Abstract

When the sun is in an active state and coronal mass ejections reach the top of their speed of ejection and strength of imbedded magnetic field, encounters with Earth subject the thermosphere to direct magnetic coupling to the speeding ejecta with little mediation by the magnetosphere. The force applied to drive thermospheric winds on such occasions is amplified (seemingly paradoxically) an order of magnitude over the force that the solar wind actually exerts. The situation on such occasions differs so markedly from normal that one needs different names to distinguish the normal from the disturbed state. We use “Chapman-Ferraro” and “Dungey-Alfvén,” respectively, for reasons that will be made clear. This is a seminar about the Dungey-Alfvén (DA) situation. The DA state is so rare (~1% of time) that its discovery as a distinct state was possible only with the advent of global numerical simulations, which allow one to isolate it and study it separate from vestiges of the CF state. One of its distinguishing features, as mentioned, is the direct coupling between the solar wind and the thermosphere. We will focus on this coupling, present its properties as revealed by global simulations, explain its paradoxical force, discuss its consequences for thermospheric research, and present evidence for its existence.