Abstract

The x-ray emitting, million-degree hot corona and its solar wind, permeated with the Sun’s external magnetic field, are the prototype of a universal astrophysical phenomenon (Parker, 1994). Most Sun-like stars in the galaxy are observed to have x-ray coronas that are presumably expanding into ste... Show moreThe x-ray emitting, million-degree hot corona and its solar wind, permeated with the Sun’s external magnetic field, are the prototype of a universal astrophysical phenomenon (Parker, 1994). Most Sun-like stars in the galaxy are observed to have x-ray coronas that are presumably expanding into stellar winds. The varieties of stellar and galactic winds with their acceleration mechanisms are common astrophysical concepts. With the broader astrophysical interests in mind, this chapter presents a constructive understanding of the observed corona as a fully ionized hydromagnetic atmosphere (Parker, 1979; Low, 1996, 2001; Aschwanden, 2004). We focus on understanding how the voluminous corona, a near-perfect conductor of heat and electricity, keeps pace with the photosphere in the global magnetic-field reversal of the Sun in 11-year cycles: that is, the polarity of the global dipole field reverses to repeat every complete magnetic cycle of about 22 years. This chapter presents Section 2 with preliminary remarks on the hydromagnetic corona; Section 3 on coronal phenomenology describing the photospheree chromosphereecorona structure and the relationships among prominence magnetic flux ropes, flares, helmet streamers, and coronal mass ejections (CMEs) in evolutionary interplay between magnetic self-organization and coronal polarity reversal; Section 4 on making hydromagnetic sense of the observed corona with a conceptual analysis of Parker spontaneous current sheets (CSs) and generalized magnetic helicity; and Section 5 on outstanding questions and astrophysical implications. Observational data and journal references will be cited not exhaustively but as guidance in accordance with the purpose of this chapter. Physical ideas will be treated qualitatively with attention to conceptual completeness. Hereafter, the unqualified terms "energy," "field," "field line," "flux," "flux surface," "helicity," "polarity," and "reconnection" in the context of discussion will mean the magnetic kind. Show less