UVCS/SOHO PAPERS - 1996


1996-12:   The Triple Current Sheet Model for Adjoining Helmet Streamers

Karpen, J. T.; Dahlburg, R. B.

ASP, 95, 333, ed. K. S. Balasubramaniam, Stephen L. Keil, and Raymond N. Smartt

The highly structured magnetic field and plasma properties observed in the heliospheric extension of the coronal streamer belt have been interpreted as evidence for multiple current sheets originating at coronal helmet streamers. We explore the linear stability of a simple case: a triple current sheet, as would exist above two neighboring helmets of the same polarity. The velocity and magnetic fields which characterize the three unstable modes, and the modal dependence on current sheet separation, are shown and discussed. We conclude that the magnetic topology in closely packed streamers is susceptible to instabilities with growth times on the order of hours. Our predictions indicate that the resultant plasmoid structures should be observable with the LASCO and UVCS instruments on the upcoming SOHO mission



1996-11:   Advances in solar and stellar physics: space studies

Foing, B. H.

ASP Conference Series, volume 109, p.31, edited by Roberto Pallavicini and Andrea K. Dupree

The following topics were dealt with: advances in the physics of sunspots, magnetic flux tube formation, chromospheric magnetic fields, sunspot models, radio observations of sunspots, inversion techniques, sunspot spectropolarimetric data, coordination of SOHO and ground-based observations, GOLF experiment, SUMER spectrometer observations, EIT results, UVCS results, coordinated prominence observations, SOHO and VLA observations, THEMIS capabilities, observing techniques, solar databases, 1999 eclipse, VIRGO experiment.



1996-10:   Stray light, radiometric, and spectral characterization of UVCS/SOHO: laboratory calibration and flight performance

GARDNER, LARRY D.; KOHL, J. L.; DAIGNEAU, P. S.; DENNIS, E. F.; FINESCHI, SILVANO; MICHELS, J.; NYSTROM, GEORGE U.; PANASYUK, ALEXANDER; RAYMOND, JOHN C.; REISENFELD, D. J.; SMITH, PETER L.; STRACHAN, LEONARD; SULEIMAN, R.; NOCI, G. C.; ROMOLI, MARCO; CIARAVELLA, A.; MODIGLIANI, A.; HUBER, MARTIN C.; ANTONUCCI, E.; BENNA, C.; GIORDANO, S.; TONDELLO, GIUSEPPE; NICOLOSI, PIERGIORGIO; NALETTO, GIAMPIERO; PERNECHELE, CLAUDIO; SPADARO, D.; SIEGMUND, OSWALD H.; ALLEGRA, A.; CAROSSO, PAOLO A.; JHABVALA, MURZY D.

1996, SPIE Vol. 2831, p. 2-24, Ultraviolet Atmospheric and Space Remote Sensing: Methods and Instrumentation, Robert E. Huffman; Christos G. Stergis; Eds.

The Ultraviolet Coronagraph Spectrometer is one of the instruments on board the Solar and Heliospheric Observatory spacecraft, which was launched in December, 1995. The instrument is designed to make ultraviolet spectrometric measurements and visible polarimetric measurements of the extended solar corona. Prior to launch laboratory measurements were carried out to determine system level values for many of the key performance parameters. Further measurements on instrument performance have been carried out since launch. Presented are descriptions of measurement techniques and representative results. 


1996-09:   Non-thermal velocities in flare plasmas

 E. Antonucci, C. Benna, M.A. Dodero, R. Martin

1996, Adv. Space Res. 17, 47-50

The analysis of the profile and intensity of the soft X-ray lines formed in the wavelength range from 1.8 AA to 20 AA, observed with the Soft X-ray Polychromator of the Solar Maximum Mission (SMM), shows that solar flare plasmas are characterized by non-thermal velocities. Flare plasmas in general consist of two components at different temperatures: 5-8*10/sup 6/ K and 16-25*10/sup 6/ K, respectively. The non-thermal mass motions observed in the hotter component are affected by higher velocities and they depend on temperature in a way which is varying with the flare phase. The results obtained for flare plasmas are consistent with the functional relationship between temperature and non-thermal velocity derived for the lower temperature regimes existing in the upper chromosphere, in the transition region and in the non-flaring corona.


1996-08:   EUV spectral lines from solar wind source regions: the resonance doublets of N V, Mg X and Si XII, and the Fe XII 1242 forbidden line.

SPADARO, D.; VENTURA, R.

1996, Astronomy and Astrophysics Supplement, v.115, p.531-542

We present numerical simulations of intensities and profiles of the N V {lambda}1239 and {lambda} 1243, Mg X {lambda}610 and {lambda}625, Si XII {lambda}499 and {lambda}521, and Fe XII {lambda}1242 lines expected from an equatorial and polar coronal hole at solar minimum, when observed on the plane of the sky in the 1.2R_{sun}_-8R_{sun}_ range of heliocentric distance. The radial variations of coronal temperatures, densities and outflow speeds inside these structures are those determined by Withbroe (1988) through a one-fluid steady-state radiative energy balance model. We discuss how the spectral features depend on the physical parameters of the examined structures, in order to evaluate to which extent they can put constraints on empirical models of the extended corona. We also investigate the effects of the deviations from ionization equilibrium occurring in the plasma outflowing from the considered coronal structures on the intensity and profile of the spectral lines examined. Such effects become significant at heliocentric distances higher than 3R_{sun}_. These results may contribute to prepare and interpret UVCS/SOHO future observations.


1996-07:   Interpretation of the observed plasma `turbulent' velocities as the result of magnetic reconnection in solar flares

 E. Antonucci, C. Benna, B.V. Somov

ApJ, 456, 833-839


1996-06:   Modeling High Flow Speeds in the Inner Corona

Ruth Esser and Shadia Rifai Habbal

1996, in The 8th Solar Wind Conference, Eds. Winterhalter, Gosling, Habbal, Kurth, Neugebauer, AIP-382, 133

Following recent observations which indicate the possibility of extremely high flow speeds in the inner corona, 700 - 800 km s^{-1} below 10 R_S, and he possibility of very high proton temperatures, T_p \leq 8.5 x 10^{6} K, we present a new approach to solar wind modeling. In this approach we show that if the high proton temperatures in the inner corona are genuine, then flow speeds of 700 to 800 km s^{-1} can readily be achieved at 10 R_S or even closer to the coronal base.


1996-05:   Propagation of three-dimensional Alfven waves in a stratified, thermally conducting solar wind

Y.-Q. Lou, R. Rosner, D. Lenz, G. Peres, S. Orlando

1996, J. Geophys. Res.

We model the propagation of three--dimensional, adiabatic, linear Alfven waves in the solar atmosphere and wind, taking into account relevant physical effects, including gravity stratification, thermal conduction, radiative losses, and heating (via a phenomenological term). Our magnetohydrodynamic solar wind model also accounts for the momentum deposition by a spectrum of non--WKB Alfven waves. The transmission and reflection of such waves has been previously studied by a variety of techniques, including calculations based on the computation of the ratio between the wavelength and the scale length of the Alfven speed change, and based on a globally--computed transmission coefficient. In this paper, we discuss both techniques, and show how they are related. We also discuss the physics underlying the reflection process, and the possible role wave reflection might play in the acceleration of the solar wind and the winds from other stars.


1996-04:   Volumetric Heating in Coronal Streamers

Suess, S. T., A.-H. Wang, and S. T. Wu,

1996, J. Geophys. Res., 101, 19957-19966

The addition of a volumetric heat source to a coronal streamer model produces distinct, important changes in the model. Originally such heating was added to meet the observational requirement for a thin current sheet above streamers. Here we report additional consequences of a volumetric heat source, together with the effects of redistribution of heat through thermal conduction. Specifically, we address the question of whether a heat source will allow a truly steady state to be achieved in the presence of thermal conduction, something known to be impossible for an adiabatic gas. The heat source causes a slow, continuing expansion and stripping of magnetic flux from the top of a streamer until the streamer essentially evaporates and the field is fully open to the interplanetary medium after an interval which depends on the magnitude of the source but typically varies from weeks to months. We find that thermal conduction does not quench the evaporation. We also find that the heat source, in the absence of other processes, must depend on the magnetic field geometry to simulate both the thinness of current sheets above streamers and the low density/high flow speed in coronal holes. Finally, we find that the expansion is not necessarily continuous.


1996-03:   Spectral Resolution Improvement Technique for a Spectrograph Mounting a Discrete Array Detector

C. Pernechele, L. Poletto, P. Nicolosi, G. Naletto, University of Padua

1996, Opt. Eng., Vol. 35 No. 5, 1503-1510

A new technique has been developed for improving the spectral resolution of spectrographs mounting discrete array detectors. The basic concept is to acquire various spectroscopic, each one being relatively shifted on the detector by a fraction of the pixel, i.e. the detector resolution element, and to apply suitable numerical procedures in order to extract a spectral profile with subpixel resolution. This technique has been applied to a vacuum spectrograph adopting the Johnson-Onaka configuration. The dispersion element is a concave toroidal grating hich can rotate around a pivot axis displaced from its vertex, in order to maintain a good spectral focus on the detector. The latter is a multi-anode microchannel-plate array (MAMA), and operates in the photon counting mode. Several stigmatic images of the H I Lya line at 1216 A emitted by a D2 lamp have been acquired for various rotation of the grating. The results of the application of this new technique and the numerical algorithm are presented and discussed in terms of potentialities and limitations due to the signal to noise (S/N) ratio and intrinsic spectral broadening of the signals.


1996-02:   Searching for coronal plumes in Ulysses observations of the far solar wind

Poletto, G., S. Parenti, G. Noci, S. Livi, S. T. Suess, A. Balogh, and D. J. McComas

1996, Astron. & Astrophys. 316, 374-383

In the past, from the analysis of data acquired by the Helios spacecrafts within distances


1996-01:   Latitudinal dependence of the radial IMF component - interplanetary imprint

Suess, S. T., E. J. Smith, J. L. Phillips, B. E. Goldstein, and S. Nerney

1996, Astron. & Astrophys., 316, 304-312

Ulysses measurements have confirmed that there is no significant gradient with respect to heliomagnetic latitude in the radial component, B/sub r/, of the interplanetary magnetic field. There are two processes responsible for this observation. In the corona, the plasma beta is <<1, except directly above streamers, so both longitudinal and latitudinal (meridional) gradients in field strength will relax, due to the transverse magnetic pressure gradient force, as the solar wind carries magnetic flux away from the Sun. This happens so quickly that the field is essentially uniform by 5 R/sub (.)/. Beyond 10 R/sub (.)/, beta >1 and it is possible for a meridional thermal pressure gradient to redistribute magnetic flux-an effect apparently absent in Ulysses and earlier ICE and IMP data. We discuss this second effect, showing that its absence is mainly due to the perpendicular part of the anisotropic thermal pressure gradient in the interplanetary medium being too small to drive significant meridional transport between the Sun and ~4 AU. This is done using a linear analytic estimate of meridional transport.