Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models

Motivated by new sounding-rocket wide-field polarimetric images of the Large Magellanic Cloud, we have used a three-dimensional Monte Carlo radiation transfer code to investigate the escape of near-ultraviolet photons from young stellar associations embedded within a disk of dusty material (i.e. a galaxy). As photons propagate through the disk, they may be scattered or absorbed by dust. Scattered photons are polarized and tracked until they escape to be observed; absorbed photons heat the dust, which radiates isotropically in the far-infrared, where the galaxy is optically thin. The code produces four output images: near- UV and far-IR flux, and near-UV images in the linear Stokes parameters Q and U. From these images we construct simulated UV polarization maps of the LMC. We use these maps to place constraints on the star + dust geometry of the LMC and the optical properties of its dust grains. By tuning the model input parameters to produce maps that match the observed polarization maps, we derive information about the inclination of the LMC disk to the plane of the sky, and about the scattering phase function g. We compute a grid of models with i = 28 deg., 36 deg., and 45 deg., and g = 0.64, 0.70, 0.77, 0.83, and 0.90. The model which best reproduces the observed polarization maps has i = 36 +2/-5 degrees and g ~0.7. Because of the low signal-to-noise in the data, we cannot place firm constraints on the value of g. The highly inclined models do not match the observed centro-symmetric polarization patterns around bright OB associations, or the distribution of polarization values. Our models approximately reproduce the observed ultraviolet photopolarimetry of the western side of the LMC; however, the output images depend on many input parameters and are nonunique.Comment: Accepted to AJ. 20 pages, 7 figure

EIT Observations of the Extreme Ultraviolet Sun

The Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft has been operational since 2 January 1996. EIT observes the Sun over a 45 x 45 arc min field of view in four emission line groups: Feix, x, Fexii, Fexv, and Heii. A post-launch determination of the instrument flatfield, the instrument scattering function, and the instrument aging were necessary for the reduction and analysis of the data. The observed structures and their evolution in each of the four EUV bandpasses are characteristic of the peak emission temperature of the line(s) chosen for that bandpass. Reports on the initial results of a variety of analysis projects demonstrate the range of investigations now underway: EIT provides new observations of the corona in the temperature range of 1 to 2 MK. Temperature studies of the large-scale coronal features extend previous coronagraph work with low-noise temperature maps. Temperatures of radial, extended, plume-like structures in both the polar coronal hole and in a low latitude decaying active region were found to be cooler than the surrounding material. Active region loops were investigated in detail and found to be isothermal for the low loops but hottest at the loop tops for the large loops

UV and EUV Instruments

We describe telescopes and instruments that were developed and used for astronomical research in the ultraviolet (UV) and extreme ultraviolet (EUV) regions of the electromagnetic spectrum. The wavelength ranges covered by these bands are not uniquely defined. We use the following convention here: The EUV and UV span the regions ~100-912 and 912-3000 Angstroem respectively. The limitation between both ranges is a natural choice, because the hydrogen Lyman absorption edge is located at 912 Angstroem. At smaller wavelengths, astronomical sources are strongly absorbed by the interstellar medium. It also marks a technical limit, because telescopes and instruments are of different design. In the EUV range, the technology is strongly related to that utilized in X-ray astronomy, while in the UV range the instruments in many cases have their roots in optical astronomy. We will, therefore, describe the UV and EUV instruments in appropriate conciseness and refer to the respective chapters of this volume for more technical details.Comment: To appear in: Landolt-Boernstein, New Series VI/4A, Astronomy, Astrophysics, and Cosmology; Instruments and Methods, ed. J.E. Truemper, Springer-Verlag, Berlin, 201

Imaging the solar corona in the EUV

The SOHO (SOlar and Heliospheric Observatory) satellite was launched on December 2nd 1995. After arriving at the Earth-Sun (L1) Lagrangian point on February 14th 1996, it began to continuously observe the Sun. As one of the instruments onboard SOHO, the EIT (Extreme ultraviolet Imaging Telescope) images the Sun's corona in 4 EUV wavelengths. The He II filter at 304 AË images the chromosphere and the base of the transition region at a temperature of 5 - 8 x 10^4 K; the Fe IX-X filter at 171 AË images the corona at a temperature of ~ 1.3 x 10^6 K; the Fe XII filter at 195 AË images the quiet corona outside coronal holes at a temperature of ~ 1.6 x 10^6 K; and the Fe XV filter at 284 AË images active regions with a temperature of ~ 2.0 x 10^6 K. About 5000 images have been obtained up to the present. In this paper, we describe also some aspects of the telescope and the detector performance for application in the observations. Images and movies of all the wavelengths allow a look at different phenomena present in the Sun's corona, and in particular, magnetic field reconnection

Observations of Coronal Structures Above an Active Region by EIT and Implications for Coronal Energy Deposition

Solar EUV images recorded by the EUV Imaging Telescope (EIT) on SOHO have been used to evaluate temperature and density as a function of position in two largescale features in the corona observed in the temperature range of 1.0-2.0MK. Such observations permit estimates of longitudinal temperature gradients (if present) in the corona and, consequently, estimates of thermal conduction and radiative losses as a function of position in the features. We examine two relatively cool features as recorded in EIT's Feix/x (171Å) and Fexii (195Å) bands in a decaying active region. The first is a long-lived loop-like feature with one leg, ending in the active region, much more prominent than one or more distant footpoints assumed to be rooted in regions of weakly enhanced field. The other is a near-radial feature, observed at the West limb, which may be either the base of a very high loop or the base of a helmet streamer. We evaluate energy requirements to support a steady-state energy balance in these features and find in both instances that downward thermal conductive losses (at heights above the transition region) are inadequate to support local radiative losses, which are the predominant loss mechanism. The requirement that a coronal energy deposition rate proportional to the square of the ambient electron density (or pressure) is present in these cool coronal features provides an additional constraint on coronal heating mechanisms

Association of Extreme-Ultraviolet Imaging Telescope (EIT) Polar Plumes with Mixed-Polarity Magnetic Network

SOHO EIT spectroheliograms showing the polar coronal holes during the present sunspot minimum are compared with National Solar Observatory (Kitt Peak) magnetograms taken in Fe I lambda 8688 and Ca II lambda 8542. The chromospheric lambda 8542 magnetograms, obtained on a routine, near-daily basis since 1996 June, reveal the Sun's strong polar fields with remarkable clarity. We find that the Fe IX lambda 171 polar plumes occur where minority-polarity flux is in contact with flux of the dominant polarity inside each polar hole. Moreover, the locations of "plume haze" coincide approximately with the patterns of brightened He II lambda 304 network within the coronal hole. The observations appear to be consistent with mechanisms of plume formation involving magnetic reconnection between unipolar flux concentrations and nearby bipoles. The fact that minority-polarity fields constitute only a small fraction of the total magnetic flux within the polar holes suggests that plumes are not the main source of the high-speed polar wind