Solar Active Region Physical Parameters Inferred From a Thermal Cyclotron Line and Soft X-Ray Spectral Lines

We present simultaneous high resolution observations of coronal loops are presented at 20 cm wavelength with the Very Large Array and at soft X ray wavelengths. The images at 20 cm and soft X-ray wavelengths have nearly identical sizes and ellipsoidal shapes, with a linear extent of about 5 cm. This emission stretches between and across regions of opposite magnetic polarity in the underlying photosphere. Complete X ray coronal loops can therefore be imaged at 20 cm, and the VLA maps describe the radio wavelength counterpart of X ray coronal loops. X ray spectral lines were used to obtain values of electron temperature and electron density averaged over the emitting area. The absence of detectable circular polarization is consistent with an optically thick plasma. The observed brightness temperature was roughly equal to the electron temperature of the coronal plasma. The VLA maps at 10 closely spaced frequencies between 1440 and 1720 MHz describe the same coronal loops or arcades of loops. A plot of the maximum brightness temperature of these loops as a function of observing frequency exhibits a line like feature with a central frequency of 1650 MHz and a half width of 80 MHz. This spectral feature is attributed to a thermal cyclotron line and indicates that the optical depth of thermal gyrd-resonance radiation must be greater than that of thermal bremsstrahlung at these frequencies.
Prepared in cooperation with Lockheed Missile and Space Co., Palo Alto, CA. Palo Alto Research Lab.