Author name code: libbrecht-ken ADS astronomy entries on 2022-09-14 author:"Libbrecht, Ken G." OR author:"Libbrecht, Kenneth G." year:1980-2003 ------------------------------------------------------------------------ Title: Origin of irradiance variations from disk photometer data Authors: Woodard, Martin F.; Libbrecht, Ken G. Bibcode: 2003ESASP.517..117W Altcode: 2003soho...12..117W No abstract at ADS Title: Spatial and temporal variations in the solar brightness Authors: Woodard, M. F.; Libbrecht, K. G. Bibcode: 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. Title: A Search for CO Absorption in the Transmission Spectrum of HD 209458b Authors: Brown, Timothy M.; Libbrecht, Kenneth G.; Charbonneau, David Bibcode: 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. 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. Bibcode: 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. Title: LIGO: Hearing the Gravitational-Wave Universe Authors: Libbrecht, Kenneth G. Bibcode: 2001Mercu..30e..24L Altcode: A team of physicists is opening a new window to the universe, the realm of ripples in space-time. 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. Bibcode: 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 10-3 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. Title: Photometric measurements of the solar disk at BBSO. Authors: Woodard, M. F.; Libbrecht, K. G. Bibcode: 2000BAAS...32..803W Altcode: No abstract at ADS Title: Photometric Measurements of the Solar Disk at BBSO Authors: Woodard, M. F.; Libbrecht, K. G. Bibcode: 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. Title: LIGO: Present Status and Future Improvements. Authors: Libbrecht, Ken G. Bibcode: 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<10-21/√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. 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. Bibcode: 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 10−4 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. 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. Bibcode: 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 10-5. For a 3-year data set, the fractional error is expected to be 3 x 10-6. 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. Title: A Fiber-Fed Echelle Spectrograph for the Hale 5-m Telescope Authors: Libbrecht, K. G.; Peri, M. L. Bibcode: 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 ) 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 Bibcode: 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. Title: Advances in Helioseismology Authors: Libbrecht, K. G.; Woodard, M. F. Bibcode: 1994snft.book..428L Altcode: No abstract at ADS Title: A Search for Acoustic Oscillations on eta CAS with the Palomar 200'' Echelle Spectrograph Authors: Peri, M.; Libbrecht, K. Bibcode: 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. Title: Observations of Time Variation in the Sun's Rotation Authors: Woodard, M. F.; Libbrecht, K. G. Bibcode: 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. Title: Preliminary Assessment of Stellar RV Variation Measurements with the Palomar East Arm Echelle Spectrograph Authors: Peri, M.; Libbrecht, K. Bibcode: 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. 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. Bibcode: 1993ASPC...40...93G Altcode: 1993IAUCo.137...93G; 1993ist..proc...93G No abstract at ADS 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. Bibcode: 1993ASPC...42..213G Altcode: 1993gong.conf..213G No abstract at ADS Title: A New Echelle Spectrograph for Asteroseismology Authors: Peri, M. L.; Libbrecht, K. G. Bibcode: 1993ASPC...42..489P Altcode: 1993gong.conf..489P No abstract at ADS Title: Solar Activity and Oscillation Frequency Splittings Authors: Woodard, M. F.; Libbrecht, K. G. Bibcode: 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. Title: A New Echelle Spectrograph for Astroseismology Authors: Peri, M.; Libbrecht, K. Bibcode: 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. Title: On the Ultimate Accuracy of Solar Oscillation Frequency Measurements Authors: Libbrecht, K. G. Bibcode: 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. Title: Nonfacular Solar Luminosity Variations Authors: Kuhn, J. R.; Libbrecht, K. G. Bibcode: 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. Title: Advances in Helioseismology Authors: Libbrecht, K. G.; Woodard, M. F. Bibcode: 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. Title: Is There an Acoustic Resonance in the Solar Chromosphere? Authors: Woodard, M. F.; Libbrecht, K. G. Bibcode: 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. Title: Short-Term Changes in Solar Oscillation Frequencies and Solar Activity Authors: Woodard, M. F.; Kuhn, J. R.; Murray, N.; Libbrecht, K. G. Bibcode: 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. Title: A Search for Acoustic Amplitude Deficit at the Antipodes of Sunspots Authors: Peri, Michal L.; Libbrecht, Ken G. Bibcode: 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. Title: The solar rotation. Authors: Libbrecht, K. G.; Morrow, C. A. Bibcode: 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. Title: Frequencies of Solar Oscillations Authors: Libbrecht, K. G.; Woodard, M. F.; Kaufman, J. M. Bibcode: 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. Title: Comparison of Solar p-Mode Oscillations in Surface Brightness and Velocity Authors: Libbrecht, K. G. Bibcode: 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. Title: Solar-cycle effects on solar oscillation frequencies Authors: Libbrecht, K. G.; Woodard, M. F. Bibcode: 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. Title: Solar Cycle Effects on Solar Oscillation Frequencies Authors: Libbrecht, K. G. Bibcode: 1990BAAS...22..890L Altcode: No abstract at ADS Title: A Search for Acoustic Amplitude Deficit at the Antipodes of Sunspots Authors: Peri, M. L.; Libbrecht, K. G. Bibcode: 1990BAAS...22..856P Altcode: No abstract at ADS Title: Observations of Solar Cycle Variations in Solar p_Mode Frequencies and Splittings Authors: Libbrecht, K. G.; Woodard, M. F. Bibcode: 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. Title: Seismology of Solar Oscillation Line Widths Authors: Christensen-Dalsgaard, J.; Gough, D. O.; Libbrecht, K. G. Bibcode: 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. Title: Inside the Spinning Sun Authors: Libbrecht, K. Bibcode: 1989S&T....77..584L Altcode: No abstract at ADS Title: The Radial Gradient in the Sun's Rotation Authors: Dziembowski, W. A.; Goode, Philip R.; Libbrecht, K. G. Bibcode: 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. Title: Solar p-Mode Frequency Splittings Authors: Libbrecht, K. G. Bibcode: 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. Title: The excitation and damping of solar oscillations. (Invited review). Authors: Libbrecht, Ken G. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 104. Tables of published frequency measurements for l ≤ 5 are also included. Title: A Search for Radial Velocity Oscillations in Procyon Authors: Libbrecht, K. G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1987ApJ...318..451D Altcode: The solar oblateness measured in 1985 is Δr = req-rp = 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. Bibcode: 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~ Tv2/Φ~0.1 K, where T is an average temperature (5,700 K) and v2/Φ is the ratio of the rotational kinetic and potential energy density of the photosphere. Detailed calculations1-3 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. Bibcode: 1987BAAS...19Q.933L Altcode: No abstract at ADS Title: Fine Structure in Solar Oscillation Spectra Authors: Libbrecht, Ken G. Bibcode: 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. Bibcode: 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. Bibcode: 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 2, and stochastic excitation by turbulent convection2-4. 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 observations4. 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 ~1034erg. Title: Properties of Intermediate-Degree Solar Oscillation Modes Authors: Libbrecht, K. G.; Zirin, H. Bibcode: 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. Bibcode: 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 Harvey1 and by Brown2; 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. Bibcode: 1986Natur.319..128K Altcode: Although there have been several claims for the detection of solar g-modes with periods between 2 and 10 hours1-3 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. Bibcode: 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. Bibcode: 1985SoPh...99..371L Altcode: Techniques for generating large-order Ylm(θ, ϕ) are discussed. Title: Oblateness of the Sun in 1983 and relativity Authors: Dicke, R. H.; Kuhn, J. R.; Libbrecht, K. G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 1984PhDT.......133L Altcode: No abstract at ADS Title: The Shape of the Sun Authors: Libbrecht, K. G. Bibcode: 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. Bibcode: 1983Natur.304..326D Altcode: Schatten and Sofia1 have recently reconsidered the question of whether the 1966 solar ellipticity measurements2 were seriously contaminated by excess brightness of faculae near the solar limb3-7. 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 data8 supports this result. We disagree with the earlier conclusion1, 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. Bibcode: 1983BAAS...15..717L Altcode: No abstract at ADS