L. D. Gardner

UVCS/SOHO Empirical Determinations of Anisotropic Velocity Distributions in the Solar Corona

We present a self-consistent empirical model for several plasma parameters of a polar coronal hole near solar minimum, derived from observations with the Solar and Heliospheric Observatory Ultraviolet Coronagraph Spectrometer. The model describes the radial distribution of density for electrons, H0, and O5 + and the outflow velocity and unresolved most probable velocities for H0 and O5 + during the period between 1996 November and 1997 April. In this Letter, we compare observations of H I Lyα and O VI λλ1032, 1037 emission lines with spatial models of the plasma parameters, and we iterate for optimal consistency between measured and synthesized observable quantities. The unexpectedly large line widths of H0 atoms and O5 + ions at most radii are the result of anisotropic velocity distributions, which are not consistent with purely thermal motions or the expected motions from a combination of thermal and transverse wave velocities. Above 2 R, the observed transverse, most probable speeds for O5 + are significantly larger than the corresponding motions for H0, and the outflow velocities of O5 + are also significantly larger than the corresponding velocities of H0. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration.

An Empirical Model of a Polar Coronal Hole at Solar Minimum

We present a comprehensive and self-consistent empirical model for several plasma parameters in the extended solar corona above a polar coronal hole. The model is derived from observations with the SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) during the period between 1996 November and 1997 April. We compare observations of H I Lyα and O VI λλ1032, 1037 emission lines with detailed three-dimensional models of the plasma parameters and iterate for optimal consistency between measured and synthesized observable quantities. Empirical constraints are obtained for the radial and latitudinal distribution of density for electrons, H0, and O5+, as well as the outflow velocity and unresolved anisotropic most probable speeds for H0 and O5+. The electron density measured by UVCS/SOHO is consistent with previous solar minimum determinations of the white-light coronal structure; we also perform a statistical analysis of the distribution of polar plumes using a long time series. From the emission lines we find that the unexpectedly large line widths of H0 atoms and O5+ ions at most heights are the result of anisotropic velocity distributions. These distributions are not consistent with purely thermal motions or the expected motions from a combination of thermal and transverse wave velocities. Above 2 R☉, the observed transverse most probable speeds for O5+ are significantly larger than the corresponding motions for H0, and the outflow velocities of O5+ are also significantly larger than the corresponding velocities of H0. Also, the latitudinal dependence of intensity constrains the geometry of the wind velocity vectors, and superradial expansion is more consistent with observations than radial flow. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration.

The Ultraviolet Coronagraph Spectrometer for the Solar and Heliospheric Observatory

The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The purpose of the UVCS instrument is to provide a body of data that can be used to address a broad range of scientific questions regarding the nature of the solar corona and the generation of the solar wind. The primary scientific goals are the following: to locate and characterize the coronal source regions of the solar wind, to identify and understand the dominant physical processes that accelerate the solar wind, to understand how the coronal plasma is heated in solar wind acceleration regions, and to increase the knowledge of coronal phenomena that control the physical properties of the solar wind as determined by in situ measurements. To progress toward these goals, the UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliocentric height of 12 solar radii.

First Results from the SOHO Ultraviolet Coronagraph Spectrometer

The SOHO Ultraviolet Coronagraph Spectrometer (UYCS/SOHO) is being used to observe the extended solar corona from 1.25 to 10 R from Sun center. Initial observations of polar coronal holes and equatorial streamers are described. The observations include measurements of spectral line profiles for H I Lα and Lβ, O VI 1032 A and 1037 A, Mg × 625 A, Fe XII 1242 A and several others. Intensities for Mg × 610 A, Si XII 499 A, and 520 A, S × 1196 A, and 22 others have been observed. Preliminary results for derived H0, O5+, Mg9+, and Fe11+ velocity distributions and initial indications of outflow velocities for O5+ are described. In streamers, the H0 velocity distribution along the line of sight (specified by the value at e-1, along the line of sight) decreases from a maximum value of about 180 km s-1 at 2 R to about 140 km s-1 at 8 R. The value for O5+ increases with height reaching a value of 150 km s-1 at 4.7 R. In polar coronal holes, the O5+ velocity at e-1 is atout equal to that of H0 at 1.7 R and significantly larger at 2.1 R. The O5+ in both streamers and coronal holes were found to have amsotropic velocity distributions with the smaller values in the radial direction.

Elemental Abundances and Post-Coronal Mass Ejection Current Sheet in a Very Hot Active Region

A peculiar young active region was observed in 1998 March with the Ultraviolet Coronagraph Spectrometer (UVCS) over the southwest limb. The spectra showed strong emission in the λ974 line of fluorine-like iron, [Fe XVIII], which is brightest at an electron temperature of 106.8 K, and lines of Ne IX, [Ca XIV], [Ca XV], Fe XVII, [Ni XIV], and [Ni XV]. It is the only active region so far observed to show such high temperatures 0.5 R☉ above the solar limb. We derive the emission measure and estimate elemental abundances. The active region produced a number of coronal mass ejections (CMEs). After one CME on March 23, a bright post-CME arcade was seen in EIT and Yohkoh/SXT images. Between the arcade and the CME core, UVCS detected a very narrow, very hot feature, most prominently in the [Fe XVIII] line. This feature seems to be the reconnection current sheet predicted by flux rope models of CMEs. Its thickness, luminosity, and duration seem to be consistent with the expectations of the flux rope models for CME. The elemental abundances in the bright feature are enhanced by a factor of 2 compared to those in the surrounding active region, i.e., a first ionization potential enhancement of 7-8 compared to the usual factor of 3-4.

EUV Spectral Line Profiles in Polar Coronal Holes from 1.3 to 3.0 R

Spectral line profiles have been measured for H I λ1216, O VI λλ1032, 1037, and Mg X λ625 in a polar coronal hole observed during 1997 September 15-29, at projected heliographic heights ρ between 1.34 and 2.0 R☉. Observations of H I λ1216 and the O VI doublet from 1997 January for ρ=1.5-3.0 R☉ are provided for comparison. The O VI lines are well fit to a narrow and broad component which appear to be associated with regions of higher and lower spectral radiance, respectively. The narrow components dominate at low heights and become a small fraction of the lines at higher heights. Mg X λ625 is observed to have a narrow component at ρ=1.34 R☉ which accounts for only a small fraction of the observed spectral radiance. In the case of the broad components, the values of v1/e for O VI are only slightly larger than those for H I at ρ=1.34 R☉ but are significantly larger at ρ=1.5 R☉ and much larger for ρ>1.75 R☉. In contrast, the Mg X values are less than those of H I up to 1.75 and then increase rapidly up to at least ρ=2.0 R☉ but never reach the values of O VI.

Solar and Heliospheric Observatory Observations of a Helical Coronal Mass Ejection

The EUV Imaging Telescope (EIT), Large Angle Spectrometric Coronagraph (LASCO), and Ultraviolet Coronagraph Spectrometer (UVCS) instruments aboard the SOHO satellite observed a prominence eruption (coronal mass ejection) on 1997 December 12. Ejected plasma moved at about 130 km s-1 in the plane of the sky and showed Doppler shifts between -350 and +30 km s-1. The eruption appeared as a strongly curved arch in EIT images low in the corona. Emission in ions ranging from Si III to O VI in the UVCS spectra indicates a temperature range between 104.5 and 105.5 K. The morphology of the bright emission regions seen by all three instruments suggests several strands of a helical structure of moderate pitch angle. A reasonable fit to the spatial structure and the velocity evolution measured by UVCS is provided by a left-handed helix untwisting at a rate of about 9 × 10-4 radians s-1.

Measurement of Hydrogen Velocity Distributions in the Extended Solar Corona

H I Lyα spectral line profiles have been measured in polar regions of the solar corona at projected heliocentric heights of 1.8-3.5 R☉. Observations were made with the Ultraviolet Coronal Spectrometer on the Spartan 201 satellite from 16:52 to 04:04 UT on 1993 April 11-12 (south pole) and from 12:28 to 22:09 UT on 1993 April 12 (north pole). In general, the coronal profiles cannot be accurately curve-fitted with a single-Gaussian function. The fits with two Gaussians yield most probable velocities of 158 and 322 km s-1 (south) and 98 and 266 km s-1 (north). These parameters vary by less than 10% (1 σ) over the observed heights. The observations are consistent with a line-of-sight model that attributes the narrow component to background streamers and to sites within polar coronal holes or a surrounding diffuse corona, and the broad component to the polar coronal holes and/or plumes. This interpretation suggests that there are regions within the observed coronal holes that have hydrogen and proton kinetic temperatures of (4-6) × 106 K, which is 4-10 times higher than the expected electron temperatures at the same heights. However, other models with, for example, local non-Maxwellian velocity distributions are also consistent with the observations.

First results from UVCS/SOHO

Abstract We present here the first results obtained by the Ultraviolet Coronagraph Spectrometer (UVCS) operating on board the SOHO satellite. The UVCS started to observe the extended corona at the end of January 1996; it routinely obtains coronal spectra in the 1145 A – 1287 A, 984 A – 1080 A ranges, and intensity data in the visible continuum. Through the composition of slit images it also produces monocromatic images of the extended corona. The performance of the instrument is excellent and the data obtained up to now are of great interest. We briefly describe preliminary results concerning polar coronal holes, streamers and a coronal mass ejection, in particular: the very large r.m.s. velocities of ions in polar holes (hundreds km/sec for OVI and MgX); the puzzling difference between the HI Ly-α image and that in the OVI resonance doublet, for most streamers; the different signatures of the core and external layers of the streamers in the width of the ion lines and in the OVI doublet ratio, indicating larger line-of-sight (l.o.s.) and outflow velocities in the latter.

Solar Wind at 6.8 Solar Radii from UVCS Observation of Comet C/1996Y1

The comet C/1996Y1, a member of the Kreutz family of Sun-grazing comets, was observed with the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO) satellite. The Lyα line profile and spatial distribution are interpreted in terms of the theory of bow shocks driven by mass-loading. At the time of the observation, the comet was 6.8 R☉ from the Sun in a region of high-speed wind, a region difficult to observe directly with the SOHO instruments but an important region for testing models of solar wind acceleration and heating. We find a solar wind speed below 640 km s-1 and a constraint on the combination of solar wind speed and proton temperature. The total energy per proton at 6.8 R☉ is 50%-75% of the energy at 1 AU, indicating that significant heating occurs at larger radii. The centroid and width of the Lyα line generally confirm the predictions of models of the cometary bow shock driven by mass-loading as cometary molecules are ionized and swept up in the solar wind. We estimate an outgassing rate of 20 kg s-1, which implies an active area of the nucleus only about 6.7 m in diameter at 6.8 R☉. This is likely to be the size of the nucleus, because any inert mantle would have probably been blown off during the approach to the Sun.