explanation blue bibcodes open ADS page with paths to full text
Author name code: libbrecht-ken
ADS astronomy entries on 2022-09-14
author:"Libbrecht, Ken G." OR author:"Libbrecht, Kenneth G." year:1980-2003
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Title: Origin of irradiance variations from disk photometer data
Authors: Woodard, Martin F.; Libbrecht, Ken G.
2003ESASP.517..117W Altcode: 2003soho...12..117W
No abstract at ADS
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Title: Spatial and temporal variations in the solar brightness
Authors: Woodard, M. F.; Libbrecht, K. G.
2003SoPh..212...51W Altcode:
We have investigated long-term variations of solar brightness as
a function of both time and solar latitude using eight years of
ground-based photometric data in conjunction with space-based irradiance
data. In particular, we have examined whether the combination of sunspot
brightness deficits and facular brightness excesses is sufficient to
explain the solar cycle irradiance variations. After correcting for
the contribution from sunspots, we find that the irradiance data can
be adequately explained by a model in which the remaining brightness
variations are due entirely to facular contributions confined to
the magnetically active latitudes. Thus we find no support for the
hypothesis that there are convectively driven hot bands in the active
latitudes, and our data show brightness variations that are well
described by a facular contrast function.
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Title: A Search for CO Absorption in the Transmission Spectrum of
HD 209458b
Authors: Brown, Timothy M.; Libbrecht, Kenneth G.; Charbonneau, David
2002PASP..114..826B Altcode: 2002astro.ph..5246B
We observed one transit of the extrasolar planet HD 209458b with the
NIRSPEC spectrograph on the Keck II telescope. Using time series of
low-noise observations in the wavelength range 2.0-2.5 μm, we searched
for extra absorption from the first-overtone rotation-vibration band
of CO near 2.3 μm. This was not detected with a detection limit that
fails to test simple models of the planetary atmosphere by a factor
of about 3. Great improvements in the detectability of the CO spectrum
features could be realized by observing a transit that is centered near
stellar meridian passage, and in better weather. Since it appears that
similar observations taken under better circumstances might succeed,
we describe our analysis procedures in detail.
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Title: Evolution of the sun's near-surface asphericities over the
activity cycle
Authors: Goode, P. R.; Didkovsky, L. V.; Libbrecht, K. G.; Woodard,
M. F.
2002AdSpR..29.1889G Altcode:
Solar oscillations provide the most accurate measures of cycle dependent
changes in the sun, and the Solar and Heliospheric Observatory/Michelson
Doppler Imager (MDI) data are the most precise of all. They give us
the opportunity to address the real challenge — connecting the MDI
seismic measures to observed characteristics of the dynamic sun. From
inversions of the evolving MDI data, one expects to determine the nature
of the evolution, through the solar cycle, of the layers just beneath
the sun's surface. Such inversions require one to guess the form of
the causal perturbation — usually beginning with asking whether
it is thermal or magnetic. Matters here are complicated because the
inversion kernels for these two are quite similar, which means that we
don't have much chance of disentangling them by inversion. However,
since the perturbation lies very close to the solar surface, one can
use synoptic data as an outer boundary condition to fix the choice. It
turns out that magnetic and thermal synoptic signals are also quite
similar. Thus, the most precise measure of the surface is required. We
argue that the most precise synoptic data come from the Big Bear
Solar Observatory (BBSO) Solar Disk Photometer (SDP). A preliminary
analysis of these data implies a magnetic origin of the cycle-dependent
sub-surface perturbation. However, we still need to do a more careful
removal of the facular signal to determine the true thermal signal.
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Title: LIGO: Hearing the Gravitational-Wave Universe
Authors: Libbrecht, Kenneth G.
2001Mercu..30e..24L Altcode:
A team of physicists is opening a new window to the universe, the
realm of ripples in space-time.
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Title: Seeking the Atmospheric Transmission Spectrum of HD209458b
Authors: Brown, T. M.; Butler, R. P.; Charbonneau, D.; Noyes, R. W.;
Sasselov, D.; Libbrecht, K. G.; Marcy, G. W.; Seager, S.; Vogt, S. S.
2000AAS...197.1105B Altcode: 2000BAAS...32Q1417B
Transiting extrasolar giant planets such as HD209458b should impress
a spectroscopic signature on the light that is transmitted through
the outer parts of their atmospheres. Theory suggests that the depths
of absorption features resulting from this effect may be as large as
about 10<SUP>-3</SUP> of the parent star's continuum intensity. Such
spectral features could provide important diagnostics concerning the
composition and physical state of the planetary atmosphere. Accordingly,
we have obtained low-noise spectra of HD209458 during two transits
of its planet, once in visible light using the HIRES spectrograph at
the Keck I telescope, and once in the near infrared using the NIRSPEC
spectrograph at Keck II. We describe the methods employed and the
results of searches for spectral signatures of neutral atomic sodium,
carbon monoxide, and other atomic and molecular species.
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Title: Photometric measurements of the solar disk at BBSO.
Authors: Woodard, M. F.; Libbrecht, K. G.
2000BAAS...32..803W Altcode:
No abstract at ADS
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Title: Photometric Measurements of the Solar Disk at BBSO
Authors: Woodard, M. F.; Libbrecht, K. G.
2000SPD....31.0112W Altcode:
Precise photometric measurements of the Sun have been obtained with the
BBSO Solar Disk Photometer (SDP) since 1993. We describe our program
to characterize sunspot, facular, and non-facular contributions to
the spatial and temporal variations in solar irradiance using these
data. This work is supported by NSF and NASA.
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Title: LIGO: Present Status and Future Improvements.
Authors: Libbrecht, Ken G.
1998APS..APR.R1003L Altcode:
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a
joint project being undertaken by Caltech and MIT, aimed at detecting
gravitational waves emitted from astrophysical sources. LIGO is
presently under construction, and will include three power-recycled
Michelson-type interferometers using Fabry-Perot arm cavities: two
interferometers located at Hanford, Washington, with 2-km and 4-km
arm lengths, and a single 4-km interferometer at Livingston Parish,
Louisiana. By spring of 1998 construction will be about three-fourths
complete, with detector installation beginning at the Hanford site. The
initial interferometers should achieve the designed strain sensitivity
of Δ L/L<10<SUP>-21</SUP>/√Hz over a bandwidth of ~ 40-400 Hz in
2001, at which point the first extended observing run will begin. This
talk will review the LIGO detector and construction progress, new
interferometer configurations and other ideas aimed at improving
the instrument sensitivity, and the latest results on the prospects
for observing the gravitational-wave signatures from gamma-ray burst
sources.
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Title: Spatial Dependence of Solar-Cycle Changes in the Sun's
Luminosity
Authors: Taylor, S. F.; Varsik, J. R.; Woodard, M. F.; Libbrecht, K. G.
1998SoPh..178....1T Altcode:
We report observations of the large-scale spatial dependence of the
Sun's luminosity variations over the period 1993-1995. The measurements
were made using a new scanning disk solar photometer at Big Bear Solar
Observatory, specially designed to measure large-scale brightness
variations at the 10<SUP>−4</SUP> level. Since the level of solar
activity was very low for the entire observation period, the data
show little solar cycle variation. However, the residual brightness
signal ΔI/I (after subtracting the mean, first, and second harmonics)
does show a strong dependence on heliocentric angle, peaking near the
limb. This is as one would expect if the residual brightness signal
(including the excess brightness coming from the active latitudes)
were primarily facular in origin. Additional data over the next few
years, covering the period from solar minimum to maximum, should
unambiguously reveal the large-scale spatial structure of the solar
cycle luminosity variations.
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Title: The Solar Acoustic Spectrum and Eigenmode Parameters
Authors: Hill, F.; Stark, P. B.; Stebbins, R. T.; Anderson, E. R.;
Antia, H. M.; Brown, T. M.; Duvall, T. L., Jr.; Haber, D. A.;
Harvey, J. W.; Hathaway, D. H.; Howe, R.; Hubbard, R. P.; Jones,
H. P.; Kennedy, J. R.; Korzennik, S. G.; Kosovichev, A. G.; Leibacher,
J. W.; Libbrecht, K. G.; Pintar, J. A.; Rhodes, E. J., Jr.; Schou, J.;
Thompson, M. J.; Tomczyk, S.; Toner, C. G.; Toussaint, R.; Williams,
W. E.
1996Sci...272.1292H Altcode:
The Global Oscillation Network Group (GONG) project estimates
the frequencies, amplitudes, and linewidths of more than 250,000
acoustic resonances of the sun from data sets lasting 36 days. The
frequency resolution of a single data set is 0.321 microhertz. For
frequencies averaged over the azimuthal order m, the median formal
error is 0.044 microhertz, and the associated median fractional error
is 1.6 x 10<SUP>-5</SUP>. For a 3-year data set, the fractional error
is expected to be 3 x 10<SUP>-6</SUP>. The GONG m-averaged frequency
measurements differ from other helioseismic data sets by 0.03 to 0.08
microhertz. The differences arise from a combination of systematic
errors, random errors, and possible changes in solar structure.
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Title: A Fiber-Fed Echelle Spectrograph for the Hale 5-m Telescope
Authors: Libbrecht, K. G.; Peri, M. L.
1995PASP..107...62L Altcode:
We describe a new fiber-fed echelle spectrograph which is now in
operation on the Hale 5-meter telescope at Palomar Observatory. The
instrument is optimized for high spectral stability, necessary for
asteroseismology measurements of small periodic variations in stellar
radial velocities. It features a 7-element all-spherical 610-mm
focal length f/3.0 lens system and prism cross-disperser in a compact
double-pass quasi-Littrow configuration. Light enters the system at
prime focus, and is channeled to the spectrograph by a fiber optic
cable. The instrument rests on a fixed-orientation optical bench inside
the telescope's East Arm. It can be operated in a low-resolution mode,
with resolution R=20,000 and overal efficiency (including atmospheric
seeing) e~5 percent at 550 nm, or a high-resolution mode, with R=40,000
and e~1.5 percent at 550 nm; both modes have R_pixel=100,000. With
large-format CCD detector (2048X2048 with 27-micron pixels), the entire
visible spectrum from 400-1000 nm can be recorded in two exposures, with
no gaps. We have also incorporated a novel imaging system to produce
a circularly symmetric guide image of the input fiber tip. Using a
fiber-optic double scrambler, the instrument produces radial-velocity
measurements which are stable at the ~1 m/sec level over short (<~
30 minute) time periods. (SECTION: Astronomical Instrumentation )
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Title: The Global Oscillation Network Group Site Survey - Part Two
Authors: Hill, Frank; Fischer, George; Forgach, Suzanne; Grier,
Jennifer; Leibacher, John W.; Jones, Harrison P.; Jones, Patricia
B.; Kupke, Renate; Stebbins, Robin T.; Clay, Donald W.; Ingram,
Robert E. L.; Libbrecht, Kenneth G.; Zirin, Harold; Ulrichi, Roger
K.; Websteri, Lawrence; Hieda, Lester S.; Labonte, Barry J.; Lu,
Wayne M. T.; Sousa, Edwin M.; Garcia, Charles J.; Yasukawa, Eric
A.; Kennewell, John A.; Cole, David G.; Zhen, Huang; Su-Min, Xiao;
Bhatnagar, Arvind; Ambastha, Aashok; Al-Khashlan, Abdulrahman Sa'ad;
Abdul-Samad, Muhammad-Saleh; Benkhaldoun, Zouhair; Kadiri, Samir;
Sánchez, Francisco; Pallé, Pere L.; Duhalde, Oscar; Solis, Hernan;
Saá, Oscar; González, Ricardo
1994SoPh..152..351H Altcode:
The Global Oscillation Network Group (GONG) Project will place a
network of instruments around the world to observe solar oscillations as
continuously as possible for three years. The Project has now chosen the
six network sites based on analysis of survey data from fifteen sites
around the world. The chosen sites are: Big Bear Solar Observatory,
California; Mauna Loa Solar Observatory, Hawaii; Learmonth Solar
Observatory, Australia; Udaipur Solar Observatory, India; Observatorio
del Teide, Tenerife; and Cerro Tololo Interamerican Observatory, Chile.
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Title: Advances in Helioseismology
Authors: Libbrecht, K. G.; Woodard, M. F.
1994snft.book..428L Altcode:
No abstract at ADS
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Title: A Search for Acoustic Oscillations on eta CAS with the Palomar
200” Echelle Spectrograph
Authors: Peri, M.; Libbrecht, K.
1993AAS...18311108P Altcode: 1993BAAS...25.1458P
We have begun a program to observe acoustic oscillations in solar-type
stars. Timeseries of radial velocity variations in the stellar
absorption lines are acquired using the high-resolution echelle
spectrograph on the Palomar 200” telescope. These measurements are
Fourier transformed to search for periodicities which are the key
signature of p-mode oscillations. Initial tests indicate that our
technique is likely to provide sufficient sensitivity to measure the
sub-m/s Doppler amplitudes predicted for the oscillations. Our first
target star, eta Cas (G0 V, m_v=3.44), was observed during 6 nights
in Aug-Sept 1993. We present results of those observations.
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Title: Observations of Time Variation in the Sun's Rotation
Authors: Woodard, M. F.; Libbrecht, K. G.
1993Sci...260.1778W Altcode:
Observations of solar p-mode frequency splittings obtained at Big
Bear Solar Observatory in 1986 and during 1988-90 reveal small (~1
percent) changes in the sun's subsurface angular velocity with solar
cycle. An asymptotic inversion of the splitting data yields the latitude
dependence of the rotation rate and shows that the largest changes in
the angular velocity, ≈4 nanohertz, occurred between 1986 and the
later years, at high (≈60^circ) solar latitudes. Earlier helioseismic
observations suggest that solar cycle changes in the ratio of magnetic
to turbulent pressure in the solar convection zone are large enough to
account for the magnitude of the observed angular velocity variations
but a detailed model of the phenomenon does not exist.
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Title: Preliminary Assessment of Stellar RV Variation Measurements
with the Palomar East Arm Echelle Spectrograph
Authors: Peri, M.; Libbrecht, K.
1993AAS...182.5109P Altcode: 1993BAAS...25..887P
One of the most promising techniques for detecting acoustic oscillations
in solar-type stars is the measurement of minute radial velocity
shifts produced by the oscillations. Photon noise limitations can
be alleviated by using an echelle spectrograph to observe Doppler
shifts in a large number of spectral lines simultaneously. The
new East Arm Echelle Spectrograph on the the Palomar 200-inch Hale
telescope is an excellent instrument for measuring radial velocity
variations, providing high resolution over a broad wavelength
range. The instrument is fiber-fed with a double-scrambler option
to enhance stability, and incorporates a molecular absorption cell
for instantaneous calibration. In this poster we present preliminary
stellar observations, assess instrumental performance, and discuss
the potential for asteroseismology measurements.
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Title: Seismic evidence of modulation of the structure and angular
velocity of the Sun associated with the solar cycle
Authors: Gough, D. O.; Kosovichev, A. G.; Sekii, T.; Libbrecht, K. G.;
Woodard, M. F.
1993ASPC...40...93G Altcode: 1993IAUCo.137...93G; 1993ist..proc...93G
No abstract at ADS
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Title: The Form of the Angular Velocity in the Solar Convection Zone
Authors: Gough, D. O.; Kosovichev, A. G.; Sekii, T.; Libbrecht, K. G.;
Woodard, M. F.
1993ASPC...42..213G Altcode: 1993gong.conf..213G
No abstract at ADS
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Title: A New Echelle Spectrograph for Asteroseismology
Authors: Peri, M. L.; Libbrecht, K. G.
1993ASPC...42..489P Altcode: 1993gong.conf..489P
No abstract at ADS
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Title: Solar Activity and Oscillation Frequency Splittings
Authors: Woodard, M. F.; Libbrecht, K. G.
1993ApJ...402L..77W Altcode:
Solar p-mode frequency splittings, parameterized by the coefficients
through order N = 12 of a Legendre polynomial expansion of the mode
frequencies as a function of m/L, were obtained from an analysis
of helioseismology data taken at Big Bear Solar Observatory during
the 4 years 1986 and 1988-1990 (approximately solar minimum to
maximum). Inversion of the even-index splitting coefficients confirms
that there is a significant contribution to the frequency splittings
originating near the solar poles. The strength of the polar contribution
is anti correlated with the overall level or solar activity in the
active latitudes, suggesting a relation to polar faculae. From an
analysis of the odd-index splitting coefficients we infer an upper limit
to changes in the solar equatorial near-surface rotatinal velocity of
less than 1.9 m/s (3 sigma limit) between solar minimum and maximum.
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Title: A New Echelle Spectrograph for Astroseismology
Authors: Peri, M.; Libbrecht, K.
1992AAS...18110116P Altcode: 1992BAAS...24.1283P
One of the most promising techniques for detecting acoustic oscillations
in solar-like stars is the measurement of minute radial velocity shifts
produced by the oscillations. Photon noise limitations can be alleviated
by observing a large number of spectral lines simultaneously. To this
end we have recently completed construction of a new high-resolution
echelle spectrograph for the Palomar 200” Hale telescope. The
instrument has a 2-pixel resolving power of R=45000, and currently
uses a 1024*E24 CCD to record 19 orders per frame. In this poster
we describe the spectrograph design and construction details, its
performance, and its potential for asteroseismology measurements.
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Title: On the Ultimate Accuracy of Solar Oscillation Frequency
Measurements
Authors: Libbrecht, K. G.
1992ApJ...387..712L Altcode:
If one assumes that solar p- and f-mode oscillations are stochastically
excited, then the measured mode properties and solar background
noise can be used to calculate maximum-likelihood uncertainties in
mode frequency measurements for a given observation time. It should
be possible with a 3-yr continuous observation to measure individual
mode frequencies to accuracies as high as a factor of about 5 times
better than current best one-season measurements. The most precise
measurements should be for low-l modes in the 1-2 mHz range, and the
longest periods observable in a 3-yr observation will be approximately
20 minutes. These fundamental limitations in the eventual accuracy of
p-mode frequency measurements are set by the solar background noise
and the stochastic nature of the driving mechanism.
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Title: Nonfacular Solar Luminosity Variations
Authors: Kuhn, J. R.; Libbrecht, K. G.
1991ApJ...381L..35K Altcode:
Solar limb photometry data obtained in 1990 shows a decline in the
active region facular contribution to the solar irradiance from
the previous year - in accord with the maximum in the sunspot cycle
occurring in late 1989. In contrast, the nonfacular and nonsunspot
contribution to the solar flux increased compared to 1989. This third
component of the irradiance variation is consistent with a significant
increase in the total mean solar irradiance from the value observed
near the time of sunspot maximum. Evidently, the irradiance maximum,
like other solar activity indices, exhibits a phase lag with respect
to the solar activity maximum as measured by the sunspot number.
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Title: Advances in Helioseismology
Authors: Libbrecht, K. G.; Woodard, M. F.
1991Sci...253..152L Altcode:
Globally coherent oscillation modes were discovered in the sun about
a decade ago, providing a unique seismological probe of the solar
interior. Current observations detect modes that are phase-coherent for
up to 1 year, with surface velocity amplitudes as low as 2 millimeters
per second, and thousands of mode frequencies have been measured
to accuracies as-high as 1 part in 10^5. This article discusses the
properties of these oscillation modes and the ways in which they are
adding to our understanding of the structure and dynamics of the sun.
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Title: Is There an Acoustic Resonance in the Solar Chromosphere?
Authors: Woodard, M. F.; Libbrecht, K. G.
1991ApJ...374L..61W Altcode:
By comparing helioseismology data from 1986, 1988, and 1989, it was
found that the frequecy dependence of the frequency perturbation
of solar p-modes caused by solar activity drops abruptly for modes
of frequency above approximately 3.9 mHz. The drop in the frequency
dependence of the frequency shift may result from solar cycle changes
in the chromosphere, provided that the chromosphere acts as a cavity
in which p-modes are trapped (Goldreich et al.). No evidence is found
in the temporal power spectrum of a time series of narrow-band Ca II
K-line filtergrams of a resonance which would reveal the existence
of a chromospheric cavity. This circumstance constrains the possible
physical explanations of the frequency shifts.
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Title: Short-Term Changes in Solar Oscillation Frequencies and
Solar Activity
Authors: Woodard, M. F.; Kuhn, J. R.; Murray, N.; Libbrecht, K. G.
1991ApJ...373L..81W Altcode:
It is shown that the frequencies of solar rho-mode oscillations
change significantly over periods as short as one month. These changes
correlate significantly with variations in the strength of surface solar
activity as measured by the average, over the sun's visible surface,
of the magnitude of the line-of-sight magnetic field component from
magnetograms. The frequency and mean magnetic variations are found to
obey a linear relationship. It is seen that the mean frequency shift at
any time depends on the history of solar activity over an interval of,
at most, several months prior to the measurement and conclude that the
dominant mechanism of the frequency shift is correlated with surface
magnetic activity.
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Title: A Search for Acoustic Amplitude Deficit at the Antipodes
of Sunspots
Authors: Peri, Michal L.; Libbrecht, Ken G.
1991SoPh..132..223P Altcode:
We present a search for the acoustic oscillation deficit which may
exist at the antipodes of sunspots. Dopplergrams from Big Bear Solar
Observatory 1988 helioseismology data were selected for five days
on which large sunspots were known to be on the unseen hemisphere of
the Sun. Acoustic oscillation amplitudes in the antipodal regions of
these sunspots were compared with amplitudes in surrounding quiet-Sun
regions. We did not detect a statistically significant acoustic
amplitude deficit in our data. Our results indicate that the amplitude
deficit at the sunspot antipodal points is limited to no more than 3%
of the acoustic amplitude in the region, for solar oscillation modes
of spherical harmonic degree l ≲ 200. We conclude that no strong
acoustic deficit exists at the antipodes of sunspots. A more sensitive
search, requiring more elaborate observations than we have performed,
would be desirable in order to determine if a weak acoustic amplitude
deficit exists at some level at the antipodes of sunspots, perhaps
at higher spatial frequencies of oscillation. The noise level in
any signals detected by such observations would probably limit their
usefulness as seismic probes. However, information on the lifetimes of
solar oscillation modes can be deduced even if no acoustic amplitude
deficit is detected.
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Title: The solar rotation.
Authors: Libbrecht, K. G.; Morrow, C. A.
1991sia..book..479L Altcode:
The authors review the rotation of the Sun, with emphasis on the
observations. While it has been known for centuries that at the solar
surface the equator rotates with a higher angular velocity than the
poles, different surface measurements suggest that the near-surface
rotation is not as simple as one might expect. Recently we have been
able to infer the interior rotation rate from helioseismology. These
new observations indicate that surface-like rotation extends through
the convection zone, changing to solid-body rotation in the upper part
of the radiative interior; observations are inconclusive regarding
the rotation of the deep core of the Sun. The dynamics that produces
the observed solar rotation profile remains a mystery.
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Title: Frequencies of Solar Oscillations
Authors: Libbrecht, K. G.; Woodard, M. F.; Kaufman, J. M.
1990ApJS...74.1129L Altcode:
Solar oscillations have been observed at three different spatial scales
at Big Bear Solar Observatory during 1986-1987 and, using three data
sets, a new and more accurate table of solar oscillation frequencies
has been compiled. The oscillations, which are presented as functions
of radial order n and spherical harmonic degree l, are averages over
azimuthal order and therefore approximate the normal mode frequencies
of a nonrotating, spherically symmetric sun, near solar minimum. The
table contains frequencies for most of the solar p and f modes with l
between 0 and 1860, n between 0 and 26, and oscillation mode frequencies
between 1.0 and 5.3.
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Title: Comparison of Solar p-Mode Oscillations in Surface Brightness
and Velocity
Authors: Libbrecht, K. G.
1990ApJ...359..232L Altcode:
Solar p-modes with degrees l = 30-60 were observed from Big Bear
Observatory as surface velocity and surface brightness oscillations,
the former at a depth of tau (5000) roughly 0.05, and the latter
using a 10 A bandpass filter centered at 6687 A. Comparison of the two
observations shows that the brightness/velocity oscillation power ratio
for these low-l modes depends only weakly on frequency between 2 and 4
mHz. This result contradicts a similar measurement recently published
by Jimenez et al. (1988), who find nearly an order of magnitude of
variation in the p-mode power ratio over the same frequency range
for modes with l = 1 or less. Since it is unlikely that the different
l-values measured would yield a different power ratio, the cause of
the discrepancy remains unknown.
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Title: Solar-cycle effects on solar oscillation frequencies
Authors: Libbrecht, K. G.; Woodard, M. F.
1990Natur.345..779L Altcode:
Measurements of solar oscillations taken in 1986 and 1988 show
systematic changes in the Sun's acoustic-mode frequencies of the order
of 1 part in 10,000. These data reveal that the frequency shifts are
the result of latitude-dependent changes in the structure of the Sun
which are correlated with the Sun's magnetic-activity cycle.
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Title: Solar Cycle Effects on Solar Oscillation Frequencies
Authors: Libbrecht, K. G.
1990BAAS...22..890L Altcode:
No abstract at ADS
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Title: A Search for Acoustic Amplitude Deficit at the Antipodes
of Sunspots
Authors: Peri, M. L.; Libbrecht, K. G.
1990BAAS...22..856P Altcode:
No abstract at ADS
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Title: Observations of Solar Cycle Variations in Solar p_Mode
Frequencies and Splittings
Authors: Libbrecht, K. G.; Woodard, M. F.
1990LNP...367..145L Altcode: 1990psss.conf..145L
We discuss here two sets of helioseismology data acquired at Big Bear
Solar Observatory during the summers of 1986 and 1988. Each data
set consists of roughly 60,000 fulldisk Doppler images of the sun,
accumulated over a four-month time span. These data clearly show
that solar p-mode frequencies change with time, and that the measured
frequency shifts v = v 88 - v 86 depend strongly on frequency and only
weakly on for 5 60. The frequency dependence is well described by v ∞
M-1(v), where M(v) is the mode mass for low-ℓ modes. Such a frequency
dependence is expected if the effective sound speed perturbation is
located predominantly near the solar surface. It should be possible to
invert the frequency shift measurements to determine some aspects of the
structure of solar activity as a function of depth. The data also show
that the even-index splitting coefficients depend strongly on frequency,
again being well described by α2j (v) ∞ M-1(v). This functional
form is expected if the sound speed perturbation responsible for Δv is
localized in solar latitude. Latitude inversions of the time-dependent
splitting and Δv measurements show that the perturbation is strongest
in the active latitudes, but includes a weak polar component.
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Title: Seismology of Solar Oscillation Line Widths
Authors: Christensen-Dalsgaard, J.; Gough, D. O.; Libbrecht, K. G.
1989ApJ...341L.103C Altcode:
Recent measurements of line widths of solar p-modes are compared with
damping rates computed under several different assumptions. There
is reasonable agreement with a calculation taking some account of
perturbations in the convective fluxes induced by the oscillations,
whereas calculations neglecting these flux perturbations are further
from the observations. This opens up the prospect of using observations
of solar oscillations to test theories of time-dependent convection. The
results should be of importance to studies of other types of pulsating
stars.
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Title: Inside the Spinning Sun
Authors: Libbrecht, K.
1989S&T....77..584L Altcode:
No abstract at ADS
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Title: The Radial Gradient in the Sun's Rotation
Authors: Dziembowski, W. A.; Goode, Philip R.; Libbrecht, K. G.
1989ApJ...337L..53D Altcode:
The solar oscillation data of Libbrecht (1989) are inverted, and it
is found that there is a sharp radial gradient in the sun's rotation
at the base of the convection zone. The existence of a sharp radial
gradient there may be used to suggest that it is the site of the dynamo
which drives the sunspot cycle.
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Title: Solar p-Mode Frequency Splittings
Authors: Libbrecht, K. G.
1989ApJ...336.1092L Altcode:
Measurements of solar p-mode frequency splittings based on 100 days
of solar Doppler observations are presented. The measurements have a
high S/N ratio and show the dependence of splitting on radial order
as well as spherical harmonic degree. The data confirm that the
solar rotation rate is not constant on cylinders but is more closely
represented as having only slight variation with radius throught the
convection zone. Also, it is shown that the frequency splitting vary
with solar cycle.
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Title: The excitation and damping of solar oscillations. (Invited
review).
Authors: Libbrecht, Ken G.
1988ESASP.286....3L Altcode: 1988ssls.rept....3L
The author reviews the present status of our understanding of the
excitation and damping of solar p-mode oscillations. While a few simple
statements can be made about the expected properties of p-modes,
there is currently no complete theory which explains the observed
mode amplitudes and linewidths. He discusses the growing evidence that
the p-modes cannot be overstable and therefore self-excited, but are
instead probably stochastically excited by turbulent convection. The
author discusses the present status of the latter model for exciting
p-modes, and some possible future directions.
---------------------------------------------------------
Title: The GONG site survey.
Authors: Hill, F.; Ambastha, A.; Ball, W.; Duhalde, O.; Farris,
D.; Fischer, G.; Hieda, L.; Zhen, Huang; Ingram, B.; Jackson, P.;
Jones, H.; Jones, W.; Kennewell, J.; Kunkel, W.; Kupke, R.; Labonte,
B.; Leibacher, J.; Libbrecht, K.; Lu, W.; Morrison, L.; Odell, C.;
Pallé, P.; Saá, O.; Sousa, E.; Stebbins, T.; Xiao, Suming; GONG
Site Survey Team
1988ESASP.286..209H Altcode:
The Global Oscillation Network Group (GONG) project is planning to
place six observing stations around the world to observe the solar
oscillations as continuously as possible. This paper describes the
procedures that are being used to select the six sites. The latest
results of measurements of cloud cover obtained by networks of 6
(out of 10) radiometers show a duty cycle of over 93%, with the first
diurnal sidelobe in the window power spectrum suppressed by a factor
of 400. The results are in good agreement with the predictions of a
computer model of the expected cloud cover at individual sites.
---------------------------------------------------------
Title: On the measurement of solar rotation using high-degree p-mode
oscillations.
Authors: Woodard, Martin F.; Libbrecht, Ken G.
1988ESASP.286...67W Altcode: 1988ssls.rept...67W
The authors describe the progress made and some of the difficulties
encountered in measuring the solar rotation rate with p-modes
of degree 100 ⪉ l ⪉ 400, using a set of high-resolution solar
images taken at Big Bear Solar Observatory. The main conclusion drawn
from an analysis of one day of data is that the equatorial angular
velocity is essentially equal to the observed surface rate over the
radius range 0.87 ⪉ r/R ⪉ 0.99 to within a few percent. Because
of likely systematic errors at the 1% level these data do not allow
us to distinguish between a surrface rotation rate equal to that
measured using magnetic tracers and that based on the Doppler shift
of photospheric spectral lines.
---------------------------------------------------------
Title: Solar p-mode frequency splittings.
Authors: Libbrecht, Ken G.
1988ESASP.286..131L Altcode: 1988ssls.rept..131L
The author discusses here new measurements of solar p-mode frequency
splittings, based on 100 days of solar Doppler observations from Big
Bear Solar Observatory. The splittings were determined accurately for
individual mode multiplets for the firs time, and show a dependence
of splitting on radial order n as well as degree l. Two inversions,
which infer the solar rotation rate as a function of depth and latitude
from the measurements, are also discussed, and present the following
picture of the sun's internal rotation: (1) The rotation rate in the
convection zone is roughly independent of depth, showing a latitudinal
differential rotation equal to that seen at the solar surface; (2) There
is a relatively sharp transition zone at the base of the convection
zone, where the differential rotation in the convection zone gives
way to approximately solid body rotation in the radiative interior.
---------------------------------------------------------
Title: The Surface Temperature of the Sun and Changes in the Solar
Constant
Authors: Kuhn, J. R.; Libbrecht, K. G.; Dicke, R. H.
1988Sci...242..908K Altcode:
The solar distortion telescope has been used to measure the limb shape
and latitude dependence of the limb brightness during the summer months
for a 5-year period--a time base sufficient to detect solar cycle
trends in the data. Comparison of these observations with spaceborne
measurements of the solar constant suggests that a significant part
of the solar cycle variation in irradiance is a result of temporal
changes in the latitude-dependent surface temperature of the sun.
---------------------------------------------------------
Title: Solar p-Mode Phenomenology
Authors: Libbrecht, K. G.
1988ApJ...334..510L Altcode:
New observations of solar p-mode amplitudes and line widths are
presented, along with a phenomenological discussion of the excitation
and damping of the modes. A surprising amount of structure is seen
in the measured mode line widths as a function of frequency, which at
present is unexplained. It is shown that the energy flux required to
keep the p-modes excited to their observed amplitudes is of the same
order of magnitude as the nonthermal energy flux required to heat the
chromosphere. Thus it is not inconceivable that acoustic modes play
some role in heating the solar atmosphere.
---------------------------------------------------------
Title: Solar and stellar seismology
Authors: Libbrecht, Ken G.
1988SSRv...47..275L Altcode:
This article reviews solar and stellar seismology, with emphasis on
the enormous progress which has been made recently in the observation
and understanding of solar p-modes. Precision measurements of p-mode
frequencies and frequency splittings allow a greater understanding
of the structure of the solar interior, while p-mode amplitudes and
linewidths shed light on the mode excitation mechanism, which is
probably stochastic excitation by turbulent convection. The prospects
for making similar measurements on other stars are also discussed.
---------------------------------------------------------
Title: Radial Velocity Observations Reveal Multimode Oscillations
in Gamma Equulei
Authors: Libbrecht, K. G.
1988ApJ...330L..51L Altcode:
Radial velocity oscillations were observed in the rapidly oscillating
Ap star Gamma Equulei. Two peaks in the power spectrum are clearly
significant, at v = 1366 and 1427 micro-Hz, in contrast to the single
peak at 1339 micro-Hz from an earlier observation reported by Kurtz
(1983). These observations suggest that several modes of Gamma Equ
with different (l, n) are excited at various times, with a fundamental
spacing of roughly 58 micro-Hz. The ratio of velocity to brightness
oscillations is inferred to be of order 30 km/s/mag or greater, similar
to the approximately 80 km/s/mag observed for the much lower amplitude
solar p-mode oscillations.
---------------------------------------------------------
Title: Frequencies of Solar p-Mode Oscillations
Authors: Duvall, T. L., Jr.; Harvey, J. W.; Libbrecht, K. G.; Popp,
B. D.; Pomerantz, M. A.
1988ApJ...324.1158D Altcode:
Acoustic oscillations of the Sun were observed by measuring Doppler
shifts at Big Bear Solar Observatory in 1985 and by measuring intensity
fluctuations at the geographic South Pole in 1981. These data are
reduced to spectra in frequency and spherical harmonic degree, l,
by averaging over azimuthal order after removing frequency shifts
caused by rotation. Distinct spectral features are identified and
fitted with models to produce estimates of multiplet frequencies and
errors. The authors present a table of measured frequencies for 4 ≤
l ≤ 99, with measurement uncertainties of the order of one part in
10<SUP>4</SUP>. Tables of published frequency measurements for l ≤
5 are also included.
---------------------------------------------------------
Title: A Search for Radial Velocity Oscillations in Procyon
Authors: Libbrecht, K. G.
1988IAUS..132...83L Altcode:
The author describes a new technique for precision radial velocity
measurements, using an iodine absorption cell in front of the slit
of a high resolution spectrograph. A CCD in a continuous readout mode
records a timeseries of stellar spectra with a duty cycle of unity. In
January 1987, observations were obtained on two clear nights with bad
seeing. No oscillations were detected in the data, but the technique
shows promise for significantly better results under better conditions.
---------------------------------------------------------
Title: Frequencies of High-Degree Solar Oscillations
Authors: Libbrecht, K. G.; Kaufman, J. M.
1988ApJ...324.1172L Altcode:
Acoustic oscillations of the sun were observed at Big Bear Solar
Observatory by measuring Doppler shifts at three different spatial
resolutions, in order to measure the frequencies of solar p-modes
at low, intermediate, and high values of spherical harmonic degree
l. Distinct, sharply peaked spectral features were not identified
in these data, and instead the positions of broad ridges in the l-v
power diagram were measured, from which p- and f-mode frequencies
were derived. A table of oscillation frequencies with l in the range
of 30-132O is presented. Systematic errors in determining p-mode
frequencies from ridge positions were examined for l less than 100
by comparing the measured ridge positions with frequencies obtained
from the analysis of sharply peaked spectral features. This led to
the identification of two types of systematic errors, which were
subsequently removed from the data. Residual systematic errors in the
measured frequencies are estimated for the three data sets.
---------------------------------------------------------
Title: What do the Observations Tell us about the Excitation of
Solar Oscillation Modes
Authors: Libbrecht, K. G.
1988IAUS..123..359L Altcode:
Present theories suggest two different classes of excitation mechanisms
which may be responsible for the observed amplitudes of solar p-mode
oscillations - self-excitation of the modes (e.g. the κ mechanism),
and stochastic excitation by turbulent convection. The author discusses
here the agreement and disagreement between the predictions of these
two mechanisms and the observed mode amplitudes and linewidths.
---------------------------------------------------------
Title: Observations of a Solar Latitude-dependent Limb Brightness
Variation: Erratum
Authors: Kuhn, J. R.; Libbrecht, K. G.; Dicke, R. H.
1987ApJ...319.1010K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Is the Solar Oblateness Variable? Measurements of 1985
Authors: Dicke, R. H.; Kuhn, J. R.; Libbrecht, K. G.
1987ApJ...318..451D Altcode:
The solar oblateness measured in 1985 is Δr =
r<SUB>eq</SUB>-r<SUB>p</SUB> = 14.6±2.2 arc ms, where the error is
only a formal standard deviation assuming normally distributed and
uncorrelated errors. The above result is significantly greater than
the 1984 value which, in turn, is significantly less than the 1983
and 1966 values. The differences may be physically significant and are
consistent with the hypothesis that the oblateness oscillates with the
11.14 yr period of the solar cycle. The data at present only weakly
support this hypothesis.
---------------------------------------------------------
Title: Evidence of global circulation currents from solar-limb
temperature variations
Authors: Kuhn, J. R.; Libbrecht, K. G.; Dicke, R. H.
1987Natur.328..326K Altcode:
The temperature distribution in a turbulent rotating photosphere is
non-spherical. Dimensional arguments for the Sun suggest that such a
temperature modulation may have an amplitude ΔT~ Tv<SUP>2</SUP>/Φ~0.1
K, where T is an average temperature (5,700 K) and v<SUP>2</SUP>/Φ
is the ratio of the rotational kinetic and potential energy density of
the photosphere. Detailed calculations<SUP>1-3</SUP> generally support
this expectation. Here we report new observations that should help to
understand the solar global dynamics problem. From about 1,400 h of
solar-limb data obtained during the summers of 1983-85 we find that
the solar-limb temperature variation is not spherically symmetric and
is ~1 K. Our results also indicate that the limb temperature departs
from its expected l = 2 spatial harmonic form and has, at most, a weak
dependence on solar cycle.
---------------------------------------------------------
Title: Progress in Helioseismology
Authors: Libbrecht, Ken G.
1987BAAS...19Q.933L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Fine Structure in Solar Oscillation Spectra
Authors: Libbrecht, Ken G.
1987ASSL..137...59L Altcode: 1987isav.symp...59L
The paper addresses two aspects of helioseismology which require precise
measurements of the relative frequencies of solar oscillation modes:
rotational and magnetic mode splittings and solar cycle changes in the
mode frequencies. Shifts in solar oscillation mode frequencies between
1985 and 1986 are analyzed as a function of nu and l. It is shown that
the frequencies were consistent between the summers of 1985 and 1986
at the 0.02 microHz level.
---------------------------------------------------------
Title: The Variable Oblateness of the Sun: Measurements of 1984
Authors: Dicke, R. H.; Kuhn, J. R.; Libbrecht, K. G.
1986ApJ...311.1025D Altcode:
The solar oblateness measured in 1984 appears to be significantly less
than the 1983 value. This is in turn substantially less than the 1966
value. The observations of 1983 and 1984 were made with a modified and
improved version of the Princeton Solar Distortion Telescope used in
1966. Its design is discussed. The sources of error and the analysis
technique are discussed. The observed changes in oblateness of the
sun are believed to be real and significant.
---------------------------------------------------------
Title: The excitation and damping of solar oscillations
Authors: Libbrecht, K. G.; Popp, B. D.; Kaufman, J. M.; Penn, M. J.
1986Natur.323..235L Altcode:
Present theories suggest two classes of excitation processes which
may be responsible for the observed amplitudes of solar p-mode
oscillations-self-excitation of the modes via an overstability mechanism
such as the K mechanism <SUP>2</SUP>, and stochastic excitation by
turbulent convection<SUP>2-4</SUP>. We now have data which stand in
support of the latter mechanism. Linear overstability calculations are
still ambiguous, because of the uncertainties involved in including
turbulent viscous damping of the modes. Nevertheless no calculation
predicts overstable f-modes, and these modes are observed on the
Sun. Furthermore no nonlinear damping mechanism has yet been proposed
which would limit the growth of overstable modes to their observed
amplitudes. Assuming the modes are stably damped, the theory of mode
excitation by convective turbulence now gives mode amplitudes that are
in agreement with observations<SUP>4</SUP>. In this letter we use data
from Big Bear Solar Observatory to compare theory and observation in
detail. In particular, our data show that the energy per oscillation
mode is nearly constant at low mode frequencies, and is approximately
independent of degree at low degrees. The total energy in all the
oscillation modes is estimated at ~10<SUP>34</SUP>erg.
---------------------------------------------------------
Title: Properties of Intermediate-Degree Solar Oscillation Modes
Authors: Libbrecht, K. G.; Zirin, H.
1986ApJ...308..413L Altcode:
During the summer of 1985, 8042 high-resolution full disk Doppler
images of the Sun were obtained at Big Bear Solar Observatory in a 12
day period. These images were analyzed for solar oscillation modes with
spherical harmonic degrees 5 ≤ l ≤ 20. The frequencies, lifetimes,
and amplitudes of these modes are presented.
---------------------------------------------------------
Title: Is there an unusual solar core?
Authors: Libbrecht, K. G.
1986Natur.319..753L Altcode:
Various attempts have been made in the past two decades to determine the
properties of the core of the Sun, by measurements of solar neutrino
radiation, the distortion of the solar gravitational potential
and frequency splittings of solar oscillation modes. In the latter
category, the two best measurements have been made by Duvall and
Harvey<SUP>1</SUP> and by Brown<SUP>2</SUP>; both showed splitting
roughly independent of spherical harmonic degree l, and both had a
peculiar peak in the measured splitting at l = 11. We present here new
results, based on the analysis of 6,656 individual oscillation modes
for 5 <= l <= 20. These data yield a splitting spectrum which
is consistent with previous measurements, but without the unusual peak
at l = 11, thus suggesting that a simple standard model for the solar
core is essentially correct.
---------------------------------------------------------
Title: Solar ellipticity fluctuations yield no evidence of g-modes
Authors: Kuhn, J. R.; Libbrecht, K. G.; Dicke, R. H.
1986Natur.319..128K Altcode:
Although there have been several claims for the detection of solar
g-modes with periods between 2 and 10 hours<SUP>1-3</SUP> and
although the present sensitivity of the Princeton Solar Distortion
Telescope should allow these low frequency modes to be observed, solar
oblateness data from the summers of 1983 and 1984 show no evidence of
such oscillations with periods between 1 and 5 hours. In about 250
days (nearly 1,000 hours) of observations, we find no evidence for
significant spectral power associated with g-modes. In particular,
there is no evidence of a 160.01-min period solar oscillation.
---------------------------------------------------------
Title: On the facular contrast near the solar limb
Authors: Libbrecht, K. G.; Kuhn, J. R.
1985ApJ...299.1047L Altcode:
Libbrecht and Kuln (1984) have conducted measurements of the continuum
contrast of solar faculae in wavelength bands centered at 800 and
525 nm. Some of the obtained results did not agree with observations
reported by Chapman and Klabunde (1982). The present paper has the
objective to provide new data, taking into account a demonstration of
a procedure for obtaining the facular contrast near the solar limb on
the basis of a simple analysis. The findings confirm the results of
Libbrecht and Kuln that the contrast in the region near the extreme
solar limb decreases with decreasing distance to the limb. Attention
is given to exposed limbs and flux ratios for three occulting disks,
seasonal average normalized flux profiles Delta F/I for the three
disks in two colors, and excess facular signal Delta F/I plotted for
various disks using 1984 data.
---------------------------------------------------------
Title: Practical Considerations for the Generation of Large Order
Spherical Harmonics
Authors: Libbrecht, Ken G.
1985SoPh...99..371L Altcode:
Techniques for generating large-order Y<SUB>l</SUB><SUP>m</SUP>(θ,
ϕ) are discussed.
---------------------------------------------------------
Title: Oblateness of the Sun in 1983 and relativity
Authors: Dicke, R. H.; Kuhn, J. R.; Libbrecht, K. G.
1985Natur.316..687D Altcode:
Measurements of the solar oblateness obtained during 1983 from Mount
Wilson, California, yield a value with an upper bound only half of
that observed in 1966. This difference may support the conjecture
that the solar quadrupole moment slowly oscillates. A knowledge of
the character of such an oscillation, if it occurs, would be needed
to test Einstein's relativity theory using Mercury's orbital motion.
---------------------------------------------------------
Title: Observations of a Solar Latitude-dependent Limb Brightness
Variation
Authors: Kuhn, J. R.; Libbrecht, K. G.; Dicke, R. H.
1985ApJ...290..758K Altcode:
A small latitude-dependent photospheric excess brightness variation has
been observed from 131 days of data obtained with the Princeton Solar
Distortion Telescope. Using an analytic model to separate the influence
of faculae from the brightness signal, a temperature difference of 0.6 +
or - 0.1 K between the poles and the regions at + or - 53 solar latitude
was obtained for the summer of 1983, a temperature difference of 0.6 +
or - 0.1 K between the poles and the regions at + or - 53 deg solar
latitude was obtained for the summer of 1983, with the polar regions
being hotter.
---------------------------------------------------------
Title: Observations of a Solar Latitude Dependent Photospheric
Brightness Variation
Authors: Kuhn, J. R.; Libbrecht, K. G.; Dicke, R. H.
1984BAAS...16..451K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A new measurement of the facular contrast near the solar limb
Authors: Libbrecht, K. G.; Kuhn, J. R.
1984ApJ...277..889L Altcode:
Photometer measurements of solar faculae taken at 525 nm and 800 nm over
a five-month period in the summer of 1982 are presented. They reveal a
contrast function which decreases with decreasing mu near the extreme
limb, in striking disagreement with the result of Chapman and Klabunde
(1982). It is shown that systematic errors in the Chapman-Klabunde
data analysis could account for the discrepancy. The instrument and
observing program of this study and the procedure used to identify
faculae near the limb are described. The effectiveness of different
analysis programs for determining the contrast function from photometer
data is discussed and the analysis procedure used on the presented
data is described in detail. The results are in good agreement with
the facular model of Spruit (1976).
---------------------------------------------------------
Title: The shape of the sun
Authors: Libbrecht, Kenneth George
1984PhDT.......133L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Shape of the Sun
Authors: Libbrecht, K. G.
1984PhDT.........3L Altcode: 1984DiAbI..45..230L
This thesis describes the design, construction, and operation of a
telescope capable of measuring the shape of the sun to an accuracy
of about one milliarcsecond. The instrument is similar to that
used by Dicke in 1966, but with added angular resolution around
the solar limb to detect the presence of photospheric faculae and
sunspots. Data were collected on 131 days in the 5-month period
May-September 1983 at Mt. Wilson, California. A solar oblateness of
(DELTA)r = r(,equator)-r(,pole) = 12.8 (+OR-) 2.4 milliarcseconds was
measured for the period, where the sun's radius is 960 arcseconds. The
12-day rotating distortion seen in '66 was not detected in the
present data set, nor were any higher-frequency oscillations. These
results are consistent at the 2(sigma) level with the oblateness
(DELTA)r = 7.8 milliarcseconds expected from the sun's surface
rotation alone, and inconsistent with Dicke's '66 results. There is
some indication in the data that there exists a polar temperature
excess of T(,pole)-T(,equator) (TURNEQ) 4(DEGREES)C in the quiet
photosphere. Solar faculae and anomalous lensing effects in the earth's
atmosphere were found to be the major sources of systematic error in
the measurement, but neither of these error sources can explain the
'66 data. In fact no explanation (other than a changing sun) was found
for the discrepancy between the two measurements.
---------------------------------------------------------
Title: Facular influences on the apparent solar shape
Authors: Dicke, R. H.; Kuhn, J. R.; Libbrecht, K. G.
1983Natur.304..326D Altcode:
Schatten and Sofia<SUP>1</SUP> have recently reconsidered the question
of whether the 1966 solar ellipticity measurements<SUP>2</SUP> were
seriously contaminated by excess brightness of faculae near the solar
limb<SUP>3-7</SUP>. They considered several different functions for
the variations of the facular contrast with position relative to the
solar limb. With their own facular contrast function, Schatten and Sofia
obtain only a small contribution of faculae to the 1966 apparent solar
ellipticity, but with the Chapman function they obtain a substantial
contribution. New observations of faculae during the summer of 1982
and a novel analytical technique determine a facular contrast which is
constant or decreasing towards the limb, consistent with the Schatten
and Sophia function but inconsistent with Chapman's function. We show
here that the statistical analysis of the 1966 data<SUP>8</SUP> supports
this result. We disagree with the earlier conclusion<SUP>1</SUP>,
that with an acceptable facular contrast function one can obtain “an
acceptable fit to the oblateness measurements” as a purely facular
effect. For 20-30% of the observational days in 1966 only a few small,
weak facular patches were present at the limb, but the ellipticity
signal was present and it was not reduced in magnitude for those days.
---------------------------------------------------------
Title: A New Measurement of the Facular Contrast Near the Solar Limb
Authors: Libbrecht, K. G.; Kuhn, J. R.
1983BAAS...15..717L Altcode:
No abstract at ADS
