The workshop was very fruitful for WGD. Jon Linker, Zoran Mikic, Yuri Pisanko, Lika Guhathakurta, Todd Hoeksema, and Sarah Gibson were all in attendance.
Our first result came from the 3-d magnetic field modelers. Both Linker&Mikic (LM) and Hoeksema&Zhao (HZ) found that by using the photospheric field as a boundary condition and no other observational input they were able to create a coronal field which qualitatively matched many of the 3-d coronal features observed by the coronagraphs and imaging telescopes. The techniques used were a numerical MHD model (LM) and photospheric extrapolations, both potential field/source surface and bulk current/equatorial current sheet, (HZ). The presence of the "Elephant's Trunk" equatorial coronal hole was shown by plotting the foot-points of open field lines on the solar surface. Another way of viewing this open field region was shown by plotting isocontours of the closed magnetic field lines, which clearly outlined the elephant's trunk at the solar surface, and showed how its boundary expanded with height . Such isocontours can be compared to coronal emission observations and the 3-d coronal reconstructions of Working Group A.
Moreover, both LM and Pisanko found correlation between the solar wind velocities predicted by their MHD models and the passage of the Elephant's Trunk coronal hole. Specifically, LM found that an increase in in situ wind speed could be traced back to the solar surface via open field lines to the boundaries of the Elephant's Trunk. Perhaps even more interesting, both LM and Pisanko found additional high-speed wind streams connected to southward extensions of the north coronal hole located approximately half of a solar rotation away from the Elephant's Trunk. Although coronal emission doesn't really show these additional low-latitude coronal holes, there are fast wind speeds and Corotating Interaction Regions (CIRs) associated with the Carrington longitudes where the MHD models, and also the photospheric extrapolations of HZ , predict low-latitude open field. Thus, coronal field modeling can show where the field is open and so likely to speed up the solar wind, even when coronal emission doesnt show a "coronal hole".
Although the photospheric field is boundary condition enough to capture much of the general morphology of the 3-d corona, it is not enough information to get results that compare quantitatively with coronal observations. Specifically, the density contrast between the coronal hole and streamer and the velocities and mass fluxes at 1 AU found by the MHD models are sensitive to the thermodynamics assumed, and also to the boundary conditions at the coronal base. The photospheric expansions make assumptions about the coronal currents, specified in terms of model parameters. Features such as the height of closed magnetic field regions and expansion of open field coronal holes depend upon the choices of these parameters.
Incorporating all of the 3-d coronal observations into the 3-d MHD and photospheric extrapolation models is a formidable task. However, by limiting ourselves to the "solar minimum" portion of the WSM rotation where coronal structures do not change with longitude for approximately 90 degrees (CROT 1912 90 - 180 deg), we are able to locally apply 2-d models. Guhathakurta has a 2-d MHD model of the polar coronal hole, which uses white light observations of coronal streamers to define the expansion of the hole, and is constrained to match mass flux, velocity, and magnetic flux observed by the Ulysses spacecraft. Using Gibson, Bagenal, and Low, 1996 , a 2-d magnetostatic model which self-consistently contains the equatorial streamer belt within current sheets at the closed field/open field interface, we are able to match densities observed in white light in both the streamer and the polar hole. We plan to use the results of these 2-d models to help constrain the 3-d models. For example, the height of the cusp of the streamer belt, and the shape of the closed field and expansion of the coronal hole will constrain the model parameters of the photospheric expansion. Moreover, the variation of the density and temperature at the base of the streamer belt will be used as a boundary condition on the MHD models. Our goal is to determine a 3-d field which is consistent not only with the general morphology of the observed corona, but also with the quantified density, temperature, and velocity profiles that Working Group B will provide.
Sarah Gibson
