Corona V
Presiding: J Karpen, Naval Research Laboratory; J Brosius, The Catholic University of America
SP33A-01 13:30h
Dissertation Talk: The Impact of Sunspots on Modelling Coronal UV Observations
Existing model studies of O VI 1032 and 1037 A spectral lines from UVCS/SOHO observations often conclude that O5+ ions in coronal streamers have no significant outflow velocity at heights below 3 Rs, and large increases above this height. These observations were modelled with a typical O VI quiet Sun disk spectrum, and the subsequent results were interpreted as an indication of different flows arising from closed and open magnetic field regions within streamers. Given that the O VI solar disk spectrum emitted from sunspots is very different from the spectrum emitted from the quiet Sun or coronal holes, we show how the inclusion of a contribution from sunspots in the incident disk radiation, which excites the coronal O5+ ions, has a significant impact on the intensity and intensity ratio of the coronal O VI spectral lines. Such a result has important implications for the calculation of the outflow velocity of O5+ ions in streamers. Through the analysis of UVCS observations of a solar maximum active region streamer with a large sunspot cluster at its base, we show how the inclusion of a 3% contribution from sunspots in the modelled quiet disk spectrum allows agreement between the observed and modeled intensity ratios at lower heights with non-zero outflow velocities. Such a result cannot be achieved with a standard quiet Sun disk spectrum. Taking the sunspot contribution into account yields O5+ ions with an outflow velocity of about 80 km/s at a height of 3 Rs, increasing linearly to 260 km/s at a height of 8 Rs. These results imply that the presence of sunspots on the solar disk concurrent with streamer observations requires a revision of published results.
SP33A-02 INVITED 14:00h
On the Incompatibility Between UVCS/SOHO Observations of Polar Coronal Holes and Isotropic Oxygen Velocity Distributions
We present a reanalysis of UVCS/SOHO observations of the O VI 1032, 1037 emission line doublet at large heliocentric distances in polar coronal holes during the last solar minimum (1996-1997). The traditional interpretation of the broad line widths and unusual intensity ratios has been that the oxygen ions exhibit a strong temperature anisotropy, with the temperature perpendicular to the magnetic field being much larger than the temperature parallel to the field. However, a recent paper by Raouafi and Solanki suggested that it may be possible to model the observations using an isotropic velocity distribution of (still very hot) oxygen ions. In this presentation we show that the standard interpretation of an anisotropic distribution is the only one that is fully consistent with the observational data. Using the same electron density and magnetic field models assumed by Raouafi and Solanki, we varied the 3 main ion properties (outflow speed and the 2 bi-Maxwellian temperature components) in a 3D data-cube that exhaustively treated all possibilities. This data-cube spans the parameter space of both earlier UVCS/SOHO empirical models and the new proposal of Raouafi and Solanki. Even so, we find that above about 2.5 solar radii the only points in the data-cube that reproduce the actual observed line widths and intensity ratios are those with substantial temperature anisotropies.
SP33A-03 14:15h
EUV Observations of Active Region Dynamics
Data collected during SoHO JOP 146, in collaboration with TRACE, is used to investigate the physical characteristics of coronal active region loops as a function of time and position along and across loop structures. These data include TRACE images in all three EUV passbands, and simultaneous CDS spectroscopic observations. Preliminary measurements of the loop temperature both along the loop half-length and loop cross-section are presented as a function of time. We will show the temperature and density profiles of several structures as a function of position, show changes in temperature and density with time and characterize the coronal background emission. Questions raised by these results will be greatly advanced with the high resolution spectra available from the EIS on Solar-B.
SP33A-04 14:30h
Self and Mutual Helicities in Coronal Magnetic Configurations
Together with the magnetic energy, the magnetic helicity is an important quantity used to describe the nature of a magnetic field configuration. The most meaningful value of helicity is the relative magnetic helicity (relative to a reference field) which describes the linkage of the field lines even if the volume of interest is not bounded by a magnetic surface. In addition if the magnetic field can be decomposed into the sum of a closed field and a reference field (following Berger 1999), we can introduce three other helicity values: the self helicity of the closed field, the mutual helicity between the closed field and the reference field, and the vacuum helicity (self helicity of the reference field). To understand the meaning of those quantities, we derive them from the potential field (reference) and the nonlinear force-free field computed with the same boundary conditions for three different cases: (i) a single twisted flux tube derived from the extended Gold-Hoyle solutions, (ii) a simple magnetic configuration with three balanced sources and a constant distribution of the force-free parameter, and (iii) the AR 8210 magnetic field observed at 19:40 UT on May 1, 1998. The self and mutual helicities corresponds to the twist and writhe of confined flux bundles, and the crossing of field lines in the magnetic configuration respectively. The vacuum helicity is interpreted as a measure of the topological complexity of the field.
SP33A-05 14:45h
Measuring and Modeling Solar EUV Flux for Thermospheric and Ionospheric Modeling
In the near future, NOAA will begin long-term measurements of the solar EUV flux for use in models of the thermosphere and ionosphere. The requirement is to determine the flux between 10 and 122 nm however, the spectral region between 40 and 100 nm is difficult to measure using the diode/filter technology that is typically applied to the problem. It has been proposed that a combination of measurements and modeling may satisfy the requirements in this region of the spectrum. Using the NASA TIMED SEE data, we will investigate how well observations in the region of the solar EUV spectrum that is readily observed will track the important spectral features in the part of the spectrum that is more difficult to measure. We will also examine how any uncertainties or errors that are introduced by this approach might translate into errors in modeling of the upper atmosphere. This empirical approach will be examined on hourly to daily time scales and over much of the range from solar maximum to the present, near minimum conditions.