1</SUP>. Our value for D<SUB>m</SUB> agrees with that required
in the model of Sheeley, Nash, & Wang, but these features are
too large to have their random walks propelled by the supergranular
convection. Furthermore, analysis of their relative contributions
to the background field implies they decay at a rate consistent
with a smaller diffusion constant D<SUB>s</SUB>~=250 km<SUP>2</SUP>
s<SUP>-1</SUP>. This agrees with the value determined in high-resolution
studies, which suggests that the mesoscale features are aggregates
of small-scale features undergoing random walks as well, like those
observed in these studies.
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Title: The effects of meridional motion on the determination of
rotation by tracer tracking
Authors: Snodgrass, Herschel B.; Smith, Adam A.
2000SoPh..191...21S Altcode:
We explore a systematic error that arises in feature-tracking
measurements of time-average rotation. It stems from the flows of
features across latitudes, and as these flows vary with the solar
activity cycle, the error has a pattern of variation which mocks the
torsional oscillation. We develop a series expansion for this error
and evaluate the leading terms for the example case of cycle 21. It
grows with the time lag; for a 30 day lag it is ≲1%, depending on how
the correlations are done and interpreted. We conclude that the mock
pattern cannot, however, account for the magnetic-rotation torsional
oscillations pattern found in recent analyses of magnetograms from Kitt
Peak and Mount Wilson. For the 1-day time lag in the Kitt Peak study,
the error is negligible, and for the ∼30-day time lag in the Mount
Wilson study, it represents at most about 30% of the signal.
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Title: Observations of the Polar Magnetic Fields During the Polarity
Reversals of Cycle 22
Authors: Snodgrass, H. B.; Kress, J. M.; Wilson, P. R.
2000SoPh..191....1S Altcode:
The Mount Wilson synoptic magnetic data for the period September 1987
through March 1996 are completely revised and used to provide polar
plots of the solar magnetic fields for both hemispheres. This period,
from Carrington rotations 1793 to 1906, covers the reversals of the
polar magnetic fields in cycle 22. Comparison of our plots with the
presently available Hα filtergrams for this period shows that the
polarity boundaries are consistent in these two data sets where they
overlap. The Mount Wilson plots show that the polar field reversals
involve a complex sequence of events. Although the details differ
slightly, the basic patterns are similar in each hemisphere. First the
old polarity becomes isolated at the pole, then shortly thereafter,
the isolation is broken, and the polar field includes unipolar regions
of both polarities. The old polarity then reclaims the polar region,
but when the isolation of this field is established for a second time,
it declines in both area and strength. We take the reversal to be
complete when the old polarity field is no longer observed in the
Mount Wilson plots. With this criterion we find that the polar field
reversal is completed in the north by CR 1836, i.e., by December 1990,
and in the south by CR 1853, i.e., March 1992.
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Title: Comment on “Absence of Correlation between the Solar Neutrino
Flux and the Sunspot Number”
Authors: Snodgrass, H. B.; Oakley, D. S.
1999PhRvL..83.1894S Altcode:
A Comment on the Letter by Guenther Walther, Phys. Rev. Lett. 79, 4522
(1997). The authors of the Letter offer a Reply.
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Title: Observations of 44i Bootis.
Authors: Jurgenson, C.; Price, M. E.; Pereira, M.; Macinnes, D.;
Snodgrass, H. B.
1999BAAS...31..954J Altcode:
No abstract at ADS
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Title: Observations of 44i Bootis
Authors: Jurgenson, C.; Price, M. E.; Pereira, M.; Macinnes, D.;
Snodgrass, H. B.
1999AAS...194.7505J Altcode:
Seven times of minimum were obtained for the W Ursae Majoris type system
44i Bootis; three primary and four secondary. These observations were
taken using a 10-inch Newtonian reflector and 1P21 photomultiplier
at Lewis & Clark College. Our times of minimum are used with
previously observed minimum times for plotting an (o-c) curve to
generate a function that describes how the system's ~ 6.5 hr. period
is changing over time. After applying a parabolic least squares fit
to the (o-c) curve we obtain a quadratic function F(E), where E is the
eclipse number. Adding this to the 1991 ephemeris of Oprescu, we obtain
a corrected non-linear ephemeris JD 2443604.5919 + 0.26781665 E + 5.9
x10(-11) E(2) . From this we find, in agreement with past observations,
that the period of 44i Bootis is increasing over time, and determine the
rate of increase. This work was supported by Partners in Science Grant
HS0485, an M. J. Murdock Charitable Trust Award of Research Corporation.
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Title: Simulations of Photospheric Magnetic Fields
Authors: Smith, A. A.; Snodgrass, H. B.
1999AAS...194.9402S Altcode: 1999BAAS...31Q.991S
We have run plots of artificial data, which mimic solar magnetograms,
through standard algorithms to critique several results reported
in the literature. In studying correlation algorithms, we show that
the differences in the profiles for the differential rotation of the
photospheric magnetic field stem from different methods of averaging. We
verify that the lifetimes of small magnetic features, or of small
patterns of these features in the large-scale background field, are
on the order of months, rather than a few days. We also show that a
meridional flow which is cycle dependent creates an artifact in the
correlation-determined magnetic rotation which looks like a torsional
oscillation; and we compare this artifact to the torsional patterns
that have been reported. Finally, we simulate the time development of a
large-scale background field created solely from an input of artifical,
finite-lifetime 'sunspot' bipoles. In this simulation, we separately
examine the effects of differential rotation, meridional flow and
Brownian motion (random walk, which we use rather than diffusion), and
the inclination angles of the sunspot bipoles (Joy's law). We find,
concurring with surface transport equation models, that a critical
factor for producing the patterns seen on the Sun is the inclination
angle of the bipolar active regions. This work was supported by NSF
grant 9416999.
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Title: Multipole Decomposition of the Solar Magnetic Field
Authors: Collord, J.; Snodgrass, H. B.
1999AAS...194.9401C Altcode: 1999BAAS...31..991C
We do a multipole expansion of the photospheric magnetic field,
determined by least-squares fits to the radial component plotted on
Carrington maps of Mount Wilson magnetograph data. We study these
moments in each hemisphere separately and also for the Sun as a
whole, and follow their evolution over three solar cycles. The
axial and equatorial components of the dipole each have a roughly
21-yr. sinusoidal variation and, as expected, are 90(o) out of
phase. The equatorial component is strongest around solar maximum,
and its direction can suddenly shift. Between such shifts, it
rotates at a rate that varies during the cycle. The north and south
axial components reflect the different times of the north and south
polar field reversals, and also a persistent north-south asymmetry,
for the southern axial dipole lags in its time development, but is
stronger at maximum. Although the polar reversals seem to involve
some `redirecting' of the hemispheric dipoles, our evidence suggests
that the field reversals cannot be thought of as rotations of these
dipoles. The behavior of the quadrupole moments is also discussed,
as is the overall method for doing this expansion. This work was
supported by NSF Grant AST9416999.
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Title: Interactions between solar neutrinos and solar magnetic fields
Authors: Oakley, D. S.; Snodgrass, Herschel B.
1997APh.....7..297O Altcode: 1996hep.ph....4252O
We attempt to correlate all of the available solar-neutrino data
with the strong magnetic fields these neutrinos encounter in the
solar interior along their Earth-bound path. We approximate these
fields using the photospheric, magnetograph-measured flux from central
latitude bands, time delayed to proxy the magnetic fields in the solar
interior. Our strongest evidence for anticorrelation is for magnetic
fields within the central ±5° solar-latitude band that have been
delayed by 0.85 ± 0.55 yr. Assuming a neutrino-magnetic interaction,
this might indicate that interior fields travel to the solar surface
in this period of time. As more solar-neutrino flux information is
gathered, the question of whether this result arises from a physical
process or is merely a statistical fluke should be resolved, providing
that new data are obtained spanning additional solar cycles and that
correlation studies focus on these same regions of the solar magnetic
field.
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Title: Meridional Motions of Magnetic Features in the Solar
Photosphere
Authors: Snodgrass, Herschel B.; Dailey, Sara B.
1996SoPh..163...21S Altcode:
We cross-correlate pairs of Mt. Wilson magnetograms spaced at intervals
of 24-38 days to investigate the meridional motions of small magnetic
features in the photosphere. Our study spans the 26-yr period July
1967-August 1993, and the correlations determine longitude averages of
these motions, as functions of latitude and time. The time-average of
our results over the entire 26-yr period is, as expected, antisymmetric
about the equator. It is poleward between ∼ 10° and ∼ 60°, with
a maximum rate of 13 m s<SUP>−1</SUP>, but for latitudes below ±10°
it is markedly equatorward, and it is weakly equatorward for latitudes
above 60°. A running 1-yr average shows that this complex latitude
dependence of the long-term time average comes from a pattern of motions
that changes dramatically during the course of the activity cycle. At
low latitudes the motion is equatorward during the active phase of the
cycle. It tends to increase as the zones of activity move toward the
equator, but it reverses briefly to become poleward at solar minimum. On
the poleward sides of the activity zones the motion is most strongly
poleward when the activity is greatest. At high latitudes, where the
results are more uncertain, the motion seems to be equatorward except
around the times of polar field reversal. The difference-from-average
meridional motions pattern is remarkably similar to the pattern of the
magnetic rotation torsional oscillations. The correspondence is such
that the zones in which the difference-from-average motion is poleward
are the zones where the magnetic rotation is slower than average, and
the zones in which it is equatorward are the zones where the rotation
is faster.
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Title: Meridional Flow, Torsional Oscillations and Random Walk of
Photospheric Magnetic Features
Authors: Snodgrass, H. B.; Dailey, S. B.
1995AAS...18710110S Altcode: 1995BAAS...27.1427S
We have used two-dimensional cross-correlations of Mount Wilson
coarse-array magnetograms, spaced at 24-38 days, to determine
the pattern of meridional drifts for photospheric features in the
large-scale background field during the period 1967-1993. The flow
pattern is linked to the butterfly diagram, and varies markedly during
the activity cycle. The dominant trend is motion away from regions
of high flux concentration. Our results are consistent with a picture
in which magnetic features of size comparable to a few supergranules
behave like 'particles' undergoing a Brownian motion on the solar
surface. They appear neither to be tightly bound to subsurface field
structures, nor to evaporate, and the diffusion that appears to
propel them about evidently does not extend to small enough scales to
take them apart. Comparison with the magnetic torsional oscillation
suggests that the torsional pattern is an artifact of the meridional
drift pattern rather than an actual East-West flow. {abstract}
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Title: On the Correlation of Solar Surface Magnetic Flux with Solar
Neutrino Capture Rate
Authors: Oakley, David S.; Snodgrass, Herschel B.; Ulrich, Roger K.;
Vandekop, Toni L.
1994ApJ...437L..63O Altcode:
We correlate the Homestake solar neutrino capture rate with magnetograph
measured photospheric magnetic flux, using all available data from
1970-1991. We find that the anticorrelation of the capture rate with
the flux is stronger than the (previously studied) anticorrelation with
sunspot number, and that the anticorrelation and its significance
improve markedly when the flux is taken from near the center of
the solar disk. Furthermore, we find that there is no significant
correlation when the near-disk-center flux is excluded. This supports
an hypothesis that there is an interaction between the outgoing
solar neutrinos and the magnetic fields they encounter along their
flight paths. We find the suggestion of a similar pattern with the
Kamiokande neutrino data, although noise level and time span do not
permit conclusive results.
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Title: Real and Virtual Unipolar Regions
Authors: Snodgrass, H. B.; Wilson, P. R.
1993SoPh..148..179S Altcode:
Difficulties in relating magnetograph measurements to the actual
solar magnetic field are discussed. After a brief review both of
problems inherent in the nature of the measurements and of sources
of instrumental error, we show that field measurements taken within
the photosphere can map out large-scale regions of a single magnetic
polarity even though these regions contain no footpoints of large-scale
magnetic structures, but instead only aggregates of small, unresolved
bipoles. This may occur wherever the density of unresolved bipoles has
a preferred orientation and a spatial variation along the direction
of that orientation. We call these regionsvirtual unipolar regions,
as they are not connected to regions of opposite polarity by field
loops or lines passing through the corona. Investigation of these
regions shows that they can arise at widely separated locations, and
that they may evolve into real unipolar magnetic regions which are
connected to the chromospheric and coronal fields. These results can
explain a number of puzzling aspects of magnetograph observations of
the solar background magnetic field.
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Title: A Photometric Investigation of the Eclipsing Binary V505
Sagittarii
Authors: Chambliss, C. R.; Walker, R. L.; Karle, J. H.; Snodgrass,
H. B.; Vracko, Y. A.
1993AJ....106.2058C Altcode:
V505 Sgr is a classical Algol system consisting of an A2 V primary and
a G5 IV secondary that fills its Roche lobe. New times of minimum light
are presented. The period of the eclipsing system (1.18287d) varies,
due in part to an orbital light-time effect. A third component has been
detected that orbits the eclipsing pair. This investigation uses the
SIMPLEX algorithm (Kallrath & Linnell, 1987) and the Differential
Correction code (Wilson, 1979) to analyze two separate datasets. The
results indicate the third component, an F8 V star, contirbutes about 5%
of the light to the system. The minimum projected distance between the
third component and the eclipsing pair is 37 AU. This implies an orbital
period of about 105 years, a value that differs with the O-C data. The
photometric solution, combined with recent spectroscopic data yields
R<SUB>1</SUB> = 2.14 solar radius and R<SUB>2</SUB> = 2.24 solar radius
and M<SUB>1</SUB> = 2.20 solar mass and M<SUB>2</SUB> = 1.15 solar mass.
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Title: Is the Flux of Solar Neutrinos Correlated with the Solar
Magnetic Activity Cycle?
Authors: Vandekop, T.; Snodgrass, H. B.
1993BAAS...25.1194V Altcode:
No abstract at ADS
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Title: Full-Disk Magnetogram Cross-correlations at Long Time Lags
Authors: Snodgrass, H. B.; Metcalf, T.; Vandekop, T.
1993BAAS...25.1194S Altcode:
No abstract at ADS
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Title: Telluric water vapor contamination of the Mount Wilson solar
Doppler measurements
Authors: Carter, Christopher S.; Snodgrass, Herschel B.; Bryja, Claia
1992SoPh..139...13C Altcode:
It has been shown that the solar line λ5250.2 (FeI) is weakly blended
with a telluric line in the water vapor spectrum, and that magnetograms
taken using this line are therefore inaccurate. We investigate the
effects of this contamination on the Mount Wilson synoptic magnetograph
data, which is based on λ5250.2. Using spectrum scans taken at Kitt
Peak, we model the contamination and develop a procedure that would
correct for it, whenever the slant water vapor along the line of
sight to the Sun is known. As this information is not available for
the data collected thus far at Mount Wilson, we use the variation of
determined quantities with airmass to obtain an average, or first-order,
correction. Concentrating on the fitted coefficients for the solar
rotation, the correction is found to be very slight, ∼ 0.5%, raising
the value for the A coefficient, averaged over the period 3 December,
1985 to 22 July, 1990, from 2.8289 to 2.8422 μrad s<SUP>-1</SUP>,
The correction also removes a slight annual variation that has become
discernible in the data collected since 1986.
---------------------------------------------------------
Title: Smokestacks and Balloonmen: A Magnetic Rotation Controversy
Authors: Snodgrass, Herschel B.
1992ASPC...27...71S Altcode: 1992socy.work...71S
No abstract at ADS
---------------------------------------------------------
Title: Synoptic Observations of Large Scale Velocity Patterns on
the Sun
Authors: Snodgrass, Herschel B.
1992ASPC...27..205S Altcode: 1992socy.work..205S
No abstract at ADS
---------------------------------------------------------
Title: A Torsional Oscillation in the Rotation of the Solar Magnetic
Field
Authors: Snodgrass, Herschel B.
1991ApJ...383L..85S Altcode:
The pattern rotation rate for the sun's magnetic field, determined by
cross-correlating Mount Wilson full disk 5250.2 (Fe I) magnetograms
spaced a full solar rotation apart, closely parallels at all latitudes
the photospheric plasma rotation profile determined from the Doppler
shifts of the same spectral line. When an 11-yr running mean is
subtracted, a torsional oscillation is revealed, in the form of
an equatorward-migrating pattern of fast and slow zones. Although
the magnetic rotation torsional pattern is similar enough to its
much-studied Doppler counterpart to provide confirmation, there
are significant differences between the two - the magnetic pattern
is strongest (about 20 m/s) at high latitudes, weakens at sunspot
latitudes where the Doppler pattern is strongest, and is offset at all
latitudes by about 10 deg toward the equator, so that its slow zones
approximately coincide with the maximal shear zones of the Doppler
pattern. These zones appear to be fore-runners to the wings of the
magnetic flux (sunspot) butterflies of the activity cycle.
---------------------------------------------------------
Title: The CUREA 1992 Summer Program in Astrophysics at Mount Wilson
Observatory
Authors: Snider, J.; Bracher, K.; Briggs, J.; Mickelson, M.; Mitchell,
W., Jr.; Pasachoff, J.; Snodgrass, H.; Yorka, S.
1991BAAS...23.1437S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Torsional Pattern in the Rotation of the Solar Magnetic Field
Authors: Snodgrass, H. B.
1990BAAS...22.1233S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Reversal of the Solar Polar Magnetic Fields - Part One
Authors: Wilson, P. R.; McIntosh, P. S.; Snodgrass, H. B.
1990SoPh..127....1W Altcode:
Some theoretical difficulties confronting the current model of the
polar magnetic reversal by cancellation with the flux remnants of
decaying active regions are discussed. It is shown that the flux
transport equation does not adequately describe the essential physical
consequences of the transport of large-scale fields, linked to deep
subsurface toroids, over distances comparable with the solar radius. The
possibility that subsurface reconnections may release these fields
to form U-loops is discussed but it is shown that, in this event,
the loops will quickly rise to the surface. Mechanisms whereby the
flux may escape through the surface are considered.
---------------------------------------------------------
Title: Rotation of Doppler Features in the Solar Photosphere
Authors: Snodgrass, Herschel B.; Ulrich, Roger K.
1990ApJ...351..309S Altcode:
The pattern rotation rate for the line-of-sight velocity features
in the solar photosphere is determined by cross-correlating Doppler
residual coarse arrays from magnetograph observations. From the
latitude dependence and the approximately one-day lifetime of the
correlation amplitudes, it is concluded that the dominant velocity
pattern producing the correlation is the supergranulation network. The
rotation rate average over the entire period is determined. The rate at
all latitudes is about 2 percent faster than the magnetic and sunspot
rates and about 4 percent faster than the Mount Wilson spectroscopic
rate. Comparing this coarse array determination with Duvall's (1988)
earlier result indicates that the supergranulation pattern may be
a very sensitive indicator of large-scale motions at the top of the
solar convection zone.
---------------------------------------------------------
Title: Photometric Observations of Short-Period Eclipsing Binaries
Authors: Vracko, Y. A.; Snodgrass, H. B.; Karle, J. H.
1990BAAS...22..831V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An Extended Activity Cycle Picture of the Sun's Polar
Magnetic Fields
Authors: Snodgrass, H. B.; Wilson, P. R.
1990BAAS...22Q.855S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: CUREA: The Consortium for Undergraduate Research and Education
in Astronomy
Authors: Snider, J.; Bracher, K.; Meyers, K.; Mickelson, M.; Mitchell,
W., Jr.; Naftilan, S.; Pasachoff, J.; Snodgrass, H.; Yorka, S.;
Zook, A.
1989BAAS...21.1065S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: SYSTEMATIC OBSERVATIONS OF THE SUN (In honour of Helen Dodson
Prince): Observations
Authors: McIntosh, P.; Snodgrass, H.; Mouradian, Z.; Harvey, K.;
Altrock, R.; Simon, P.; Legrand, J. -P.; Alissandrakis, G.; Neckel,
H.; Petropoulos, P.; Poulakis, X.; Gokhale, M. H.; Sivaraman, K. R.;
Pap, J.
1989HiA.....8..672M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Rotation Measurements at MT.WILSON - Part Five
Authors: Ulrich, Roger K.; Boyden, John E.; Webster, Larry; Snodgrass,
Herschel B.; Padilla, Steven P.; Gilman, Pamela; Shieber, Tom
1988SoPh..117..291U Altcode:
This paper describes a thorough reevaluation of the procedures for
reducing the data acquired at the Mt. Wilson Observatory synoptic
program of solar observations at the 150-foot tower. We also describe
a new program of acquiring as many scans per day as possible of the
solar magnetic and velocity fields. We give a new fitting formula
which removes the background velocity field from each scan. An
important new feature of our reduction algorithm is our treatment
of the limb shift which permits time variation in this function. We
identify the difference between the limb shift along the north-south
axis and the east-west axis as potentially being a result of meridional
circulation. Our analysis interprets the time variation in the east-west
limb shift as being the result of changes in a vertical component of
the meridional circulation.
---------------------------------------------------------
Title: The extended solar activity cycle
Authors: Wilson, P. R.; Altrocki, R. C.; Harvey, K. L.; Martin, S. F.;
Snodgrass, H. B.
1988Natur.333..748W Altcode:
The solar cycle has been defined in terms of a sequential periodic
variation in sunspot numbers, the period being the interval between
successive minima, currently averaging 11.2 years. But a number of
observations have indicated that the activity cycle may begin at
higher latitudes before the emergence of the first sunspots of the
new cycle. Here we report results from sunspot cycle 21 concerning
the ephemeral active regions, the coronal green-line emission and the
torsional oscillation signal, which confirm the earlier suggestions. In
particular, we report the appearance of a high-latitude population of
ephemeral active regions in the declin-ing phase of sunspot cycle 21,
with orientations that tend to favour those for cycle 22 rather than
21. Taken together, these data indicate that sunspot activity is simply
the main phase of a more extended cycle that begins at high latitudes
before the maximum of a given sunspot cycle and progresses towards
the equator during the next 18-22 yr, merging with the conventional
'butterfly diagram' (the plot of the latitudes of emerging sunspots
against time) as it enters sunspot latitudes. We suggest that this
extended cycle may be understood in the perspective of a model of
giant convective rolls that generate dynamo waves propagating from
pole to equator.
---------------------------------------------------------
Title: Evidence for a solar cycle.
Authors: Snodgrass, H. B.
1988PhT....41a..11S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Photospheric Magnetic and Velocity-Feature Rotation in λ5250.2
Authors: Snodgrass, H. B.
1987BAAS...19R1118S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Telluric Contamination Mount Wilson λ5250. 2 Magnetograph
Observations
Authors: Carter, C. S.; Snodgrass, H. B.
1987BAAS...19.1117C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar torsional oscillations as a signature of giant cells
Authors: Snodgrass, H. B.; Wilson, P. R.
1987Natur.328..696S Altcode:
Although the existence of giant cells<SUP>1</SUP> as the fundamental
mode of solar convection has long been proposed on theoretical grounds,
attempts to detect them observationally have been unsuccessful. During
one search, using Mount Wilson magnetograph data, Howard and
LaBonte<SUP>2,3</SUP> discovered a pattern of latitudinal velocity bands
that move from the poles towards the equator in synchrony with the
sunspot cycle, and they interpreted this pattern as a torsional wave
or 'oscillation' with wavenumber k=2 hemisphere<SUP>-1</SUP>. Here we
suggest that this signal is not in fact an oscillation but represents
a modulation of the mean differential rotation caused by a system of
giant convective rolls which start at the poles at 11-yr intervals and
migrate to the equator in a period of 18-22 yr. Additional evidence
for the presence of these rolls is found in the zero offsets in the
Mount Wilson data<SUP>4</SUP> and in latitude variations of the limb
temperature<SUP>5</SUP>. Thus we argue that the fundamental mode
of giant-cell convection in the sun takes the form of equatorward
migrating azimuthal rolls. This differs from the 'banana cell' mode
suggested by Gilman<SUP>6</SUP>, and from the poleward propagating
rolls reported by Ribes et al.<SUP>7</SUP>.
---------------------------------------------------------
Title: Azimuthal Rolls and the Solar Cycle
Authors: Snodgrass, H. B.; Wilson, P. R.
1987BAAS...19Q.935S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spectroscopic Evidence for a Moving Pattern of Azimuthal
Rolls on the Sun
Authors: Snodgrass, Herschel B.
1987ApJ...316L..91S Altcode:
A migrating pattern of solar surface motions associated with the
torsional oscillation is revealed through the analysis of latitudinal
'zero offset' data from Mount Wilson full-disk Dopplergrams taken
over the past 19 yr. This new pattern, which registers meridional
flow in high latitudes and vertical flow in low latitudes, contains
a line-of-sight velocity signal of about + or - 4 m/s. The vertical,
meridional, and rotational flows, taken together, constitute the pattern
of surface motions expected from a system of giant azimuthal rolls. The
implication is that there is a system of from three to four rolls per
hemisphere, which migrate from near the poles to the equator over
a period of about 18 yr. This observational result provides strong
supporting evidence for recent azimuthal convective-roll models of
the solar cycle.
---------------------------------------------------------
Title: Torsional Oscillations and the Solar Cycle
Authors: Snodgrass, Herschel B.
1987SoPh..110...35S Altcode:
Both the net torsional pattern and its derivative, the shear
oscillation, are studied in relation to the solar activity cycle using
data collected at Mount Wilson from 1967-1986. The shear is seen as
the better quantity for study, since it is both more fully determinable
with these data and has straighter ties to the zones of activity. The
shear zones run from pole to equator, clearly indicating that the cycle
begins at the poles. Total transit, roughly at constant speed, takes
roughly 18 years, and the active zones emerge to span the zones of shear
enhancement after the latter have reached sunspot latitudes. This 18-yr
transit time is seen as the proper duration of the cycle: successive
cycles begin roughly 11 years apart and thus overlap. The polar origin
of the torsional pattern is found to be phenomenologically connected
with variations in the polar field amplitude. It is also noted in both
the magnetic and torsional patterns that, for the past few cycles,
the activity portion begins earlier and thus lasts longer in the
northern hemisphere.
---------------------------------------------------------
Title: Correcting for Atmospheric Water Vapour Interference of the
λ5250 (Fe I) Solar Line
Authors: Bryja, C.; Snodgrass, H. B.
1986BAAS...18..932B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Torsional Oscillation Update and a Search for Longitudinal
Structure
Authors: Shieber, T. R.; Snodgrass, H. B.
1986BAAS...18R1011S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Polar Genesis and Propagation of the Torsional Shear
Oscillation
Authors: Snodgrass, H. B.
1986BAAS...18Q1011S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Torsional Oscillations of the Sun
Authors: Snodgrass, H. B.; Howard, R.
1985Sci...228..945S Altcode:
The sun's differential rotation has a cyclic pattern of change that is
tightly correlated with the sunspot, or magnetic activity, cycle. This
pattern can be described as a torsional oscillation, in which the solar
rotation is periodically sped up or slowed down in certain zones of
latitude while elsewhere the rotation remains essentially steady. The
zones of anomalous rotation move on the sun in wavelike fashion,
keeping pace with and flanking the zones of magnetic activity. It is
uncertain whether this torsional oscillation is a globally coherent
ringing of the sun or whether it is a local pattern caused by and
causing local changes in the magnetic fields. In either case, it may
be an important link in the connection between the rotation and the
cycle that is widely believed to exist but is not yet understood.
---------------------------------------------------------
Title: Solar torsional oscillations - A net pattern with wavenumber
2 as artifact
Authors: Snodgrass, H. B.
1985ApJ...291..339S Altcode:
A net solar torsional oscillation pattern is uncovered through a new
analysis of Mount Wilson Doppler data. This pattern, found from zonal
fits, without subtraction of a global fit, consists of a relative polar
spin-up around solar maximum, alternating with a single traveling
wave that runs from mid latitude to low latitude during the rest of
the cycle. It is suggested that these are separate phenomena, and
thus that the previously inferred pole-to-equator traveling pattern
with wavenumber 2 per hemisphere may be a mathematical artifact. The
new pattern retains aspects of the original pattern's relationship to
magnetic activity, and agrees better with model predictions.
---------------------------------------------------------
Title: Torsional Oscillations of Low Mode
Authors: Snodgrass, H. B.; Howard, R.
1985SoPh...95..221S Altcode:
Standing wave torsional oscillations of wavenumber 1/2 and 1
hemisphere<SUP>−1</SUP> are studied using an improved fit to Mount
Wilson magnetograph data. These oscillations are seen to be in phase
with each other and with the magnetic activity cycle, and seem best
represented as a flexing of the differential rotation curve. Superposing
them gives a differential rotation which at solar minimum is slightly
flattened at the equator but considerably (∼ 5%) steepened at the
poles, and also tends to produce a travelling wave with wavenumber
1 hemisphere<SUP>−1</SUP> that moves from pole to equator as the
cycle progresses.
---------------------------------------------------------
Title: Absolute Torsional Oscillations of the Sun
Authors: Snodgrass, H. B.
1984BAAS...16..978S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Limits on photospheric Doppler signatures for solar giant cells
Authors: Snodgrass, H. B.; Howard, R.
1984ApJ...284..848S Altcode:
Mount Wilson solar-velocity data taken since improvement of the
spectrograph in May 1982 are analyzed to search for photospheric traces
of persistent velocity patterns that are anticipated in recent model
predictions. The method involves time-averaged autocorrelations and
cross correlations of the residuals that remain after least-squares fits
for differential rotation, limb shift, and meridional circulation are
extracted from the daily-magnetogram velocity arrays. It is argued that,
owing to the supergranular motions in the photosphere, the sensitivity
in applying the present method to the new Mount Wilson data is close to
the ultimate sensitivity possible for detection of this phenomenon. The
following limits are currently established through this analysis: (1)
there is no sharply peaked power spectrum with amplitude above about 1
m/s per wavenumber, and (2) there is no broad-band power spectrum for
which the total integrated power is greater than about 10 sq m/sq sec.
---------------------------------------------------------
Title: Separation of large-scale photospheric Doppler patterns
Authors: Snodgrass, H. B.
1984SoPh...94...13S Altcode:
Mount Wilson solar Doppler data spanning January 1967 to March 1984 are
refit with an expanded set of functions representing the line-of-sight
components of rotation, limbshift and meridional flow. The `ears'
are not included, and a constant term, formerly regarded as the
relative instrumental zero, is reclassified as representing an aspect
of the limbshift. The long-standing problem of crosstalk among the
fit-determined coefficients is eliminated by orthonormalization
with respect to the solar disk of the function space representing
each motion class. Examination of the new coefficients shows clear
evidence for their variation over the solar cycle: for the rotation
coefficients, this variation is a low mode torsional oscillation,
and for the limbshift, it appears consistent with the suppression of
small-scale convection by magnetic activity. The meridional flow is
found to be poleward and slightly faster at low latitudes. Also seen
in all coefficients is a dramatic reduction of day-to-day scatter
following recent major modifications to the Mount Wilson 150-ft tower
spectrograph.
---------------------------------------------------------
Title: Recalibration of Mount-Wilson Doppler Measurements
Authors: Snodgrass, H. B.; Howard, R.; Webster, L.
1984SoPh...90..199S Altcode:
A new calibration of the spectrograph at the Mount Wilson 150-foot
Tower Telescope demonstrates that all reported solar Doppler rates
to date measured at λ5250.2 with this instrument are too high by a
factor of 0.55%.
---------------------------------------------------------
Title: Limits on Giant Cell Signatures in the Photosphere
Authors: Snodgrass, H. B.; Howard, R.
1983BAAS...15..953S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic rotation of the solar photosphere
Authors: Snodgrass, H. B.
1983ApJ...270..288S Altcode:
Magnetograms made at Mt. Wilson Observatory from January 1967
to May 1982 are crosscorrelated in 34 latitude strips at 1-4-day
increments to determine the rotation of magnetic features in the
solar photosphere. The data are smoothed by averaging corresponding
correlations and calculating rotation from the displacement of the
averaged-correlation maximum; the usefulness and validity of this
procedure are discussed. No significant time variation or field
dependence is found for the period of the observations, at least to
the accuracy of the calculated means (variance of from about 2 m/sec at
low latitudes to about 10 m/sec near the poles). The rotation function
omega at solar latitude phi is shown to be 2.902 0.464 sin sq phi -
0.328 sin to the 4th phi microrad/sec, in agreement with the Mt. Wilson
Doppler profile near the poles and with the sunspot determination of
Newton and Nunn (1951) at sunspot latitudes, where the Doppler estimate
is about 30 m/sec slower.
---------------------------------------------------------
Title: Large-Scale Doppler Shifts in the Solar Photosphere
Authors: Snodgrass, H. B.; Howard, R.
1983BAAS...15..719S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Rotation of Solar Magnetic Fields
Authors: Snodgrass, H. B.; Bruning, D. H.
1981BAAS...13R.906S Altcode:
No abstract at ADS
