The magnetic field in the corona above sunspots at the eclipse of 1991 July 11

We have used the frequency-agile solar array at the Owens Valley Radio Observatory (OVRO) and the rocket-launched soft X-ray imager NIXT to observe the corona above an active region during the solar eclipse of 1991 July 11. The uncovering of the active region AR 6718 by the Moon allows us to obtain an angular resolution of 1″.8 over the frequency range of the OVRO telescope, 1-18 GHz, comparable to the less than 1″ resolution of NIXT. The dominant features of AR 6718 are two leading spots of positive polarity followed by two spots of negative polarity ∼3′ to the east. Bright (T_b ≈ 2 × 10⁶ K) radio emission coincides with the positions of the sunspots, attributable to gyroresonance radiation from ambient electrons above the spots. Using a simplified model of the source as a function of frequency based on the interferometer fringe amplitudes, we obtain brightness temperature spectra for the emission associated with the sunspots. From these data we are able to deduce that the magnetic field strength at the base of the corona above the leading spots was ∼1200 G, and ∼1100 G above the following spots. The magnetic field spreads and its strength decreases with height in the corona, with the 1200 and 360 G fields covering areas of ∼15″ and 28″ in diameter, respectively. Lower down, in the transition region where T_e ≈ 2 to 3 × 10⁵ K, the field strength was ∼1800 G. The soft X-ray brightness above the sunspots was very low, ∼30 times lower than that of the adjacent plage-associated emission. From the X-ray data, supplemented by the electron temperature derived from the radio data we find that the electron density at the base of the corona above the sunspots was ≈1 × 10⁹ cm^-3. Combining the X-ray and radio data, we derive an upper limit to the gradient of field strength with height: |∇B| ≲ 1.5 × 10^-6 G cm^-1.