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T. R. Metcalf (Lockheed Martin Solar \& Astrophysics Lab.), D. L. Mickey (University of Hawaii, Institute for Astronomy)
The energy source for the heating of the solar corona and for solar flares is widely believed to be the solar magnetic field. While most observations of the solar magnetic field are of the photospheric magnetic field, there are a number of advantages to observing the magnetic field in the chromosphere. Most of these derive from the fact that the magnetic field in the chromosphere is force-free more than a few hundred km above the photosphere. This was first demonstrated by Metcalf et al. (ApJ, 439, 474, 1995) using NaI D-line observations from the Haleakala Stokes Polarimeter.
When analyzing the magnetic field in an active region, the field is usually assumed to be force-free. The fact that it is not restricts the usefulness of such analyses. For example, the magnetic field is universally assumed to be force-free when extrapolations of the field into the corona are computed. Clearly, if the measured field really is force-free, such extrapolations are more robust. Also, when the field can be shown to be force-free, new analysis tools are available. For example, the free energy in the magnetic field can be measured using the magnetic virial theorem. This is impossible with photospheric measurements of the magnetic field (Metcalf et al., 1995).
The objective of this project is to measure the solar chromospheric magnetic field using the NaI D line observed with the University of Hawaii's Imaging Vector Magnetograph (IVM). Since the magnetic field observed in the chromosphere is known to be force-free, we plan to use the data to measure the magnetic free energy in active regions. This data set will enable us to look for relationships between the free energy and coronal heating rates and flare rates. This work is supported by NASA contract NAG5-7438.
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