A USER'S GUIDE TO UVCS/SOHO

Section 2
Diagnostic Capabilities and Results

1. Coronal Velocity Distributions
2. Coronal Outflow Velocities
3. Coronal Morphology and Evolution
4. Coronal Mass Ejections
5. Stars
6. Solar System Objects

Coronal Velocity Distributions

• The most surprising initial results from UVCS have been the extremely broad profiles of H I (neutral hydrogen) atoms and minor ions such as O VI (oxygen 5+) and Mg X (magnesium 9+).

• Also, the unresolved microscopic speeds (along the line of sight) for O VI are much higher than for H I and protons. Neither thermal equilibrium nor common transverse wave motions can explain this.

• The O VI motions are highly anisotropic, implying kinetic temperature anisotropies of order (T_perpendicular / T_parallel) > 100.

Empirical line widths and microscopic velocities

(Click on small image for larger version with captions)

Coronal Outflow Velocities

• The Doppler dimming method allows us to measure the bulk outflow velocity of observed ions.

• For resonantly-scattered lines formed in a moving medium, the local scattering profile is Doppler-shifted with respect to the solar-disk profile (i.e., the origin of the photons), and the ``supply'' of photons is depleted.

• For isolated lines like H I Lyman alpha, higher outflow causes lower intensity (Withbroe et al. 1982).

• For lines neighboring other solar-disk lines, like O VI 1037, higher outflow causes dimming, followed by regions of ``pumping'' when the scattering profile lines up with the neighboring solar-disk lines ( Noci et al. 1987, Li et al. 1998, submitted).

Empirical outflow velocities

(Click on small image for larger version with captions)

Coronal Morphology and Evolution

• Daily synoptic observations are made with UVCS/SOHO which allow us to characterize the large-scale spatial distribution of plasma parameters in the solar corona between 1.5 and 3 solar radii.

• The 27-day solar rotation provides ``synoptic maps'' of latitude versus Carrington longitude.

• Lines measured by UVCS sample the optically-thin plasma along an extended region, effectively integrating along the line of sight.

• Assuming a time-steady system, these measurements can be used to tomographically reconstruct the 3D structure of the corona.

Example Synoptic Images and 3D Maps

(Click on small image for larger version with captions)

Coronal Mass Ejections

• UVCS has measured coronal plasma parameters (at large coronal radii) in several CMEs in 1996 and 1997.

• UVCS can isolate spectral lines from both hot (coronal) and cool (chromospheric) material, and measure intensities, Doppler shifts, and line shapes.

• Observed ``knots'' of emission are strongly Doppler shifted, exhibiting a complex 3D velocity structure (see Antonucci et al. 1997 and Ciaravella et al. 1997).

CME Ultraviolet Spectra vs. time and space

(Click on small image for larger version with captions)

Stars

• UVCS can observe any UV-bright star which passes within 10 solar radii of the Sun (in the plane of the sky) during the course of the year.

• SOHO's orbit at L1 provides continuous time coverage.

• Wavelength range extends below that observed by IUE or HST.

• Stars observed so far: 38 Aqr, TT Ari (CV), 53 Tau, 103 Tau, 121 Tau, tau Tau, zeta Tau, alpha Leo, rho Leo, HD 142883, omega Sco, delta Sco, beta Sco, HD 164794, HD 164492, HD 164816.

Example Stellar Spectrum

(Click on small image for larger version with captions)

Solar System Objects

• Sun-grazing comets: In May 1997, UVCS observed Comet SOHO-8/Stezelberger (C/1997 H2), in one of the first high-resolution UV spectroscopic measurements ever made of a comet. The size of the coma at several different positions was measured in H I Lyman alpha (see composite image below).
  UV Images of Comet SOHO-8
  
  (Click on small image for larger version with captions)

• Interplanetary Emission: Both neutral hydrogen, backscattered from the local interstellar gas, and neutral helium, enhanced in a heliospheric ``gravitational focusing cone,'' have been observed.

• Planets: Venus occulted the corona in June 1996, and was observed with UVCS as a localized dimming. Jupiter was observed in opposition in January 1997, mainly for photometric calibration, but H I Lyman alpha emission was measured from the disk.

GO BACK to the User's Guide Section 1
GO FORWARD to the User's Guide Section 3