The Photospheric Boundary of Space Weather Models

Soft X-ray images of the solar corona suggest that coronal mass ejections (among the primary drivers of space weather here at earth) tend to arise in connection with active regions exhibiting sheared and or twisted coronal loops. It is understood that these features are signs of stressed magnetic fields containing a substantial amount magnetic free energy that can be released by simple evolution, by some destabilizing event such as local emergence of new magnetic flux from below the photosphere, or by changes in magnetic connectivity due to events elsewhere on the Sun. Large scale space weather modeling efforts such as those of CISM (The Center for Integrated Space Weather Modeling) and SolarMURI must therefore overcome both the problem of self-consistently incorporating photospheric magnetic fields and flows into the lower boundary of dynamic models of the corona and the problem of initiating these simulations with realistic, potentially eruptive active-region topologies.

Coronal Extrapolations

A qualitative comparison between a potential field (left) and non-constant alpha force free extrapolation (middle) to an MHD simulation of flux emergence (right). The extrapolations were generated from slices through the MHD dataset at a heights corresponding to the model's chromosphere (top row) and photosphere (bottom row). (from Abbett et al. 2004).

The challenge of incorporating measurements of the vector magnetic field at the photosphere into numerical models of the solar corona (in a physically self-consistent manner) has recently become a topic of interest in the literature. One way of incorporating vector data into models --- dubbed ILCT (for "Inductive Local Correlation Tracking") --- has been introduced by Brian Welsch, George Fisher, myself and Stephane Regnier in Welsch et al. 2004. This technique provides a means to update the electric field at the photospheric boundary of a model corona in such as way as to be consistent with both the magnetic induction equation and flows obtained via local correlation tracking applied to magnetic elements.

The challenge of initiating a simulation with an initial magnetic topology that compares favorably to soft X-ray observations of the corona --- and the associated problem of introducing magnetic flux into a model corona with strong, pre-existing magnetic structures --- is addressed in Abbett et al. 2004.