Magnetic free energy in NOAA active region 10486 on 2003 October 29

We calculate the total and the free magnetic energy for solar NOAA active region 10486 on 2003 October 29 using chromospheric vector magnetograms observed with the Imaging Vector Magnetograph at Mees Solar Observatory in the Na I lambda 5896 spectral line. The magnetic energy is derived from the magnetic virial theorem using observations spanning the X10 flare that occurred at 20: 39 UT. Although poor atmospheric seeing prevented us from discerning changes in the free energy associated with the flare, there was an unusually large amount of free magnetic energy in NOAA AR 10486: (5.7 +/- 1.9) x 10(33) ergs, which is consistent with the very high level of activity observed in this active region. It is thus plausible that the extreme activity was powered by the magnetic free energy.