On the energy balance of solar active regions

Abstract

The cause of sunspots has long been an important, unsettled problem in solar physics. Biermann¹ suggested that the strong magnetic field of a sunspot inhibited convection, allowing the sunspot to cool. Parker², on the other hand, proposed that a sunspot was cooled by the generation of waves that carried away the thermal energy. The solar ‘constant’ has been measured with a daily uncertainty of about 10 parts per million (p.p.m.) by the Active Cavity Radiometer Irradiance Monitor (ACRIM) on board the Solar Maximum Mission (SMM) satellite launched in February 1980. The ACRIM has shown that the solar constant changes with solar activity, showing dips of the order of 0.1–0.3% associated with sunspots. This discovery has provoked renewed attempts to understand the sunspot phenomenon. Two divergent views are emerging: that the missing energy is stored in the convection zone for long periods of time³ or that the missing energy is re-radiated fairly quickly at different angles by faculae⁴, non-spot magnetic fields that often accompany sunspots. The consequences are that in the first case an 11-yr modulation is expected in the solar output whereas in the second case it is not. I point out here that facular emission may equal the missing energy from sunspots, over a period of some weeks or months, suggesting that the missing energy is stored by being converted from kinetic into magnetic energy after which it decays back into thermal energy in the faculae.