SOHO JOP128 received: June 15, 2000 Title: VLA/SOHO Investigation of the Evolution of the Active Region Corona J. Newmark (Emergent at GSFC): newmark@eitv2.nascom.nasa.gov A. Vourlidas (NRL): vourlidas@nrl.navy.mil J. Cook (NRL): cook@nrl.navy.mil J.W. Brosius (Raytheon ITSS at GSFC): brosius@comstoc.gsfc.nasa.gov Instruments: EIT, VLA, MDI, CDS Dates: July 1,3,5 & 8, 15:20-21:20 UT each day EOF contact: Jeff Newmark Background and Scientific Justification: The objective is to obtain VLA/EUV observations of the disk passage of an active region in order to clarify discrepancies between EUV and radio observations previously reported, e.g. the higher than observed free-free radio emission predicted by the EUV/SXR data. Additionally temporal evolution of the active region corona will be examined. We will use the recently developed differential emission measure (DEM) method (Cook et al. 2000) using the four EIT bands to measure the amount of coronal plasma in the temperature range, log T = 4.6-6.5. This temperature regime covers precisely the range where the majority of the radio free-free emission is formed. The CDS data, with its high spectral resolution will be used to calculate accurate DEM curves over the smaller area and used as a calibration/comparison point for the EIT data. The SXT instrument will also participate and provide the emission measure of the hotter component which contributes to the observed radio emission but is not measured with the cooler EIT lines. MDI magnetograms will be used to model the gyroresonance contribution to the observed radio emission. The recent upgrade of the frequency-agile Solar Array at OVRO has improved the imaging over the 1-18 GHz range and will provide much needed spectral constraints for the EUV/VLA comparisons. The combination of instrumentation and data analysis techniques provide the means to address the problems of the past and distinguish between various models for the discrepancy, e.g. high Fe coronal abundance (Zhang et al 1999, White 2000), negative temperature gradients (Vourlidas and Bastian (1996), gyroresonance effects (Brosius et al 1997),etc. Ground based observations: VLA: Configuration DnC. Wavelengths: 20, 6, 3.5 cm. OVRO: 1-18 GHz multi-frequency imaging observations. SOHO Observations: EIT: full disk, 4 bands, 2x2 binned, repeat sequence every ~20 minutes. MDI: full disk magnetograms, during this period MDI should be obtaining 1 minute cadence, no special observations CDS: (taken from Jop 100) 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 6 hour VLA observing period to build up statistics, or else cover a wider area during the VLA observing period. TRACE: (taken from jop 100) 6.4 arcmin by 6.4 arcmin images of target active region for tracing field lines and for identifying mass motions. Use 1.0 arcsec pixels for 171, 195, 284, and 1550 wavebands; Separate C IV from the UV waveband. Obtain images at about 90 sec cadence.