Photospheric electric current and transition region brightness within an active region

Distributions of vertical electric current density (J _z) calculated from vector measurements of the photospheric magnetic field are compared with ultraviolet spectroheliograms to investigate whether resistive heating is an important source of enhanced emission in the transition region. The photospheric magnetic fields in Active Region 2372 were measured on 6 and 7 April, 1980 with the MSFC vector magnetograph; ultraviolet wavelength spectroheliograms (L and Nv 1239 Å) were obtained with the UVSP experiment aboard the Solar Maximum Mission satellite. Spatial registration of the J _z (5 arc sec resolution) and UV (3 arc sec resolution) maps indicates that the maximum current density is cospatial with a minor but persistent UV enhancement, but there is little detected current associated with other nearby bright areas. We conclude that although resistive heating may be important in the transition region, the currents responsible for the heating are largely unresolved in our measurements and have no simple correlation with the residual current measured on 5 arc sec scales.National Research Council Resident Research Associate.National Oceanic and Atmospheric Administration Space Environment Laboratory; currently at MSFC/SSL.     

UBV photometry at geographic latitude 69°22′ North

UBV measurements of the light of the night sky in the auroral zone during three seasons are presented. The mean brightness of the night sky in theV band is found to be equal one 18^m1 star (arc sec)^–2, with considerable variations. The observed meanB-V andU-B indicies are +0 _. ^m 7 and –1 _. ^m 6, respectively.Light curves of variable stars during strong auroral activities are also shown. On the basis of measurements we briefly discuss the possibility of accurate stellar photometry in the auroral region.     

A high-resolution study of SGR-B2 at CM-wavelengths

The radio source Sgr-B2 was studied with the radio telescope of the USSR Academy of Sciences, RATAN-600, at wavelengths 13, 8.2, 3.9, 2.08, and 1.38 cm. The brightness distributions obtained allow us to defined spectra of individual components and to revise a total spectrum of the source itself. It was shown that the source contains no less than four ultracompact details with electron densities exceeding 2×10⁴ cm^–3. An extended detail with a flux density at 1.38 cm equal to 12.2 f.u. and an angular size 58 arc sec was found in a nuclear region. Excitation parameters of all compact details were shown to correspond to stars of a spectral type earlier than O6 (U60). A relation (logN _e –logd) for all Hii regions in the field around Sgr-B2, including G 0.6-0.0 and G 0.5-0.0, was constructed and shown to be nearly linear, with a slope –1.2.     

Evidence for a coronal magnetic bottle at 10 solar radii

The Faraday rotation of a radio source (Pioneer 6) occulted by the solar corona has been measured by Levy et al. (1969). During the course of these measurements, three large-scale transient phenomena were observed. These events were preceded by subflares and class 1 flares. These transient events are interpreted as evidence for a coronal magnetic bottle at 10 R _⊙. The velocity of propagation for the disturbance is set at 200 km/sec; the dimension of the region, 10 R _⊙; field strength at 10 R _⊙, 0.02 G; particle density, 2.0 × 10⁴/cm³; Alfvén speed, 320 km/sec. From the nature of the observations and the lack of related effects from similar flares on the interplanetary sector pattern observed at 1 AU, it is suggested that such coronal magnetic bottles expand to perhaps 10–30 R _⊙ and then contract to a few solar radii. Such a phenomena is evidence for an expansion of the corona with a sub-Alfvénic velocity. It is further suggested that such magnetic bottles may be important in the storage and diffusion of solar generated cosmic ray particles. NAS-NRC Postdoctoral Resident Research Associate.     

Observations of the Solar Limb Shape Distortions

Two new campaigns devoted to the observation of the solar limb distortions were made at the Pic-du-Midi Observatory, in September 2000 and September 2001, by means of the scanning heliometer. This apparatus can be used now routinely to accurately determine solar limb profiles (at two wavelengths), at any heliographic latitudes. Each measurement is made within 44 milliseconds (of time) which permits to record a limb profile together with the seeing. Scans are automatically rejected for seeing larger than 1.3 arc sec. Such conditions are essential to perform high-quality observations necessary to obtain the quadrupole term (l=2) in the polynomial expansion of the radius contour R() _{= constant} = R ₀ left(1+_l c _l P _l()right). Exceptional meteorological conditions in September 2001 (seeing of the order of 18 cm, for a 50 cm clear aperture of the refractor) enabled us to determine c ₂ and c ₄ (see Table I) with an accuracy of a few milli-arc-sec. Results indicate a distorted solar shape, the departures from a pure spherical body not exceeding 20 milli-arc-sec. We propose a model to interpret such results (the combination of a nearly uniform rotating core with a prolate solar tachocline and an oblate surface), which is briefly discussed. Our results are confronted to those obtained from space. We conclude that measurements of the quadrupole term from the ground are possible, but of high difficulty and can be obtained only during excellent weather conditions. The hexadecapole term should be only obtained from space. We show that an astrometric satellite would be required, whose mission would be also to accurately determine the solar rotation profiles (both surface and in depth) in order to unambiguously determine the inertia moments of the Sun through the J _n terms. Such values are also briefly discussed.     

The San Fernando Observatory Video Spectra-Spectroheliograph

We describe recent work in the development of the San Fernando Observatory (SFO) Video Spectra-Spectroheliograph (VSSHG), a spectrum-based instrument for the measurement of the solar Stokes profiles. Its most important features are: simultaneous measurement of Stokes I plus one of Stokes Q, U, or V; spatial sampling of 0.5 arc sec; spectral sampling of 8.8 mÅ; and time sampling of one minute (for one pair of Stokes profile) to three minutes (for all four profiles). Routine data processing is carried out using a moments technique; tests of this technique show it to be reasonably accurate. Sample data are shown and briefly discussed: a longitudinal magnetogram and Dopplergram of NOAA 5573 observed on 17 August, 1989, and a vector magnetic field map and Dopplergram of NOAA 6659 observed on 10 June, 1991.     

Absolute intensity calibrations of solar K-line profiles

Individual K-line profiles from elements of fine structure on the surface of the Sun are calibrated absolutely. The continuum calibrations of Labs and Neckel and of Houtgast and Namba are considered, and the average K-profile is scaled to that of White and Suemoto.The ranges of intensities across a high-resolution spectrogram are tabulated for various parts of the line profile. Although the spatially-averaged value for K ₃ of 4.2% of the continuum corresponds to a brightness temperature of 4155 K, minimum and maximum values were 3980 and 4360 K, respectively. Similarly, K _2v ranges from 4200 to 4560K, and K _2r from 4180 to 4460K in small elements about 1 arc sec across.     

Time variability and structure of quiet Sun sources at 6 cm wavelength

Using the Westerbork Synthesis Radio Telescope (WSRT) we produced a synthesized map of a quiet Sun region on June 15, 1976, and studied the structure and time variability of the quiet emitting regions at 6 cm wavelength with a spatial resolution of 6 arc sec. Comparison of the 12hr synthesis map with Ca⁺ K filtergram shows that bright and dark features on the 6 cm quiet Sun synthesized map correspond to the chromospheric networks and cells observed in Ca⁺ K. All 6 cm bright features lie over bright Ca⁺ K network elements. The reverse correlation is not true, that is, not all bright Ca⁺ K network features have their 6 cm counterparts. Comparison with the photospheric magnetogram shows that about 72% of the photospheric magnetic field enhancements (¦B¦ 5 G) are coincident with 6 cm emissive regions. Only one 6 cm feature could be positively identified with a bipolar magnetic structure. This implies that no more than 20–25% of the 6 cm emitting features could be associated with X-ray bright points. Intercomparison of our 12hr two-dimensional synthesis map, a 4hr two-dimensional synthesis map (around meridian) and the one-dimensional fan beam scans of the quiet Sun region at 6 cm, along with the Ca⁺ K filtergram and photospheric magnetogram shows that: (1) All of the 15 time-varying elements at 6 cm were located on Ca⁺ K networks; (2) about 40% of the 15 time varying elements at 6 cm are coincident with enhancements of the photospheric magnetogram; (3) individual time-varying sources have minimum source size (FWHM) of 15 arc sec and maximum brightness temperature of 10⁵ K; (4) the life time of the time varying sources varies from a few minutes to several tens of minutes; (5) the intensity of the sources varies by factors of 2 to 7 over time periods of 1 min to tens of minutes; and (6) the sources tend to disappear for periods of up to tens of minutes and to reappear at the same locations.     

EUV observations of quiescent prominences from Skylab

We report measurements of line intensities and line widths for three quiescent prominences observed with the Naval Research Laboratory slit spectrograph on ATM/Skylab. The wavelengths of the observed lines cover the range 1175 Å to 1960 Å. The measured intensities have been calibrated to within approximately a factor 2 and are average intensities over a 2 arc sec by 60 arc sec slit. We derive nonthermal velocities from the measured line widths. The nonthermal velocity is found to increase with temperature in the prominence transition zone. Electron densities and pressures are derived from density sensitive line ratios. Electron pressures for two of the prominences are found to lie in the range 0.04–0.08 dyn cm^–2, while values for the third and most intense and active of the three prominences are in the range 0.07–0.22 dyn cm^–2.     

CCD surface photometry of the Seyfert 1 galaxy NGC 4151

We present CCD surface photometry of the central region of the bright Seyfert 1 galaxy NGC 4151. The observations were obtained under excellent seeing conditions (0.75 FWHM). Morphological parameters for the bulge of the galaxy are estimated from the observed broad bandV, R, andI surface brightness profiles.From the analysis of the colour maps two distinct nuclear structures emerge perpendicular to each other. One of them is elliptically elongated at PA angle 50°. This direction is close to the direction of the radio jets and coincides with the Extended Narrow Emission Line Region (ENELR). Its extension is about 7 arc sec and its colours are bluer than the surronding bulge. The second structure is smaller in extension (about 1 arc sec) and is elongated at PA130°. Its colours are redder than those of the surrounding bulge. It is suggested that this might constitute areal extension of the Broad Line Region (BLR).Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Persistent 1.5 s oscillations superimposed to a solar burst observed at two mm-wavelengths

Long-enduring quasi-periodic oscilations (1.5s) superimposed upon a solar burst have for the first time been observed simultaneously at two different mm-wavelengths (22 GHz and 44 GHz). The oscillations were present throughout the burst duration (about 10 min), and were delayed at 44 GHz with respect to 22 GHz by 0.3 s. The relative amplitude of the oscillation was of about 20% at 44 GHz and of about 5% at 22 GHz. Interferometer measurements at 10.6 GHz indicated the burst source position stable within 1 arc sec. An He i D₃ line flare showed two persistent small spots separated by about 10 arc sec. The 22/44 GHz burst position corresponds well with the location of the He i D₃ spots. The oscillations display features which distinguish them from ultrafast time structures found in other bursts. One possible interpretation is a modulation of the synchrotron emission of trapped electrons by a variable magnetic field on a double burst source, optically thin at 44 GHz and with optical thickness 0.3 at 22 GHz.     

Rocket observation of the EUV images of a solar flare and active regions

Images of a flare and active regions were obtained in the extreme ultraviolet emission lines such as CIII 977 Å, Ne VIII 770 Å, and HI L, and hydrogen Lyman continua with a spatial resolution of less than ten seconds of arc together with one-dimensional scanning at 1650 Å. A microchannel plate was used as a detector, and pointing accuracy was, for about half of the observation time, around 0.5 arc sec.The relationship between the shape of the flare and the structure of the photospheric magnetic field is discussed. A map of the electron temperature distribution derived from the intensity ratio of the Lyman continua at 880 Å and 815 Å showed a lower temperature in regions of higher activity. A very small geometrical thickness of 50–500 m in the C III emitting region of the flare was found. And the layer emitting the continuum in 1650 Å is shown to be at a temperature of 5300 K in the flare and 4700 K in active regions.     

Radio emission from the Sun at 843 MHz

The Molonglo Observatory Synthesis Telescope (MOST) has recently been modified to permit observations of the Sun. With a collecting area of 18000 m², MOST makes high-sensitivity measurements in right-hand circular polarisation over a 3 MHz bandwidth at 843 MHz. The maximum baseline of the multi-element interferometer is 1600 m, so that one-dimensional spatial resolution as fine as 32 arc sec is available. A resistor array produces simultaneously a set of 64 beams separated by 22 arc sec, which may be offset electronically to cover the entire Sun in a few seconds. Observations may be made with a beam shape corresponding to either a multiplying or an adding interferometer. By exploiting the technique of Earth-rotation synthesis the telescope may be used to make two-dimensional maps of the Sun at the time of the austral solstice with a synthesized beamwidth of 43 × 110 arc sec. This paper describes the instrument and the procedures used to make various types of solar observations, and exhibits some of the first data collected.     

A search for millimeter wave spectral lines from comet Sugano-Saigusa-Fujikawa (1983e)

The search for radio spectral lines from Comet Sugano-Saigusa-Fujikawa (1983e) was conducted using the 45-m telescope of Nobeyama Radio Observatory. The frequency ranges of 44.0–46.0 and 47.5–49.5 GHz were surveyed down to $\text{ΔT}{}_{\mathrm{<mtext>A</mtext>}}{}^1\text{(}\text{rms}\text{) = 20–30}\text{mK}$, with a beam size of ～35 arc sec. Upper limits have been established for spectral lines of atomic hydrogen, CS, OCS, SO₂, H₂CO, CH₃OH, HCCCCCN, HCOOCH₃, CH₃OCH₃, and CH₃CH₂CN. The J = 5?4 line from HCCCN in the vibrational ground state possibly has been detected but not confirmed. The suggested total amount of HCCCN in the coma is consistent with the possible picture that HCCCN is the main parent molecule of CN.     

Hα Doppler brightening and Lyman-α Doppler dimming in moving Hα prominences

We consider the effect that coherent motion has on the observed brightness of moving clouds above the photosphere. We find that steady state clouds (constant N _e and T _e ) that are moving perpendicular to the line of sight will appear brighter in H for speeds between 8 and 100 km/sec and dimmer for speeds greater than 135 km/sec. The brightening and dimming are due to apparent Doppler shifts of the respective H absorption and the Lyman- emission profiles seen by the absorption profile of the moving cloud.We apply this analysis, along with optical depth and geometrical considerations, to the observed brightness variations of the 1 March 1969 limb eruptive prominence. We find that all of the observed brightening and dimming can be explained by the motions, and that no significant change in the prominence N _e or T _e was necessary during the observed H event. This conclusion is significant in interpreting an X-ray burst that began as the prominence velocity increased abruptly at the time of maximum H intensity. The thermal X-ray peak occurred 150 sec later when the prominence had become faint again. There was no associated flare that was visible in H. We discuss the relative brightness of H and D ₃ in a specific moving prominence knot.We note that the observed range of limb speeds (30–150 km/sec) may be due to the combined H Doppler brightening and Lyman- dimming effects. We also discuss generally the H brightness of disk surges (bright and dark) and flares, and sprays and puffs that occur at or near the limb.Now at the Dept. of Physics and Astrophysics, University of Colorado, and High Altitude Observatory (NCAR) Boulder, Colo., U.S.A.     

The EUV chromospheric network in the quiet Sun

Investigations on the structure and intensity of the chromospheric network from quiet solar regions have been carried out with EUV data obtained from the Harvard spectroheliometer on the Apollo Telescope Mount of Skylab. The distribution of intensities within supergranulation cell interiors follows a near normal function, where the standard deviation exceeds the value expected from the counting rate, which indicates fine-scale structure below the 5 arc sec resolution of the data. The intensities from the centers of supergranulation cells appear to be the same in both quiet regions and coronal holes, although the network is significantly different in the two types of regions. The average halfwidth of the network elements was measured as 10 arc sec, and was independent of the temperature of formation of the observing line for 3.8 < log T _e < 5.8. The contrast between the network and the centers of cells is greatest for lines with log T _e 5.2, where the network contributes approximately 75% of the intensity of quiet solar regions. The contrast and fractional intensity contributions decrease to higher and lower temperatures characteristic of the corona and chromosphere.     

Determining the minimum sum of the component masses for a binary with a known parallax from observations of a short arc of its apparent motion

An inequality that allows the minimum sum of the component masses for a binary, M₀, to be determined was derived from simple geometric considerations. This quantity satisfies the observed orbital motion according to Newton's law with a known parallax. The M₀ value can be calculated if the apparent-motion parameters for the components, including the curvature of the observed short orbital arc, were determined from observations. We estimated M₀ for 14 Pulkovo program stars for which the apparent orbital arc covered with observations was no less than 10°. We compare M₀ with the masses estimated from the mass-luminosity relation. A significant mass excess was found for the star ADS 10329.     

Observational studies of the solar intensity profile in the far infrared and millimeter regions

Observations of the intensity distribution near the solar limb at 2.43 and 22.5 , show the absence of limb brightening to within 1 or 2 arc sec of the limb. Observations at 1.2 mm indicate limb brightening at this wavelength. These results are compared with the Utrecht Reference Photosphere and with existing data on the solar flux in the millimeter range, and suggest that the temperature minimum is broad and extends above ₅₀₀₀ = 2 × 10^–3. A sharp rise of temperature is required above ₅₀₀₀ = 10^–5.     

An infrared imaging search for low‐mass companions to members of the young nearby β Pic and Tucana/Horologium associations

We present deep high dynamic range infrared images of young nearby stars in the Tucana/Horologium and β Pic associations, all ∼10 to 35 Myrs young and at ∼10 to 60 pc distance. Such young nearby stars are well‐suited for direct imaging searches for brown dwarf and even planetary companions, because young sub‐stellar objects are still self‐luminous due to contraction and accretion. We performed our observations at the ESO 3.5m NTT with the normal infrared imaging detector SofI and the MPE speckle camera Sharp‐I. Three arc sec north of GSC 8047‐0232 in Horologium a promising brown dwarf companion candidate is detected, which needs to be confirmed by proper motion and/or spectroscopy. Several other faint companion candidates are already rejected by second epoch imaging. Among 21 stars observed in Tucana/Horologium, there are not more than one to five brown dwarf companions outside of 75 AU (1.5″ at 50 pc); most certainly only ≤5% of the Tuc/HorA stars have brown dwarf companions (13 to 78 Jupiter masses) outside of 75 AU. For the first time, we can report an upper limit for the frequency of massive planets (∼10 M_jup) at wide separations (∼100 AU) using a meaningfull and homogeneous sample: Of 11 stars observed sufficiently deep in β Pic (12 Myrs), not more than one has a massive planet outside of ∼100 AU, i.e. massive planets at large separations are rare (≤9%). (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

GAIA Photometric System: Evaluation of Performance

We present a method and results of evaluation of the photometric systems (PSs) proposed for the GAIA mission. The method, however, can be applied for the analysis of virtually any multicolour PS designed for the global investigation of the Galactic stellar populations. Performance of the 1F, 2A and 3G PSs is evaluated taking into account their ability to simultaneously determine the main stellar parameters,T _eff, log g, [M/H] and E _B-V, for a large variety of stars down to G ∼ 20. The representative Galactic stellar populations were constructed and employed for evaluation of the PSs. Despite the fact that the 2A and 3G PSs perform significantly better than the 1F (presently adopted as a baseline PS for GAIA), we conclude that still there is no photometric system proposed to date, which would allow to achieve the scientific objectives of the GAIA mission. This revised version was published online in July 2006 with corrections to the Cover Date.