ar040.klimchuk06 Posted: 06-Apr-93 Updated: 05-Sep-93, 25-Mar-94, 8-Nov-94 Events specified: NOAA AR 6891
Collaboration: J. Klimchuk (Stanford), D. Gary, G. Hurford, J. Lim, H. Zirin (Caltech), N. Nitta (LPARL)
The Owens Valley Radio Observatory (OVRO) Solar Array has recently made the first microwave observations of the Sun that are both spectrally and spatially resolved. The microwave spectra can be analyzed to infer the mechanism of emission (free-free or gyroresonance), as well as the temperature and emission measure of the radiating plasma. Furthermore, this analysis can be performed at individual pixels within a two-dimensional grid so as to produce maps of temperature and emission measure.
The validity of the microwave results has not yet been established. We therefore plan to compare temperature and emission measure maps made from OVRO data with corresponding maps made from SXT data. Differences will be studied in terms of the inherent uncertainties in the microwave and X-ray measurements and in terms of physical conditions in the corona which might affect the microwaves and X- rays differently (e.g., temperature variations along the line-of- sight).
We will begin by using exiting observations of Active Region NOAA 6891 made on 1991 Oct 24 (15:50 - 23:30 UT). These observations are not ideally suited to this type of analysis, however, due to the complexity and activity of the region. We will therefore propose future coordinated observations of a more suitable target.
Required Observations: Microwave observations from OVRO and concurrent soft X-ray observations from SXT spanning a period of sevaral hours. The SXT images should be PFI images with a field-of- view of 320x320 arcsec2 (64x64 half resolution pixels or 128x128 full resolution pixels) and must be made with at least two different cool filters (preferably Al.1 and AlMg). The images within each filter set should be as concurrent as possible, but the different filter sets can be separated by several minutes.
Update 8-Nov-94
We have completed our investigation of Active Region NOAA 6891 and submitted our results to the Astrophysical Journal. The abstract of the paper follows.
We have determined active region temperatures and emission measures using both broad-band soft X-ray images from the Yohkoh satellite and spatially and spectrally-resolved microwave data from the Owens Valley Radio Observatory (OVRO). This work differs from previous work in that the microwave temperatures and emission measures are directly measured from the microwave spectrum, and are not model dependent. The soft X-ray temperatures and emission measures are approximately 2.5 times greater than the corresponding microwave values, on average. Detailed error analysis indicates that the temperature differences are real, but that the emission measure differences may not be. We have simulated Yohkoh and OVRO observations of idealized plane- parallel and nested-loop coronal models. The plane-parallel model reproduces the observed temperature differences if the coronal temperature decreasees exponentially with height from a maximum value of approximately 4 MK at the base to an asymptotic value of approximately 1 MK. The nested-loop model, which assumes quasi-static loop equilibrium, also reproduces the observed temperature differences and indicates that the volumetric coronal heating rate varies inversely with loop length to a power > 2. Both models predict microwave emission measures that are larger than observed. We suggest that a more complex model is required to explain the observed emission measures and that more than one coronal heating mechanism may be operative in solar active regions. We present derivations of the temperature and emission measure uncertainties that result from random and systematic errors in the Yohkoh observations. The expression for random error emission measure uncertainties is different from that used previously and is especially important for observations of nonflaring plasmas.
Update 25-Mar-94
We have completed our study of NOAA 6891 and have a first draft of the paper describing our results. We have constructed several simple models of the corona and have simulated observations of these models by both SXT and the Owens Valley Radio Observatory. Comparison with actual solar observations indicates that temperature decreases with height in the corona. We have concluded that the coronal heating rate varies inversely with loop length to at least the second power.
Update 05-Sep-93
Active region NOAA6891 has been analyzed, and the results were presented at the June meeting of the Solar Physics Division of the American Astronomical Society held at Stanford University. We find that temperatures and emission measures inferred from SXT data are systematically larger than those inferred from OVRO microwave data. This result can be explained by a coronal temperature which decreases with height.