Including the transition region in models of the large-scale solar corona

In traditional multidimensional models of the solar corona, the boundary of the calculation closest to the solar surface is placed well into the corona (at temperatures of 1-2 x 10(6) K). We describe a large-scale MHD model that includes the transition region in the calculation. In this model, we simulate the solar atmosphere from the top of the chromosphere (at 20,000 K), through the transition region, into the corona, and extending out into the inner heliosphere. Our model includes parallel thermal conduction, coronal heating, and radiation losses in the energy equation. For simplicity, we describe a two-dimensional (axisymmetric) implementation of our model. This model enables us to study the large-scale structure of the transition region. In particular, we contrast the variation of the structure of the transition region underneath a closed magnetic arcade with that in an open-field region. We discuss how the inclusion of the transition region and upper chromosphere into the model modifies the time constants. We compare our results with a model in which the calculation is started near the top of the transition region (at 500,000 K) using a "radiative energy balance" condition, and we find that the two models agree well in open-field regions and for long loops. However, a model that includes the transition region is required to properly model short loops in closed-field regions.