Magnetospheres of Solar Active Regions Inferred from Spectral-Polarization Observations with High Spatial Resolution

Abstract

The strong magnetic fields of active regions organize both the plasma structures and energy processes in the chromosphere and corona. Recent radio observations with high spatial resolution permit measurements of the magnetic fields in these regions and also localize regions of thermal and nonthermal energy release. They can additionally be used to determine temperatures and electron densities in these regions. The results of such diagnostics suggest the term magnetosphere for the space surrounding an active region in the solar atmosphere where the basic structures and physical processes are controlled by the magnetic fields/electric currents of the particular region. The physical parameters of quiescent, or nonflaring, structures in the low solar corona and upper chromosphere have been inferred from nearly simultaneous spectral polarization observations (RATAN 600) and high-spatial-resolution radio observations (VLA). They have been compared with images from the Soft X-ray Telescope (SXT) aboard the Yohkoh satellite, indicating that bright radio (20 cm) and X-ray structures coincide, but that there is radio emission that is not detectable at X-ray wavelengths. Variable soft X-ray emission on time scales of hours suggests continued, varying, low-level heating or particle acceleration in localized areas of active regions. The RATAN-600 observations have been combined with the theory of thermal cyclotron emission to infer magnetic field strengths in the low corona above practically all large sunspots with an accuracy of 2 or 3 %. They indicate that the magnetic field strength of the thermal plasma at the million-degree level above large sunspots is 75%-80% of the magnetic field strength in the underlying photospheric sunspots. The evolution of the magnetic structures is specified. Coronal potential field extrapolations are also provided, suggesting that the magnetic fields in the corona diverge more slowly than expected from a simple dipole located below the surface. Theoretical models are compared with multiple-wavelength VLA observations and potential field extrapolations, indicating that the radio emission from one active region can be explained by thermal gyroresonance radiation in a conductive flux model. However, the high brightness temperature and steep spectrum of the radio emission of another active region cannot be explained by conventional thermal models, and instead suggest long-lasting nonthermal heating in localized coronal sources above the magnetic neutral line in the underlying photosphere Gyrosynchrotron radiation of nonthermal electrons cannot explain the observations of one such source, but heating within a localized neutral current sheet might account for them. Long-lasting radio sources with high brightness temperatures ≥ 10<SUP>7</SUP> K and steep radiation spectra are often associated with active regions with a multipolar δ configuration of the photospheric sunspots. These "peculiar" coronal radio sources appear above the magnetic neutral line in the photosphere, and appear to require nearly continuous acceleration of energetic nonthermal electrons by a yet unknown process.