 The calculated 20 cm image with calculated (dotted) and observed (solid) contours of 0.25, 0.5, 1.0, and 1.5 MK. |
 The calculated 6 cm intensity image with contours of 0.1 and 0.25 MK |
We observed NOAA region 7563 simultaneously with SERTS and the VLA on 1993 August 17. SERTS obtained 280-420 Å spectra, and images in the lines of Mg IX 368.1, Fe XV 284.1, and Fe XVI 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 if all the other quantities are measured. We used the SERTS data to derive the relevant plasma parameters, and then fit the radio observations to a magnetic field model to determine the field structure.
We used the method of Monsignori-Fossi & Landini (1991, 1995) and the coronal elemental abundances of Feldman et al. (1992) to derive the differential emission measure (DEM) curve for region 7563 from numerous EUV emission lines in spatially averaged SERTS spectra. A similar curve was estimated for each pixel in the region by scaling the average DEM curve with corresponding pixel intensities in the Mg IX, Fe XV, and Fe XVI images. We integrated each such DEM over narrow temperature ranges to obtain the column emission measure (CEM) as a function of temperature, CEM(T). We obained electron density measurements from line intensity ratios, and derived a functional relation between density and temperature (n_e(T)).
We derived the temperature dependence of the coronal magnetic field (B(T)) at each point in the two dimensional region by incorporating CEM(T) and electron density into expressions for the thermal bremsstrahlung and the 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 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.
Key Points from Coronal Magnetography StudiesWe derived the three-dimensional active region coronal magnetic field B(x,y,T) by calculating the temperature (T) dependence of the coronal magnetic field at each (x,y) position in the region.
The coronal magnetic field derived with our method typically exceeds the field extrapolated with a simple potential model.
The magnetic field above a small sunspot is nearly potential.
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NOTE: Click on above image for a larger view! |
Comparison of the height variation of the magnetic field strength derived with our method using a filling factor of 0.01 (solid), with that derived from the potential model (dashed), at (a) the 6 cm intensity peak, (b) the 20 cm intensity peak, and (c) a ``typical'' pixel with a 20 cm intensity of about 10^6 K (1 MK). The y-axis is the field strength in Gauss, and the x-axis is the height in units of 10,000 km. For heights from which the contributions to the 20 cm emission are greatest (vertical dotted lines), the ratios between our calculated coronal magnetic field strengths and the corresponding extrapolated potential fields are (a) 1.32, (b) 2.27, and (c) 3.33.
Coronal Magnetography References
"Coronal Magnetography of a Solar Active Region Using Coordinate SERTS and VLA Observations," J. W. Brosius, J. M. Davila, R. J. Thomas, and S. M. White, Ap. J. 488, 488 (1997).
"Simultaneous Observations of Solar Plage with SERTS, the VLA, and the Kitt Peak Magnetograph," J. W. Brosius, J. M. Davila, W. T. Thompson, R. J. Thomas, G. D. Holman, N. Gopalswamy, S. M. White, M. R. Kundu, and H. P. Jones, Ap. J. 411, 410 (1993).
Last Revised: Wednesday, 29-Nov-2006 07:36:52 EST
Responsible NASA Official: joseph.davila@gsfc.nasa.gov
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