Multi-Spectral Solar Telescope Array Research Articles

The Solar Chromosphere/Corona Interface. I. Far‐Ultraviolet to Extreme‐Ultraviolet Observations and Modeling of Unresolved Coronal Funnels

A coronal funnel model, developed by D. Rabin, was tested against a calibrated spectroheliogram recorded in the 170-1 75 A bandpass. This image was recorded on board a sounding-rocket experiment flown on 1994 November 3, called the Multi-Spectral Solar Telescope Array II (MSSTA II). MSSTA, a joint project of Stanford University, the NASA Marshall Space Flight Center, and the Lawrence Livermore National Laboratory' is an observing platform composed of a set of normal-incidence, multilayer-coated optics designed to obtain narrow-bandpass, high-resolution images (1 sec.- 3 sec.) at selected far-ultraviolet (FUV), extreme-ultraviolet (EUV), and soft X-ray wavelengths (44-1550 A). Using full disk images centered at 1550 A (C IV) and 173 A (Fe IX/X), the funnel model, which is based on coronal back-heating, was tested against the data incorporating observed constraints on global coverage and measured flux. Found was a class of funnel models that could account for the quiescent, globally diffuse and unresolved emission seen in the 171-175 A bandpass, where the funnels are assumed to be rooted in the C IV supergranular network. These models, when incorporated with the CHIANTI spectral code, suggest that this emission is mostly of upper transition region origin and primarily composed of Fe IX plasma. The funnels are found to have constrictions, Gamma approx. 6-20, which is in good agreement with the observations. Further, the fitted models simultaneously satisfy global areal constraints seen in both images; namely,that a global network of funnels must cover approx. 700 - 95% of the total solar surface area seen in the 171-175 A image, and a 5% of the disk area seen in the 1550 A bandpass. These findings support the configuration of the EUV magnetic network as suggested by Reeves et al. and put forth in more detail by Gabriel. Furthermore, the models are in good agreement with differential emission measure estimates made of the transition region by J. C. Raymond & J. G. Doyle for temperatures 250,000 K less than or = T less than or = 650,000 K, based on full-disk observations made on board Skylab.

High resolution imaging with multilayer telescopes: resolution performance of the MSSTA II telescopes

The Multi-Spectral Solar Telescope Array (MSSTA) is a sounding rocket-borne observatory composed of a set of normal-incidence multilayer-coated telescopes that obtained selected bandpass spectroheliograms (44 to 1550 Aa) of the solar atmosphere. These spectroheliograms were recorded on specially fabricated XUV and FUV 70-mm Kodak film. Rocket launches of this instrument payload took place in 1991 (MSSTA I) and 1994 (MSSTA II) at the White Sands Missile Test Range in New Mexico, sponsored by the National Aeronautics and Space Administration (NASA) sounding rocket experiment program. Immediately prior to the 1994 launch, visible light focusing tests of each telescope were performed in situ using a 1951 standard Air Force high-resolution test target, to measure optical resolution performance. We determined that the MSSTA II telescopes performed at diffraction-limited resolutions down to 0.70 arcsec at visible wavelengths. Based on these measurements, we calculate an upper bound to the focusing errors that incorporate the sum of all uncorrelated system focus errors that affect resolution performance. Coupling these upper bound estimates with the in-band diffraction limits, surface scattering errors and payload pointing jitter, we demonstrate that 11 of 19 MSSTA II telescopes--having negligible figures of focus errors in comparison to the corresponding visible diffraction limits--performed atmore » sub arcsecond resolution at their operational FUV/EUV/XUV wavelengths during flight. We estimate the in-band performance down to 0.14{+-}0.08 arcsec. (c) 2000 Society of Photo-Optical Instrumentation Engineers.« less

Observation and Modeling of the Solar Transition Region. I. Multi‐Spectral Solar Telescope Array Observations

We report on observations of the solar atmosphere in several extreme-ultraviolet and far-ultraviolet bandpasses obtained by the Multi-Spectral Solar Telescope Array, a rocket-borne spectroheliograph, on flights in 1987, 1991, and 1994, spanning the last solar maximum. Quiet-Sun emission observed in the 171-175 A bandpass, which includes lines of O V, O VI, Fe IX, and Fe X, has been analyzed to test models of the temperatures and geometries of the structures responsible for this emission. Analyses of intensity variations above the solar limb reveal scale heights consistent with a quiet-Sun plasma temperature of 500,000 ≤ Te ≤ 800,000 K. The structures responsible for the quiet-Sun EUV emission are modeled as small quasi-static loops. We submit our models to several tests. We compare the emission our models would produce in the bandpass of our telescope to the emission we have observed. We find that the emission predicted by loop models with maximum temperatures between 700,000 and 900,000 K are consistent with our observations. We also compare the absolute flux predicted by our models in a typical upper transition region line to the flux measured by previous observers. Finally, we present a preliminary comparison of the predictions of our models with diagnostic spectral line ratios from previous observers. Intensity modulations in the quiet Sun are observed to occur on a scale comparable to the supergranular scale. We discuss the implications that a distribution of loops of the type we model here would have for heating the local network at the loops' footpoints.

Observation and Modeling of Soft X‐Ray Bright Points. II. Determination of Temperature and Energy Balance

The Multispectral Solar Telescope Array was launched on 1991 May 13, in order to obtain narrowband images of the solar corona and transition region. Calibrated EUV and H Lyman-α images are used to constrain a simple loop model of the energy balance in 23 X-ray bright points (XBPs). The results are consistent with substantial back heating of the lower transition region by thermal conduction from the corona. Statistical analysis shows that length and temperature are uncorrelated for XBP loops; implications of this finding for coronal heating are discussed. Some evidence is found for a systematic imbalance between radiative and conductive losses. Inferences of effective loop cross section by modeling suggest that the coronal filling factor is nearly unity.

CHROMOSPHERIC AND CORONAL STRUCTURE OF POLAR PLUMES – I. Magnetic Structure and Radiative Energy Balance

The Multi-Spectral Solar Telescope Array (MSSTA), a rocket-borne solar observatory, was successfully launched from White Sands Missile Range, New Mexico, on May 13, 1991 at 19:05 UT. The telescope systems onboard the MSSTA obtained several full disk solar images in narrow bandpasses centered around strong soft X-ray, EUV, and FUV emission lines. Each telescope was designed to be sensitive to the coronal plasmas at a particular temperature, for seven temperatures ranging from 20000 K to 4000000 K. We report here on the images obtained during the initial flight of the MSSTA, and on the chromospheric and coronal structure of polar plumes observed over both poles of the Sun. We have also co-aligned the MSSTA images with Kitt Peak magnetograms taken on the same day. We are able to positively identify the magnetic structures underlying the polar plumes we analyze as unipolar. We discuss the plume observations and present a radiative energy balance model derived from them.

Observation and Modeling of Soft X-Ray Bright Points. I. Initial Results

The Multi-Spectral Solar Telescope Array was launched from White Sands Missile Range, New Mexico, on 1991 May 13 at 1905 UT. Full-disk, high-resolution solar images were obtained in a variety of soft X-ray and far-ultraviolet wavelengths. The 193 A (FeXII) and 44 A (SiXII) images show a large number of coronal bright points. The high spatial resolution of the FeXII image allows many of the bright points to be resolved as tiny loops. Co-alignment of the soft X-ray images with the 1216 A Ly{alpha} image reveals that all the coronal bright points have counterparts in the transition region, often resolved as a pair of footprints, which are brighter than neighboring elements of the chromospheric network. Moreover, comparison with the KPNO magnetogram shows dipole structures coincident with nearly all of the bright points. We present a quantitative analysis based on preliminary photometry of four of the bright points that were observed. By fitting a simple, numerical loop model to the photometric data, we estimate the magnitude of the coronal heating in these structures. The rate of heating per unit footpoint area is found to be similar to previous measurements for much larger coronal structures. Implications for heating of the chromosphere andmore » lower transition region are also discussed. {copyright} {ital 1996 The American Astronomical Society.}« less

Multi-Spectral Solar Telescope Array V: Temperature diagostic response to the optically thin solar plasma

The compact soft X-ray/EUV/FUV multilayer coated telescopes developed for solar chromosphere, corona, and corona/solar-wind interface studies permit the use of conventional (Cassegrain, Herschelian, etc.) configurations. The multilayer coatings also allow a narrow-wavelength band to be selected for imaging. NASA's Multi-Spectral Solar Telescope Array is composed of 17 of these compact telescopes; attention is given to their ability to obtain temperature-diagnostic information concerning the solar plasma.

EUV/FUV response characteristics of photographic films for the Multi-Spectral Solar Telescope Array

The photographic film employed by NASA's Multi-Spectral Solar Telescope Array must have high-to-ultrahigh resolution; since the spacecraft bearing the telescope must be evacuated to prevent the failure of delicate EUV and soft X-ray filters due to acoustic vibration during launch, the films must also have very low outgassing rates. An account is presently given of the properties of important new emulsions selected for flight, together with response-characteristics data for the experimental XUV 100 film and an uncoated Spectroscopic 649 emulsion.

Multi-Spectral Solar Telescope Array IV: the soft x-ray and extreme ultraviolet filters

NASA's Multi-Spectral Solar Telescope Array uses various combinations of thin foil filters composed of Al, C, Te, Be, Mo, Rh, and phthalocyanine to achieve the requisite radiation-rejection characteristics. Such rejection is demanded by the presence of strong EUV radiation at longer wavelengths where the specular reflectivity of multilayer mirrors can cause 'contamination' of the image in the narrow band defined by the Bragg condition.

Multi-Spectral Solar Telescope Array II: soft x-ray EUV reflectivity of the multilayer mirrors

We have developed seven compact soft x-ray/EUV (XUV) multilayer- coated and two compact FUV interference-film-coated Cassegrain and Ritchey-Chrétien telescopes for a rocket-borne observatory, the Multi-Spectral Solar Telescope Array. We report here on extensive measurements of the efficiency and spectral band pass of the XUV telescopes carried out at the Stanford Synchrotron Radiation Laboratory.

Astronomical observations with normal incidence multilayer optics: recent results and future prospects

We have obtained high resolution (~1 arcsec) X-ray and EUV images of the solar corona with two rocket-borne normal incidence Cassegrain multilayer telescopes. The Cassegrain images correspond to material at ~1 000 000 K (λ ~ 173 Å) and ~ 80 000 K (λ ~ 256 Å). Other images, at somewhat lower resolution, were obtained with Herschelian multilayer telescopes and Wolter-Schwarzschild hybrid grazing incidence/multilayer optical systems. We discuss the Cassegrain and Herschelian images, emphasizing the analysis of polar plumes observed at the limb of the Sun. Polar plumes are thought to represent structures that contribute significantly to the flow of coronal material into the solar wind. We will also comment briefly on other coronal features observed, including coronal holes, the chromospheric network, active region loops, and prominence cavities. Multilayer technology represents a major breakthrough in astronomical X-ray and EUV imaging; we will discuss the application of specific multilayer techniques to astronomical observations, emphasizing observations of the sun. We will also report on current efforts in our laboratories to develop a comprehensive Multi-Spectral Solar Telescope Array rocket instrument for the study of a wide variety of problems related to the solar atmosphere.

Multi-Spectral Solar Telescope Array

The development of multilayer optics has profound implications for soft x-ray/extreme ultraviolet (XUV) astronomy. During the October 1987 flight of the Stanford/Marshall Space Flight Center Rocket X-Ray Spectroheliograph, narrow wavelength band, low scatter soft x-ray/extreme ultraviolet spectroheliograms were obtained with -1 arcsec spatial resolution at X - 173 A (Fe IX, Fe X) and at is. - 256 A (He II Ly-3). Although the Cassegrain telescopes used in this experiment were small (63.5 mm diameter) and utilized spherical rather than paraboloidal/hyperboloidal mirrors, the images produced exceed in quality any XUV spectroheliograms previously obtained with either normal or grazing incidence techniques. We describe a new rocket spectroheliograph instrument, the Multi- Spectral Solar Telescope Array (MSSTA), that is currently being prepared for launch in August 1990. This instrumentwill utilizetrue Ritchey-Chrétien optics of 127 mm diameter and parabolic Herschelian optics of 40 mm diameter, which will allow spectroheliograms to be obtained over the soft x-ray/extreme ultraviolet/far ultraviolet spectral range (40 to 1550 A). The performance of this new instrument should definitely demonstrate the unique combination of ultrahigh spatial resolution and spectral differentiation that multilayer optics afford for astronomical observations. The MSSTA will also represent the first astronomical use of an important new optical device, the multilayer grating. The MSSTA should obtain unprecedented information regarding the structure and dynamics of the solar atmosphere in the temperature range i04 to i07 K.

Performance of compact multilayer coated telescopes at soft x-ray/EUV and far ultraviolet wavelengths I

We have developed compact soft x-ray/EUV (XUV) and farultraviolet (FUV) multilayer coated telescopes for the study of the solar chromosphere corona, and corona/solar wind interface. Because they operate at short wavelengths (-40 < X < 1550 A), the modest apertures of 40 to 127 mm allow observations at very high angular resolution (0.1 to 0.7 arcsec). In addition to permitting traditional normal incidence optical configurations such as Cassegrain, Ritchey-Chrétien, and Herschelian to be used at XUV wavelengths, multilayer coatings also allow a narrow wavelength band (X/zX - 30 to 100) to be selected for imaging. The resuIting telescopes provide a very powerful and flexible diagnostic instrument for the study of both the fine scale structure of the chromosphere/ corona interface and the large scale structure of the corona and corona/solar wind interface. In previous papers we described a new solar rocket payload, the Multi-Spectral Solar Telescope Array (MSSTA), composed of 17 of these compact telescopes. In this paper we report on the performance of the seven MSSTA Ritchey-Chrétien telescopes.