Radio Wavelength Observations: Structure, Strength and Variability of Magnetic Fields in the Transition Region and the Solar Corona

Abstract

The 6 cm radiation of solar active regions marks the legs of dipolar loops which have their footpoints in lower-lying sunspots. The temperatures T ≈ 10 6 K and longitudinal magnetic field strengths H ℓ ≈ 600 Gauss at heights h ≈ 4 × 10 9 cm above sunspot umbrae. The circularly polarized emission at 6 cm delineates the magnetic structure above sunspot penumbrae. The 20 cm radiation of solar active regions delineates the ubiquitous coronal loops previously detected at X-ray wavelengths. We infer semilengths L ≈ 5 × 10 9 cm, maximum electron temperatures T e (max) ≈ 3 × 10 6 K, emission measures and electron densities N e ≈ 10 9 cm −3 for the 20 cm bremsstrahlung. Future V.L.A. observations at 20 cm may be used to determine the magnetic field strength of coronal loops. Changes in temperature and magnetic structure before and during solar bursts are briefly discussed.