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Session 1 - Chromosphere, Corona, Flares.
Display session, Friday, June 27
Ballroom B, Chair: Charles Kankelborg

[1.35] Coronal Magnetography of a Solar Active Region Using Coordinated SERTS and VLA Observations

J. W. Brosius (Hughes STX/NASA GSFC), J. M. Davila, R. J. Thomas (NASA GSFC), S. M. White (UMD)

We observed NOAA region 7563 simultaneously with Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph (SERTS) and with the Very Large Array (VLA) on 1993 August 17. SERTS obtained spectra in the 280 to 420 Å\ wavelength range, and images in the lines of Mg IX \lambda 368.1, Fe XV \lambda 284.1, and Fe XVI \lambda 335.4. The VLA obtained microwave images at 20 and 6 cm wavelengths. The microwave emission depends upon the coronal temperature, density, column emission measure, and magnetic field; therefore, the coronal magnetic field can be derived when all of these other quantities are measured. Here we demonstrate this approach by using the SERTS data to derive all the relevant plasma parameters and then fitting the radio observations to a magnetic field model in order to determine the magnetic field structure. We derived the temperature dependence of the coronal magnetic field (B(T)) at each point (i.e., each pixel or each spatial location) in the two dimensional region by incorporating the corresponding column emission measure (CEM(T)) and electron density (n_e(T)) into expressions for the thermal bremsstrahlung and gyroresonance opacities, and varying B(T) so as to minimize the difference between the calculated and the observed microwave intensities. The resulting calculated 20 and 6 cm microwave intensity images reproduce the observed images very well. Thermal bremsstrahlung emission alone is not sufficient to produce the observed microwave intensities: gyroemission is required. Further, contrary to several earlier studies, we found no evidence for cool, absorbing plasma in the solar corona above the active region. The coronal magnetic fields derived with our method typically exceed the coronal fields extrapolated with a simple potential model, suggesting the presence of coronal electric currents. However, in the diminutive sunspot which dominates the 6 cm emission this difference is relatively small, suggesting that the sunspot magnetic field itself is nearly potential. (This work was supported by NASA grant NASW-4933.)


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