Leonard Strachan

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.

EMPIRICAL DENSITIES, KINETIC TEMPERATURES, AND OUTFLOW VELOCITIES IN THE EQUATORIAL STREAMER BELT AT SOLAR MINIMUM

We use combined Ultraviolet Coronagraph Spectrometer and Large Angle Spectroscopic Coronagraph data to determine the O5+ outflow velocities as a function of height along the axis of an equatorial streamer at solar minimum and as a function of latitude (at 2.3 R☉ from Sun center). The results show that outflow increases rather abruptly in the region between 3.6 and 4.1 R☉ near the streamer cusp and gradually increases to ~90 km s-1 at ~5 R☉ in the streamer stalk beyond the cusp. The latitudinal variation at 2.3 R☉ shows that there is no outflow (within the measurement uncertainties) in the center of the streamer, called the core, and that a steep increase in outflow occurs just beyond the streamer legs, where the O VI λ1032 intensity relative to H I λ1216 (Lyα) is higher than in the core. Velocity variations in both height and latitude show that the transitions from no measurable outflow to positive outflow are relatively sharp and thus can be used to infer the location of the transition from closed to open field lines in streamer magnetic field topologies. Such information, including the densities and kinetic temperatures derived from the observations, provides hard constraints for realistic theoretical models of streamers and the source regions of the slow solar wind.

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.

Far-ultraviolet spectra of fast coronal mass ejections associated with X-class flares

The Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory satellite has observed very fast coronal mass ejections (CMEs) associated with X-class flares. These events show spectral signatures different from those seen in most other CMEs in terms of very rapid disruption of the pre-CME streamer, very high Doppler shifts, and high-temperature plasma visible in the [Fe XVIII] emission line. This paper describes three very similar events on 2002 April 21, July 23, and August 24 associated with X-class flares. We determine the physical parameters of the pre-CME streamers and discuss the geometric and physical nature of the streamer blowouts. In the April 21 event, the hot plasma seen as [Fe XVIII] is not related to the structure seen in [Fe XXI] by the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) instrument at lower heights. It has the form of a rapidly expanding fan, quite likely a current sheet. In the August event, on the other hand, the [Fe XVIII] is probably a bubble of hot plasma formed by reconnection in the wake of the CME. C III emission from the July 23 flare is detected as stray light in the UVCS aperture. It precedes the hard X-ray brightening by about 2 minutes.

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.

A Doppler dimming determination of coronal outflow velocity

Outflow velocities in a polar coronal hole are derived from observations made during a 1982 sounding rocket flight. The velocity results are derived from a Doppler dimming analysis of resonantly scattered H I Ly-alpha. This analysis indicates radial outflow velocities of 217 km/s at 2 solar radii from sun-center with an uncertainty range of 153 to 251 km/s at a confidence level of 67 percent. These results are best characterized as strong evidence for supersonic outflow within 2 solar radii of sun-center in a polar coronal hole. Several means for obtaining improved accuracy in future observations are discussed.