Slowly-Varying Microwave Emissions from the Solar Corona

Abstract

Measurements of the total microwavef emission from the solar disc at single and multiple wavelengths provide a useful indicator of solar activity, and correlate well with other indices of solar activity and with solar luminosity.Since activity in the solar atmosphere is driven by the emergence, redistribution and disappearance of magnetic flux, the close and linear correspondence between the microwave flux densities, as observed at 10.7 and 21 cm wavelengths, provide a convenient and objective indicator of solar magnetic activity.Microwave emissions from the “non-flaring” solar corona originate in a wide range of sources. These generally lie in active regions, and comprise two main types: bright, compact sources a few arc-seconds across and brightness temperatures of the order of 106K, and weaker, diffuse sources arc-minutes in size and having brightness temperatures of about 104K. There is also evidence of a weaker, widespread emission covering large areas of the solar disc. The compact sources are probably due to gyroresonance, although some could be non-thermal. The diffuse widespread emission are almost certainly free-free thermal emission from plasma concentrations in the corona.The strong correlation between the 10.7 cm flux and other activity indices, even when they are produced by disimilar processes and at different heights in the solar atmosphere, may at least partially be due to the overiding role played by magnetic activity in dictating the structure of the chromosphere and corona. However, the consistent correspondences over wide ranges of activity suggest dependences upon relatively few quantities and that the microwave emission from the non-flaring Sun is dominated by a single process, which is closely and simply-related to density and temperature, such as free-free (bremsstrahlung) emission from plasma concentrations trapped in coronal and chromospheric magnetic structures. Attempts to “budget” the total microwave emission at 2.8, 10.7 and 21 cm wavelengths suggest that this is the case.Where ambient magnetic fields exceed 3000/λ Gauss, gyroresonance can produce much higher optical depths than free-free emission, and accounts for the bright emission observed in the vicinity of sunspots. Although not the dominant contributor to the total emission from the solar disc, these sources stand out brightly in high-resolution maps of active regions.To examine the assumptions of dominance of free-free thermal emission, and simple dependences upon few parameters, we compute the spectrum for a range of coronal density models, and compare the results with observations. The agreement is good, suggesting that the total disc flux is dominated by thermal, free-free emission, and that the intensity is controlled mainly by the density at the base of the corona, but is otherwise fairly tolerant of the density model used.