The Properties of the Solar Corona and Its Connection to the Solar Wind

Abstract

The corona is a layer of hot plasma that surrounds the Sun, traces out its complex magnetic field, and ultimately expands into interplanetary space as the supersonic solar wind. Although much has been learned in recent decades from advances in observations, theory, and computer simulations, we still have not identified definitively the physical processes that heat the corona and accelerate the solar wind. In this review, we summarize these recent advances and speculate about what else is required to finally understand the fundamental physics of this complex system. Specifically:• We discuss recent sub-arcsecond observations of the corona, some of which appear to provide evidence for tangled and braided magnetic fields, and some of which do not. • We review results from three-dimensional numerical simulations that, despite limitations in dynamic range, reliably contain sufficient heating to produce and maintain the corona. • We provide a new tabulation of scaling relations for a number of proposed coronal heating theories that involve waves, turbulence, braiding, nanoflares, and helicity conservation.An understanding of these processes is important not only for improving our ability to forecast hazardous space-weather events, but also for establishing a baseline of knowledge about a well-resolved star that is relevant to other astrophysical systems.

The evolution of the solar wind

“…2.7). There is still no consensus at how precisely the corona and the wind are heated to such high temperatures and it is possible that a combination of different mechanisms heat the plasma in different regions and at different times, as discussed in the recent review on solar coronal and wind heating by Cranmer and Winebarger (2019).…”

“…The zero-age main sequence is shown by the grey line. Image reproduced with permission from Cranmer and Winebarger (2019), copyright by Annual Reviews have hotter winds (on the order of 6 K) and low mass-loss rates, while on the top right corner of the HR diagram cool stars have colder winds that can reach temperatures of 10 4 K and maybe even lower, and high mass-loss rates. There is a relatively smooth transition between these two groups, with stars that show signs of weak/warm coronae belonging to an intermediate 'hybrid' group, and they perhaps have a combination of wind driving mechanisms (thermal and mechanical, Ó Fionnagáin et al 2021).…”

The Solar Orbiter mission

“…A few years later, Parker (1958) realised that the latter is closely linked to the presence of a hot corona surrounding the Sun. While the detailed physical mechanisms that heat the Sun's corona to millions of degrees are still not fully understood, most coronal heating mechanisms proposed attribute key roles to the Sun's convective motions in the photosphere and its magnetic field, which can release energy via magnetic reconnection and/or guide waves that might dissipate energy in the corona (for reviews, see Klimchuk 2006;Reale 2010;Cranmer & Winebarger 2019). The hot plasma of the solar corona travels outward into interplanetary space to form the solar wind, thereby blowing a cavity in the interstellar medium that is known as the heliosphere.…”

Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)

“…De Pontieu et al, 2009;Moore et al, 2011), resonant interactions with ion-cyclotron waves (Hollweg and Isenberg, 2002), and others (e.g. Cranmer and Winebarger, 2019, and the references therein), but these are to date poorly constrained by observations. Similarly, the slow wind has variously been proposed to originate from the edges of coronal holes, closed-open field boundaries within and bordering active regions, streamers (particularly streamer tops), or small coronal holes, but while these locations are all associated with magnetic reconnection between closed magnetic flux systems and open ones that connect to the wind, there is no consensus on the dominant underlying field configuration or acceleration mechanism (e.g.…”