G. Noci

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.

Solar wind diagnostics from Doppler-enhanced scattering

Solar wind ions can attain sufficient outflow speed, w, to cause line excitation by chromospheric or transition region radiation in a nearby line. It is shown that this extends the diagnostic possibilities of a coronal EUV line to much larger values of w than would be possible if pumping were limited to radiation from the same spectral line. For the 1037.6 A coronal line of O VI, the pumping effect of the chromospheric C II 1037.0 A line is efficient for w between 100 and 250 km/s. An approximate expression for the line ratio for a doublet of the Li or Na isoelectronic sequences is derived, and the diagnostic capabilities of doublet line ratios, either by themselves or combined with the observation of other quantities, are discussed. In particular, that the determination of doublet line ratios at several heights can be sufficient to yield the solar wind velocity at those heights together with a constraint on other coronal parameters. 18 references.

ULTRAVIOLET CORONAGRAPH SPECTROMETER OBSERVATIONS OF DENSITY FLUCTUATIONS IN THE SOLAR WIND

Recent Ultraviolet Coronagraph Spectrometer (UVCS) white-light channel (WLC) observations on board the Solar and Heliospheric Observatory (SOHO) indicate quasi-periodic variations in the polarized brightness (pB) in the polar coronal holes. This is the first observation of possible signatures of compressional waves high above the limb (at heliocentric distances in the range 1.9-2.45 R☉). The Fourier power spectrum of the pB time series at 1.9 R☉ shows significant peak at about 6 minutes and possible fluctuations on longer timescales (20-50 minutes). The observation at 1.9 R☉ is the only currently available WLC data set with sufficient cadence to resolve the 6 minute period. These preliminary observations may result from density fluctuations caused by compressional waves propagating in polar coronal holes. We stress that our results are preliminary, and we plan future high-cadence observations in both plume and interplume regions of coronal holes. Recently, Ofman & Davila used a 2.5 D MHD model and found that Alfven waves with an amplitude of 20-70 km s-1 at the base of the coronal hole can generate nonlinear, high-amplitude compressional waves that can contribute significantly to the acceleration of the fast solar wind. The nonlinear solitary-like waves appear as fluctuations in the density and the radial outflow velocity and contribute significantly to solar wind acceleration in open magnetic field structures. The motivation for the reported observations is the MHD model prediction.

The effect of temperature anisotropy on observations of doppler dimming and pumping in the inner corona

Recent observations of the spectral line profiles and intensity ratio of the O VI λλ1032 and 1037.6 doublet by the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO), made in coronal holes below 3.5 RS, provide evidence for Doppler dimming of the O VI λ1037.6 line and pumping by the chromospheric C II λ1037.0182 line. Evidence for a significant kinetic temperature anisotropy of O5+ ions was also derived from these observations. We show in this Letter how the component of the kinetic temperature in the direction perpendicular to the magnetic field, for both isotropic and anisotropic temperature distributions, affects both the amount of Doppler dimming and pumping. Taking this component into account, we further show that the observation of the O VI doublet intensity ratio less than unity can be accounted for only if pumping by C II λ1036.3367 in addition to C II λ1037.0182 is in effect. The inclusion of the C II λ1036.3367 pumping implies that the speed of the O5+ ions can reach 400 km s-1 around 3 RS, which is significantly higher than the reported UVCS values for atomic hydrogen in polar coronal holes. These results imply that oxygen ions flow much faster than protons at that heliocentric distance.

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.

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.

UVCS WLC OBSERVATIONS OF COMPRESSIONAL WAVES IN THE SOUTH POLAR CORONAL HOLE

Recent SOHO Ultraviolet Coronagraph Spectrometer (UVCS) white light channel (WLC) observations of the south polar coronal hole plumes and interplume regions produce signatures of quasi-periodic variations in the polarized brightness (pB) at a heliocentric distance of 1.9 solar radii (R☉). The Fourier power spectrum of the pB time series shows significant peaks at about 1.6-2.5 mHz and additional smaller peaks at longer and shorter timescales. Wavelet analysis of the pB time series shows that the coherence time of the fluctuations is about 30 minutes. The new observations strongly suggest that the fluctuations are compressional wave packets propagating in the coronal hole high above the limb. The presence of compressional waves may have important implications that help to explain the heating of coronal holes and the fast solar wind acceleration.

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.