Source Surface Models and Their Impact on Solar Wind Research III
Presiding: J B Gurman, NASA Goddard Space Flight Center; M J Aschwanden, Lockheed Martin Advanced Technology Center
SH24A-01 15:30h
Observed and Modeled Coronal Holes
We combine relative intensity images of the solar chromosphere and corona with magnetograms to identify coronal hole regions on the Sun. We compare them with the coronal holes - defined as the foot-points of magnetically open field lines - derived with two different coronal models: a PFSS model (Arge and Pizzo, 2000) and a 3D MHD model (Riley, Linker, and Mikic, 2001) that use the same magnetic field maps as photospheric boundary. The fraction of the observed coronal holes found by coronal models gives us a means to validate how well models reproduce observations. Cases for different level of solar activity during solar cycle 23 are presented and discussed. (This work is supported by CISM, which is funded by the STC program of the National Science Foundation under Agreement Number ATM-0120950.)
SH24A-02 INVITED 15:45h
Applications of the Potential Field Source Surface Model in CME and solar wind studies*
With various collaborators we have used the PFSS model to: 1) investigate topological changes in the coronal magnetic field related to active region strength, location and orientation within the background photospheric field; 2) investigate the relationship between large scale coronal magnetic field changes and the occurrence of CMEs; and 3) investigate the solar cycle variation of solar wind sources. Our experiences with the model, which is extremely versatile and easy to implement relative to magnetogram-based global force-free or MHD models, have been surprisingly positive considering the known physical shortcomings of the underlying assumptions. As long as one always keeps in mind the expected limitations of the potential field and spherical source surface assumptions, PFSS model makes it possible to approximate coronal open field regions as accurately as MHD models (or more so at the current state of MHD global modeling, depending on the latter's spatial resolution and resistivity properties), to describe coronal features observed in some CME event case studies, and to visualize the characteristics of a spherical geometry on null points and topological field domains related to a particular photospheric field distribution. Global potential model has often been used to be the initial state of a MHD model simulation. When viewed as a tool in the arsenal of solar physics, rather than as an end model, the PFSS approach opens the door to many applications that set the stage for later more sophisticated modeling. *work suppoted by DOD/solarMURI, the Center for Integrated Space Weather Modeling (an NSF Science and Technology Center), and NSF/ATM.
SH24A-03 16:05h
Predicting the Solar Wind
A theory has been developed for the acceleration of the solar wind, which couples the acceleration to the behavior of the open magnetic flux of the Sun. The open flux is transported in the corona by various diffusive processes, which in turn can impart energy into and accelerate the solar wind. The theory thus provides an opportunity to relate the final speed of the solar wind to the processes governing the behavior of the open flux, or equivalently, it provides the basis for predicting the solar wind flow from observed solar parameters. Comparison of the results of this approach with observations, and the relationship of this theory with other methods to predict the solar wind will be considered.
SH24A-04 16:20h
What Determines the Solar Wind Speed ?
Recent observations by Interplanetary Scintillation measurements by Nagoya-STEL group (Hirano et al.2003; Kojima et al.2004) show that solar wind speed is well-correlated with B/f, where B is radial magnetic field strength at the solar surface and f is a super-radial expansion factor of open flux tubes. We show that this correlation is nicely explained by dissipation of Alfven waves no matter what types of the wave dissipation processes operate. B determines the input energy flux of Alfven waves and f controls adiabatic loss of the wave energy, so that B/f is an important control parameter which determines the solar wind speed. (reference ) [1] Hirano, M., Kojima, M., Tokumaru, M., Fujiki, K., Ohmi, T., Yamashita, M, Hakamada, K., and Hayashi, K. 2003,, Eos Trans. AGU, 84(46), Fall Meet. Suppl., Abstract SH21B-0164 [2] Kojima, M., K. Fujiki, M. Hirano, M. Tokumaru, T. Ohmi, and K. Hakamada, 2004, "The Sun and the heliosphere as an Integrated System", Giannina Poletto and Steven T. Suess, Eds. Kluwer Academic Publishers, in press
SH24A-05 16:35h
Modeling of the Radio Metric Emission of the Quiet Sun Corona Using Potential Field Source Surface Extrapolations
We present the result of a modeling of the Quiet Sun corona in the metric radio range (F~150-450 MHz). At these frequencies, the radio emission is dominated by non-thermal emissions (plasma emissions) due to populations of accelerated electrons, and, when the solar activity is low or moderate, by the thermal emission of the corona (bremsstrahlung). While this emission mechanism is well known, depending only on the electron density and temperature, the difficulties arise from the refraction that affects the radio waves propagating in the corona, which depends on the electron density distribution. In order to build a realistic description of the electron and temperature distribution in the corona for a given date, we have used Potential Field Source Surface extrapolations, based on synoptic maps of the photospheric magnetic field. The density and temperature distribution is based on scaling laws which depend on the field strength as well as the length of the loops. We use radio data from the Nancay Radioheliograph to constraint the free parameters of these scaling laws, and make qualitative and quantitative comparisons between the radio images and the simulations.