Kitt Peak National Research Articles

Assignments and Intensities of 14NH3 Hot Bands in the 5- to 8-μm (3ν2−ν2, ν2+ν4−ν2) and 4-μm (4ν2−ν2, ν1−ν2, ν3−ν2 and 2ν4−ν2) Regions

To complete the analysis of the complex infrared spectra of 14NH3 in the 4- to 8-μm region, we assigned nearly 1200 transitions in the 3ν2−ν2, ν2+ν4−ν2, ν1−ν2, ν3−ν2, 2ν4−ν2 and 4ν2−ν2 hot bands using the upper state energy levels determined by our previous studies at 4 and 3 μm (J. Mol. Spectrosc.173, 120–145 (1995); 193, 46–71 (1999)). Line intensities of those hot bands were analyzed using measurements retrieved from 0.0056 and 0.011 cm−1 unapodized resolution FT spectra recorded at Kitt Peak National Observatory. By a least-squares fit of vibrational-inversion transition moments, over 770 intensity measurements are modeled to ±7.2% and ±8.9% in the 5- to 8-μm and 4-μm regions, respectively. The values of the transition dipole moment were determined to obtain the band strengths of hot bands. In the 5- to 8-μm region, the band strengths in cm−2 atm−1 at 296 K of 3ν2−ν2 (a←s), 3ν2−ν2 (s←a), ν2+ν4−ν2 (s←s) and ν2+ν4−ν2 (a←a) were found to be 0.000075(23), 0.351(22), 0.689(32), and 0.525(44), respectively. In the 4-μm region, for the ν3−ν2 (s←s), ν3−ν2 (a←a), 2ν4l4=±2−ν2 (s←s), and ν4l4=±2−ν2 (a←a), they were estimated to be 0.0703(32), 0.0599(27), 0.00547(34), and 0.00473(29), respectively at 296 K. Also, about 100 line intensities of the ν1−ν2 (a←s), ν1−ν2 (s←a), 2ν4l4=0−ν2 (a←s), 2ν4l4=0−ν2 (s←a), and 4ν2−ν2 (s←a) weak hot bands were measured and reported. The prediction generated by the present study is available for the detection of 14NH3 in exoplanet atmospheres and brown dwarf stars with relatively high temperature (∼1000 K). A few self-broadened linewidths are reported to demonstrate that there is a large variation with the vibrational quanta.

First Assignment and Line Strengths of the 4ν4 Band of 12CH4 near 1.9 μm

The investigation of the methane spectrum in the region 1.6–2 μm has provided the first assignment and analysis of the 4ν4 band near 1.9 μm. Hamiltonian and dipole moment operators written in tetrahedral formalism and adapted to the extrapolation method have been used to fit the spectra recorded at the Kitt Peak National Observatory/National Solar Observatory. Nearly 190 line positions and 160 measured intensities have been modeled with standard deviations of 0.107 cm−1 and 18.5%, respectively.

Large Bodies in the Kuiper Belt

We present a survey for bright Kuiper Belt objects (KBOs) and Centaurs, conducted at the Kitt Peak National Observatory (KPNO) 0.9 m telescope with the KPNO 8K Mosaic CCD. The survey imaged 164 deg2 near opposition to a limiting red magnitude of 21.1. Three bright KBOs and one Centaur were found, the brightest KBO having red magnitude 19.7, about 700 km in diameter, assuming a dark Centaur-like 4% albedo. We estimate the power-law differential size distribution of the classical KBOs to have index q = 4.2, with the total number of classical KBOs with diameters larger than 100 km equal to 4.7 × 104. Additionally, we find that if there is a maximum object size in the Kuiper Belt, it must be larger than 1000 km in diameter. By extending our model to larger size bodies, we estimate that 30 Charon-sized and 3.2 Pluto-sized classical KBOs remain undiscovered.

Evidence of a Third Star Orbiting the Eclipsing Binary δ Librae

ABSTRACTRadial velocity observations obtained at the Kitt Peak National Observatory in 1981, 1983, 1984, and 1991 confirm the suggestion made by McLaughlin six decades ago that the bright Algol‐type binary δ Librae has a long‐period companion. An analysis of this eclipsing system’s barycentric velocity for these four epochs and four earlier ones where published observations were available yield these spectroscopic elements for its center of mass: P = 2.762 ± 0.001 yr, K = 5.46 ± 0.15 km s−1, e = 0.28 ± 0.05, ω = 198° ± 7°, and T = 1983.24 ± 0.04. From these results, it can be inferred that the third star’s mass and spectral type are 0.95 ± 0.04 M⊙ and G9 and that the entire δ Lib system is even more similar to the archetypal eclipsing binary β Persei (Algol) than was previously recognized. The spectroscopic orbital elements for the B9.5 V primary star in the eclipsing system are also updated.

10μm ethylene: spectroscopy, intensities and a planetary modeler's atlas

FTS and TDL spectra of ethylene in the 10 μm region have been observed, measured, calibrated, assigned and intensities have been measured. The ultimate goal of this work is the production of a planetary modeler's atlas. A spectrum taken in double-pass configuration at the McMath–Pierce FTS instrument at Kitt Peak National Observatory has been frequency calibrated using CO 2 laser bands. Results of a previous analysis (Cauuet et al., J Mol Spectrosc 1990;139:191) have enabled the assignment of the FTS spectrum wherein we have measured over 500 line intensities in the 900– 1000 cm −1 region. These FTS intensities have been calibrated against 13 isolated transitions, taken as secondary intensity standards. These standard lines have been measured independently using TDL (tunable-diode-laser) spectrometers at University of Tennessee and Goddard Space Flight Center. A calculated spectrum, including mixing coefficients for ν 4, ν 7, ν 10 and ν 12, and calculated relative intensities, and the TDL-calibrated FTS line intensities were used as data in a non-linear regression analysis to determine the 296 K vibrational band intensities of S 0–7 v =321.69±0.36 cm −1/ cm atm , S 0–10 v =1.16±0.47 cm −1/ cm atm , and S 0–12 v =31.60±9.80 cm −1/ cm atm . These vibrational band intensities combined with the theoretical spectral-line atlas make possible the generation of an ethylene spectrum at an arbitrary temperature. Such spectra prove useful to the planetary-atmosphere modeling-community. A web site is available where an individual can interact with the model and download a custom atlas. The URL is http://aurora.phys.utk.edu / ̃ blass/ethyatl/.

Analysis of the Interacting Octad System of 12CH4

The octad system of methane 12CH4 has been analyzed with a fourth-order (459 parameters) Hamiltonian for energy levels and a third-order (28 parameters) dipole moment expansion for infrared intensities. Nearly 8000 assigned positions and 2500 measured line intensities (obtained with the Fourier transform spectrometer located at Kitt Peak National Observatory/National Solar Observatory) have been considered in the modeling. Infrared (Octad–Ground State) and hot bands (Octad–Dyad) data have been used. While the model does not reproduce the measurements at their inherent experimental precisions, it has been sufficient to interpret and substantially assign this spectral region of methane for the first time in over 30 years of investigation.

Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). I. New CFHT MOS Spectroscopy and the Composite Database

We present a comprehensive database of kinematic, photometric, and positional information for 352 objects in the field of M87 (NGC 4486), the central giant elliptical galaxy in the Virgo Cluster; the majority of the tracers are globular clusters associated with that galaxy. New kinematic information comes from multislit observations with the Multi-Object Spectrograph (MOS) of the Canada-France-Hawaii Telescope (CFHT), an investigation that has added 96 new velocities to and confirmed many of the earlier values in a preexisting data set of 256 velocities published elsewhere. The photometry, consisting of magnitudes and colors in the Washington (T1, C-T1) system, is based on CCD observations made at the Cerro Tololo Inter-American Observatory (CTIO) and the Kitt Peak National Observatory (KPNO). The composite database represents the largest compilation of pure Population II dynamical tracers yet identified in any external galaxy; moreover, it extends to larger spatial scales than have earlier investigations. The inclusion of photometric information allows independent study of the distinct red and blue subpopulations of the bimodal globular cluster system of M87. In a companion paper we use this powerful data set to analyze the present dynamical state of the M87 globular cluster system and consider the question of its interaction and formation history.

Pilachowski is AAS President-Elect

Catherine Pilachowski, a scientific staff member at the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona, takes office as presidentelect of the American Astronomical Society next month. In 2002, Pilachowski will succeed Anneila Sargent, a professor of astronomy at Caltech, for a two-year term as AAS president.The biggest challenge facing AAS in the next few years, according to Pilachowski, is combining budget constraints with the opportunities presented by the recent decadal survey of astronomy and astrophysics (see Physics Today, July 2000, page 45). “Our scientific frontiers are expanding at a breathtaking pace. It’s an exciting time for astronomy,” she says.Pilachowski earned her MS and PhD degrees in astronomy from the University of Hawaii, Manoa, and then worked for four years as a postdoctoral fellow at the University of Washington, Seattle. Since 1979, she has worked for Kitt Peak National Observatory, in Tucson, and NOAO. Her research focuses on the chemical composition of stars and star clusters in the Milky Way, using high-resolution spectroscopy to better understand the evolution of stars.Also taking office next month for a three-year term as vice president is Joseph A. Burns, a professor of astronomy at Cornell University. Arlo U. Landolt, a professor of physics and astronomy at Louisiana State University, was reelected for three years as AAS secretary. And elected for three-year terms as AAS councilors were Thomas R. Ayres, a research professor at the Center for Astrophysics and Space Astronomy at the University of Colorado, Boulder; Dana E. Backman, an associate professor of astronomy at Franklin and Marshall College in Lancaster, Pennsylvania; and Susana Lizano, a researcher at the National Autonomous University of Mexico’s Institute for Astronomy in Morelia. Pilachowski PPT|High resolution© 2001 American Institute of Physics.

Continuum Variability of Active Galactic Nuclei in the Optical–Ultraviolet Range

The variability of the continuum spectral energy distribution has been analyzed for a complete magnitude-limited sample of quasars in Selected Area 57, observed at two epochs in the photographic U, BJ, F, and N bands with the Mayall 4 m telescope at Kitt Peak National Observatory. Changes δα of the spectral slope α appear correlated with brightness variations δ log fν, indicating an average hardening of the spectrum in the bright phases. This confirms that the correlation of variability with redshift, found in a single observing band, is due to intrinsic spectral changes. The average observed δα-δ log fν relation is consistent with the spectral change due to temperature variation of a blackbody of about 2.5 × 104 K.

Oxygen Abundances in Metal‐poor Stars (−2.2 < [Fe/H] < −1.2) from Infrared OH Lines

Infrared OH lines at 1.55 - 1.56 um in the H-band were obtained with the Phoenix high-resolution spectrograph at the 2.1m telescope of the Kitt Peak National Observatory for a sample of 14 metal-poor stars. Detailed analyses of the sample stars have been carried out, deriving stellar parameters based on two methods: (a) spectroscopic parameters; (b) IRFM effective temperatures, trigonometric gravities and metallicities from Fe II lines. The Fe I lines present in the H-band region observed showed to be well fitted by the stellar parameters within $\Delta$[Fe/H] < 0.15 dex. The oxygen abundances were derived from fits of spectrum synthesis calculations to the infrared OH lines. CO lines in the H- and K-bands were obtained for a subsample in order to determine their carbon abundances. Adopting the spectroscopic parameters a mean oxygen-to-iron ratio of [O/Fe] ~ +0.52 is obtained, whereas using the IRFM temperatures, Hipparcos gravities and [FeII/H], [O/Fe] ~ +0.25 is found. A mean of the two methods gives a final value of [O/Fe] ~ +0.4 for the metallicity range -2.2 < [Fe/H] < -1.2 of the sample metal-poor stars.

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. VII. Additional Variables in the Field M33A Discovered with Image Subtraction

DIRECT is a project to directly obtain the distances to two Local Group galaxies, M31 and M33, which occupy a crucial position near the bottom of the cosmological distance ladder. As the first step of the DIRECT project, we have searched for detached eclipsing binaries (DEBs) and new Cepheids in the M31 and M33 galaxies with 1 m class telescopes. In this paper, we present a catalog of variable stars discovered in the data from the follow-up observations of the DEB system D33J013346.2+304439.9 in field M33A (α = 2355, δ = 3072; J2000.0), collected with the Kitt Peak National Observatory's 2.1 m telescope. In our search covering an area of 108 arcmin2, we have found 434 variable stars: 63 eclipsing binaries, 305 Cepheids, and 66 other periodic, possible long-period, or nonperiodic variables. Of these variables, 280 are newly discovered, mainly short-period and/or faint Cepheids. Their light curves were extracted using the ISIS image subtraction package. For 85% of the variables, we present light curves in standard V and B magnitudes, with the remaining 15% expressed in units of differential flux. We have discovered a population of first-overtone Cepheid candidates, and for eight of them we present strong arguments in favor of this interpretation. We also report on the detection of a nonlinearity in the KPNO T2KA and T1KA cameras. The catalog of variables, as well as their photometry (~7.8 × 104 BV measurements) and finding charts, is available electronically via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

On the Nature of AX J2049.6+2939/AX J2050.0+2914

AX J2049.6+2939 is a compact X-ray source in the vicinity of the southern blow-up region of the Cygnus Loop supernova remnant (Miyata et al. 1998a). This source was the brightest X-ray source inside the Cygnus Loop observed during the ASCA survey project. The X-ray spectrum was well fitted by a power-law function with a photon index of $-2.1 \pm 0.1$. Short-term timing analysis was performed and no coherent pulsation was found. Follow-up observations with ASCA have revealed a large variation in X-ray intensity by a factor of $\simeq$ 50, whereas the spectral shape did not change within the statistical uncertainties. In the second ASCA observation, we found another X-ray source, AX J2050.0+2941, at the north east of AX J2049.6+2939. During the three ASCA observations, the X-ray intensity of AX J2050.0+2941 varied by a factor of $\simeq$4. No coherent pulsations could be found for AX J2050.0+2941. We have performed optical photometric and spectroscopic observations in the vicinity of AX J2049.6+2939 at the Kitt Peak National Observatory (KPNO). As a result, all objects brighter than $B$-band magnitude of 22 in the error box can be identified with normal stars. Combined with the X-ray results and the fact that there are no radio counterparts, AX J2049.6+2939 is not likely to be either an ordinary rotation-powered pulsar or an AGN. The nature of AX J2049.6+2939 is still unclear and further observations over a wide energy band are strongly required. As to AX J2050.0+2941, the long-term X-ray variability and the radio counterpart suggests that it is an AGN.

Geocoronal Hα intensity measurements using the Wisconsin Hα Mapper Fabry‐Perot facility

The Wisconsin Hα Mapper (WHAM), a remotely operable, semi‐automated Fabry‐Perot located at Kitt Peak Observatory, has been making an all‐sky survey of interstellar hydrogen Balmer α(Hα) emissions since 1997. Using the annular summing spectroscopy technique, WHAM has acquired ∼37,000 spectra to date, spanning almost 100 nights of observations. Since all of the galactic emission spectral data contain the terrestrial Hα (6562.7 Å) emission line, these measurements constitute a rich source of geocoronal data for investigating natural variability in the upper atmosphere. The WHAM observations also serve as a benchmark for comparison with future data. Analysis of the first year of WHAM data shows only small day‐to‐day variations after shadow altitude variations are taken into account. For example, at shadow altitudes of 2000 and 3000 km, the RMS scatter is within approximately +/− 20%; this variability is expected to be reduced with accurate accounting of the smaller‐scale effects of observational slant path, zenith angle, and azimuth on the Hα intensity. This result is consistent with past midlatitude Wisconsin data sets but different from observations made by other observers and instruments at the low‐latitude Arecibo site. The multiple viewing geometries of the observations provide stringent modeling constraints, useful in testing current modeling capabilities. Modeling of the WHAM data with a global nonisothermal resonance radiation transport code (lyao_rt) indicates that the signal‐to‐noise of the data is sufficient to determine relative variations in upper atmospheric atomic hydrogen column densities to better than 5%. This paper describes the WHAM aeronomy program and its observational scheme, analysis procedures, and results from data taken in 1997. Case study comparisons are made with past data sets and with predictions from the lyao_rt resonant radiation transport modeling code of Bishop [1999].

Observational Constraints on Higher Order Clustering up toz≃ 1

We present constraints on the validity of the hierarchical gravitational instability theory and the evolution of biasing based on measurements of higher order clustering statistics in the Deeprange Survey, a catalog of ~710,000 galaxies with IAB ? 24 derived from a Kitt Peak National Observatory (KPNO) 4 m CCD imaging survey of a contiguous 4? ? 4? region. We compute the three- and four-point angular correlation functions using a direct estimation for the former and the counts-in-cells technique for both. The skewness, s3, decreases by a factor of 3-4 as galaxy magnitude increases over the range 17 ? I ? 22.5 (0.1 z 0.8). This decrease is consistent with a small increase of the bias with increasing redshift, but not by more than a factor of 2 for the highest redshifts probed. Our results are strongly inconsistent, at about the 3.5-4 ? level, with typical cosmic string models in which the initial perturbations follow a non-Gaussian distribution; such models generally predict an opposite trend in the degree of bias as a function of redshift. We also find that the scaling relation between the three- and four-point correlation functions remains approximately invariant over the above magnitude range. The simplest model that is consistent with these constraints is a universe in which an initially Gaussian perturbation spectrum evolves under the influence of gravity combined with a low level of bias between the matter and the galaxies that decreases slightly from z ~ 0.8 to the current epoch.

The Relationship Between Solar Activity and the Large-Scale Axisymmetric Magnetic Field

To investigate the relationship between solar activity and the large-scale axisymmetric magnetic field of the Sun, we inferred from sunspot data over the period 1964–1985 a latitude–time distribution of magnetic field associated with active regions. This has been done allowing for both bipolar structure of the active regions and inclination of their axes to parallels of latitude, so the inferred magnetic field characterizes latitudinal separation of magnetic polarities which might be related to the large-scale magnetic field of the Sun according to the Babcock–Leighton model. The inferred magnetic field, Az, is compared with the longitude-averaged (zonal) magnetic field of the Sun, Bz, derived from series of magnetograms obtained at Mount Wilson Observatory in the years 1964–1976, and at Kitt Peak National Observatory during the period from 1976 to 1985. The inferred magnetic field, Az, exhibits a complex structure distribution of magnetic polarities with respect to latitude and time. Apart from concentration of the different polarity magnetic fields inside the high- and low-latitude portions of the sunspot belts, bipolar active regions produce an intensive, shorter-scale component of the magnetic field which varies on the time scale of about 2 years. Such a short-term variation of Az reveals substantial correlation with the short-term component of Bz which has the form of the poleward-drifting streams of magnetic field. Most significant correlation takes place between the short-term variations of Az occurring at latitudes below 20° and those of the large-scale magnetic fields occurring at middle latitudes of 40–50°. Moreover we analyze harmonic coefficients al and bl obtained by expanding Az and Bz into series in terms of the spherical harmonics. Power spectra of the time-dependent harmonic coefficients indicate that both Az and Bz reveal a number of resonant modes which oscillate either with the 22-year period in the case of the anti-symmetric (odd-l) modes or with periods of about 2 years in the case of the symmetric (even-l) modes, but the resonant modes of Az have significantly larger values of the spherical harmonic degree l (and, hence, smaller spatial scales) as compared to those of Bz. It is found that there is a close relationship between the harmonic coefficients bl and am for which either m−l≈16 (even l=4,...,10) or m−l=4 (odd l=5,...,15).

Line Positions and Intensities in the 2ν2/ν4 Vibrational System of 14NH3 near 5–7 μm

Line positions and intensities belonging to the vibrational system 2ν2/ν4 of ammonia 14NH3 are measured and analyzed between 1200 and 2200 cm−1 in order to improve the molecular database. For this, laboratory spectra are obtained at 0.006 and 0.011 cm−1 unapodized resolution and with 4% precisions for the intensities using Fourier transform spectrometers located at the Kitt Peak National Observatory and the Jet Propulsion Laboratory. The observed data contain transitions of the ν4 fundamental band near 1626.276(1) and 1627.375(2) cm−1 (for s and a inversion upper states, respectively) and the 2ν2 overtone band near 1597.470(3) and 1882.179(5) cm−1 (for s and a inversion states, respectively). A total of 2345 lines with J′ ≤ 15 is assigned from which 2114 line positions with J′ ≤ 15 are fitted using an effective rotation–inversion–rotation Hamiltonian to achieve an rms of 0.003 cm−1 with 57 molecular parameters. Over 1200 intensity measurements are modeled to ±4.7% using 16 terms of the dipole moment expansion. A dyad model is used in order to model all the interactions expected within the 2ν2/ν4 system. The bandstrengths of 2ν2 (s ← a), 2ν2 (a ← s), and ν4 (s ← s and a ← a) are estimated to be 6.68(24), 0.201(5), and 116(3) cm−2 atm−1, respectively, at 296 K. The prediction generated by this study is available for planetary studies.

First Results from the Large-Area Lyman Alpha Survey

We report on a new survey for z ≈ 4.5 Lyα sources, the Large-Area Lyman Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume and sensitivity by using narrowband filters on the new 81922 pixel CCD Mosaic camera at the 4 m Mayall telescope of Kitt Peak National Observatory. Well-detected sources with flux and equivalent width matching previously known high-redshift Lyα galaxies (i.e., observed equivalent width EW > 80 Å; 2.6 × 10-17 ergs cm-2 s-1 < line + continuum flux < 5.2 × 10-17 ergs cm-2 s-1, and a small uncertainty on the equivalent width, δEW < EW/4) have an observed surface density corresponding to 11,000 ± 700 deg-2 per unit redshift at z = 4.5. Variations in this surface density are apparent on comparison between counts in 6561 ± 40 and 6730 ± 40 Å filters. Early spectroscopic followup results from the Keck telescope included three sources meeting our criteria for good Lyα candidates. Of these, one is confirmed as a z = 4.52 source, another remains consistent with either z = 4.55 or z = 0.81, and the third is an [O III] λ5007 emitter at z = 0.34. These pilot spectroscopic results suggest that approximately one-third of our good candidates are bona fide Lyα emitters, implying a net density of ~4000 Lyα emitters per square degree per unit redshift.

Experimental Line Parameters of the Oxygen A Band at 760 nm

To support atmospheric remote sensing applications, line positions, intensities, self- and nitrogen-broadened linewidths and their temperature dependences and pressure-induced shifts in line positions at room temperature were measured up to J′ and N′ = 22 for the oxygen A band at 13 122 cm−1. Line intensities were obtained with 1% precisions and 2% absolute accuracies using absorption spectra recorded at Doppler-limited (0.02 cm−1) resolution with the McMath Fourier transform spectrometer (FTS) located at Kitt Peak National Observatory/National Solar Observatory in Arizona. The oxygen line positions were calibrated using near-infrared transitions of the 2–0 and 3–0 bands of CO as secondary standards. The intensities and positions of seven H2O lines near 13 900 cm−1 were also remeasured to validate the FTS performance. The O2 intensities fell within 1% of the values currently assumed for the molecular databases, but it was found that broadening coefficients and line positions should be revised for the A band of molecular oxygen.

On the relationship between polar faculae and large-scale magnetic field

We investigate a latitude–time distribution of polar faculae observed at Ussuriysk Observatory in years 1966–1986. The distribution is compared with the longitude-averaged (zonal) magnetic field of the Sun calculated from the data obtained at Mount Wilson Observatory in the years 1966–1976, and at Kitt Peak National Observatory during the period from 1976 to 1985. We found that slow, poleward-directed migration of the polar faculae zones occurring during the course of the solar cycle is not a continuous process, but it contains several episodes of appearance and fast poleward drift of new zones of polar faculae. At the rising phase of the solar cycle, new zones of polar faculae appear at latitudes as low as 40°, but the ones observed during the declining phase of the solar cycle originate at higher latitudes of 50–55°. Such episodes of appearance and fast migration of the polar faculae zones are associated with the poleward-directed streams of magnetic field originated at low latitudes. Moreover, we found some evidence for existence of an additional component of the polar faculae activity that reveals an equatorward migration during the course of the solar cycle. We also investigated a relationship between the number of polar faculae, n, and absolute magnetic flux Φz of the zonal mode of the solar magnetic field. We found that within the polar zones of the Sun, substantial correlation between temporal variations of n and Φz takes place both on the time scale of the solar cycle and on a shorter time scale of 2–4 years. The relationship between the number of polar faculae and magnetic flux may be approximated by a linear dependence n=0.12Φz (where Φz is expressed in 1021 Mx), except for time interval 1977 through 1980 for which the factor of proportionality is found to have a systematically larger value of 0.20.

3-D magnetic configurations for filaments and flares: The role of “magnetic dips” and “bald patches”

The 3-D magnetic configuration of a filament and of a low energy flare is reconstructed, using linear magnetohydrostatic ( lmhs) extrapolations. In both cases, we find observational signatures of energy release at the locations of computed “bald patches” separatrices, characterised by field lines which are tangent to the photosphere. The filament was observed on Sept. 25, 1996, in Hα with the MSDP on the German VTT, Tenerife, as well as in Si iv with SOHO/SUMER. It is modeled as a twisted flux-tube deformed by the magnetic polarities observed with SOHO/MDI. The shape and location of the computed dipped field lines are in good agreement with the shape of the filament and its feet observed in Hα. Some “bald patches” (BPs) are present where the distribution of dips reaches the photosphere. We show that some of the large scale field lines rooted in BPs can be related to bright fine structures in Si iv. We propose that the plasma there is heated by ohmic dissipation from the currents expected to be present along the BP separatrices. The flare was observed on May 18, 1994, in soft X-rays with Yohkoh/SXT, and in Hα at Mitaka (Japan). The magnetic field is directly extrapolated from a photospheric magnetogram from Kitt Peak Observatory. The intersections with the photosphere of the computed separatrices match well the bright Hα ribbons. The later are associated to three BPs, with overlaying dipped field lines. We show that enhanced densities are present in these dips, which can be correlated with dark Hα fibrils. Both cases show the importance of dipped field lines and BPs in the solar atmosphere. Energy release via ohmic dissipation as well as reconnection along BP separatrices is proposed to provide heating observed as UV brightenings in filament channels and even as small flares.