SOHO JOP #100 Extensive Temperature Coverage of Active Region Differential Emission Measures (DEM) for Coronal Heating and Coronal Magnetography Studies Short title: Active Region DEM for Heating and Magnetography Studies J.W. Brosius (Raytheon ITSS at GSFC): brosius@comstoc.gsfc.nasa.gov W.T. Thompson (SAC at GSFC): thompson@orpheus.nascom.nasa.gov S.M. White (U. Maryland): white@astro.umd.edu SCIENCE OBJECTIVES ------------------ This JOP provides data needed to (i) derive three-dimensional active region coronal magnetograms, and (ii) explore the nature of coronal heating. (i) Active Region Coronal Magnetography The coronal magnetographic method of Brosius et al (ApJ 488, p. 488, 1997) requires reliable measurements of plasma properties such as density, emission measure, and filling factor throughout an active region (that is, in each spatial pixel) so that one can everywhere accurately separate contributions from the different mechanisms responsible for the microwave emission. Although Brosius et al (1997) used coordinated SERTS EUV and VLA microwave observations of active regions, the same method can be applied to CDS and coordinated VLA observations as well. This method exploits the dependence of the microwave emission upon the coronal magnetic field, and derives the field strength and orientation angle as functions of temperature for each spatial pixel in the field of view by minimizing the differences between the calculated and the observed brightness temperatures simultaneously for all microwave observing frequencies. The height dependence of the coronal magnetic field is obtained by combining densities, column emission measures, and filling factors (all of which can be calculated from the observed EUV emission lines) to obtain the thicknesses of the emitting volumes at various temperatures; height is obtained by summing the thicknesses of successive volumes. Since accurate results can only be obtained if the temperature distribution of the emitting plasma is well known, we seek the widest possible temperature coverage to derive the differential emission measure (DEM) distribution. Thus we obtain (i) emission lines from CDS/NIS2 to derive the low-temperature portion of the DEM, (ii) emission lines from CDS/NIS1 to derive the high-temperature portion of the DEM, and (iii) broad band soft X-ray filter images from Yohkoh/SXT to constrain the highest temperature contributions to the DEM. A long-term goal of this work is the routine derivation of active region coronal magnetograms. (ii) Exploring the Nature of Coronal Heating Lee et al (1997) explored the nature of coronal heating with an empirical approach involving a comparison of active region coronal temperatures with magnetic data to determine which physical quantity correlates best with observed temperature measurements on magnetic field lines. An initial application of this technique revealed that the field lines with the largest current density entering at their footpoints showed the highest temperature. This comparison was made purely with radio temperature measurements, but the technique can be improved by adding (i) measurements of the temperature distribution of the radio-emitting plasma from CDS and Yohkoh/SXT, and (ii) additional tests of the magnetic field model using field lines traced by EIT and/or TRACE images. Thus this effort combines radio, magnetic, EUV, and soft X-ray data for an active region in order to further test an empirical approach to understanding coronal heating, and to address major questions posed by the observation that the corona is not isothermal, but rather shows a mixture of temperatures along any given line of sight. Observing Strategy ------------------ (a) CDS Use NIS4W to obtain 4 arcmin by 4 arcmin active region spectral data with an exposure time of 50 sec. Spatial pixel size is 4 arcsec by 6.8 arcsec. Spectra obtained over the full NIS1 and NIS2 wavebands. Entire sequence takes a little over 2 hours to complete. It is likely that the He I 584 and the O V 630 lines will saturate, but this is necessary in order to bring up the weaker Fe X - XV lines in NIS1. We would either observe the same target region multiple times (4 plus or minus 1) during the 10 hour VLA observing period to build up statistics, or else cover a wider area during the VLA observing period. (b) VLA Obtain D-configuration observations at 2, 3.5, 6, and 20 cm between 1400 and 2400 UT. (c) Yohkoh/SXT Obtain broad band soft X-ray observations with thin Al, AlMg, thick Al, Mg, and Be filters. This will enable widest possible temperature coverage. May need relatively long exposures with Be filter, but short compared to CDS exposure duration. (d) EIT Full-disk images in all four wavebands for context information and for tracing field lines. Additional 11 arcmin by 11 arcmin images of the target region in all four wavebands at midtimes of the CDS raster scans, i.e., 15, 17, 21, and 23 UT. (e) TRACE 6.4 arcmin by 6.4 arcmin images of target active region for tracing field lines and for identifying mass motions. Use 0.5 arcsec pixels for 171, 195, and 1550 wavebands; use 1.0 arcsec pixels for 284 waveband. Separate C IV from the UV waveband. Obtain images at about 90 sec cadence. (f) Photospheric Magnetographs Photospheric longitudinal (from Kitt Peak Spectromagnetograph) and vector (from Advanced Stokes Polarimeter) magnetograms for extrapolating coronal magnetic fields for comparison with other data. (g) MDI If MDI is not otherwise committed, it would be useful to obtain coordinated high-resolution magnetograms. Multiple magnetograms during the 10-hour VLA observing period would be useful to search for canceling and/or emerging flux.